ISSN:0308-6003    

DOI:10.1039/PR97875FX001

出版商:RSC    

年代:1978

数据来源: RSC

ISSN:0308-6003    

DOI:10.1039/PR97875BX003

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

2 PhysicaI Chemist ry of Proteins By A. GAFNI Department of Chemical Physics The Weizmann Institute of Science Rehovot Israel 1 Introduction In this chapter recent developments in several areas of the physical chemistry of proteins will be reviewed with special emphasis on optical spectroscopic techniques. These techniques and their applications continue to develop very rapidly; hence only a small fraction of the total work done can fit within the scope of this review. The topics to be covered deal with (a)folding processes in proteins (b) the application of optical activity techniques in studies of protein structure and association (c) the environment of aromatic amino-acids in proteins and (d) the determination of distances in proteins by energy-transfer measurements.2 Protein Folding Unfolding and Denaturation The hypothesis that all the information needed to form the three-dimensional structure of a protein molecule is contained in the sequence of its amino-acids has been suggested many years ago.”* The mechanism by which the folding process takes place still is the goal of many studies. Based on both equilibrium and kinetic studies it has been suggested that the reversible denaturation of proteins involves essentially two major stable conformational states native and denatured; i.e. to be a highly co-operative pro~ess.~ However an ever increasing number of studies have shown that many protein-folding and -unfolding reactions do not follow a simple two-state mechani~m,~ and stable intermediate states were identified along the reaction pathway.’ In some cases the unfolding mechanism was found to depend on the denaturing conditions.Thus the heat denaturation of cy -1actalbumin at neutral pH is described as a two-state transition6 while the unfolding by guanidine-HC1 M. Sela F. H. White and C. B. Anfinsen Science 1957 125 691. C.B. Anfinsen Science 1973,181,223. (a)C. Tanford Adv. Protein Chem. 1968 23 121; (b) J. F.Brandts in ‘Structure and Stability of Biological Macromolecules’ ed. G. Fasman and S.Timasheff Marcel Dekker N.Y. 1969,p. 213;(c)C. N.Pace C.R.C. Crit. Rev. Biochem. 1975,3,1;(d)J. Bandekar Internat.J. Peptide Protein Res. 1978 11 191; (e) F.Ahmad and P. McPhie Biochemistry 1978 17 241. (a)R.L. Baldwin Ann. Rev. Biochem. 1975,44,453;(6)T.M.Li J. W. Hook III. H. G. Drickamer and G. Weber Biochemisrry,1976,15,5571; (c) T. Y.Tsong ibid.,p. 5467;(d)P. J. Hagerman and P. L. Baldwin ibid. p. 1462. (a)D.A. Chignell A. Azhir and W. B. Gratzer EuropeanJ. Biochem. 1972,26,37;(6)E.A. Carrey and R. H. Pain Biochim. Biophys. Acta 1978,533,12; (c) K. P.Wong and C. Tanford J. Biol. Chem. 1973,248,8518. K. Kuwajima and S. Sugai Biophys. Chem. 1978,8 247. 5 A. Gafni involves an intermediate form (which has the same amount of helical structure as the native form) and cannot be explained by a two-state mechanism.’ In many proteins groups of amino-acid residues along the polypeptide chain are found to cluster into relatively rigid domains the interactions inside domains being stronger than inter-domain interactions.A multi-domain model has recently been proposed for peni~illinase,~~ in which the domains can separate without appreciable change of secondary structure. The presence of structural domains distributed along the flexible polypeptide chain of cell surface protein has also been suggested from denaturation studies by surfactants and denaturants.’ The existence of these domains in proteins is believed to play a major role in the folding mechanism since it suggests that parts of a continuous polypeptide chain can fold independently to form nucleation sites for protein folding. The nucleation process in which locally ordered segments of the polypeptide chain are formed is the initial stage of folding and the experimental and theoretical evidence for this view has been reviewed by Anfinsen and Scheraga.’ A method for predicting nucleation sites for protein folding based on hydrophobic interactions was introduced.lo The nucleation sites are predicted by searching the amino-acid sequence of the protein for pockets of non-polar residues whose net free energy of interaction is negative. The predictions of the model were tested for several proteins. Kanehisa and Tsong” have treated the mechanisms of folding and unfolding of globular proteins also assuming locally ordered regions of the polypeptide chain (which they termed ‘clusters’) at an early stage of the folding process. The presence of clusters of amino-acid residues in the denatured state of proteins was verified by evaluating the heat capacity and volume changes which accompany protein denaturation.l2 The published values of these parameters are in accord with those estimated for a denatured state in which hydrophobic side-chains remain clustered and out of contact with solvent water. A statistical-mechanical treatment of protein folding has been pre~ented’~ based on the theory of a one-dimensional lattice gas with long-range many-body inter- actions. The model was applied to three typical proteins and their folding pathways were traced. The roles of both short- and long-range interactions in protein folding were studied by a Monte-Carlo simulation method within the framework of a lattice m0de1.l~ The protein molecule was represented by a self-avoiding chain polymer consisting of N units located at the lattice points of the square lattice and connected by linear bonds whose length is the same as the lattice constant.Two types of forces were considered to operate; short-range and long-range interactions. It was concluded that while the short-range interactions accelerate the folding and unfold- ing transitions the highly co-operative stabilization of the native structure is achieved by the long-range interactions. In a second study” a three-dimensional lattice model designed to assimilate the native conformation of lysozyme was tested ’M. Nozaka K. Kuwajima K. Nitta and S. Sugai Biochemistry 1978 17 3753. G. Colonna S. S. Alexander Jr. K. M. Yamada I. Pastan and H. Edelhoch J. Biol. Chem. 1978,253 7787. C.B. Anfinsen and H. A. Scheraga Adv. Protein Chem. 1975,29,205. lo R. R. Matheson Jr. and H. A. Scheraga Macromolecules 1978,11,819. M. I. Kanehisa and T. Y. Tsong J. Mol. Biol. 1978 124 177. J. Bello J. Phys. Chern. 1978,82 1607. l3 H.Wako and N. Saito J. Phys. SOC.Japan 1978,44,1939. l4 N.Go and H. Taketomi Proc. Nut. Acad. Sci. U.S.A.,1978,75 559. Is Y. Ueda H. Taketomi and N.Go Biopolymers 1978 17 1531. Physical Chemistry of Proteins 7 by computer simulation; the process of unfolding as well as the denatured states of the model were discussed. The effect of structural heterogeneity inside globular proteins on their structure and folding has been analysed. l6 This heterogeneity arises from the presence (in the interior of proteins) of clusters of polar residues which separate domains of non-polar residues.From the amino-acid sequence the residues occurring in non-polar clusters were predicted satisfactorily for lysozyme. It was suggested that formation of large non-polar domains in the protein interior is preceded by the formation of small clusters by local interactions. The independent re-folding of domains was studied experimentally in elastase.” By limited proteoly- sis using trypsin a fragment of elastase containing residues 126-245 was prepared and its complete denaturation was achieved by 6M guanidine-HCl in the presence of mercaptoethanol. Upon removal of denaturants spontaneous re-folding of the fragment to its native form took place and the ability to bind elastin was restored.Ribonuclease A.-Several of the studies published in 1978dealt with the re-folding reactions of ribonuclease A the first protein shown to be able to resume its native structure after denaturation and complete reduction of its disulphide bonds.18 The extended two-state model for the re-folding of ribonuclease A has been tested. 19*20 This model assumes that the only species to be found during the folding process of proteins are the native structure and multiple forms of the completely unfolded protein.21 Multiple denatured forms of lysozyme have recently been found by both laser Raman22 and ultrasonic absorption23 studies. Two unfolded species of ribo-nuclease A (U and U,) have indeed been and it has been proposed that these differ only in the cis-trans isomers of the four proline residues.21 One of these two species (U,) the fast-re-folding species is postulated to contain all the proline residues in the same conformations as in the native enzyme while U1 consists of several species each containing one or more proline residues in a conformation different from that found in the native enzyme.The slow folding of U1is rate-limited by the interconversion to U2 which upon formation folds rapidly. Nall et a1.l’ found that the activation enthalpy of the U1eU2interconversion is much too small to be attributed to proline isomerization and that the reaction rate changes sharply with guanidineVHC1 concentration (while the rate of proline isomerization in model compounds does not). Thus proline isomerization may not be the rate-limiting step in the U1 eU2 interconversion.The authors also found a rapid formation of partial folding in re-folding conditions thus casting doubt on the validity of the extended two-state model for re-folding of ribonuclease A. A kinetic study of the folding of the two unfolded forms of ribonuclease A as a function of solvent viscosity involved adding various concentrations of glycerol or sucrose.2o No dependence on solvent viscosity was found for either the fast folding reaction or for the U1 eU2 intercon- J. Crampin B. H. Nicholson and B. Robson Nature 1978,272 558. l7 C. Ghelis M. Tempete-Gaillourdet and J. M. Yon Biochem. Biophys. Res. Comm. 1978,84,31. C. B. Anfinsen E. Haber M. Sela andF. H. White Jr. Proc. Nut. Acad. Sci. U.S.A.,1961,47 1309.l9 B. T.Nall J. R. Garel and R. L. Baldwin J. Mol. Biol. 1978,118,317. *’ T. Y.Tsong and R. L.Baldwin Biopolymers 1978,17 1669. 21 J. F. Brandts H. R. Halvorson and M. Brennan Biochemistry 1975 14,4953. 22 R. S. Porubcan K. L. Watters and J. T. McFarland Arch. Biochem. Biophys. 1978 186 255. 23 K.Yamanaka H. Nakajima and Y. Wada Biopolymers 1978 17 2159. 24 J. R. Garel and R. L. Baldwin Proc. Nut. Acad. Sci.,U.S.A.,1973,70 3347. 25 J. R.Garel B. T. Nall and R. L. Baldwin Proc. Nut. Acad. Sci. U.S.A. 1976,73 1853. 8 A. Gafni version; hence neither reaction is rate-limited by external frictional effects (due to the solvent) on segmental motion. From the effect of various denaturants on the rate of the fast folding reaction it was concluded that a structural intermediate is involved in the rate-limiting step of this reaction.Early formation of a folding intermediate in ribonuclease A has also been shown by 'H n.m.r. spectroscopy which revealed that the slow-folding species U1 rapidly becomes partly folded upon initiation of re-folding.26 The appearance of enzymatic activity of ribonuclease A was also used to monitor re-folding in a study devoted to the early steps of this process.27 A low level of activity (about 0.04% that of the native enzyme) was found at very early stages of the folding and was shown to be due to partially (or completely) reduced molecules. The findings were explained as the result of an equilibrium between native and unfolded conformations in reduced ribonuclease A.The presence of an intact folded region in the reduced enzyme was also shown in measurements of fluorescence quantum yield and lifetime of the tyrosine residues.28 This folded region was found to be hydrophobic and to contain one tyrosine residue. Oligomeric Proteins.-In oligomeric proteins composed of two or more identical subunits it is usually found that re-folding of the individual subunits precedes their re-association and is the rate-limiting step in the generation of native ~tructure.~~ However in some cases the re-association step is found to be the major barrier in renaturation. This was found to be the case in bovine seminal ribon~clease.~' The two subunits of this dimeric enzyme are similar in structure to ribonuclease A and are connected by two disulphide bonds.Fully reduced and denatured seminal ribo- nuclease was regenerated using the glutathione redox system and the major product was found to be the monomer which was twice as active towards yeast RNA as the native enzyme. The circular dichroism of the monomer differed from that of the native enzyme and resembled the c.d. of ribonuclease A. The inability of the subunits to associate is the result of blocking of the cysteine residues which form the disulphide bonds by glutathione. Active subunits have also been observed upon dissociation of yeast tran~ketolase~~ In the latter case and superoxide di~mutase.~~ only dissociation occurred in 8M urea with no further denaturation suggesting that dissociation precedes disruption of the tertiary structure.The re-folding and re-activation of horse liver alcohol dehydrogenase (LADH) was studied after denaturation of the enzyme in 6M guanidine~HC1.~~ Dissociation of the dimeric enzyme was monitored by its enzymatic activity by circular dichroism and by fluorescence. Re-folding and re-activation were achieved by diluting the solutions thereby reducing the concentration of denaturant. The rate of re-activation of LADH was found to depend strongly on the addition of but was 26 A. D. Blum S. H. Smallcombe and R. L. Baldwin J. Mol. Biol. 1978,118 305. 27 J. R. Garel J. Mol. Biol. 1978 118 331. N. Barboy and J. Feitelson Photochem. Photobiol. 1977,26 561. 29 (a) I. Bjork and C. Tanford Biochemistry 1971 10 1289; (b) L. Bornmann B. Hess and H.Zimmermann-Telschow Roc. Nut. Acad. Sci. U.S.A. 1974,71,1525; (c)H. Tennenbaum-Bayer and A. Levitzki Biochim. Biophys. Acta 1976 445 261. 30 G. K. Smith G. D'Alessio and S. W. Schaffer Biochemistry 1978,17 2633. 31 G. A. Kochetov and 0.N. Solovieva Biochem. Biophys. Res. Comm. 1978 84 515. 32 J. V. Bannister A. Anastasi and W. H. Bannister Biochem. Biophys. Res. Comm. 1978,81 469. 33 J. Gerschitz R. Rudolph and R. Jaenicke European J. Biochem. 1978 87 591. 34 R. Rudolph J. Gerschitz and R. Jaenicke European J. Biochem. 1978.87 601. Physical Chemistry of Proteins 9 not affected by the presence of coenzyme NAD' (coenzyme concentration did however affect the reaction yield by promoting the formation of inactive aggregation products).The rate of re-activation showed strong dependence on enzyme concen- tration; hence an association reaction must be involved in the process. The irrever- sible heat denaturation of LADH was used to study its interaction with the reduced coenzyme NADH.35 Binding of NADH in the presence of the substrate analogue isobutyramide protects the dimeric enzyme against heat denaturation at 75 "C. The denaturation was followed at various degrees of occupation of the binding sites by NADH and it was concluded that the two subunits bind the coenzyme indepen- dently. Binding of NADH to one subunit protects both subunits against heat denaturation probably by shifting the equilibrium that exists between intact enzyme and the heat-labile dissociated subunits towards the former.That the dissociation of LADH to its subunits is the first step in the heat denaturation process is consistent with the experimental observations. Heat denaturation was followed in Micrococcus lysodeikticus adenosine triphosphatase (ATpa~e).~~ Upon heating the multi- subunit enzyme undergoes a major unfolding transition. The denaturation was irreversible; however no evidence for dissociation into subunits was found after cooling to room temperature and the products retained a large degree of secondary structure. Unfolding of the subunits thus seems to take place without dissociation pointing to a very compact quaternary structure apparently stronger than the tertiary structure of the subunits. ATP CaC12 or high concentrations of ATPase increased the stability of enzyme against denaturation.Coenzyme binding was found to protect pig heart lactate dehydrogenase against inactivation by y-ray~.~' The formation of ternary complexes of enzyme :coenzyme :substrate analogue provided maximal protection apparently by stabilizing the enzyme conformation. Stabilization of dihydrofolate reductase against heat denaturation by the coenzyme NADPH has been In this case too the formation of ternary complexes with coenzyme and inhibitors enhanced the stabilization effect. Several enzymes which form covalent intermediates with their substrates during catalysis were stabilized against pH-induced denaturation in the presence of substrates owing to the stability of these intermediates3' Yeast D-glyceraldehyde 3-phosphate dehydrogenase a tetrameric enzyme was found to be reversibly inactivated and dissociated to its subunits by lowering the temperature from 25 to 0°C in the presence of ATP and mer~aptoethanol.~' The inactive monomer was capable of binding one NAD' molecule which indicates a high degree of residual secondary and tertiary structure in it.The dissociation reaction at 0 "C followed first-order kinetics with a T1/2 of 180minutes. The re-activation at low enzyme concentration showed a higher reaction order with a limiting value of 2. Association of the inactive monomers is therefore a prerequisite for enzymatic activity. The kinetic parameters resemble the ones obtained for re-activation after full denaturation in 6M guanidine~HC1.~~ Thus a common intermediate is suggested " A.Gafni and L. Brand Biochim. Biophys. Acra 1978 537,446. 36 J. A. Ayala and M. Nieto Biochem. J. 1978.169 371. 37 M.Saito Internat. J. Radiation Biol. 1978 34 95. B. Bielaski-Kitchell and R. W. Henkens Biochim. Biophys. Acra 1978,534,89. 39 M. Volini and S. F. Wang. Arch. Biochem. Biophys. 1978,187 163. 40 P.Bartholmes and R. Jaenicke European J. Biochem. 1978,87,563. 41 R. Rudolph I. Heider and R. Jaenicke European J. Biochem. 1977,81 563. 10 A. Gafni in re-activation processes following dissociation of the enzyme under widely differing conditions. Tetrameric Escherichia coli aspartase dissociates and denatures upon heating to 55"C and does not recover its activity upon Treatment of the denatured enzyme with 6M guanidineqHC1 followed by dilution of the denaturant was found to restore a significant fraction of the original enzymatic activity as well as the quaternary structure.A similar re-activation was found for acid-denatured aspartase. Extensive unfolding with a potent denaturant of the entangled poly- peptide chains formed by irreversible denaturation reactions may thus lead to regeneration of the native structure upon removal of denaturant. Another tetra- meric enzyme rabbit muscle aldolase was dissociated to its subunits at alkaline pH (pH >12).43The dissociation was followed by denaturation both reactions being of first order. The free subunits were considered to show partial activity. The re- activation kinetics were described by sequential interconversion and association reactions with first- and second-order rate constants respectively.Changes in the absorption spectrum and the enhancement of tryptophan fluores- cence were used to follow the denaturation of Euglena cytochrome c-552caused by urea and g~anidine.HC1.~~ The accessibility of the two tryptophanyl residues to iodide was studied by fluorescence quenching and was found to be heterogeneous at low denaturant concentrations (which however were enough to cause significant changes in the protein structure as judged from the fluorescence enhancement). Upon complete denaturation the two tryptophanyl residues were equally exposed. These results indicate that intermediates with partial native structure exist along the denaturation pathway and hence that the unfolding of cytochrome c-552does not follow a two-step mechanism.Conformational changes which precede denaturation by guanidineSHC1 were also reported for rabbit skeletal (Y(Y -tropomyosin using pyrene excimer flu~rescence.~~ Partially unfolded intermediates exist at low concentrations of the denaturant. A complex mechanism of unfolding was also found for human gly~ophorin~~ by circular dichroism viscosity and fluorescence of 1-anilino-8-naphthalenesulphonatebound to the protein. A simple two-state mechanism of denaturation was found for chymotrypsinogen A that was immobil- ized by covalent attachment to derivatized porous glass beads.47 Denaturation was achieved by urea and guanidineSHC1 and first-order kinetics for both unfolding and re-folding were monitored by changes in the enzyme fluorescence.3 Conformations and Interactions Studied by Optical Activity Techniques Circular Dichroism of Intrinsic Chromophores.-Circular dichroism (c.d.) was used to study the secondary structure of lipophilin (a hydrophobic protein purified from human central nervous system myelin) in aqueous and lipid ~olutions.~~ The secondary structure composition was calculated assuming that the contributions to 42 M. Tokushige and G. Eguchi Biochim. Biophys. Acta 1978,522 243. 43 R. Rudolph A. Haselbeck F. Knorr and R. Jaenicke Hoppe-Seyler'sZ. Physiol. Chem. 1978,359,867. 44 I. Aviram and C. Weissmann Biochemistry 1978,17 2020. 45 S. L. Betcher-Lange and S. S. Lehrer J. Biol. Chem. 1978 253 3757. 46 D.M.Byers and J. A. Verpoorte Biochim. Biophys. Acta 1978 533,478. 47 H. E.Swaisgood V. G. Janolino and H. R. Horton Arch. Biochem. Biophys. 1978,191,259. 48 S. A.Cockle R. M. Epand J. M. Boggs and M. A. Moscarello Biochemistry 1978,17 624. Physical Chemistry of Proteins 11 the c.d. spectrum in the peptide region (190-250 nm) from a-helix @-sheet and unordered conformations were additive. When the protein was introduced into phosphatidylcholine vesicles about 75% a-helical conformation was found. A similar content of a-helix was found for the water-soluble form of lipophilin obtained by dialysis from 2-chloroethanol to aqueous solutions devoid of lipid. Other water-soluble preparations obtained by dialysis from phenol-acetic acid-urea had lower helical contents.The amount of @-sheet was minimal for lipophilin incorporated into vesicles. The far-u.v. c.d. spectra of eight lectins have been studied with the aim of resolving contributions from the various structural forms to their secondary All eight proteins appeared to have secondary struc- tures dominated by @-pleated sheet which is generally true for lectins. An attempt to quantitate the three structural components (a-helix @-sheet and unordered) met with difficulties for some of the lectins due among other reasons to the unusual c.d. spectra which show considerable ellipticity above 225 nm a region where no electronic transition of peptides is known to take place. Alteration in the secondary structure of mouse brain tubulin upon interaction with the tranquillizing drug chlorpromazine was studied by ~.d.~' Upon binding of one mole of the drug large changes in the far-u.v.c.d. (200-250 nm) of the protein were observed while no change in the c.d. above 260 nm could be detected. These findings apparently reflect changes in secondary structure (reduction in a-helix content and increase in @-structure) without extensive changes in tertiary structure or in the state of association of the subunit protein. Binding of additional chlorpromazine molecules to tubulin did not induce further changes in secondary structure. The observation of an unperturbed tertiary structure is in accord with the conclusion reached by Lee etal.'l that in calf brain tubulin a disulphide bond maintains a highly stable structural domain containing aromatic chromophores.A large increase in the helical content of troponin C was found to accompany the binding of Ca2'.52 The increase was biphasic about 62% occurring with Ca2' binding to a class of sites with Kca of 2.7 x lo71mol-' and the remaining change with Ca2' binding to a class of sites having Kcaof 3.1 x lo51 mol-'. No effects of salt concentration on the c.d. spectrum of human myeloma immunoglobulin G between 205 and 250nm were detected while difference spectra indicated a change in the degree of exposure of tyrosine residues.53 Increasing salt concentrations thus affect the tertiary (but not the secondary) structure of this immunoglobulin molecule. The contribution of phenylalanine and tyrosine side-chains to the far-u.v.c.d. of peptides and proteins has been calc~lated.'~ The interaction of the aromatic ring with neighbouring peptide bonds generates rotatory strength in its La transition. From the preferred backbone and side-chain conformations it was concluded that the most probable conformations have positive Labands. Thus,a significant positive contribution due to the rotatory strength of aromatic residues in globular proteins is 49 M. S. Herrmann C. E. Richardson L. M. Setzler W. D. Behnke and R. E. Thompson Biopolymers 1978,17,2107. A. G. Appu-Rao D. L. Hare and J. R. Cann Biochemistry 1978,17,4735. '' J. C. Lee D. Corfman R. P. Frigon and S. N. Timasheff Arch. Biochem. Biophys. 1978 185,4. 52 J. D. Johnson and J. D. Potter J. Biol. Chem. 1978,253 3775.53 V. P. Zavyalov S. Yu. Tetin V. M. Abramov and G. V. Troitsky Biochim. Biophys. Acta 1978,533 496. 54 R. W. Woody Biopolyrners. 1978.17 1451. 12 A. Gafni to be expected from nearest-neighbour interactions without invoking stacking interactions among the aromatic side-chains. This was confirmed by calculations for proteins of known conformations at the nearest-neighbour level. The effect is found in several snake venom toxins where side-chain contributions from tyrosine and tryptophanyl residues manifest themselves as positive c.d. bands in the far-u.v. region. Bush et al? studied the c.d. of two groups of cyclic hexapeptides having p turns whose geometry can be firmly established by X-ray crystallography and n.m.r. spectroscopy.They found that no significant contributions to the c.d. spectra in the 195-240 nm region were made by the aromatic phenylalanyl chromophore. Circular Dichroism of Haem-containing Proteins.-The far-u.v. c.d. spectrum of yeast cytochrome c peroxidase indicates that its secondary structure contains roughly equal amounts of cy -helix p -structure and unordered ~tructure.~~ Removal of the haem group did not affect the secondary structure to any significant extent. The c.d. spectra of the haem group under various conditions suggest that this moiety is located in a region tightly surrounded by amino-acid residues of the protein. Interaction between the two haem groups (haem c and haem dl) which are found in each subunit of the dimeric enzyme cytochrome oxidase was studied by magnetic circular di~hroism.~’ This technique and some of its biological applications have been reviewed during 197EL5*The spectral range of 350-700 nm was studied for the cytochrome oxidase in the oxidized reduced and reduced-carbon monoxide forms.All the spectra were simple sums of contributions from haem c and haem dt Also no effect of ligand binding to ferrous haem dl was observed in the magnetic c.d. spectrum of haem c in contrast to a previous rep~rt.’~ Thus no interaction between the two haem groups in each subunit was detected. The intrinsic c.d. spectrum of ferric cytochrome b562isolated from E. coli reflected the presence of 52% a-helix and 48% random structure while a slightly lower cy -helix content (49%) was found for the reduced form of the pr~tein.~’ In neither case was the presence of p-structure detected.The near-u.v. and visible c.d. spectra (up to 600 nm) of the oxidized and reduced forms of the proteins were also studied and it was concluded that conformational alterations in the haem group occurred upon change in the valency of the metal ion. The oxidation-reduction process was also found to change the environment of the aromatic chromophores in the cyto- chrome molecule. Changes in the tertiary structure of a cytochrome molecule which depend on its redox state were also reported to occur in the cytochrome b-cl complex from yeast.61 In this case changes in the c.d. spectrum of cytochrome c1 were induced by reduction of the cytochrome b molecule indicating conformational interactions between the two cytochrome molecules.This interaction was not influenced by binding of the inhibitor antimycin A to the complex; hence the interaction does not seem to be involved in the mechanism of electron transport through the complex. 55 C. A. Bush S. K. Sarkar and K. D. Kopple Biochemistry 1978,17,4951. 56 G. Sievers Biochim. Biophys. Acta 1978 536 212. ” L.E.Vickery G. Palmer and D. C. Wharton Biochem. Biophys. Res. Comm. 1978,80,458. 58 B. Holmquist and B. L. Vallee in ‘Methods in Enzymology’ ed. C. H. W. Hirs and S. N. Timasheff Academic Press New York 1978 Vol. 49 p. 149. s9 Y. Orii H. Shimada T. Nozawa and M. Hatano Biochem. Biophys. Res. Comm. 1977,76,983. 6o P.A. Bullock and Y. P.Myer Biochemistry 1978,17 3084.61 J. Reed T. A. Reed and B. Hess European J. Biochem. 1978,91,255. Physical Chemistry of Proteins 13 Circular Dichroism and Circular Polarization of Luminescence in Protein-Ligand Interactions.-The c.d. spectra of thionicotinamide adenine dinucleotides (SNAD’ and SNADH) were investigated in solution and in the complexes formed with horse liver alcohol dehydrogenase (LADH).62.63Cleaving experiments of the coenzyme analogue in aqueous solution by phosphodiesterase revealed the existence of stacking interactions between the adenine and thionicotinamide rings in the intact coenzyme. The c.d. spectrum of SNADH in the ternary complex with LADH and isobutyramide had features resembling the spectrum obtained for the cleaved coenzyme suggesting an open conformation for the bound coenzyme.The larger c.d. signal observed for the bound SNADH relative to the spectrum of the free species was ascribed to a higher degree of conformational rigidity of the bound form. A highly immobilized dihydropyridine ring of NADH was also suggested to be present in the ternary complex between coenzyme UDP-galactose-4-epimerase and UMP.64This suggestion is based on the large increase in the c.d. spectrum of the coenzyme that is observed upon binding. Some interactions between proteins and small ligand molecules were studied using the circular polarization of luminescence (c.p.1.) technique. C.p.1. is a measure of the optical activity of fluorophores in their electronically excited state i.e. a c.p.1. spectrum is related to the molecular conformation in the excited state in a way similar to that in which the c.d.spectrum is related to the molecular conformation in the ground state. Both the instrumental aspects of the c.p.1. technique and its biochemi- cal and biophysical applications have been reviewed during 1978.65*66 A comparison between the c.d. and c.p.1. spectra of NADH in aqueous solution showed them to be markedly different.67 However since cleavage of the coenzyme molecule by phos- phodiesterase did not affect the spectra it was concluded that the differences originate in conformational changes of the excited state of the nicotinamide ribose fragment. Significant differences were also found between the c.d. and c.p.1. spectra of the LADH :NADH binary complex as well as between the corresponding spectra of the coenzyme bound to beef heart lactate dehydrogenase (LDH) in binary and ternary complexes (the latter with the inhibitor oxalate).These observations indicate that in all three complexes the nicotinamide ring enjoys some freedom of movement in its electronically excited state and hence is not fully immobilized in this state. In contrast the c.d. and c.p.1. spectra of NADH in the ternary complex with LADH and isobutyramide (IBA)were found to be very similar indicating that in this case the environment of the nicotinamide ring does not change upon excitation. The coenzyme is therefore rigidly bound in the ternary complex. This conclusion is supported by the very high degree of linear polarization of fluorescence observed for NADH in the ternary complex.While the c.p.1. spectra of LADH:NADH and LADH :NADH :IBA differ substantially from one another their c.d. spectra are identical verifying that binding of the substrate analogue immobilizes the nico- tinamide ring without affecting its environment in the binding site. The increased 62 R. Joppich-Kuhn and P. L. Luisi European J. Biochem. 1978,83 587. R. Joppich-Kuhn and P. L. Luisi European J. Biochem. 1978 83,593. 64 S. S.Wong J. Y. Cassim and P. A. Frey Biochemistry 1978,17 516. 65 I. Z. Steinberg in ‘Methods in Enzymology’ ed. C. H. W. Hirs and S. N. Timasheff Academic Press New York 1978,Vol. 49 p. 179. 66 I. Z. Steinberg Ann.Rev. Biophys. Bioeng. 1978 7 113. 67 A. Gafni Biochemistry 1978,17 1301.14 A. Gafni rigidity of the ternary LADH :NADH :IBA complex may explain its high stability against heat denat~ration.~’ The observed similarity in the c.d. spectra of the binary and ternary complexes of NADH with LADH is not a general feature of complexes of the coenzyme. Thus the c.d. spectra of NADH bound to dihydrofolate reductase in binary and ternary complexes (formed with inhibitors) were found to differ markedly,68 reflecting differences in the environment of the bound nicotinamide group. The signs of both c.d. and c.p.1. spectra of NADH bound to LADH were opposite to those of the LDH-bound coenzyme.67 Structural differences between the two nicotinamide- binding sites are thus evident and undoubtedly result from the proximity of these sites to the substrate-binding sites.The latter may be expected to differ being constructed to accommodate very different substrates. The mode of interaction between a series of oligosaccharides dansylated at their reducing end and the homogeneous mouse IgM secreted by MOPC-104E was studied by various technique^.^^ The c.p.1. of all the dansyloligosaccharide-MOPC-104E complexes was within the experimental error of the instrument proving that the dansyl chromophore was not in a chiral environment. The dansyl group appears to protrude out into the solvent and does not interact effectively with the oligosac- charide-binding site. 4Environment of Amino-acid Residues in Proteins Fluorescence Quenching.-The quenching of intrinsic tryptophanyl fluorescence in proteins by compounds of low molecular weight may serve to distinguish between residues exposed to the solvent (which may be quenched) and those buried inside the protein (which are not accessible to the quenching agent).70 The quenching of exposed tryptophans by iodide ions is dynamic and may be described by the Stern-Volmer equation F,/F = 1+&[I-] where Foand F are the fluorescence intensities in the absence and presence of iodide and KOis a constant.However if a second population of tryptophan residues not accessible to I- is present the plot of Fo/F us. [I-] will deviate from linearity. Lehrer70C proposed a modified Stern-Volmer plot to describe the quenching data in such cases FOIW = (1/CI-Ifa&J +(Wa) (2) here hF =Fo-F; i.e.the change in fluorescence intensity observed at a concen- tration of quencher [I-] while fa is the fraction of accessible trytophan residues whose quenching constant is KO. ‘* A. V. Reddy W. D. Behnke and J. H. Freisheim Biochim. Biophys. Acru 1978,533,415. 69 G. Schepers Y. Blatt J. Himmelspach and 1.Pecht Biochemistry 1978,17 2239. 70 (a)S. S. Lehrer Biochem. Biophys. Res. Comm. 1967,29,767; (6) E. A. Burstein Biofiziku 1968,13 433; (c)S. S. Lehrer Biochemistry 1971,10,3254; (d)R. F. Steiner R. E. Lippoldt H. Edelhoch and V. Frattali Biopolymers 1964 Symp. 1 p. 355; (e) S. S. Lehrer in ‘Biochemical Fluorescence Concepts’ ed. R. F. Chen and H. Edelhoch Marcel Dekker New York 1975 Vol. 2 p. 515; (f)S. S. Lehrer and P. C. Leavis in ‘Methods in Enzymology’ ed.C. H. W. Hirs and S. N. Timasheff Academic Press New York 1978 Vol. 49 p. 222. Physical Chemistry of Proteins 15 The effect of iodide on the fluorescence of the oxygen-carrier protein haemocy- anin from the snail Levantina hierosolima was In the apoprotein which is devoid of the copper ions about half of the tryptophanyl fluorescence was accessible to quenching by I-. No quenching by iodide of the fluorescence of oxyhaemocyanin was detected and it was concluded that the residual fluorescence observed in this case originates exclusively in buried tryptophans. Fluorescence decay measure- ments showed that the introduction of copper into the apoprotein to form deoxy- haemocyanin did not affect the emissive properties of the protein while 55% of the tryptophanyl emission was quenched upon the introduction of oxygen.It appears that the copper-binding site in haemocyanin is near the exterior of the protein close to the tryptophans which are accessible to the solvent. Iodide was used as a quencher of exposed tryptophans in antithrombin a single-chain glycoprotein of mol. wt. 65 000 which is a protease inhibitor in the plasma. The quenching curves deviated from the Stern-Volmer equation but followed the modified relation of equation (2). From the data a value of 0.6 was obtained for the fraction of fluorescence due to exposed tryptophan residues in this multi-tryptophan-containing protein. Upon denaturation in 6M guanidine. HCI all the tryptophans were exposed to the solvent. Fluorescence difference spectra revealed that the quenchable tryptophans are located in a relatively polar environ- ment on the protein surface.Upon binding of heparin to antithrombin 111 the pattern of quenching by I- changed drastically. It was concluded that a con- formational change of the protein following heparin binding radically changes the number of exposed tryptophans. The fluorescence of the heparin-antithrombin I11 complex may be dominated by a single tryptophan residue. Horse liver alcohol dehydrogenase contains two tryptophan residues per subunit Trp-15 and Trp-314. From X-ray data it appears that while Trp-314 is buried inside the enzyme Trp-15 is probably close to the surface. 33% of the fluorescence was found to be quenchable by I-.73 The spectrum that was obtained after maximal quenching had an emission maximum at 320nm characteristic of a tryptophan residue in a hydrophobic environment.When this spectrum was subtracted from the enzyme emission in the absence of I- a red-shifted spectrum was obtained with a maximum at 340 nm; hence this emission originates in a tryptophan residue located in a polar environment. Thus only the exposed Trp-15 appears to be accessible to the quencher. The quantum yields of the 'blue' and 'red' tryptophans (i.e.Trp-314 and Trp-15) were evaluated from the data to be 0.37 and 0.19 respectively. In egg-white apo-riboflavin-binding protein no quenching of tryptophanyl fluorescence by I- or Cs' could be detected.74 Both anionic and cationic agents effectively quenched the fluorescence of the denatured protein in 4M guanidineSHC1.All the tryptophan fluorescence in apo-riboflavin-binding protein is therefore due to residues buried inside the protein. Quenching of tryptophan fluorescence of yeast hexokinase isoenzyme B by Cs' I- and glucose has been performed at various pH values.75 The 7' N. Shaklai A. Gafni and E.Daniel Biochemistry 1978 17,4438. 72 R. Einarsson Internat. J. Peptide Protein Res. 1977,10,342. 73 M.A. Abdallah J. F. Diellrnann P. Wiget R. Joppich-Kuhn and P. L. Luisi European J. Biochem. 1978,89,397. 74 Y. Nishina K. Horiike K. Shiga and T. Yarnano J. Biochem. (Japan) 1977,82 1715. D. C.Kramp and I. Feldman Biochirn. Biophys. Actu 1978 537,406. 75 16 A. Gafni data indicated that the four tryptophan residues of the monomer subunit may be classified as (a) one highly accessible surface tryptophan (6) one surface tryp- tophan possibly in a crevice with restricted accessibility (c) one tryptophan residue inside a cleft (partially shielded) and (d)one buried tryptophan in the hydrophobic interior of the protein.An uncharged quenching agent of tryptophan fluorescence in proteins is acryl- amide. This molecule has been used to study the exposure to solvent of tryptophan residues in several immunoglobulin^.^^*^^ Schreiber et al.76studied the quenching in three homogeneous rabbit antipeptidoglycan antibodies in the absence and in the presence of their specific ligands. The fluorescence of the Fc and F(ab‘),fragments is quenched more effectively than that of the intact protein.Binding of the specific haptens to the antibodies has a small effect on the quenching by acrylamide apparently resulting from shielding of tryptophan residues which are located in or near the combining sites. Middaugh and Litman77 studied acrylamide quenching of tryptophan fluorescence in six cold-soluble IgM proteins and in two IgM types which possess cryoglobulin properties (abnormal cold insolubility). The quenching curves deviated from the Stern-Volmer relation and showed an upward curvature. The quenching was accompanied by a blue shift of the emission maximum. The spectra of the IgM molecules in 6M guanidineqHC1 were typical of tryptophans exposed to aqueous environment having a maximum at 350 nm.The quenching data were resolved into static and dynamic components by a modified Stern-Volmer equation. While the static components for quenching of the two cryo-immunoglobulins tested were similar to those of the non-cryo-immunoglobulins some differences between the two classes of IgM were found in the kinetic quenching components. Quenching of tyrosine fluorescence by acrylamide was used to study the effects of an inter-chain disulphide cross-link introduced into rabbit skeletal tropomyosin on the structure of the 52% of the tyrosine fluorescence in the folded state of tropomyosin was accessible to acrylamide quenching while in the unfolded state (in 5M guanidine-HCl) the accessibility was 88%. No increase in accessibility of tryosines to the quencher was found between 0 and 2 moles of denaturant despite the increase in flexibility and partial unfolding of the protein as shown by other techniques.Thus the quenching data suggest that the disulphide cross-link keeps the chains together in the region where most of the tyrosines are located i.e. the C-terminal half of the molecule. Trichloroethanol is an efficient quencher of tryptophanyl fluorescence in pro- tein~,’~ and was used to probe these fluorescent residues in wheat-germ agglutinin.*’ At low quencher concentrations the Stern-Volmer law was followed with a rate constant for quenching of 1.2x lo91 mol-’ s-l. In the presence of trichloroethanol the two fluorescent tryptophan residues in each polypeptide chain of the dimeric protein are photochemically modified.This modification leads to a reduction in the haemagglutinating activity of the protein and in its affinity towards chitin oligomers. It appears from the data that tryptophan residues are located in the binding sites of 76 A. B. Schreiber A. D. Strosberg and I. Pecht Zmmunochemistry 1978,15,207. 77 C. R. Middaugh and G. W. Litman Biochim. Biophys. Acra 1978 535 33. ” S. S. Lehrer J. Mol. Biol. 1978,118 209. 79 M. R. Eftink and C. A. Ghiron J. Phys. Chem. 1976,80,486. J. P. Privat and M. Charlier European J. Biochem. 1978.84,79. Physical Chemistry of Proteins wheat-germ agglutinin. This proposition is supported by experiments in which mercuriated oligosaccharides were bound to the protein and the heavy-atom effect on the phosphorescence of tryptophan residues was studied." The mercuriated glycoside derivatives bind to wheat-germ agglutinin in the same way as the cor- responding sugars.The binding results in a drastic quenching of tryptophan fluores- cence with a concomitant increase in the intensity of phosphorescence at 77K. These changes are due to the heavy-atom effect which leads to enhanced inter- system-crossing rates in excited chromophores whose distances from the heavy atom are of the order of van der Waals radii.82 It is therefore evident that a tryptophan residue in the binding site is in close contact with the ligand. Dynamic quenching effects induced in a fluorescent pyrene conjugate of acetyl- cholinesterase by iodide nitromethane and thallous ion were studied and The derivative pyrenebutyl methylphosphonofluoridate (l),conjugates (1) with acetylcholinesterase with a stoicheiometry of one fluorescent label per subunit.The quenching constants of the conjugated pyrene chromophore by all three quenchers are much smaller than those of the pyrene moiety in solution proving the pyrene-binding site to be partially shielded from the solvent. However the decrease in the rate constants for quenching was found to be much smaller for T1' than for I- indicating that the pyrene is located in an anionic environment. Upon binding of (l) the intrinsic tryptophanyl fluorescence of acetylcholinesterase is largely quenched whereas when propidium (a peripheral-site ligand) binds to the protein 80-90% of the pyrene fluorescence is lost.These two quenching processes are considered to result from non-radiative energy transfer in which the pyrene serves as acceptor or donor of energy. Thermal quenching of fluorescence in a series of proteins was studied over the temperature range 5-75 "C and an attempt was made to correlate the quenching with intramolecular structural mobility.84 For proteins containing one or two fluorescent tryptophan residues the following relation was found to hold l/q=a+b. T/q (3) where q is the quantum yield a and b are temperature-independent constants T is the temperature and q is the viscosity of the solvent (water). This dependence of q on T/q (whose value is proportional to the diffusion coefficient in water) was found both in proteins whose tryptophans are exposed to the solvent and those in which " M.Monsigny F. Delmotte and C. Helene Proc. Nut. Acad. Sci. U.S.A.,1978,75 1324. 82 W.C.Galley and R. M. Purkey Proc. Nut. Acad. Sci. U.S.A. 1972,69,2198. *' H.A.Berman and P. Taylor Biochemistry 1978,17 1704. 84 T.L.Bushueva. E. P. Busel and E. A. Burstein Biochim. Biophys. Acta 1978,534,141. 18 A. Gafni these residues are shielded. Deviations from linearity of the l/q us. T/q plots occur only in the temperature ranges in which denaturation of the proteins takes place. The results indicate that the mobility of protein structures is controlled by the diffusion characteristics of the solvent and that the intramolecular collisions between excited chromophores and quenching groups occur during fluctuations of the protein’s structure.Different values were obtained for the constants [a and b of equation (3)] in different proteins reflecting differences in their structures. Solvent Perturbation.-The environment of tryptophan residues in proteins may be studied from perturbations of their absorption spectra induced by solvent mole- cule~.~~ This technique was used in a comparative study of bovine human and guinea-pig a-lactalbumins.86 The perturbing solvents used were ethylene glycol glycerol and dimethyl sulphoxide (20% in each case). The number of exposed tryptophans was calculated from the ratio of A&for the proteins to A& of N-acetyl-L- tryptophan ethyl ester (A& is the difference betwe.cn extinction coefficients in the region 291-294 nm in the presence and absence of perturbing solvent).The results showed almost the same degree of exposure of the tryptophan residues in all three a-lactalbumins both at 25 “C and at 2 “C. Since Trp-26 and Trp-60 of bovine a-lactalbumin are replaced by other amino-acids in the human and guinea-pig protein species these residues cannot be the exposed ones and they must be inaccessible to solvent. This conclusion is based on the assumption that the three lactalbumins have similar conformations and evidence for this was found in the very similar far- and near-u.v. c.d. spectra. A cationic detergent was also used to perturb the tryptophan absorption. From the similar difference spectra obtained for a-lactalbumin and lysozyme it was suggested that the former has a cleft-like region similar to the active-site cleft of lysozyme.The difference spectra indicated that two tryptophan residues are located in this region of the protein. Exposure of tyrosine residues to the solvent in ovomucoid was studied by difference spectroscopy following solvent perturbation by glycerol ethylene glycol and dimethyl ~ulphoxide.~’ The difference spectra in 3.5M guanidineaHC1 showed the same number of tyrosine residues to be exposed as in the absence of denaturant; hence although the protein was significantly unfolded it retained a degree of native structure. Upon increasing the concentration of guanidineSHC1 to 6M complete loss of native structure occurred and all six tyrosine residues were exposed. 5 Determination of Distances by Non-radiative Energy Transfer The quantum-mechanical theory of resonance energy transfer by dipole-dipole interaction between a pair of donor and acceptor chromophores has been developed by Forster.88 The possible use of energy-transfer efficiency as a ‘spectroscopic ruler’ for the determination of distances (of up to several nm) among chromophores in macromolecules was discussed by Stryer and Ha~gland.~~ In the past few years 85 T.T. Herskovits in ‘Methods in Enzymology’ ed. C. H. W. Hirs Academic Press New York 1967 vol. 11 p. 748. 86 K. Takase R. Niki and S. Arima J. Biochem. (Japan) 1978 83 371. M. A. Baig and A. Salahuddin Biochem. J. 1978 171 89. Th. Forster in ‘Modern Quantum Chemistry’ ed. 0.Sinanoglu Academic Press New York 1965 Part 111 p.93. 8y L. Stryer and R. P. Haugland Proc. Nar. Acad. Sci. U.S.A..1967 58 719. Physical Chemistry of Proteins 19 energy transfer has been used in an ever-increasing number of studies and several review articles have been published dealing with various aspects of this technique.” The energy-transfer process by depopulating the electronically excited state of the donor directly competes with light emission and with non-radiative de-excitation processes. The transfer efficiency T,in terms of the donor’s fluorescence intensity is given by T = 1-FIFO= 1/[1+(r/Ro)6] (4) where F and Foare the fluorescence intensities of donor in the presence and absence of the acceptor r is the distance between donor and acceptor and R is that distance between the donor-acceptor pair at which the energy-transfer efficiency is 0.5.According to Forster,88 Ro (in Angstroms) is given by here 4 is the fluorescence quantum yield of the donor in the absence of acceptor K’ is the orientational factor given by K~ = (cos 8DA -3 cos 8A cos 8D)2 where 8DA is the angle between the dipoles of donor and acceptor 8D and 8A are the angles between the dipoles of the donor and acceptor respectively and the line joining them n is the index of refraction of the medium while JDA the overlap integral is given by JDA=([om&A(A) ‘fD(A) ’ A4dA)/IOmfD(n)an (6) where E~(A)is the extinction coefficient of the acceptor and f&) is the relative (corrected) emission intensity of the donor per unit wavelength interval.In order to obtain the distance r between donor and acceptor one has to determine the values of F F, and R,. In most cases it is the determination of R which limits the accuracy of distances determined from energy-transfer measurements mainly due to ~~~*~ uncertainty in the value to be used for K~. Theoretically the orientational factor can range between 0 and 4; however in many cases spectroscopic or structural information regarding the participating chromophores enables one to reduce considerably the range of possible values thereby increasing the accuracy of the determined distance. Haas treated the effect of mixed polarizations in the electronic transitions of donor and acceptor on the accuracy of distances determined from energy-transfer measurements.Mixed polarizations occur when the absorption or emission spectra are characterized by two or more incoherent transition dipole moments. This situation is not uncommon and may result from the presence of several electronic transitions in the pertinent spectral range of absorption of the acceptor or from the partially forbidden character of the electronic transitions of the donor or acceptor. In the latter case the various vibronic transitions of an electronic band may have (a) I. Z. Steinberg Ann.Rev. Biochem. 1971,40,83; (6)R. E. Dale and J. Eisinger in ‘Biochemical p. 115; (c) P. W. Schiller ibid. p. 285; (d) R. F. Fairdough and C. R. Cantor in ‘Methods in Fluorescence Concepts’ ed. R. F. Chen and H. Edelhoch Marcel Dekker New York 1975 Vol.1 Enzymology’ ed. C. H. W. Hirs and S. N. Timasheff Academic Press New York 1978 Vol. 48 p. 347; (e)L. Stryer Ann.Rev. Biochem. 1978,47,819. 91 E. Haas E. Katchalski-Katzir. and I. Z. Steinberg Biochemistry 1978 17 5064. 20 A. Gafni different polarization proper tie^.^' In a chromophore whose electronic transitions have mixed polarizations there is no unique orientation of a transition dipole moment within the chromophore for a given electronic transition. Formally this situation is equivalent to that of a fast but restricted Brownian rotatory motion during the lifetime of the excited state; this has been discussed by Dale and Ei~inger.~" In the extreme case when both donor and acceptor have three perpen- dicular transition moments of equal intensities the value of K will be independent of the orientation of the chromophores (as it is in the case of a fast unrestricted Brownian rotatory motion).A value of f is to be used for K~ in this case. Haas et d9'determined the transition dipole moments of naphthalene and dansyl chromo- phores from their polarization spectra in the spectral range involved in energy transfer between them. The results were applied to evaluate the distribution of end-to-end distances in a series of oligopeptides in viscous solutions using the energy transfer between the two chromophores which were attached to the molecu- lar ends. Due to the mixed polarizations of the naphthalene and dansyl groups the calculated end-to-end distribution function is only very slightly affected by the orientational dependence of the efficiency of energy transfer.In a second paper by Haas et~zl.,~~ the kinetics of fluorescence decay of the donor in the homologous series of oligopeptides labelled at their ends by donor and acceptor was studied in mixtures of glycerol and trifluoroethanol of various viscosities. A disturbance of the equilibrium distribution of end-to-end distances in the population of excited molecules is to be expected due to the energy-transfer process whose efficiency decreases rapidly with distance. This efficiency is enhanced when rear- rangement by diffusion of the molecular ends relative to one another follows excitation. Indeed the fluorescence decay rate of the donor was found to increase with decreasing solvent viscosity.The diffusion coefficients for the Brownian motion of the molecular ends were evaluated from the fluorescence decay data and their values were found to be considerably smaller than those expected for the free chromophores in solvents of the same viscosity. It thus appears that the polymeric chains possess appreciable internal friction which slows down the rate of the Brownian motion. The diffusion coefficients of the end-to-end motion were found to increase with chain length reflecting smaller internal friction in longer chains. A theoretical study of the unperturbed chains Tyr(Ala) Tyr with n =4 or 9 revealed no correlation between end-to-end distances and the relative orientations of the chromophores at the chains ends.94 The dipole moments of the tyrosine residues are very nearly randomly oriented relative to the vector connecting the peptide ends.In a second a Monte-Carlo method was used to generate oligopeptide chains composed of 4,9,or 14repeating units in the random-coil state labelled with tyrosine or tryptophan. Interactions with the solvent (water) were taken into account and the chains represented oligopeptides composed of hydrophobic or hydrophilic amino-acid residues. For all the chains considered the values of K~ were not far from 2. 92 I. Z. Steinberg in 'Biochemical Fluorescence Concepts' ed. R. F. Chen and H. Edelhoch Marcel Dekker New York 1975,Vol. 1 p. 79. 93 E. Haas E. Katchalski-Katzir and I. Z. Steinberg Biopolymers 1978 17 11. 94 A.Englert and M.Leclerc Proc. Nat. Acad. Sci. U.S.A. 1978,75 1050. " M. Leclerc. S. Premilat and A. Englert Biopolymers 1978.17.2459. Physical Chemistry of Proteins 21 Energy transfer was used to evaluate the distances between the AMP- and Mn2'-binding sites in E. coli glutamine ~ynthetase.~~ The fluorescent derivative 1,N6-etheno-AMP(EAMP) was used as donor of energy while the Mn2' ions were replaced by Co2+ which served as the acceptor. The efficiency of energy transfer was followed as a function of Co2' concentration. A biphasic titration curve was obtained and analysed in terms of two Co2'-binding sites. The dissociation constants of Co2' from these sites were calculated from the data. The distances from the E adenine ring to the two bound Co2' ions were evaluated to be 11 and 13 A.The value to be used for Rowas computed assuming a value of $for u2. The absorption band of Co2' in the region of overlap with the ethenoadenine emission spectrum is composed of three orthogonal almost degenerate transition moments; hence the acceptor's dipole moment is effectively isotr~pic.~~*~~ This limits the possible values of K* to a range between f and $(the first value applies when the donor's emission dipole is perpendicular to the line joining the donor and acceptor while the latter value holds when the emission dipole is along this line). The authors concluded that the uncertainty in the distance determination due to the possible range of K values was not more than ~12%.This seems to be a conservative estimate since the electronic transition involved in emission from the E adenine chromophore has mixed polarizations and is not associated with a single dipole moment.99 Co2' was also used as an energy acceptor when substituting for the Zn2' ion of thermolysin molecules in which a luminescent Tb3' ion replaced Ca2' ions in two sites and served as the energy donor.100 A value of $was used for u2,based on the isotropy of the electronic transitions of the two ions in the spectral region pertinent to energy transfer.In an extension of previous studie~,~~,~~ the distance between Tb3' and Co2' was determined as a function of temperature and was found to increase from 14 A at 25 "C to about 21 A at 80 "C. Gradual changes in the structure of the protein thus occur upon heating leading to the increase in distance between the metal-binding sites.These proposed structural changes were not detected by tryptophan fluorescence enzyme activity or optical rotation. The ethenoadenine chromophore was also used as the energy donor when bound (as EATP) to the unique nucleotide-binding site of G-actin."' The acceptor was a non-fluorescent dinitrophenyl derivative bound to the sulphydryl group of Cys-373. While the E ATP molecule was found to be immobilized in its binding site in the time scale of the donor's excited state lifetime the acceptor seemed to enjoy partial rotational freedom. Based on a possible range of K~ values between 5 and $ the distance between the nucleotide-binding site and Cys-373 was calculated to be between 26 and 33 A.Energy transfer was used to study the distance between specific sites on bacterial luciferase."' This enzyme is a heterodimer designated ap where cy is the catalytic subunit. The individual subunits are inactive and in the intact enzyme they are bound with a high affinity. Bacterial luciferase was labelled by allowing an essential 96 J. J. Villafranca S. G. Rhee and P. B. Chock Proc. Nut. Acud. Sci. U.S.A. 1978,75 1255. 97 V. G. Berner D. W. Darnall and E. R. Birnbaum Biochem. Biophys. Res. Comm. 1975,66,763. 98 W. D. Horrocks Jr. B. Holmquist and B. L. Vallee Proc. Nut. Acud. Sci. U.S.A.,1975,72,4764. ''A. Gafni. J. Schlessinger and I. Z. Steinberg J. Amer. Chem. SOC. 1979 101 463. loo S. M. Khan E. R. Birnbaum and D. W. Darnall Biochemistry 1978,17,4669.lo' M. Miki and K. Mihashi Biochim. Biophys. Actu 1978,533 163. '02 S. C. Tu C. W. Wu and J. W. Hastings Biochemistry 1978 17 987. 22 A.Gafni sulphydryl group on the a subunit to react with fluorescent maleimide derivatives. Each of these derivatives was used as donor while 8-anilino-1 -naphthalenesul- phonate (ANS) which binds to the enzyme in a molar ratio of 1:1 served as the acceptor. Energy-transfer efficiencies were determined from the enhancement of ANS fluorescence and by estimating the range of possible values for IC~from fluorescence polarization the distance was evaluated to be between 21 and 37 A. Bound ANS was also used as donor of energy to the natural coenzyme FMN. Since bound FMN is not fluorescent the transfer efficiency was determined from the fluorescence decay rate of the ANS.The distance between the two chromophores was found to be between 30 and 58 A. Energy transfer between chromophores bound to two interacting proteins was found to play a role in Renilla biolumines~ence.'~~ The interaction between luciferase and the green fluorescent protein is highly specific and the energy transfer from enzyme-bound oxyluciferin to the green fluorescent protein is efficient. The distances between individual components of yeast cytochrome c oxidase cytochrome c complex were also evaluated from measurements of energy tran~fer."~ Cytochrome c oxidase is composed of seven distinct subunits and the cytochrome cmolecule is bound to subunit 3. The electron-transferring moiety of the protein contains two haem groups (haem a).Subunit 2 was covalently labelled NHCH,CH,NHCCH,I II 0 with the fluorescent dyes N-(iodoacetamidoethy1)-1-aminonaphthalene-5-sulphonic acid [1,5-AEDANS (2)] or with the 2,6-isomer. These dyes attach to reactive sulphydryl groups. The fast decay of the emission anisotropy of the bound AEDANS indicates that the fluorophore is highly mobile in the complex. A value of $ was assumed for IC~in calculating the distances from AEDANS to the haem a groups or to the haem of cytochrome c. Further justification for using the value for IC'may be found in the fact that the distances measured with the two AEDANS isomers which have quite different molecular symmetries were identical. The distances obtained were subunit 2 to haem a 52A;subunit 2 to cytochrome c (bound to subunit 3) 35A; cytochrome c to haem a 25A.Based on these distances it appears that electron transfer from the haem of cytochrome cto haem a groups of cytochrome coxidase must bridge a considerable distance. Pulse fluorometry was used to study energy transfer from tryptophan residues to NADPH in beef liver glutamate dehydrogenase (GDH).'" Both ternary GDH :NADPH :L-glutamate and quaternary GDH :NADPH :L-glutamate:GTP complexes were studied. Since several tryptophan residues of GDH may serve as energy donors a detailed description of all the transfer processes was not given. The W. W. Ward and M. J. Cormier Photochem. Photobiol. 1978,27 389. M. E.Dockter A. Steinemann and G. Schatz J.Biol. Chem. 1978,253,311. J. C. Brochon Ph. Wahl J. M. Jallon and M. Iwatsubo Biochim. Biophys. Acta 1977,462,759. Physical Chemistry of Proteins 23 tryptophan residues may however be divided into two classes having different energy-transfer rates. Ligand binding induces a conformational change in the enzyme leading to partial quenching of tryptophanyl fluorescence. Energy transfer from tryptophan residues to NADH and NAD' bound to liver alcohol dehy- drogenase was also studied.73 The fluorescence of the two tryptophans in each subunit of the dimeric enzyme is largely quenched upon NADH binding and to a lesser extent when NAD' binds. Energy transfer from tryptophan to NADH accounts for less than half of the observed quenching the rest being apparently caused by conformational changes induced in the enzyme by NADH binding.In spite of the small overlap between tryptophan emission and NAD' absorption it was concluded that energy transfer can contribute to the quenching observed upon NAD' binding. However also in this case a sizeable part of the fluorescence quenching is due to conformational changes which follow coenzyme binding. The fluorescence of tryptophan in proteins decreases with increasing pH,Io6and from this dependence it was suggested that the quenching mechanism is by energy transfer from excited tryptophan to the tyrosinate anions.1o7 Indirect evidence to support this hypothesis was found in p-trypsin from the effects of viscosity and chemical modification of the protein on the fluorescence intensity and lifetime.'0s Quenching by Trp +Tyr energy transfer was the only mechanism consistent with all the experimental results.The minimum distances among the active sites of the four enzyme components of the pyruvate dehydrogenase multi-enzyme complex (from Azotobacter uinlandii) have been estimated from fluorescence energy transfer. lo9 No energy transfer could be detected between thiochrome diphosphate bound to the active site of pyruvate dehydrogenase and the FAD in the active site of lipoamide dehydrogenase. Also no energy transfer was observed from several fluorescent sulphydryl labels bound to the lipoyl moiety of lipoyl transacetylase to either the FAD or to the thiochrome diphosphate. Thus all the active centres of enzymes in the complex are more than 40 A apart at least during some stages of catalysis.The distances between active sites on different catalytic subunits of aspartate trans- carbamoylase and between active sites and sulphydryl groups (which are located near the active sites) on different catalytic subunits were determined by energy transfer.'*' The enzyme of molecular weight 300 000 is composed of two catalytic subunits each being a trimer of mol. wt. 100000 and three regulatory subunits. The energy transfer was measured using hybrid enzyme molecules in which the binding sites of one catalytic subunit were covalently labelled with pyridoxamine phosphate while those of the second catalytic subunit were labelled with pyridoxal phosphate. Energy transfer between these two chromophores and also from pyridoxamine phosphate to 2-mercuri-4-nitrophenol bound to the sulphydryl groups of the second catalytic subunit was measured.In both cases the transfer of energy was efficient and was not affected by the presence of allosteric effectors or of the substrate carbamoyl phosphate. Assuming that the active sites of each catalytic subunit define an R. F. Steiner and H. Edelhoch Nature 1961 192 873. 107 J. W.Longworth in 'Excited States of Proteins and Nucleic Acids' ed. R. F. Steiner and I. Weinryb Plenum Press New York 1971,p. 319. N. Ramachadran and C. A. Ghiron Biochim. Biophys. Acta 1978,532,286. W.H. Scouten A. C. De Graaf-Hess A. Dekok H. J. Grande A. J. W. G. Visser and C. Veeger European J. Biochem.1978,84 17. 110 L.Hsien E.Hahn and G. G. Harnmes Biochemistry,1978.17 2423. 24 A. Gafni equilateral triangle and that the planes of the two triangles are parallel the distance between the binding sites on different catalytic subunits was calculated to be about 30 A. Migration of excitation energy in the highly ordered macromolecular aggregate in phycobilisomes was studied by measuring quantum yields and fluorescence polarization.'" The mean transfer time among the constituent phycobiliproteins was calculated to be about 280 ps. This corresponds to an average of 28 jumps of the excitation energy in the phycoerythrin layer before being captured by phycocyanine. Triplet energy transfer was observed from p-benzophenone that is covalently linked to bovine serum albumin to low-molecular-weight water-soluble quen- The number and locations of the bound benzophenone chromophores were limited by the conditions of preparation and the acceptor molecules were reversibly bound in a protein-quencher complex.Triplet excited acetone is generated in the oxidation of isobutanal catalysed by horseradish peroxidase. 113*114 The triplet species is generated inside the enzyme and is considerably protected from quenching by oxygen. As a result acetone phosphorescence may be observed. Long-range triplet +singlet energy transfer was observed from the excited acetone to the flavin chromophore of FMN or FAD. The sensitized emission of the flavin was indepen- dent of FAD concentration. This emission may therefore arise from a fraction of FAD molecules tightly bound to the enzyme.J. Grabowski and E. Gantt Photochem. Photobiol. 1978 28,47. 'lZ G. I. Glover P. S. Mariano andR. A. Hildreth Photochern. Photobiol. 1978,28 7. 'I3 0.M. M. Faria-Oliveira M. Ham N. Duran P. J. O'Brien C. R. O'Brien E. J. H. Bechara and G. Cilento J. Biol. Chem. 1978 253,4707. '14 M. Ham N. Duran and G. Cilento Bwchem. Biophys. Res. Comm. 1978,81,779.

ISSN:0308-6003    

DOI:10.1039/PR9787500005

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

3 Kinetic Studies of Metal Ion Catalysis of Heterolytic Reactions By D. P. N. SATCHELL Department of Chemistry University of London King’s College Strand London WC2R 2LS and R. S. SATCHELL Department of Chemistry Queen Elizabeth College Campden Hill Road London W8 7AH 1 Introduction There are essentially just four ‘electronic’ types of elementary reaction.’ Out of them in different combinations virtually all chemical reactions however complex are built up. The four types of electronic reaction are (i) those involving no change in electron ownership (e.g. conformational or order-disorder transformations) (ii) those involving complete electron transfer (iii) those involving bond homolysis and its reverse (free radical reactions) and (iv) those involving bond heterolysis and its reverse (Lewis acid-base reactions).Free or ligated metal ions can enter into all four types and a metal ion route for some overall change will sometimes prove to be a catalytic route. The tendencies of different metal ions to engage in these four different types of process depend upon the detailed electron configurations of the ions and upon their resident ligands. A masterly and penetrating start on the rationalization of these matters has been made by Halpern2 and it is difficult to conceive of anything that illustrates better the value of thinking of reactions in terms of electronic types. In a recent review Hipp and Busch3 have made a valiant effort to consider all the various types of metal ion-catalysed reactions in a systematic way by outlining many of the different possible functions of the metal.In this Report we are confining attention to reactions which involve only Lewis acid-base steps. In Lewis acid-base reactions nucleophiles either co-ordinate with or separate from electrophiles. The overall role of a catalyst is therefore to assist nucleophilic attack or departure or perhaps both. A proton the simplest Lewis acid catalyst acts in this respect either by attachment to a suitable site on an electrophile so making it more electrophilic (role I) or by attachment to a departing nucleophile ’ D. P. N. Satchell Nuturwiss.. 1977,64 113. J. Halpern Chem. Eng. News 1966,44,68; Adv. Chem. Ser. 1968 70 1. C. J. Hipp and D. H. Busch in ‘Coordination Chemistry’ ed.A. E. Martell Vol. 2 A.C.S. Monograph 174 Washington 1978 Ch. 2. 25 D. P. N. Satchell and R. S. Satchell thus rendering the latter less nucleophilic (role 11). Metal ions acting as Lewis acid catalysts can and do function in the same two general ways but for them other mechanisms of catalysis are possible. In one of these the metal acts as a centre for the simultaneous attachment of both an electrophile and the attacking nucleophile thus converting an inter- into an intra-molecular process (template catalysis role 111). In another catalytic mechanism of some importance the metal ion leads to the produc- tion of more or occasionally3 improved nucleophile usually by induced ionization of protons from ambient nucleophiles (role IV). Related to this is metal ion-induced ionization of protons from the electrophile which can result in a poorer electrophile and therefore in the easier departure of an attached nucleophile.(The proton as catalyst cannot achieve these last three effects for fairly obvious reasons.) Metal ion-induced conformational changes can also lead to catalysis. This role is probably mainly confined to enzymatic systems; these are excluded from this Report. Apart from the mechanisms of catalysis not available to protons there are certain other aspects in which catalysis by metal ions differs from that by protons. Compared with all metal ions the proton is small and highly polarizing. As a result a kinetically significant concentration of protonated reactant is often readily formed whilst the presence of even a substantial concentration of di- or tri-positive metal ions fails to produce detectable catalysis.Clearly only substrates carrying groups which permit the formation of a kinetically significant concentration of metal ion-substrate adduct will be susceptible to catalysis. In practice this often means that the substrate must act as a chelating ligand and perhaps largely be converted into the adduct. This is particularly the case with Class A (hard4) metal ions and with substrates which are 0-bases. Findings related to the relatively weak inherent acidity of metal ions are (i) the frequent ineffectiveness of monopositive (especially Class A) metal ions and (ii) the infrequency of mechanisms of metal ion catalysis analogous to slow proton transfer? with metal ions pre-equilibrium adduct formation with a reactant is the norm.Another important peculiarity of metal ion catalysis is that often it is not really catalysis at all in that the metal species is not quantitatively regenerated but remains attached to a product. Strictly speaking these examples are metal ion-promoted reactions but they are normally considered together with genuinely catalytic systems. (Even using protons promoted reactions are more common than is usually realised.) As is well known metal ions can be categorized as either inert or labile.6 By choosing an inert ion as catalyst a researcher can begin with a substrate-metal ion adduct of known structure and with it therefore demonstrate more certainly the presence or absence of specified catalytic effects.In the hands particularly of Buckingham Sargeson and co-workers this approach has greatly strengthened belief in the reality of types of mechanism postulated much earlier using labile ions. Labile ions of course continue to be studied; they represent the majority and contain those especially interesting biologically.’ R. G. Pearson Science 1966 151,172. ’ M. L. Bender ‘Mechanismsof Homogeneous Catalysis from Protons to Proteins’,Interscience New York 1971. R. G. Wilkins ‘The Study of Kinetics and Mechanisms of Reactions of Transition Metal Complexes’ Allen and Bacon Boston 1974. ’ Metal Ions in Biological Systems’ ed. H. Sigel Dekker Basle 1976 Vol. 5. Kinetic Studies ofMetal Ion Catalysis ofHeterolytic Reactions In the following sections we deal with the recent literature back to about 1974.For some of the topics the period prior to this has been thoroughly reviewed; for others it has not. It will be found that the general points made in the foregoing paragraphs are supported by current work and in particular the four main roles (I-IV) available to metal ions as catalysts for heterolytic reactions are all well represented. Many types of reaction fall into the heterolytic category. Three (heterolytic hydrogenation carbonylation and skeletal rearrangement) have especially close ties with their radical and electron-transfer counterparts3 and are therefore more conveniently discussed separately. They are omitted from the present Report.2 Carboxylic Ester Hydrolysis Ever since Kroll’s discovery’ that the hydrolysis of amino-acid esters is catalysed by metal ions ester hydrolyses have contributed much to metal ion studies. Hay and Morris’ have reviewed some aspects of the topic up to about 1974. Work with inert complexes has established that an ester co-ordinated by its electrophilic half can undergo hydrolysis in aqueous alkali by three different routes typified by equations (1)-(3) from which all other ligands have been omitted for clarity. Which route(s) MM++NH2CH2C02R+ OH-Mn++NH2CH2CO; + ROH (1) will obtain in given circumstances depends upon the metal ion upon the ester structure and for equation (3) upon the availability of a cis-OH group. When complexes such as [cis-CO(~~)~(H~I)(NH~(CH~),CO~R)]~+ are hydrolysedg-12 in the absence of other metal catalysts there occurs (principally) first a relatively rapid SNICB dissociation6 of halide (via dissociation of one of the en protons) to give a five-co-ordinate intermediate which when n = 1(glycine esters) very rapidly leads to a mixture of compounds (1)and (2) the ester carbonyl group competing with external hydroxide for the vacant Co site.There then follows a relatively slower H. Kroll J.Amer. Chem. SOC.,1952,74 2036. R. W. Hay and P. J. Morris in ref. 7 Ch. 4. lo D. A. Buckingham D. M. Foster and A. M. Sargeson,J. Amer. Chem. SOC.,1969,91,4102. l1 R. W. Hay R. Bennett and D. P. Piplani J.C.S. Dalton 1978 1046. l2 R. W. Hay R. Bennett and D. J. Barnes J.C.S.Dalton 1972 1524.D. P. N. Satchell and R. S. Satchell hydrolysis of the ester function via routes like equations (2) and (3),both leading to chelated glycine. When n =3 the hydrolysis follows the same pattern except that the ester carbonyl does not successfully compete with external hydroxide and the ester hydrolysis is entirely via a route like equation (l),leading to a hydroxypentamine compound (3). Kinetic studies have shown that for routes like (2) and (3) rate accelerations (compared with the rate of hydrolysis of the uncomplexed ester by OH-) are ca. lo6 and >lo7 respectively. For routes like (1) the acceleration depends upon the distance of the ester group from the metal;"*'2 for n =3 it is ca. seven-fold at 25°C. Compared with the OH- rate for the protonated ester (N'H,(CH2),C02R) the Co route (1) involves a three-fold deceleration when n = 3.The very great reactivity of a directly co-ordinated carbonyl group in these reactions means that any ambient base can be a successful nucleophile and general base catalysis of nucleophilic attack is unnecessary." The majority of recent studies have involved Cu2+ complexes. With this ion at high pH dipeptide esters give terdentate complexes for Etglygly (4 R =H) for Etgly-L-leucine (4 R = CH2CHMe2) and for Etgly-P-alanine (5). Kinetic st~dy'~ + 'OEt (4) (5) shows that hydrolysis of the ester function occurs for both the aquo and hydroxy (neutral) versions of these complexes at rates of ca. 2 x 103-fold and ca. 3 x 102-fold faster respectively than for the uncomplexed esters at 25 "C.These values suggest (i) moderately weak carbonyl co-ordination and (ii) external OH- attack [equation (2)]. All the esters give similar rates which suggests that for Cu2+ a six-membered ring is as convenient as a five-membered ring. Angelici Nakon and their colleagues have contin~ed'~-'~ their kinetic studiesg of Megly hydrolysis promoted by highly chelated Cu2+ Ni2+ and other divalent ions e.g. by [CUL]'~-")' where L"-is (~-P~CH~)~NH, (NH2CH2CH2)2NH (C0;CH2)2NH (CO;CH2)3N (pyr), etc. These tri-and tetra-chelated complexes generally co-ordinate one Megly molecule in aqueous solution in an essentially unidentate fashion with the possibility of some weak bidentate interaction. The rate of hydrolysis is normally given by Rate = k[MLglyMe'2-"'+][OH-].The sequence of l3 D. A. Buckingham J. Dekkers A. M. Sargeson and M. Wein J. Amer. Chem. SOC.,1972,94,4032. l4 R. W. Hay and K. B. Nolan J.C.S. Dalton 1974,2542. R. Nakon P. R. Rechani and R. J. Angelici J. Amer. Chem. SOC.,1974,96,2117. l6 D. E. Newlin M. A. Pellack and R. Nakon J. Amer. Chem. SOC.,1977,99,1078. S. A. Bedell and R. Nakon Inorg. Chem. 1977.16 3055. l8 R. D. Wood R. Nakon and R. J. Angelici Inorg. Chem. 1978,17 1088. l9 J. K. Walker and R. Nakon J. Amer. Chem. SOC.,1978,100,1151. Kinetic Studies of Metal Ion Catalysis of Heterolytic Reactions effectiveness of metals is generally Cu >Zn >Ni >Co (Irving-Williams) and save for yrtain of the Cu2+ complexes the observed rates are slightly slower than for NH3CH2C02Me.Clearly little activation of the carbonyl group is occurring and the suggested mechanism is like equation (1). The effects of the various ligands L"-on k for a given metal ion are apparently determined by (a)the resulting overall charge on the complex and (5) the effective basicity of the ligand. The more strongly the ligand is held the poorer is the complex at promoting ester hydrolysis. This effect is a recurring theme in this group's work and is clearly demonstrated for a series of [CuLMegly12' complexes (of fixed charge)." For the Cu2' complexes which are more effective than the proton it is suggested that weak C=O co-ordination (at an apical position) is involved. A similar conclusion is drawn from a comparative study of ester hydrolysis and water exchange" in complexes (6).Here the same sequence of metal ion effectiveness (Pb >Cu >Zn >Co >Ni) is observed for both reactions and the magnitudes of AH' for the two processes are similar. This result suggests that in the ester hydrolysis the carbonyl group first displaces a water molecule from an apical position and that the contribution of this step dominates the overall AH' value AH*and AS* for the subsequent intermolecular OH- attack having little influence on AG*. For those foregoing examples in which carbonyl co-ordination is thought negligible AS' for the attack of OH- is considered to be the principal factor determining AG'. Divalent metal ions have been shown2' to be powerful promoters of the hydrolysis of methyl-8-hydroxyquinoline-2-carboxylate(EH) by OH-.Both complexes (7 MEH2+) and (8,ME') lead to reaction and it is believed that the carbonyl group i + 0-M+O must be engaged by the metal (M2'). Attack by external hydroxide is suggested and supported by measurements using D20. The activity of M2' is in the sequence Cu>Co> Ni> Zn>Mn. In aqueous solution at 25"C the complexes CUE' and MnE' are ca. 106-fold and ca. 103-fold more rapidly hydrolysed respectively than is E-. '"R. W. Hay and C. R. Clark J.C.S. Dalton 1977 1866. D. P. N. Satchell and R. S. Satchell The foregoing studies are all examples of metal ion activation of the electrophilic part of the ester in hydrolysis (role I) and sometimes also of template action (role 111).In contrast we turn now to cases involving a metal's influence on the attacking nucleophile (role IV). Obviously if a metal ion is attached to the attacking nucleo- phile in a bimolecular Lewis acid-base reaction that nucleophile is going to suffer reduced nucelophilicity. The question is how much? We have already seen for Co promotion that if both the ester and the attacking nucleophile are attached to the same metal centre in suitable juxtaposition any reduction in nucleophilicity can be more than off -set by the gain in AS'. It appears that even when the free ester reacts with a metal-bound nucleophile in a positive neutral or negatively charged complex the nucleophile can show surprising strength; attachment to a metal does not reduce nucleophilicity so much as attachment to a proton.Thus OH- in [Co(NH3),(OH)I2' and deprotonated imidazole (Im-) in [CO(NH~)~I~]~' have pK values of ca.6.4 and 10 respectively and react withp-nitrophenylacetate in aqueous solution at rates2' expected for bases of such pK values. Similar results are found for OH-complexes of Pd Pt Hg and other metal^.^^-^^ The use of such complexes therefore permits the employment of substantial concentrations of useful nucleophile in circumstances (e.g.non-aqueous solvents or low pH) where little of the corresponding free nucleophile is available. For instance in aqueous solution at pH6.5 and 25"C about half of any [Pt(en)(H20)I2+ or [CO(NH~)~(H~O)]~' will be in the OH form while the concentration of free OH-will be <lo-'.Some especially interesting complex metal hydroxide catalysts have been reported by Werber and Shaliti~~~~'~~ who used phen bipy dien and pentamethyl- dien hydroxy metal complexes (9) to hydrolyse p-nitrophenyl esters of various substituted acetic acids. Species (9) is surprisingly reactive in simple bimolecular reaction with esters. I+ r Me Finally in the ester section there are the examples where a metal ion assists the departure of the leaving nucleophile (role 11). It is of course only in this sort of promotion that an Al-like mechanism' is possible. In the other three sorts of promotion (I 111 and IV) either an inter- or an intra-molecular A2- scheme' must always operate. Leaving groups sufficiently basic to attach a kinetically significant amount of metal ion are found in various S-esters when the metal ion is very soft.21 J. MacB. Harrowfield V. Norris and A. M. Sargeson J. Amer. Chem. Soc. 1976,98,7282. 22 M. M. Werber and Y. Shalitin Bioinorg. Chem. 1973 2 275. 23 M. M. Werber and Y. Shalitin Bioorg. Chem. 1975,4 149. 24 M. C. Lim and R. B. Martin J. Inorg. Nuclear Chem. 1976,38,1911;see also M. A. Wells G. A. Rogers and T. C. Bruice J. Amer. Chem. SOC.,1976,98,4336. Kinetic Studies of Metal Ion Catalysis of Heterolytic Reactions Thus the hydrolyses of thiol [equation (4)] thion esters26 [equation (5)],and thiolbenzimidate esters2' [equations (6)and (7)]are all powerfully promoted by Hg2+ p-RC6H4COSEt+Hg2++2H20 + p-RC6&C02H +H30' +HgSEt' (4) p-RC6H,CSOEt +Hg2' +2H20 + p-RC6H4C02Et+2H30++HgS (5) PhC(=&HZ)SEt +Hg2++2H20 -+ PhCN +2H30++HgSEt' (6) PhC(=&HR')SEt +Hg2++3H20 -+PhCONHR'+ 2H30+ +HgSEt' (7) ions in aqueous solutions of pH <3 at 25 "C(conditions under which the proton catalysed reactions are negligible).When R =OMe in equation (4) the reaction follows an A 1-like route equation (8);when R =NO an A2-like scheme prevails. Hg2+ f p-MeOC6H4COSEt+Hg2+ .%-COS slow --+ -CO+HgSEt' '::+ + \ Et p-MeOC6H4C02H+H30++HgSEt+ (8) An Al-like mechanism is also found in reaction (6) which has a complex rate equation owing to the necessary ionization of the N-bound protons. Three slow unimolecular product-forming steps have been identified steps (9),(lo),and (11).In slow __* Ph&H +HgSEt+ H20 b PhCN +H30++HgSEt' (9) PhC+ Et NH Et / slow S' -/k PhCN +HgSEt' (10) PhC Et vHg+ Hgz+ slow 's /\ ___* PhCN +Hg2++HgSEt+ (11) PhC Hg+ N all these Hg2+-promoted reactions of S-esters only a small extent of pre-equilibrium complex formation oc~urs.~~-~~ This is in contrast to the reactions discussed in the preceding paragraphs where usually one is working with a pre-formed complex or under conditions where saturation complexation is attainable at moderate metal ion concentrations.All the Hg2'-promoted S-ester hydrolyses are kinetically first order in the S-ester and in the Hg2+ concentrations and route (1 1) underlies a kinetic term in [Hg2'I2 found in the S-imidate reactions.It is common in metal-assisted nucleo- phile departures to find when the metal ion-substrate adduct has only a single ''D. P. N. Satchell and I. I. Secemski J. Chem. SOC.(B), 1970 1306. 26 D. P. N. Satchell M. N. White andT. J. Weil Chern. andInd. 1975 791. *' A. J. Hall and D. P. N. Satchel] J.C.S. Perkin ZZ 1976 1274 1278. 32 D. P. N. Satchell and R. S. Satchell positive charge that routes involving the help of a second metal ion are indicated. For example a similar effect is found for the A2-like Ag'-promoted hydrolyses of thiol esters" for which the rate equation is -d[S-esterlldt = (kl[Ag'] +k2[Ag']')[S-ester]. Here again two metal ions are very probably attached to one sulphur atom. A last point concerns the relative efficiencies of soft metal ions in these S-ester reactions.Usually Hg2' and Ag' are ca. 106-fold and lo3-fold respectively more effective than the proton. T13' and AuCl have also been shown to be of comparable activity to Hg2' in the S-imidate hydrolyses and are doubtless also very efficient with other types of S-esters. Other supposedly rather soft metal ions such as Cd" Cu2+ and Pb" lead to comparatively negligible promotion at pH 3 and are probably less effective than the proton. Metal ion promotion of reactions of various organo- sulphur compounds has been reviewed.28 A cobalt-ester adduct [~is-Co(en)~(Hal){NH~(CH~)~0COMe)l2', analogous to those considered above but with the cobalt attached to the leaving nucleophile has been studied by Hay and CO-workers." Hydrolysis by OH- in aqueous solution occurs as expected in two stages first there is a relatively rapid SNICB loss of halide which is followed by a slower hydrolysis of the ester group by free OH-via an intermolecular route like that in equation (1).The product is [cis-Co(en)JOH)- (NH2(CH2)'0H)I2+. At 25 "C the ester group hydrolysis occurs ca. 18-fold faster than for the uncomplexed ester and ca. 3-fold faster than for the N-protonated ester. No intramolecular or chelation effects intrude; these are clearly unfavourable when the carbonyl group is this far away from the cobalt atom. Cases of metal ions affecting a reaction by chelation at N-containing leaving groups are discussed by Fife and Squillacote3' (see also Section 3). One of these may conceivably involve intramolecular OH- attack via a metal attached to an ester's leaving group (10).Little complex forms in this system and no catalysis is detectable at pH <6; this is probably because at low pH (i) there exists less MOH' and (ii) protons compete more effectively for the N atom. A clearer example of this type of effect has been provided31 by Hay and Clark as shown in Scheme 1. In this system powerful (almost enzymatic) acceleration is found which supports the notion of an intramolecular route. Zn2' is more effective than Cu2' which being a stronger Lewis acid binds OH- more strongly. Available experiments on ester hydrolysis therefore suggest that various metal ion-promoted inter- and intra-molecular A2-like routes are possible and for esters co-ordinated via their leaving nucleophiles there is also the possibility of an A 1-like mechanism.3 Carboxylic Amide Solvolysis Parts of this topic have been reviewed' up to about 1974. Buckingham et al.32have reported on a system which by itself exhibits for amide hydrolysis analogues of all three mechanisms found for ester hydrolysis when 28 D. P. N. Satchell Chem. SOC.Rev. 1977,6 345. 29 K. B. Nolan B. R. Coles and R. W. Hay J.C.S. Dalton 1973 2503. 30 T. H. Fife and V. L. Squillacote J. Amer. Chem. SOC.,1978,100,4787. 31 R. W. Hay and C. R. Clark J.C.S. Dalton 1977,1993. 32 D. A. Buckingham A. M. Sargeson and F. R. Keene. J. Amer. Chem. SOC.,1974,96,4981. 33 Kinetic Studies of Metal Ion Catalysis of Heterolytic Reactions 0 L 0-M-0 t OCOMe 1 OH Scheme 1 a metal is attached to the acyl part of the substrate.In aqueous alkali [cis-C~(en)~(Br)glyglyOC~H~]~+ is hydrolysed in two stages (Scheme 2). As for the similar species discussed in Section 2 the first stage in an SN1CB loss of Br- leading to two products (11)and (12) both of which undergo amide hydrolysis in the second stage; (11)by external OH-attack on the chelate (a known route') and (12) by both intra- and inter-molecular routes which are about equally efficient at pH 10. The final Co-bound products are thus the hydroxypentamine (14) and the glycine chelate (13). General base catalysis of the intramolecular route is observed and in the presence of sufficient added base (e.gl phosphate) this route can be ca.10'O-fold faster than hydrolysis of the unco-ordinated substrate. Buckingham and co-worker~~~ have also found that dimethyl formamide is a sufficiently strong 0-base to co-ordinate to cobalt in [CO(NH~)~O=CHNM~~]~+ 33 D. A. Buckingham J. MacB. Harrowfield and A. M. Sargeson J. Amer. Chem. SOC.,1974,96 1726. D. P. N. Satchell and R. S. Satchell J[(en)2Co(OH)NH~CH2C02]++ RNHz 11 (14) NHR NHR H+&-OH- RNHz + (13) (11) Scheme 2 without chelation. It then undergoes hydrolysis via external OH-attack ca. 104-fold faster than does the free amide mainly as a result of a favourable AS* value. The conformations of Cu2+and Ni2' pentapeptides suggest that weak interaction of their terminal carbonyl groups with metal apical positions is possible whereas such interaction is impossible for tetra pep tide^.^^ In agreement with this it is found that the hydrolysis of pentapeptides is slightly (ca.10-fold) accelerated by co-ordination to these ions whereas tetrapeptides are unaffected. A somewhat qualitative of the glycolysis of benzamide at 139 "Creveals catalysis by the acetates and similar derivatives of various metals (including Na'). Fractional reaction orders are observed and the system is not easy to interpret mechanistically. A number of studies have appeared in which the metal ion facilitates leaving group departure. In one equation (12) attack of HzO on the (unchelated) Co-bound substrate is found to be general acid and base catalysed.21 This is an A2-like scheme.m m [(NH3),CoNwNCOMe]'+ + H,O --+ [(NH3),CoNwNHl3+ + MeC02H (12) The presence of Co increases the rate of H20 (or OH-) attack by ca. 20-fold rn compared with [HN-NCOMe]' but as expected greatly reduces the contribu- tion of routes involving catalytic protonation of the leaving nucleophile. The bimolecular hydrolysis (and aminolysis) of penicillin36 is powerfully (ca. 10') catalysed by Cu2' ions owing to the possibility of leaving group chelation (15). Certain buffer components are found to deactivate the Cu2' ions. 34 J. J. Czarnecki and D. W. Margerum Inorg. Chem. 1977,16 1997. 35 J. Malek and E. ZelCna Coll. Czech. Chem. Comm. 1976,41,395. 36 N. P. Gensmantel E. W. Gowling,and M. I. Page J.C.S. Perkin II 1978 375. Kinetic Studies of Metal Ion Catalysis ofHeterolytic Reactions RCoNH Kinetic studies have been made of the promoted hydrolysis and decomposition of thioamides by various soft metal ions in aqueous sol~tion.~~-~~ Regardless of the metal used N-unsubstituted amides (RCSNH2) lead to RCN and the metal sulphide by usually somewhat complicated routes the details of which depend upon the metal ion.With most of the metals used ionization of N-bound protons from meta1-S- amide adducts is involved prior to a unimolecular decomposition of species such as RC(=NH)SM'"-"' e.g. equation (13). With N-substituted S-amides the product is ,S-& PhC -+ PhCiH + AgS-% PhCN + H30++ AgS-(13) %H necessariIy the analogous 0-amide formed via A2-type routes e.g. equation (14). /S-Hg+ PhC +H2O + PhC'6H2 +HgS % PhCONHR+H30'+HgS (14) %R \NR With Hg2' T13' AuCl; and Ag' stoicheiometric adduct formation between S-amides and the metal ion is observed but with Cu2+ Cd2' Pb2+ and Co2' little adduct is formed.At pH e3 the sequence of reactivities is Hg2+ -Ti3+-AuCI > Ag'>>Cu2'>>Pb2' Cd2+ Ni2' Tl' and Cs'. The Hg2' group of ions provide substantial accelerations (ca. lo6) over the corresponding proton-catalysed hydrolysis whereas the ions beyond Cu2' in the sequence have only comparable or less reactivity than the proton. Similar results are found in S-ester hydrolysis (Section2). At ca. pH 6 in buffer solutions some catalysis is observed for Pb2+ Cd2+ and Co2' ions but interpretation is complicated by the presence of M2'-buffer and possibly of MOH' com~lexes.~~ In all these studies Cd2+ is anomalously ineffective in view of its supposed softness.Two interesting features of the reactions are (i) the greater reactivities (ca. 10-fold) of AuC1;- and AuC130H2 compared with AuC1; and (ii) the independence of the Au"'- and T13+-promoted reactions of [H,O+] which means that ionization of M-bound water or amide protons is kinetically unimportant with these ions. Therefore tertiary S-amides (RCSNR2') can be hydrolysed by them as readily as primary and secondary derivatives (which is not the case with the other soft ions). Lastly two studies by Fife and Squillac~te~~*~~ suggest that when a metal species can co-ordinate to both the electrophile and nucleophile parts of a substrate its 37 A.J. Hall and D. P. N. Satchell J.C.S. Perkin 11 1975 778 953 1273 1351. '' A. J. Hall and D. P. N. Satchell J.C.S. Perkin 11 1977 1366. 39 0.M. Peters N. M. Blaton and C. J. DeRanter J.C.S. Perkin 11 1978 23. 4o T. H. Fife and V. L. Squillacote J. Amer. Chem. Soc. 1977 99 3762. D. P. N. Satchell and R. S. Satchell presence will not accelerate decomposition. For example the presence of Co2+ Cu2+,Zn2' and especially Ni2' leads to marked inhibition of the intramolecular nucleophilic route to the hydrolysis of N-(2-phenanthroyl)phthalamicacid without providing any additional catalysis (16); clearly the metal effectively holds the two halves of the substrate together. 4 Reactions of Nitriles and Imines Once again we can do no better than to begin with the recent work of the Australian s~hool.~' Following earlier with the same systems it is now proposed that CN)]" the M"'-promoted hydrolysis at acid pH of [ci~-Co(en)~(Hal)(NH~(CH~) (where Hal = C1 or Br x = 1 or 2 and M"' = Hg2+ Ag' Hg22+ Zn2' or Cd2') proceeds principally by Scheme 3 to give 0-chelated amide.With an excess of M"' r NH2(CH2),CNI2+ 11H'O (er~)~Co NH2(CH2).CN] 2' 'OH \ Scheme 3 present the rates of these reactions are usually first-order in [M"'] and inversely proportional to [H30+]. The details of the final slow (intramolecular) step are unknown except that (interestingly) with Ag' an important kinetic term second- order in [Ag+] is also found suggesting that two silver ions can beneficially be 41 D.A. Buckingham P. Morris A. M. Sargeson and A. Zanella. Znorg. Chem. 1977 16 1910. 42 D. A. Buckingham A. M.Sargeson and A. Zanella J. Amer. Chem. SOC.,1972,94 8262. 43 K. B. Nolan and R. W. Hay J.C.S. Dalton 1974 914. Kinetic Studies of Metal Ion Catalysis of Heterolytic Reactions attached to the CN group. Very large (>lo8)accelerations are observed compared with the rate of hydrolytic cyclization of the M"'-free hydroxy derivative (HO(CH,),CN) especially when x = 2. The relative efficiencies of the soft ions are in the sequence Hg2' 3Hg22' >> Ag' > Cd2' > Zn2'. As found for many S-substrates,28 Hg2+ is ca. 103-fold superior to Ag' at 25 "C; a surprising result is the finding that Cd2+ and Zn2+ are only ca. 10 and 102-fold less effective than Ag' in this system whereas with S-substrates their contribution is relatively very When x = 1 a second hydrolysis route is present (Scheme 4) which involves intermolecular attack of water on the chelated nitrile.The relative importance of this route which leads to the N-chelated amide depends upon the nature of Hal and' not upon that of M"'. Scheme 4 In alkaline solution competition to hydrolysis arises from an intramolecular attack on the nitrile by a deprotonated NH group of en equation (15). + slow Hal Hg (15) The kinetics of some analogous intermolecular hydrolyses (by OH-) of Co-bound nitriles in [Co(NH3),NCRI3' (where R = 4-CNC6H5 4-CHOC6Hs etc.) have been described.44 The hydrolyses are first-order in each reactant and large accelerations (>lo6)are found.The dominant effect is that of Co3+:substituents in the benzene ring have little effect on kOb. The great importance of the Lewis acidity of the metal centre is also illustrated by similar of benzonitrile and acetonitrile hydrolyses by OH- at pH 8-9 in aqueous solution at 25 "C using the complexes [M(NH3)5NCR]"+. For MeCN it is found that Ru'" >> Ru" and Ru"'> Rh'" = CO'~ while for PhCN Ru"' > Co"' 3Rh'''3 I?". The Ru"' complexes lead to accelera- tions of ca. 108-fold compared with the metal-free nitriles. The sequences are 44 R. J. Balahura P. Cock and W. L. Purcell,J. Amer. Chem. SOC.,1974,96,2793. 45 A.W.Zanella and P. C. Ford J.C.S. Chem. Comm.. 1974,795. 46 A.W.Zanella and P. C. Ford Inorg.Chem. 1975,14 700. D. P. N. Satchell and R. S. Satchell discussed in terms of d-electron content and ion size. MeCN is also hydrolysed4' in the presence of Pt'" and a qualitative kinetic account of its hydrolysis promoted by Hg2+ in concentrated MeCN-H20 mixtures is also available.48 Added ions (AcO- NO3-) which tie up the metal inhibit reaction. Two interesting related reactions which involve the metal ion-promoted hydra- tion of oximes and imines have recently been reported. The first shows that the hydrolyses equation (16),of the Cu2+ complexes of (17)and (18)are rapid but n H2O-Cu2+ NHz '7 H20Cu2+ n ____) s)J NH NH -S-aldehyde -S-aldehyde (17) (181 are not due to relief of strain in quadri- and ter-dentate ligands since only the N atoms are co-ordinated.The second report" suggests that the rearrangement of oximes catalysed by Ni" and Pd" complexes proceeds uia oxime adducts which undergo dehydration followed by intramolecular hydration e.g. equation (17). CHR 1+ These metal-promoted imine and nitrile hydrolyses differ from those of esters and amides in that activation of the CN multiple-bond towards nucleophilic attack always appears to require direct attachment to the metal. In other systems direct attachment to a leaving nucleophile often provides unimolecular slow steps but since with nitriles it is the opening of the multiple-bond which corresponds to leaving group departure unimolecular Al-like schemes may be expected to be less common." 5 Dehalogenation Dehalogenation of both organic and of inorganic compounds by metal ions has long been used for preparative purposes.Its kinetic study provides a link between these two branches of chemistry and it is encouraging to find that a number of common features exist. Best results will clearly be obtained with soft metal ions (which form very stable halides) and such ions are commonly used. The reactions as a whole form the clearest category of processes in which the metal ion assists departure of the leaving nucleophile. Very often A 1-like mechanisms obtain. 47 A. K.Johnson and J. D. Miller Inorg. Chirn. Acru 1977 22 219. 48 Y-K. Sze and D. E. Irish Cunud. J. Chern. 1975 53,427. 49 A. C. Braithwaite C. F. E. Rickard and T. N. Walters J.C.S. Dulron 1975 2149.A. J. Leusink T. G. Meerbeck and J. G. Noltes Rec. Truu. chirn. 1977,96 142; ibid. 1976.95 123. " See also E. N. Zilberman V. I. Trachenko S.M. Danov and N. R. Shipmova Izuesr Vyssh. Uchebn. Zuued. Khirn. Tekhnol.. 1977 20 1141 (Chem.Abs.,87 183830). Kinetic Studies of Metal Ion Catalysis of Heterolytic Reactions 39 Examples have already been given (e.g.Schemes 3 and 4) in which the aquo- and ring-closed form of a complex cobalt halide has been produced via metal (normally Hg2') ion-promoted loss of halide.52 Usually these reaction^^'^'^' involve an A 1-like mechanism rapid pre-equilibrium attachment of the metal ion to the halide is followed by the rate-determining formation of a five-co-ordinate transient inter- mediate which then quickly undergoes intra- or inter-molecular nucleophilic attack.[This intermediate is not identical with that formed in the corresponding base catalysed unimolecular substitution (SNICB) e.g. Scheme 2 and the two do not necessarily lead to the same distribution of inter- and intra-molecularly formed products.'] A number of recent studies have looked at the relative effects of different soft metals in promoting the aquation of various inorganic halides. For [ReCl6I2- and [ReBr612- with M"' in aqueous solution the simple kinetic form kobs= kl + k2[Mm+] is k representing the unpromoted rate. For Hg2' and T13'k2[Mn'] >> kl,but for Cd2+ the two rates make comparable contributions and for In3'k2-0. For [CO(NH~)~C~]~+ efficiencies are54 in the sequence Hg2+ -HgCl' > T13' -T10H2+> T1C12' the overall factor being ca.60. Normally Hg2'>T13' but this order can be reversed for anionic cobalt complexe~.~~ When one or more halogen atoms is cis to the others in a substrate bridged metal ion adducts can be detected as intermediates. With only one halogen atom little adduct is detectable. Recent work with organic halides has mostly involved promotion by Ag'. Besides removing halogen atoms in promoted solvolyses Ag' ions can also catalyse the rearrangement of suitable unsaturated or strained hydrocarbons which contain no halogen. The balance between these roles is a concern of two studies of halocyclo-propanes in methan01.~~*~' As for the inorganic compounds the removal of halogen from organic halides usually involves slow unimolecular heterolysis.The resulting carbocation can then suffer attack at carbon by any ambient nucleophile (e.g. solvent) or by losing a proton to the nucleophile form olefin. For a variety of solvents (MeOH MeN02 MeCN) a reasonably self-consistent pattern of behaviour RHal + Ag+$RHalAg+ slow R+Hal-Ag' +R++ AgHal (route A) II.-IS Y-R+Hal-Ag+ alkene + SH' + AgHal (route B) 1 Y- RY+AgHal 4 Scheme 5 s2 See also V. Tinner and W. Marty Helv. Chim. Acta 1977 60 1629; V. I. Belevantsev E. I. Evdokimova and B. I. Peshchevitskii Zzvest Sib.Otd. Akad. Nauk. S.S.S.R.Ser. Khim. Nauk. 1978,38. s3 J. Burgess and S. J. Cartwright J.C.S. Dalton 1976 1561. 54 S. W. Foong B. Kipling and A. G. Sykes J. Chem. SOC.(A),1971 118." S. F. Chan and S. L. Tan Znorg. Nuclear Chem. Letters 1975,11,435. 56 G. M. Blackburn and C. R. M. Ward J.C.S. Chem. Comm. 1976,79. " D. B. Ledlie J. Knetzer and A. Gitterman J. Org. Chem. 1974.39 708. 40 D. P. N. Satchell and R. S. Satchell is beginning to This is shown in Scheme 5 in which S =solvent and Ag'Y-is any particular silver salt. The relative importance of the different paths depends upon (i) the dielectic constant and other properties of the solvent (ii) the tendency of R' to give olefin and (iii) the nature of Y-. Usually the reactions are first-order in organic halide but have terms of the first- and of the second-order in [Ag+]stoich. Since [Y-] = constant X [Ag']stoich the second-order term in silver ion can represent the contribution of the A2-like route B,but it is believed that it also and often mostly reflects promotion via a route involving two silver ions attached to the halogen atom.The kinetic parallel here with Ag'-promoted reactions of S-compounds and co-ordinated nitriles is as striking (see Sections 2 and 4)as that between the sequences of metal ion efficiency found for inorganic complex halides and for S-compounds (see above). Not only typical soft metal ions but other ions too can speed-up the reactions of halogen especially in solvents of low dielectric constant. Two interest- ing examples of powerful accelerations by LiC104 in diethyl ether have been recently provided by Pocker and Ellsworth.64 The exact role of the Li' ion in these reactions is perhaps uncertain.6 Dealkylation A metal ion-promoted reaction analogous to solvolysis via dehalogenation in that it (i) involves only assistance of leaving nucleophile departure and (ii) requires soft promoters is the solvolysis of organometal complexes via dealkylation. Reaction (18)is typical. This process is found6' to be first-order in the chromium complex and [(H20)&RI2' +Hg2+(or R'Hg+) %[Cr(H20),l3'+ RHg+ (or R'HgR) (18) in the mercury promoter. Various species R'Hg' are all ca. 102-fold less reactive than Hg2+ and electron withdrawal by R slows down the reaction. Steric effects are observed and other factors suggest that the reaction involves slow carbanion transfer followed by rapid attack of water on [Cr(H20)J3'. This process equation (19) is [(H20)5CrR]2' +[Hg(0H2),l2' [(H20)5Cr13' +[RHg(OH2)x-il' +HzO therefore a metal ion analogue of slow proton transfer to carbon.The relative reactivities of promoting species in this type of system depend to some extent on whether R carries a net charge (e.g.as in CH2-) but in general show that Y. Pocker and W. H. Wong J. Amer. Chem. SOC.,1975 97,7097,7105. 59 D. N. Kevill V. V. Likhite and H. S. Posselt J.C.S. Perkin IZ 1975 911. 6" R. D. Bach and C. L. Willis J. Amer. Chem. Soc. 1975,97 3844. 61 D. N. Kevill and R. F. Sutthoff J.C.S.Perkin 11 1977 201. 62 V. V. Zamashchikov E. S. Radakov I. R. Chanysheva and S. L. Litvinenko Dopov.Akad. Nauk. Akr. R.S.R. Ser. B. Geol. Khim. Biol. Nauki 1978.2 125 (Chem. Abs. 88 189528). " V. N. Plathotnick Katal.Katal. 1975,13,63;V. N. Plathotnik L. V. Boguslavskaya and V. V. Varekh ibid. 1977 15 41 (Chem. Abs. 88 177768). " Y. Pocker and D. L. Ellsworth J. Amer. Chem. SOC.,1977 99,2276,2284. 65 J. P. Leslie and J. H. Espenson J. Amer. Gem. SOC.,1976 98 4839. Kinetic Studies of Metal Ion Catalysis of Heterolytic Reactions Hg2+> T13+. The addition of aniomic ligands normally reduces promoting power in the sequences Hg2+ > HgCl' > HgC1,-> HgC142- and T13+> T10H2+> T1C12+> T1Clz+>TlCl > TlC14- but when R carries a positive charge the reactivity spread is greatly reduced or even reversed.66 Soft ligands can greatly lower the effectiveness of a metal ion for one demethylation it is observed67 that Hg(OAc)z >> HgC12> HgBrz>> Hg(SCN)zHg(CN)z.An interesting intramolecular slow metal transfer to carbon6* is provided by reaction (20). c1I1 +d+PY+PY c1 -+ P;CHzCHz-Pt+PYI Ic1 I CI 7 Phosphate Hydrolysis This topic has been reviewed6' up to about 1974. Phosphates are rather difficult to hydrolyse and rate accelerations are often modest. Parts of the field especially the hydrolysis of ATP and similar compounds are still in a state of flux. Recent ~~rk~~-~~ on the cleavage of the terminal phosphate group from ATP (e.g.reaction 21) catalysed by Cu2+ Zn" and Ni2+ has re-emphasized (i) the importance of both ATP4-+ OH-M2+ w ADP2-+ P3-(21) dimeric 1 1-and 2 1-complexes [i.e. (MATP)z4- and (M,ATP),] as the reactive species and (ii) the role of the N-7 atom of the adenosine moiety in engaging the first M2+ ion.This type of phosphate has an embarrassing number of potential sites for a metal ion. However although charge neutralization by the metal ion is obviously an important part of its function it seems unlikely that attachment at the &-positions will be ideal since this will tie the cleaving fragments together (19). Perhaps the 000 II. llp II Ad-O-P-O-P-O-PY-O-I I 0-0-0-hd M2+ (19) reason for the importance of the (MzATP)2 species is that one M2+ ion can be attached to the terminal phosphate group and the other to the a@ phosphate groups and the adenosine nitrogen atom so providing a two-way stretch. Too great a concentration of OH- or other ligands (e.g. bipy) are considered to reduce the hydrolysis rate by competition for the metal 66 D.Dodd M. D. Johnson andD. Vamplew J. Chem. Soc. (B),1971,1841. b7 Y.Yamamoto T. Yokoyama J. Chen and T. Kwan Bull. Chem. SOC.Japan 1975,48 844. " I. M.Al-Najjar and M. Green J.C.S. Chem. Comm. 1977 926. 69 B. S.Cooperman in ref. 7,Ch. 2. 70 D. H.Brisson and H. Sigel Biochem. Biophys. Acta 1974,343,45. 71 H.Sigel and P. E. Amsler J. Amer. Chem. SOC.,1976,98 7390. 72 P.E.Amsler D. H. Brisson and H. Sigel 2. Naturforsch. 1974,29,680. 73 See however M. M. T. Khan and M. S. Mohan Indian J. Chem. 1976,14A,945. D. P. N. Satchell and R. S. Satchell of the hydrolysis of the phosphate esters (20) and (21) show that chelation of a metal ion to either the ester or the phosphate half of the substrate leads as expected to an increase in hydrolysis rate.The extent of involvement of the phosphate oxygen atom with the Zn2' ion in (20) is probably slight. A particularly H interesting example76 of phosphate ester hydrolysis involves (unsurprisingly) [CO(~~)~~~(P=O)OC~H~NO~]+ where tn =trimethylenediamine and the phos- phate group is chelated to cobalt (with ring strain). It is found that simultaneous ester hydrolysis and aquation (oia ring-opening) occur according to the outline in Scheme 6. The rate of the direct hydrolytic route provides very powerful (ca. lo9)promotion compared with the free phosphate ester. 0 00 OH-' /\4 __* (tn)*Co \P '0-+ (tn)2~o P \/\ 0 0-OH 0 I 'OPNP 0-0-Scheme 6 Ligand effects have been reported77 for the Cu2-catalysed hydrolysis of acetyl phosphate.At pH 5 whereas amines terpy and amino acids decrease the catalytic activity of Cu2+ bipy and phen increase its activity; Zn2+ Co2+,and Ni2+ are not affected in the same way by phen and bipy. 74 J. E. Loran and P. A. Naylor J.C.S. Perkin 11 1977,418. 75 C-M. Hsu and B. S. Cooperman J. Amer. Chem. SOC.,1976,98,5652,5659. 76 B. Anderson R. M. Milburn J. MacB. Harrowfield G. B. Robertson and A. M. Sargeson. J. Amer. Chem. SOC.,1977,99,2652. 77 M. Murakata Kinki Daigaku Rihogakuba Kenkyu Hokoku 1977.53 (Chem.Abs. 87,167 135). Kinetic Studies ofMetal Ion Catalysis ofHeterolytic Reactions 8 Decarboxylation Important contributions to our understanding of the Zn2'- and Cu2+-catalysed decarboxylation of oxaloacetic acid have recently come from Leussing and This reaction has often been studied and has been reviewed,'l together with related decarboxylations up to about 1974.It well illustrates two points made earlier (i) that metal ion promotion of substrates which are 0-bases usually require (at least in aqueous solvents) a chelating substrate and (ii) that tying the cleaving halves of a substrate together is unlikely to facilitate decomposition. Thus in water only 0-keto-acids containing another potential ligand (e.g. another carboxy-group) can be decarboxylated using metal ions. The essentials of the reaction are well-established:81 it proceeds from the 1:1-Py-chelate of the keto form of the oxsloacetic dianion (22)to the chelated pyruvate enol [23] which then ketonizes Scheme 7.[The negatively charged enolic chelate (24) Scheme 7 does not decarboxylate.] Normally two consecutive reactions one appreciably faster than the other and both involving changes in visible absorption are measure- able but there has been debate as to exactly what parts of Scheme 7 these spectral changes reflect. These matters are clarified by Leussing's kinetic work which also deals with effects of Zn2' and Cu2' ions on the establishment and position of the initial keto-enol equilibrium. It is known that inactive 2 1-adducts (25) are present in the reaction mixtures especially at high pH and high metal ion concentrations. Leussing suggests that the 'I3 W. D. Covey and D. L. Leussing J. Amer. Chem. SOC.,1974,96,3860.79 D. L. Leussing and N. V. Raghavan J. Amer. Chem. Soc. 1974,% 7147. N.V.Raghavan and D. L. Leussing I. Amer. Chem. SOC.,1977,99,2188. 81 R.W.Hay in ref. 7 Ch. 3. 44 D. P. N. Satchell and R. S. Satchell effects of added ligands (L),such as amino-acids,82 bipy and phen which are found to increase the effectiveness of M2+(especially Cu”) is not normally due to an increase in the acidity of ML2+ (compared with free M2+) towards the substrate but is due usually to the effect of L in reducing the concentration of (25). A kinetic of the Cu+-catalysed decomposition of aromatic acids in pyridine has appeared. The reaction is thought to be heterolytic. 9 Schiff Base Formation Schiff base formation equation (22),is promoted by metal ions if both the amino and carbonyl components are attached to the metal it is an example of template action (role 111).Using a series of M2+ ions Leussing and c011eagues~~*~~ discovered that Pb2+ Mn2+ and Zn2’ were effective but that Co2+ Cu2+ and Ni2+ were ineffective as promoters. Leussing has argued that this is because the reacting ligands are more rigidly held by some ions the most strongly acidic ions and therefore less free to react with each other than when more loosely held on other ions. However the issue is a complicated one since many factors are involved the loss of freedom and loss of nucleophilicity on being bound the gain in electrophilicity on being bound the effect of binding one basic ligand on the metal’s acid strength towards the other the relative amounts of the two ligands attached and the detailed stereochemistry of reaction.All these effects must be involved and it appears unlikely that a single rationale will be applicable to all template catalyses. Leussing’s most recent work86 emphasizes that the acceleration provided by template action by metal ions arises chiefly from the conversion of an intermolecular into an intramolecular process and not from directly acidic (polarization) effects on the substrates. Studies of pyridoxyl catalysed decompositions of P-hydroxy amino-acids and similar compounds show that they are promoted by metal ions and involve Schiff base intermediates formed from the amino-acid and pyridoxyl on the metal as a template.87*88 10 Acetal Hydrolysis and Related Reactions The kinetics of metal ion-promoted acetal hydrolysis have been examined very little.One or two studies have appeared recently. Simple U-acetals although potentially chelating are weak bases and even highly charged ions (e.g.,Fe3+) are little better89 in aqueous solution than the proton as catalysts. The hydrolysis of the O-acetal(26) ’* Y. Yasuhiro Y. Nobuyuki 0.Tadashi and M. Motoichi Yakugaku Zasshi 1977,97,70,76(Chem. Ah.,86 171 826). 83 T. Cohen R. W. Berninger and J. T. Wood J. Org. Chem. 1978,43,837. 84 D.Hopgood and D. L. Leussing J. Amer. Chem. SOC.,1969,91,3740. 85 B.E.Leach and D. L. Leussing J. Arner. Chem. SOC.,1971,93,3377. 86 R.S.McQuate and D. L. Leussing J. Amer. Chem. Soc. 1975,97 5117. ’’ Y. Murakami and H. Kondo Bull.Chem. SOC.Japan 1975,48,541. ’’ K. Tatsumoto and A. E. Martell J. Amer. Chem. Soc. 1978 100 5549. 89 G.Wada and M. Sakamoto Bull. Chem. SOC.Japan 1973,46,3378. Kinetic Studies of Metal Ion Catalysis of Heterolytic Reactions 45 which has improved chelating possibilities is promoted by Cu2+ Ni2+ and Co2+ at pH 6 and 70 "C. The observed" rates are a substantial improvement on those found in the absence of the metal ions whose relative efficiencies are Cu" :Ni2+:Co2' 1380 :15:1. Only small amounts of adduct are formed and an Al-like route Scheme 8 is proposed. HO + cu2+ d fast lH2* HO Scheme 8 In contrast to 0-acetals S-acetals are very readily hydrolysed in the presence of soft metal ions and large rate accelerations (e.g. ca. lo6compared with the proton) are found.With S-acetals such as (27) only a small extent of pre-equilibrium adduct formation occursg1 even with Hg2+ but for substrates such as (28) stoicheiometric SR / Ph,C \ SR (28) 1:1-adduct formation can occur even with the weaker Ag+ and this is another system in which kinetic terms reflecting paths involving two Ag' ions co-ordinated to the substrate are obser~able.~~ Both A 1and A2-like schemes have been proposed for these S-acetal reactions. The topic was reviewed" in 1977. Somewhat related to the acetal hydrolyses is the M2'-promoted hydrolysis and halogenation of 2-pyridyloxiran reaction (23). In aqueous solutions at pH 5 the sequence of metal ion effectiveness is Cu2' >Co2+>Zn2+>NiZ+ with Cu2+ accelerating the rate by ca.lo4-fold. Little chelate is formed especially at low pH 90 C. R. Clark and R. W. Hay J.C.S. Perkin 11 1973 1943. L. R. Fedor and B. S. R. Murty J. Amer. Chem. SOC.,1973,95,8407. 92 D.P.N. Satchel1 and T. J. Weil Inorg. Chim. Acta Letters 1978 29 L239. D. P. N. Satchell and R. S. Satchell when protons compete for the nitrogen atom. Cleavage is probably only at the p-C-0 bond in an A2 H H 11 Miscellaneous Reactions Examples of a number of other types of metal ion-promoted reactions have also been reported recently. These include hydrogen exchange and racemization of optically active the mutarotation of glucose,96 the hydrolysis of quinoline ~ulphate,~' (in some the glycolysis of organic acids and related e~terification~~~-'~' of which M' ions appear to be effective catalysts) and the ethanolysis of N-sulphinylanilines.lo2 Two particularly interesting reactions are (i) the cleavage of P-diketones with methanol (to give a monoketone and an ester) for which a reactivity sequence Zn2+>Co2' >Ni2+>Cu2' has been e~tablished,"~ and (ii) the hydration of a~etaldelyde"~ catalysed by complexes of Zn2' designed to provide general base catalysis vis template action e.g.(29). Me (29) 93 R. P. Hanzlik and W. J. Michaely J.C.S. Chem. Comm. 1975 113. 94 L. G. Stadther and R. J. Angelici Inorg. Chem. 1975 14 925. 95 P. R. Norman and D. A. Phipps Inorg. Chim. Acta Letters 1978 28 L161. q6 S.Kirschner R. V. Moraski and G. Dragulescu J. Indian Chem.SOC. 1977,54,29. 97 R. W. Hay,.C. R. Clark and J. A. G. Edmonds J.C.S. Dalton 1974 9. 98 N. E. Khomatov B. Ya. Eryshev B. P. Yatsenko and T. A. Smirnova Zhur. Vses. Khim. O-ua 1978 23,118 (Chern. Abs. 88 1692 852). 9q 0.M. 0.Habib and J. Malek Coll. Czech. Chem. Comm. 1976,41 2724. loo N. S. Antonenko E. P. Kovsman G. N. Freidlin G. A. Tarakhanov and A. I. Gravschenko Zhur. priklad. Khim. 1975. 48,692 (Chem. Abs.. 82 138 868). J. Vejrosta E. ZelCna and J. Malek Coll. Czech. Chem. Comm. 1978 43,424. "* W. K. Glass I. J. King and A. Shiels Inorg. Chim. Acta 1977 25 157. K. Uchara F. Kitamura and M. Tanaka Bull. Chem. SOC. Japan 1976,49,493. P. Woolley J.C.S. Chem. Comm. 1975. 579. Kinetic Studies of Metal Ion Catalysis of Heterolytic Reactions 12 Conclusions In the introduction we mentioned the four principal roles played by metal ions in catalysing heterolytic reactions (at least in non-enzymatic systems) and noted some of the ways in which metal ion catalysis is similar to and differs from catalysis by protons.The recent literature can be considered to exemplify these opening remarks. Apart of course from the existence of roles I11 and IV the chief difference between metal ion and proton catalysis (via roles I and 11) is probably the lack of examples of mechanisms which involve successive catalyst-substrate equilibria in which the catalyst first activates the electrophilic part of the substrate and subsequently shifts to stabilize the departing nucleophile. This type of mechanism is generally considered to be very common in proton-catalysed reactions of for example carboxylic acid derivatives.' The reason underlying this difference is that usually only one half of the substrate (the electrophile or the nucleophile part) is capable of forming a kinetically significant amount of adduct with the metal.Substrates which can chelate via both parts are not as we have seen normally decomposed effectively since the products of decomposition are still joined together. This latter effect is sometimes expressed somewhat differently and perhaps over-simply by saying that the attachment of the metal ion to the reactant makes formation of the transition state for decomposition more difficult. The 'tying-up' of a reactant by the metal ion in a less reactive form (so leading to inhibition) is a general possibility which is sometimes observed but is probably no more common than the deactivation of reactants by protons (e.g.the removal of OH-as H,O); this is the impression given by the current literature.One further aspect of metal ion catalysis which differs from that by protons is that for metal ions the rate of reaction is frequently not only dependent on metal ion concentration terms but also upon terms involving [H30+],kobssometimes being a complicated function of [M"'] and [H30']. This effect can arise not only when the concentration of attacking nucleophile is controlled by [H30+],but when any metal-induced ionization of ligand or substrate protons affects the catalysis. Complications of this nature are rarely central to the understanding of the mechanism of catalysis and have not been stressed in the foregoing discussions of individual systems.The reader needs to be prepared however for the occasional elaborate rate equation. One desirable aim in this field is to be able to predict the best metal ion to promote any given heterolytic reaction. For processes promoted by soft-soft interactions which to date have mainly concerned reactions involving assistance to nucleophile departure (role 11) the sequences of M"' reactivities obtained in different experi- mental contexts are very largely self-consistent as are the effects of ligands on the metal's reactivity nearly always ligands deactivate the softest the most. Such promoted reactions usually involve a straightforward acid-base effect.For hard (Class A) metal ions the picture is cloudier. Again if just role 11 or role I is involved usually the relative acidities of the ions (e.g. Irving-Williams series) towards that particular class of substrate seem paramount in determining reactivity but for roles I11 and IV competing effects of the metal are present. The effects of ligands (substituents) both unidentate and chelating on a hard metal's reactivity seem again often to be straightforward (the most basic leading to the greatest lowering of D. P. N. Satchell and R. S. Satchell reactivity) but not infrequently the co-ordination of a neutral base or even a negative ion leads to an apparent increase in catalytic reactivity. The reasons for this will sometimes be nothing directly to do with modified Lewis acidity but sometimes apparently they are.There seems to be a need for more understanding of the effects of substituents on the Lewis acidity of metal ions and generally for more systematic information on the relative acidities of a wide range of metal ions towards different classes of base especially unidentate bases. There are grounds therefore for believing that we still have some way to go before our predictions of M"' reactivity will be at all reliable over most of the field. The main current motivation behind studies of metal ion catalysis is their relevance as background chemistry to enzymatic catalysis. Many of the papers reviewed finish with a paragraph or two in which possible enzymatic implications are mooted.

ISSN:0308-6003    

DOI:10.1039/PR9787500025

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

4 Infrared Laser Photochemistry By R. T. BAILEY and F. R. CRUICKSHANK Department of Pure and Applied Chemistry University of Strathclyde Glasgow GI 1x1,Scotland 1 Introduction The discovery of the powerful efficient (-20°/0) CO laser induced investigations into the possibility of initiating novel chemical reactions as long ago as the late 1960’s. Since then a great deal has been done particularly with reference to isotope separation where the process promises to compete economically with the traditional uranium separation techniques. Recently a book has appeared devoted entirely to megawatt i.r. laser photochemistry.’ A number of reviews have also on data up to 1977 and the energy transfer processes which constitute the photophysics behind the chemistry have also been reviewed.4d These i.r.photophysical data now form a coherent picture and current theories are making progress in describing the processes involved. However the situation is entirely different in the i.r. photochemical work. A large number of unconnected studies have been carried out which are often reported without relevant experimental details such as laser beam flux absorption coefficient pressure dependence or cell core temperatures. Occasionally a more coherent study emerges as for SF or OsO, but even then there are some anomalies as yet unexplained. Accordingly we have attempted in this Report to assemble recent data (up to September 1978) on a variety of compounds sufficient to indicate the typical results obtained from i.r. photochemistry and the deductions usually made from such results.Most i.r. photochemistry is performed with TEA lasers and these studies are listed under the relevant compound heading. In some cases e.g. hydrocarbons all the work on one compound type is listed under one heading. Continuous-wave laser (c.w.) or chopped C.W. laser work is discussed for all compounds under the C.W. laser heading. The interpretation of these studies is entirely different from that of the pulsed laser work although it is at least as complex. ’ ‘Megawatt Infrared Laser Chemistry’ E. Grunwald. D. F. Dever and P. M. Keehn J. Wiley New York 1978. ‘Chemical and Biochemical Applications of Lasers’ ed. C. B. Moore Academic Press London 1977 VOl. 111. S. Kirnel and S. Speiser Chern.Rev. 1977,77,437. R. T. Bailey and F. R. Cruickshank in ‘Molecular Spectroscopy’ ed. R. F. Barrow D. A. Long and D. J. Millen (Specialist Periodical Reports) The Chemical Society London 1974 Vol. 2 p. 262. R. T. Bailey and F. R. Cruickshank Applied Spectroscopy Reviews 1975 10 1. ‘R. T. Bailey and F. R. Cruickshank in ‘Gas Kinetics and Energy Transfer’ ed. P. G. Ashmore and R. J. Donovan (Specialist Periodical Reports) The Chemical Society London 1978 Vol. 3 p. 109. 49 50 R. T. Bailey and F. R. Cruickshank Space does not permit an exhaustive treatment of the topic but current theories are presented together with a discussion of the many areas where further extensive research is required. A section outlining the most important features of lasers relevant to laser chemistry is also included.Since several figures use Torr as the pressure unit and since this unit is used by all principal groups in the i.r. photochemical field we have used it also (1Torr = 1mm Hg pressure = 133.3 N m-2). 2 Experimental Techniques Laser Sources.-Unlike a photon in the u.v.-visible region an i.r. photon carries relatively little energy. For example a mole of photons at 1000 cm-’ carries only an energy of 12.01 kJ. Because of this relatively low photon energy the absorption of more than one photon is needed to produce photochemical reactions. As a consequence of the small photon energy most i.r. photochemical reactions take place in the ground electronic state. However relatively high power lasers are necessary to produce the photon flux required for i.r.photochemistry. Because of this high power requirement very few i.r. lasers are suitable for photochemical studies. Some of the most important are the CO, chemical lasers such as HF-DF and CO although others such as NH3lasers are undergoing extensive development’ and should soon reach sufficient power levels for i.r. photochemistry. Tunable lasers such as the parametric oscillator or spin flip Raman laser are useful for specialized applications where their relatively low power output can be utilized. It is convenient to divide i.r. lasers into two types C.W. and pulsed. In the case of C.W. lasers the power is generally limited to a few kW cm-’ since above this level atomization occurs. Pulsed lasers are available with a wide range of characteristics but typically the output consists of -1 J cm-’ with a pulse duration of 0.15-1 ps with a beam diameter of a few centimetres.This corresponds to power levels of 1-5 J cm-2 for the unfocused beam but when focused power levels of gigawatts (GW) per square centimetre can be easily achieved near the focal point. The characteristics of the main types of laser used for i.r. photochemistry are now briefly reviewed. The CO Laser. By far the greatest number of studies have been carried out using the CO laser. This is due to its suitability in terms of energy power availability and convenience of operation. Many different versions of this laser are available com- mercially both C.W. and pulsed. Continuous powers of over 10 kW and pulsed outputs of many megawatts are currently available.Normal C.W. operation occurs near 10.6 pm but with a grating in the cavity the laser can be tuned to about one hundred discrete vibration-rotation transitions between 886 and 1096 cm-’. The C.W. laser can also be pulsed and Q-switched to provide short high power pulses typically 10 kW in pulses 300ns wide for a 2m cavity. Shorter pulses can be obtained using mode-locking techniques.* A significant advance in i.r. laser development was the introduction of the transversely excited atmospheric pressure (TEA) laser. By operating the active medium at atmospheric pressure vacuum problems are eliminated and the output ’C. K. N. Patel T. Y. Chang J. T. Nguyen Appl. Phys. Letters 1976 28 603.’P. K. Cheo in ‘Lasers’ ed. A. K. Levine and A. J. De Maria Dekker New York 1971 Vol. 3 p. 111. Infrared Laser Photochemistry power per unit volume is increased. To achieve uniform excitation at atmospheric pressure it is necessary to preionize the gas to initiate a uniform glow discharge between the main electrodes. A typical pulse from a TEA CO laser varies from 70 to 300 ns in width and powers of several megawatts are readily available. When shorter pulse widths are required active mode locking techniques can be employed.’ Single pulses can then be selected from the train by use of a suitable switching element. Typically pulses of 1-2 ns duration with an energy in the mJ range are obtained when an intracavity loss modulator is used. Passive mode-locking using SF6 as saturable absorber can also be used to produce short pulses in the 1.5 to 10 MW range.” Bonds which absorb in the CO,laser region (9.2-10.9 pm) include C-C C-0 C-F P-0 W-0 S-F Si-F and S-H.For laser chemistry it is common to use long pulse TEA CO lasers operating at a repetition rate of S1Hz since laser fluence rather than peak power seems to be the important quantity in laser photochemistry. These lasers can deliver up to about 10 J in a pulse width of between 250 and 80011sin a beam 34cm in diameter. This corresponds to an energy fluence of -1 J ern-, and a power of about 2 MW ern-,. The energy level diagram for CO showing the lasing transitions is illustrated in Figure 1. OtherLasers. Chemical lasers” such as the HF-DF laser operating between 2.8and 4.0 pm can produce large output powers and fluences.However as yet they have been little used in laser chemistry. This is due partly to operating problems which involve the handling or disposal of corrosive gases and partly to the lack of flexibility compared with the COz laser. A large number of chemical lasers have now been studied giving rise to a large number of laser lines in the mid-i.r. region. The characteristics of these lasers are generally similar to the CO laser but with lower powers. With some commercial TEA CO lasers it is possible to change the optics and electrical discharge parameters and operate the laser with a different gas mixture generating a number of different laser lines. The CO laser (5.1-5.6 pm) is generally operated C.W.but at lower powers than the CO laser. It can only be used in specific applications where its relatively low power is useful. Tunable lasers.-Rapid advances have been made in the technology of i.r. tunable lasers in recent year^,'^.'^ but their output powers are still rather too low in most cases to be of practical use in laser chemistry. Several tunable laser systems offer some promise in chemical applications; the parametric oscillator and the spin-flip Raman laser are two such lasers. Parametric oscillators utilizing powerful Nd/YAG 1.06 p m lasers as pumps are now available commercially. These devices use angle tuning of a LiNb03 crystal to tune through the 1.4-4.45 pm region of the i.r. The optical parametric oscillator can be used to achieve relatively high tunable coherent power levels in the near i.r.13,14 Optical parametric oscillation has been observed in M. C. Richardson Optics Comm. 1974 10 301. lo R. Fortin F. Rheault J. Gilbert M. Blanchard and J. L. Lachambre Cunud. J. Phys. 1973 51 414. R. Ward and T. Y. Chang Appl. Phys. Letters 1972 20 77. ’’ A. Mooradian in ‘Tunable Lasers and Applications’ ed. A. Mooradian T. Jaeger and P. Stokseth Springer-Verlag Berlin 1976. ” M. Colles and C. R. Pidgeon Reports Progr. Phys. 1975 38 329. l4 R. L. Byer and R. L. Herbot in “on-Linear Infrared Generation’ ed Y. R. Shen Springer-Verlag Berlin 1977 p. 81. 52 R. T. Bailey and F. R. Cruickshank 3000 I I J I 131 27 23 2500 -I I V 2000 J 28 24 20 16 " 1I J 28*: 'Oo0FERMI\-I I 0200 RESONANCE Figure 1 Energy level diagram showing the lasing transitions of the 10.6 pm band of COz (Reproducedby permission from 'Lasers' ed.A. K. Levine A. J. De Maria Dekker New Yorker 1971 Vol. 3 p. 111) about ten different crystals with tunable output from 0.4to almost 17 pm. The long wavelength limit is currently 16.5 pm achieved by pumping CdSe with an HF 1a~er.l~ Typical line widths are of the order of 0.1 to 1cm-'. Optical parametric oscillators are likely to find increasing application in i.r. photochemistry. The i.r. region from 2.2 to 24 prn can now be covered by difference frequency generation in various non-linear materials. However the power outputs are presently too low to be of use in i.r.photochemistry. In spin-flip Raman lasers a fixed frequency pump laser beam (CO or CO) is inelastically scattered by the stimulated Raman effect from conduction electrons in a semiconductor crystal16at 4 K. The output is tuned by varying the large magnetic field to which the semiconductor is subjected. In the use of InSb for magnetic fields Is S. Rockwood in ref. 12 p. 140. R. J. Butcher R. B. Dennis and S. D. Smith Proc. Roy. Soc. 1975,344A,541 Infrared Laser Photochemistry 53 of 25 to 100 kG pulsed output powers of the order of 1kW in the first Stokes tens of watts in the first anti-Stokes and a few watts in the second Stokes lines are typical. The output covers the regions 5.0-6.5 pm and 9.0 to ca. 14 pm; the latter limit has recently been extended to 16.8 pm by pumping with an ammonia laser at 780 cm-'.17 The spin-flip Raman laser can also be operated C.W.using the CO laser as pump. Other semiconductors such as InAs and Hgl-,Cd,Te'8 can also be used. Spin-flip Raman lasers offer moderate power narrow line widths ( cm-' C.W. and 10-4-10-5 cm-' pulsed) and a reasonable tuning range. However the equipment is complex requires liquid helium cooling and does not have sufficient output power for most photochemical applications. In some applications where the high power per unit bandwidth can be utilized the device is valuable. Although the CO laser can be line-tuned over a large number of transitions near 10 pm for some applications greater tunability is desirable. This can be achieved by operating the laser at high pressures when pressure broadening effects result in continuous absorption over the whole vibration-rotation band.Continuous tunability is then possible over the whole band with high output powers. High pressure electrical excitation of CO and other simple molecules has been demonstrated using a variety of discharge techniques.* Electron beam sustained COzlasers at pressures up to 15 atm have been operated with efficiencies of about 2% and cavity energy densities of 60 J 1F'. This technique clearly offers the potential of tunable very high power single pulse energies and is well suited to photochemical applications. Diagnostic Techniques.-Diagnostic techniques can be divided into two groups; (i) static techniques depending on the analysis of the products of a system after irradiation for a given time at a particular power level and laser fluence and (ii) dynamic techniques which monitor the progress of a photochemical reaction both in the short time regime during and immediately after a laser pulse and in the longer time regime as a function of the number of laser shots.The dynamic techniques are much better probes of the physical and chemical processes occurring in the molecular system and are far easier to interpret. The static techniques are often complicated by the absorption of laser energy and subsequent further reactions of the primary photolysis products. Standard analytical techniques such as g.1.c.-m.s. and i.r. spectroscopy are commonly used to analyse the products in a static reaction system.In the dynamic case time resolved i.r. fluorescence from both reactants and products is a valuable technique for monitoring the progress of a reaction. Since the effect is usually very weak sensitive fast response i.r. detectors and high aperture optical systems (or filters) are necessary. The techniques of i.r. fluorescence are well established in energy transfer work.6 Typically a time resolution of 10-6-10-7 s can be obtained. The spectral resolution is generally limited by the available energy and normally a wide bandpass is used; however in C.W. work with the larger energies available a resolution down to 1cm-' can be achieved. A more sensitive technique where it is applicable is to monitor the concentration of a particular species reactant or product by absorption of an i.r.laser tuned to an i.r. band in the molecule. The time resolution is again limited by the overall rise time of the i.r. detector-amplifier assembly. Tunable lasers such as the spin-flip Raman laser are particularly useful for l7 C. K. N. Patel T. Y. Chang and V. T. Nguyen Appl. Phys. Letters 1976 28,603. 18 P. Norton and P. W. Kruse Optics Comm.. 1977 22 147. 54 R. T. Bailey andR R. Cruickshank this purpose since high powers are not required. Another very sensitive and versatile technique is the visible dye laser excited fluorescence of molecular species.” The U.V. visible fluorescence from the particular species is excited by a tunable dye laser either pulsed or C.W.and monitored as a function of time and/or frequency. Since photomultipliers can be employed in this spectral region much better time resolution and greater sensitivity can be achieved. The technique is however limited to species having absorption-fluorescence bands in the appropriate spectral region. In certain cases visible chemiluminescence is observed directly from excited product species at high laser fluences. A very versatile technique for exploring the mechanisms of laser activated reactions in the collisionless or near collisionless regime uses molecular beam-mass spectrometry techniques. 20*21 The experiments are conducted in a differentially pumped beam sampling mass spectrometer designed to allow in situ optical and mass spectral analysis of the photolysis products generated by each laser pulse.A low pressure (typically 5-100 mtorr) beam of molecules is passed through the focused beam of a TEA C02laser into a quadrupole mass spectrometer (Figure 2) where the gas fLow laser beam -D /i-1 to pump I-3 x lo5 Torr L-signal beam quad .9’ divergence I I Figure 2 Schematic diagram of the mass spectrometer beam sampling system (Reproduced by permission from J. Chem. Phys. 1978,68,777) photolysis products are analysed. With this technique the primary dissociation channels can be studied directly in the absence of complicating secondary reactions. In some photochemical reactions particularly those involving laser isotope separation of heavy atoms high power densities are required at the low gas pressures employed.At these power levels power broadening of the absorption lines frequently becomes greater than the isotopic shift and isotopically selective excit‘a- l9 S. E. Bialkowski and W. A. Guillory J. Chem. Phys. 1978,68 3339. Aa. A. Sudbo P. A. Schulz E. R. Grant Y. R. Shen and Y. T. Lee J Chem. Phys. 1978,68. 1306. 21 J. W. Hudgens J. Chem. Phys. 1978,68,777. Infrared Laser Photochemistry 55 tion is lost. To overcome this difficulty a double photon technique has been used." Many substances cannot be excited directly because they do not have i.r. absorp- tion bands that match the frequencies of suitable laser systems. Such molecules can be activated by the addition of an i.r. sensitizer which absorbs the laser radiation and transfers it by V-V R/T processes to the molecule of interest.Suitable i.r. sensi- tizers include SF6 SiF4 BCl etc. SiF is particularly convenient because it absorbs strongly at 1025 cm-' and is chemically inert.' SF6is also a very strong absorber and is efficient at very low pressures but at megawatt power levels it is not inert and reacts with molecules containing hydrogen. 3 Theoretical Considerations Diatomic Molecules.-The effect of vibrational excitation on the rate of reaction of diatomic molecules depends on the sign of AHo for the reaction. In thermoneutral reactions e.g. reactions (1)and (2) HCl+ He- H2+ C1. AH" = -4.6 kJ mol-' (1) HCI + Oq3P) + *OH+ CIS AH" = + 4.2kJ mol-' (2) energy transfer processes seem to play a major role and the acceleration of reaction consequent on raising the HCl to u = 1is not large.24 With endothermic processes the acceleration of rate can be dramatic e.g.in reaction (3) K+ HCl + KCI + Ha AH" = +4 kJ mol-' (3) there is only ca. -ten-fold increase in rate if HCl is in u = 1instead of u = 0 but in reaction (4) Br. + HCl -+ HBr + C1. AH" = 65.2 kJ mol-' (4) the rate is increased by a factor of -10" if the HCl is in u = 2 instead of u = 0. This reaction is exothermic for HCl at u = 2. [AE HCl(u = 1)-HCl(u = 0) = 34.5 kJ mol-' and AE HCl(u = 2)-HCl(u = 1)= 33.3 kJ mol-'I. These results are to be expected in that for endothermic processes Hammond's postulatez5 leads us to expect a 'late' barrier in the potential surface and thus that the vibrational energy of the reactant diatomic molecule would be efficiently converted into translational energy of separation of the products.However in the simple case of the diatomic/atom reaction the density of states and the volume of phase space available to the reactants is very much less than for polyatomic systems. (Even the above discussion is over simplified in taking no account of the variations possible in the type of potential energy function operating between the various atoms.) Exothermic reactions would be expected to be slower when the activation energy is stored largely in vibrational degrees of freedom. An example is the case given in reaction (5). *O(3P)+CN-+CO+.N('D) AH"= -78 kJ mol-' (5) 22 R. V. Ambartzumian N. P. Furzikov Yu.A. Gorokhov V.S. Letokhov G. N. Makarov and A. A. Puretzky Optics Letters 1977 1 22. 23 K. J. Olszyna E. Grunwald P. M. Keehn and S. P. Anderson Tetrahedron Letters 1977 19 1609. 24 J. Wolfrum Ber. Bunsen gesellschaftphys. Chem. 1977,81 114. 2s See e.g. 'Fundamentals ot Organic Keaction Mechanisms' J. M. Harris and C. C. Wamser J. Wiley New York 1976 p. 124. R. T. Bailey and F. R. Cruickshank Already the simple arguments begin to fail however for the above result is true only for the reaction path via the complex NCO. An alternative reaction path is significant viz. reaction (6) SO(~P) + CN + CO + sN(~S) AHo= -309 kJ mol-’ (6) where the reaction cross-section increases with CN vibrational excitation. This has been rationalized on the basis of ‘induced attraction’ on the grounds that in the limit when CN is dissociated the C attracts 0 and at large vibrational amplitudes some measure of this attraction should develop.This is really a rationale for a complex potential surface. For a four-centre process (two diatomics) the activation energy is equal to several quanta of i.r. laser radiation and in practice other processes than the elementary reaction step play a significant role e.g. energy transfer. The distribution of vibra- tional energy between the molecules affects the reaction rate. In the case of reaction (7) H,+D -* 2HD (7) stimulated Raman pumping was used to excite H2 and sequential (multiple) photon absorption was used to explain the results.26 With a dye laser tuned to the t = 0 +6 transition HCl was excited but no evidence was found for reaction (8).However reaction (9) HCl(U = 6)+ D2 + DCl +HD (8) HCl(u = 6)+ C4Hs + Me3CCl (9) occurs at room temperature. Polyatomic Molecules.-Larger polyatomic molecules present a far more complex picture but for the interpretation of CO laser i.r. photochemistry several broad features appear to be common to most systems. Experiments principally on SF and MeF have been performed by many groups. Theoretical models based on several approaches now tend to support the deductions of the experimentalists. 27*28 With the CO laser one quantum at -1000cm-’ is much less than the activation energy of most molecular reactions so that -34 quanta are needed for example to decompose SF,. In addition to the complexities arising from the increased volume of phase space available to molecules such as SF or MeF there will be the complexities of energy transfer processes-very fast for species excited to v -34.In order to slow down energy transfer processes it is usual to lower the pressure i.e. the gas collision frequency. The resultant low number of joules absorbed by the system is insufficient to cause reaction and this is compensated for by increasing the intensity of the laser used until multiple photon absorption ensues. The high intensity required demands that pulsed lasers be used. A ‘rule-of-thumb’ threshold of -1 MW cm-* exists below which reaction rarely occurs at the usual 1-1000 mTorr pressures. The reason why the fluence absorbed [Jcm- or ‘dose’ as distinct from flux (J cm- s-’) all per unit path length] increases has been investigated in great detail.26 S. H. Bauer D. M. Lederman E. L. Resler and E. R. Fisher Inrernat. J. Chem. Kinetics 1973,5 93. ” M.Quack J. Chem. Phys. 1978,63 1282. J. G.Black E. Yablonovitch N. Bloembergen and S. Mukamel Phys. Rev. Letters 1977,38 1131. Infrared Laser Photochemistry It is now clear that every system studied is different in the details of this mechanism but some common features will be discussed. A multiple photon absorption is required to get -34quanta into any one molecule (N.B. not a multiphoton absorption which is a term reserved for the instantaneous absorption of several photons). Since the molecular vibrations are anharmonic 34 different sizes of quanta should be needed for this process of ascending the energy level ladder.In practice substantial reaction occurs at one laser frequency! In most cases the mechanism is a combination of the following events. There is evidence that rotational levels can compensate for the quantum mismatch at least in the lower levels. A P-branch transition in v = 0-1 will possibly match a Q-or R-branch transition in v = 1-2 etc. if the rotational quantum is of the appropriate size. Ambartzumian has ascribed this mechanism to the Os04decom-position where the luminescent decomposition failed to occur when laser lines overlapping with the R-branch of Os04were used to pump the OsO,. This was despite the significant absorption of these lines.29 The accessible states of the molecule can be considered to be relatively few at low energy i.e.up to u = 3 or 4.Thereafter a rapid rise in the density of states results in a Figure 3 Schematic energy level diagram for SF6 i.r. laser photochemistry (Reproduced by permission from Phys. Rev. Letters 1977 38 1131) 29 R. V. Ambartzumian Y. A. Gorokhov G. N. Makarov A. A. Puretzkii and N. P. Furzikov Chem.Phys. Letters 1977 45 231. R. T. Bailey and F. R. Cruickshank condition described as the quasicontinuum. Ultimately this leads to the dissociation limit (Figure 3). This model readily explains the isotopic selectivity in requiring accurate quantum matching over the first three or four quanta of the energy ‘ladder’. For a given laser line (linewidth -0.03 cm-’) matching the o = 0-1 energy gap it is clear that the 32SF6 and 34SF6absorption lines will be differentiated (isotope shift 17 cm-’); however it is necessary to explain how the narrow laser line can also match the v = 1-2 efc.In addition to the Ambartzumian mechanism which can operate at low field intensities at high laser intensities a currently accepted rationale is power broadening. The concepts involved are not easy and should be discussed briefly. The molecular model for power broadening is that of an absorbing molecular vibration surrounded by a ‘lattice’ of other vibrational modes. As for the analysis of n.m.r. linewidths the absorbing mode relaxes to the molecular ‘lattice’ of energy levels with a vibration-to-vibration energy transfer relaxation time T, (cf.spin-lattice). An absorbing mode dephasing relaxation time T2,(cf. spin-spin) is also present. The absorption linewidth of the gas (normally -0.002 cm-’ Doppler limited) due to the high incident power is increased and is given by AvR = wR/2.rrc (cm-’> Here wn is the Rabi frequency given by wR= 2rIF121EO/h where Ip12)is the transition moment (typically 0.1-0.5 D 1D = 3.3 x lop3’ C m). Eois the electrical field in V cm-’ associated with the laser beam intensity and this is the source of power broadening. It is caused by the high voltage electrical field of the laser light itself. The cycle average theorem coupled with Poynting’s theorem gives I,” = $E~c~~E(?, t)I2 where 7is the refractive index and eo the permittivity of free space.Thus Eoin the previous equation is given by Eo= 27.5 I”2 V cm-’ where I is in W cm-2. At 5 MW cm-2 Eo= 6.1 X lo4V cm-’ and AYR = 0.5 cm-’. Increasing the flux to -1GW cmP2 to make the process more efficient is not always satisfactory as the power broadening AvR becomes 2-10 cm-’ for UF6 i.e. greater than the isotopic shift of -1 cm-’ for 235UF6 and 238UF6. To avoid this problem the two frequency technique was developed (see Os04,SF6). In the case of Os04 the 956.2 cm-’ laser line was used to raise the population of the o = 1level with isotopic selectivity and the 944cm-’ laser line was used at higher power to promote the species so excited to the top of the quasicon- tinuum and then to the dissociation limit. The power broadening effect described above is not to be confused with dynamic Stark broadening due to the a.c.field. In the case of SF, where no permanent dipole exists there can be no first-order Stark effect. The second-order Stark effect gives rise to a broadening of -0.1 cm-’ for the 10 V cm-’ field of -1 GW cm-2 laser power flux. However the value of AvR is 5.00 cm-’ under these condition^.^' 30 ‘Physics of Quantum Electronics’ Ed. S. F. Jacobs M. Sargent M. 0.Scully and C. T. Walker 4 ‘Laser Photochemistry’ ‘Addison Wesley’ 1976. Infrared Laser Photochemistry 59 The Rabi frequency is the optical nutation frequency and can be obtained as a beat frequency between emission from the macroscopic dipole in the system resulting from electrical alignment of the molecules and the slightly different frequency of the incident laser itself as in a Stark pulse e~periment.~' It would be wrong to assume from the above that isotopic selectivity is entirely due to the u = 0-1step since it has been shown3 that even in the quasicontinuum the absorption coefficient varies markedly with frequency over frequency spans as small as 10 cm-'.So far the discussion has centred on SF,. Another well studied molecule is MeF. Being a symmetrical top with a large dipole moment MeF has a first-order Stark effect with a shift of several cm-' and a AvR-30 cm-' at 10 V cm-'. Also MeF has a lowest vibrational frequency of 1049cm-' and a high rotational constant B of -0.7 cm-' yielding about three lines per wavenumber as against the several hundreds per wavenumber of SF6 in the CO laser frequency region.It has been found that even 50cm-' either side of the P(20) C02 laser line resonance at 1047 cm-' MeF and C1 can be induced to react whereas even 5 cm-' off resonance SF has been reported not to react.33 However because of obvious fluence depen- dence of the level 'pumping' and absorption coefficients the differences between SF and MeF are probably not as simple as seems from the above. The validity of the quasicontinuum model is governed by Fermi's Golden In the polyatomic molecule the density of accessible states rises rapidly with total molecular energy content The Golden Rule states that the full Schrodinger equation description of the system can be replaced by a rate equation description provided that the transition rate to the quasicontinuum is such that3' liiu(~)l-' << transition rate << T,' Thus as the total molecular energy content rises u (E)rises and the Rule will be validated.This point determines the onset of the quasicontinuum. The relevant rate equation is where W is the probability of being in that group of stationary states nh v above the initial level (ground state); KE KZ are absorption and stimulated emission coefficients; and Ktiss is the reaction rate constant. The rate equation can be multiplied through by dt and thus the time evolution of the dissociation rate is seen to depend only on I dt i.e. energy fluence. This at first sight unlikely result has been confirmed now by comparison of observed reaction rate with laser pulse width.For constant fluence the reaction rate seems to vary little for sF6for laser pulse widths from 30 ps to 200 ns.36s37Because of this fact it seems likely that the spontaneous mode-locking which is almost invariably present in CO TEA laser pulses is probably of no consequence. It is still perhaps rather too soon however to be sure 31 R. G. Brewer and R. L. Schoemaker Phys. Rev. Letters 1971 27,631. 32 A. V. Nowak and J. L. Lyman J. Quant. Spectroscopy Radiative Transfer 1975 15 1945. 33 A. M. Ronn 'Laser Focus' 1976 p. 53. 34 'Fundamentals of Quantum Electronics,' R. H. Pantell and H. E. Puthoff Wiley New York 1969. 35 E. R. Grant P. A. Schulz A. S. Sudbo and Y. T. Lee Phys. Rev. Letters 1978 40 115. 36 P. Kolodner C.Winterfield and E. Yablonowitch Optics Comm. 1977,20 119. 3' H. S. Kwok and E. Yablonovitch Phys. Rev. Letters 1978.41 745. R. T. Bailey and F. R. Cruickshank that this is true for all reactions. Even if a molecule has sufficient energy to react a finite time will elapse on average before it finds the correct region of phase space for reaction. The Kassel equation for reaction rate constant X,is x=wo n!(n-rn +N -l)! (n-m)! (n+N-l)! where wois the laser frequency or the high pressure A factor in the ergodic regime m the minimum number of photons required for reaction and N the number of normal modes (N.B. n is not the quantum number of the ‘pumped’ normal mode v3 but is the level in the quasicontinuum which is a composite of all modes). If m = 34 and excess energy = n -rn = 10 the reaction occurs in about lo-’ s.Excess energy of the reaction fragments has been shown to be consistent with this Figure 4 shows the reaction yield as a function of mean excitation (n) (photons Figure 4 Reaction yield Y,for SF6 decomposition as a function of photons absorbed per molecule ((n)or (n’))for two laser pulse widths. The yields are normalized that for the 500 ps pulse being 30% greater than the 100ns value under similar conditions (Reproduced by permission from Physics Today 1978 May 25) absorbed per molecule). Since (n) corresponds to the ‘centre of gravity’ of the population distribution over energy this is an experimental measure of the shape of the energy distribution function. The shorter pulse with large Rabi broadening and energy boosts the system directly into the quasicontinuum and so the energy distribution should then be close to that of thermally excited SF6,since the ‘bottle- necking’ in the discrete levels has been avoided.The bandshape of high quantum number transitions can now be examined by laser photoacoustic or thermal lens spectroscopy and gives information on the energy distribution in those states. TIcan be thought of as the time needed for the molecule to acquire ergodic behaviour. For mode-specific reaction the activation energy would have to be supplied and the reaction would have to occur in less than TI. 38 N. J. Coggiola P. A. Schulz Y. T. Lee and Y. R. Shen Phys. Rev. Letters 1977,38,17. 39 E. R. Grant M. J. Coggiola,Y.T. Lee P. A. Schulz and Y. R. Shen Chern. Phys. Letters 1977,52,595. Infrared Laser Photochemistry 61 There are indications that Tl is of the order of a few psa4’ In practice this will require high excess energies and number of normal modes (Kassel equation). In turn this will require large energies in the laser pulse as has already been predicted. Recently cell geometries and photon ‘usage’ have been in~estigated.~~ The laser beam was admitted to the cell via a small hole (2 mm diameter) at the focus of a lens and the subsequently divergent beam reflected off the cell walls. A relatively short cell (20 cm) with multiple passes during the laser pulse (200 reflections) was found best but still lo4incident photons were needed for each SF6molecule to react.Our experience indicates that simple lens systems behave rather badly at -1000cm-’ and since no optical element is infinite diffraction effects will abound especially for apertures in the millimetre diameter range.42 Such effects can lead to very high local energy densities and these would be expected in the cell designs above. These details are included here to illustrate that the apparently simple experiment can be difficult to interpret without a full knowledge of the optics inside the cell used. Beam profile scanning is essential in this context but extremely difficult to do with a 200 ns laser pulse. There seems to be some confusion in the literature on the effects of adding inert gases to a cylindrical cell containing an absorbing gas and irradiated with a coaxial laser beam.In many cases it is assumed that the increased heat capacity of the irradiated volume will result in lower temperatures of the absorbing medium. However it has been and recent work supports the quantitative accuracy of the description to the 1% level that the relaxation time T,of the thermalized hot gas in the geometry described above is given by d2C,P 7=-4RTK where C is the heat capacity per mole of the gas mixture d is the l/e radius of the TEMoo laser beam P is the gas pressure R is the gas constant T the temperature in degrees Kelvin and K is the thermal conductivity coefficient. Clearly TOCC,P i.e. increased gas pressure and/or C (per mole) of the mixture will slow down the rate of heat loss at the cell core.Increase of P tends to accelerate vibration-translation (V-T) energy transfer and the combined effect of an increase in P is frequently to increase greatly the temperature on axis in the cell and not to decrease it. These considerations are particularly important in high repetition rate M.W. pulsed laser and in the high pressure C.W. i.r. laser experiments. Typical gas cooling times to within -1% of ambient temperature are -50 ms at a few Torr pressure. The simple argument based on C only applies where this kinetic control of the temperature is inoperative i.e. low sample pressures. 4 Sulphur Hexafluoride Sulphur hexafluoride is probably the most studied molecule in i.r. laser photo- chemistry. This is the result of its similarity to UF and its strong absorption of CO 40 J.D. Rynbrandt and B. S. Rabinovitch J. Phys. Chem. 1971,75,2164. 41 J. L.Lyman S. D. Rockwood and S. M. Freund J. Chem. Phys. 1977,67,4545. 42 R. T.Bailey F. R. Cruickshank D. Pugh and W. Johnstone Chem. Phys. Letters 1978,59,324. 43 R. T.Bailey F. R. Cruickshank D. Pugh and W. Johnstone ‘Lasers in Chemistry’ ed. M. West Elsevier London 1977,p. 257. 44 R. T. Bailey F. R. Cruickshank D. Pugh and W. Johnstone unpublished data. R. T.Bailey and F. R. Cruickshank laser radiation. SF6 is in other ways an unfortunate choice having a very complex i.r. absorption spectrum extending from 936-953 cm-'. Additionally the thermal decomposition of SF6 is poorly characterized. Coupled with the difficulties of high field intensity spectroscopy the above problems are the reason why after five years study there is still considerable doubt about the details of the decomposition mechanism.In most studies now a TEA C02laser pulse on the P(20)line (944.2cm-') is used to 'pump' the SF,. Absorption occurs from the ground vibrational state to the v3(u=1,944 cm-') level. At 300 K the z16 (v =1,363 cm-') level contains -0.52 of the population and can also be 'pumped' to the v3+ v6 (u = 1 1328cm-') There is little doubt however that most molecules will be 'pumped' up the v3 vibrational ladder for a few discrete levels before attaining the quasicontinuum of the Bloembergen-Yablonovitch model." Table 1lists the calculated anharmonic level frequencies of the SF6 v3ladder.46 The exact level at which the quasicontinuum can be considered to begin is obviously of interest and work is still being carried out to verify the existence of this quasicontinuum.Table 1 Calculated anharmonic level frequencies of the SF6 v3 ladder v3quantumnumber Level type 1 quantumnumber Frequency /cm-' 0 1 A',Flu 0 1 0 947.97 0 1886.4 2 {gig 2 1889.0 3 F2n F;I{FL F2u 2 1 3 3 1896.9 2820.3 2837.5 2839.0 '42U 3 2846.9 3744.0 3744.4 3767.8 4 3773.4 A tg 3778.5 3780.5 3789.0 Above v3 (u =lo) the vibrational level density is such (10' per cm-') that there is complete overlapping of the vibrational lines within their natural linewidth of -lO-'cm-'. In the zero electric field condition it is reasonable to assume that Au = 1 AJ = 1 transitions are -8 cm-' wide in the P-branch and -5 cm-' wide in the R-branch by extrapolation from the absorption spectrum.Within this total band contour width of -13 cm-' it can be that there are several resonant paths in the v3ladder from u =0 to u =4-5 by Av =1 processes. At fixed fluence plots of SF6 dissociation rate as a function of laser line frequency typically maximize at -P(24) (-941 cm-'). This result is consistent with the u =4-5,u =5-6 absorp-tions and the above argument that the number of resonant paths is an important D. S. Frankel and T. J. Manuccia Chem. Phys. Letters 1978 54,451. 4h J. Dupre J. Dupre-Maquaire. P. Pinson and C. Meyer Infrared Physics 1978.18 185. Infrared Laser Photochemistry 63 feature. Rabi power broadening is invoked to bridge the gap between v =4-5 and u = 10.This study further quantifies the Bloembergen-Yablonovitch and certainly explains the success and observations of the twin laser experiment^.^^'^^ In these a low power laser (100kW cm-2 at 947 cm-') was used to promote the SF6 to the v3(v =4-5) level. A second CO laser pulse coincident in time was used at higher power (58 MW cm-*) and not at a resonant absorption frequency (1084cm-'). This would not broaden the lower levels but had sufficient field intensity to give the necessary Rabi broadening to promote the molecules excited by the first laser beam from the v =4-5 levels to the quasicontinuum. Given that the v3 mode is excited to high energies as above it still remains to explain how this energy can be rapidly channelled into the other modes even at low pressures (collisionless regime),49 and giving a variety of products (At 150 K and a molecular beam pressure of Torr SF5 SF3,SF2,S and F are all detected by mass spectrometer).A series of experiments were performed4' in which fluores- cence at -600cm-' was monitored when SF6 (0.002Torr) was excited by a long (-2ps) CO laser pulse on the P(20) line. The risetime of this fluorescence was faster than the 2 ps pulse width even when the time between collisions was as long as 60 ps! Under these conditions this fluorescence risetime was also faster than the rotational relaxation rate5' (p~ = 36 ns Torr) and 300 times faster than the collisional V-V energy transfer rate." With 0.028 Torr SF6 and 2 Torr He the fluorescence relaxes at the V-T rate but not to zero showing that significant bulk heating occurs after 40 ps.It is stated that the amplitude of this part of the signal (due to thermalized hot SF6)decreases as more He is added due to the increased heat capacity of the system consequent upon the rare gas addition. However this frequently used explanation may not be correct. From observation~~~ at even lower laser powers it is found that the effect of added rare gas buffers is to increase the V-T rate and decrease the thermal diffusivity. The result is to increase the temperature of the thermalized gas at the cell core. This effect can overcome that of increased C,. The conclusions of Frankel etal. may stem from the fact that as the thermal diffusivity is reduced it becomes far less easy to tell when the fluorescence intensity has returned to the 'baseline' (zero).At laser repetition rates in excess of -10 Hz this heating effect will integrate. At high repetition rates the absorbing gas temperature could be quite high. This would lead to a significant increase in population of the 363 cm-' level and a change in the 'pumping' mechanism over a period of minutes. This added complication is best avoided by low laser repetition rates. The importance of the fast risetime of the 600 cm-' fluorescence lies in that it is a clear demonstration of fast collisionless intramolecular energy transfer. It may also be that the fluorescing state is directly pumped. (The 2v4 state is within 60 cm-' of the v3+ v6 state which can be directly 'pumped' from the 363 cm-' level.Fluorescence at 600 cm-' would then arise from the v3+ v6/2v4+ v4 transition. At the 100kW cm-' laser intensity and low (50-100 mJ) energy used Rabi broadening and quasicon- tinuum processes should be unimportant in the Frankel experiment. Thus there 47 R. V. Arnbartzumian N. P. Furzikov Y. A. Gorokhov V. S. Gorokhov V.S. Letokhov G. N. Makarov and A. A. Puretskii Optics Comm. 1976 18 517. 48 M. C. Gower and T. K. Gustavson Optics Comm. 1977,23,69. 4y F. Brunner T. P. Cotter K. L. Kompa and D. Proch J. Chem. Phys. 1977,67 1547. P. F. Moulton D. M. Larsen J. N. Walpole and A. Mooradian Optics Letters 1977,1 51. 5' J. T. Knudtson and G. W. Flynn J. Chem. Phys.. 1973 58 1467. R. T. Bailey and F.R. Cruickshank seems to be a mechanism of fast vibrational thermalization in the ‘discrete’ level v = 0 +t = 5 region. Clearly this requires further investigation. The dependence of the probability P of dissociation of an irradiated molecule on incident laser flux I,(W cmP2) depends on whether measurements are made above or below the critical flux I,,at which this probability saturates. Below I,,P varies as 13,suggesting that v = 3 is the average excitation of molecules stuck in3the discrete level region,52 this agrees with Ambartzumian’s value.53 Above I,,PK I2as expected from saturation and the conical geometry. When a 50 ns strongly self mode-locked pulse was rather different results were obtained. It should be noted that in this study a ‘uniform portion’ of the laser beam was ‘selected by a 9.5 mm diameter aperture’ and the laser beam was not focused.This aperture will give rise to Fresnel diffra~tion.~~ Without knowledge of the distance from cell to aperture it is not possible to assess the significance of this diffraction but it could lead to very high local intensities in a complex pattern in the cell. Low pressures (10-3Torr) of hydrocarbon were added to give a luminescent reaction. The intensity of this luminescence was interpreted as a measure of rate of reaction of F atoms with hydrocarbon and thus of pressure of SF6 dissociated It was found that for the P(20) C02 laser line the luminescence intensity varied as (n)43*’3((n) = average number of photons absorbed per molecule).The variation of (n)with fluence data agrees with the values of the Harvard group2* for their 100 ns pulse. Given the complexities of the chemistry involved and the length of the laser pulse which will cause ‘bottlenecking’ the very high value of the exponent of (n) cannot be regarded as a reliable measure of the energy distribution. That there is a 1ps delay after the laser pulse before luminescence begins to be observed and that it peaks 200 ps after this suggests that the luminescence rises with a time constant very like a translational heating process. We should not therefore like to use these data to examine the nature of the energy distribution and we believe the results of the Harvard group are more reliable i.e. that a Planck type distribution prevails.The behaviour in the quasicontinuum has been outlined in Section 3. Here transitions are those of incoherently ‘pumped’ oscillators and a Planck distribution is to be expected with a temperature which can be deduced from the energy absorbed from the laser beam. Carefully calibrated optoacoustic measurements of the vibra- tional temperature in short pulse (500 ps) experiments2’ give results in agreement with this model. It should be realised however that significant Fermi-resonance type mixing of wave functions must occur in this region in order that i.r. inactive states can be ‘pumped’ (i.e.all levels except v3and v4). Vibrational thermalization is achieved in the quasicontinuum in 30 ps or less.37 The absorption cross-section CT of the SF molecule can be expressed in terms of the average number of photons absorbed per molecule with short laser pulses (n’) viz CT =d((n’)hv)/dF ’* D.Tal U. P. Oppenheim G. Koren and M. Okon Chem. Phys. Letters 1977,48,67. ” R. V. Ambartzumian Y. A. Gorokhov V. S. Letokhov G. N. Makarov and A. A. Puretskii J.E.T.P. Letters 1976 23 22. 54 S. Speiser and J. Jortner Chem. Phys. Letters 1976 44 399. ” M. Rothschild W. S. Tsay and D. 0.Ham Optics Comm. 1978,24 327. Infrared Laser Photochemistry 65 where F is the fluence (J cm-’). As the molecule is energized in the quasicontinuum this absorption cross section falls from lo-’* cm’ to well below cm2 at F = 10 J cm-’. This drop is due to the broadening and shift in the absorption as the temperature of the molecule rises.32 Clearly if F is raised to for example 100 J cm- (1GW cm-2 for a typical CO laser pulse) the background i.r.absorption cross section (-lo-’’ cm’) will begin to absorb significant amounts of energy. Thus at this power and energy level any molecule can be dissociated by C02laser radiation even if it has ‘no’ i.r. absorption frequency in the C02 laser region. It must be noted that for CO laser pulses longer than 500ps at the threshold fluence of 1.4 J cm-’ considerable ‘bottlenecking’ will occur in the lower levels because insufficient Rabi broadening will have increased the significance of anhar- monicity. If under these conditions (n)(average number of photons absorbed per molecule with ‘bottlenecking’) is used to calculate vibrational temperature the calculated a will disagree with the thermal value contrary to the 500 ps pulse data.Additionally departures from RRKM behaviour will be shown if (n)is used instead of (n’). The difference between (n) and (n’) is attributed to the fraction f of molecules which cannot reach the quasicontinuum from their initial vibration- rotation state under long pulse conditions f-((n‘) -(n))/((n)- 3) where it is assumed that 3 hv is the average energy of those molecules which are always stuck in the discrete levels (see above). The value of f is about $ at 1.4 J cm- and tends to zero at > 10J cm-*. In the quasicontinuum vibrational temperatures ((n)hv/Nk)of -1700 K can be reached. A careful study has been made of 32SF6 decomposition where the 32SF6 pressure was monitored by absorption of a weak C.W.C02 P(20),laser beam as a continuous function of time.52 Since the stable photofragments SF4 S2F2 S2FI0 S02F2 and SOF show no i.r. absorption in the 944 cm-’ region only the 32SF6 was monitored The data are summarized in Figure 5. Obviously there is a process in the case of pure SF6 which very slowly restores some of the 32SF6 to the monitoring beam path. The fraction of 32SF6 which reacts varies with the number of laser pulses (Figure 6) in a different way depending on whether H2 scavenger is present or not. The curves in Figure 6 intersect at high numbers of laser pulses so that enrichment of 34SF6 with respect to 32SF6 becomes very difficult to interpret unless monitored as a function of time.This study underscores the need for detailed careful work on all aspects of the physics as well as the chemistry of multiple photon laser decompositions. The quantum yield (molecules dissociated per photons absorbed) of 32SF6 dis- sociation for a single pulse is 6 X lop5for pure SF6. Addition of a scavenger reduces the quantum yield. It has been known for some year^^^.^' that intense CO laser radiation could selectively remove for example ?3F6 by reaction to leave the 34SF6species having undergone reaction to a lesser extent or not at all if the pressure was sufficiently low (Figure 7).49,58 The recent revelation that the reaction with or without a scavenger ’‘ R. V. Ambartzumian Y. A. Gorokhov V. S. Letokhov and G. N. Makarov J.E.T.P.Letters 1965,21 171.57 J. L. Lyman R. J. Jensen J. P. Ruik C. P. Robinson and S. D. Rockwood,Appl. Phys. Letters 1975,27 87. 58 H. N. Rutt Culham Laboratory ‘Preprint CLMlP470’ 1977. R. T. Bailey and F. R. Cruickshank loolo Ii 0.10 b 0.20 4 0.30 ' 40.40 5 10 TIME /minutes (a) 0.05 +. v) 0.10 k LL 0 W 3 a -0.15 % W0z W a s !z3i zU I- 40 n Q-0.30 ' -0.20 ~ t-Lz - 40.40 0 5 10 TIME / minutes (b) Figure 5 The variation of SF transmission of a low power C02 laser probe beam as a function of initial SF6 pressure and time (a)with no scavenger present (b)with -1 Torr H2 as added scavenger. Laser pulse (5 J) repetition rate 25 per min (Reproduced by permission from Chem. Phys. Letters 1977 48 67) Infrared Laser Photochemistry 1 o 0.7 z \ E z -0.5 0 + 2 0.4 z 0 F 0.3 a a t- Z W u g 0.2 0.1 torr SF \ 0 e.g.HZ, is time variant in a complex way has necessitated a complete reappraisal of the studies of isotope enrichment factor (34SF6/32 sF6 after reaction 34SF6/32SF6 before reaction) as a function of number of laser pulses. The data of Figure 6 could explain why Dupre et al.59 report a higher enrichment factor with scavenger after 1000 pulses than observed without a scavenger. At a small number of pulses the dissociation rate of 32SF6without a scavenger is higher than with a scavenger so that under these conditions the Cotter and Fuss6' data which appeared to show that the scavenger decreased the 32SF6dissociation rate are consistent with the apparently contradictory Dupre results.A scavenger can also appear to cause higher 32SF dissociation if analysis is carried out several minutes after irradiation because reverse processes then become very significant. In an isotope separation plant it would be useful if the SF6pressure could be raised to improve throughput. Predictably it is observed61 that the excited 32SF6,absorbing 59 J. Dupre P. Pinson J. Dupre-Maquaire and C. Meyer Comptes Rend. 1976,282 B 357. "' T. P. Cotter and W. Fuss Optics Comm. 1976,18 31. 61 S. T. Lin S. M. Lee and A. M. Ronn Chem. Phys. Letters 1978. 53 260. R. T. Bailey and F. R. Cruickshank WAVENUMBEP. (cm-! ) 1000 900 1000 900 -n n n n n l -1 -1 500 1000 1500 2000 2500 3000 3500 LASER PULSES Figure 7 The i.r.spectra of 1Torr SF6+ 1Torr H2irradiated for a series of numbers of (2 J x 1 ps) laser pulses. The 947.9 cm-' absorption is due to 32SF6 and that at 930.5 cm-' to 34SF6 (Reproduced by permission from Chem. Phys. Letters 1978 53 260) the P(2O)CO2laser line gives up its energy to 34SF6more readily as pressure is increased so that the 32SF6percentage decomposition is reduced while the 34SF6 decomposition rises. Long time analysis was used in this study so that all reverse processes can be assumed to have occurred. However the CO laser pulse used was unusually long at 1ps (2 J fluence 50 J ern-,). In agreement with the results of Tal et a1 the decreased observed dissociation of 32SF6and 34SF6with H2 added increases towards the collisionless values at large numbers of laser pulses.It was also shown that addition of for example 20 Torr Ar does not prevent isotopic enrichment. It should be noted however that sample heating under these conditions will be severe,44 altering the rotational band contour and thus absorption coefficient. This effect will confuse the V-T/R energy transfer argument used by Ronn et al. to interpret the results. As a further example of the experimental artefacts that can arise it is appropriate to mention here the paper by Turner et a1.62 in which CO laser irradiated SF in Ar or CO matrices at 12 K was apparently enriched isotopically in 34SF6. This reproduced the data of Ambartzumian et al.63 and was shown to be an artefact due to ablation of the matrix.'* B. Davies. M. Poliakoff K. P. Smith and J. J. Turner Chem. Phys. Letters 1978 58,28. 63 R.V. Ambartzumian Y. A. Gorokhov G. N. Makorov A. A. Puretskii and N. P. Furzikov in 'Laser Spectroscopy' 3rd Edn. ed. J. L. Hall and J. L. Carlsten Springer Verlag Berlin 1977. Infrared Laser Photochemistry 69 5 Hydrogen Halides The interaction of vibrationally excited molecules with reactive atoms will frequently involve competition between reaction and relaxation pathways. In some cases the reactive channel may account for a large fraction of the total vibrational depletion so that vibrational excitation may be employed to overcome the endothermicity in a reaction between a molecule and an atom. Thus vibrationally enhanced selective photochemistry becomes a possibility with isotopic selection in appropriate cases.Hydrogen Chloride.-The competition between reaction and relaxation in vibra-tionally excited HC1 was first studied by Arnoldi and W~lfrum.~~,~~ These authors investigated the reactions of HCl(u) with H D C1 and Br atoms using the laser excited fluorescence technique combined with a discharge flow reactor. By deter- minations of the absolute concentration of the vibrationally excited molecules time resolved observation of the atom concentration and mass spectrometric analysis of the products from laser induced reactions the reactive and inelastic pathways could be distinguished. A HCl laser was used to provide vibrational excitation. In the case of Br atom reaction there was a dramatic increase in the rate constant (k3)when HCl was excited to the u = 2 level (i.e.ca.lO"-fold). The main processes are given in reactions (lo) (1l),and (12). HCl(u = 1)+ Br(2P:) -+ HCI(u = 0)+ Br(2P:) (10) kl = (2.5 *0.6) x 10" cm3mol-' s-l + HCI(u =O)+Br*(*P:) (11) k2 = (1.7* 0.4) x 10l1cm3mol-' s-l HCI(u =2)+Br(2P:) + HBr(v =O)+Cl k3=(9*6)x1O1'cm'mol-*s-' (12) Reaction (12) was used to provide an enrichment of 35Cl of up to 90% using the sequence reaction (13) reaction (14). H35Cl(u= 2)+ Br + HBr(u = 0)+ 'kl (13) 35 CI+Br -+ Br35C1+Br (14) A similar study on the HCl (IJ= 1)system was carried out by Leone et a1.66 Again a HCl chemical laser was used to produce HCl(u = l) but a tunable parametric oscillator was used to excite HCl directly from u = 0 to IJ = 2.With direct 0-2 excitation the kinetics of the deactivation of HCl(tl= 2) by Br atoms are simple and the rate constant reliable. Also the high excitation efficiency of the parametric oscillator allowed a working pressure as low as 0.04 Torr to be employed making the V-V rates comparable to those of atom deactivation. The fluorescence from the HCl(v = 2 -+ 1)was used to monitor the HCl (u = 2) population. The rate constants for the deactivation of HCl(v = 1)and HCl(u = 2) by Br atoms at 294 K were found to be (2.8f0.52)x cm' molecule-' s-' and (1.8f0.33) x cm' molecule-' s-' respectively. The rate constant for deactivation of HCl(t = 1)by Br 64 D. Arnoldi K. Kaufman and J. Wolfrum Phys.Rev. Letters 1975 34 1597. 65 D. Arnoldi and J. Wolfrum Ber. Bunsen gesellschuft Phys. Chem. 1976,80 892. " S.R.Leone R. G. MacDonald and C. B. Moore J. Chem. Phys. 1975,63,4735. 70 R. T. Bailey and F. R. Cruickshank was also measured and found to be (3.26*0.13) X cm3 molecule-'s-l at 295 K. The vibrational relaxation of HC1 by Br is about ten-times faster than by Br2 and about 105-times faster than by Ar67 illustrating the importance of the reactive pathway in the case of Br. The results of this study support the conclusions that the deactivation of HCl(u = 1) occurs throbgh V -+ T R energy transfer while the deactivation of HCI(u = 2) occurs largely by reaction. Attempts to produce isotope enrichment of 35Cl were not successful in the case of HCl(u = 1).Hydrogen Bromide.-A large rate enhancement was also observed for the reaction of vibrationally excited HBr with iodine atoms by Badcock et aL6*The rate of reaction (15) HBr(u)+I + HI+Br (15) was found to increase by at least a factor of lo9when HBr(u = 0) is excited to HBr (u22). The bromine atoms produced in this reaction are rapidly scavenged by undissociated I2 molecules by reaction (16) kl Br+12 -+ BrI+I which produces a stable easily detected bromine product and kl 2 3 X cm3 molecule-' s-l. Ground state iodine atoms were produced in the reactor by 0.4 to 0.8 J cm-' pulses from a pulsed dye laser. Vibrational excitation of HBr was achieved using a TEA laser operating on H2 and Br2. With multiline operation both u = 1 and t = 2 levels were populated but single line operation populated only the u = 1level.Reaction products were analysed using a quadrupole mass spectrometer. Acceleration in reaction rate was observed for both multiline (u> 1) and single line (u = 1)excitation. Since reaction occurs from HBr (u > l) acceleration also occurs after collisional pumping which populates the higher vibrational levels. The acceleration is sufficient to allow the reaction to compete effectively with the rates of vibrational isotopic equilibration and isotopic scrambling reactions in order to produce the observed small bromine isotopic enrichment in BrI. 6 Halogenated Hydrocarbons The C02 laser induced decomposition of the halogenated hydrocarbons is one of the most extensively studied areas of i.r.laser photochemistry. From this work it has become obvious that conflicting results can be obtained for the dissociation of even simple molecules like CCI2F2 and CFC13. These difficulties arise mainly from the different experimental conditions employed by various groups of workers. These include different laser lines laser powers and energy fluences and sampling the products at different intervals after irradiation so that further reactions/dissociations can occur to varying degrees. A variety of gas pressures have also been employed which affect the amounts of laser energy absorbed by the gas and the kinetics of the reaction. The decomposition of CC12F2 at low pressures by a pulsed TEA C02 laser has been studied by Hill Grunwald and Keehn.69 This molecule absorbs at 921 cm-' 67 R.V. Steele and C. B. Moore J. Chem. Phys. 1974,60 2794. 68 C. C. Badcock W. C. Hwang and J. F. Kalsch Chem. Phys. Letters 1977 50 381. 69 G. A. Hill E. Grunwald and P. Keehn J. Amer. Chem. Soc. 1977 99,6521. Infrared Laser Photochemistry when a CCl stretch is excited and at 1088cm-' where a CF stretching mode is excited. Two competing reaction channels are expected for CC12F2 as shown in reactions (17) and (18). CC12F2 -+ CClF2+ C1 AH = 350 kJ mol-' (17) CC12F2 -+ CF2+ C12 AH = 3 10 kJ mol-' (18) These two reactions should be easily distinguished by the nature of the reaction products. For the first case the formation of ClF,CCClF and other free radical products such as CClF would be expected whereas in the second case C2F4 would be the predicted product.Laser irradiation at both 921 cm-' and 1088cm-' produced identical results with ClF2CCClF2 and CClF accounting for -83% of decomposition. There was no evidence for the formation of C2F4. Thus the primary reaction mechanism involves the breaking of the C-Cl bond. When CBr2F2 was irradiated the only product detected was CBrF2-CBrF indicating the primary step to be C-Br bond fission. Also when a mixture of CC12F2 and CBr2F2 was excited at 921 cm-' (CBr2F2 does not absorb at this frequency) the dominant products were CClF,CClF, and CBrF2CBrF2. There was no evidence for the formation of CC1F2CBrF2 which should have been formed had the free radical species -CClF2 and *CBrF2 been present simultaneously.The evidence thus suggests that the reaction occurs in two stages the photochemical decomposition of CC12F2 occurring very soon after excitation from a non-equilibrium molecular energy distribution while CBr2F2 reacts thermally to produce CBrF after V-T/R relaxation of the absorbed energy. CC12F2 does not react during the thermal reaction period due to the markedly greater dissociation energy of the C-Cl relative to the C-Br bond. Using a molecular beam-mass ~pectrometer~~ Sudbo et al." have studied the multiple photon dissociation of a number of halogenated hydrocarbons with pulsed CO laser radiation. Both angular and time-of-flight distributions were measured for the fragments to identify the major dissociation channels. Some of the molecules studied are listed in Table 2 together with results and some experimental parameters.The nozzle was heated in some experiments to increase hot-band absorption and thus shift the absorption frequency closer to a CO laser line. In the case of the CF3X series (X = C1 Br or I) the products were CF and X with velocity distributions correlated by the conservation of linear momentum in the dissociation process. The molecules CF2C12 CF2Br2 and C,HCl were also found to dissociate mainly by rupture of a carbon-halogen bond in agreement with the theory by Hill et al. For CFCl however as previously reported for SF6,71 an ionization pattern was observed that depended on laser energy fluence and the product translational energy. At moderate (-5 J cmP2) fluence the products were identified as CFCl and C1*.At higher energy fluence however ( -20 J cm-') further dissociation of -CFC12 to CFCl and .C1 was indicated. With some of the other molecules listed in Table 2 HC1 elimination was observed. In the case of molecules with two carbon atoms competing dissociation channels were observed but the dissociation always pro- ceeded along the channel of lowest threshold. Thus the results were consistent with 'O Aa. S. Sudbo P. A. Schulz E. R. Grant Y. R. Shen and Y. T. Lee J. Chem. Phys. 1978,63 1306. '' E. R. Grant P. A. Schulz Aa. S. Sudbo Y. T. Lee and Y. R. Shen in 'Multiphoton Processes' ed. J. H. Eberly and P. Lambropoulos Wiley New York,1977. R. T. Bailey and F. R. Cruickshank Table 2 The results and some of the experimental parameters used in the study of the multiple photon dissociation of halogenated hydrocarbons with pulsed CO laser radiation Absorption Exciting Beam nozzle Molecular Dissociation frequency /cm-' frequency /cm-' temperature 1°C CH3Cl + CF3*+C1* 1106 1090.0 240 CF3Br + CF3-+Br.1082 1078.6 25 CF31 -+CF3.+I. 1076 1073.3 25 CF2C12 + CF2Cl*+C1. 1098,923 1089.0,925.1 25 CF2Br2 + CF2Br.+Br. 1090 1084.6 25 CHFzCl + CFZ +HCl 11 16,1160 1082.3 280 CHFC12 4 CFCl +HCl 1070 1055.6 290 CFC13 + CFCl,.+CI* CFC12 + CFCl +C1. } CZF2+ HCl 1090 1074.6 25 /* CHClCF2 970 967.7 25 L 'CHCF2 + C1. C2HC13 -B C2HC12 + C1* 930 929.1 80 CF3CF2. + C1. /* CF3CF2Cl 982 978.5 25 L CF2Cl. + CF3. CH3CF2C1 -+ CH2CF2 + HCl 963 956.2 280 CH3CC13 -+ CH2CC12+ HCl 1075 1073.3 25 a random distribution of the excitation energy and the statistical theory of uni- molecular reactions appears to hold.Hudgens7' also observed the multiple photon decomposition of CF2C12and CFC13 by a strong i.r. laser field under collisionless or near collisionless conditions. Here the pressure ranged from 5-80 mTorr and laser fluences at the focus between 10 and 140 J cm-2. The techniques used were similarly to those employed by Sudbo et ala70 and generally similar conclusions were reached. However in the case of CF2C12,a small quantity (-3%) of :CF radicals were produced by reaction (19). CF2C12 -B :CF2 + C12 (19) When CFCl was irradiated it was found to decompose exclusively according to reaction (20).CFC13 -+ *CFC12+Cl. (20) No molecular C1 was detected in contrast to the result of Dever and Gr~nwald.'~ This illustrates the difficulties involved in postulating reaction mechanisms from an analysis of secondary reaction products in a static system. In the case of CF2Cl2 a '' J. W. Hudgens. J. Chem. Phys. 1978 68,777. 73 D. F. Dever and E. Grunwald J. Amer. Chem. Soc.. 1976,98,5055. Infrared Laser Photochemistry 73 laser energy threshold has been shown to This was found to be about 23 MW cm-2 which is similar to the value found for SF6. The i.r. laser induced decomposition of CHClF2 has also been studied by Grun- .~~ wald er ~ 1 Pressures from 5.8 to 100Torr and laser energy fluences up to 0.5 J cm-2 at 1088 cm-’ were used.Only C2F4 and HCl products were observed in agreement with other ~ork.~’,~~ By means of monitoring the concentration of :CH2 radicals as a function of time by U.V. absorption a delay of about 0.7 ps was noted between the laser pulse and the rise in :CF2 concentration. This is consistent with a photochemical rather than thermal mechanism for the dissociation of CHClF,. Further evidence for the formation of :CF2 radicals in CF2C12 decompositions was found in a preliminary of the laser induced reactions of 02,HCl NO and Me2C=CH2 with CF2C12. Isotopic selectivity was indicated arising from the initial isotopically selective formation of this reactive intermediate difluoro-carbene (:CF,). In a further study,77 the CO laser induced reactions of CF2C12 and CF2Br2 with isobutene propene and ethene were investigated.Maximum power densities of the order of 100 MW cm-’ in P(36) R(18) R(24) and R(26) C02 laser lines were used. In the Me2C=CH2-CF,Br2 system evidence for the formation of laser produced difluorocarbene was provided by the identification of the :CF2-olefin addition compound in the products. The yield of this material from the Me2C=CH2-CF2C12 system was dependent upon the laser irradiation frequency. No analogous gem-difluorocyclopropanes were detected in the CF2Br2-MeCH=CH2 and CF2Br2-C2H4 systems. Carbon isotopic segregation was found in all the experiments. Thus it appears that the :CF2 radical is only produced at the higher laser energy fluences. Evidence for difluorocarbene radical formation was also found in the laser induced dissociation of difluoromethane to give C2F4 as a major This conclusion was substantiated by scavenging with O2 when CF20 was found to be the major product.In the presence of molecular chlorine at least two distinct mechanisms in different pressure regimes were found. The principal product was CHF2Cl which was formed very efficiently. No power threshold was obvious since the reaction was driven by the unfocused laser beam towards a single product. With large numbers of pulses other products such as C2F4C12 appeared by further reaction. Products were also observed when the laser was tuned away from resonance by 10cm-’ to the R(34) 9.6 pm line as well as 30 cm-’ away to the P(20) 9.6 pm transition. No strong dependence of product concentration or nature on laser frequency was detected.Although energy transfer information” predicts a more reactive C-F channel the experimental evidence favours the C-H channel almost exclusively and these products are thermodynamically preferred. The selective i.r. photoisomerization of 1,2-dichloroethylene has been studied by .~~ two groups. Ambartzumian et ~1 have observed the selective isomerization of 74 M. C. Gower and K. W. Billman Appl. Phys. Letters 1977 30 514. 75 E. Grunwald K. J. Olszyna D. F. Dever and B. Krishkown J. Amer. Chem. SOC.,1977 99,6515. 76 J. J. Ritter and S. M. Freund J. C. S. Chem. Comm. 1976 811. 77 J. J. Ritter J. Amer Chem. SOC.,1978 100 2441. S. T. Lin and A. M. Ronn Chem. Phys. Letters 1977 49 255. 79 L.A. Gamss and A. M. Ronn Chem. Phys. 1975,9 319. R. V. Ambartzumian N. V. Chekalin V. S. Doljikov V. S. Letokhov and V. N. Lokhman Optics Comm. 1976,18,400. 74 R. T.Bailey and F. R. Cruickshank trans-C2H2C12 with 10.6 pm radiation from a TEA CO laser at power levels greater than lo9W cm-'. At these power levels however other processes such as dis- sociation of truns-C2H2C12 to give acetylene :CH and C2 radicals also occur. Photoisomerization of both cis- and trans-C2H2C12 have also been observed when SF is added as a sensitizer." The direct photoisomerization was also observed for the trans which absorbs at 898 cm-' overlapping the P( 16-20) lines of the 10.6 pm band of the CO laser. The reaction rate of the sensitized photo- isomerization was about six times that of the direct process owing to more efficient pumping of the 898 cm-' band.The C02 laser induced decomposition of chloroethylene to acetylene and hydrogen chloride has been studied by Willis and Back.82 Since chloroethylene could be selectively decomposed by the 1041.34 and 1045.04 cm-' laser lines deuterium enrichment was achieved. Deuteriated chloroethylenes are transparent to laser radiation at these frequencies. At pressures below 1 Torr however C-Cl bond scission appears to increase at the expense of molecular fragmentation and gives rise to radical processes which are isotopically non-selective. The CO laser photolysis of dilute samples of various alkyl halides in helium (0.5 to 2 Torr in up to 50 Torr He) was studied using the P(36) line of the 10.6 pm CO band.83 The pyrolysis of the isobutyl halides yielded isobutene methyl acetylene acetylene and other compounds.The ethyl halides yielded ethylene and acetylene but the former is also photolysed with acetylene as the only product. The reaction pathways were always dissociation into the lowest thermal dissociation channels of the molecule. The molecules photolysed were found to be not thermally equilibrated with the bath or with each other and the molecular-specific nature of the laser excitation was demonstrated. The multiple photon dissociation of CF31 has been achieved with a CO TEA laser producing a maximum of 1J single line output in a 60 ns A I3C enrichment factor of nearly 600 was obtained by irradiating 0.1 Torr of CF31 at 193 K with the R(14) line of the 9.6 pm CO laser transition.The dependence of enrichment factor on gas pressure indicated that collisions during the dissociation were effective in destroying the selectivity. The multiple photon dissociation was found to be quite efficient; at a laser fluence of 1.2 J ern-, one in every eleven absorbed photons contributed its energy to the breaking of the C-I bond. Octafluorocyclobutane absorbs CO laser radiationg5 and its thermal decom- position has been well characterized both thermally and chemically. Thus the thermal reaction yield can be accurately predicted and any increase due to vibra- tional (laser induced) enhancement should be detected. The photolysis of C4Fs at low pressures using the P(14) line of the 10.6 pm band of a TEA C02 laser has been shown to yield C2F4 cleanly according to reaction (21) CyClO-C4F8 -+ 2C2F4 (21) both thermally and by i.r.photolysis.g6 Addition of 20 Torr of argon to 1 Torr of " K. Nagai and M. Katayarna Chem. Phys. Letters 1977,51 329. 82 A. Gandini C. Willis and R. A. Back Canada. J. Chem. 1977 55,4156. 83 W. Braun and W. Tsang Chem. Phys. Letters 1976,44 354. 84 S. Bittenson and P. L. Houston J. Chem. Phys. 1977,67,4819. '' T. G. Roberts Rev. Sci.Instr. 1976 47 257. 86 J. M. Preses R. E. Weston and G. W. Flynn Chem. Phys. Letters 1977,46 69. Infrared Laser Photochemistry C4F8 was found to decrease the decomposition by about one-sixth by increasing the heat capacity (sic). However for the thermal reaction a decrease of 10-7-10-s was predicted and the above result was interpreted as showing that vibrationally excited molecules were involved in the reaction.As indicated in Section 3 however the above argument is not necessarily valid. With unfocused pulsed CO laser radiation hexafluorocyclobutene (1)decom-poses to the less stable isomer hexafl~orobutadiene.'~ unfocustd F -aF With focused beams however other reaction products appear in addition to C4F6 including C2F4 and polymers. It is not clear if these products result from decom- position of the C4F6 or are formed as a parallel reaction to the formation of the diene. At a pressure of 1Torr the yield of diene product approaches 60% but on the addition of 16 Torr of helium to 1Tom of the reactant the reaction goes to completion.This is interpreted as showing that collisional deactivation of the diene by helium atoms is so effective that the reverse reaction is quenched. However a temperature increase could also be responsible. The thermal decomposition of octafluorocyclobutane (3)'' occurs smoothly between 360 and 560°C but side reactions with the wall of the vessel give in addition to C2F4 [reaction (22)] CO CO, and SiF,. By contrast however laser induced decomposition of (3) at 949 cm-' with laser powers in the megawatt region leads to clean decomposition. The example illus- trates one of the major advantages of i.r. laser induced chemistry; the virtual absence of wall effects in gas phase reactions. 7 Hydrocarbons Many different kinds of CO laser induced reactions have been carried out with hydrocarbons.These include dissociation rearrangement and isomerization reac- tions. Reactions have also been carried out under a variety of conditions; for example the reactions of ethylene have been studied at atmospheric pressure using a C.W. CO laser to excite the CH2-wagging mode." Products obtained in a 20 :80 ethylene :butadiene mixture at 1atm included cyclohexane at 25 W and cyclo- pentene and cyclopentadiene at 40 W. On the other hand the i.r. multiple photon 87 A. Yogev and R. M. J. Benmair Chem. Phys. Letters 1977,46 296. J. N. Butler J. Amer. Chem. SOC.,1962,84 1343. 89 J. W. Robinson P. J. Moses and P. M. Boyd. Spectroscopy Letters 1974,7 395. 76 R. T. Bailey and F.R. Cruickshank dissociation of C2H4 at low pressures and high powers suggested that C2 was eliminated intact possibly by atomization of all four H-atoms in a collision-free time.90 In the low pressure regime (<1Torr) unimolecular reactions usually predominate and isotopic selectivity can sometimes be achieved with appropriate choice of i.r. laser wavelength. An early i.r. laser study of hydrocarbons involved the gigawatt cis-trans iso-merization of but-2-ene91 [reaction (23)]. Me Me Me H \ / 10.6wm c=c e‘c=c / (23) . H ’ H / ‘H ‘Me cis trans This process was accompanied by partial decomposition. When a 1:1 mixture of both isomers was irradiated at 4Torr a 15% enrichment of the cis isomer was observed. At 14Torr however no change in the ratio was observed.Ther-modynamic data show that the cis :trans ratio should be less than unity at all temperatures. In the presence of SiF as sensitizer allene can be isomerized to methyl on irradiation with a C02 laser at 1025 cm-’ [reaction (24)]. SiF, CHz=C=CH?-MeC_CH 1025 cm-’ The equilibrium constant for this isomerization is 13.4 at 298 K 3.4 at 1000 K and 2.7 at 2000 K. On irradiation of a mixture of 20 Torr allene with 5 Torr SiF4 at an energy fluence of 0.73 J cmP2 the ratio methylacetylene :allene reaches about 1.6. In both these cases the i.r. photostationary state was different from the thermodynamic ratio. The retro-Diels-Alder reaction of 1-limonene (4) to isoprene (5) was one of the first laser induced organic reaction The symmetry selection rules dictate that this reaction is allowed only in the electronic ground state confirming the laser induced reaction to be a ground state reaction.The early work was done with a 5 W C.W. laser at 943 cm-’ resulting in 90 R. V. Ambartzumian N. V. Chekalin V. S. Lotokhov and E. A. Ryabov Chem. Phys. Letters 1975,36 301. 91 A. Yogev and R. M. J. Lowenstein-Benmair J. Amer. Chem. SOC., 1973,958487. 92 P.Keehn and C. Cheng J. Amer. Chem. SOC.,1977,99 5808. 93 A.Yogev R. M. J. Lowenstein and D. Amar J. Amer. Chem. SOC.1972,94 1091. Infrared Laser Photochemistry other products including benzene toluene and several other compounds as well as the major product isoprene. The chemistry was found to be less complicated when the reaction was conducted with added SiF sensitizer under pulsed megawatt conditions.Norbornadiene (6)94also undergoes retro-Diels-Alder reaction when sensitized with SiF at 1025 cm-' and a fluence of 0.3 J cmP2 as shown in reaction (26). Irradiation of a C7Hs-SiF4 mixture (12.5 Torr-5.5 Torr) resulted in almost complete reaction. However under the same conditions in a C5H6-SiF4 mixture (10Torr-13Torr) 68% of the diene was decomposed. A Since C-H and C-D vibrations occur at different i.r. frequencies deuterium labelled compounds may sometimes be cleanly rearranged by i.r. laser radiation so that the D atom moves to another site in the molecule. For example the Cope rearrangement of hexa-lS-diene (7) reaction (27),95 has been induced by focused gigawatt i.r.laser irradiation at 926 cm-'. D D When an equilibrium mixture of the two isomers is irradiated only (7a) absorbs and the mixture is greatly enriched in (7b). The reaction is clean at 5-16 Torr but at higher pressures decomposition leading to acetylene formation is observed. The mole fraction of (7b) in the equilibrium mixture is greatest at a pressure of 5 Torr. Studies with other deuterium labelled isomers of hexa- 1,5 -diene have given consis- tent results and confirmed that the reaction was a genuine Cope rearrangement rather than dissociation to two alkyl radicals followed by recombination. The thermal decomposition of cyclopropane as well as the U.V. photolysis has been well ~haracterized.~~ Several vibrational modes including the CH2-wag CH3-rock and ring deformation may be pumped by the CO laser.The i.r. laser photolysis of cyclopropane was carried out using focused gigawatt power from a TEA C02laser at 9.552 pm P(20).96 Two processes were observed an apparently non-Boltzmann high energy decomposition giving rise to acetylene propylene methane and ethylene as major products and a typical flame reaction characterized by the luminescence of the C2+(d3wg)Swan band. The latter process played only a minor 94 D. Garcia and P. M. Keehn personal communication quoted in ref. 1 p. 77. 95 I. Glatt and A. Yogev J. Amer. Chem. SOC.,1976 98 7087. 96 M. L. Lesiecki and W. A. Guillory J. Chem. Phys. 1977 66. 4317. 78 R. T.Bailey and F. R. Cruickshank role in terms of the percentage consumption of cyclopropane.Both temporal and frequency resolved spectroscopy were used for the identification of the emitting species as well as in the characterization of the elementary reactions producing them. When mixtures of cyclopropane and NO were photolysed CN' C2' CH' and NH' were the observed emitters whereas with cyclopropane-0 mixtures only C2+ and CH emissions were observed. Previous work has indicated that C2 was eliminated intact by the multiple photon dissociation of ethylene." To obtain information of this process Hall ef af.97 have examined the mechanism for the production of C2 generated by the laser induced photolysis of 12CH2 13CH2 as a function of initial sample pressure. The laser induced fluorescence spectra of l3.l3C2 12*13C2 and 12*12C2 indicated that at pressures greater than -0.2Torr the C2 resulted from collisional processes such as C=C scission followed by recombination of carbon fragments or a possible four-centre reaction between a pair of highly excited ethylene molecules.At pressures below -0.2 Torr Cz was produced primarily by direct elimination of C2 from a single C2H4 molecule. Photolysis of a 0.30Torr sample of C2H4 (in natural abundance) resulted in an almost 100% enrichment of 12CHi2CH2. The delay times between the CO laser pulse and the onset of observed C2 were studied as a function of pressure and extrapolated to the collisionless pressure regime to give an estimated energy fluence of 50* 12 J cm-2 for direct elimination of C2. 8 Alcohols Ethers and Esters The i.r.muhiple photon dissociation of MeOH has been studied in two different pressure In the relatively high pressure regime (1.7-10 Torr) the focused output of a TEA C02laser was used to dissociate both pure MeOH and with added NO as a free radical scavenger.98 The energy fluence was calculated to be about 150 J cm-2 at the 0.05 cm diameter focal point resulting in both molecular and free radical initiated products. The decomposition was assumed to follow the course in reactions (28) and (29) CH30H -P CH20*+H2 (28) CH2O* + CO+H2 (29) which accounted for -90% of the consumed MeOH and the radical initiated process in reaction (30) CH30H + CH3.+OH (30) which ultimately results in stable products C2H4 C2H2 and CH4 accounted for the remaining -10% of the MeOH consumed.Visible luminescence observed from the focal zone was associated with the decay of OH* CH* C2* and possibly CH20". The concentration of stable products as well as the visible luminescence due to the electronically excited diatomic radicals were followed as a function of pressure time and the addition of the free radical scavenger gas NO. Both major photodecom- position routes appeared to be non-Boltzmann. 97 J. H. Hall M. L. Lesiecki and W. A. Guillory J. Chem. Phys. 1978,68,2247. 98 S. E.Bialkowski and W. A. Guillory J. Chem. Phys. 1977,67 2061. 99 S.E.Bialkowski and W. A. Guillory J. Chem. Phys. 1978,68,3339. Infrared Laser Photochemistry In the low pressure regime the collisionless dissociation of MeOH between 1000 and 10 mTorr was examined using similar laser energy fl~ences.~~ The laser-induced fluorescence technique was used to monitor the concentrations of product species.The appearances of OH 50* 20 ns after the laser pulse independent of initial MeOH pressure (50-200 mTorr) suggested that the primary dissociative channel was reaction (3l) CH3OH -+CH3*+OH (31) although CH3*could not be detected. The appearance of CH 70k20 ns after the appearance of OH independent of initial MeOH pressure (70400mTorr) suggested secondary collisionless dissociation. The observation of the relaxation of OH over a lops interval after radical appearance allowed the separate charac- terization of collisional rotational relaxation and translational diffusion.Ethyl vinyl ether has been shown to undergo two thermal reactions a retroene reaction [reaction (32)] and homolytic cleavage of the C-0 bond [reaction (33)]. H Recently Rosenfeld et af.loo proposed that the radical disproportionation k4 in reaction (33)alone competes with reaction (32) when ethyl vinyl ether is subjected to an intense i.r. radiation field at 10.6 pm (0.3 J per pulse focused). By measuring the ratio acetaldehyde :ketone as a function of pressure they found that the reaction was independent of pressure and consistent with a thermal distribution of energy. In subsequent work Brenner"' evaluated the effect of energy fluence and pulse duration on the branching ratio and yields of reactions (32)and (33). The conditions used for the photolysis were different from those used previously and different results were obtained.At a pressure of 0.4Torr using the 1041 cm-' laser line at a fluence of 0.91 J cm-2 acetaldehyde butyraldehyde and ethylene were the major products. At a high energy fluence the butyraldehyde decomposed and ketene trapped as ethyl acetate was observed. Evidence that the butyraldehyde was derived from reaction (33) was provided by the observation that the addition of NO quenched its formation. Under these conditions the yield of acetaldehyde decreased by only a minor extent consistent with the concerted nature of (32). By changing the laser pulse duration it was found that the yields and the branching ratio were varied and also that both reaction channels were observed at threshold when the pulse duration lo" R.N. Rosenfeld J. I. Braurnan J. R. Barker and D. M. Golden J. Amer. Chem. Soc. 1977,99,8063. lo' D.M.Brenner Chem. Phys. Lerters 1978.57 357. R. T.Bailey and F. R. Cruickshank was 0.2 ps. There was also evidence to suggest that a statistical energy distribution was not achieved during laser pumping when T,,= 0.2 ps. When ethylacetate is irradiated with a pulsed CO laser ethylene and acetic acid are the sole products under both partially focused and non-focused conditions. lo2 Under non-focused conditions Daren et a1.'02 found that at a fluence of 0.7-0.8 J cm-* ethylene was produced at 12-16% conversion per pulse nearly all of which was due to thermal processes. The elimination of HBr from isopropyl bromide was used as an internal standard to monitor thermal effects.Under partially focused conditions (fluence of 0.8-8 J cmP2) laser induced non-equilibrium chemistry was demonstrated by showing that ethylene was produced in an amount that was greater than expected via the thermal route. The results indicated that both low pressure and high energy fluences were necessary for the non-equilibrium pathway. The comparative single pulse shock tube-laser induced reactions of ethyl acetate were examined by Gutman et al.'03 Laser energy fluences of 4.5J cm-2 at 9.28 pm were used for the laser induced studies. In both cases ethylacetate was mixed with a second compound either allylmethylether or isopropyl bromide so that relative unimolecular rate constants could be determined.The decomposition of these compounds yields stable products reactions (34)-(36) under these conditions iso-C3H7Br -* C3H6+ HBr (34) C3H50CH3 -+ C3H6 + HCHO (35) CH3COOC2H.j + C2H4 +CH3COOH (36) avoiding the complicating effect of secondary reactions. It was found that although the products of the thermal and laser induced reactions were identical the relative rate constants showed variations. In the ethylacetate-isopropyl bromide system only the former compound absorbed laser radiation and the ratio of unimolecular rate constants almost reached the thermal value at the highest pressures studied (50 Torr). At lower pressures however (2 Torr) the decomposition of the pumped reactant became dominant in the laser experiments. A closer adherence to the thermal rate constant was observed where both molecules absorbed the laser radiation (allylmethylether with ethylacetate).The results suggest rapid intramolec- ular energy equilibrium prior to the onset of decomposition with identical mechanisms for both the thermal and laser induced reactions. Tetramethyl-1,2-dioxetane.-The competition between collisional energy transfer processes and chemical reaction plays a crucial role in determining the mechanism of a laser initiated chemical reaction. Tetramethyl-l,2-dioxetane(TMD) is an interes- ting molecule for studying such processes since it possesses some unique features. These include the quantitative i.r. photochemical decomposition to produce acetone reaction (37) accompanied by the emission of blue light at about 410nm and the 0-0 0 II I1 Me-C-C-Me + ,C + hv (37) I1 Me Me Me Me "'* W.C. Daren W. D. Munslow. and D. W. Setser J. Amer. Chem. SOC.,1977,99,6961. ln3D. Gutman W. Braun and W. Tsang J. Chern. Phys. 1977,67,4291. Infrared Laser Photochemistry 81 well established thermochemistry of the decomposition. The reaction dynamics can be conveniently probed after excitation by monitoring the time resolved visible emission from the acetone. Two groups of workers'04*'05 have looked at different aspects of the laser induced decomposition. Haas and Yahavlo4 used focused 0.5 J 80 ns 10.2 pm pulses from a TEA C02 laser to decompose TMD at a pressure of 0.35 Torr. Visible chemi- luminescence was observed with a monochromator-photomultiplier system with a time constant of 5 ns.The results indicated that TMD dissociated to yield an electronically excited product which decayed by at least two mechanisms a truly unimolecular one and another involving collisions. In subsequent work,'06 these authors used -3 ns pulses from a mode locked CO laser and replaced the sample in the cell after each shot. The gas pressure was reduced to 0.1 Torr with no buffer gas present. Two distinct emissions were observed in this case at 420nm attaining maximum intensity simultaneously with the laser pulse and at 460 nm appearing 140 ps after the laser pulse. The latter emission was assigned to triplet acetone and the former to singlet acetone fluorescence mixed with some triplet emission. In a related experiment Farneth eta1.'05 used 9.6 pm radiation from a TEA CO laser to initiate TMD decomposition by energy transfer from MeF.In this case unfocused radiation (300 mJ in 1ps pulses) was used so the power density was far below that used by Haas and Yahav.'06 The reaction was followed by monitoring the time resolved visible emission from acetone the time resolved spontaneous i.r. emission from MeF and the time resolved translational temperature changes (using the thermal lens technique). Attempts were made to generate luminescence using a variety of other sensitizers but only SF6 was effective. Typically pressures of -1 Torr TMD and 2-30 Torr MeF were employed so that collision induced processes probably dominated the mechanism. This was supported by the results which indicated that the reaction was initiated by i.r.absorption into MeF and that the visible light generated by the decomposition of TMD was produced on an energy transfer timescale. 9 Miscellaneous Compounds Hydrazoic Acid (HN3 and DN3).-The i.r. multiple photon dissociation of HN3 and DN has been studied at low pressures using the focused output of a TEA CO laser.lo7 The D-N bending fundamental in DN is centred at 954 cm-' while the v2-Y4 hot band in HN3 is found at 950 cm-'. Excitation within the P-branches of this band is possible with many P-branch transitions of the 10.6 pm CO laser. Hart- fordlo7 used the P(18) line for excitation using 100mTorr of DN3 and 150 mTorr of HN3. In the case of DN relatively strong emission was observed in the visible region at wavelengths greater than 560 nm.This was attributed to emission from ND,(*A,) produced by reactive collisions of ND(a 'A) produced in the primary photolysis step (38). 104 Y. Haas and G. Yahav Chem. Phys. Letters 1977,48 63. ")' W. E. Farneth G. Flynn R. Slater and N. J. Turro J. Amer. Chem. SOC.,1976,98,7877. '06 Y. Haas and G. Yahav J. Amer. Chem. SOC.,1978,100,4885. In' A. Hartford Chem. Phys. Letters 1978 57 352. R. T. Bailey and F. R. Cruickshank The multiple photon dissociation of HN by irradiation of the vZ-V~ hot band also produced emission attributable to However owing to the small cross section the hot band emission could only be observed at pressures greater than 150 mTorr of HN,. The smaller dissociation of HN3 compared with DN3 under identical conditions was suggested as a possible basis for a deuterium isotope enrichment process.Ammonia.-The dissociation of ammonia into ground electronic state NH2 radicals using high intensity CO laser radiation has been observed by Campbell et a1.1°8 This was accomplished using the P(32) CO laser line which is close to resonance with a line in the v2 band of NH,. In subsequent work this group reported results on the dissociation of NH3 into ground state NH using different CC2 laser lines.lo9 Many of the 60 laser lines used were out of resonance with absorption features in the NH3 spectrum. Letokhov et al. previously studied the multiple photon dissociation of NH under relatively high pressures (90Torr) using a series of CO laser lines and found emission from the electronically excited NH fragment only when the laser frequency was in coincidence with an absorption line of NH,.In the experiments of Campbell et a1.l" ammonia at pressures ranging from 0.05 to 7 Torr was dissociated with the focused radiation of the multimode TEA CO laser line tunable over the range 9.2-10.9 pm. The laser output ranged from 1.3 to 4.5 J per pulse about 100ns wide. Ground state NH radicals were detected by laser fluorescence excitation using a tunable dye laser at 597.7 nm to excite the radicals and a photomultiplier to detect the fluorescence. The maximum fluorescence intensity was found to occur for CO lines within the (00'1-10'0) band close to the densely packed v2( +) and v2( -) band heads but not in direct resonance the laser excited fluores- cence maxima being apparently shifted to lower wavenumbers.Signals observed from CO lines in the (00'1-02'0) band were of lower intensities. The fluorescence intensities for the sequence of lines R(6) to R(16) of the (00'1-10'0) CO laser band are shown in Table 3 together with the observations of Letokhov."' The laser energy kept constant at 3 J per pulse was focused into NH at a total pressure of 6 Torr. Letokhovll' only observed strong fluorescence with the laser lines which were close to NH3 absorption features whereas Campbell et al.lo9 observed comparable intensities for all the lines. These differences are probably due to the higher power employed in the latter work with a consequent increase in power broadening effects.The laser powers used (-10 GW cm-*) would give rise to a power broadening of about 6 cm-' for the strongest NH lines. This is sufficient to account for substantial pumping of the v2 vibrational level by virtually all CO laser lines at these intensities. Furthermore the observation of collisionless dissociation implies that the CO laser frequency and the broadened vibration-rotation NH lines must overlap significantly to enable the molecule to climb the vibrational ladder. 1')8 J. D. Campbell G. Hancock J. B. Halpern and K. H. Welge Optics Comm. 1976 17 38. '09 J. D. Campbell G. Hancock J. B. Halpern and K. H. Welge. Chem. Phys. Letters 1976 44 404. V. S. Letokhov E. A. Ryabov and 0.A. Tumanov. Soviet Phys.J.E.T.P. 1973.36 1069. Infrared Laser Photochemistry 83 Table 3 Fluorescence intensities of some lines of the 00'1-10'0 CO laser band Fluorescence intensities Campbell et al.'09 Letokhov"o 2.3 strong 1.3 weak 1.1 weak 1 .o none 1.2 strong 1.3 none The pumping and relaxational processes in the lower levels of NH have been studied by Tablas et al.' " Various levels of the v2mode were pumped by a CO laser and the level populations monitored by U.V. absorption spectroscopy. The ground state depletion was found to correspond well to the excited state populations but only a fraction of the absorbed photons were present in excited molecules indicating very fast V-T relaxation. Direct optical pumping appears to pump primarily the 1(+) level but strong collisional coupling with the 1(-) and 2( -) levels populates these also.Boron Trich1oride.-In addition to the considerable amount of C.W. laser work on boron compounds some high pressure pulsed laser studies have been carried out with particular reference to isotope separation. As expected from the C.W. laser results 'OB is enriched. The reactions have the additional merit that only the enriched 'OB species remain in the gas phase. Two separate studie~"~*~'~ give similar 10 B isotopic enrichment factors of 1.5-1.7. In one case'' BC13 (10 Torr) hydrogen (20 Torr) and Ti metal (-1 g used as a product scavenger) were irradiated (1.5 h) with the 947.7 em-' CO TEA laser line with -7 MW pulses at a 5 Hz repetition rate.This was absorbed by "BC13 only. In the other case,'13 BC13 (2 Torr) and H,S and D,S (2-40 Torr) were similarly irradiated except that the laser was focused by a 25 ern focal length lens at the centre of a near spherical 500 cm3 cell. The 5-10 h reaction times were typical. It was also found that if the 982 cm-' laser line was used being selectively absorbed by "BCl, a "B isotopic enrichment factor of 1.4 could be produced. The product of this latter study was HSBCI,. The chemistry is however not apparently simple. When D,S was used instead of H2Sin the 947.7 cm-' irradiation experiment the 'OB 'enrichment' factor became 0.8 i.e.the opposite effect to that of the H2S experiment was observed. The reversal did not occur in the 982cm-' irradiation experiments.It is not clear how these results correlate with those of Arnbart~umian''~et al. who report two excitation processes in TEA CO laser irradiation of BC13 and Houston et al.'" who report efficient energy transfer between "BC13 and "BC13 in less than 0.5 ps. ''I F. G. M. Tablas W. E. Schmid and K. L. Kompa Optics Comm. 1976 16 136. T. Lin T. D. Z. Adams and F. B. T. Pessine J. Appl. Phys. 1977,48 1720. S. M. Freund and J. J. Ritter Chem. Phys. Letters 1975 32 255. R. V. Ambartzumian N. V. Chekalin V. S. Doljikov V. S. Letokhov and E. A. Ryabov. Chem. Phys. Letters 1974 25 515. 'Is P. L. Houston A. V. Nowak and J. I. Steinfield J. Chem. Phys. 1973 58 3373. R. T. Bailey and F. R. Cruickshank OsmiumTetroxide.-A C02 TEA laser has been used to excite Os04at several laser frequencies and fluence levels from -0.02 to 1J cm-2.The oso4so excited reacts to give luminescence products and it has been shown'I6 that this luminescence intensity is proportional to the dissociation rate. This unusual but important result makes Os04a very convenient compound to study. The experimental conditions optically as well as chemically have been very carefully m~nitored.~~ The laser beam is collimated not focused so that spatial effects are eliminated. Only about 10% of the laser energy is absorbed in the 100cm long cell so that (n)can be calculated with good precision. The dissociation rate as a function of laser frequency is shown in Figure 8. Clearly no reaction occurs if R-branch absorption lines are used where the I I 1 1 I 0.2Torr Os04 I3 65 MW cm-2 Q / I\ I /I II \ \ \ \ \ 1.0 950 970 v/cm" Figure 8 The Os04 luminescence intensity (IF)dependence on laser frequency at constant laser power flux. The dotted line is the low i.r. intensity absorption spectrum (After Chem. Phys. Letters 1977 45 231 with permission) small signal absorption coefficient of these frequencies is quite as large as for the P-branch lines. As discussed in Section 3 a rotational compensation for anharmonicity is pro- posed for the multiple photon absorption. A P-branch line (u = 0 + 1)may match a P- or Q-branch line (u = 1+ 2) and a Q-or R-branch line (u = 2 +3)thus allowing pumping of the discrete levels. For the proposed mechanism P+Q + R tran-sitions or Q -P R transitions it is necessary that 12 BJ-Aval<AvR.For P+ R transitions 14BJ-AvaI < AuR is required. Ava is the frequency shift of the u = 2 level due to anharmonicity and AvR is the power broadened width of the transition. Above u = 3 the transition width is of the order of the interstate spacing so that a quasicontinuum can be assumed to exist above this level. A two pulse laser technique has been used to probe the details of the proposed mechanism. The lower power (-0.2 J cm-2) TEA laser (El)was tuned to a P-branch transition frequency and was expected to 'pump' the Os04 to u = 3. The higher power TEA laser (E,)(-0.7 J cm-2) was expected to 'pump' the oso4from u = 3 to the dissociation limit i.e. it was to 'pump' in the quasicontinuum region.Figures 9 'I6 R. V. Ambartzumian V. S. Letokhov. G. N. Makarov and A. A. Puretzkii Optics Comm. 1978,25,69. Infrared Laser Photochemistry lo4 n u) Y .-5 lo3 5 2 .w .-2 .2 Id 1. c .-ul al c .-9 10 al u) C .-5 JI 910 920 930 940 950 960 970 980 Exciting frequency 9, /cM' Figure 9 The Os04 luminescence intensity dependence on El laser frequency (vl) with El and E2fluencesfixed at the values shown and v2= 932.96 cm-' as indicated by the arrow. The Os04pressure is 0.18 Torr. The upper and lower curves are on log and linear scales respectively and the low i.r. intensity absorption spectrum is shown for comparison (Reproduced by permission from Optics Comm.1978 25,69) al 10 al u) .-C s dl 910 920 930 940 950 960 970 980 Dissociating frequency Se/CM-' Figure 10 The Os04 luminescence intensity dependence on E2laser frequency (v2)with El and E2fluencesfixed at the values shown and v1 = 954.5 cm-' as indicated by the arrow. The Os04pressure is 0.18 Torr. The laser pulse shapes were identical to those used for Figure 10 (Reproduced by permission from Optics Comm. 1978 25 69) R. T. Bailey and F. R. Cruickshank and 10show clearly that the rate of reaction is strongly dependent on the frequency of El, and this yield spectrum is highly structured. The yield spectrum of E2is much less structured and is strongly shifted to the low frequency end of the small signal spectrum.This is as expected for both Eland Ezfrom the proposed mechanism. The low frequency shift of the EZspectrum is characteristic of a high anharmonic level being ‘pumped’ to some upper state. The structureless appearance is characteristic of a wide choice of upper states with small energy gaps being favoured over larger but with otherwise little difference in transition probabilities i.e. a quasicontinuum does indeed exist here. When the delay between the two laser pulses El and E2was lengthened from the 0.5 ps used in the above experiments it was possible to probe the decay kinetics of the upper states in the discrete level region; Figure 11shows the result. The El laser Delay time /ps Figure 11 The oso4 luminescence intensify dependence on the delay between El and E2 laser pulses.Laser El was set at 954.5 cm-’ E2 at 924.97 cm-’. The oso4 pressure was 0.2 Torr (Reproduced by permission from Optics Cornrn. 1978 25,69) pulse was shortened to 15f5 ns FWHM and El = 0.03 J cm-’ with E2=0.3 J cm-* (90ns FWHM 1ps tail). The upper discrete level population obviously decays on a ps timescale. It was calculated that only 2/7 of the initial molecules were excited by the El laser pulse at the fluence used and that an energy level equivalent to 7 CO laser quanta was achieved in these molecules. This work is in its early stages yet but the technique promises to reveal a great deal about the details of the mechanism. It is unfortunate that the chemistry of OsO is not better characterized. Fe(C0)4 (isolated in low temperature matrices).-In contrast with room tempera- ture i.r.photochemistry low temperature (-20 K) inert gas matrix isolated compounds can undergo reactions of very low activation energy to products which will be stable. Reaction rates will be very slow for activation energies considerably lower than one i.r. quantum of CO laser radiation. Infrared Laser Photochemistry The most systematically studied reaction series of this type is that involving Fe(CO),. 117-'19 A C.W.CO laser was used (1.5-3 W bandwidth<0.001 cm-' 1919-1880 cm-'). The Fe(CO) was produced by prolonged U.V. photolysis of Fe(CO)Sin an Ar matrix. This reaction is only partially reversible if the matrix is irradiated with broadband i.r. from a Nernst glower and this is interpreted as a demonstration that most of the CO ejected from the Fe(CO) has diffused away from the Fe(CO) product.These 'isolated' Fe(CO) molecules enriched in 13C'80could be induced to isomerize as follows as shown in reaction (39) where X is 13C'80. These species are distinguishable being of C2"symmetry with bond angles of -145" and 120". It was shown that dimer participation or reaction with the CO ejected from the Fe(CO) was extremely unlikely. Only one CO laser photon at 1880 cm-' (-22 kJ mol-') seems to be necessary to isomerize the mole-cule with no phonon-vibration energy transfer occurring so that multiple photon transitions are not involved. The possibility that the absorbed energy resulted in a - local 'melting' of the matrix reaction (40) was considered for 3ci8o).When the laser beam was plane polarized substantial dichroism developed during the photolysis i.e. the Fe(C0)4 molecules did not rotate during the reaction timescale (hours). Thus orientation-specific laser photolysis occurred ruling out localized melting which would allow free rotation. With a tunable spin flip Raman laser it was possible to irradiate Fe(C0)4 in specific matrix sites since the site differences were reflected in a fine splitting of the i.r. absorption band. The reaction induced was between Fe(CO) and the CO ejected from the parent Fe(CO)5after only 15 s of U.V. photolysis i.e. the reverse of the preparative reaction. The tunable laser enabled the absorption linewidth of the matrix isolated Fe(C0)4to be measured (0.2 cm-') and these lines were found to be so narrow that they are less than the slit-widths of most spectrometers.Thus the degree of overlap between an i.r. absorption line (measured on a highest quality spectrometer) and for example a CO laser line is extremely difficult to estimate. Clearly the continuously tunable spin-flip Raman laser will greatly facilitate i.r. photochemistry of matrix isolated species. B. Davies A. McNeish M. Poliakoff and J. J. Turner J. Amer. Chem. SOC., 1977,99,7573. 'I8 B. Davies A. McNeish M. Poliakoff,M. Tranquille,and J. J. Turner Chem.Phys. Letters 1977,52 477. 'I9 (a)M. Poliakoff N. Breedon B. Davies A. McNeish and J. J. Turner Chem. Phys. Letters 1978 56 474; (b)M. Poliakoff Chem. SOC.Revs.1978,7 527. R. T.Bailey and F. R. Cruickshank Sulphur Pentafluorochloride (SF,Cl).-Although there are many similarities between SF,Cl and the more extensively studied SF6 there are several differences which serve to illuminate the mechanism of absorption and reaction. First the S-Cl bond in SF,Cl is weaker than the S-F bond in SF6 (255 versus 385 kJ mol-') and secondly if the decomposition of SFsCl involves the breaking of a S-Cl bond intramolecular vibrational energy transfer from the excited S-F to the S-Cl bond must occur. The lower symmetry of SFsCl also results in a different vibrational spectrum. Isotopically selective reactions of SF,Cl induced by the focused radiation from a TEA COz laser at several frequencies near the S-F stretching frequency at 909 cm-' were observed by Leary et al.'" The experiments were performed at low pressures (0.25-4.0 Torr) with pure SF,Cl and with several SF5Cl-diluent mixtures using pulse energies of the order of 1J for all the lines.The reaction yield product distribution and isotopic selectivity were determined for various reaction conditions using a quadrupole mass spectrometer. Isotopic selectivity was found to be poorer than for SF6 due in part to intermolecular V-V energy transfer and other scrambling processes. The primary photolysis reaction involved the breaking of the weakest bond reaction (41) a reaction which is isotopically selective. Thus the V-V transfer processes are fast enough even at the lowest pressures to transfer excitation from the S-F to the S-C1 modes before S-F bond cleavage can occur.Mode selective chemistry does not therefore appear to be possible with this molecule at least under the conditions used and the presence of 'collisionless' V-V energy transfer rate processes is in agreement with SF6results. Methyl Cyanide (MeCN).-The MeCN molecule is interesting in that it exhibits two different absorption bands (Me rock and C-C stretch) within the tuning range of the CO laser. The effect of mode selective pumping on the decomposition can thus be studied. The i.r. laser induced photolysis of MeCN has been studied by Lesiecki and Guillory.l2l The MeCN at pressures of 0.5-5.0 Torr was pumped with the focused output of a TEA C02 laser using the P(20) line of the 00'1-02'0 transition to pump u7 (Me rock) of MeCN or P(32) of the 00'1-10"0 transition to pump u4 (C-C stretch).The powers at the focus were estimated to be about 1GW cm-'. Both electronically excited and ground state CN are produced by the dissociation of MeCN. The former was detected by luminescence and the latter by its dye laser excited fluorescence.'22 A pulsed N2-laser pumped dye laser was used to excite the fluorescence. It was found that excitation of the Me vibrational mode was at least three-orders of magnitude more effective in producing fragmentation of MeCN than was the excitation of the C-C stretch. This could not be accounted for by the simple effects of differences in absorption coefficient and power differences. However the products and relative product ratios were the same irrespective of the mode pumped; that is there was no detectable mode specificity.I2O K. M. Leary J. L. Lyman L. B. Asprey and S. M. Freund J. Chem. Phys. 1978,68 1671. 12' M. L. Lesiecki and W. A. Guillory Chem. Phys. Letters 1977,49,92. '22 M. L. Lesiecki and W. A. Guillory J. Chem. Phys. 1977,66,4239. Infrared Laser Photochemistry Methyl Isocyanide (MeNC).-The thermal isomerization of MeNC to MeCN known to be exothermic to about 14.7 kcal mol-' has been suggested as an ideal reaction for testing thermal explosion the0~ies.l~~ It has been the subject of extensive kinetic studies as a result of which it has emerged as a model unimolecular reactant. Recently the states of MeNC having sufficient energy to effect iso- merization were pumped directly by a C.W.dye 1a~er.l~~ This experiment led to an unambiguous determination of the unimolecular rate constant k for the states selected. In a subsequent MeNC vapour was irradiated with a TEA CO laser tuned to coincide with the v4 fundamental of this molecule. At pressures between 10and 100Torr at room temperature more than 50% conversion of the gas to its isomer MeCN was observed. For a given pressure a sharp threshold in CO laser power was found to exist above which isomerization occurs. The threshold for thermal isomerization was also shown to be pressure dependent (Figure 12). These 1 I T1 I 50 40 \ -3 E 30 -\ -5 W 20 10 0 1 I I I I 0 20 40 60 80 I00 PRESSURE (Torr) Figure 12 Variation of threshold energy with pressure; 0 threshold energies at the entrance window of the cell A threshold energies at the beam focus in the centre of the cell (Reproduced by permission from Chem.Phys. Letters 1978 57 479) results were interpreted in terms of a thermal explosion triggered by laser induced heating (cf. EtC1 EtI). 10 Continuous (Low Power) I.R. Laser Photochemistry As in the case of high power pulsed i.r. laser photochemistry few systematic studies have as yet been carried out with C.W. i.r. lasers which by their nature must have lower powers typically in the 50-400 W range. One well-studied system is the reaction between ozone and nitric oxide. The H. 0.Pritchard and B. J. Tyler Canad. J. Chem. 1973 51 4001. 124 R. V. Reddy and M.J. Berry Chem. Phys. Letters 1977,52 11 1. 12' D. S. Bethune J. R. Lankard M. M. T.Loy J. On and P. P. Sorokin Chem. Phys. Letters 1978,57,479. R. T.Bailey and F. R. Cruickshank effects of vibrationally (CO laser) excited'26 03(001)and vibrationally (CO laser) excitedlz7NO on the rate of reaction have been examined separately. The reaction is highly exothermic and electronically excited N02('B1)is produced as well as ground state N02(2A1).For reactions (42)-(45) O,+NO A N02(2Bl)+02 (42) B' +NO -03* N02(2B1)+ 0 (43) A O,+NO -N02(2Al)+02 (44) A' 03* + NO -+ NO,(~AJ + 0 (45) the room temperature rate constant enhancement factors where 03*is very probably O,(OOl) are kBl/ kg = 4.1f2.0 and kA'/ kA = 17.1f4.3. A flow system was used to study these reactions with -2.4 Torr O, 0.7 Torr NO and 0.1 Torr 0 excitation by a CO laser (2 W P(30) 9.6 pM,modulated at 240 Hz).It is calculated from the observed enhancements that 50% of the vibrational energy in the O3is used in B' and 85%of the available energy is used in A. In a similar flow system at similar pressures except that the NO pressure was typically an order of magnitude smaller a CO laser was used [up to -0.4 W cm-2 1884 cm-' P(13) modulation 100 Hz] to excite the NO to o = 1. For reactions (46)-(49) NO+O -!+ N02(2Bl)+02 (46) NO* + o3-L NO,(~B,)+ 0 (47) NO+03 2N02('A1)+02 (48) NO* + o3-2 NO,(,A 1) + 0 (49) the enhancement factors are kl,/kl= 4.7* :: and k2,/k2s 18. While the NO fluorescent state may not be created by thermal heating the arguments used in this paperlZ7 to show that thermal effects are absent are not too convincing.For example it is stated that the maximum temperature rise that could occur is given by (energy absorbed)/C of the irradiated volume and that addition of up to 4 Torr N2 has little effect on the fluorescence of the NO although this increased the heat capacity of the system by a factor of 3.3. As we have pointed out already in connection with the high power laser photochemistry the highest temperature will occur on the laser beam axis and will be much larger than that calculated by Stephenson et al. Additionally the effect of added heat capacity is to slow the thermal diffusivity which tends to preserve the high thermal gradient to which reaction exothermicity also contributes.We feel that a thermal contribution cannot be excluded. From these results it is deduced that in the transition state the NO vibration must transfer energy to the 02-0reaction co-ordinate. The fact that the NO vibration contains nearly twice the quantum energy of the 03(001),but gives similar rate 126 R. J. Gordon and M. C. Lin Chem. Phys. Letters 1973 22 262. 12' J. C. Stephenson and S. M. Freund J.Chem. Phys. 1976,65,1893. Infrared Laser Photochemistry enhancements is ascribed to several factors e.g. the probability of energy flow into the reaction co-ordinate could be low or the NO quantum energy which is rather less than twice the 03(001)quantum energy may not be sufficient to excite the mode of the complex into u = 2 (This is almost certainly true since the laser energy is too low to allow such a multiphoton transition any large probability.) Similar experiments on reaction (50) o3+ SO -+ SO,('B~) + o,(~c,) (50) using vibrationally excited O3Iz8yield an enhancement factor for the resultant SO2 fluorescence of 2.5 stO.6 at 300 K.The 630*200 cm-' blue shift of fluorescence was interpreted in terms of vibrational energy transfer from the 0,to the SO2('Bl) vibrational manifold. C.W.i.r. laser radiation is known to cause explosions in for example the irradiation of C2HsC1'29 and C~HSI.'~~ These are purely thermal processes and almost certainly rely on no phenomena unique to laser deposition of energy. The escalating reaction rate following an induction period and leading to the explosion results in both cases from the increased i.r.absorption coefficient of the product ethylene. Thus the reactant temperature increases with extent of reaction and all the observed phenomena can be explained. There remains only a difficulty in measuring the temperature in such systems e.g. -1100 K from i.r. emission band contours and -900 K from reaction rate in the case of ethyl chloride. Product distributions differ from the isothermal distributions but this is not surprising given the large thermal gradients in the system. Similarly in the reactions between atomic oxygen and C.W. CO laser vibrationally excited C2H4 and OCS no rate enhancement was observed which could not be explained on the basis of heating effect^.'^' In a study of the C.W.C02 laser [400W P(20) 10.6 pm] promoted decomposition of CF2C12,132 calculated temperatures Teqof -1300 K are attained. The 300 K CF2C12 laminar flow system operates at just over one atmosphere pressure with CFzClZ-50% and the shield gas Ar -50% of this pressure. Conditions were so arranged that the energy input to the CF2Cl was kept constant at 100k 0.2 kJ mol-' ( Teq-1300 K). When 31'/0 SF6 was added to the system the CF2C12 decomposition was totally inhibited. It was concluded that this was evidence of a laser specific effect. However at these high pressures of Ar and CF2C12 and at > 1Torr SF6 the penetration of the COJaser beam into the CF,C12 stream would be confined to a very thin boundary layer adjacent to the laser input port.Even if all the CF2C12 in this layer decom- posed it would represent a trivial overall yield. This totally thermal explanation seems to us the most probable reason for the observed phenomena. Work with such an optically thick sample always makes reliable interpretation difficult. In more A. Kaldor W. Braun and M. J. Kurylo J. Chem. Phys. 1974,61 2496. ''' R. T. Bailey F. R. Cruickshank J. Farrell D. S. Horne A. M. North P. B. Wilmot and Tin Win. J. Chem. Phys. 1974 60 1699. 130 J. C. Bellows and F. K. Fong J. Chem. Phys. 1975 63 3035. ''I R. G. Manning W. Braun and M. J. Kurylo. J. Chem. Phys. 1976 65 2609. '32 M. P. Freeman D. N. Travis and M. F. Goodman J. Chem. Phys. 1974,60 231. 92 R. T. Bailey and F. R.Cruickshank recent on CH2Cl2 and CC1F3 all the laser intensity was absorbed within the sample cell. A higher conversion rate resulted from C.W. irradiation of CF2C12 with the R(40) (9.4pm) laser line than when the P(36) (10.6 pm) line was used. This observation has been as evidence of the non-thermal nature of the reaction since the extinction coefficient of the R(40)line is lower than that of the P(36) line. However the lower extinction coefficient will allow greater penetration of the optically very thick sample and since the CF2C12 can only be dissociated this could explain the increased conversion. A measure of the relative irradiated volumes of sample above the reaction laser flux threshold would have been very helpful in this work. As the CF2C12 is converted into CF3Cl light is emitted.It was found that the intensity of the emission correlated very poorly with reaction rate. This is almost certainly true of similar work both in short pulse M.W. laser studies and in C.W. studies. In the SF sensitized decomposition of C2H6134 (static system) promoted by CO laser radiation [300-600 W ern-, P(20) 944.2 cm-' c.w.] a pressure was observed at which the reaction rate maximized; at 263 Torr C2H6,3.3 Torr SF, and 51.7 Torr A this occurs at 600 W cm-2 laser power; the thermalization factor x,is found to minimize at this pressure. x is given by r/h where T =E/Eo (E =thermocouple reading just outside the irradiated zone with Ar SF6 and C2H6 present Eo= the same measurement with only Ar and SF in the cell).A = (Po-P)/Po (P=transmitted laser power for the Ar SF, C2H6 mixture Po=transmitted laser power for the Ar and SF only mixture). The coincidence of these pressures is held to be evidence of a laser specific process. However an examination of heat flow in the cell would indicate that as the pressure of C2H6 is increased the heat capacity increases and thermal diffusivity decreases. Thus over the 1 s irradiation period used T will decrease and A will rise since P drops as C2H6 pressure increases due to the relaxation of the absorbing SF6 by the C2H6 as discussed by the authors. Thus a minimum in x is to be expected and must coincide with a maximum in the cell core temperature i.e. in reaction rate. In view of these doubts we do not feel that speculation on the photochemical mechanistic details of this work is justifiable.Isotope separation in C.W. i.r. laser supported reactions has also been reported. Deactivation processes can apparently be used successfully to compete with level 'pumping' of a particular isotopic species. Early work almost certainly involved substantial thermal eff ects13' as already discussed. 136*137 Bauer et al.I3' have shown that H D and "B "B can be separated in multiple i.r. photon induced dissociation of D3BPF3using a CO laser. All precautions were taken to minimize thermal effects. Deactivation of the laser excited molecule MeBr reacting with atomic chlorine was arranged to be faster than V-V energy exchange between MeBr decreased quantum efficiency of course occurs. The P(10) and R(14)lines of the 133 V.Slezak J. Caballero A. Burgos and E. Quel Chem. Phys. Letters 1978 54 105. 134 J. T. de Maleissye F. Lempereur C. Marsal and R. I. Ben-Aim Chem. Phys. Letters 1976 42,46. S. W. Mayer M. A. Kwok R. W. F. Gross and D. J. Spencer Appl. Phys. Letters 1970,17 516. 136 C. B. Moore Accounts Chem. Res. 1973,6 323. 13' C. Willis R. A. Back R. Corkum R. D. McAlpine and F. K. McClusky Chem. Phys. Letters 1976.38 336. 13* K. R. Chien and S. H. Bauer J. Phys. Chem. 1976,80,1405. 139 T. J. Manuccia M. D. Clark and E. R. Lory J. Chem. Phys. 1978,68 2271. Infrared Laser Photochemistry 10.6pm CO laser band were used to excite (intracavity 160 Wcm-,) pre-dominantly Me8'Br and Me79Br respectively. The reaction scheme is that given in reactions (51) to (53).laser MeBr ___* MeBr* C1. +Me8'Br* + HCl+ sCH,~'B~ (preferentially) (52) sCH,~'B~ +C12 -+ CH2C18'Br+C1. (53) The 79Br/81Br enrichment factor was found to be 1.05 (183K) for the R(14) line and for the P(10) line the 81Br/79Br enrichment factor was 1.04 (183K). Typical conditions for the separation were MeBr = 0.0023 Torr Ar =0.7 Torr C1 = 0.028 Torr. The estimated cost of 6000 eV per enriched molecule was calculated by the authors for a suitable reaction. This would represent a significant advance over present methods. This work was almost certainly true i.r. photochemistry since varying the laser frequency altered the isotope enriched. Very similar experiments with the U.V. initiated bromination of Me irradiated with a C02 laser showed that the 12C or 13C molecule could preferentially react with a suitable choice of laser freq~ency.'~' Typical conditions were; Br2 0.075 Torr Ar 2.9 Torr MeF 0.01-0.15 Torr CO laser power 10-100 W.A considerable number of useful syntheses in boron chemistry have been carried out using low power C.W. C02laser radiation. There appears to be strong evidence that several of these are essentially i.r. photochemical processes with little or no thermal contribution. Thermally H2S reacts with diborane to give (HBS), a polymeric solid. However irradiated at 973.3 cm-' by a 6-7 W C.W. CO laser where B2H6 is the only absorber the reaction sequence of (54) to (57) is initiated.I4' BH3.+H2S + HSBH,+H2 (55) HHH H2S +HSBH2 + (HS)2BH+H2 (57) CO laser radiation of 1.5W 973.3 cm-' produces icosaborane &OH16 from 200 Torr B2H6 by a laser-triggered chain ~eaction.'~' The p-HSB2H5 which is otherwise difficult to synthesise was obtained in up to 30% yield in about 30 minutes.Y. N. Molin V. N. Panifilov and V. P. Strunin Chem. Phys. Letters 1978 56,557. H. R.Backmann F. Backmann K. L. Kompa H. Noth and R. Rinck Chem. Ber. 1976,109,3331. '42 H.R. Backrnann H. Noth R. Rinck and K.L. Kompa Chem. Phys. Letters 1974 29,627. 14' R. T.Bailey and F. R. Cruickshank A temperature probe was used in a trimethyl boron reaction sequence in order to estimate the thermal contribution. For the sequence (58),(59) (60) BMe3+HBr + BMezBr+CH4 (58) (970.5 cm-' laser thermal reaction temperatures 150-1 80 "C) BMe2Br+HBr + BMeBr +CH4 (59) (1039.4 cm-' laser thermal reaction temperatures >250 "C only) BMeBr2+HBr -+BBr3+ CH4 (60) (970.5 cm-' or 1039.4 cm-' laser thermal reaction temperatures >450 "C only) reaction (60)occurs only at temperatures significantly above that at which reaction (59) is significant.BMeBr (100 Torr) was irradiated (60 min 4.5 W 970.5 cm-') with HBr (200 Torr) and BMe,Br (20 Torr). Only the BMeBr absorbed the laser radiation. Although BBr3 was produced in significant yield the probe gas BMe2Br was unchanged in concentration clearly demonstrating the non-thermal nature of the process. Further evidence of the synthetic utility and i.r. photochemical nature of these C.W. C02 laser boron reactions is provided by the BCI promoted trimerization of C2C14.Only the BC1 (150-200 Torr) absorbed the -6 W 940 cm-' laser radiation; 0.1 mmol C2C14 per hour were converted c&& being recovered in 88% yield. BCl loss was 0.015 mmol h-'. This reaction of czCl4 occurs thermally between 700 and 725 "C. However when SF6 is used instead of BCl no reaction whatever is observed despite the increased absorbed energy.144 In the presence of 02,c2c14 produces phosgene at 1200 "C via the intermediate C2C12. With BCI, O, and C2C14 under laser irradiation however CO was the main product and no phosgene was found. If SF was used in these experiments instead of the BCl then the phosgene was produced. This seems to demonstrate that the BC13 promoted reaction is truly i.r.photochemical whereas the SF6promoted reaction is most probably thermal. A further example of this type of reaction is the removal of phosgene from BCl (15-100 Torr 100W C.W. C02 laser 944.2 cm-'). Only the BCl absorbed the laser radiation and in 3 4 s all the COCl (1.6% in BCI,) had reacted to CO and C1,. No appreciable loss of BCl was detected. Replacement of BCl by the similarly absorbing C2H4 produced no reaction between the C2H4 and COC1,. A reduced rate of decomposition of the COCl was observed This work does of course explain why COC1 was absent in the previous study. In a study of reaction (61) 2BC1 +BMe3 + 3MeBCl2 (61) induced under similar conditions to the above by C.W. CO laser radiation a long wavelength shift was observed in the laser frequencies giving optimum yield compared with the BCl absorption spectrum.145 Irradiation of the v3 absorption band of "BCl produced a more efficient con- version than when the "BC13 band was irradiated. These results were interpreted in 143 H. R. Backmann H. Noth R. Rinck and K. L. Kompa. Chem. Phys. Letters. 1975,33,261. 144 H.R. Backmann R. Rinck H. Noth and K. L. Kompa Chem. Phys. Letters 1977,45,169. 14' F.Backmann H. Noth R. Rinck W. Fussand K. L. Kompa Ber. Bunsen gesellschaft Phys. Chem. 1977 81.313. Infrared Laser Photochemistry terms of a rate determining absorption step by vibrationally excited BCl,. The vibrational absorption maximum of these molecules derived from the yield spec- trum indicated that the energy distribution in the excited BCl was far from Boltzmann.11 Conclusion In the pulsed TEA laser studies there has been considerable interest in whether the reacting molecules are in Boltzmann equilibrium with respect to their internal degrees of freedom particularly vibrational. Some studies seem to show that the internal distribution of energy is non-Boltzmann e.g.cyclopropane ethyl vinyl ether (in long pulse work) SF (with long laser pulses only) and methanol. Other work (e.g. ethyl vinyl ether and SF6with short laser pulses) seems to show that the reactant is in Boltzmann equilibrium internally. It could be unwise to attempt a general conclusion i.r. photochemistry still being in its infancy but it seems likely that the reactant molecule is in Boltzmann equilibrium internally.The rapid V-V energy transfer in the collision-free regime e.g. in SF, occurs by an as yet unknown mechanism. However its occurrence seems effectively to remove the possibility of mode specific chemistry in pulsed laser work (e.g. selective bond breaking by ‘pumping’ the molecule with a frequency corresponding to that ‘bond stretching’ vibration). More subtle C.W. ‘pumping’ of vibrational populations in a system where kinetic control allows establishment of a photostationary state may well succeed in promoting mode specific i.r. photochemistry. If the reactant molecule is in thermal equilibrium internally and also translation- ally with a substantial number of its nearest neighbours then its chemistry ceases to fall under the title of i.r.photochemistry and becomes thermal instead. Considerable effort has been expended in many studies to show whether or not the chemistry was thermal albeit with substantial thermal gradients in the reaction cell. That this proof is not easy is obvious from the work described above. In the case of pulsed laser work it is difficult to see how a reaction occurring before any collisions could be thermal as defined here. On the other hand it is difficult to see how any C.W. laser study at pressures up to tens of Torr and higher could be other than thermal. Nevertheless in both types of work as we have indicated phenomena occur which seem to dispute this simple picture. Clearly much more work must be done on the careful characterization of the optics of the system its thermal history and its chemical history with the timing of these events accurately correlated.The extremely low quantum yield (-6 x lo-’ for SF6)must also be critically examined. The conversion efficiency seemed to be much higher in the molecular beam experi- ment,49 but otherwise very low values obtain even given that the reaction is a multiple photon process. At these low levels careful consideration will have to be given to the number of collisions the reactant molecule is likely to undergo at ‘collisionless’ pressures given that very few energised molecules react. Wall collisions may also be important especially when high efficiency reaction cells are used where the laser beam impinges on the walls of the vessel. As outlined above simple heat capacity arguments are totally inadequate for estimating a mixture’s thermal behaviour particularly in sensitized systems with added non-absorbing gas.Bulk heating is known to occur with SF6(0.028Torr)+ R. T.Bailey and F. R. Cruickshank He(2Torr) and the C effect of added He must be tempered with consideration of kinetic control of the core temperature. Additionally the pressure dependence of the extinction coefficient is rarely mentioned. Figure 13 shows the variation of extinc- *O I 0 100 200 P/ Torr-Figure 13 Absorption of CO2 laser Q-switched pulses at -10 Hz (100 W 942.4 cm-') by -0.040 Torr SF6 as a function of added argon pressure (P) tion coefficient of SF6 with pressure of added argon.44 The SF6pressure was very low (-40 mTorr) and as yet not accurately known.However the change in absorbed energy as argon is added to the constant SF pressure is very large. Although obviously important in i.r. photochemistry such observations are never reported. Statements that absorbed power is kept constant require amplification. Unless the absorbed power is measured for each run the statement could be quite untrue. It is not enough to assume that the behaviour of such systems is totally predictable. As for SF6 any molecule operating near to saturation of its laser energy absorbing transition will have an extinction coefficient strongly dependent on pressure due to kinetic factors. The value of the fluence is frequently calculated from assumed optical charac- teristics of lenses.This is rarely adequate. Experience of simple lens systems indicates to us that the laser beam diameter and profile must be measured to ensure the validity of fluence calculations. At the long wavelengths involved in i.r. photo- chemistry diffraction effects can be very important. Thermal lens effects parti- cularly at high laser pulse repetition rates will alter the fluence as a function of reaction cell length. These effects depend on refractive index temperature coefficient heat capacity laser beam profile and diameter and thermal diff usivity. They will be expected to be very important in C.W. laser photochemistry. In many of the systems examined by Grunwald et al. the conversion per laser pulse has been shown to follow an Arrhenius type expression A exp -@a/&,) where Eab is the laser energy absorbed and A and Eaare constants.Eavalues near the threshold Infrared Laser Photochemistry energies of the thermal reaction systems were required to fit the data and this has been interpreted as indicating that the relevant temperature calculated from Eabis the Boltzmann equilibrium value not the higher value associated with a restricted number of vibrational modes less than the maximum. This would be indicative of ergodic reactant molecules. However in view of the above experimental problems it is probably better to reserve judgement on this deceptively simple argument. The applications of i.r. photochemistry are certain to spread rapidly. Even if mode selective reactions are rare molecule specific processes definitely occur for example in isotope separation.Freedom from wall effects will be attractive in many reactions. Equilibrium forcing in either direction i.e. independent of thermodynamic con- straints is therefore also possible. Far more subtle possibilities will no doubt be exploited soon. Already C02 laser pumping of levels gives sufficiently large photo- stationary excited state populations for them to lase in the far i.r. The i.r. lasers used in i.r. photochemistry can be readily scaled up to give very large beam diameters (several inches) and their efficiency is already sufficiently high to attract commercial users. We can undoubtedly look forward to some exciting developments in the i.r. photochemistry field in the near future.

ISSN:0308-6003    

DOI:10.1039/PR9787500049

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

5 Electronic Energy Transfer in Polymers By J. R. MACCALLUM Chemistry Department University of St. Andrews Fife 1 introduction There are many areas of practical application in which electronic energy transfer (EET) is significant. Transference of electronic energy from the point of photon capture to some other site within a macromolecular structure by non-radiative means is of vital importance in photostabilization of synthetic polymers.’ Non-chemical dissipation of the energy results in effective protection and conversely efficient conversion leading to chemical change gives the basis for synthesizing photodegrad- able plastics. An understanding of the mechanism of electronic energy transfer will result in better design of materials for particular environmental conditions.EET is also one of the essential steps in the process of photosynthesis where a chromophore is situated in conditions more akin to attachment to a polymer molecule than to a small molecule. Diffusional movement is restricted and neighbouring groups are determined by the chemical composition of the macromolecule. The fact that in Nature molecular aggregates are involved in conversion of radiative energy into chemical reactions has prompted some polymer chemists to investigate the feasibility of employing synthetic polymers as media for using solar energy to promote chemical reactions by a mechanism of photon harvesting.* The rather special properties of macromolecules may well endow to attached chromophores some specific advan- tages not possessed by small-molecule analogues.It is reasonable to predict that the behaviour of polymer molecules in EET processes should be somewhere between that of small molecules where the most common mechanism involves random collisions of energy-donor and -acceptor and that of crystalline solids in which exciton migration within the highly ordered structure plays a dominant role. Luminescence of crystalline organic solids has just been re~iewed.~ If the above comparison is restricted to the solid phase then the obvious question is How does electronic energy transfer from a donor to an acceptor both bound to a polymer molecule compare with the same process involving on the one hand fixed randomly distributed small molecules and on the other the same donor-acceptor pair incorporated in a crystalline lattice? Even to attempt to answer that question requires a clear understanding of the significance of donor-acceptor separation and orientation in the efficiency of transfer.’ B. Rhby and J. F. Rabek ‘Photodegradation Photo-Oxidation and Photostabilisation of Polymers’ John Wiley New York 1975. J. E. Guillet Pure Appl. Chem. 1977 19,249. J. 0.Williams Ann Reports (A),1977 51. 99 J. R. Maccallum This Report will attempt first to outline the theories advanced to account for the mechanism of EET and their relation to experimental measurements and then review the work which has been carried out involving solid polymers either as donor or as donor and acceptor. For a variety of reasons much interest in EET has been generated among biochemist^.^ Since this area of research is concerned with macromolecules in solution and therefore under conditions in which translational movement albeit highly restricted of the donor or acceptor group can occur thereby altering the separation/orientation parameters in time it will warrant reference only when proposals are made or conclusions drawn which are relevant to the theories of EET.2 Theory The process under consideration can be described as D*+A + D+A* (1) The transfer of electronic energy from the excited donor D* to the acceptor A takes place by a non-radiative mechanism. The transfer of energy is complete since no back transfer takes place and it is possible to identify on which species the energy resides.Normally D and A are different chemical types but given the correct conditions D and A may be chemically identical molecules. The mechanisms involved in process (1)require that the transition D* + D be of comparable energy to A + A* and that the initial state (D* +A) interacts in some way with the final state (D+A*). Normally the former requirement means that vibrational levels in both species contribute to the transfer of energy. The mechanisms of transfer generally described as resonance transfer processes which are considered to predominate in macromolecules are assumed to involve vibra- tionally relaxed species.’ The implication of such an assumption is simply that the rate of transfer is much slower than the rate of vibrational relaxation.The converse condition of transfer before relaxation requires very fast rates and takes place in organic crystals. This type of EET was covered in Williams’ Report last year and is satisfactorily explained by exciton the~ry.~ 3 Rate of Energy Transfer The conditions for energy transfer being much slower than relaxation are U << AE AE (2) in which U is the interaction energy between initial and final states AE the transition energy and A& the electronic bandwidths. A consequence of these requirements is that the spectral characteristics of donor and acceptor are unchanged in each others’ presence. It can be shown that the probability of transfer from the initial state to the final state depends on the interaction U between the two states and on the density of L.Stryer and R. P. Haugland Proc. Nut. Acad. Sci.,U.S.A.,1967 58 719; J. Eisinger Biochemistry 1969 8 3902. Th. Forster Discuss. Faraday SOC.,1959 27 7. Electronic Energy Transfer in Polymers states ps.6 Thus the probability of EET PDA is expressed in the form where h is Planck's constant. The density of states is measured from the spectral overlap of the donor emission and acceptor absorption both normalized to unity. Two types of interaction are significant in resonance transfer of energy; (i) dip~le-dipole,~ and (ii) exchange.6 4 Dipole-Dipole Interaction The first complete derivation of a relationship between the specific rate of EET and experimentally available parameters was made by For~ter.~ Briefly he proposed that the interaction between D and A would be totally dipolar in nature when D and A were separated by more than approximately 1nm.Thus cJc€[3$5]2 (4) in which UDand UAare the dipoles for the transitions in D and A on transfer and R is the D-A separation. The dipoles are treated as points. UDand UAare related to the radiative transitions of each species i.e. the radiative lifetime of the donor 7D the donor quantum yield QD and the molar extinction coefficient of the acceptor Forster derived the relationship where K is a constant factor to allow for the orientation of UDand UA,n is the refractive index of the medium and B = [FD(u)&A(u)/(u)4] du. A specific D-A separation Ro,is defined such that for this condition the probability of transfer is equal to the probability of decay by radiative and non-radiative processes.Under these circumstances ~ETTD= 1 and then Normally K is averaged over all possible orientations and consequently (K)~ is equal to 0.66. Equation (6)now becomes &/nm = 2.89 x lop5[-(7) Q;B]i The terms on the right-hand side of the equation can be evaluated from the spectroscopic properties of the donor and acceptor thereby allowing a prediction of Ro from such data. Obviously the overlap factor B is the key parameter in determining Ro. Equation (5)can now be rewritten as Th. Forster Ann. Physik 1948 2 55. Th. Forster 2.Naturforsch. 1949 4a 321. J. R. Maccallum It is convenient to define the efficiency of transfer QET in an analogous way to the definition of quantum efficiency thus or Equation (8)gives the specific rate of transfer for a single pair separated from each other by a distance R.Experimentally what is studied is an assembly of D-A pairs involving a distribution of R values for a given sample. Furthermore the D-A distance must remain constant during the lifetime of the donor otherwise allowance must be made for a time-dependent R resulting from spatial diffusion of the chromophores involved in EET.8 Thus the next step in the development of Forster's theory was to average over all the D-A pairs in a given sample. Indeed an averaging procedure has already been operated in allotting a value of 0.66 to (K)~,a situation which has provoked some comment and to which reference will be made later.' The equations derived by Forster in which R is replaced by the concentration of acceptor C are now given QET = Ji x exp(x)*[I -erf (XI] (12) where x = 0.54; (C/Co).Equation (11)describes the decay of donor fluorescence intensity with time and equation (12) relates the efficiency of transfer to varying acceptor concentration; Co is the 'equal probability' value corresponding to a D-A separation of R,. Experi-mentally C is obtained by fitting measured data to either equation (1l),if flash excitation is used or equation (12) if steady-state illumination is used. Ro is calculated from equation (13) which is derived by considering the volume of the sample as comprising uniform spheres each containing one acceptor species. This treatment cannot be applied for intramolecular EET.Ro= 0.7346(Co)' (13) Thus from equation (7)a spectroscopic prediction of Ro is available and can be compared with the value obtained from energy-transfer experiments through equa- tions (13) and (11)or (12). In this way Forster's theory can be tested and in the event of discrepancy between the two approaches the possibility of phenomena such as bulk diffusion and energy migration before or after transfer can be investigated. It is worth making an observation on equation (12). On solving for QET when C=Co a value of 0.72 is obtained. Now Co is the concentration of acceptor corresponding to the D-A separation Ro [equation (13)] and from equation (lo) when ~ET(TD)-' = 1 i.e. R = Ro,then QET= 0.5. The explanation for this apparent disagreement is not available.* D. L. Dexter J. Chem. Phys. 1953,21 836. R. E. Dale and J. Eisinger Proc. Nat. Acad. Sci. U.S.A.,1976 73 271. Electronic Energy Transfer in Polymers 103 Diffusion and Energy Migration.-Forster did not extend his treatment to account for the effect of bulk diffusion of D and A during the lifetime of D. When this occurs R becomes a function of time and as kETK(R)-6,the specific rate can vary considerably. Several papers have been published developing this aspect of Forster's original theory. With regard to bulk diffusion the most notable contributions are those from Yokota and Tanimoto" and more recently from Gosele et al." Since macromole- cules are generally studied in the solid phase and even in solution their diffusion is slow this effect will not be considered in this review.Energy migration on the other hand is a more likely possibility in a macromole- cular environment than in an assembly of small molecules randomly distributed in a matrix. D/A chromophores attached to the same polymer molecule which is itself suspended in an inert medium may have a very low overall concentration and therefore an apparently high R. However at the molecular level the actual separation of the chromophores is governed by their disposition on the polymer molecule and the correct R can be a fraction of the apparent R. In effect each polymer molecule contains a high local concentration of D and A. The number of such pockets is governed by the molecular weight of the polymer and the weight per cent suspended in the matrix.Somewhat more difficult interpretations of concen-tration in relation to R have to be made when the donor species is part of one polymer molecule and the acceptor is part of another. Indeed this example is one which has not been completely resolved. For the case in which the matrix is itself the donor then energy migration may well be significant with the overall efficiency following the predictions of exciton theory rather than single-step resonance-transfer theory. Energy migration is also an important step in photosynthesis. No complete theory has been evolved to relate QET with concentration of acceptor although Zwanzig has proposed an approximate relationship.'* If the mechanism of resonance transfer from one chromophore to another be that considered by Forster then it would seem that the movement of the quantum of energy would be very similar to the anisotropic diffusion of an elec- tronically excited molecule as treated by Gosele,' the anisotropy being introduced by the requirements of orientation of dipoles.Indeed Altmann Beddard and Porter have used computational techniques to analyse the diffusion equation relevant to energy transfer in chlorophyll. l3 Although the computer-simulated results are for a two-dimensional model the agreement with experimental observation is encouraging. On considering the possible mechanisms for EET between like molecules psmust be the governing factor in determining the efficiency [vide equation (3)].Since this parameter is normally very small it is probable that energy migration along polymer chains containing donor species attached to the repeat unit proceeds by the exchange mechanism rather than by the dipole-dipole. 10 M. Yokota and 0.Tanimoto J. Phys. SOC.Japan 1967,22,779. U. Gosele M. Hauser U. K. A. Klein and R. Frey Chem. Phys. Letters 1975,34,519; U. Gosele U. K. A. Klein R. Frey and M. Hauser ibid. 1976 41 139; U. Gosele ibid. 1976 43 61. 12 S. W. Haan and R. Zwanzig J. Chem. Phys. 1978,68 1879. l3 J. A. Altmann G. S. Beddard and G. Porter Chem. Phys. Letters 1978 58 54. J. R.Maccallum 5 Exchange Interaction The exchange interaction arises from the spatial overlap of the donor and acceptor wavefunctions and since normally the molecular electronic wavefunction decreases exponentially the expression for this type of interaction would be expected to include a term exponential in the D-A separation.Dexter has attempted to solve the wavefunctions for overlap and he proposed the following expression for the specific rate of EET;' (14) where Z2is dependent on R such that Z2= Yexp (-2R/L) (15) Y is a constant with the dimensions of energy and L another constant described as 'the effective average Bohr radius'. The overlap integral is normalized such that du (donor emission) = ~~(0) 1FD(u) du (acceptor absorbance) = 1. Using a similar approach as adopted for dipole-dipole interaction a D-A separa-tion Rois defined such that for this value EkET(~D)-l= 1 and consequently EkET= (71-l exp -(R/RO)I} (16) in which d = 2R0/L and Rois the so-called critical transfer distance.The specific rate of EET falls off exponentially with increasing D-A separation; however the theory has not yet reached a stage of development which allows direct estimation of Rofrom spectroscopic properties of D and A. The physical significance of L is not too clear and the application of equation (16) leaves L as an adjustable parameter adjustment being restricted within certain limits. 6 Comparisonof the Two Mechanisms The decrease of specific rate of transfer with increasing D-A separation is less severe for dipole-dipole than for exchange interaction. For this reason EET involving the former mechanism can take place over distances up to 10 nm which in molecular terms is considerable.The efficiency of transfer for the dipole-dipole mechanism is governed by the strength of the transition in the donor and acceptor chromophores particularly the donor whereas for exchange this does not apply as the overlap function is normalized. Thus considering only spin-allowed processes the following transfers can be postulated D*(singlet)+A(sing1et) -D D(sing1et)+A*(singlet) (17) D*(singlet)+A(trip1et) -D D(sing1et)+A*(triplet) (18) D*(triplet)+A(sing1et) + D(sing1et)+A*(triplet) (19) D*(triplet)+A(trip1et) -+ D(sing1et)+A*(singlet) (20) Processes (17) and (18)will be more feasible by the dipole-dipole mechanism as the transitions in both the donor and acceptor are allowed although the exchange Electronic Energy Transfer in Polymers 105 mechanism is possible if less likely.On the other hand (19)and (20)are more likely to proceed through exchange interaction as both the donor and acceptor transitions are forbidden. Since ground-state triplet species are uncommon and transfer to excited acceptors is unusual (18) and (20) are experimentally improbable though possible. However having stated the generalization it is worth noting the exception which involves oxygen a triplet species in the ground state. The lowest singlet form of oxygen has been the subject of much investigation and is responsible for some unusual chemical reactions.14 7 Singlet-Singlet EET As indicated above this type of transfer generally proceeds by the dipole-dipole mechanism.The most common donors are aromatic groups attached to the polymer main chain for example polystyrene poly(vinylnaphthalene) and poly(viny1- carbazole) with small molecule acceptors suspended in the donor itself or with the D-A pair dispersed in an inert matrix usually poly(methy1 methacrylate) for studies at room temperature. No experimental data appear to have been reported for a polymeric acceptor or for both polymeric donor and acceptor. The use of an inert matrix can lead to experimental difficulties in that the theory requires a random distribution of acceptors around the excited donor and the attainment of such a distribution may present problems since donor and acceptor can have quite different compatibilities with the matrix.In this context the actual configuration of the macromolecule within the solid solution must be of some significance. For example a chromophore buried in the centre of a spherical macromolecule will perforce be much farther from an acceptor than the same chromophore on the periphery yet both have the same probability for excitation. A situation could also arise whereby the acceptor species is preferentially distributed within the polymer coils; in effect the converse of the previous condition. Attempts have been made to allow for such possibilities in studies involving fluid but not for solid solutions in which the above theories are applicable. With a homopolymer containing pendant chromophores in close proximity to each other there arises the possibility of same-species transfer along the polymer chain giving the phenomenon of energy migration which must be considered as a possible prior occurrence to EET from polymeric donors.Diffusion of the singlet energy before transfer results in a higher efficiency of transfer and a simple test for energy migration is to compare the spectroscopic value of Ro with that determined by experiment. When the latter is much larger than the former this is considered as good evidence for energy migration. Another somewhat special property of donors attached to addition polymers is that since they are made up of multi-units comprising 1,3-disubstituted propane blocks the possibility of forming excimers is greatly enhanced. The properties of such excited states in polymer and small molecules have been reviewed by Klopf- fer.16 The formation of excimers is raised at this point to illustrate a further possible mechanism for energy migration which does not involve EET by dipole-dipole or 14 ‘Singlet Oxygen’ ed.B. Rhby and J. F. Rabek John Wiley New York 1978. Is J. DanEcEk P. HrdloviE and I. Lukac European Polymer J. 1976 12 513. W. Klopffer ‘Organic Molecular Photophysics’ ed. J. Birks John Wiley New York 1973 p. 357. J. R. Maccallum exchange interactions thus IS* I S I S I cs I Sl* I S CH2 H CH2 H / \y \A/ Excimer/CH2 H CH2\A/ \ I S IS* IS S* represents an excited singlet or triplet substituent aromatic residue. The requirement of a face-to-face conformation in forming the excimer places definite restrictions on the number of transfers possible on a macromolecule held rigidly within a glass.However relatively unrestricted main-chain rotation in solution should enhance this mechanism. Efficient migration over many substituent groups implies that the excimer is not a trap from which the electronic energy is dissipated as radiative or vibrational energy. Polystyrene.-Two excited singlet species are found in polystyrene at room temperature; monomer which has A,, emission at 280 nm with a quantum yield -0.004 and excimer with A,, emission at 330 nm and a quantum yield of approximately 0.02. The first report of EET involving polystyrene as matrix and donor was made by Basile who used tetraphenylbutadiene as acceptor." From the concentration of acceptor at which QET= 0.5 he found Roto be 2.18 nm.His data fitted the Forster equation (12) very well but R (spectroscopic) was not determined. Later Geuskens et al. repeated this work using polystyrene and a styrene-methyl methacrylate copolymer giving Ro (experimental) of 1.95 nm and 2.2 nm respectively for QET = 0.72. They concluded that both monomer and excimer were acting as donors. The agreement between the two sets of results is reasonable and the mechanism would appear to be dipole-dipole transfer. Geuskens and co-workers proposed that singlet energy migration takes place along the polymer chain from phenyl unit to phenyl unit until an excimer-forming site is reached where the migrating energy is trapped." Evidence for this happening is usually based on comparing R (spectroscopic) with R,(experimental).However another method which can be used is measurement of the polarization p of the emitted radiation using polarized exciting radiation. Energy migration provided it is spatially random results in depolarization of the emitted radiation. Two sets of measurements have been performed on solutions of copolymers of styrene and methyl methacrylate in glasses of methyltetrahydrofuran at 77 K.'9*20 The emission studied was that from monomer and it was found that the extent of depolarization increased with increasing styrene content suggesting that the L. J. Basile Trans. Faraday SOC., 1965 42 3163. '* C. David M. Piens and G. Geuskens European Polymer J. 1973,9,533.l9 C.David D. Baeyens-Volant and G. Geuskens European Polymer J. 1976,12,71. 2o R. F.Reid and I. Soutar J. Polymer Sci.,Polymer Phys. Edn. 1978,16,231. Electronic Energy Transfer in Polymers 107 captured photon could migrate from unit to unit before emission. The mechanism of EET must be either dipole-dipole or exchange as no excimer emission was observed. Both investigations also measured the ratio of the intensity of excimer emission to that from monomer in fluid solutions at room temperature and each concluded that the results obtained indicated the presence of energy migration. However the plots on which the conclusions were based differ. Geuskens found IE/IM(emission intensity of excimer to monomer) to be linear with the fraction of styrene-styrene links,lg whereas Soutar obtained a linear plot for IE/IM versus the fraction of styrene-styrene links multiplied by the mean length of styrene seg- ments2’ Somersall carried out similar measurement with styrene/acrylonitrile and his data agreed with the relationship found by Geuskens.The occurrenceof energy migration would in fact be more likely to give a linear plot in the form reported by Soutar. The matter is further complicated by decay measurements performed by Phillips using very sophisticated apparatus.22 The decay of monomer emission in fluid solution follows exponential behaviour which would not be the case when significant EET takes place from one species to another of the same chemical type. Polarization measurements for polymer solutions do not throw any light on the problem as they are not meaningful when the emitting species can move within its own lifetime.The polarization value obtained for excimer the energy trap in thin films of polystyrene was 0.77 indicating that the emitting species is also the absorbing species thus precluding energy migrati~n.’~ However similar measure- ments on the monomer emission in thin films of styrene/methyl methacrylate copolymers pointed to site transfer of the singlet It seems that the occurrence of energy migration within solid polystyrene although frequently proposed is by no means proved. Neither the exchange nor the dipole-dipole mechanism would be expected to be very temperature-dependent yet the available results suggest efficient migration at 77 K but none at room tempera- ture.Phillips’ results for dilute solutions22 give strong support to the room-tempera- ture polarization data from films.23 The matter remains to be resolved. Poly(vinylnaphthalene).-Polymers of both 1-~inylnaphthalene~’ and 2-vinyl- naphthalene26 have been studied and no notable difference in photophysical prop- erties has been recorded. The emission spectrum for poly(vinylnaphthalene) PVN comprises monomer (A,= == 340 nm) and excimer (A,, 400 nm) with no quantum yields reported for either emission. At room temperature the predominant emission is that from excimer. The general interest in the photophysical behaviour of PVN has been directed towards investigating naphthyl-naphthyl rather than naphthyl-acceptor EET.However David Demarteau and Geuskens have studied the system PVN (donor)- benzophenone (acceptor) at 77 K2’ and measured R (experimental) as 1.5 nm compared with Ro (spectroscopic) of approximately 1.4nm an agreement which 2’ A. C. Somersall J. Polymer Sci. Polymer Chem. Edn. 1977 15 2013. 22 K. P. Chiggino R. D. Wright and D. Phillips. J. Polymer Sci. Polymer Phys. Edn. 1978 16 1499. 23 J. R. MacCallum and L. Rudkin Nature 1977 266 338. 24 C. David N. Putman-De Lavareille ,and G. Geuskens European PolymerJ. 1977,13 15. 25 R. F. Reid and I. Soutar J. Polymer Sci. Polymer Letters Edn. 1977 15 153. 26 N. F. Pasch and S. E. Webber Chem. Phys. 1976,16,361. 27 C. David W. Demarteau and G. Geuskens European Polymer J. 1970,6 1397. 108 J.R. Maccallum tends to eliminate significant energy migration. However a number of studies of the homopolymer and copolymers have indicated the likelihood of singlet energy hopping along the polymer chain. Geuskens measured the depolarization ratio for a series of methyl methacrylate copolymers and homopolymer as rigid glasses and as films; the results strongly suggested energy migration.24 Soutar performed similar experiments extending the range of copolymer composition and also measuring IE/IM He for solutions as a function of the naphthyl content in the cop01ymer.~~ concluded that extensive energy migration occurred among the aromatic moieties. A very thorough investigation of the ratio of intensity of excimer emission to monomer emission over a temperature range 4.2-373K was carried out by Frank and Harrah.28 They developed a model which incorporated an element of energy transfer from unit to unit by dipole-dipole interaction accounting qualitatively for the behaviour at low temperatures.A cautionary note should be added to the effect that none of these authors considered the contribution of delayed fluorescence resulting from triplet-triplet annihilation an effect which has been clearly demon- strated (see later). Although this report is primarily devoted to solid-state EET it is appropriate to refer to the use of time-resolved measurements of emission spectra. Using short- and long-time gates following flash excitation of solutions of poly(1-vinyl-naphthalene) Phillips showed22 that the excimer dissociated to produce excited monomer thereby demonstrating that EET could proceed by the third mechanism indicated above.Extension of such measurements to polymer films should help resolve some of the problems associated with assessing the significance of energy migration. Poly(vinylcarbazole).-The fluorescence characteristics of poly(vinylcarbazole) PVK are complicated by the occurrence of two excited states of very similar energies neither of which has a ground-state analogue. Thin films of PVK show a very broad structureless emission centred around 410 nm which comprises two emissions; the low-energy part of the band has a decay time of 20 ns and is due to excimer while the high-energy portion is attributed to dimer structures with a lifetime of 11ns.There is marked overlap of the fluorescence of the two emitting states. No monomer emission has been observed for films of PVK at room ternperat~re,~~ whereas dilute suspensions of PVK in poly(methy1 methacrylate) films show only monomer.3o Since the formation of excimer is due to intramolecular association of pendant groups the macromolecules must be held in the matrix in such a way that face-to-face configurations are eliminated. The first investigation of EET with PVK as donor was reported by Klopffer who used films of the polymer containing small amounts of perylene hexachloro-p- xylene (HCX) and 2,4,7-trinitrofluorenone as acceptor^.^' Addition of small amounts of acceptor resulted in very efficient EET in fact much higher values than could be accounted for by either dipole-dipole or exchange energy transfer.Indeed the absorption spectrum of HCX has no overlap with the polymer emission and yet 28 C. W. Frank and L. A. Harrah J. Chem. Phys. 1974,61 1526. 29 W. Klopfer and D. Fischer J. Polymer Sci.,Symposium Edn. 1973,40,43. 30 A. M. North and M. F. Treadaway European Polymer J. 1973,9,609. 31 W. Klopffer J. Chem. Phys. 1969 50 2337. Electronic Energy Transfer in Polymers this substance was an efficient quencher of polymer excited states. Klopffer inter- preted his results in terms of exciton diffusion following photon absorption until the mobile exciton reached a trap in the form of an excimer-forming site or in the case of added materials a guest trap. He calculated that the exciton ranged over one thousand polymer units intramolecularly and intermolecularly during its lifetime.This proposed mechanism is similar to the type of energy migration which takes place in crystals of aromatic compounds when strong ground-state intermolecular inter- actions favour diffusion of the exciton. In these circumstances the ground-state absorption spectrum of the donor system shows clear differences from that for the same molecules in solution. However films of PVK have the same absorption spectrum as solutions of the polymer and also model compounds. It is not clear what mechanism allows the exciton to move so freely in competition with monomer deactivation. Later Powell made lifetime measurements on films of PVK doped with ~erylene.~* He proposed that as well as exciton migration into excimer dimer and acceptor traps EET took place from dimer structures to closely associated acceptor molecules (perylene) by a dipole-dipole mechanism.The efficiency of this process was almost independent of temperature in the range 13 K to room tempera- ture. North and Treadaway measured EET from the carbazole units in vinylcar- bazole/methyl acrylate copolymers to anthra~ene.~' Following Yokota and Tani- moto," they characterized the energy migration in terms of a migration coefficient analogous to a diffusion coefficient and their results are summarised in Table 1. As Table 1 Electronic energy migration coefficient for vinylcarbazolelmethyl acrylate copolymers." Singlet EET Mole fraction of vinyl- carbazole (donor) Migration coe cient AX 1o9lrnP in polymer Acceptor Matrix' s-' 1 Anthracene PMMA 7.3 1 Anthracene PS 7.8 0.84 Anthracene PMMA 6.2 0.79 Anthracene PS 5.6 0.70 Anthracene PMMA 3.9 0.70 Anthracene PS 4.2 a A.M. North and M. J. Treadaway European Polymer J. 1973 9 609; PMMA is poly(methy1 methacrylate) PS is polystyrene. previously noted an interesting feature of this work is that no significant excimer emission was observed for dilute solutions of PVK in poly(methy1 methacrylate) and polystyrene matrices. Monomer (A,, 350 nm) was the only emission detected and transfer from excimer to the acceptor was discounted. Although neither KlOpfferz9 nor Mika~a~~ observed monomer emission using films of pure PVK this could be explained by self-absorption of monomer fluorescence within the sample.A molecular energy-transfer distance r can be defined such that r = AT^) 32 G. E. Venikouas and R. C. Powell Chem. Phys. Letters 1975 34 601. 33 M. Yokoyama T. Tamamura T. Nakano and H. Mikawa I. Chem. Phys. 1976,65,272. J. R. Maccallum where A is the donor excitation energy migration coefficient and T~ is the donor fluorescence lifetime in the absence of additive. Assuming T~ to be around 11ns yields a value of 12.1 nm for r for the homopolymer. This compares reasonably well with Klopffer’s estimate of a random-walk distance of approximately 20 nm for the exciton in pure PVK films in which intermolecular hopping must extend the range compared with the essentially intramolecular conditions used by North and Treadaway.30 Poly(phenylacetylene).-Macromolecular polyenes are provoking some interest; in particular their unusual electrical properties have led to speculation about the extent of delocalization of electrons within the polymer The photophysical behaviour of one of the more stable polyenes poly(phenylacetylene) PPA and copolymers derived from this monomer has been studied in some detail since the electronic structure might facilitate exciton migration. The absorption and emission spectra of samples of PPA are broad characterless bands with low quantum yields for emission (-0.02) and radiative lifetimes around 5 ns. Using poly(methy1 methacrylate) as matrix North and co-workers measured Ro (experimental) as 4.4nm compared with R (spectroscopic) of 2.5 nm for PPA (donor) and Rhodamine B (a~ceptor).~’ The Ro values for a series of copolymers of styrene and phenylacetylene (donors) with perylene (a~ceptor)~~ are shown in Table 2 giving the Table 2 Comparison of Ro(experimentul) with R,(spectroscopic) for styrenelphenyl- acetylene copolymers.” Singlet EET Mole fraction of phenyl-acetylene (donor) 1experimen ta ,( R ) Ro(spectroscopic) in polymer Acceptor Matrix /nm /nm 0.23 Perylene PMMA 7.80 2.61 0.66 Perylene PMMA 7.52 3.28 0.76 Perylene PMMA 5.90 3.31 0.80 Pery1en e PMMA 6.10 3.29 0.23 BBOT PMMA 12.87 3.86 0.76 BBOT PMMA 8.57 3.81 0.80 BBOT PMMA 10.63 2.98 a L.Rudkin Ph.D.Thesis University of St. Andrews 1976; PMMA is poly(methy1 methacrylate) BBOT is 2,5-bis-[5’-t-butylbenzoxazolyl(2‘)]thiophen. same range of differences between experimental and spectroscopic values as obser- ved by North et al. Later measurements showed that the decay of the fluorescence was non-exponential and using time-resolved spectroscopy Guillet Hoyle and MacCallum were able to prove that the emission from a phenylacetylene/styrene copolymer was due to two emitting ~tates.~’ The value of this experimental tech- nique is illustrated by this particular example for which the demonstration of two excited singlet states was a key factor in interpreting the EET behaviour. The time-resolved spectra for a phenylacetylene/styrene copolymer are shown in Figure 1.34 H. Shirakawa T. Ito and S. Ikeda Makrornol. Chern. 1978 179 1565. ” A. M. North D. A. Ross and M. F. Treadaway European Polymer J. 1974 10,411. 36 L. Rudkin Ph.D. Thesis University of St. Andrews 1976. 37 J. E. Guillet C. E. Hoyle and J. R.MacCallum Chern. Phys. Letters 1978 54 337. Electronic Energy Transfer in Polymers 350 400 450 500 550 Wavelength / nm Figure 1 Time-resolved fluorescence spectra of SPA in polystyrene film. Exciting wavelength is 313 nm. The lower and upper limits for the time-resolved spectra are given as the interval from the lamp maximum (a) upper 0.23 ns (b)lower 1.15 ns; upper 4.37 ns (c)lower 4.6 ns; upper 7.82ns (d) lower 20.2 ns; upper 25.1 ns. Spectra adjusted to fit on same scale. (Reproduced by permission from Chem.Phys. Letters 1978 54 337) North and co-workers concluded that efficient energy migration occurs within the polyene system before EET to the acceptor.35 Comparison of Ro(experimental) and Ro(spectroscopic) in Table 2 gives strong support to this conclusion. However the demonstration that two emitting states are responsible for the polyene emission indicates an alternative explanation. Berlmann has commented on the relationship between natural lifetime and radiative lifetime when fluorescence is due to two emitting states of similar energy.38 For diphenyl-terminated polyenes Dalle and Rosenburg showed that the natural lifetime T~, == ~/20Q,where Tis the radiative lifetime and Q is the fluorescence quantum yield.39 Now in the deduction of the Forster equations the specific rate of EET is shown to be proportional to (natural lifetime)-' and the assumption is usually made that TN = T/Q resulting in the Forster relationship [equation (5)].However for polyenes a modification must be made to accommodate Dalle and Rosenburg's observations. The outcome of the correction is that Ro (spectroscopic) ,as shown in Table 2 and calculated by North must be multiplied by 1.65 i.e. (20)",to yield the real value. The correction factor should be regarded as approximate but when taken into account the process of EET involving PPA as donor can be rationalized by a dipole-dipole mechanism with no energy migration. This explanation is supported by polarization measurements reported by MacCallum and Rudkin on emission from a copolymer of styrene and phenyla~etylene.~~ 38 I.Berlman 'Handbook of Fluorescence Spectra of Aromatic Molecules' Academic Press New York 1971 p. 60. 39 J. P. Dalle and B. Rosenberg Phorochzrn. and Photobiol. 1970 12 151. J. R. Maccallum 8 Triplet-Triplet EET The process of an electronically excited triplet donor producing a triplet-state acceptor [equation (19)] is spin-allowed and most probably proceeds by the exchange mechanism. The possibility of the dipole-dipole mechanism operating is very much reduced by a low molar extinction coefficient and a long natural lifetime for the donor change of state. The spatial requirements for the exchange interaction demand a close approach of a DA pair with the consequent effect of reducing R to within the range 1.0-1.5 nm.This type of transfer has received relatively little attention from polymer chemists and few studies of polymer (donor)-small molecule (acceptor) systems have been reported. Polymers containing Ketone Groups.-Guillet and Dan studying solid solutions of poly(pheny1 vinyl ketone) and poly(methy1 isopropenyl ketone) to which varying amounts of 1-cis,3 -cis- cyclo-octadiene and piperylene had been added as quen- chers found evidence for energy migration in the former but not in the latter polymer.4o They compared the efficiency of quenching of a series of model compounds with equivalent solutions of the polymers and found the aromatic-based polyketone to be more effectively quenched. A number of polymers containing aryl ketones have been investigated by Geus- kens and co-workers.Efficiency of EET to naphthalene as acceptor at 77 K has been measured for poly(vinylben~ophenone),~~poly(pheny1 vinyl ketone),42 and poly(methy1 vinyl ketone),43 with R, values shown in Table 3. For the exchange Table 3 R,(experimental) for ketone-containing polymers. Triplet EET Poly(viny1benzophenone)" Poly(pheny1 vinyl ketone)b Polymer (donor) Naphthalene Naphthalene Acceptor ss ss Matrix* 3.6 2.6 Ro(experimental)/nm Poly(methy1 vinyl ketone)' Poly(vinylnaphthalene)d Naphthalene Penta-1,3-diene ss SS 1.1 1.5 Poly(styrene/vinyl- Naphthalene ss 3.0 benzophenone)' * SS is solid solution at 77 K. C. David W. Demarteau and G. Geuskens European Polymer J.1970 6 537; C. David W. Dernarteau and G. Geuskens ibid.,p. 1405; C. David N. Putman M. Lernpereur and G. Geuskens ibid. 1972,8,409; C. David M. Lempereur and G. Geuskens ibid. p. 417; 'C. David V. Naegelen W. Piret and G. Geuskens ibid. 1975 11 569. mechanism there is no means of estimating Ro (spectroscopic) and thus experimen- tal Re's have to be compared with what could be defined as the range of likely values 1.0-1.5 nm. The distances evaluated from Geusken's data for the triplet-triplet exchange gave strong evidence for energy migration. Polarization measurements on styrene/vinylbenzophenone cop01ymers~~ supported the conclusion that quite extensive migration of the triplet energy was taking place within the polymer chains the extent increasing with increasing benzophenone content.The phosphorescence 40 E. Dan D. C. Sornersall and J. E. Guillet Macromolecules 1973 6 228. 41 C. David W. Demarteau and G. Geuskens European Polymer. J. 1970,6,537. 42 C. David W. Demarteau and G. Geuskens European Polymer J. 1970,6 1405. C. David N. Putnam M. Lempereur and G. Geuskens European Polymer J. 1972,8,409. Electronic Energy Transfer in Polymers 113 decay of the same copolymers was studied at 77 K in glassy solutions and as For the solutions the decay was exponential giving a lifetime of 5 ms for the whole range of copolymers. On examining the phosphorescence decay of the powders it was found that those containing more than 50 mole per cent vinylben- zophenone exhibited bi-exponential decay with lifetimes of 2.2 ms and 20 ms for the two components.It was proposed that the long-living triplet was a trap into which the migrating energy flowed. As the number of traps increased with increasing vinylbenzophenone content the authors proposed that the trap was a benzophenone unit perturbed by neighbouring benzophenone units. 9 Delayed Fluorescence Delayed fluorescence results from triplet-triplet annihilation [equation (20)]. Basically delayed fluorescence is a biphotonic process with a lifetime half that of the sample phosphore~cence.~~ For polymers either in glassy solutions or as films the excited chromophore is incapable of translational motion and therefore the incidence of delayed fluorescence is indicative of triplet energy migration.This phenomenon was first observed by Fox and Cozzens in a solid solution of poly(vinylnaphtha1ene) and was interpreted as being due to triplet-triplet reaction following migration of triplet energy.46 Delayed fluorescence has also been observed at low temperatures from poly(naphthy1 metha~rylate)~’ and poly(vinylcarbaz01e.)~~ Poly(vinylnaphthalene).-Cozzens and Fox explained the emission of delayed fluorescence from solid solutions of poly( 1-vinylnaphthalene) as resulting from intramolecular triplet EET.46 The relatively high efficiency of quenching by addi- tives supported their mechanism. More recently Pasch and Webber have studied solid solutions ( M in chromophore) of poly(2-~inylnaphthalene).~’By measur- ing the efficiency of quenching of phosphorescence by piperylene they showed that sample molecular weight played an important role in the migration mechanism.Their interpretation of this effect was that the probability of a triplet exciton encountering a quencher species increased as its intramolecular diffusion length increased. Once the chain-length of the polymer exceeded the diffusion length (ca. 700 units) then molecular weight made no difference to the efficiency of quenching. Table 4shows the values obtained for A for a range of molecular weights. A very interesting application of the properties of the kinetics of the annihilation process was proposed by Avakian who used magnetic fields to vary the rate of migration.49 From the dependence of the intensity of delayed fluorescence on the strength of the applied magnetic field he was able to derive structural information about the macromolecules.Poly(2-naphthyl methacrylate) PNMA.-Guillet and Somersall observed T-T annihilation leading to delayed fluorescence in solid solutions of PNMA and they 44 C. David D. Baeyens-Volant P. Macedo de Abren and G. Geuskens European Polymer. J. 1977,13 841. 45 C. A. Parker and C. G. Hatchard Trans. Faraday SOC. 1963 59 284. 46 R. F. Cozzens and R. B. Fox,J. Chem. Phys. 1969 50 1532. 47 J. E. Guillet and A. C. Somersall Macromolecules 1973 6 218. 48 N. F. Pasch R. D. MacKenzie and S. E. Webber Macromolecules 1978 11,727. 49 P. Avakian R. P. Groff A. Suna and H. N. Cripps Chem. Phys. Letters 1975 32,466. J. R. Maccallum Table 4 Electronic energy-migration coefficient for series of polymers.Triplet EET Polymer (donor)* Poly(viny1naphthalene)" DP = 100 Acceptor Piperylene Matrix t ss Migration coe cienr ~xlo'*/mBss-l 2.41 DP = 273 DP = 664 DP = 3250 Poly(vinylcarbazole)b Poly(naphthy1 methacrylate)' Piperylene Piperylene Piperylene Naphthalene Piperylene ss ss ss ss ss 5.17 5.50 7.63 320.0 13.0 * DP is degree of polymerization; f SS is solid solution at 77 K N. F. Pasch R. D. MacKenzie and S. E. Webber Macromolecules 1978,11,733;* M. Yokoyama T. Tamamura T. Nakano and H. Mikawa J. Chem. Phys. 1976,65,272; E. Dan A. C. Somersall and J. E. Guillet Macromolecules 1973 6 228. studied the relative transfer rate of triplet energy by the addition of triplet quen- hers.^' The delayed fluorescence was biphotonic but excimers did not involve nearest neighbours.This work was extended by Pasch and Webber who found that the intensity of delayed fluorescence increased with increasing molecular weight while the intensity and lifetime of phosphorescence decreased." The decays of phosphorescence and delayed fluorescence were non-exponential with the 'lifetime' of the latter very much less than half that for phosphorescence while the relative intensities did not follow the biphotonic relationship predicted for straightforward T-T annihilation. The authors suggested that the behaviour was consistent with a model involving two types of triplet a trapped and a mobile species. The relatively slow rate of migration of the triplet exciton governed the decay rate for delayed fluorescence.The effect of increasing molecular weight was primarily to increase the probability of two triplets being simultaneously generated on one chain thus facilitating T-T annihilation following intramolecular energy migration. Fox et al. using a series of copolymers both random and alternating showed that intramolecular triplet exciton migration in alternating copolymers was as efficient as in the corresponding homopolymers.5' They observed that in solid solutions intramolecular migration was the principal mechanism of EET while in films of the pure polymer inter-chain hopping was occurring. Each hop operated on exchange interaction. Copolymers incorporating alternating vinyl aromatic units/methyl methacrylate did not show any excimer emission; this was explained by the existence of charge-transfer interactions between aromatic chromophore and adjacent ester carbonyl groups.Poly(vinylcarbazole).-It was for PVK that the first observation of the significance of chain-length on the ratio of intensity of delayed fluorescence to phosphorescence was noted.31 Solid solutions of a range of molecular weights were investigated at 77 K and it was observed that phosphorescence could not be detected for samples of high molecular weight. In a later paper Klopffer and co-workers used fractionated samples and found that the intensity of phosphorescence was equal to that of delayed fluorescence at a molecular weight of 110 OO0.52 They measured the phos- phorescence decay as a first-order process with a lifetime of 8.2s whereas the N.F. Pasch and S. E. Webber Macromolecules 1978 11,733. " R. B. Fox T. R. Price R. F. Coyzens and W. H. Echols Macromolecules 1974,6 218. '' W. Klopffer D. Fischer and G. Naudorf Macromolecules 1977 10 450. Electronic Energy Transfer in Polymers 115 delayed fluorescence had a lifetime of 34 ms. It was concluded that the triplet population comprised two species; long-living immobile triplets and shorter-lived mobile triplets. Delayed fluorescence resulted from self-annihilation of the latter. A parallel study under comparable conditions was carried out by Yokoyama et al. who used a range of samples of similar molecular weights to Klopffer's They added naphthalene as a triplet quencher and observed that the lifetime of delayed fluorescence decreased but the lifetime of phosphorescence was unchanged at 7.6 s despite a reduction in intensity.It was proposed that phosphorescence was emitted from traps and not from migrating triplet excitons which were responsible for delayed fluorescence and were quenched by naphthalene whereas the trapped triplet was unaffected by this quencher. The molecular weight at which the intensi- ties of phosphorescence and delayed fluorescence were equal was about 40 000 and no delayed fluorescence was detected from samples of molecular weight less than 10 000. For the homopolymer they calculated an energy-migration coefficient A of 3.2 x m2s-'. The kinetics of delayed fluorescence and triplet energy migration in films of PVK have been studied by B~rkhart.~~ Some notable differences in behaviour compared with solid solutions were observed.The relative intensity of delayed fluorescence was Siphotonic and that of phosphorescence monophotonic; the lifetimes were in the ratio 0.5 :1,as predicted by basic theory. The molecular weights of his samples were constant but on varying the temperature from 198 to 123 K the ratio of the intensities of delayed fluorescence and phosphorescence changed markedly with the maximum at 173 K. At this temperature two triplet emitting species were detected one with Amax at 475 nm and T of 475 ms and the other with A,, at 505 nm and T of 574ms. Both decays were first order. The two emissions were explained by postulating the existence of two types of triplet; the lower-energy species was associated with immobile trap sites and the higher-energy emission was attributed to mobile triplets.Burkhart proposed the following annihilation steps (mobile triplet) +(traptriplet) = (singlet)'+ (trap singlet)' Polystyrene.-George investigated the relative efficiency of quenching by added and copolymerized additives.54 Films of polystyrene suffered identical quenching of phosphorescence when naphthalene was present as a free molecule or as a comonomer. For intramolecular energy migration the comonomer quencher would be expected to be more effective. The above author concluded that for the condition of a slow final energy-transfer step following rapid energy migration copoly- merization was no improvement on simple molecular dispersion of an additive.He extended his conclusions to triplet EET in poly(viny1 phenyl ketone). It can be seen that when as in films inter- as well as intra-molecular energy migration becomes significant differences in behaviour are observed. In general terms the process of triplet energy migration is understood but some of the details are still to be unravelled. 53 R.D. Burkhart Macromolecules 1976 9 234. 54 G. A. George,J. Polymer Sci. Polymer Phys. Edn. 1972 10,1361. J. R. Maccallum 10 Orientation Factor In the Forster derivation relating specific rate of energy transfer to measurable parameters [equation (5)] a term K is introduced to allow for D-A spatial orientation. When this is completely randomized during the transfer time K’ takes a value of 0.66.For intermolecular transfer in solutions of low viscosity this is a realistic condition. However for intramolecular transfer and transfer in solids such as films or glasses the appropriate value for K may well be quite different; indeed depending on the disposition of the donor and acceptor moments K may vary from 0 to 4. The question of the appropriate value for K~ and even the applicability of the Forster equation have been discussed in a series of papers by Eisinger.’’ The thrust of his arguments is more directed to biochemical studies of EET but there seems to be a good case for consideration of the relevance of his criticisms to systems involving polymers in films and glasses. None of the papers which have been referred to in this Report and which used the Forster equations made any allowance for possible orientation effects.In Dexter’s treatment of the exchange mechanism for EET there is no explicit term which allows for orientation of the D/A pair [equation (14)]. Judeikis and Siegel proposed that the specific rate of transfer EkET,should be modified such that in which /3 is set equal to cos”8 with 8 the angle between donor and acceptor molecular planes.56 /3 was introduced as an arbitrary factor but the authors designated EkkT as a slow-moving function (m =O) or a fast-moving function (m large) of 8. Polarization measurements have been made on benzophenone-phenanthrene mixtures in poly(methy1 methacrylate).” In the circumstances used triplet EET took place by the exchange mechanism.It was concluded that the function was slow-moving and the dominant factor in determining EET was that of distance dependence. The relevance of orientation requirements in EET processes has been highlighted in theoretical treatments but there seems to be a need for more experimental data to assess the real significance of this factor in both the dipole-dipole and exchange mechanisms of EET. 55 J. Eisinger and R. E. Dale ‘Excited States of Biological Molecules’ Proc. Int. Conf. ed. J. Birks Wiley Chichester 1974 p. 579. s6 H. S. Judeikis and S. Siegel J. Chem. Phys. 1970 53 3500. 57 A. Adamczyk S. W. Beavan and D. Phillips ‘Excited States of Biological Molecules’ Proc. Int. Conf. ed. J. Birks 1974 p.39.

ISSN:0308-6003    

DOI:10.1039/PR9787500099

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

6 Electron Spin Resonance Spectroscopy M.C. R. SYMONS Department of Chemistry The University Leicester LE 1 7RH 1 Introduction It is now some 17 years since I last covered the topic of e.s.r. spectroscopy in Annual Reports.’ I found re-reading this Report both interesting and nostalgic. It contained 183references the present Report is meant to be limited to ca. 100references (this aim was not achieved!) and in contrast with the earlier Report it simply comprises what seem to me to be recent highlights which I hope will be of general interest. My own bias is towards chemistry rather than physics and inevitably this emerges in the present article. I have endeavoured to confine attention to results that are uniquely rather than peripherally enlighted by e.s.r.studies. By far the most important recent event for e.s.r. spectroscopists has been the publication’ of Landolt-Bornstein ‘Magnetic Properties of Free Radicals’ covering all the data the e.s.r. spectroscopist needs to know in this huge field. A similar publication on transition-metal complexes would not come amiss! The Chemical Society’s Specialist Periodical Reports on e.s.r. spectroscopy3 are invaluable to practitioners in this field each volume covering about 1; years. A book on e.s.r. spectroscopy that stresses chemistry rather than physics may be useful to those starting in this field or wishing to know about its ~tility.~ E.s.r. spectrometers have steadily improved and the use of computers to store spectra to remove noise and to simulate complex spectra is now widespread.Also good ENDOR spectrometers are now commercially available and are quite widely used as an aid in spectral analysis and to help pick up weak hyperfine interactions. Spin-echo techniques of the type now routinely used in n.m.r. spectroscopy are only just being introduced. One potential advantage of this technique seems to be in the study of transients in flash-photolyses or pulse-radiolyses for which conventional e.s.r. methods are limited by broadening from uncertainty in life-times. Also a variety of sophisticated techniques that are not strictly e.s.r. spectroscopy but which give the same information have been developed. These include optically detected magnetic resonance methods (ODMR) applied with great success to triplet states and laser magnetic resonance (LMR) spectroscopy.The latter technique has ’ M. C. R. Symons Ann.Reports 1962,59,45. Landolt-Bornstein ‘Magnetic Properties of Free Radicals’ Vol. 9 Part a eds. H. Fischer & K.-H. Hellwege Springer-Verlag 1977. ‘Electron Spin Resonance’ Vol. 1-3 ed. R. 0.C. Norman; Vol. 4 and 5 ed. P. B. Ayscough (Specialist Periodical Reports) The Chemical Society London 1972-1979. M. C. R. Symons ‘Chemical and Biochemical Aspects of Electron Spin Resonance Spectroscopy’ Van Nostrand Reinhold Co. Ltd. 1978. 117 M. C.R. Symons played a major role in the recent upsurge of work on gas-phase species. Gas-phase applications have been covered admirably in Carrington's recent book.' In addition to the wide range of chemical and physical applications e.s.r.spectroscopy has become part of the armoury of many biologists and some geologists. Even archaeologists are interested and it seems possible that e.s.r. methods may prove to be useful in dating studies.6 The dominance of U.V. and e.s.r. spectroscopy in the detection of radical inter- mediates is not as great as it was. 1.r. spectroscopy is most effective especially in low-temperature matrix studies and photoelectron spectroscopy is a powerful tool in gas-phase studies. Resonance Raman spectroscopy promises to be an extremely powerful tool in specific cases when there is an optical transition of suitable wavelength. The technique is an order of magnitude more sensitive than normal vibrational spectroscopy and gives only vibrational details for oscillators directly connected with the chromophore.' Nevertheless e.s.r.spectroscopy still holds its own as I hope is illustrated by .the following highlights. 2 Inorganic and Organometallic Radicals Trapped and Solvated Electrons.-This continues to be a flourishing field as is evidenced by the extensive coverage in a recent issue of Canad.J. Chem.devoted to the proceedings of an international conference at Banff in 1976.8 This and similar reviews show however that e.s.r. spectroscopy is only one of very many tools needed to obtain physical insights into the nature of these elusive entities. This is because the most powerful feature of e.s.r. spectra -hyperfine coupling -is usually not resolved in e.s.r. spectra of these species.For this reason the observation of well resolved 14N hyperfine components for excess electron centres on the surface of magnesium oxide treated with ammonia or amines is of considerable interest.' There was no indication of 'H hyperfine coupling and the 14N coupling was remarkably isotropic. High coverage gave a three-nitrogen centre the three coupling constants being identical. There was no evidence for specific solvent anion (*NH3-) formation. This is important since the concept that these electron excess centres are after all merely solvent anions 4-, has recently been renewed." The arguments against relaxed radical anions with the excess electron in a u* orbital on S seem to me to be overwhelming. Thus for example *NH3- would give a very large 'H hyperfine coupling and hexamethylphosphoramide would give a very large 31P hyperfine coupling.The possibility remains that the electron migrates so rapidly that the solvent molecules have no time to relax. In that case possibly an outer Rydberg type orbital might be involved but then however to call the species *S-would seem to be highly inappropriate since there would be no reason for localization. A. Carrington 'Microwave Spectroscopy of Free Radicals' Academic Press London 1974. G. V. Robins N. J. Seeley D. A. C. McNeil and M. C. R. Symons Nature 1978,276,703. ' R. E. Hester in 'Advances in Infrared and Raman Spectroscopy' Vol. 4 ed. R. J. H. Clark and R. E. Hester Heyden and Son Ltd. London 1977 p. 1. Cunud.J. Chem. 1977,55 No. 11. M.C. R. Symons D. R. Smith and P. Wardman J.C.S. Chem. Comm. 1978,71. lo T. R. Tuttle and P. Graceffa. J. Phys. Chem. 1971,75,843; J. Chem. Phys. 1966,44,3791; S. Golden and T. R. Tuttle J. Phys. Chem. 1978 82 944. Electron Spin Resonance Spectroscopy For trapped electrons in glassy solids e.s.r. lines are broadened by hyperfine interactions but rarely resolved. Kevan and co-workers" have probed these interactions using the powerful ENDOR and spin-echo techniques. Their results seem to favour the cavity model thus for electrons trapped in lOM-NaOH/H,O glasses they conclude that six equivalent water molecules define the cavity each with one 0-H bond oriented towards the cavity. This strongly supports the 'solvated anion' model and again does not accord with the *H20-model.Atoms and Related Species.-It is well established that hydrogen atoms trapped in matrices often exhibit a weak interaction with matrix molecules. Examples include He in CaF2 showing hyperfine coupling to six equivalent F-ions and H- in xenon showing coupling to six Xe atoms. The proton coupling is often greater than that for gas-phase hydrogen atoms and hence cannot be used for these systems to estimate spin-densities. This is probably a specific effect arising from the extreme sensitivity of this coupling to small changes in the radial extension of the 1s wavefunction. Alkali-metal atoms are far less sensitive to this effect and hence their hyperfine coupling constants can be taken as a measure of spin-density. Catterall and Edwards found that when solutions of alkali metals in hexamethylphosphoramide were cooled to 77 K well resolved spectra exhibiting hyperfine coupling to the metal nuclei were obtained.'* In general a single 'atomic' species dominated with ca.70% of the gas-phase hyperfine coupling but minor species generally with lower spin-densities were also detected. A simple-minded view of these results is that there is some delocalization onto the nearest neighbour solvent molecules the various centres representing various types of solvate such as M(S), M(S)5,M(S),=, etc. However Catterall and Edwards prefer to discuss their results in terms of intermediate impurity states within the band gap of an amorphous solid. Unfortunately and interestingly no 31Phyperfine coupling was detected.Lithium atom centres trapped in argon in the presence of water and ammonia molecules have been even more inf~rmative.'~ The Li'OH2 and Li'NH3 species detected have a u*structure the major spin density (ca. 60%)being on the metal. However hyperfine coupling to 14N (ca.14 G) was remarkably isotropic which does not seem to accord with the simple a* model for which some anisotropy would surely be expected. Hyperfine coupling to the protons was only detected for the water complexes. There has been an interesting controversy about the nature of silver atoms in aqueous mat rice^.'^ When prepared from gas-phase atoms the Ago centres are clearly unsolvated atoms. However electron attachment to aquated Ag' ions at 4.2 K gave centres whose isotropic couplings to lo9Ag and Io7Ag were ca.88% of the atomic values. This shows that the solvation characteristic of Ag' ions is at least partially retained on electron capture. On annealing to 77 K the isotropic coupling fell to ca. 75% of the atomic values and hyperfine coupling to a single proton appeared. This is viewed as an atom solvation process by Kevan and as a partial desolvation of the original Ag' by Symons. Detection of 170 hyperfine coupling" See for example S. Schlick P. A. Narayana and L. Kevan J. Chem. Phys. 1976,64,3153. Is R. Catterall and P. P. Edwards Chem. Phys. Letters 1976 42 540 and references therein. l3 P. F. Meier R. H. Hauge and J. L. Margrave J. Amer. Chem. SOC.,1978,100 2108. l4 M. C. R. Symons J. Chem. Phys. 1978,69 3443; and L.Kevan ibid. p. 3446. D. R. Brown and M. C. R. Symons J.C.S. Faraday I 1977.1490. M. C. R. Symons shows clearly that some solvent molecules are retained at 77 IS,and this is confirmed by spin-echo and related st~dies.'~ In my view one solvating water molecule moves in such a way as to remove its bonding electrons from the vicinity of the original bond by a rotatory motion that makes one of its protons move towards the silver. Concurrently the wavefunction extends out into this region by admixture of the 5p (a)orbital thus explaining the fall in 5s-character. Silver atoms have also been studied in cyanomethane glasses. When formed from dissolved Ag' their e.s.r. spectra show well defined superhyperfine coupling to four equivalent 14N nuclei.16 Thus in this case the original [Ag(MeCN),]' unit is retained at 77 K (Figure 1).1 3200G (9.112GHt) Gain x 2.5 x lo3 I!II Gain x 10 1 'I y' Figure 1 First derivative X-band e.s.r. spectrum for silver atoms formed from silver perchlorate in cyanomethane. The low field multiplet is due to Ag"; the prominent low and high field doublets are due to Ag'; the outer set from lo9Agand the inner from IQ7Ag.The fine structure is due to hyperfine interaction with four equivalent 14N nuclei. The intense central features are from solvent radicals Although by no means 'atomic' species it is convenient to mention some most interesting reactions of atoms (Al Ag Cu) with C2H4 and C2H2 studies by Kasai and McLeod," in rare-gases at 4.2K. The Ag-C2H4 complex showed only a small coupling to silver and no 'H coupling.It was suggested that the unpaired electron is in a non-bonding d,,-p hybrid on silver. The A1-C2H4 complex exhibited a highly anisotropic coupling to 27Al showing that the unpaired electron is strongly confined to one 3p orbital on aluminium with only slight delocalization onto ethylene. Silver gave a somewhat similar complex with acetylene but aluminium gave an adduct thought to have the 'vinyl' structure (1).The 27Al coupling was almost isotropic as expected for this structure and the 'H coupling constants also accorded well with expectation for structure (1).Copper atoms reacted with acetylene to give a range of interesting species. One having a high spin-density in the copper 4s orbital (ca.'' D. R. Brown G. W. Eastland and M. C. R. Syrnons Chem. Phys. Letters 1979,61,92. l7 P. H. Kasai and D. McLeod J. Amer. Chem. SOC.,1975,97,5609; 6603; 1978,100,625; P. H. Kasai D. McLeod and J. Wanatabe J. Amer. Chem. SOC.,1977,99 352. Electron Spin Resonance Spectroscopy 121 0.67) is clearly quite different from either the silver or aluminium complexes. The other having a very small interaction with copper involves two acetylene molecules. Neither structure is yet clearly understood. (ca. 85 G) A1 \ (55G)H' H(20G) Diatomic and Related Radicals.-Weltner and his co-workers'8 have studied a wide variety of neutral diatomic radicals and related species in rare-gas matrices. Recent examples include MnO YbH MgOH and BeOH. The MnO radical was formed together with Mn02 MnO, and MnO from Mn atoms and 02,N20 or 0,.It has the expected 'C.+ ground state and a zero-field splitting close to that for the gas-phase molecule. YbH is a 0* species with ca. 80% spin-density in the 6s orbital on Yb.19 As expected MgOH and BeOH are almost ionic the spin-density on the hydroxide ion being very small. Two groups of silver molecules AgMg AgCa AgSr AgBa and AgZn AgCd and AgHg have been prepared in argon at 4.2 K. These three electron a*-radicals make an interesting comparison with some one electron 0-radicals such as Ag-Ag' Ag-Cd2+ and Ag-Hg2+ that we had previously studied.20 In all cases the Ag hyperfine coupling is nearly isotropic showing that even for the 0* species the pz contribution is small.This is also true for the other metals when their nuclei are magnetic. This is not an obvious result in view of the extensive s-p mixing found for diatomic molecules with a greater number of valence electrons. For the first series the 5s character on silver falls from 46% to 12% on going from Mg to Ba. For the latter it rises from 73% to 86% from Zn to Hg. These trends follow the ionization potentials of the bivalent metals thus clearly establishing the antibonding character of the unpaired electron. The superoxide ion partly because of its great biochemical significance has been widely studied. Because of the degeneracy of its T orbitals its g-tensor components are defined by the environment and rapid fluctuations make its liquid-phase spectrum too broad to detect.Nevertheless it often has a well defined solid-state spectrum indicating quite precise solvation. When 02-is generated from O2 in protic matrices at 4.2 K no e.s.r. spectrum can be detected. However on warming to 77 K well defined spectra almost identical with those obtained from frozen solutions of 02-,grow in.21 These results showing the growth of specific anion solvation parallel earlier work on the growth of solvation for trapped electrons. They have obvious implications for time resolved studies. Triatomic Radicals and Related Species.-A common first order method for analysing e.s.r. results for *AB2 and -AB3 radicals having high spin-density on the l8 J. M. Brom and W. Weltner J. Chem. Phys. 1973 58 5322; 1976 64 3894; R. F.Ferrante J. L. Wilkerson W. R. M. Graham and W. Weltner J. Chem. Phys. 1977,67 5904; R. J. Van Zee M. L. Seely and W. Weltner J. Chem. Phys. 1977 67 861. l9 P. H. Kasai and D. McLeod J. Phys. Chem. 1975,79,2324; 1978,82 1554. *' R. J. Booth H. C. Starkie and M. C. R. Symons J. Chern. SOC. (A),1971,3198;M. C. R. Symons and I. N. Marov RussianJ. Inorg. Chem. 1972,17 1362. 21 G. W. Eastland and M. C. R. Symons J. Phys. Chem. 1977,81,1502;J. Chem. Research 1977 (S),254; (M)2901. 122 M. C. R. Symons central atom is to assume orbital following and hence to use the estimated s and p characters to derive bond angle^.^ As discussed further in Section 4 the whole concept of orbital following has been under fire recently. Since this concept is intuitively satisfying and is used very widely in the thinking of most chemists it is important to attempt an assessment of these criticisms.For *AB2 and *AB3 radicals ab initiu calculations22 have helped to reveal some misconceptions in the previous critical and we conclude firmly that the simple procedure is qualitatively valid for such radicals. Potentially the most interesting triatomic radical recently announced is H20- since this is a u*radical of a previously unknown class of great pertinence to the solvated electron problem. Unfortunately it has been shown to be NO3’-formed from fortuitous nitrate ion imp~rities.~~ The ‘molecule’ Kr-F-Kr is also a novelty. This species formed by fluorine atoms in solid Krypton” is probably linear (2Xu)and is ‘isoelectronic’ with the well known F3’-anion.As expected the unpaired electron is strongly confined to the 2p orbital on fluorine. Incidentally I should mention that Morton and Preston26 favour a new set of atomic parameters differing slightly from those normally ad~ocated.~ When -AB2 or -AB3 radicals are formed in the presence of :AB2- or :AB3- species (the charges are not significant) u*‘dimers’ are often formed. The same ‘dimers’ can frequently be prepared by electron addition to B2A-AB2 molecules. A recent study of NzO4 showed that N204- ions are indeed stable.*’ The results are of interest in that they confirm an observation that I have made for previous ‘dimer’ species namely that the u*electron is far more strongly confined to the two central atoms (A) than is the case for the monomer radical *AB2 (*NO2in this case).Tetra-atomic Radicals and Related Species.-One electron u1 ‘dimers’ are far less common. N204+ probably also formed from N204,is an example.27 A better defined example is the radical [(MeO),B zB(Me0)3]+ formed by irradiating (Me0)3B at low temperatures.28 Again the electron is strongly confined to the u-bonding orbital. Two sulphur radicals make an interesting contrast. Roberts et have studied (Me0)3S- radicals which are expected to be isostructural with *SF3radical^.^' These 27-valence electron species almost certainly have a planar T structure with only two equivalent ligands. The problem that arises is concerned with the nature of the orbital of the unpaired electron. The liquid-phase data3’ are not sufficient to give a clear distinction between the rival structures shown in (2) and (3).Structure (2) (2B1 in CZ0symmetry) is in effect a T structure and structure (3) (*A1in C2u)could be described as u.Roberts et al.favour the intuitively more acceptable structure (2). ’* B. Burton T. A. Claxton S. J. Hamshere H. E. Marshall R. E. Overill and M. C. R. Symons J.C.S. Dalton 1976 2446. 23 F. J. Owens Chem. Phys. Letters 1973 18 158; Y. Takahata T. Eri and Y. J. I’Haya Chem. Phys. Letters 1974 26 557. 24 M. C. R. Symons D. R. Brown and J. C. Vedrine Chem. Phys. Letters 1977,52 133. ’’ A. R. Boate J. R. Morton and K. F. Preston Chem. Phys. Letters 1978,54 579. 26 J. R.Morton and K. F. Preston J. Magn. Resonance 1978 30 577. ” D.R. Brown and M.C. R. Symons J.C.S. Dalton 1977 1389. ’* R. L. Hudson and F. Williams J. Amer. Chem. SOC.,1977,99,7714. 29 J. W. Cooper and B. P. Roberts J.C.S. Chem. Comm. 1977,228. 30 J. R. Morton K. F. Preston and S. J. Strach J. Chem. Phys. 1978,69 1392. Electron Spin Resonance Spectroscopy (2) (3) This is more attractive since the two 'non-bonding' electrons will have considerable s-character and clearly play an important role in determining the stereochemistry of the molecule. The absence of direct s-character in the unpaired electron's orbital on sulphur is to be expected. For structure (3),the 'non-bonding' electrons are forced into a T level and cannot have s-character which seems to be an unacceptable situation for a molecule with highly electronegative ligands.Nevertheless Morton et al. make a good case for an in-plane orbital comprising only sulphur 3p and fluorine 2p orbitals. Unfortunately the much needed 33Shyperfine coupling could not be detected. A further confusing factor is that weak coupling to a fourth 19F nucleus was detected. This was assigned to one of the BF4- atoms (the radical was studied in SF3'BF4- crystals) but it must be borne in mind that *SF5 and -PF5- both have fluorine ligands showing extremely small hyperfine coupling so the possibility of fluorine transfer cannot be dismissed. Penta-atomic Radicals and Related Species.-By far the most studied .AB4 radicals are the phosphoranyl radicals. Much of the extensive e.s.r. work on these species has been reviewed recently.31 Detailed mechanistic arguments can be derived from the results which probably represent one of the best examples of the use of e.s.r.in mechanistic chemistry. An impressive recent example is the work of Roberts and co-w~rkers,~~ who were particularly concerned with pseudo rotations in the radicals (4)-(7). H2 Me \ lH*H4 OR2 (4) (5) Me OR' OR' (6) (7) Phosphoranyl radicals were amongst the radical species detected in irradiated organic phosphate Monoalkyl esters gave on electron addition only alkyl radicals and *P032-radicals. However diethyl esters gave (RO),PO,*-radicals in 31 P. Schipper E. H. J. M. Jansen and H. M. Buck Topics in Phosphorus Chem. 1977,9,407. 32 J. W. Cooper M. J. Parrott and B. P. Roberts J.C.S.Perkin ZI 1977 730. 33 D. J. Nelson and M. C. R. Symons J.C.S. Perkin ZI 1977,286. M. C. R. Symons addition to R-and (RO)P02- and trialkyl phosphates gave predominantly (RO),PO-radicals. *AB3 and *AB6 Radicals.-Perfluoro-radicals dominate these classes largely because of the outstanding work of Morton and Idealized structures for *AF, -AF5 and *AF6radicals are shown structures (8)-(10). The SOMO for -AF4 F I .,F "A\ IF F (8) (9) (10) and OAF,species is an s-p hybrid on A and is delocalized onto axial ligands (OAF,) and equatorial ligands (OAF,). Spin-density on the fifth ligand in *AF5is negligible. In contrast *AF6radicals are always highly symmetrical the SOMO comprising only s on A with equal p(u) on all six ligands.Delocalization follows expectation for an antibonding electron as can be judged from the remarkably comprehensive set of data given in Table 1. Many of the central atom hyperfine coupling constants are extremely large and Morton and Preston have given a very satisfying account of the underlying theory required for spectral interpretati~n.~~ Table 1 Central-atom hyperfine interactions (in MHz) and ns spin densities for various hexapuoride radicals Cfromref. 34) MF~- MF~- MF~- MF; MF6 M 27~1 29~i 31P 33s 35~1 1807 2211 0.52 0.39 0.055 0.16 69Ga Ge73 75As Se77 Br79 -1799 9403 10 222 11773 0.75 0.64 0.51 0.37 0.063 0.091 0.14 0.17 115In Sn119 lZ1Sb lZ5Te 1271 -29745 21390 -28 318 17 550 0.68 0.61 0.51 0.42 0.088 0.11 0.16 205~1 '07Pb '09Bi Po At aM 125010 47868 36 020 M(6s) F (2P) 0.68 0.050 0.59 0.074 0.46 0.094 3 Transition-metal Complexes Introduction.-This field has been extremely active in the past few years.Much of the work is in areas of specific current interest to Physicists but of only peripheral See for example ACSSymposium Series 1978,66,386; A. R. Boate J. R. Morton andK. F. Preston J. Phys. Chem. 1976,80,2954; Chem. Phys. Letters 1977,50,65; J. Chem. Phys. 1977,67,4302; J. Phys. Chem. 1978,82,718. 3s A. R. Boate J. R. Morton and K.F. Preston J. Magn. Resonance 1976 24 259. Electron Spin Resonance Spectroscopy 125 interest to most Chemists. Areas such as the effects of very high pressures techniques of acoustic paramagnetic resonance and optical detection of magnetic resonance resonances in semiconductors lanthanide compounds doped oxide glasses and superconducting systems fall in this area.E.s.r. spectroscopy is just one of many tools needed to probe these systems. Jahn-Teller distortions remain an important area of research especially when they are co-operative (the co-operative Jahn-Teller effect). Much of this work has recently been reviewed by Porte in ‘Electron Spin Resonance’ Vols. 4 and 5. Chemists have been more interested in systems of relatively low symmetry such as are often encountered in compounds containing complex organic ligands. Some of the interpretative problems that then arise have been considered by Golding and St~bbs,,~ and by Figgis et al.37 Much of this work is peripheral so far as e.s.r.spectroscopy is concerned. Generally the interest centres on the complex itself and its e.s.r. spectrum normally accords well with expectation based in X-ray structure determination i.r. and U.V. spectroscopic measurements etc. I have selected some areas rather arbitrarily for specific mention. Areas with a strong biochemical link are discussed in Section 6. Small Molecules.-I have already mentioned some of the work done by Weltner and co-workers in this area.** Much of this work involves the transition metals recent examples being MnO MnO, MnO, Mn04 and TiF3. These species are very well defined by their e.s.r. spectra and much detailed information has been forthcoming. In particular MnO and TiF have planar D3,,,structures the unpaired electron being in an s-ds2 hybrid orbital with an outstandingly large s-orbital content giving rise to very large isotropic coupling constants.Ligand Interactions.-The extent to which ligand hyperfine coupling contributes to the spectrum depends upon the degree of covalency and on the symmetry. Very large hyperfine coupling constants can be observed if the unpaired electron is in a o* orbital and the bond covalent. An important recent example is for the complex HNi(CN),2- for which an isotropic proton coupling of ca. 150 G indicates a delo- calization of ca. 0.3 onto the ligand.,* At the other extreme ENDOR spectroscopy has been used to probe weak remote interactions in a manner complementary to the use of n.m.r. spectroscopy. An outstanding example is the study by Hutchison and co-workers of the nicotinic acid derivative of Nd3+.39 A detailed mapping of the 16 protons within a 5 8 radius of the metal ion gave locations correct to 0.003 A.This precision is remarkable and clearly the technique could have wide significance in studies on metallo enzymes and of metal-doped biomolecules. Abnormal Valences.-Electrochemical studies of transition-metal complexes now frequently include polarography coupled with e.s.r. spectroscopy. Paramagnetic high or low valence states are often sufficiently stable to give well defined e.s.r. spectra. An example is the electrochemical reduction of the square planar dithiomaleonitrile complexes of Ni Pd and Pt to give d9 complexes in which the unpaired electron is found to be strongly localized in the metal dx~-y2 orbital as 36 R.M. Golding and L. C. Stubbs Proc. Roy. SOC.(London) 1977 A354 223. 37 B. N. Figgis B. W. Skelton and A. H. White Austral. J. Chem. 1978 31 57. M. C. R. Symons M. M. Aly and D. X. West J.C.S. Chem. Comm. 1979 51. 39 C. A. Hutchison jun. and D. B. McKay J. Chem. Phys. 1977,66 3311. 126 M. C. R. Symons e~pected.~'Another good example is the electrochemical reduction of the tri-cobalt clusters RC[CO(CO),]~ discussed below.41 Another important method for generating unusual valence states of complexes is to use high energy radiation. At sufficiently low temperatures this may cause electron transfer to give isolated electron-gain and electron-loss centres that have not undergone major changes in ligands.On annealing specific changes can be picked up by e.s.r. spectroscopy. An interesting example is that of Mn2(CO)10. Electron-gain gives the anion [Mn2(CO)lo]- the unpaired electron being strongly confined to the metal-metal (T*orbital with equal density on the two metal However electron-loss causes a drastic change even at 77 K the resulting species having a 6S state on one Mn atom the other -Mn(CO)5 group acting as a 'ligand' and giving only a minor contribution to the hyperfine pattern. Metal Clusters.-The example just given42 illustrates two of the situations that may arise in clusters in which an unpaired electron may be shared between two or more metal ions or may be confined to one of the ions only on the e.s.r. time-scale.In the latter case electron transfer may still be simple being controlled by a shape difference that may be lost for transfer to occur. For example salts of [{(NH&Ru)~~~~]~+ (11) have spectra which show that the unpaired electron is confined to one Ru atom but is transferred via the pyrazine bridge to the other Ru atom at a rate that is fast on the n.m.r. time-scale but slow by e.s.r. standard~.~~ -(ll)(n = 4,5 or 6) A very interesting class of cobalt clusters RC[CO(CO),]~ readily accept electrons to give stable anions with well defined e.s.r. The RC-groups lie along the symmetry axes above the plane of the cobalt atoms each of which has near octahedral bonding. The excess electron is strongly confined to metal d-orbitals in the cobalt plane being equally shared between the three metal atoms.The group R can be hydrogen alkyl aryl or halide or RC can be replaced by S or Se with very little effect on the e.s.r. parameters. A more complicated situation arises when two or more paramagnetic ions are part of a cluster and e.s.r. spectra can be extremely involved. Examples are the dimeric species where R = CC13,CF3,et~.~~ [Fe(sali~ylideniminato)RCO,]~ These materials are antiferromagnetic and the exchange parameters J are between -4.5 and -7.0cm-'. Exchange coupling between pairs of Cu2' ions is observed in the pyridine-N-oxide complex C~(py0)~Cl~,H~0,~~ in which the metal ions are bridged by oxygen atoms of the (py0) ligands. Partial replacement of Cuz+ by Mn2' gave Cu2+-Mn2+ exchange-coupled pairs with an S = 2 ground state.Interestingly all three g-values were less than the free electron value. In most models for exchange 40 W. E. Geiger jun. C. S. Allen T. E. Mines and F. C. Senftleber Znorg. Chem. 1977,16,2003. B. M. Peake B. H. Robinson J. Simpson and D. J. Watson Znorg. Chem. 1977 16,405. 42 S. W. Bratt and M. C. R. Symons J.C.S. Dalron 1977 1314. 43 B. C. Bunker R. S. Drago D. N. Hendrickson R. M. Richrnan and S. L. Kessell J. Amer. Chem. Soc. 1978,100,3805. 44 R. G. Wollrnann and D. N. Hendrickson Znorg. Chem. 1978,17,926. '' D. A. Krost and G. L. McPherson J. Amer. Chem. Soc. 1978,100,987. Electron Spin Resonance Spectroscopy the exchange interaction parameter J is interpreted in terms of a ferromagnetic component JFand an antiferromagnetic component JAF.It seems that JAF is now reasonably well understood but JF is not. In most cases when the local metal orbitals are not strictly orthogonal Jm dominates and JF cannot be determined accurately. New complexes C~VO(fsa)~en is a bichelat- CH30G [where (f~a)~en~- ing ligand from a Schiff base] have the required strict ~rthogonality.~~ The orbitals involved are shown in (12). In this case the experimental J-value is equal to JF. Paramagnetic Ligands.-Yet another interesting coupling situation arises when a ligand in a paramagnetic complex is itself paramagnetic. Complexes having a single unpaired electron largely or totally localized on a ligand are well known and have been reviewed in Landolt-Bornstein Vol.9a2 When the metal ion is also magnetic composite e.s.r. spectra are usually obtained. For example bis(hexafluoroacety1- acetonate) complexes of Mn2+ and V02+ with pyridylimino and pyridylnitronyl nitroxide ligands have been st~died.~' The ortho-nitroxides (13) gave diamagnetic 0 complexes in which possibly an electron has been largely transferred from the metal to the ligand. However the meta- and para-pyridyl complexes gave well defined e.s.r. spectra with 'separate' strongly broadened components from the metal and ligand. High-Low Spin Equilibria.-A variety of Fe'" dithiocarbamates Fe(S2CNR1R2)3 have structures such that the high spin (6A1)and low spin (2T2)forms are of comparable energy. Thus their magnetic properties depend strongly on tempera- ture pressure and environment.In most cases these forms co-exist in equilibrium. However in some cases especially in hydrogen-bonding solvents structures with S =$ may be formed and for [Fe(m~d)~1CH~Cl~ [mcd =morpholinocarbodithioato-SS'] the S = 5 state is the ground In certain Co" complexes the symmetry may be such that states with S =$ are almost equal in energy to the normal S=3ground states. An interesting example is 46 0.Kahn P. Tola J. Galy and H. Coudanne J. Amer. Chem. SOC. 1978 100 3931. 47 P. F. Richardson and R. W. Kreilick J. Phys. Chem. 1978,82 1149. 48 R. J. Butcher J. R. Ferraro and E. Sinn J.C.S. Chem. Comm. 1976 910. 128 M. C.R. Symons for solutions of CO(TAAB)(NO~)~ [TAAB = tetrabenzo(b,f,j,n)( 1,5,9,13)tetra- azacyclohexadecine].In MeOH Me2C0 and dimethylformamide this is a normal low-spin d7complex with All >>A,("Co) and gl >> gll. When pyridine is added this becomes a ligand and )A111approaches 1A,I and g approaches gll. When 2:l complexes formed no signal could be detected. This was explained in terms of an s=".-S = 1 equilibrium. Cull Configurations.-Complexes of Cur' with six ligands generally distort by elongation along one axis (z),giving a dt2-y~structure. This rule is not invariant and there are rare examples in which tetragonal 'compression' is favoured giving a d) ground state. One recent example is for Cu(pyraz01e)~Cl~ complexes in Cd(pyra-20le)~Cl~ In this case it is perhaps misleading to talk of a compressive distortion since the ligands themselves provide the required symmetry and the 'hole' clearly seeks the most suitable orbital to give maximum stability.However for Cur' in Ba2ZnF6 crystals the parent ZnF64- ions have a crystal-induced tetragonal compression and the CuII ions are induced to take up the same distortion giving a di2 distortion. When [Cu"] is >35'/0 the requirements of the copper dominate and the normal tetragonally expanded ions are formed.50 There is wide interest in Cu" complexes having near tetrahedral structures since this is thought to be the symmetry approached by Cu" in blue copper-proteins which sometimes have very unusual e.s.r. spectra. What is observed is a gain in the amount of 4s-admixture on moving from square-planar towards tetrahedral symmetry.The magnitudes of the hyperfine coupling constants are thereby reduced. There are many examples such as Cuxr in Zn[C(NH2)3]2(S04)2," and dichloro[2-(2-dimethyl- aminoethy1)pyridinel ~opper(H7W811).~* Nevertheless in all these cases the 63Cu and 65Cu splitting remains quite large and is readily measurable. Mechanism.-There is still much activity with catalytic systems involving transition- metal complexes although the e.s.r. spectra are not always directly significant. For example work on Ziegler-Natta systems has been extended.53 Hydrogenation of olefins using (cyc10pentadienyl)~TiCl~ and sodium naphthalenide butyl magnesium bromide or butyl lithium indicates that the catalytically active units are the bridged hydride cornple~es.~~ Tsou and Koch? have used a variety.of techniques including e.s.r.spectroscopy to study reductive coupling that is induced by electron loss. An example is given in equation (1)for planar nickel(I1) complexes Et,P Ar \Ni + 21rC162-+ArPEt3++ BrNiPEt,' + 2IrCI63-(1) Br/ \PEt3 At low temperatures transient optical and e.s.r. spectra were obtained which were assigned to Ni"' complex ArNiBr(PEt,),'. 49 J. A. C. Van Ooijen P. J. Van der Put and J. Reedijk Chem. Phys. Letters 1977,51 380. J. Reedijk Chem. Weekblad. Mag. 1977 97. 51 R. Kirmse W. Dietzsch and B. V. Solov'ev J. Inorg. Nuclear Chem. 1977,39 1157. 52 V. G. Krishnan J. Chem. Phys. 1978,68,660. 53 G. Giunchi E. Clementi M. E. Ruiz-Vizcaya and 0.Novara Chem. Phys. Letters 1977 49 8. 54 V. V. Saraev F.K. Shmidt N. M. Ryutina V. A. Makarov and A. V. Gruznykh Koord. Khim. 1977,3 1364. " T. T. Tsou and J. K. Kochi J. Amer. Chem. Soc. 1978 100 1634. Electron Spin Resonance Spectroscopy 129 4 Organic Radicals Structure This remains one of the most prolific areas of research but in many studies the e.s.r. spectra do not supply novel structural information. Often they are more concerned with mechanism solvation or biochemical problems and some are discussed under these headings below. Here I have selected results which seem to me to have novel structural significance. Two books concerned primarily with e.s.r. spectra of organic radicals formed by ionizing radiation have appeared.56*57 The former is particularly useful in giving a summary of Russian work in this area and the latter although limited to a relatively small number of examples illustrates very well the way in which ENDOR spectroscopy has helped to enlighten this field.Perfluoroanions.-One of the most exciting developments has been the discovery that a variety of perfluoro compounds add electrons to give relatively stable The precise electronic structures of these species remain somewhat ambiguous but there can be no doubt that the expected dissociative electron capture does not occur. Williams and co-worker~~~ studied a range of cyclic compounds (CnFZn)- the best results being for C,F,-. All the fluorine nuclei are equivalent showing that the electron is effectively delocalized through the molecular frame- work. The combined electron-withdrawing effect of the fluorine atoms must leave the carbon framework sufficiently denuded of electrons that an extra electron is welcome.These results nicely illustrate that the popular concept of .rr-delocalization and u-localization is unsatisfactory for open-shell species. A similar result was obtained for (C,F,)- in an adamantane matrix.59 Well resolved isotropic spectra were obtained showing again that the six fluorine nuclei interact identically the isotropic coupling of 137 G being far greater than could possibly be explained in terms of a conventional T*structure. In this case however there is a possible alternative to the u* structure postulated-namely that the planarity of the C,F ring is lost on electron addition.,’ That this might occur is supported by the fact that the radical CF is markedly pyramidal as judged by its large isotropic 13C hyperfine coupling.(This argument presupposes orbital follow- ing.) Another example in which a u*structure has been postulated but for which a non-planar pseudo-.rr structure is a possible alternative is the anion C2F4-.61 We have also studied the e.s.r. spectrum for this anion and support the results of Williams and co-workers. However in our view a non-planar structure is as acceptable as the planar u*structure proposed. a*-Radicals.-This term is generally used to describe radicals (AIB)- having the unpaired electron largely confined to a specific u*orbital rather than being delo- calized through the u-framework. Examples studied by e.s.r.spectroscopy include FIF- RILIR+ (R-C0)2Nlhal- RC=CII- NECLBr- etc. These species may be formed by electron attachment to A-B or by electron loss from A- or B- S. Ya. Pshezhetskii A. G. Kotov V. K. Milinchuk V. A. Roginskii and V. I. Tupikov ‘EPR of Free Radicals in Radiation Chemistry,’ Wiley New York 1974. 57 H. C. Box ‘Radiation Effects ESR and ENDOR Analysis,’ Academic Press New York 1977. 58 A. Hasegawa and F. Williams Chem. Phys. Letters 1977,45 275; Faraday Discuss. Chem. SOC.,1977 63 157. 59 M. B. Yim and D. E. Wood J. Amer. Chem. Soc. 1976,68,2053. 6o M. C. R. Symons R. C. Selby I. G. Smith and S. W. Bratt Chem. Phys. Letters 1977 48 100. 61 R. I. McNeil M. Shiotani F. Williams and M. B. Yim Chem. Phys. Letters 1977 51 433. M.C.R. Symons followed by reactions to give (ALB)- Examples of equations (3) and (4) are given under ‘Sulphur Radicals’ below an example of equation (2) that links with the preceeding section id2 C6F51+e-+(C6F5‘I)-(5) Here the unpaired electron is clearly confined to the localized C-I u*orbital rather than being delocalized. A very significant set of examples are the F3CLhal- (CI Br I) anions where again the excess electron is primarily in the C-ha1 u*orbital.63 These results make an interesting contrast with those for simple alkyl halides here there is no evidence for u*anion formation even when the R* and hal- products of dissociative electron capture remain trapped in the same solvent cavity there is no tendency for u*radical formation.64 This contrast reflects an electronegativity difference coupled to a geometrical factor in the radical R-.The more electronegative the group R the more the u electrons are located on R and the u*electron on the halogen atom hence the smaller the tendency to form R- +hal-. Also when R- is planar as is the case for alkyl radicals (see below) but not for CF3radicals addition of e- to R-ha1 results in a lengthening of the C-ha1 bond and a concurrent flattening at carbon this process evidently continues to give planar Re. Note that in all the examples of (RLha1)- u* radicals given above the orbital of the unpaired electron in the ‘free’ radical R* remains s-p hybridized rather than becoming pure p. Competition between T* and u*electron addition can be significant. Thus some halogenated uracil derivatives gave both 7r* and u* anions prior to undergoing dissociative electron ~apture.~’ The results have led to the proposal that T* anions are not the direct precursors of dissociation and that a 7r*+u* change is a necessary preliminary step.Radicals that are structurally related to 7r* and u* anions are p-halogen alkyl radicals (hal=Cl Br or I). It is clear from liquid- and solid-state studies that R2C-C(C1)R2 radicals have a strong preference for structure (14) where u-T or 0 7’ (14) R-C-C-R R‘O ‘R hyperconjugative delocalization is maximized. However although one might expect P-bromo and p-iodo radicals to exhibit a similar preference some recent isotropic spectra showing well defined bromine and iodine hyperfine features with very small coupling constants of ca.6 G have been interpreted in terms of such radicals which clearly cannot have structure (14).66Instead structure (15)was proposed to explain 62 M. C. R. Symons J.C.S. Chem. Comm. 1977,408. A. Hasegawa M. Shiotani and F. Williams Faraday Discuss. Chem. SOC. 1977,63 157. 64 M. C. R. Symons J. Chem. Research (S),1978,360. 65 H. Riederer J. Huttermann and M. C. R. Symons J.C.S. Chem. Comm. 1978 313. 66 D. E. Wood and R. V. Lloyd J. Amer. Chem. SOC. 1975 97 5986; Tetrahedron Letters 1976 345. 131 Electron Spin Resonance Spectroscopy (2 1.4G)CH CH (2 1.4G) h (42.8G) (15) the e.s.r. data. In fact we had previously assigned an e.s.r. spectrum to P-bromo radicals which exhibited the expected large hyperfine coupling to 79Br and 81Br but this was evidently deemed to be unsatisfactory.A-major reason for Wood and Lloyd’s assignment was that Me2CHCH2Cl gave Me2CCH2Cl unambiguously under conditions in which Me2CHCH2Br gave the species they believe to be (15). In fact our species with a large bromine coupling is also formed under their conditions. For this and for many other reasons we67 consider that Wood and Lloyd’s radicals are Me3C*/hal- adducts formed by electron attachment followed by rearrangement Me2CHCH2Br+e-+Me2CHCH2/Br-(6) Me2CHCH2/Br-+Me3C*/Br-(7) Me3C*/Br-+Me3C. +Br-(8) It is significant that when their radical decays Me3C radicals are formed. Alkyl Radicals Planar or Non-planar?-The structure of the t-butyl radical has been much exposed to the slings and arrows of outrageous fortune the crucial question being is it planar or non-planar? I have long maintained that it must be effectively or chemically planar for a variety of reason^^-^^ one of which is the conclusion that *BH3- is planar.Nevertheless some have concluded that it has a nearly ‘tetrahedral’ geometry but that the barrier to inversion (ca. 2 kJ mol-’) is relatively This conclusion well supported by ~al~~lation~,~~*~~ is having repercussions an important example being the conclusion that ‘orbital following’ must be abandoned as a good chemical c~n-cept.~~ This concept which has been under fire from other quarters was discussed in Section 2. If as I still maintain Me3C* is effectively planar the keystone to this argument is removed and a very useful chemical concept is preserved.There may be times when there has to be a compromise between orbital following and structural resistance to changes in bond angles. An example is the phenyl radical. If the C-C-C angle remained at 120” orbital following would require a p :s ratio of ca. 2. If on the other hand this bond angle could open up towards that for vinyl (H2C=k ) the I3C isotropic ‘H 67 D. J. Nelson and M. C. R. Symons Tetrahedron Letters 1975 2953; I. G. Smith and M. C. R. Symons J.C.S. Perkin submitted for publication. ‘’T. A. Claxton E. Platt and M. C. R. Symons Mol. Phys. 1976,32 1321. J. B. Lisle T. F. Williams and D. E. Wood J. Amer. Chem. SOC.,1976 98 227. 70 P. J. Krusic and P. Meakin J.Amer. Chem. SOC.,1976 98 228. ” C. L. Reichel and J. M. McBride J. Amer. Chem. SOC., 1977,99,6758;J. M. McBride J.Amer. Chem. SOC., 1977 99 6760. M. C. R. Symons coupling would be ca. 107 G. In fact the coupling is ca. 135 G. My own guess would be that there has been a considerable degree of opening out but that orbital following is not complete in this case. Perhaps the only strong argument in favour of non-planarity for Me3C* radicals is that the isotropic 13C coupling constant displays a minimum on cooling. It is clear that the large changes originally are incorre~t,~*~~~ but nevertheless a minimum is still found even though the changes are small. (The total change is ca. 0.6 G.) This can still be fitted to a double minimum which requires non-planarity.However it now seems to be agreed that deviations from planarity are It must be remembered that although there is 'free' rotation of the methyl groups in Me&* at the temperatures used methyl group rotation takes on a tunnelling character at low temperatures. This together possibly with the umbrella bending mode of the CH groups may combine with the out-of-plane bending for an essentially planar radical in such a way as to give the anomalous temperature behaviour. Also the role of the environment must be considered. Minor deviations from planarity for electronic reasons implies a very subtle balance of forces for this radical; planarity requires no such fortuitous balance and hence is more probable. The change in Ais,is also very subtle and it seems to me to be dangerous to insist that the one establishes the other conclusively.It is interesting to consider the behaviour of (Me,N*)+ in this context. This ion should have a greater preference for planarity than Me3C-. Also medium effects should be greater because of the charge. Michaut and Roncin7? have studied this ion in a variety of environments and have observed almost no effect on Ai,(14N) from the media. They did observe a small temperature effect Aisobeing unchanged between 77 and 168 "C but falling slightly at higher temperatures. Theory predicts that Aimshould increase monotonically on heating. This serves to underline my conclusion that it is unsafe to explain subtle temperature effects rigidly in terms of a single theory.Certain cyclic radicals such as (16) do not appear to acquire planarity at the radical centre.73 This is thought to occur in part because the C(l)C(7)C(4) angle is ca. 90" this means that the 2s contribution is largely distributed between the orbital of the unpaired electron and the C-H cT-orbital. However charge transfer structures such as (17) are also thought to contribute to the marked tendency to bend which is indicated by the small a-H coupling of (-) 4.49 G. Alkane Radical Cations.-It is normally observed that alkyl radicals are formed directly after electron loss by alkanes. Indeed for methane in zeolites methyl radicals exhibiting a small extra doublet splitting are thought to be H3C- -H+ in '' D. Griller K. U. Ingold P. J. Krusic and H.Fischer J. Amer. Chem. SOC.,1978 100,6750. '2a J. P. Michaut and J. Ronchin Canad. J. Chem. 1977,55 3554. 73 Y. Sugiyama T. Kawamura and T. Yonezawa J.C.S. Chem. Comm. 1978,804. Electron Spin Resonance Spectroscopy 133 which the proton is bound to the In the absence of any proton acceptors one might hope to obtain the parent radical cations and it has been tentatively suggested that electron loss from Me3C-CMe gives a cation whose e.s.r. spectrum is remarkably (suspiciously?) close to that predicted for two Me3C* groups sharing a single If this is the cation and this remains to be proven then its structure can be simply represented as C-C 0,with a single electron largely confined to the central C-C bond. This work has subsequently been modified but the conclusions remain the same.Olefin radical cations are in contrast well known. A group of sterically hindered (‘persistent’) cations (R2CACR2)+ have been studied by e.s.r. spectroscopy where R = CH or CH2SiR3.76 These alkene cations are related to vinyl radicals thus CH2GCH2+ is the conjugate acid of CH,=C’H. Further loss of a proton gives H2C=C- which is isoelectronic with the well known and relatively stable radical H2C=N. This anion has recently been prepared from ethylene on magnesium It exhibits a large (58 G) coupling to the two protons as expected. Thus the four isoelectronic radicals H2C=C- H,C=N H2C=0)’ and H2B=0 are now established. cwcw-Dioxygen Substituted Alkyl Radicals.-Electronegative a-substituents encourage alkyl radicals to become pyramidal though the extent to which this is inductive or due to .rr-delocalization effects is a matter of controversy.Thus R2COH or R2CO- radicals may deviate slightly from planarity but (RC02)2- (RC(0R)O)-and RC(OR) radicals are certainly non-planar. These species can be prepared either by electron addition to carboxylates or carboxylic acids or by hydrogen atom abstraction from 1 1 diols or diethers. Thus (RO),CH radicals have variable positive a proton coupling constants in the region of 12 G and INDO calculations are in good accord with this.78 Similar coupling constants were observed for a-methyl protons but marked linewidth effects suggested that there must be a preferred orientation for the methyl group. This was confirmed by solid-state studies which showed that when rotation was slow one hydrogen gave a large coupling and the other two gave small couplings.Again INDO calculations reproduced this result most satisfactorily. Alkoxy-radicals.-It is curious that the first alkoxy-radicals to be detected by e.s.r. spectroscopy were RCH20 radicals formed in irradiated serine and in various nucleosides and nucleotides such as 3’-cytidylic acid 5-halodeoxyuridine adenosine hydrochloride and deoxyadenosine m~nohydrate.~~ As expected these radicals have large and variable values for gll and very large &proton hyperfine coupling constants. The Me00 radical has now been detected in irradiated methanol at 4.2 K.80 The proton coupling of 52 G and gil value of 2.088 are quite reasonable for 74 M.Shiotani F. Yuasa and J. Sohma J. Phys. Chem. 1976 79 2669. 75 M. C. R. Symons J.C.S. Chem. Comm. 1978,686. 76 H. Bock and W. Kaim Tetrahedron Letters 1977 27 2343. 77 M. C. R. Symons Y. B. Taarit and A. J. Tench J.C.S. Faraday Z 1977,73 1149. 78 C. Gaze and B. C. Gilbert J.C.S. Perkin ZZ 1977 1161; C. Gaze B. C. Gilbert and M. C. R. Symons ibid. 1978 235. 79 J. Y. Lee and H. C. Box J. Chem. Phys. 1973,59,2509; W. A. Bernard D. M. Close J. Huttermann and H. Zehner J. Chem. Phys. 1977.67 1211. *’ M. Iwasaki and K. Toriyama J. Amer. Chem. SOC.,1978,100 1964. 134 M. C.R. Symons MeO-. The spectrum in the parallel region actually comprised seven lines charac- teristic of a methyl group undergoing tunnelling rotation. This result finally settles a long-standing controversy regarding the intermediacy of MeO- radicals in the radiolysis of methanol.Sulphur Radicals.-Thiyl radicals RS. have been almost equally elusive as the alkoxy-radicals for the same reasons. It is now clear that they are usually formed in the radiolysis of RSH compounds,81 and that gll is again a function of the environment. Ag(gl1-2.0023) is larger than for RO* radicals partly because of the larger spin-orbit coupling constant for sulphur and partly because of the weaker hydrogen bonding that is reponsible for lifting the orbital degeneracy. When RSH compounds are irradiated in methanol glasses the g-values are independent of the group R and become characteristic of RS- radicals. RS* radicals react reversibly with RS- ions to give (RS-LSR)- u*radicals which are also formed by electron addition to disulphides.It is possible that they also react with R,S or RSH molecules to give weakly bound complexes RSLSR having properties similar to RS- radicals but g-values characteristic of the complex. (Species X in ref. 81 this species has often been wrongly identified as RS..) There is controversy over this assignment however Hadley and GordyS2 showed conclusively that radical X has two inequivalent sulphur atoms but they decided the species must be RSS. Unfortunately many of the e.s.r. parameters to be expected for these two structures turn out to be similar. Also since either theory requires considerable movement within the host crystals measured directions give no firm lead to identification.The RSiSR2 theory is more attractive chemically but the RSS-theory accommodates the absence of coupling to protons in the R-groups of the -SR2 half of the radical. Detailed arguments for and against both theories have been presented." The radicals F3CSLSR have been studied in the liquid phase,83 (R = alkyl). They have g,,=2.013 which is smaller than that for species X (ca. 2.028). Also proton coupling from the R-group protons in the region of 5 G was obtained. These results suggested that if X is indeed RSzSR2 then the spin must be more confined to the RS group and the u-bond must be weaker when R in RS is alkyl than when it is CF3. This is not unreasonable. Finally I should stress that polymer radicals have been very extensively studied by e.s.r.spectroscopy. No attempt is made to cover this field herein but the Reader is referred to a very thorough monograph on the 5 Organic Radicals Mechanism Mechanism is probed in two ways by the e.s.r. spectroscopist. On the one hand primary intermediates and subsequent radicals can be detected and identified. If they are too transient or their e.s.r. spectra too broad to detect they can be indirectly detected by the use of spin-traps which react to give radicals of long life. On the other hand the kinetic course of radical generation modification and loss can be D. J. Nelson R. L. Petersen and M. C. R. Symons J.C.S. Perkin 11 1977 2005. J. H. Hadley and W. Gordy Proc. Nat. Acad. Sci. U.S.A.,1974 71 3106. 83 J. R. M. Giles and B.P. Roberts J.C.S. Chem. Comm. 1978,623. 84 B. Ranby and J. F. Rabek 'ESR Spectroscopy in Polymer Research,' Springer-Verlag Berlin 1977. Electron Spin Resonance Spectroscopy 135 measured. For a very wide range of radicals in fluid solution life-times are diffusion controlled and hence rate measurements are not very informative. One well tried method of increasing life-times is then to use high viscosities another is to have suitable bulky substituents that sterically inhibit reaction. This latter method has been systematically exploited by Ingold and co-w~rkers.~~ They recommend that such protected radicals be described as ‘persistent’ and specifically not as ‘stabilized’. A symposium on organic radicals held at Aix-en-Provence has been published,86 and so have the proceedings of two conferences on Radiation Proces~es.~’*~~ Reviews by Sealy in Vols 4 and 5 of the Specialist Periodical Report ‘Electron Spin Resonance’ cover the field of recent liquid-phase organic mechanisms very fully.3 The CIDEP effect constitutes a powerful mechanistic probe this is discussed briefly below (Section 9).Hydrogen Atoms.-Reactions of hydrogen atoms have been widely studied especially in solid-state systems at very low temperatures. It is clear that hydrogen atoms are important intermediates in the radiolysis of alkanes but they generally abstract hydrogen so efficiently that they are not detected by e.s.r. spectroscopy. A penetrating experiment involved a 2 mol % solution of isobutene in neopentane at 77 K H-atoms added to the solute to give Me3C- radicals as well as reaction with the solvent.However at 4.2 K only neopentyl radicals were obtained at a concen- tration equal to the sum of the concentrations of the two 77 K radicals. It is suggested that hot Ha radicals are efficiently captured by R-H at 4.2 K but that at 77 K long-range reactions with the olefin occur. This was supported by linewidth and power saturation rnea~urements.~~ Further insight into the possible role of ‘hot’ hydrogen atoms comes from the photolytic studies of Kinugawa et aL90 When HI is photolysed in a xenon matrix in the presence of i-CSHI0 Hat and *C4H9 radicals are detected. On annealing to 77 K the H* atom signal was lost and that of C4H9 decayed greatly. However the same exposure at 77 K gave a high yield of C4H9 radicals far in excess of that obtained after annealing.This strongly suggests that ‘hot’ hydrogen atoms react at 77 K which must mean that their excess energy can survive migration through the xenon lattice. Incidentally multiple satellite lines from hyperfine coupling to ‘29Xe and 131Xe confirmed that the hydrogen atoms are trapped in a substitutional site in xenon.9o These workers have also contrasted the behaviour of hydrogen atoms generated by radiolysis of alkanes with those formed by HI photoly~is.~~ At 4.2 K alkane radiolysis gave H atoms that reacted with neighbouring solvent molecules. At 77 K they react more selectively as expected. However He from HI reacted more selectively even at 4.2 K. This result seems to imply that H atoms are ‘hotter’ in the photolysis experiments.Thus it seems that after being trapped ‘cold’ H atoms do not attack R-H molecules at 77 K or below but react preferentially with other radicals. However when not trapped they react with R-H molecules more efficiently at 4 K than at 77 K. This temperature effect D. Griller and K. U. Ingold Accounts Chem. Res. 1976,9 13. 86 M. C. R. Symons ‘Radicaux Libres Organiques,’ CNRS Edition 1978 p. 105. 87 S. P. Mishra and M. C. R. Symons Faraday Discuss. Chem. SOC., 1977,63 175. Proceedings of the Fourth Tihany Symposium on Radiation Chemistry ed. P. Hedvig and R. Schiller AkadCmiai Kiad6 Budapest 1977. M. Iwasaki K. Toriyama K. Nunome M. Fukaya and H. Muto J. Phys. Chem. 1977,81 1410.90 K. Kinugawa T. Miyazaki and H. Hase J. Phys. Chem. 1978,82 1697. 91 K. Kinugawa T. Miyazaki and H. Hase Radiation Phys. Chem. 1977 10 341. M. C. R. Symons may of course reflect the relative looseness of the medium at 77 K in effect the 'hot' atom may become trapped prior to loss of its excess energy at 4.2 K and hence have time to extract hydrogen from one of its cage molecules. Hydrogen addition radicals may be formed by direct reaction with H-or by electron addition followed by protonation. Generally the end products are the same however for pyrimidines it is postulated that H-addition occurs at C-5 [see structure (IS)] whilst protonation after electron addition occurs at C-6 [structure (19)]. Photolysis with visible light converts the C-5 radical into the C-6 radical.With adenine derivatives He addition is at C-8 protonation of the anion occurs at C-2. Again photolysis converts C-8 into C-Lg2 H H (18) (19) Electron Addition.-Mechanisms for electron addition have been discu~sed.*~~~~ Most alcohols provide efficient physical traps for electrons and when they do react it is generally supposed that they do so to give hydrogen atoms. However Me3COH gives Me3C- in good yield in solid-state radiolyses and by using MeOD in CD30D to slow up the rate of hydrogen extraction it has now been shown that *CH3 radicals are also formed from methan01:~~ CH30H+e-+.CH3+OH-(9) Also fluorinated alcohols react in the solid state to give dissociative electron capture products both by loss of F- but more significantly by loss of OH-.95 These reactions have never been found to be significant in pulse radiolysis studies of aqueous solutions.This may be because of the lower reactivity of aquated electrons compared with the 'dry' electrons thought to be responsible for the solid-state reactions. Organic sulphides also react by a dissociative electron capture process. It has been suggested that direct capture without dissociation can occur to give for example (RSH)- anions.96 Whilst this is possible my own opinion is that the species reported are RS. radicals since I would have expected much smaller g-shifts for the anions and also relatively large splittings from the S-H protons. Our own results" show that in frozen protic media the reaction RSH +e-+R.+SH-(10) is quite efficient. This process is no doubt aided by hydrogen bonding to the solvent which provides the required stabilization of SH-. In the particular case of peni- cillamine we found that in methanol at 77K HS-was lost and Me2CCH(NH3')C02-radicals were detected. This contrasts interestingly with 92 E. Westhof W. Flossmann H. Zehner and A. Miiller Faraday Discuss. Chem. SOC.,1977,63 248. 93 S. P. Mishra and M. C. R. Symons Faraday Discuss. Chem. Soc. 1977,63 175. 94 (a)M. C. R.Symons and K. V. S. Rao RadiationPhys. Chem. 1977,10,35;(6)M. C. R. Symons and G. W. Eastland J. Chem. Research 1977 (S) 147; (M),2901. 95 M. C. R. Symons J. Chem. Research 1978 (S),288; (M),3565. 96 J. H. Hadley and W. Gordy Proc. Nut. Acad. Sci. U.S.A.,1977 74 216.Electron Spin Resonance Spectroscopy 137 results for the pure compound at 4.2 K when electron addition to the C02-group occurred. Furthermore on annealing NH was lost rather than SH-.97 Isotope Effects in C-H Attack.-It is clear that radical attack on C-H bonds is far faster than that on comparable C-D bonds. For example the trapping of *CH3 radicals in CD30Dmentioned above does not occur in CH30Hbecause of efficient H-atom extraction. This effect has been examined quantitatively in CH,CN + CD3CN mixtures by S~rague,~~ who found that reaction (11)occurred far more efficiently than reaction (12) CH3 +CH3CN -+ CH4 +H2CCN (11) CD3+CD3CN -+ CD4 +D2CCN (12) The huge kinetic isotope effect of ca. 28 000 can only be explained in terms of a tunnelling mechanism.Photo1yses.-There is no doubt that the use of the solid state can be a great aid in the study of mechanisms and organic chemists are now rediscovering this fact. In the main radical intermediates are not involved. If however the substrate itself is a radical then e.s.r. spectroscopy can be a powerful tool in studying their photolytic responses. One recent example of this is the photolysis of radicals formed in y-irradiated esters at 77 K.99 Esters R'-COOR2 have been reported to give either R'-or R2*radicals after such treatment. By comparing the response of esters having combinations of R' and R2=CH3and C2H5,it was established that equations (13)-( 16)satisfactorily explain the results. 0 0-Y R-C // + R-C ./ +(RCOOCHXY)+ (13) 'OCHXY \OCHXY //O (RCOOCXY)+ -R-C + (H') (14) \OCXY 0-R-C ./ vis hv b RC02-+CHXY (15) 'OCHXY 0 280 <A420 nm R-C // b RCO+XYC=O (16) \OCXY R.1+CO Spin Trapping.-Direct detection of radicals in the liquid-phase by e.s.r.spec- troscopy as a mechanistic tool has its pitfalls since relatively stable or persistent radicals may accumulate and dominate the spectrum even when they are produced by some minor side route or even from fortuitous impurities and hence false deductions may be made. The use of spin traps also has many pitfalls for at least two reasons one is that the difference between two alternative spin-trapped radical spectra may be so small that false identifications are made. Another is that the rates 97 E.E. Budzinski and H. C. Box,J. Phys. Chem. 1971,75,2564. 98 E. D. Sprague J. Phys. Chem. 1977,81 516. 99 R. L. Hudson and F. Williams J. Phys. Chem. 1978 82 967. M. C.R. Symons of formation of trap adducts may vary greatly and hence an incorrect balance is produced sometimes to the virtual exclusion of significant radicals. The former problem can in principle be overcome by the use of liquid-phase ENDOR. This is an increasingly popular technique which has now been successfully applied to 'H 'H 13C 14N 19F 31P and 133Cs. The latter problem 7Li 23Na 8s'87Rb has been quantified in an important study by Schmid and Ingold,'OOwho report rate constants for the addition of primary alkyl radicals to most of the spin traps currently used.They used a cunning competition procedure using the [ l-13C]-5-hexanyl radical as a primary standard. This radical in part adds to the spin trap to give an e.s.r. spectrum with a 13C splitting and in part it undergoes cyclization to give the cyclopentylmethyl radical which on addition to the trap does not give a 13C splitting ("?+T -P *T(13CR) c +T + *T(R) The tabulated data should prove useful in the selection of suitable traps. Some conclusions are (i) Nitroso compounds react more rapidly than nitrones. (ii) Aromatic nitroso traps react faster than nitrosoalkanes unless there is strong steric hindrance. In fact nitrosodurene had the highest rate constant of all the traps studied. (iii) Aliphatic nitrones reacted faster than aromatic nitrones.In all cases the traps that react rapidly are the most effective. These rate constants for alkyl radicals are compared with those of other radicals such as those reported by Janzen and co-workers. A good example of the problems that can arise in the use of spin traps is contained in the controversy over the radiolysis of methanol mentioned above. Various workers have come to a diverse range of conclusions. These are discussed in refs. 94b and 101. Another example is in the radiolysis of t-butanol. Spin-trap studies revealed the formation of Me3CO- Me3C* Me* HOCMezCH2- and H* at 30 "C. However after exposure at 77 K only the adduct of HOCMe2CH2* was detected.lo2 In contrast the direct use of e.s.r. spectroscopy showed the formation of Me&* HOCMe2CH2* and Me3CO- at 77K and the growth of a signal from Me.on annealing.94 Finally I wish to mention an interesting use of spin traps in a study of the photo-Kolbe region lo3 RC02-5RC02. + R*+CO (19) loo P. Schmid and K. U. Ingold J. Amer. Chem. SOC.,1978,100,2493. lo' M.Shiotani S. Murabayashi and J. Sohrna Radiation Phys. Chem 1978 11 203. lo* S.P. Yarkov V. N. Belevskii V. E. Zubarev and L. T. Bugaenko Khirn. Vys. Energ. 1978,12,131. lo3 B. Krautler C. D. Jaeger and A. J. Bard J. Amer. Chem. SOC., 1978,100,4903. Electron Spin Resonance Spectroscopy The reaction is heterogeneous and occurs on an illuminated TiO electrode. Methyl radicals from CH,CO,- ions were clearly trapped with a nitrone trap when Ph3C-C02-ions were used Ph3C- radicals were detected directly.6 Biological Applications Introduction.-Growth of e.s.r. publications in many areas of biology has been outstanding in recent years. McConnell's invention of 'spin-labelling' with nitroxide radicals has blossomed into a technique that competes with the use of phos- phorescent labels as a method for probing specific environments and motions in bio-molecules and membranes. E.s.r. and ENDOR spectroscopy are powerful tools for identifying paramagnetic metal centres in proteins and for probing their environment. Also 'doping' with a paramagnetic metal ion can be useful and the study of lanthanum nicotinate dihydrate using Nd3'-proton double resonance mentioned in Section 3 promises to be the forerunner of many studies which will probe proton locations around the metal ion very precisely.In addition many organic molecules which participate in redox processes usually with semiquinone type structures are paramagnetic. Ionizing radiation is widely used to generate radicals in biological systems not only to discover the origins of radiation damage in living cells but also as a method of exploring local structure and for studying redox processes. Spin Labels.-This large amorphous field has recently been reviewed by Peake (ref. 3 Vol. 5). In principle the isotropic 14N hyperfine coupling and g, values give information about environment. This is because A( 14N) increases as R2N-0*-H hydrogen bonding increases and fortunately water induces a very large shift. However it is linewidth measurements that are most widely used.These are related via the g-and A-anisotropies to the mean correlation time of the nitroxide. Careful analysis may reveal that the tumbling is anisotropic and the appearance of x y and z features are indicative of highly restricted motion. The new technique of 'saturation transfer e.s.r.' is able to probe motion in the 10-4-10-7s range thus greatly extending the power of the spin-label technique.'04 In this method the dispersion signal is detected using high microwave powers and rapid modulation. The signal is detected 90"out of phase. If the radical moves during the modulation period this is picked up in the spectrum. Biological systems studied include (i) Lipids and membranes in which lipid organization and fluidity phase transitions and lipid-protein interactions have been studied.(ii) Drugs insofar as they affect membranes. (iii) Proteins including haemoglobin serum albumin ATPases dehydrogenases etc. (iv) Nucleotides and DNA. In some cases however it is not easy for the layman to understand to what extent the systems are really illuminated by these studies. '04 J. S. Hyde and L. R. Dalton Chem. Phys. Lerfers 1973,16,568; J. S. Hyde and D. D. Thomas AnniNew York Acad. Sci.,1972,222,680;D. D. Thomas L. R. Dalton and J. S. Hyde I. Chem. Phys. 1976,65 3006. M. C.R. Symons Photosynthesis.-Chloroplasts seem to comprise very highly organized ladder systems which electrons or holes climb or descend in order to acquire the correct energy to accomplish the overall photosynthetic process.E.s.r. and ENDOR have been used to study triplet-states and the parent cations and anions of chlorophyll. Thus for example the proton ENDOR spectra of the anion and cation radicals of bacteriochlorophyll have been studied by Fajer et al.loS The assignment of the various proton coupling constants was accomplished using model compounds partial deuteriation and theory. Organic Radicals.-Normal e.s.r. spectroscopy usually reveals only broad singlets for the organic radical species encountered in redox enzymes. ENDOR will undoubtedly prove to be of help in this area especially if 13C coupling can be used. A nice example of what can be done is the 13C ENDOR and electron nuclear-nuclear tripk resonance study of 13C-labelled galvinoxyl by Kurreck and co-workers.lo6 Metallo-Proteins.-The power of the ENDOR technique is well illustrated by the studies of Scholes and co-workers on haem derivatives in frozen organic media. Dipolar and quadrupolar hyperfine interactions of 14N in high-spin ferric proto- hemin dimethylesters were derived. Also both 'H and I4N ENDOR signals were obtained from low-spin ferric haemopr~tein."~ These and other data have been treated theoretically by Mun eta1.lo8 Thus the electronic structure of these systems is relatively well understood. Dioxygen derivatives of haemoglobin or myoglobin are low-spin and hence diamagnetic. At room temperature electron addition results in immediate decom- position. However at 77 K these molecules readily accept electrons generated by ionizing radiation."' The excess electron is largely confined to the (Fe-00) unit being extensively delocalized onto oxygen.Use of 170 revealed that the oxygen atoms are inequivalent. Two types of intermediate were detected one possibly being the protonated form (Fe-OOH). This decomposed on warming to give high-spin Fe"' together with HO,-. Single crystal studies of the myoglobin derivative gave a measure of the direction of tilt of the dioxygen ligand. The haemoglobin derivative gave separate signals from centres in the a-and &chains thus enabling us to study the effect of pH and inositol hexaphosphate on the relative electron affinities of the two chains. The most studied material containing cobalt is vitamin BI2,which in its reduced form gives a well defined e.s.r.spectrum for Co". An unusual spectrum has been detected during a variety of vitamin BI2enzyme reactions comprising a broad feature at g =2.3 and a pair of lines in the g =2 region with a 2 :1intensity distribution and separated by ca. 100 G."' Theie spectra were interpreted in terms of spin-spin lo' J. Fajer A. Forman M. S. Davis L. D. Spaulding D. C. Brune and R. H. Felton J. Amer. Chkm. Soc. 1977,99,4134;D. C. Borg A. Forman and J. Fajer ibid. 1976,98,6889. lo6 B. Kirste H. Kurreck W. Lubitz,and K. Schubert J. Amer. Chem. Soc. 1978 100 2292. lo' H. L. van Camp. C. P. Scholes and C. F. Mulks J. Amer. Chem. SOC.,1976,98,4094:C. P. Scholes and H. L. van Camp Biochim. Biophys. Actu 1976 434 290.log S. K. Mun J. C. Chang and T. P. Das Biochim. Biophys. Actu 1977 490 249. Io9 M. C. R. Symons and R. L. Petersen Proc. Roy. SOC.,1978 B201,285; Biochim. Biophys. Acta 1978 535 241; ibid. 1978 537 70. 'lo J. F. Boas P. R. Hicks J. R. Pilbrow andT. D. Smith J.C.S. Furuduy ZI 1978 417. Electron Spin Resonance Spectroscopy coupling between Co'' and substrate radicals thus leading to a measure of their separations. Copper-proteins and model systems have been extensively studied. E.s.r. spectra can distinguish between various types of Cu" one of which is clearly Cu" in normal square-planar sites and another in sites which are between square-planar and tetrahedral in nature. As this distortion occurs 4s-character is mixed into the formally d:2+ structure thus decreasing the magnitude of the isotropic hyperfine coupling to 63*65Cu.In some extreme cases such as one of the Cu" units in cytochrome c oxidase the copper hyperfine coupling is apparently too small to be resolved. This rather unprecedented situation which must reflect a very unusual environment for copper is unfortunate since one can no longer be quite sure that the signal is actually due to Cu". These units give rise to low wavelength bands of high extinction which make these proteins an intense blue colour. The Cu" has an unusually large positive redox potential and is readily converted into Cur. A variety of model compounds having sulphur and nitrogen ligands such as (20) have been synthesized in structures that endeavour to retain a 'tetrahedral' conformation.'" However the resulting data [e.g.for N-mercaptoacetyl-L-histidine-Cu" gll= 2.301 gl =2.069 A~I(~~CU) = 93 GI still fail to reproduce the small hyperfine coupling constants found in some natural systems.Beinert and co-workers112 have made a thorough study of cytochrome c oxidase which is a very high molecular weight protein whose structure is still poorly understood. They detected a rapid reduction process with electron-gain at a low-spin ferrihaem component and at two Cu'' centres that give no e.s.r. signal. This is followed by a slow further reduction in which a Fer1' high-spin feature at g = 6.2 appears which is rapidly lost when oxygen is added. An intriguing model to explain these and many other results has been put forward by Palmer et ai.'I3 A very interesting set of copper proteins are the oxygen-carrying haemocyanins.These bind oxygen in the ratio of one molecule to two copper atoms. The deoxy form contains two Cu' ions and it is thought that an oxygenation partial electron transfer occurs to give a derivative that can be depicted as Cu"--O-O--Cu". This derivative is devoid of ex. features and the two Cu'' ions are clearly strongly exchange coupled. Reaction with NO2-results in a mononuclear Cu" centre whose e.s.r. spectra establish a rhombic distortion and a poorly defined extra splitting assigned to hyperfine coupling to two I4N nuclei.'14 Y. Sugiura and Y. Harayama J. Amer. Chem. SOC.,1977,99 1581. C. R. Hartzell and H. Beinert Biochim. Biophys. Actu 1976,423,323; H.Beinert R. E. Hansen and C. R. Hartzell ibid. 1976 423 339. G. Palmer G. T. Babcock and L. E. Vickery Proc. Nut. Acud. Sci. U.S.A.,1976,73,2206. A. J. M. Schoot Uiterkamp H. van de Deen H. C. J. Berendsen and J. F. Boas Biochirn. Biophys. Actu 1974,372,407. M. C.R. Symons We have found that electron addition at low temperature yields a Cu" centre presumably by converting one of the Cu" ions into CU'.''~ This result confirms the presence of two coupled Cu" ions and also suggests that they are inequivalent or that electron addition causes marked distortion at one of the copper sites. Super- hyperfine structure on the parallel 63*65Cu hyperfine features was tentatively inter- preted as being due to the second copper nucleus rather than to 14N.Another important transition metal ion is Mn". This is thought to be involved for example in certain photosynthetic systems and in some forms of superoxide dismutase. Also Mn" can often replace Mg2+ without causing loss of activity and hence e.s.r. spectroscopy can be used to probe binding sites. Proton n.m.r. (and presumably ENDOR) can be used to estimate the detailed proton distribution around the Mn" ions. Another important trace metal is molybdenum. Bray who has played a leading role in studying this element has written an extensive review that accentuates e.s.r. studies."6 He and others have developed a rapid-freeze flow system in which aqueous suspensions of the enzymes are mixed with substrate or chemical reducing agents followed immediately by freezing.Using xanthine oxidase from bovine milk they detect e.s.r. signals from MoV in various environments from two types of Fe-S units and from FADH- radicals. One of the MoV centres is distinctive in having a well defined coupling to a single proton (ca. 10 G). ENDOR studies support the concept that the molybdenum is close enough to one of the Fe-S units to give a weak spin-spin coupling."' By the use of redox mediators the mid-point potentials for the centres involved in redox processes were obtained:"* Fe-S centre I = 343 mV; Fe-S centre I1 = 303 mV; FAD-FADH- = 35 1mV; FADH-FADH2 = 236 mV; MO~I-MO~ = 355 mV; MO~-MO'~ = 355 mV. The method of low temperature radiolysis has been applied to xanthine oxidase and these same centres have been detected on electron capture."' In particular a MoV centre known as 'very rapid' 'l5 was found to be converted into the 'rapid' centre showing 'H hyperfine coupling on slight annealing above 77 K.The first centre has one g-value >2.0023 whilst the second has only low g-values as expected for a d' system. It was suggested that the former has some delocalization onto one sulphur ligand and that the rapid change is due to protonation of this ligand. This would reduce the spin-density on sulphur and lift the v-orbital degeneracy thus removing the positive contribution to the g-shift. It also accounts for the unique proton hyperfine coupling. This study reveals aspects of the kinetic control of electron transport thus both the MoV and Fe-S' centres transferred their electrons to the Fe-S" centre on annealing.Radiation Processes.-As stressed elsewhere there have been a large number of e.s.r. studies of single crystals and glassy solutions of various simple organic compounds that are building blocks for biopolymers. Single crystal studies are of course more informative but for some systems aqueous glasses may more closely 11' M. C. R. Symons and R. L. Petersen Biochim. Biophys. Acta 1978,535,247. R. C. Bray in 'The Enzymes' ed. P. Boyer Academic Press New York 3rd. Edition 1975 Vol. XII p. 299. 11' D. J. Lowe and J. S. Hyde Biochim. Biophys. Acta 1975 377 205. R. Cammack M. J. Barber and R. C. Bray Biochem. J. 1976,157,469. '19 M. C. R. Symons and R. L. Petersen J. Chem. Research 1978 (S) 382; (M),4549.Electron Spin Resonance Spectroscopy approach the environment to be expected in living cells. Thus both types of study are valid from a biological stance. The systems that are fruitfully studied are gradually becoming more complex. Many excellent examples are given in the recent book on radiation effects in DNA,120and in Box's book.57 I have decided to illustrate the field and the problems that arise by describing one arbitrarily selected study in some depth. Single crystals of 6-methylmercaptopurine riboside (21) were irradiated at 77K.12* One major product was formed by loss of hydrogen from methyl (H,CSR). H' This is probably not a primary product. Two sulphur-centred radicals R3 and R5 were detected. Also an alkoxy-radical R2CH0,with a 41.5 G proton coupling was detected.This is interesting since in previous studies of nucleosides and nucleotides RCH20 radicals were invariably formed. The structure is probably (22). s The sulphur radical R5 is clearly an RS. radical [g-values 2.154,2.003 1.9891. This is unlikely to be CH3S*since no 'H hyperfine coupling was detected and hence it was probably formed by loss of methyl (23). This is not a simple thiyl radical since the degeneracy of the 3p orbitals on sulphur should be lifted by winteraction with the ring. The other sulphur radical R3 had g-values of 2.0608,2.0211 and 2.004 which are quite close to those for radicals described above as X.*l The favoured structure for R3 was RSS*,where R is the purine group. As discussed above this is one of the alternatives for X the other being the (+* species RSI-SR,.It is interesting that in formulating a mechanism for the formation of RSS. Sagstuen and Alexander suggested the following reactions RSCH3 + R*+.SCH3 *SCH3+RSCH3 -* CH3S'S(R)CH3 CH3SLS(R)CH3 + RSS*+C2H6 (22) 120 'Effects of Ionizing Radiation on DNA' ed. J. Bertinchamps Springer-Verlag Berlin 1978. 121 E. Sagstuen and C. Alexander J. Chem. Phys. 1978,68,762,and references therein. M. C. R. Symons This is of course possible but nevertheless I find it surprising that loss of C2H6 should be ready at 77 K. The intermediate CH3S-Y3(R)CH3 certainly ought to exhibit hyperfine coupling to the methyl protons in fact another radical was detected with g, =2.060 and a 1:3 :3 :1proton interaction of ca.9.5 G splitting. This could of course quite well be CH3SS*. Equally radical R3 with no proton splitting could possibly be (CH3),S4R so nothing definitive has been established. I have not begun to do justice to this rapidly growing field of biochemistry. Reviews in ref. 3 Vols. 14,which cover this area in great depth should be consulted for further information. 7 Aspects of Solvation Introduction.-One of the great achievements of this technique has been the light that it has shed upon ion-pairing and ion-clustering in low dielectric solvents. The huge activity in this field during the 1960's which went hand-in-glove with the study of aromatic radical anions is now over. The small numbers of papers still produced in this area are focusing attention on the rather neglected thermodynamic parameters that can be derived from temperature studies and on their relationship with parameters deduced by the use of more conventional electrochemical tech- niques.This field has recently been reviewed.122 Ion-Pair Formation.-Hirota and co-workers continue to publish definitive work in this field. A recent example of their work is on the system 2,5-di-t-butylbenzo- quinone; Na+; propan-2-01; tetrahydrofuran (THF).'23 AGO for the propan-2-01 solvation process was -2.27 kcall mol-' at 25 "C and the forward rate constant was >3.5 x lo81mol-' s-' at -40 "C. The rate of cation migration from one oxygen site to the other was enhanced by the solvating alcohol molecule mainly because of a reduction in the activation energy.Further addition of alcohol caused the ion-pair to dissociate. Of the two reasonable mono-solvate structures (24) and (25) the ex. data clearly favour (25) since A('H)6 increases and A('H) decreases making the :.-M 0 X IHOR M+ 0-.c" 0 0.. .HOR two proton coupling constants approach each other. (This change would also result if the alcohol solvated M' but this is unlikely since the cation but not the anion is already strongly solvated by THF.) It might be supposed that since the alcohol solvation needs to be switched when the cation moves for structure (25) the reaction would be slower than in pure THF. However the rates of gain and loss of ROH are lZ2 M. C. R. Symons Pure and Appl.Chem. 1977,49 13. lZ3 K. S. Chen and N. Hirota J. Phys. Chem. 1978,82 1133. Electron Spin Resonance Spectroscopy large compared with the cation migration rate. Solvation of the ‘free’ oxygen site removes negative charge from the cation-site thus reducing the M+ --O-bonding and encouraging cation loss. Hence the change is in the enthalpy term rather than the entropy term. Clearly careful studies of this type continue to shed detailed light on ion-pair formation and solvation. Solvation of Neutral Radicals.-Although nitroxides are used widely as environ- mental probes in biological systems they have received far less attention from solvation chemists. They are in effect basic aprotic molecules with a solution ability somewhere between ketones and dimethylsulphoxide.The advantage of e.s.r. spectroscopy is that it can monitor nitroxide probes at very low concentrations the disadvantageis that in mixed media the nitroxide is sampling all modes of solvation so rapidly that the e.s.r. spectrum is on the fast exchange extreme. In this sense i.r. spectroscopy is a more powerful technique. A comprehensive study of (Me3C)2N0 in a range of mixed solvents and in the presence of a variety of electrolytes has given some insight into the factors that control the I4N hyperfine coupling and the 1ine~idths.l~~ A key result was that in most systems changes in linewidths were well correlated with changes in A(14N). Differential broadening connected with the anisotropy in the g-and A-tensors was generally insignificant at room temperature but was greatly enhanced on cooling aqueous solutions especially in the presence of certain additives.For aqueous systems changes in Aiso were dominated by the equilibrium R,NO--HOH+B $ R2NO+B--HOH (23) the role of the basic co-solvent being to desolvate the nitroxide. The linewidths which were dominated by spin-rotational relaxation increased as A(I4N)fell that is as the extent of hydrogen bonding fell. This was interpreted to mean that only the non-hydrogen bonded radicals were sufficiently free to contribute significantly to this width. Electrolytes made two major contributions they shifted equilibrium (23) and multivalent cations co-ordinated to the oxygen or to solvent molecules hydro- gen-bonded to the nitroxide thereby increasing the bond-strength and hence A(14N).For R4N+salts the role of the anions was the same as B in equation (23) desolvation resulting in rapid fall in A(14N)and a concomitant increase in linewidth. In a most interesting study Griller has investigated the effect of pressure on the solution spectra of nitroxide radicals. 12’ The result for solutions in cyclopentane was dA(14N)/dV= -0.5 1f0.04 G/L. It would be useful to know the gas-phase value for A(14N) in this context. Perhaps the most important finding is that the increase in A(14N) observed on cooling solutions in cyclopentane is almost entirely due to the contraction in volume the temperature effect at constant volume being negligible. Surprisingly this was also true for solutions in ethanol.In this case I would have expected an equilibrium involving free and hydrogen bonded nitroxide to occur and be temperature sensitive. The lack of a real temperature effect suggests that effectively all the nitroxide molecules are hydrogen bonded. Y. Y.Lim E. A. Smith and M. C. R. Symons J.C.S. Faraday I 1976,72,2876; S. E. Jackson E. A. Smith and M. C. R. Symons Faraday Discuss. Chem. SOC.,1978,64 173. ’*’ D. Griller J. Amer. Chem. Soc. 1978 100 5240. M. C.R. Symons 8 Triplet-State Species Introduction.-Ground-state triplet species are generally studied by e.s.r. spec- troscopy but excited-state triplets are being increasingly studied by optically- detected double-resonance techniques (PMDR and ODMR). The former have been reviewed by Wasserman and Hutton,126 and the latter by Dobkowski et aZ.127 Important reviews are also to be found in Vols.4 and 5 of ref. 3. Photoexcited Triplets.-This is a field of great activity and considerable complexity. A lot of studies have their inspiration in the desire to understand subsequent chemistry and in particular CIDEP effects (Section 9). Zero-field splitting parameters together often with hyperfine coupling constants give great insight into the distribution of the magnetic electrons. Also life-times and rate-constants for energy transfer can be measured and interpreted. The 3Blustate of benzene continues to excite interest because of its pseudo Jahn-Teller instability and consequent marked sensitivity to small environmental changes.12' Thus for example (3~1,)C6~6 in C6H6 having a single C6D6 neighbour has a much lower value for E than has (3B1u)C6H6 doped into C6D6. Triplet-states in charge-transfer complexes are also of interest. Also porphins of various structures are widely studied because of their biological significance chlorophyll being a prime example. Ground-State Triplets.-Following the discovery by Wasserman and co-workers that carbenes and nitrenes can be prepared in low-temperature matrices and studied by e.s.r. spectroscopy the methylene molecule was sought and ultimately detected. Its ground-state structure is as a consequence remarkably well understood and theoretical agreement with the data is excellent. Recent interest has been concerned with the motional behaviour of CH2 and CD2 at very low temperature^.^^^ It is concluded that CH can jump to a new orientation with a much lower activation energy (14 cm-l) than can CD (30 cm-l).This is thought to be a consequence of its higher zero-point energy. Trimethylene methane (26) and its methyl substituted nitrogen analogue (27)have the properties expected for the structures shown except that the former apparently exhibits a small x -y splitting (non-zero E term) at relatively high temperatures which is absent at low temperature. The origin of the effect is not yet fully underst~od,'~~ but may be due to libratory motion. lZ6 E. Wassermann and R. S. Hutton Accounts Chem. Res. 1977,10 27. J. Dobkowski J. Herbich and B. Kozankiewiez Wiad.Chem. 1977 31 181. 12* Ph.J. Gergragt and J. H. van de Waals Mol. Phys. 1977,33,1507;Ph. J.Gergragt J. AKoote and J. H. van de Waals ibid. 1977,33,1523; R. L. Christensen and J. H. van de Waals Chem. Phys. Letters 1977 45,221. lZ9 R. A. Bernheim and S. H. Chien J. Chem. Phys. 1977,66,5703. 130 P. Dowd and M. Chow J. Amer. Chem. Soc. 1977,99,2825. Electron Spin Resonance Spectroscopy 147 It is interesting that Me3SiCH seems to be linear in contrast with CH2.I3' This may be due to 3d orbital participation as suggested but it may also be simply an electronegativity effect. The phenomenon is comparable with that which makes CH2=CH bent at the radical centre but CH2=C-SiMe 1'inear. There has long been controversy regarding the possibility that phenyl cations might be ground-state triplets.The 'vacant orbital' of the singlet structure can accept an electron from the v-system to give (28),which could well be more stable. This stability could be enhanced by ?r-electron substituents in the 0-and p-positions and such triplet-cations have been prepared by photolysis of the diazonium cations132 (i.e. 29). -_-.-(+ ON* I. Radical-Pairs.-It is now quite common for solid-state e.s.r. spectroscopists to detect features from pair-trapped radicals in effective triplet-states in the wings of the main features from doublet state species. Indeed care must be taken not to confuse such features with for example features from radicals containing low- abundant magnetic nuclei such as 33S. That such pairs should be common in photolyses is clear especially for reactions such as hu R-N=N-R R*/N*/R.(24) If excited states are formed in radiolyses the same processes can occur but it seems clear that pairs are also formed via electron-loss and electron capture. An example is the formation of pairs in irradiated benzene naphthalene and anth~acene.'~~ 9 Chemically Induced Dynamic Electron Polarization (CIDEP) Introduction.-This phenomenon involving e.s.r. spectra having features in emis- sion or in enhanced absorption is the analogue of the better known n.m.r. phenomenon CIDNP. The effect was discovered some 16 years ago but it excited curiously little attention until recently. Then a burst of activity in a small number of laboratories has resulted in a number of experimental and theoretical studies and a remarkably large number of review articles.Indeed almost all the practitioners in this field have written one or more reviews. The latest by Hore Joslin and McLauchlan in ref. 3 volume 5 is thorough and very clear and is strongly recommended to those wishing to understand this phenomenon. Both the e.s.r. and n.m.r. effects stem from the fact that at equilibrium the separate levels involved in the resonance are almost equally populated. Thus even minor changes in these populations can cause huge changes in intensity. In the n.m.r. 131 M. R. Chedekel M. Skoglund R. L. Kruger and H. Schechter J. Amer. Chem. SOC.,1976,98,7846. 13' A.Cox T. J. Kemp D. R. Payne M. C. R. Symons and P. P. de Moira J. Amer. Chem. SOC.,1978,100 4779.133 T.Matzuyama and H. Yamaoka J. Chem. Phys. 1978,68,331. M. C. R. Symons field nuclei take a long time to reach thermal equilibrium and the experimentalist has plenty of time to pick up the unusual polarizations that occur when the molecules are created from radicals. To observe CIDEP however time-resolved techniques must be used because decay is governed by the electron spin-lattice relaxation time which is only a few microseconds. The effect stems from pair-wise interactions of radicals to give triplet states and an understanding of triplet states is essential for an understanding of CIDEP effects. Two mechanisms have now been distinguished for the acquisition of spin polariza- tion the triplet mechanism (TM) and the radical-pair mechanism (RPM).In the former polarization originates in triplet-state molecules which react to give radicals. It is therefore the dominant mechanism in most photolyses and is only observed in such systems. It can give rise to very large enhancements of populations relative to equilibrium populations. The latter (RPM) depends on the relative rates at which radicals in various spin-states react together. In general these differ thus depleting certain states faster than others so that signals from the remaining radicals are polarized. Enhancements by this mechanism are generally much smaller. It applies to thermal redox and radiolytic generation of radicals. CIDEP effects can yield rate constants for radical and triplet reactions and also spin-lattice relaxation times for both radicals and triplet precursors.It also provides a type of label for following reactions since it can be carried through from one radical to another (and eventually to their non-radical products via CIDNP). The Triplet Mechanism.-When excited singlet molecules change to the triplet-state by intersystem crossing some levels are populated more than others. Which levels are favoured depends upon the molecular symmetry for the much studied carbonyl compounds T (TIJ is strongly favoured. When this occurs in the magnetic field used in the resonance experiment this becomes predominantly 1+1)if D < 0 or 1-1) if D > 0. For ketones D is positive and the signals are in emission. Polarization is at a maximum when ID1 = wo,where wois the resonant frequency.It falls to zero at the low and high field limits. In order that this triplet polarization be carried through to radicals the reaction must be fast enough to compete with relaxation of the triplet to its thermodynamic populations. Since this is rapid only very fast reactions will exhibit CIDEP. It is important to note that by this mechanism the polarization should be independent of the nuclear spin-states so that all components of a hyperfine multiplet have the same polarization. The situation envisaged is crudely summarized in Figure 2. Radical-Pair Mechanism.-This mechanism can operate whatever the method of generation of radicals and may often occur together with the TM. However polarization from the TM is usually so much greater that this dominates in photo- chemical processes involving triplet molecules.Generally a key step is mixing of singlet and triplet states of pairs of radicals by the S + Toroute in a magnetic field though interchange with T, and T- can sometimes contribute. The next key step is that only pairs in singlet combined states can react together thus generally the [To]> [S]. The S $ Tointerchange is fastest when the effective g-values have greatest separation. It is important to realise that there is now no overall spin polarization but only a separation or sorting of spins. The result is that one component may be in emission but another will then be in enhanced absorption Electron Spin Resonance Spectroscopy )Singlet molecule +Tr ip1e t molecule -t-Radi -1s1ca I ...x ; I -I> 01 zero field in magnetic field Rate -10’ s-l Rate !-lo6 s-’ Figure 2 A summary of the triplet mechanism (TM) giving no net gain or loss. For two radicals having singlet spectra one line will be in emission and the other in absorption depending on the sign of Ag. However if lines are separated by say a doublet hyperfine splitting then for a given radical one feature will usually be in emission and the other in absorption. Simple rules have been derived to show which way these polarizations will occur. The underlying theory is complicated since initial encounters and re-encounters need to be consi- dered magnetic interactions being important at relatively large separations and chemical interactions depending on close encounters.Recent theories use the Stochastic Liouville equation to include magnetic interactions and relative radical m0~ements.l~~ The scheme in Figure 3 is an attempt to summarize the RPM. S-Reaction S-Reaction 1 Separated m n Pair $--To Splitting may be Sor 1 or random 1 Separated Polarized Radicals Figure 3 A scheme for the radical-pair mechanism (RPM) Experimental Studies.-Much effort has gone into the development of radical generation techniques that will give optimum detection of CIDEP. Also many experiments have been designed to test rival theories rather than to use the phenomenon as a mechanistic probe. Three types of experiment are conducted; time resolved with rapid response time resolved with slow response and continuous flow 134 See for example J.B. Pedersen and J. H. Freed J. Chern. Phys. 1974.61 1517. 150 M. C.R. Symons systems with continuous monitoring systems. In particular pulsed laser photolysis gives intense TM polarizati~n,'~~ whereas pulse radiolysis gives polarization by RPM.'36 Some negative results reported by Verma and Fessenden are particularly reveal- ing.137 They found that eiq has zero magnetization (N = No)at birth. The growth of magnetization depended upon the nature of the other radicals showing that cross relaxation is important. Radicals formed from e also had zero initial magnetiza- tion. In complete contrast radicals formed from -OH had equilibrium magnetiza- tions showing that TI (*OH)< 1ns. This result is significant since it is diagnostic of formation via *OH which because of its almost unique orbital degeneracy relaxes exceptionally rapidly and is not seen by liquid-phase e.s.r.spectroscopy. When secondary radicals exhibit CIDEP this can be used as a subtle probe of mechanism. The key point is that spin transfer cannot affect polarization and hence one can argue back to the initial polarization state. Thus the third important species in the radiolysis of water the hydrogen atom is polarized and this can be passed on to reaction products. 137 Interestingly the secondary radical H,CCO,H formed by attack of H* atoms in aqueous acetic acid solutions showed ST- polarization. 13' A detailed study of the biacetyl radical anion has been published which nicely illustrates the possibilities of polarization tran~fer.'~~ Triplet Ph2C0 was generated by flash photolysis and this extracted hydrogen from Et3N to give polarized MeCH- NEt, which however was not detected.This then generated the biacetyl radical anion whose polarization was studied. Hence it was deduced that the polarization in MeCHNEt was equal within experimental error to that of Ph260H as required by the triplet mechanism. In taking this chain of events one step further Trifunac and Nelson have shown that the CIDNP effect observed from the final product may originate from electron- nuclear cross re1a~ation.l~' This is the Overhauser CIDNP mechanism that was originally proposed by Bargon and Fischer and it is satisfying to know after all that it can be significant.The system studied by pulse radiolysis is summarized in equation (25) When radicals such as C03- PO3,- or PhO. are present the RP mechanism generates eiqin an emissive state. This polarization is passkd on to *CH,CO,-which also has a spectrum predominantly in the emissive state. No normal CIDNP can originate from the [*CH2C02- -*CH,CO,-] pair and hence the strongly emissive feature obtained from succinate under these conditions can only have arisen via 135 See for example P. W. Atkins. A. J. Dobbs G. T. Evans K. A. McLauchlan and P. W. Percival Mol. Phys. 1974 27,769. 136 See for example A. D. Trifunac K. W. Johnson B. E. Clifft and R. H. Lowers Chem. Phys. Letters 1975 35 566. '37 N. C. Verma and R. W. Fessenden I. Chem.Phys. 1976,65,2139. 13* A. D. Trifunac and D. J. Nelson J. Amer. Chem. Soc. 1977,99,289. 139 K. A. McLauchlan R. C. Sealy and J. M. Wittmann J.C.S. Faraday I 1977 73,926. 140 A. D. Trifunac and D. J. Nelson J. Amer. Chem. Soc. 1978,100 5244. Electron Spin Resonance Spectroscopy cross polarization (dipolar) between the polarized electron and the nuclei in the acetate radical. 10 Radicals in the Gas Phase Introduction.-Except for atoms and a few molecular radical^,^ e.s.r. spectrometers cannot be used to study radicals in the gas phase. This arises primarily because very low pressures are needed to avoid line-broadening from rapid interconversion of rotational states which couple strongly with spin-states and it is then impossible to generate sufficient concentrations for detection.Important new techniques have been recently developed which overcome this problem and results are now being published which hold out the possibility that highly accurate magnetic and rotational data for many simple neutral radicals will soon be forthcoming. New Techniques.-Probably the most important is known as laser magnetic resonance (LMR). These spectrometers use lasers emitting in the 30-120 cm-' range and radicals can be generated within the laser cavity using a flow system. A magnetic field is then used to bring rotational transitions of the radicals into resonance with the laser beam. The sensitivity is extremely high because of the relatively large population differences. The results can give all the components of the hyperfine interactions as well as the geometries and distortion constants of the radicals.Radicals so far studied include 02,NO NO2,OH,CH HO2 NH2 and PH2. Carrington and co-workers have recently studied HCO and DCO by this technique as well as'by their microwave-Zeeman (e.s.r.) rneth~d.'~' Most parameters for this important radical including the full 'Hhyperfine tensor are now well established. There are of course alternatives to direct observation but these cannot give the wealth of information that these elegant studies provide. One is matrix isolation which has been used for many years to trap out gas-phase radicals. The other is spin-trapping which may be of use when all that is needed is an estimate of the chemical nature of the radicals and possibly a measure of their relative concen- trations.A recent example of this application is given in ref. 142. 14' J. M. Brown J. Buttershaw A. Carrington and C. R. Parent Mol. Phys. 1977,33,589;B. J. Boland J. M. Brown A. Carrington and A. C. Nelson Proc. Roy. SOC.,1978 A360,507. 14* D. B. Hibbert A. J. B. Robertson and M. J. Perkins J.C.S.Faraday I 1977 1499.

ISSN:0308-6003    

DOI:10.1039/PR9787500117

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

PART II INORGANIC CHEMISTRY 7 Introduction By D. C. BRADLEY Department of Chemistry Queen Mary College Mile End Road London El 4NS I begin this report by paying tribute to my predecessor as Senior Reporter Professor Michael Lappert and his team of Reporters for their valuable contributions during the previous three years. Also I wish to explain the change in format of this year’s Report and the coverage of the literature which this entails. Following the demise of some of the Specialist Periodical Reports concerning Inorganic chemistry it was clear that changes in the Annual Reports would be required to remedy the ensuing lack of coverage of the Inorganic field. Accordingly the Publications Board of the Chemical Society decided to expand the coverage of Inorganic chemistry in Section A beginning with the 1978 Report and this has led to a reappraisal of the content and depth of treatment of the subject.In a sense it means “putting the clock back” towards the type of treatment given before the advent of the various Specialist Periodical Reports. This year’s Report is thus a transitional stage and we crave the readers’ indulgence to any imperfections which arise and promise to do better next year. The field is divided broadly into three Sections dealing with Typical Elements (s-and p-blocks) Transition Elements (d-and f-blocks) and Organometallic Chemistry. There is inevitably and I trust justifiably some duplication of material between these Sections; this is particularly so for organometallic compounds where the same compound could be featured both in Section 3 and in the appropriate earlier Section 1or 2 pertaining to the metal involved.There will be differences in emphasis on this and other matters by the different Reporters. Looking at the overall picture presented by this year’s Report on inorganic chemistry one sees a continuation of the rapid progress in several areas which have developed in recent years; cluster molecules of various kinds involving all types of elements (e.g. polyboranes,. carbaboranes metallo-carbaboranes poly-nuclear metal carbonyls metal-sulphur clusters poly-sulphur and -selenium cations etc.); organometallic chemistry on a wide front with ever more exotic types of metal- carbon bonding; bio-inorganic chemistry; macrocyclic multidentate ligands; metal- metal multiple bonding; and the synthesis of reactive or unstable species using metal atom matrix-isolation techniques at very low temperatures.X-ray crystallography plays an increasingly important role in structural inorganic chemistry and where suitable crystals are not available the developing technique of XAFS may provide vital structural information. Laser Raman (including resonance Raman) poly- nuclear FT-n.m.r. and photon electron spectroscopic studies are also increasingly being applied to gain insight into the constitution of inorganic molecules. 155 156 D. C.Bradley Among the books published in 1978 the monographs on phosphorus’ and gold2 were referred to in last year’s Report. Monographs on Metal Alkoxides3 and Metal P-Diketonates4 have also been published. D. E. C. Corbridge ‘Phosphorus an Outline of its Chemistry Biochemistry and Technology’ Elsevier Amsterdam 1978. R. J. Puddephatt ‘The Chemistry of Gold’ Elsevier Amsterdam 1978. D. C. Bradley R. C. Mehrotra and D. P. Gaur ‘Metal Alkoxides’ Academic Press London 1978. R. C. Mehrotra R. Bohra and D. P. Gaur ‘Metal 0-Diketonates’ Academic Press London 1978.

ISSN:0308-6003    

DOI:10.1039/PR9787500153

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

The Typical Elements This chapter is primarily concerned with the chemistry of the s-block and the p-block elements. For convenience it is divided up into four Parts which are dealt with by different Reporters. By F. A. HART Depaflment of Chemistry Queen Mary College Mile End Rd London El 4NS A. G. MASSEY Department of Chemistry University of Technology Loughborough Leicestershire LEI 13TU P. G. HARRISON Department of Chemistry University of Nottingham University Park Nottingham NG7 2RD J. H. HOLLOWAY Department of Chemistry Leicester University University Rd. Leicester LEI 7RH PART I Groups I IIA and IIB By F. A. HART 1 Group1 Soft-sphere ionic radii give internuclear distances in Group I and I1 halides which agree with experiment to 0.003 A.This is a considerable improvement over the conventional hard-sphere model. The internuclear Idistaye 4 is related to the cationic (M) and anionic (X)soft-sphere radii by d' =M3+X3.The soft-sphere radii for Group I and I1 metal ions are identical with metallic radii for 12-co- ordination.' The infrared spectra of Na K Rb and Cs halides condensed together with H,O or NH3 in an Ar matrix are consistent with C3,M...NH and with pyramidal Me -.OH,. In the latter case only there is H bonding to the The He(r) photoelectron spectra have been obtained of K Rb and Cs nitrates as high-temperature vapour~.~ An X-ray structure of an interesting oxide Rb7CsI1O3 shows it to contain CsI1O3 clusters (Cs as all-face-capped trigonal prism with three 0 atoms in side-face- centering positions) arranged in columns with Rb in close-packed wavy sheets * J.B.Holbrook F. M. Khaled and B. C. Smith J.C.S. Dalton 1978 1631. * B. S. Ault J. Amer. Chem. SOC.,1978 100,2426. 'B. S. Ault J. Amer. Chem. SOC.,1978 100 5773. J. D. Allen and G. K. Schweitzer Inorg. Chem. 1978,17 3418. 157 F. A. Hart A. G. Massey P. G. Harrison and J. H. Holloway between them. Within the clusters the interatomic distances are ionic but elsewhere the distances are metallic.' LizNCN crystallizes from molten lithium after the latter has been treated with N2 and Li2C2 under an Ar atmosphere at 500 'C for 150 hours. The structure (X-ray) of the resulting product shows linear NCNZ- ions approximately tetrahedrally disposed about Li' The phenyl-bridged dimer Li2Phz(MezNCHzCH2NMez)2 has been prepared and the structure (X-ray) shows that each Li is approximately tetrahedrally co-ordinated to a bidentate amine and to one carbon atom from each of two bridging phenyl groups (Li-C-Li = 67.4'; Li-C = 2.208,2.278 A).Each phenyl group lies with its plane approximately perpendicular to the Li-Li direction.8 The majority of papers dealing with Group I metals this year are concerned with their complexes with polyethers and the rest of the report deals with this topic. These ligands may be cyclic polyethers (crown ethers) non-cyclic polyethers often contain- ing other functional groups or polycyclic polyamine-polyethers which encapsulate the metal ion (cryptate ligands). Two short reviews of cryptate ligands and their complexes with Group I (and other metal ions) have appeared.'.'' Several papers deal with X-ray crystal structures of these complexes.The metal ion may or may not lie in the plane of a crown ether usually depending on the size of the metal ion compared with the ring size. In the case of the NH4+ ion (hereby made an honorary member of Group I) it lies 1.0 8 above the ring mean plane in [NH4Br( 18-crown-6)],2Hz0. Three N-H. -0 hydrogen bonds are directed downwards to three ring oxygen atoms (N to 0= 2.884,2.857 A). A Br at 2.438 8 makes up the hexagonal pyramidal co-ordination." In two Rb crown ether complexes the metal ion is also above the mean plane. In Rb2(NCS)2 (monobenzo- 18-crown-6)z each Rb is 1.24 8 above the mean plane (mean Rb-0,3.02 A) and is also bonded to two bridging N atoms from the thiocyanate ions (Rb-N=3.04 3.05 A) giving eight-co-ordination.'2 However in Rbz(NCS)z (4-nitrobenzo-18- cr0wn-6)~ where the Rb is also above the mean plane the bridging is performed by 0 atoms from the nitro groups the NCS groups being bonded (by N) to only one Rb.13 The crystal structures of the NaNCS and KNCS complexes of an open-chain polyether diol L where L = (HOC2H40-o -C6H40CH2)2 show seven-co-ordination (60+N) for the sodium complex [Na(NCS)L] but ten co-ordination (to ten 0)for the 2 :1complex of the larger potassium ion [KL2](NCS) $CHC13.14 In the dipicrate of the ion [K2L2I2+ where L = (HOOCCH20-o-C6H40CzH4)z0, each potassium ion is co-ordinated to seven 0from the unionized polyether acid and one carbonyl oxygen bridges to the other potassium ion giving a dimer with both K eight-co-ordinated (K-0 = 2.729-2.903 A).'' Lasalocid A C34Hs308 contains three ' A.Simon W. Bramer and H.-J. Deiseroth 1978 17 875. M. G. Down M. J. Haley P. Hubberstey R. J. Pulham and A. E. Thunder J.C.S. Chem. Comm. 1978 52. 'M. G. Down M. J. Haley P. Hubberstey R. J. Pulham and A. E. Thunder J.C.S. Dalton 1978 1407. D. Thoennes and E. Weiss Chem. Ber. 1978,111,3157. J.-M. Lehn Accounts Chem. Res. 1978 11,49. lo J.-M. Lehn Pure and Applied Chem. 1978,50 871. 0.Nagano A Kobayashi and Y. Sasaki Bull. Chem. Soc. Japan 1978,51,791. '* J. HaSek and K. Huml Acta Cryst. 1978 B34 1812. l3 J. HaSek K. Huml and D. Hlavata Acta Cryst.1978 B34 416. l4 D. L. Hughes and J. N. Wingfield J.C.S. Chem. Comm. 1978 1001. I' D.L.Hughes C. L. Mortimer and M. R. Truter Inorg. Chim. Acta 1978,28 83. The Typical Elements 159 hydroxyl two ether one carbonyl and one carboxylate function and can transport ions across natural and synthetic membranes. Its complex with sodium Na2- lasalocid A),(H20), has now been submitted to X-ray examination. The two organic molecules provide cavities for the two Na' ions and the two H20 molecules where one Na' is co-ordinated to six 0and one water and the other Na' ion is co-ordinated to four 0 and two water molecules.'6 The determination of stability constants and other thermodynamic parameters has formed the subject of a number of papers. Thus K AH and AS have been determined for complexation of Na' with the polyether diamide (MeCONH- 1,2- C6H40C2H4),0 in pyridine solvent with perchlorate as anion.The method used was 23Na n.m.r. studies at 5-50 "C.The values found were AH = -71 kJ mol-'; AS = 201 J mol-' K-'; K varied from lo3to 10 1mol-' depending on temperature. The complexation is thus enthaipy-driven under these conditions." The stability constants of complexes between the 2,1,1- 2,2,1- and 2,2,2-cryptates (the numbers refer to the number of 0 atoms in each chain e.g. N(C2H40C2H40C2H4)3N is 2,2,2-cryptate) and the Group I ions have been measured in methanol by competi- tion with Ag' ion. Thus for the 2,2,1-cryptate log K values are 5.4(Li); 9.6(Na); 8.5(K); 6.7(Rb) and 4.3(Cs) 1 mol-'. This specificity for Na and K arises from the rates of dissociation rather than the rates of association.The former were measured in acid solution by stopped flow or conventional conductimetry.'* The con- formation of crown ethers and diaza crowns when complexed with Group I cations has been investigated in deuteriomethanol solution by analysis of the 'H n.m.r. coupling constants. l9 Finally as a token of the continuing interest of these systems to synthetic organic chemists it is reported that Li Na and K enolates of cyclohexanone when complexed with cryptates in ether solution are powerful bases. Thus the K-2,2,2- cryptate system converts cyclohexyl chloride into cyclohexene instantly.,' 2 GroupIIA The diatomic species Mg, CaMg SrMg and SrCa which are formed by diffusion in a solid Ar matrix have been studied by electronic absorption and (with laser excitation) emission.The observations are consistent with a van der Waals ground state but a chemically bonded excited The X-ray emission X-ray photoelectron and Auger spectra of Mg(OH)2 have been obtained and are discussed in relation to a Huckel molecular orbital treatment of the bonding in this The kinetics have been studied of the second order ligand exchange reaction of the [M(edta)]'- (M =Mg Ca or Sr) complexes which exchange with edta4-. The method used was analysis of the 'H n.m.r. line shape. AH=!=,AS+ and rate constants were obtained. In the case of the Sr2' complex a first order dissociation competes with the route which follows second order kinetics.24 l6 G.D. Smith W. L. Duax and S. Fortier J. Amer. Chem. SOC.,1978,100 6725. 17 J. Grandjean P. Laszlo F. Vogtle and H. Sieger Angew. Chem. Internat. Edn. 1978.17 856. '* B. G. Cox H. Schneider and J. Stroka J. Amer. Chem. SOC.,1978,100,4747. l9 J. C. Lockhart and A. C. Robson J.C.S. Dalton 1978,611. 2o J.-L. Pierre R. le Goaller and H. Handel J. Amer. Chem. SOC.,1978 100 8021. 21 J. C. Miller and L. Andrews J. Amer. Chem. SOC.,1978,100,2966. 22 J. C. Miller and L. Andrews J. Amer. Chem. Soc. 1978,100,6956. 23 D. E. Haycock M. Kasrai C. J. Nicholls and D. S. Urch J.C.S. Dalton 1978 1791. 24 P. Mirti J. Inorg. Nuclear Chem. 1978 40 833. 160 F. A. Hart A. G. Massey P. G. Harrison and J. H. Holloway Many compounds of the new type HMgNR (R=alkyl or aryl) have been synthesised by three routes e.g.the reaction between active MgH (from MgEt,+ LiAlH4 in ether) and Mg(NR2) in THF. Infrared and molecular weight measure- ments indicate dimeric and higher association in THF with both N and H bridgesz5 Grignard analogues RSrI and RBaI can be prepared in good yields in THF at -78 ‘C. Solids can be obtained of composition RM1,n (THF) (R = Me Et Pr” Bun; n = 2 or 3). The dicyclopentadienyls of Ca Sr or Ba can be obtained in 90-100% yield by cocondensation of metal and cyclopentadiene vapour at -196 “C followed by careful warming to room temperature.26 A number of new mixed hydrido-alkyl compounds of Li and Mg have been prepared and characterised. These include LiMgHMe and LiMgH,Me.” MgH undergoes an addition reaction with alkenes in THF under the catalytic influence of cp2TiC12 giving RCH(H)-CH(MgH)R’.The products were not isolated but were detected by reaction with D20 or MgPh2(tmed) (tmed = Me,NC2H4NMe,) forms an approximately tetrahedral mole- cule where NMgN = 82.5’; CMgC = 119.4’; Mg-C = 2.167 A; Mg-N = 2.199 2.205 The structure has been determined of an interesting related complex Li2MgzPh6(tmed), which consists of linear LiMgMgLi each pair of metal atoms being bridged by a pair of Ph groups. Each Li atom is also co-ordinated by two N from tmed to complete an approximately tetrahedral co-ordination about each metal. This compound is prepared by reaction between LiPh MgPh2 and tmed.30 In X-ray crystal structure determinations co-ordination numbers of 5 and 6 have been observed for Mg” and 6 7 and 8 for Ca”.In [Mg(Me3PO)5HzO](C104)2 there is conventional octahedral co-~rdination~~ but in [Mg(Me,PO),](ClO,) the co-ordination is square pyramidal.32 In CaC12,4H20 there are two types of Ca” ion one showing octahedral co-ordination to six C1 while the other is eight-co-ordinated to seven HzO and Cl.33 CaC1N03,2H20 again has eight-co-ordinated Ca2’ to two H20 C1 two bidentate NO3 and one monodentate A number of papers have appeared on polyether complexes of the Group I1 ions including some interesting template reactions. Thus 2,6-diacetyl-pyridine conden- ses with ethylenediamine in the presence of Ca Sr or Ba salts to give complexes of the macrocyclic hexamine (1;L). The structure of [SrCl,L],2H20 has been deter- mined and shows eight-co-ordinated Sr (six Sr-N = 2.71 1to 2.744 A; two Sr-C1 = 2.915 2.927 A somewhat similar condensation of ~yridine-2~6-dialdehyde with 1,l l-diamino-3,6,9-trioxaundecane in the presence of Ca Sr or Ba(SCN) gives complexes of the macrocyclic triaminetriether (2; L’).The structure of [Ca(NCS)2L] and [Sr(NCS),L’H20] have been determined.The eight-co-ordinated Ca is in the plane of the macrocycle with Ca-0 = 2.64 8 and Ca-N = 2.64. The larger nine- *’ E. C. Ashby and A. B. Goel Znorg. Chem. 1978 17 1862. 26 B. G. Gowenlock W. E. Lindsell and B. Singh J.C.S. Dalton 1978 657. 27 E. C. Ashby and A. B. Goel Znorg. Chem. 1978,17,322. 2a E. C. Ashby and T. Smith J.C.S. Chem. Comm. 1978,30. 29 D. Thoennes and E.Weiss Chem. Ber. 1978 111 3381. 30 D. Thoennes and E. Weiss Chem. Ber. 1978 111 3726. 31 Y. S. Ng G. A. Rodley and W. T. Robinson Actu Cryst. 1978 B34 2835. 32 Y.S. Ng G. A. Rodley and W. T. Robinson Actu Cryst. 1978 B34 2837. 33 A. Leclaire and M. M. Borel Actu Cryst. 1978 B34 900. 34 A. Leclaire and M. M. Borel Actu Cryst. 1978 B34 902. 35 J. de 0.Cabral,M. F. Cabral,W. J. Cummins M. G. B. Drew A. Rodgers and S. M. Nelson Znorg. Chim. Actu 1978 30 L313. The Typical Elements co-ordinated Sr is 0.53 8 above the ring plane with Sr-0 = 2.78 8 and Sr-N = 2.78 The identity of the mean Ca or Sr interatomic distances to N and to 0is of interest; since oxygen is the smaller atom this presumably suggests that the metal-nitrogen interaction may be at least as great as the metal-oxygen interaction.A number of X-ray structures have appeared which concern crown ether complexes. Thus in [Ba(C104)2(dibenzo-24-crown-8)], the metal ion has a co- ordination number of lo$ interatomic distances Ba-0 for the ether oxygen atoms ranging from 2.76 to 3.048, while the Ba is also co-ordinated to a unidentate C104 (2.728,) and a sesquidentate C104 (2.79 and 3.30A).37 The sexidentate benzo- 18-crown-6 also complexes with Sr and Ba perchlorates giving nine-co- ordinate [Sr(benzo-18-crown-6)(H20)3](C104)2 and ten-co-ordinate [Ba(C104)2(benzo-18-crown-6)(H20)2]. In the latter complex two water molecules and one unidentate perchlorate ion are co-ordinated on one side of the crown while the other monodentate perchlorate is co-ordinated on the other side.38 The smaller cavity of benzo-15-crown-5 allows it to form a complex [Mg(NCS)2(benzo-15- crown-5)] in which the metal ion lies in the ring mean plane giving a seven-co- ordinate pentagonal bipyramid (Mg-0 = 2.171 to 2.205 A).However in the complex [Ca(NCS)2(benzo-15-crown-5)H20] the larger Ca ion lies above the crown (Ca-0 = 2.382-2.614) with two N-co-ordinated thiocyanate ions and one water molecule situated above the metal A large number of complexes of Mg Ca Sr and Ba thiocyanates with polyethylene glycols and polyethylene glycol methyl ethers have also been de~cribed.~' 3 GroupIIB More than half the papers on zinc cadmium or mercury which were published this year contain as their sole or principal feature X-ray crystal structure deter- minations.Despite this work no really novel principles have emerged but a great deal of valuable background and illustrative information has been obtained. There have been a moderate number of publications on other aspects of these three metals particularly on studies of complexation in solution. 36 D. E. Fenton D. H. Cook I. W. Nowell and P. E. Walker J.C.S. Chem. Comm. 1978 279. 37 D. L. Hughes C. L. Mortimer and M. R. Truter Actu Cryst. 1978 B34,800. D. L. Hughes C. L. Mortirner and M. R. Truter Znorg. Chim. Actu 1978 29 43. 39 J. D. Owen J.C.S. Dalton 1978 1418. 40 S. Yanagida K. Takahashi and M. Okahara Bull. Chem. SOC.Japan 1978 51,3111. 162 E A. Hart A. G. Massey P. G. Harrison and J.H. Holloway Reviews have appeared on the aqueous solution chemistry of methylmercury and its complexes4' (72 references) and on organocadmium reagents (258 reference^).^^ When ZnC12 is prepared under strictly mhydrous conditions it has an ortho- rhombic structure different from that previously reported in that although still composed of hexagonal close packed anions with zinc in tetrahedral holes the stacking sequence is altered. The original structure reappears on exposure to air." Vacuum-dehydrated Zeolite A which has been encharged with Cd2+ ions contains zero-co-ordinated Cd2+ the nearest Cd-0 contacts being shown by an X-ray structure determination to be 3.55 A."" The phase diagram of Hg12 has been rationalised by infrared and Raman evidence that the high temperature and the high pressure forms both yellow are in fact different phases."' Solution X-ray Raman and infrared studies of Zn2+ Cd2+ and Hg2+ ions in dimethylsulphoxide have shown them all to be octahedrally co-~rdinated~~ as is the case for Cd2+ and Hg2+ in the solid The stability constants of Ph3P and Ph3As towards Hg2+ in dimethylsulphoxide favour the phosphorus donor.Thus for PPh3 log K1= 11.06 log K2= 6.55; -AH = 57 -AH; = 51 kJ mol-' while for AsPh, log K1= 6.77 log K2= 2.20; -AH = 34 -AH; = 27 kJ mol-'."' With the oxygen donor Ph3P0 the standard enthalpy change AH? for the reaction MCl,(s) +2Ph3PO(s) + [MCl,(Ph3PO)J(s) has been determined by solution calorimetry to be -51.1 (Zn) -16.3 (Cd) and -12.3 kJ (Hg). The standard enthalpies of formation AH? are -587 (Zn) -528 (Cd) and -358 kJ mol-' (Hg).It is concluded that the M-0 bond enthalpies follow the sequence Zn =Cd >Hg,48 illustrating again the comparatively weak affinity of Hg2+ for oxygen donors. Complexes of the types Hg(OOCCH3)2Ln where n = 1 and 2 and L = e.g. PPh3 PPhEt, P(p-tolyl), have been described. They are non-electrolytes in nitromethane and the bisphosphine complexes dissociate in dichl~romethane"~*~~ (see also ref. 70). A number of papers have appeared on complexes of macrocyclic ligands and cryptates. Zinc tetrabenzporphyrin has been obtained in 17% yield by an easy one-step template reaction between zinc acetate aqueous ammonia and 2-acetyl- benzoic acid in the presence of molecular sieve." The X-ray structure of a related complex chloro-N-methyl-a,P,r,S-tetraphenylporphinatozinc shows irregular five-co-ordination with the N(Me)-Zn bond very weak (2.53 A) compared with the other Zn-N bonds (2.018-2.081 A); Zn-C1 is 2.232 A.52The cyclic pent- amine CH2NHC2H4NC(Me),2,6-CSH3NC(Me)NC2H4NHkH2 (L) forms the com- 41 D.L. Rabenstein Accounts Chem. Res. 1978 11 100. 42 P. R. Jones and P. J. Desio Chem. Rev. 1978,78,491. 43 H. L. Yakel and J. Brynestad Inorg. Chem. 1978 17 3294. 44 L. B. McCusker and K. Seff J. Amer. Chem. Soc. 1978,100 5052. 45 D. M. Adams and R. Appleby Znorg. Chim. Acta 1978 26 L43. 46 M. Sandstrom I. Persson and S. Ahrland Acta Chem. Scand. 1978 A32 607. 47 S. Ahrland T. Berg and P. Blauenstein Acta Chetn. Scand. 1978 A32 933. 48 R.A. Jorge C. Airoldi and A. P. Chagas J.C.S. Dalton 1978 1102. 49 E. C. Alyea and S. A. Dias Canad. J. Chem. 1978 56 83. 50 T. Allman R. G. Goel and P. Pilon Canad. J. Chem. 1978 56,91. A. Vogler and H. Kunkely Angew. Chem. Znternat. Edn. 1978 17 760. 52 D. K. Lavallee A. B. Kopelove and 0.P. Anderson J. Amer. Chem. Soc. 1978 100 3025. The Typical Elements 163 -plex Cd(C104)2L which contains chains * * (CdL)C104(CdL)C104 -. Each C104- ion bridges two Cd by two of its 0 atoms one to each Cd. The co-ordination is thus pentagonal bi~yramidal.'~ In contrast to this cyclic quinquedentate ligand the acyclic potentially quinquedentate 2,6-C5H,N{C(Me)N.o-C6H4SMe}2 (L') forms a complex Cd12L' which is five-co-ordinated the two S atoms being free.The co-ordination is trigonal bipyramidal distorted because N-Cd-N is constrained to -68" (Cd-N = 2.33-2.41; Cd-I = 2.713 2.726 The complexes [ZnXL"]' where X = C1 Br I NCS C104 and L = ( -CH2NMeCH2CH2NMeCH2-)2 have been examined by variable-temperature 13C n.m.r. in CH3N02 and by X-ray determination (solid; X = Cl). In the latter case the co-ordination is square pyra- midal but the n.m.r. results are interpreted in terms of interconversion between two equivalent trigonal bipyramidal forms two N atoms from the cyclic tetramine being In a wide-ranging survey of stability constants between metal ions and the cryptate ligand N(CZH40C2H40C2H4)3N and related ligands where up to four oxygen atoms are replaced by NMe groups it was found that these ligands are very selective particularly for Hg2+ (log K = 18-27) but also for Cd2+ (7-12) and Zn2' (3-1 l),compared with for example K' (2-5) or Ca2' (2-5).56 In the organometallic area 'H n.m.r.has shown that the compounds ZnR2 (R = allyl 2 or 3-methylallyl 3,3-dimethylallyl) are dynamic mixtures of all isomeric ?'-ally1 forms at room temperature. Interconversion ceases below -115 OC." The triad M(CH2SiMe3)* (M= Zn Cd or Hg) has been completed by the synthesis of the Cd member by a Grignard reaction. It is thermally stable complexes with 2,2'- dipyridyl and 1 10-phenanthroline and reacts immediately with oxygen or water.58 Complex formation between CH3Hg+ and C1- Br- or NO3- in water has been studied by radiometry using 203Hg in a waterlo-xylene system and K values were obtained e.g.log Kc,-= 5.64.59 In the very popular field of X-ray crystallographic studies those of mercury predominate. We give here a selection following the sequence zinc cadmium and mercury. Zinc iodide whose structure has not been previously determined is tetragonal and shows nearly regular tetrahedral co-ordination of zinc with Zn-I = 2.58 to 2.68 Most zinc structures reported this year involve near-tetrahedral co- ordination with sulphur ligands e.g. [Zn(thiourea),](N03) (Zn-S = 2.324-2.361 and (Ph4P)2[Zn(SPh)]4 where the tetrahedron is distorted by up to 12" owing to compression along a twofold axis the analogous cadmium compound being closely similar.62 (PPh4)2[Zn(WS4)2] has [S2WS2ZnS2WS2I2- ions where the co- ordination of both Zn and W is near-tetrahedral (S-Zn-S = 96.7" and 116.2°).63 53 M.G. B. Drew S. Hollis S. G. McFall and S. M. Nelson J. Znorg. Nuclear Chem. 1978,40 1595. 54 M. G.B. Drew and S. Hollis Actu Cryst. 1978 B34 2853. 55 N. W. Alcock N. Herron and P. Moore J.C.S. Dalton 1978 1282. 56 J.-M. Lehn and F. Montavon Helv. Chim. Actu 1978,61 67. " R. Benn E. G. Hoffrnann H. Lehrnkuhl and H. Nehl J. Orgunometullic Chem. 1978,146 103. '' D. M. Heinekey and S. R. Stobart Znorg. Chem. 1978,17 1463. 59 M. Jawaid F. Ingrnan D. H. Liem and T. Wallin Acta Chem. Scand. 1978 A32 7. 6o P.H.Fourcroy D. CarrC and J. Rivet Actu Crysr. 1978 B34 3160. 61 R. Vega A. Lbpez-Castro and R. Marquez Actu Cryst. 1978 B34 2297. 62 D. Swenson N. C. Baenziger and D. Coucouvais J.Amer. Chem. Soc. 1978,100 1932. 63 I. Paulat-Boschen B. Krebs A. Miiller E. Koniger-Ahlborn H. Dornfeld and H. Schulz Znorg. Chem. 1978,17 1440. 164 F. A. Hart A. G. Massey P. G. Harrison and J. H. Holloway Cadmium structure determinations are rather few but include CdBr2(H20)4 where the Cd2+ ion in -CdBr2CdBr2. -chains is co-ordinated trans-octahedrally to four Br and two H20 (Cd-Br = 2.746;Cd-0 = 2.349A),64and [Cd(Me2SO),]- (C1O,J2which shows nearly octahedral co-ordination (0-Cd-0 = 84.5"-95.3"; Cd-0 = 2.257-2.278 A).65 Mercury(I1) is known to display a range of co-ordination numbers and geometries. It particularly favours 2,4,and 6,and in the last case shows a tendency to form two short colinear and four longer bonds (2+4co-ordination).These tendencies are well illustrated in the year's structures. Taking the simpler compounds first NEt4HgC13 shows 3+2 co-ordination. Triangular HgC13 entities (Hg-Cl= 2.426-2.444 A) are so arranged that two further long (3.054 3.017 A) bonds complete a trigonal bipyramid.66 When K2X and HgX (X = Sor Se) are heated to 360-780 "C yellow to reddish-orange K6HgX4 is obtained. The sulphur compound has nearly tetrahedral Hg (Hg-S = 2.542or 2.591A).67 Nearly octahedral co-ordination is shown by both [Hg(H20)6](C104)2 (Hg-0 = 2.341A; 0-Hg-0 = 85.8' 94.2°)68and by [Hg(Me2SO)6](ClO4)2 (Hg-0 = 2.317-2.376 which thus resembles its cadmium analogue mentioned above. However the tendency to the 'x short +y long' type of co-ordination is shown again in two other structures.[Hg(CH,COO),(P'Bu,)] has irregular five-co-ordination between Hg and P (2.371A) two nearer 0 (2.25,2.27A) and two further 0 (2.58 2.66A) from the unsymmetrically bidentate acetate gro~ps.'~ In Hg(CF,COO) (~yridine)~ we get pentagonal bipyramidal (2+5) co-ordination. Here there is a N-Hg-N axis (170.2'; 2.11 2.13A) with five 0from two bidentate and one (bridging) unidentate acetate groups (Hg-0 = 2.56-2.87 A) within at most 0.1638,of the best equatorial plane.71 In some organo-mercury complexes the tendency to '2+y ' co-ordination is quite marked. In CH,Hg(NO3)(3,3'-dimethyl-2,2'-bipyridyl) the diamine is unidentate only giving a C-Hg-N backbone (172.7";Hg-C = 2.01,Hg-N =2.11 A) which interacts with four nitrate 0 atoms (2.84-3.09 A) and may also interact with the second aromatic ring of the diamine (Hg-C = 3.11 A).72In [PhHgCN(1,10- phenanthro!ine)] there is a nearly linear (167.5') Ph-Hg-CN group (Car-Hg = 2.067A CcN-Hg = 2.063A) to which is co-ordinated the diamine (Hg-N = 2.660 2.680 A) such that the triangle HgN2 is approximately perpendicular to C-Hg-C to give a curious unsymmetrical structure where the C-Hg-C angle has resisted distortion to a pseudo-tetrahedral angle.73 Lastly in [MeHg(pyridine)]N03 the secondary bonds have become non-existent (Hg-0 = 3.16and greater) leaving two-cordinate Hg in the Me-Hg-py grouping (179.7";Hg-C = 2.04,Hg-N = 2.12 64 H.Leligny and J. C. Monier Acta Cryst. 1978 B34 5. 6s M. Sandstrom Acta Chem. Scand. 1978 A32 519. 66 M. Sandstrom and D.H. Liem Acta Chem. Scand. 1978 A32 509. 67 H. Sommer and R. Hoppe 2.anorg. Chem. 1978,443,201. 68 G. Johanson and M. Sandstrom Acta Chem. Scand. 1978 A32 109. 69 M. Sandstrom and 1. Persson Acta Chem. Scand. 1978 A32,95. 70 P. J. Roberts G. Ferguson R. G. Goel W. 0.Ogini and R. J. Restivo J.C.S. Dalton 1978 253. 71 J. Halfpenny R. W. H. Small and F. G. Thorpe Acta Cryst. 1978 B34 3075. 72 A. J. Canty N. Chaichit B. M. Gatehouse and A. Marker Acta Cryst.,1978 B34 3229. 73 A. Ruiz-Amil S. Martinez-Cartera and S. Garcia-Blanco Acta Cryst. 1978 B34 2711. 74 R. T. C. Brownlee A. J. Canty and M. F. Mackay Austral. J. Chem. 1978,31 1933.

ISSN:0308-6003    

DOI:10.1039/PR9787500157

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

The Typical Elements PART 11 Group 111 By A. G. Massey 1 Boron A comprehensive collection of IlB n.m.r. data on compounds having boron atoms with co-ordination numbers 2 3 and 4 is contained in the book by Noth and Wrackmeyer.' Besides being useful to the spectroscopist the 96 tables in the review make an excellent starting point for a rapid general survey of known boron compounds. A characteristic feature of "B n.m.r. spectra is the broadness of the peaks a quality which greatly impairs resolution. Although it might be tempting to attribute these line widths (usually 10-100 Hz) to the short relaxation time of the quadrupolar B nuclei this is not a satisfactory explanation since measurements of "B spin-lattice relaxation times have demonstrated that the 'natural linewidth' due to quadrupolar relaxation is generally less than 10 Hz.Furthermore in an analysis of the "B n.m.r. spectra of B4H10 and BSHg it has been shown that unresolved B-B spin-spin coupling is not the sole source of the extra broadness as scalar relaxation and partially collapsed spin-spin multiplets also play a significant part in broadening "B n.m.r. lines. Complications due to the presence of loB in isotopically normal samples are also possible and hence "B-enriched compounds are necessary for an accurate analysis.2a From a study of single boron crystals obtained from the hydrogen-reduction of high purity boron tribromide on a tantalum wire at 1200 "C it is concluded that there is only one modification of tetragonal boron. The structure is based on a three dimensional boron skeleton very similar to that of a-A1B12 and contains simple (B12) and twinned (B21) icosahedra linked by isolated boron atoms.2b Crystalline boron reacts with S2C12 at 800-1000 "Cto give the unstable triatomic molecule chloro- thioborine ClB=S; the methyl derivative was formed in an analogous manner using dimethyl disulphide.2' Boranes and their Derivatives The degradation of pentaborane(9) using tri- methylphosphine in toluene at 25 "C leads to a 70% yield of the diborane(4) adduct B2H4,2PMe3.It is a white crystalline solid which sublimes slowly in a vacuum at room temperature. The synthesis is particularly interesting since it uses a com- mercially available b~rane.~ Irradiation of diborane(6) with the 973 cm-' line of a CO laser gives Bl0HI4 B5H9 B5Hll (BH), and hydrogen; no BZOH16 found by previous workers could be detected in any of the experiment^.^ The results are explained in terms of a thermal reaction.' A simplified preparation of decaborane( 14) using standard laboratory ' H.Noth and B. Wrackmeyer 'N.M.R. Basic Principles and Progress' Vol. 14 ed. P. Diehl E. Fluck and R. Kosfeld Springer-Verlag 1978. * (a) R. Weiss and R. N. Grimes J. Amer. Chem. SOC.,1978,100,1401;(b) M. Vlasse M. Boiret R. Naslain J. S. Kasper and K. Ploog Compr. rend. 1978,287,C,27;(c)C. Kirby H. W. Kroto and N. P. C. Westwood J. Amer. Chem. SOC.,1978,100,3766. R. K.Hertz M. L. Denniston and S. G. Shore Inorg. Chem. 1978,17 2673. S. Shatas D.Gregory R. Shatas and C.Riley Inorg. Chem. 1978.17,163. C.Riley S. Shatas and V. Arkle J. Amer. Chem. SOC.,1978,100,658. F. A. Hart A. G. Massey P. G. Harrison and J. H. Holloway apparatus and starting from sodium tetrahydroborate has been described. Boron trifluoride etherate is heated in diglyme to ca. 105"C under nitrogen with NaBH when the BllH14- anion is formed; oxidation of this ion with dichromate results in a 40% yield of B10H14.6 For many years it has been suspected that crude commercial decaborane( 14) contains small amounts of higher boron hydrides; these have now been identified as &OH16 and B2&6 using mass spectrometry.' A structure with 22 vertices probably represents the upper limit for stable closo boron hydrides.* Dietrich has refined his 1971 estimate of the charge distribution in decaborane(l4) arising from X-ray and neutron data and compared the new results with those calculated quantum-mechanically by Lipscomb.' MNDO calculations have been carried out on the known boranes up to BI0Hl6 and borane anions up to B12H122-.The results whilst confirming the tendency of MNDO to underestimate the strengths of three-centre bonds were in sufficiently good agreement with experiment as to suggest that the technique would prove useful in further studies of borane chemistry. lo Extended molecular orbital calculations have been used to follow the changes in electronic structure which occur when idealised borane polyhedra are synthesised from each other by capping faces and edges with single BH units or by mutual approach of two faces." The infrared and Raman spectra of cis and trans 1,2-dimethyldiborane have been measured assigned and a set of fundamentals proposed.It is suggested that Raman spectra can probably be used to obtain rough quantitative estimates of the various methylated diboranes present in mixtures. l2 Monobromo-and monoiodo-diborane can be made in high yield by condensing BX3 and B2H6 at -196 "C warming the mixture to 0 "Cand holding it at that temperature for three hours; separation of the products by fractional condensation and repetition of the cycle nine times leads to an 80% conversion of the dib~rane.'~ MNDO calculations on the hydroboration of alkenes indicate that the orientation of addition to the olefins is determined primarily by steric effects.A loose r-type complex is a marginally stable intermediate the formation of this being the rate- determining step for the overall reaction. Activation energies and heats of reaction for formation of the v-complexes show an unexpected increase in the order ethylene < propene < isobutene. The electronic effects of the methyl groups are apparently outweighed by the steric repulsions between hydrogen atoms of the borane and methyl groups. Acetylene and methylacetylene also react uia inter-mediate v-complexes but these are much more stable than those in the olefin hydroborations. It might even be possible to isolate the BH3,C2H2complex in matrices at low temperatures.14 The "F n.m.r. spectrum of B4HSPF2NMe2 at room temperature shows the presence of the exo and endo isomers.At very low temperatures hindered rotation 'G. B. Dunks and K. P. Ordonez J. Amer. Chem. Soc. 1978,100 2555. ' N. N. Greenwood J. D. Kennedy and D. Taylorson J. Phys. Chem. 1978,82,623. J. Bicerano D. S. Marynick and W. N. Lipscomb Znorg. Chem. 1978,17,2041. 9 H. Dietrich and C. Scheringer Actu Cryst. 1978 B34,54. lo M.J. S. Dewar and M. L. McKee Znorg. Chem. 1978,17,1569. l1 J. Evans J.C.S. Dalton 1978 18. D. F. Eggers D. A. Kohler and D. M. Ritter Spectrochim. Actu 1978 34A,731. l3 J. E. Drake B. Rapp C. Riddle and J. Simpson Znorg. Synth. 1978,18 145. l4 M. J. S. Dewar and M. L. McKee Znorg. Chem. 1978 17 1075. The Typical Elements occurs about the P-B bond in what is probably the endo i~omer.'~" An open hypho structure is s~ggested"~ for the fluxional molecule tetramethylethylene-diaminetetraborane(8).From mass spectrometric observations on a series of proton-transfer reactions B5Hi+ R + RH' +BsHg where R is a stable molecule of known proton affinity the proton affinity of the B5Hg radical is calculated to be 184f2 kcal mol-'. Combined with other known thermo- dynamic data this gives a value of 63.8k3 kcal mol-' for AH (B5Hg).16 The two isomerizations 1-ClBSHg +Et2O 2-ClBSHg +Et2O are both approximately first order in ClB5Hg and diethyl ether. The data suggest that the isomerizations occur via a boron cage rearrangement involving a Et20,ClB5Hg complex rather than a mechanism which involves B-Cl bond cleavage." In Me2SnB,,HI2 the dimethyltin group is bonded to the decaborane cage at edge boron atoms B5-B6 and B9-Blo by what can be regarded formally as two three-centre B-Sn-B bonds.The cleavage of the Me2Sn group by excess bromine results in the facially disubstituted 5,10-dibromodecaborane( 14). When Me2SnBlOHI2 is treated with a deficiency of bromine one of the products is MezSnBrBloH12Br showing that the reaction proceeds by stepwise cleavage of the B-Sn-B three-centre bonds in an oxidative cleavage reaction process l8 H H-@H .Br &Br Br MeSnMe Br2,QQH-@H .Br H The first facile and convenient hydroboration reactions involving a polyhedral borane have been described using 6-thia-nido-decaborane(ll).The site of attack is the exo B-H bond at the 9 Is (a)T.F. Moore A. R. Garber and J. D. Odom Inorg. Nuclear Chem. Letters 1978,14,45; (b)H.M. Colquhoun J. Chem. Res. (S) 1978,451. l6 Jia-Shen Wang A. J. DeStefano and R. F. Porter Inorg. Chem. 1978,17 1374. l7 D. F. Gaines and J. L. Walsh Inorg. Chem. 1978,17 806. '* T. J. Dupont R. E. Loffredo and R. C. Haltiwanger,Inorg. Chem. 1978,17 2062. l9 (a)B. J. Meneghelli and R. W. Rudolph J. Amer. Chem. Soc. 1978,100,4626; (6)I. A. Baidina N. V. Podberezskaya V. I. Alekseev V. V. Vokov and S. V. Borisov Zhur. strukt. Khim. 1978,547. F. A. Hart A. G. Massey P. G. Harrison and J. H. Holloway The structure of 6,9-bis(ammine)decaborane B10H12(NH3)2, is very similar to those of the acetonitrile and dimethylsulphide derivatives; the B-N distances are 1.598 and 1.591 A.196 Metal1oboranes.-The iron-bridged diborane(6) derivative K'[p -Fe(C0)4B2H5]-results as a dark brown solid from the reaction.20 H [ B',h J K2'[Fe(C0),l2-+3THF,BH3-+ K' H,'*** B**aHH,+KBH4+3THF(98%) (cob When iron pentacarbonyl and pentaborane(9) are added to LiAlH in diethyl ether a deep red solution and a dark precipitate are formed.Work up of the mixture yields unreacted starting materials B4HEFe(C0)3 and a yellow-brown very air-sensitive liquid identified as B2H6Fez(C0)6 and assigned structure (1)on the basis of infrared and n.m.r. spectral evidence.21 0 C P The U.V. photoelectron spectra of several ferraboranes including B,HEFe(C0)3 BsH9Fe(C0)3 and BSH3Fe(CO), have been reported. The spectrum of the latter is consistent with two carbonyl groups being in exopolyhedral positions (2).22The PE spectrum of B,HEFe(C0)3 calculated using SCF-Xa-SW MO theory and the experimental curve are in good agreement showing that a reasonable description of the electronic structure of this type of molecule can be obtained from a first- principles one-electTon treatment without recourse to configuration intera~tion.~~ 2-Berylla-nido -hexaborane( 1l),prepared via the reaction 1-ClBSH8+Be(BH& -P B2H6 +BeC12+B5HloBeBH4 possesses a pentagonal pyramidal cage structure in which one basal position is occupied by a beryllium atom.A terminal hydrogen is attached to each of the cage boron atoms and bridge hydrogens link all adjacent basal atoms in the cage; the BH group is attached to the beryllium atom by two bridging hydrogen atoms.Treatment of the compound with hydrogen bromide at -78 "C produces BsHloBeBr from which 2,2'-commo-bis[2-berylla-nido-hexaborane( 1l)]may be obtained in 40% 2o G. Medford and S. G. Shore J. Amer. Chem. SOC.,1978 100 3953. 21 E. L. Andersen and T. P. Fehlner J. Amer. Chem. SOC.,1978 100,4606. 22 J. A. Ulman E. L. Andersen and T. P. Fehlner J. Amer. Chem. SOC.,1978 100,456. 23 D. R. Salahub J.C.S. Chem. Comm. 1978 385. The Typical Elements 169 yield by reaction with an excess of CSB& at room temperature. The structure of Be(B5H10)2 consists of two pentagonal pyramidal cages linked by a common beryl- lium atom the dihedral angle between the basal plane of the two cages being 66°.24*25 An iron atom can be 0-bonded to a basal atom in the B5H9 structure via the reaction -40" C --+Fe(775-C5H5)(CO)2(2-B5H8) K+B5H8-+ Fe(~5-C5Hs)(CO)21 -100% The product is quantitatively deprotonated by potassium hydride to yield K+[Fe( q5-CsH5)(C0),(2-B5H7)]- which reacts smoothly with another equivalent of the iodide26 K'[Fe( 77 5-C5H5)(CO)2(2-B5H7)]-+ + Fe(q5-C5H5)(CO)21 Like its B5H9 analogue the cobaltaborane 2-( q 5-CSH5)CoB4H8 undergoes depro- tonation with sodium hydride at a bridging hydrogen adjacent to the cobalt atom closo -1,2477 5-C5HSCo)2B4H4 Irradiation of 2-(q 5-C5H5)CoB4H8 in the presence of iron pentacarbonyl produces 1,2,3-(q5-C5H5)2C02(CO),FeB3H3, which is thought to have an octahedral Co2FeB3 cage in which all the metal atoms occupy the same triangular The tetrametallic air-stable cluster compounds (q'-C5H5),Ni4B4H4 and (q5-C5Hs)4Ni4B5H5 are made by stirring a mixture of NaB5H8 nickelocene and sodium amalgam in THF and separating the products by chromatography.The former brown compound possesses a closo dodecahedra1 cage containing four nickel and four boron atoms.28 A number of cobalt and nickel arsaborane derivatives have been prepared e.g. piperidine ~,~-B~OH~OAS~ + C5H6+ CoC12 PC~H~CO(B~H~AS~) However the chemistry of arsaborane-transition-metal complexes is limited in direct contrast to the rich chemistry with many other heteroborane anions. It appears that when the heavier members of a given family of elements are substituted 24 D. F. Gaines and J.L. Walsh Znorg. Chem. 1978 17 1238. 25 D. F. Gaines J. L. Walsh and J. C. Calabrese Znorg. Chem. 1978 17 1242. 26 N. N. Greenwood J. D. Kennedy C. G. Savory J. Staves and K. R. Trigwell J.C.S. Dalton 1978,237. 27 R. Weiss J. R. Bowser and R. N. Grimes Inorg. Chem. 1978 17 1522. 28 J. R. Bowser and R. N. Grimes J. Amer. Chem. SOC.,1978,100,4623. 170 F. A. Hart A. G. Massey P. G. Harrison and J. H. Holloway into a borane cage both the percentage yield of products and the number of accessible compounds drop dramati~ally.~~" The "B n.m.r. spectra of nidu-heteroboranes of formula BloH12E (E = S Se Te As- AsR PR CNMe, CH- or BH2-) have been recorded and partially assigned. The common structure of these boranes is that of an icosahedron minus one apex group E being at position 7 of the open face.Their spectra are similar and typically the resonances occur as doublets with assignments (reading upfield) being B(5) B(2,3) B(8 ll),B(9 lo) B(1) and B(4,6). The resonance of B(5)-antipodal to group E-is most sensitive to changes in E the shielding of B(5) decreasing in the order E = CH->CNMe3>PR >AsR >As->S >Se >Te.29b Borane Anions Cations and their Derivatives.-The boron spin-lattice relaxation behaviour of solid NaBH and NaBD has been studied in the region of the structural transition using "B n.m.r. The results confirmed the first order nature of the transition which is associated with the ordering of adjacent BH tetrahedra. In the high symmetry phase the orientations of the tetrahedral ions are randomly dis- tributed among two possible configurations whereas below the transition tempera- ture an orientational macroscopic order is established all the tetrahedra in a plane perpendicular to the c axis tend to have the same configuration while the orientation alternates from one plane to the At 70 "C redistribution of ligands occurs in the reaction between Bu4NBH4 and Bu4NBC14 in benzene with the formation of BHC13- BH2C12- and BHC13- the composition of products depending on the ratio of reactants.The thermal stability of the anions decreases in the order BC1,->BHC1,->B&->BH2C12->BH3Cl- the latter decomposing readily to BH2C12- and B2H7- in benzene Gas phase "B and 'H n.m.r. spectra of beryllium bis(tetrahydrob0rate) show that only the monomer is present for which a linear B-Be-B skeleton was inferred.The 11 B n.m.r. spectrum of C5H5BeBH4 consists of a quintet of quartets due to "B-H and 11 B-9Be coupling (9Be has a spin of 1). In both these tetrahydroborates the hydrogen atoms of the BH4 groups undergo rapid internal exchange.31 When diethylaluminium hydride is treated with the appropriate ratios of alu- minium tris(tetrahydrob0rate) the liquid hydride species Al(BH4)2H and Al(BH4)H2 are formed. The former is the more stable but under vacuum both compounds disproportionate evolving A1(BH4) the residues becoming solid when the concen- tration of AlH units exceeds about 78 mol% . Bis(tetrahydroborat0)alane reacts slowly with diborane at 0 "C to produce aluminium tris(tetrahydrob0rate) and forms 1:l adducts Al(BH4)2H,L with ligands such as dimethyl ether and dimethyl ~ulphide.,~ Although it has been confirmed, that dimethylgallium tetrahydroborate results when trimethylgallium and diborane are mixed a better method of pre- paration is to treat dimethylgallium chloride with lithium tetrahydroborate in the absence of solvent at -15 "C.The compound decomposes at room temperature to 29 (a)J. L. Little and S. S. Pao Inorg. Chem. 1978,17,584;(b)W. F. Wright A. R. Garber and L. J. Todd J. Magn. Res. 1978 30 595. 30 (a)A. Trokiner H. Theveneau and P. Papon J. Chem. Phys. 1978 69 742; (b)L. V. Titov L. A. Gavrilova K. V. Titova and V. Y. Rosolovskii Izvest Akad. Nauk S.S.S.R.,Ser. khim. 1978 1722. 31 D. F. Gaines J.L. Walsh J. H. Morris and D. F. Hillenbrand Inorg. Chem. 1978 17 1516. 32 P. R. Oddy and M. G. H. Wallbridge J.C.S. Dalton 1978 572. 33 A. J. Downs and P. D. P. Thomas J.C.S. Dalton 1978 809. The Typical Elements 171 give gallium metal hydrogen boron trimethyl and methylated diboranes; a ther- mally stable adduct Ga(BH4)Me2,2NH3 which is probably ionic [GaMe2(NH3)2]C[BH4]- is formed with an excess of ammonia at -80 “C. The infrared spectrum of the gaseous tetrahydroborate is consistent with a C2,structure in which the gallium is linked to a bidentate BH group. The d and f transition metal tetrahydroborates reviewed last year by Marks and K~lb,~~ continue to attract a good deal of attention. Sodium tetrahydroborate in the presence of triphenylphosphine reduces a number of cobalt(~r)~’ and nickel(11)~~ compounds to M’ tetrahydroborates stabilized by phosphine ligands e.g.CoCl2+ NaBH4+ PPh3 + CO(BH,)(PP~~)~ NiCI2+ NaBH + PPh3 + [Ni(BH,)(PPh&]2 In hydrido(tetrahydroborato)bis(tricyclohexylphosphine)nickel(II) the nickel atom is co-ordinated by two trans cyclohexylphosphine ligands a hydride hydrogen atom and two bridging atoms of the BH ligand to give a deformkd trigonal bi~yramid.~’ Yttrium has a formal co-ordination number of 1 1 in tris(tetrahydroborat0)- tris(tetrahydrofuran)yttrium(III) where three boron and three oxygen atoms are at the corners of a distorted octahedron; one of the tetrahydroborate groups is bidentate and the other two terdentate. In the terdentate Y-BH interaction the tetrahydroborate ligand is slightly tilted from local C3 symmetry resulting in inequivalent Y -H bond lengths.The gadolinium analogue is isomorphou~.~~ The infrared and Raman spectra of tetrakis(tetrahydroborato)zirconium(Iv) in which all the BH groups are terdentate have been assigned.39 Co-ordination of the bidentate tetrahydroborate groups in (q5-CH3C5H4)2Hf(BH4)2 is markedly unsymmetrical with alternating long and short X-H bonds round the four-membered HfH2B rings. Even at temperatures down to -155°C the exchange of terminal and bridging hydrogen atoms in the BH units is rapid on the n.m.r. time scale.,’ Uranium(1v) tetrahydroborate readily forms complexes with ethers the stoi- cheiometry and structure of the products depending markedly on the ether used.With dimethyl and diethyl ethers 1 1 adducts are produced as green slightly volatile crystalline solids. The structures consist of infinite chains of alternating uranium and boron atoms joined by double hydrogen-bridge bonds the remaining tetrahydroborates are attached to the uranium by triple hydrogen-bridges. In the dimethyl ether adduct successive ether molecules along the chain are trans to each other whereas in the diethyl ether complex all the ethers are cis. The total co- ordination of the uranium is 14 made up of one ether oxygen and 13 hydrogen atoms. The average U-B distance is 2.53 and 2.89 A for triple and double bridges re~pectively.~~ A pale green 2 1 complex is formed with tetrahydrofuran and is 34 T. J.Marks and J. R. Kolb Chem. Rev. 1977 77 263. ” D. G. Holah A. N. Hughes B. C. Hui and C. T. Kan Canad. J. Chem. 1978,56,814. ” D. G. Holah A. N. Hughes B. C. Hui and C. T. Kan Canad. J. Chem. 1978,56,2552. 37 T. Saito M. Nakajima A. Kobayashi and Y. Sasaki J.C.S. Dalton 1978 482. ’* B. G. Segal and S. J. Lippard Inorg. Chem. 1978,17 844. 39 B. E. Smith H. F. Shurvell and B. D. James J.C.S. Dalton 1978,710;J. C. Whitmer andS. J. Cyvin,J. Mol. Strut. 1978 50 21. 40 P. L. Johnson S. A. Cohen T. J. Marks and J. M. Williams J. Amer. Chem. SOC.,1978 100 2709. 41 R. R. Rietz A. Zalkin D. H. Templeton N. M. Edelstein and L. K. Templeton Znorg. Chem. 1978,17 653. 172 F. A. Hart A. G. Massey P. G. Harrison and J. H. Holloway unique in being monomeric in the solid state.Distorted octahedral co-ordination of four BH4 groups and two THF molecules occurs around the uranium. The U-B distance of 2.56 8 is characteristic of triply bridging tetrahydroborates the twelve hydrogen atoms being placed at the corners of a hexagonal antiprism capped by the two oxygens of the THF m01ecules.~~ With di-n-propyl ether a dimeric 1 :1adduct is formed as a light green volatile solid. The dimer is unsymmetrical the uranium atoms occurring in two different environments. One uranium is bonded to oxygen atoms of two ether molecules and eleven hydrogens of four BH groups; the other uranium has 14 neighbours from five BH groups one of which acts as bridge to the first uranium atom.43 Tetrakis(tetrahydrob0rates) of the actinides protactinium neptunium and plutonium have been made by treating the tetrafluorides with aluminium tris(tetrahydrob0rate) AnF +2Al(BH4)3 + An(BH4)4+2AlF2BH4 An =Pa Np Pu The neptunium and plutonium compounds are dark coloured volatile liquids Np(BH4) being the most volatile Np'" derivative made to date.Pa(BH4) is an orange air-sensitive solid which sublimes in a vacuum at 55 oC.44a The tetrahydro- borate group in trisindenylthorium tetrahydroborate (qS-C,H,),ThBH3 acts as a terdentate ligand.44b The photoelectron spectra of Al(BH4)3 Zr(BH4), and U(BH4) have been recorded and discussed. It is suggested that doubly and triply bridged tetrahydroborates should have qualitatively distinct photoelectron spectra so that the technique may act both as a probe of electronic properties and as a diagnostic test for the mode of bonding of the BH group.44c N.m.r.spectroscopy shows that all the hydrogen and boron atoms in the octa- hydrotriborate anion B3H8- remain equivalent down to 137 K. A possible low energy pathway for the rapid tautomerization involves the conversion of the equilibrium double bridge structure to a single bridge structure followed by rotation of the BH3 group.45 Beryllium bis(octahydrotriborate) reacts with hydrogen chloride to give tetraborane with sodium cyclopentadienide to give C5H5BeB3H8 and with zinc dimethyl to form (CH3BeB3H&. In all three beryllium octa- hydrotriborate derivatives the B3Hs groups are bound to the Be atoms in a bidentate fashion and show a range of fluxional character when studied by variable-tempera- ture n.m.r.31 Electrolysis of octahydrotriborate salts in non-aqueous solvents containing tri- phenylphosphine leads to dissolution of the copper or silver anodes and the formation of the metalloboranes Cu(B3H,)(PPh3) and Ag(B3H8)(PPh3)3.46 42 R.R. Rietz N. M. Edelstein H. W. Ruben D. H. Templeton and A. Zalkin Znorg. Chern. 1978 17 658. 43 A. Zalkin R. R. Rietz D. H. Templeton and N. M. Edelstein Znorg. Chern. 1978 17,661. 44 (a) R. H. Banks N. M. Edelstein R. R. Rietz D. H. Templeton and A. Zalkin J. Amer. Chern. Soc. 1978 100 1957; (6)J. Goffart G. Michel B. P. Gilbert and G. Duyckaerts Znorg. Nuclear Chern. Letrers 1978,14,393;(c)A. J.Downs R. G. Egdel1,A. F. Orchard andP. D. P.Thomas,J.C.S. Dalron 1978,1755." I. M. Pepperberg D. A. Dixon W. N. Lipscomb andT. A. Halgren Znorg. Chern. 1978 17 587. 46 B. G. Cooksey J. D. Gorham J. H. Morris and L. Kane J.C.S. Dalton 1978 141. The Typical Elements Metathetic reactions of the general type L (C0)MX+ Me4NB3H8 -b L,MB3H8 + CO + Me4NX L = CO C5H5 PPh3; X = C1,Br I; B3H8bidentate have led to the preparation of a variety of octahydrotriborates of Mn Fe Re Mo and W. Bromine and chlorine react with (C0)4MnB3H8 to give (C0)4Mn(B3H7X) in which the halogen substitution has occurred on the boron not involved with bonding to the manganese.47 Decarbonylation of (C0)4MnB3H8 using heat or U.V. light results in the formation of (C0)3MnB3H8 which is the first reported complex containing a terdentate octahydrotriborate group; n.m.r.studies suggest that the Mn-H-B hydrogens are static whereas the B-H and B-H-B hydrogens appear to be exchanging rapidly.48 1,4-Dichloro-1,1,3,3-tetraphenyl-cutena-di(boraphosphane),BH2ClPPh2BH2-PPh2Cl is the unexpected product arising from the reaction of tetramethylam-monium octahydrotriborate and diphenylchlor~phosphine.~~ In (PPh3)*CuB4H9 which is the first metalla derivative of the B4H9- ion the copper atom serves as a vertex of the nido-cluster framework with no evidence of Cu-H-B bridge bonding.50 K+B4H9-+ (PPh3)3CuCI -+ (PP~~)~CUB~H~ Pentaborane(9) and hexaborane( 10) are quantitatively deprotonated in etherial solvents by potassium hydride to yield the corresponding conjugate bases B5H8- and B6H9-; by adding tetra-organophosphonium or arsonium salts to the solutions it is possible to isolate R4MB5H8 and R4MB6H9 derivatives.Of the eight salts studied those with the largest cation (R = C6H5) were found to be the most table.'^ Copper and silver complexes of nido -pentaborane anions result by adding the corresponding potassium salt to M(PPh,),Cl KB5H8 +Ag(PPh3)3Cl -b Ag(PPh3)2(P-BSHS) K[ 1-BrB5H7]+ CU(PP~~)~CI -1-BrB5H7) + CU(PP~~)~(~ The structure of CU(PP~~)~(~-B~H~) was determined last year and showed that the metal atom was 2,3-v2-bonded via three-centre two-electron bonds to the B5H8- cage. A gold complex possibly Au(PPh3)(B5H8) was formed at -78 "C but proved 47 D. F. Gaines and S. J. Hildebrandt Znorg. Chem. 1978 17,794. 48 S.J. Hildebrandt D.F. Gaines and J. C. Calabrese Znorg. Chem. 1978 17,790. 49 N.N. Greenwood J. D. Kennedy and W. S. McDonald J.C.S. Dalton 1978 40. K. E. Inkrott and S. G. Shore J.C.S. Chem. Comm. 1978,866. N. N. Greenwood and J. Staves J. Znorg. Nuclear Chem. 1978.40 5. 174 F. A. Hart A. G. Massey P. G. Harrison and J. H. Holloway to be too unstable to isolate.52 Similar reactions lead to the formation of palladium(I1) and platinum(I1) derivatives KB&+ cis-MX2L2-+ cis-M(B5H8)XL2+ KX X =C1 Br I or CH3;L = organophosphines or triphenylarsine The platinum complexes were the more stable and can be stored unchanged for long periods at room temperature; the trans platinum complexes made in the same manner appear less stable than the cis. In all cases the B5Hs ligand is q2-bonded to the metal atom uia the basal borons B(2) and B(3).530 The optimum conditions for the pyrolysis of tetraethylammonium tetrahydro- borate to (NEt4)2[B10H10] have been determined.Treatment of the BloHlo2- ion with strong acids in the presence of dialkylsulphides leads to the rapid formation of B10H12(R2S)2 in good yield. Starting initially from KBH the overall yield of B10H12(R2S)2 is about 45% .53b Potassium decahydro-closo-decaboratereacts at room temperature with anhy- drous hydrogen fluoride to give &OH14 and B18H22; salts of B12H1Z2- and BloCllO2- ions do not react. On the other hand the nido derivative CsB9H14 decomposed vigorously in liquid hydrogen fluoride to form caesium tetrafl~oroborate.~~ A number of routes to Bl2Hl1SH2- have been investigated; the most favourable synthesis was nucleophilic attack on B12H122- by N-methylbenzothiazole-2-thione followed by basic hydroly~is.~’ New products formed from mercapto undecahydro- closo-dodecaborate(2-) include mixed disulphides B12H11SSR2- where R is an organic group and the disulphide monoxide B12HllSOSB12Hl14-.The ion BllH14- can now be made in 62% yield uia a one step synthesis from sodium tetrahydroborate BF,,OEt,+BH,-+ BllH14-+BF4-+Hz+EtzO On addition of tetraethylammonium bromide to the mixture Et4N[B11H14] crystal- lizes BllH14- ions react with either NaHSeO or TeO in heptane-water to produce Bl1HllSe and BllHl ,Te respectively. These compounds are assumed to have closo nearly icosahedral cage structures since their “B n.m.r.spectra are very similar to the spectrum of the known BllHllS.57 All the boron atoms in the Bl1Hll2- anion are n.m.r. equivalent even at tempera- tures down to -70 “C. In theoretical studies of this fluxional behaviour it has been shown that the previously suggested C5,structure is not a viable intermediate or transition A low energy path for the rearrangement connects C and C2 structures but due to the small energy differences between C2,C1,and C2,it is not possible to decide which is the ground state ge~metry.~~ 52 N. N. Greenwood and J. Staves J.C.S. Dalton 1978 1144. ” (a) N. N. Greenwood J. D. Kennedy and J. Staves J.C.S. Dalton 1978 1146; (b) G. Guillevic J. Dazord H. Mongeot and J. Cueilleron J. Chem. Res.(S),1978,402. 54 V. V. Volkov and K. G. Myakishev Zzvest. Sibirsk Otdel. Akad. Nauk S.S.S.R.,Ser. khim. Nauk 1978 41. ” E. 1. Tolpin G. R. Wellum and S. A. Berley Znorg. Chem. 1978 17 2867. 56 G. B. Dunks and K. P. Ordonez Znorg. Chem. 1978,17 1514. 57 G. D. Friesen and L. J. Todd J.C.S. Chem. Comm. 1978,349. D. A. Kleier D. A. Dixon and W. N. Lipscomb Znorg. Chem. 1978,17 166. The Typical Elements 175 The 'resonance energies' of closo borane anions have been calculated by assuming that the three-centre bonds in each triangular face of the polyhedra interact in a similar fashion to the pmorbitals of carbon atoms in unsaturated hydrocarbons. Each molecular orbital of B,H,,- ions can thus be expressed as a linear combination of the three-centre BBB bonding orbitals and the MO calculations carried out in an analogous manner to the Hiickel method used in organic chemistry.The resonance energy of the as yet unprepared B,H,'- ion is zero suggesting it is not aromatic; on the other hand all the known closo anions have positive resonance energies and should be aromatic. The most symmetrical ion B12H122- has the greatest resonance ~tabilization.~' In Mg(BF30H)2(THF)4 prepared from boron trifluoride-tetrahydrofuranateand RMgBr where R is a bulky group the borate groups are co-ordinated to the magnesium uia their oxygen atoms. Other derivatives obtained include Mg(BF30H)2L3 Mg(BF30H)2L2 and Mg(BF30H)2L where L = NMe, NEt, pyri- dine or THF.60 Tetraethylammonium tetranitratoborate made by adding a slight excess of N204 to Et4NBC14 is an ionic solid in which the boron atom has distorted tetrahedral co-ordination to four unidentate nitrate groups; two OBO angles are 97.6" and the other four are 115.6°.6'" In N2H5'B(C~CH)4-,N21& the anion is almost perfectly tetrahedral and the N& molecule is hydrogen-bonded to the hydrazinium cation.616 The action of chlorosulphonic acid on potassium tetrafluoroborate in thionyl chloride produces the water-sensitive KB(S03C1) in which the boron atom is co-ordinated to one oxygen in each of the four S03Cl moieties.62 Condensation of boron tris(difluor0-oxosulphimidate) onto tetraphenylphosphonium difluoro-oxosulphimidate at -80 "C followed by a slow warm up to room temperature in the formation of tetraphenylphosphonium tetrakis(difluorooxosu1-phimidate Ph,P'[B(NSOF,),]-.The hydridotris( 1-pyrazoly1)borate group acts as a terdentate N-ligand in the complex methyl[hydridotris( l-pyra- zolyl)borate]tetraflu~roethyleneplatinum.~~~ Steric effects have been studied in the addition of trialkylboranes to lithium and sodium hydrides; the reactions proceed with 1 1stoicheiometry the products being trialkylhydroborates. An increase in the size of the alkyl group results in sharp decreases in reaction rate. For example with lithium hydride at 25 "C the rates are in the order Et3B > (Bu"),B > (Bu')~B >> (BuS)),B; even in refluxing tetrahydrofuran (BU')~B and other hindered organoboranes react only sluggishly and incompletely. Sodium hydride reacts with the unhindered and a number of the hindered organo- boranes at 25 "C; the other hindered boranes add rapidly in refluxing tetrahydro- f~ran.~~ Rather unexpectedly the corresponding addition of a dialkylborane (e.g.dicyclohexylborane or disiamylborane) to LiR (R = Me Bu or Ph) does not produce 59 Jun-ichi Aihara J. Amer. Chem. SOC.,1978 100 3339. 6o P. Paetzold W. G. Druckenbrodt and A. Richter Chem. Ber. 1978,111. 189. (a)0.A. D'yachenko L. 0.Atovmyan S. M. Aldoshin K. V. Titova and V. Ya. Rosolovskii Dokludy Akad. Nauk S.S.S.R. 1978,238,1132; (b) A. I. Gusev D. Y. Nesterov A. F. Zhigach R. A. Svitsyn and E. S. Sobolev Zhur. strukt. Khim. 1978 19 180. 62 G. Mairesse and M. Drache Acta Cryst. 1978 B34 1771. 63 (a)R. Eisenbarth and W. Sundermeyer Angew.Chem. Internat. Edn. 1978,17,212; (6)N. C. Rice and J. D. Oliver Acta Cryst. 1978 B34 3478. H.C. Brown S.Krishnamurthy and J. L. Hubbard J. Amer. Chem. SOC.,1978 100,3343. F. A.Hart A. G. Massey P. G. Harrison and J. H. Holloway LiR2R'BH; instead equimolar amounts of LiBR2R and LiR2BH2 are formed.65a The structures of two more complexes Mo(q 7-C7H7)[ q6-C6H5B(C6H5)3] and E~,N(MO(CO)~[~~-C~H,B(C,H~),~), have been determined in which a phenyl ring of the tetraphenylborate anion acts as a ?r-bonding ligand. In both compounds the distance of the molybdenum from the carbon bonded to boron is significantly longer than the other five Mo-C(pheny1) The formation of such ?r-complexes can lead to complications when tetraphenylborate is used as a precipitating coun- teranion due to loss of other ligands from the central metal atom.65c In Cu(en)- (CO)BPh, where en is 1,2-diaminoethane there appears to be a weak but significant interaction between the copper and two carbon atoms of one phenyl ring of the tetraphenylborate group; the Cu-C distances are 2.919 and 2.706 A.65d Cyanoborane complexes have been prepared by first treating sodium cyano- trihydroborate with one equivalent of halogen in monoglyme nNa[H3BCN] +! X2 + nNaX + (H2BCN) + H2; X = C1 Br or I 2 2 and then adding a Lewis base such as pyridine an amine or a phosphine to the cyanoborane polymer.When chlorine is bubbled through a moist benzene solution of pyridine-cyanoborane the B -H bonds are cleaved and py,BC12CN results.66" The structures of intermediates formed in the reaction of cyanoborates with carboxyl halides have been determined and shown to be either open chain or cyclic depending on the substituents.666 R' R' \/ 4O C C HO / \ / 'RZ B N H R" R' = cyclo-hexyl; / R2 = CF3.M+RiBCN-+ R'COX \ I R+N \\ RIB CR' 0' R' = cyclo-pentyl; R2 = phenyl. A convenient synthesis of boronium cations containing phosphine ligands has been described PPh Me2SBH3+I2 (one mole) -+ Me2SBHzI -PPh3BH21(92%) PPh3BH21+ L -P (PPh3)LBH2+I-L = PMe2Ph P(~ctyl)~ NMe3 PMe3 '' (a)J. L. Hubbard and G.W. Kramer J. Organometallic Chem. 1978,156,81; (b)M. B. Hossain and D. van der Helm Inorg. Chem. 1978 17 2893; (c) L. A. Oro E. Pinilja and M. L. Tenajas J.Organometallic Chem. 1978 148 81; (d) M. Pasquali C. Floriani and A. G. Manfredotti J.C.S. Chem. Comm. 1978,921. " (a)D. R. Martin M. A. Chiusano M. L. Denniston. D. J. Dye E. D. Martin and B. T. Pennington J. Inorg. NuclearChem. 1978,40,9; (b)P. R. Mallinson D. N. J. White A. Pelter K. Rowe andK. Smith J. Chem. Res. (S),1978 234. The Typical Elements When the iodide ion is replaced by hexafluorophosphate the boronium cations can be directly chlorinated to BHCl and BC12 derivative^.^^" The methyl groups in the boronium complex [H2B(PMe3)2]+Br- are metallated by butyl-lithium in THF to give lithium boratobis(dimethylphosphinomethy1ide) :67b Me2 Me2 v2 CH , PMe P-C\H /CH,-P \ *. MX Br- H2B +/\ + LiBu *H,B / Li(THF) AH,B / M BH 2 \ \ /\ / I' PMe P-CH P-CH CH2-P Me Me2 Me2 M =Zn Cd or Hg.Carbaboranes and Metallacarbaboranes-In carbaborane and metallacarbaborane chemistry a number of isomers are often encountered which can lead to difficulties in the isolation of pure products. However a technique little used by organometallic chemists reversed phase thin layer chromatography promises to be very useful in this respect. By impregnating cellulose plates with nujol and developing the chromatograms with methanol-water mixtures Plotkin and Sneddon have achieved the separation of several isomeric cobaltacarbaboranes from mixtures which had proved inseparable by other techniques.68a The photoelectron spectra of a number of the smaller carbaboranes68b and ferracarbaboranes22 have been recorded and by dividing each molecule into ring and polar fragments an empirical model was developed to rationalize the results.Alkaline methanolysis of 6-NMe3-6-CB9H1 at 60 "C has produced the first hypho-carbaborane. Hypho-3,4-p (trimethylaminecarba)hexaborane(1l),Me,N-CB,H is an analogue of BsHll in which the 3,4-hydrogen bridge has been replaced by a =CHNMe3 bridge.69 The 'direct' synthesis of metallocarbaboranes is an interesting new development in this field. The most basic method involves cocondensation of cobalt atoms cyclo- pentadiene pentaborane(9) and 2-butyne at -196 "C when [2,3-Me2-1,2,3-(q5- C5HS)CoC2B4H4] [2,3-Me2- 1,7,2,3 -(77' -C5HS)2C02C2B3H3],and [2,5-Me2- 1,7,2 5-(q5-CSH5)2C02C2B5H5] are formed. More simply heating together cyclo- pentadienylcobalt dicarbonyl pentaborane(9) and but-2-yne in a sealed tube gives rise to [5,6-Me2-1,8,5,6-(7 5-C5H,)2C02C2B,H,] and [1,7-Mez-3,5 1,7-(q5- C5H,),Co2C2B4H4] albeit in low yield.70" More thermally fragile compounds can be made by photolysis reactions like BC4HSFe(C0)3 which is produced by the short- time photolysis of cyclobutadieneiron tricarbonyl in the presence of pentaborane(9) and has a half-life of about one hour on the probe of a mass spectrometer held at 30 "C.The compound is assumed to be one of several possible isomers based on a pentagonal pyramidal structure.706 (a) B. T. Pennington M. A. Chiusano D. J. Dye E. D. Martin and D. R. Martin J. Znorg. Nuclear Chem. 1978 40 389; (6) H. Schmidbauer G. Muller U. Schubert and 0.Orama Angew. Chem. 1978 90 126. (a) J. S. Plotkin and L. G. Sneddon J. Chromatography 1978 153 289; (b) J. A. Ulman and T. P. Fehlner J. Amer. Chem. SOC.,1978 100 449. J. Duben S. Hermanek and B. Stibr J.C.S. Chem. Comm. 1978 287. (a)G. J. Zimmerman R. Wilczynski and L. G. Sneddon J. Organometallic Chem. 1978,154 C29; (b) T. P. Fehlner J. Amer. Chem. SOC. 1978,100 3250. F. A. Hart A. G. Massey P. G. Harrison and J. H. Holloway The triple-decker sandwich metallocarbaborane[p(2,3) -1,3-C3H4-1,7,2,3-(q5-C5H5)2C02C2B3H3] possesses a Co2C2B3 pentagonal bipyramidal cage in which the two carbon atoms are bridged by an exo-polyhedral propenylene group; the cobalt atoms are in the axial positions of the cage (3).'l The molecule {[5-[q5-C,H,)Co(q5- C5H4)][2,3-Me2C2B4H3]Co[2,3-Me2C2B3H5]} is a zwitterion composed of a [Me2C,B,H3]Co"'[Me2C2B3H5]- unit and a C5H5Co"'C5H4' group with the latter substituent attached to the closo portion of the metallocarbaborane system at B(5).C P14 c pi3 C (3) (Reproduced by permission from Inorg. Chem. 1978 17 10) (4) (Reproduced by permission from Inorg. Chem. 1978,17 1447) The metallocarbaborane structure consists formally of a cobalt(rr1) ion face-bonded to a pyramidal C2B4 and a cyclic C2B3 ligand; i.e. the cobalt atom is common to both an open and a closed polyhedral system (4).'2 " J. R. Pipal and R. N. Grimes Znorg. Chem. 1978 17 10. J. R. Pipal W. M. Maxwell and R. N. Grimes Znorg. Chem. 1978 17 1447. The Typical Elements Platinum can be substituted onto a carbaborane cage via the reaction RR c-c /'A\ HB -BH -BH nido-2,3-R2-2,3-CzB,H,+ Pt(PEt,) -+ I I PEt, H-BH-J / ,Pt \ P H Et 3 (5;R = H) (6; R = Me) the carbaborane cage has the geometry of a nido-pentagonal pyramid.Holding (5) at 100"Cfor three days results in the formation of a closo pentagonal pyramidal cage in which the platinum is at an apex and the carbon atoms are in positions 2 and 4. A similar structure is formed when (6)is heated to 100"Cexcept that the carbon atoms remain coupled together in the 2,3-positions (7).73 (Reproduced from J.C.S. Chem. Comm. 1978 169) 73 G. K. Barker M. Green T. P. Onak F. G. A. Stone C. B. Ungermann and A. J. Welch J.C.S. Chem. Comm. 1978 169.F. A. Hart A. G. Massey P.G. Harrison and J. H. Holloway (7) (Reproduced from J.C.S. Chem. Comm. 1978 169) Oxidation of the nido-7,8-C2B9Hll-ion using aqueous ferric chloride is known to yield nidu- 5,6-C2BsH12; the reaction has been repeated and the by-products separated by chromatography on silica gel identified as 5,6-C2BsHl10H 5,6-C2B8HIlC1,and nido-4,5-C2B7Hll. Yields of the latter rare carbaborane can be substantially increased by using concentrations of reactants in the range 0.1-0.2 moll-’; the pure compound is spontaneously inflammable in air.74 Dicarbadodecaborane-thallium derivatives can be made either by the straight- forward reaction7’ RC-CLi RC-C \ / 1 \ / + TICI -[ ,TIC1 R=Hor CH2Cl BIOH, BIOH or by direct thallation using thallium(II1) trifluoroacetate when substitution occurs at the B(9) HC-CH HC-CH \ / + TI(OOCCF,) -\ /-9-T1(00CCF3)2 B,oH,o Blob 74 H.M. Colquhoun T. J. Greenhough M. G. H. Wallbridge S. Hermanek and J. Plesek J.C.S. Dalton 1978,944. 75 V. I. Bregadze A. Y. Usyatinskii and N. N. Godovikov Doklady Akad. Nauk S.S.S.R. 1978,241,364. ’‘ V. I. Bregadze V. T. Kampel A. Y. Usyatinskii and N. N. Godovikov Izvest. Akad. Nauk S.S.S.R. Ser. khim. 1978 1467. The Typical Elements The mercurials bis( 1,2-dicarbadodecaborane-9)mercuryand bis( 1,7-Dicarbado- decaborane-9)mercury promise to be useful precursors to other B(9)-substituted derivatives e.g. HC -CH [Hcqoca2 9-Hg + TI(COOCCF,) -+ \ /-hoH9 9-T1(00CCF3)2 (ref. 76) HC-CH \ /-g-HgClBIOH9 + Pt(PPh,) -+ HC-CH \ /-BlOH9 9-PtCl(PPh,) (ref.78) When the mercury is bound to carbon however reaction with tris(tri- pheny1phosphine)platinurn results in the formation of Pt-Hg bonded derivative^.'^ PhC-C(CH,) HgX + Pt ( PPh ,I3 -* PhC-C (CH,), HgPtX( PPh,) \/ \/ BlOHlO B,oH,o n =Oor 1 X=ClorBr The molecular conformation of 1-(met hy1mercury)- 2 -(chlorome t hyl) -1,2 -dicarba- dodecaborane(l2) is distorted so that the chlorine is near to the mercury atom the Hg-..Cl distance being 3.27 A.79 ‘Heterocycles’ containing 1,2-dicarbadodecaborane(12) residues have been made via reactions of the type8’ H,C-C‘-‘C LiC-C-(CH,) -C\ -/CLi + Me,SiCI -I \.\/ /%Me BIOHlO BIOHIO H2C-C-C \/ BIOHIO Several rneso-tetracarbaboranylporphyrins including those derived from 1,2-C2B10H1 -1-CH2-1,2,-C2B10H1 and -1-CH2-2-CH3-1,2-C2BloHlo, have been described and were mainly made by Rothemund condensation reactions.81a Substitutionof the boron atoms in 1,2-dicarbadodecaborane(12) with aryl groups has been achieved by reducing the carbabcrane with sodium to the nido-dicarba- dodecaborate(l4) dianion treating this ion with either ArMgX or ArLi and then 77 L.I. Zakharkin and I. V. Pisareva Izuest Akad. Nauk S.S.S.R.,Ser. khim. 1978 1226;V. I. Bregadze V. T. Karnpel and N. N. Godovikov J. Organometallic Chem. 1978 157 C1. ’’ L. I. Zakharkin and I. V. Pisareva Zzuest Akad. Nauk S.S.S.R. Ser. khim. 1978 252. 79 N. G. Bokii Yu. T. Struchkov V. N. Kalinin and L. I. Zakharkin Zhur. srrukt.Khim. 1978,19 380. L. I. Zakharkin and N. F. Shernyakin Zzvest Akad. Nauk S.S.S.R. Ser. khim. 1978 1450. 81 (a)R. C. Haushalter and R. W. Rudolph J. Amer. Chem. SOC. 1978,100,4628; (b)V. N. Kalinin N. I. KobeI’Kova A. V. Astakhin A. I. Gusev and L. I. Zakharkin J. Organometallic Chem. 1978 149 9. F. A. Hart A. G. Massey P. G. Harrison and J. H. Hulluway oxidising the intermediate product with copper(I1) chloride. In this way a mixture of 3- 4- 8-and 9-aryl-1,2-dicarba-closo-dodecaboranes is obtained which may be separated by g.1.c.; the structures of 1,2-Me2-9-Ph-1,2-C2BloH9 and 4-(p-tolyl)-1,2- C2B10Hl were determined and confirmed their clusu-icosahedral nature.81b Dicarbadodecaborane(12) has been bound to a rigid matrix of polystyrene by reaction of 1-Li-1 ,2-C2B10Hll with the chloromethylated copolymer C6H.5 @-CHzCI+ LiC2B loHl -@-CH2CZB10H1 + LiCl Degradation of the cage using piperidine in benzene followed by addition of tris(tripheny1phosphine)rhodium chloride produced a useful hydrogenation catalyst active at about 100 atmospheres of H2:82 (PPh3)RhCI @-CH2C2B10H1 + @-CH2C2B9Hll-@-CH2C2B9HloRh(H)(PPh3)2 A general method of preparing mercaptoheteroboranes involves treating the heteroborane with sulphur in the presence of aluminium tri~hloride~~ AICI 1,2-C2B10H12+S -8-HS-1,2-CzBloHl1 e.g.- AICl ,2-CoC2B9H1 + S 8-HS-3-(q5-C~H&3,1,2-CoC2B9H1~ 3-(v5-CSHS)-3,1 By-products in the latter reaction include the sulphide disulphide and trisulphide S,(1,7-C2B10H11)2,x = 1,2,3.The HS-group can also be replaced by -SMe SMe2' and -SOMe.84" Four of the 71 possible isomers for a compound of formula C2B9H1,-SMe2 have been isolated. Selective degradation of 1-MeS- and 9-MeS- 1,2-C2B10Hll using KOH in the presence of methanol followed by addition of methyl iodide produces the 7-Me& and 5-Me2S-7,8-C2B9Hll derivatives respec- tively. Two further isomers 9-Me2S-7,8-C2B9Hll and 8-Me2S-7,9-C2B9Hll result from the treatment of the 7,8- and 7,9-C2B9HI2 anions with dimethyl sulphoxide. The structures of these nido-carbaboranes are based on an icosahedral cage which has had one vertex removed the two carbon atoms being on the open A new series of 12-atom nidu- heterocarbaboranes Me3NCBloHloPR have been produced by treating BlOHI2CNMe3 first with triethylamine and then with RPC12 (R = Me; Et; Ph).The structure of the derivative with R = phenyl has been solved and shows that the PPh unit bridges boron atoms B(9) and B(10) in the open face of the Bloc icosahedral fragment; the CNMe group occupies position 7 of the '' E. S. Chandrasekaran D. A. Thompson and R. W. Rudolph Znorg. Chem. 1978,17 760. '' J. Plesek and S. Hermanek CON.Czech. Chem. Comm. 1978 43 1325. 84 (a)J. Plesek Z. Janousek and S.Hermanek Coll. Czech. Chem. Comm. 1978,43,1332; (b)J. Plesek Z. Janousek and S. Herrnanek Coll. Czech. Chem. Comm. 1978 43 2862; (c) W.F.Wright J. C. Huffman and L. J. Todd J. Organometallic Chem. 1978,148 7. The TypicalElements Lithium 1,2-dicarbadodecaboranyldithiocarboxylate reacts with L2MC12 complexes of the nickel triad to form derivatives in which the dithio ligand is either monodentate or bidentate RC-C-CS2Li +L2MC12 + LM(S2CC-CR)Z +L2M(S2CC-CR)Z \i \I \I BlOHlO BlOHlO BlOHlO M = Ni Pd or Pt; R = Me; PhL = tertiary phosphine The tendency to farm bis(phosphine) adducts increases in going from Ni to Pt and also appears to depend on the type of co-ordinated phosphine.Five-co-ordinate structures are assigned to LNi(S2CC2Bl0HloRj2 in which and L2Ni(S2CC2BloHloR)2 the dithio ligand exhibits bidentate and unidentate-bidentate modes of bonding respectively. On the other hand four-co-ordination is attained in the bis(phosphine) adducts of palladium and platinum through unidentate co-ordination of the dithio ligand.A rapid intramolecular interchange between the uni- and bi-dentate dithio ligands occurs at room temperature in LPd(S2CC2BloHloR)2.85 Treatment of the [q5-C,H,)CoCB7H,]-anion with NiBr2,2glyme in the presence of C5H5- leads to polyhedral expansion and the formation of several isomers of (CSH5)2NiC~CB7H8 containing formally Ni'" and Co"' atoms. One of these isomers [2,3-di-q 5-cyclopentadienyl- 1O-carba-2,3-(nickelacobalta)decaborane(8)] has the bicapped square antiprismatic geometry expected of a 10-vertex CEOSO cage. The two metal atoms were indistinguishable and occupied adjacent vertices 2.449 A apart in the same equatorial belt (8). Thermal rearrangement to give an isomer with non-adjacent metal vertices does not occur at temperatures up to 450 0C.86 CP9 3 CP (8) (Reproduced by permission from Inorg.Chem. 1978,17,1662) B. Longato F. Morandini and S. Bresadola Inorg. Chim. Acta 1978,26,157. G.E.Hardy K. P. Callahan and M. F. Hawthorne Inorg. Chem. 1978,17 1662. F. A. Hart A. G. Massey P. G. Harrison and J. H. Holloway (Reproduced by permission from Inorg. Chem. 1978 17 1658) Polyhedral expansion of [2-(q '-C5H5)-2-Co- 1,6-C2B7H9] in the presence of FeCl and C5H5- produced the 1 1 -vertex compound [l,8-di-q5-cyclopentadienyl-1-ferra-8-cobalta-2,3-dicarbaundecaborane(9)]. Although one electron short of the ideal 2n +2 electrons for closo-polyhedral bonding this molecule has closo-octadeca- hedral geometry with the iron atom in the six-co-ordinate vertex the two carbons at four-co-ordinate vertices and the cobalt atom at a five-co-ordinate position but not adjacent to the iron.The iron-boron distances are significantly longer than the cobalt-boron bonds probably due to the greater co-ordination number at the iron atom (9).87 The anion in tetramethylammonium [1-q'-cyclopentadienyl-1 -ferra-2-carbaun- decaborate(-1)] is an 11-vertex polyhedron where nine boron atoms and one carbon atom form a decaborane-like framework with six of the cage atoms bonded to the iron. The carbon atom occupies a vertex of lowest co-ordination being bonded to only four other polyhedral atoms; B(3) occupies the other low co-ordination position and this is reflected in the Fe-B(3) bond length of 2.012 8 [mean Fe-B(other) is 2.252 A] (lo)." A comparative molecular orbital study of ferrocene and [l-(q5-CSH5)-1,2,3-FeC2B9H,1]-has shown that while the bonding in ferrocene involves primarily the 7r orbitals of the C,H5 ring that of the dicarbollide (C2B9Hl12-) complex involves the u orbitals of the open pentagonal face in an important role.In both molecules the HOMO'Shave very high dz2~haracter.~~ Oxidative addition of 7,9- and 7,8-C2B9HI2- to (PPh3),RuHC1 produces [2,2- (PPh3)2-2,2-H2-2,1,7-RuC2B9Hll] (11)and [3,3-(PPh3)2-3,3-H2-3,1,2-R~C2BgHll] 87 K. P. Callahan A. L. Sims C. B. Knobler F. Y. Lo,and M. F. Hawthorne Inorg. Chem. 1978,17,1658. 88 V.Subrtova A. Linek and J. Hasek Actu Cryst. 1978,B34,2720. 89 D.A.Brown M. 0.Fanning and N. J. Fitzpatrick,horg. Chem. 1978,17 1620. The TypicalElements U (10) (Reproduced by permission from Actu Cryst.1978 B34,2720) (12) respectively; both complexes may be regarded as seven-co-ordinate formally Ru'" compounds. Compound (11) reversibly eliminates one mole of hydrogen on heating in a vacuum to give the five-co-ordinate formally Ru" derivative 2,2-(PPh3)2-2,1,7-Ru(C2B9Hll). Both (11) and (12) readily lose hydrogen in their reactions with hydrogen chloride and carbon monoxide (11)+HCl + 2,2-(PPh3)2-2-H-2-C1-2,1,7-R~C2BgHll (11)+CO + 2,2-(PPh3)2-2-CO-2,1,7-R~C2B9H11 A pyridine complex [3,3-(PPh3)2-3-H-7-CsHsN-3,172-RuC2B9Hlo] was also pre- pared the pyridine being bonded to a boron atom on the pentagonal face adjacent to the ruthenium." The full paper describing the preparation and structure of [3-diethyldithiocar- bamato-172-dicarba-3-auradodecaborane( 1 l)] has now appeared." In the complexes {3-[C2H4(NMe2)2]-3,1,2-PdC2B9Hll} (13) and [3,3-(PMe&3,1,2-PdC2B9H11](14)there is a marked influence of the ligands trans to the C2B9Hll cage in that (13) has a slipped configuration whereas (14) has much more symmetrical metal-cage binding.Since tetramethylethylene diarnine is a u-donor and tri- methylphosphine is a moderate .rr-acceptor these results provide strong support for 9" E. H. S. Wong and M. F. Hawthorne Znorg. Chem. 1978 17 2863. H.M. Colquhoun T. G. Greenhough and M. G. H. Wallbridge J.C.S.Dalton 1978 303. F. A. Hart A. G. Massey P. G. Harrison and J. H. Holloway (2 n W (14) (13 and 14 Reprinted with the author’s permission from J.C.S.Chern. Comm. 1978 322) The Typical Elements the idea that .rr-acceptor ligands reduce the anti-bonding character and thus the effect of the occupied 5e1~py)-dy,* orbital which is perpendicular to the PdL2 plane and responsible in part for deviations from the regular closo geometry observed in many d8 metallocarbaboranes. Stronger vacceptors than PMe3 on the palladium should thus give rise to eveh more symmetrical stu~tures.~~ Rather unexpectedly 3,3-(PEt,),-3,1,2-PtC2BgHll, although having a virtually identical C2B9Hllcage and similar ligands to the above Pd-PMe complex has a much less symmetrical metal-cage interaction. The structure is that of a highly distorted icosahedron in which the platinum has slipped markedly towards B(8)and the CzB3metal-bonded face folds across B(4)...B(7)away from the metal (15).93 (15) (Reproduced from J.C.S.Dalton 1978 1363) Polyhedral expansion of [3,1,2-(q5-C5H5)FeC2B9Hll] has led to the preparation of the 13-vertex polyhedral [4,5-(q5-CsHs)2-4,5-Fe2-2,3-C2B9Hll] (16) which contains two formally Fe''' atoms and is thus electron deficient. Unexpectedly it also proved to be diamagnetic even though the iron atoms are about 3.20 8 apart and thus not directly bonded; the diamagnetism was assumed to be the result of spin pairing by interaction of the two iron atoms through the carbaborane ligand. The complex was also unusual in that the lowest co-ordination position in the cage was 92 H. M. Colquhoun T.G. Greenhough and M. G. H. Wallbridge J.C.S. Chem. Comm. 1978,322. 93 D. M. P. Mingos M. I. Forsyth and A. J. Welch J.C.S. Dalton 1978 1363. F. A. Hart A. G. Massey P.G. Harrison and J. H. Holloway occupied by a boron and not a carbon atom. Another product of the polyhedral expansion reaction was the ion [C5H5FeC,B9Hl1FeC2B9Hll] in which a C5H5 ring of (16)has been substituted by a 1,2-C2B9HIl2- ligand.94 Tetracarbon 12-vertex cages have been obtained by fusion together of dicar- or bon carbaboranes. When [closo-1,2,3-(C5H5)CoC2B4H6][nido-1,2,3-(C5H5)CoC2B3H7] are treated with 10% alcoholic potassium hydroxide in the air fusion occurs to create either C5H5CoC4B7Hll or three isomers of (C5H5)2C02C4B6H10. The structure of one of the latter isomers was determined and showed the gross geometry was that of a 13-vertex closo polyhedron with one vertex eliminated to create a six-membered open face containing the four carbon atoms.The two cobalt atoms were located in different types of co-ordination sites one being (Reproduced by permission from J. Amer. Chem. SOC.,1978 100 5045) linked to six other framework atoms and the other to only five atoms (17).95 The red air-stable [( 75-C5H5)Co(CH3)4C4B,H,0Et] formed by reaction [Me2C2B4H4]2FeH2with cobalt chloride and cyclopentadiene in ethanolic KOH at 70 "C has a novel structure resembling a severely distorted icosahedron whose two halves have been partially separated creating a huge opening on one side ( 18).96The iron atoms in the 14-vertex [1,14,2,5,9,12(~~-C~H~)~Fe~(CH;)~C~B~H~] occupy the two high-co-ordination positions at opposite ends of the molecule; the four cage carbon atoms are arranged in staggered fashion in the two equatorial rings such that the mutual C-C separations are maximized.The two C5H5 rings are planar unlike the Me2C2B4H4 rings which have their two carbons drawn slightly out of plane in the direction of the other ring; all four rings are parallel to within 1" (19).97 Boron Halides.-The preparation of boron tribromide from potassium tetrafluoro- borate and aluminium tribromide has been considerably improved. By preparing the aluminium bromide in situ and having a KBF :A1Br3 ratio of about 1:10the yield of boron tribromide can be pushed to over 80% making the method suitable for the synthesis of "BBr3 from K'oBF4.98The direct synthesis of boron tri-iodide has been 94 C.G. Salentine and M. F. Hawthorne Znorg. Chem. 1978 17,1498. 95 K.S. Wong J. R. Bowser J. R. Pipal and R. N. Grimes J. Amer. Chem. Soc. 1978 100 5045. 96 J. R. Pipal and R. N. Grimes J. Amer. Chem. SOC.,1978 100,3083. 97 J.R. Pipal and R. N. Grimes Inorg. Chem. 1978 17,6. 98 H.Noth and R. Staudigh Chem. Ber. 1978 111,3280. The TypicalElements CP5 CP4 (Reproduced by permission from J. Amer. Chem. SOC., 1978,100,3083) studied in a Pyrex closed-tube reactor in the temperature range 600-1000 "C the boron being selectively heated by means of an image furnace.99 Passage of boron tri-iodide through a heated silica tube at 840-900 "C gives a-rhombohedra1 boron.100 Mixed boron trihalide adducts are the subject of a recent review containing 189 references.*" Using electron diffraction data estimates of the barrier to internal rotation in the trimethylamine complexes of boron trifluoride and trichloride have been given as ca.6.2 kcal for BF,NMe and ca. 13.7 kcal for BCl,,NMe,.''* Tetramethylethylene diamine (TMED) forms a highly insoluble complex TMED,2BF3 when treated with boron trifluoride-etherate; since the air-stable monoalkylborane complexes TMED,RBH and TMED,2RBH2 react rapidly with BF releasing the RBH2 it has been suggested that these TMED complexes may be a 99 J. Cuelleron and J. C. Viala J. Less-Common Metals 1978 58 123. '('O C. Brodhag and F. Thevenot Compt. rend. 1978 286 C 229.lo' J. S. Hartman and J. M. Miller Adv. Inorg. Chem. and Radiochem. 1978 21 147. M. Hargittai and J. Brunvoll Inorg. Chim. Acra 1978,31 L379. F. A. Hart A. G. Massey P. G. Harrison and J. H. Holloway CP!5 cpll CAZ CP144 CR43 CP145 R42 CPl41 (19) (19 Reprinted with permission from Inorg. Chern. 1978 17 6. Copyright by the American Chemical Society.) useful way of storingmonoalkylboranes.'03Fr0me.s.r. studiesat 77 Kon y-irradiated trimethylphosphine and triphenylphosphine adducts of the boron trihalides 'it is thought that R3PIBX3+ions are formed where a single electron resides in the P-B u 0rbita1.l'~ Like methyls of the other Group IV elements tetramethylsilane acts as a high- yield methylating agent for the boron trihalide~.''~ SiMe4+BX3 4 Me3SiX+MeBX2 X = Br or I 2SiMe4+BX3 -+ 2Me3SiX+Me2BX Diboron tetrafluoride and tetrachloride add to the double bonds of methyl- enecyclopropane and vinylcyclopropane forming the diboryl compounds (20) and (21) respectively.The C-C bonds of the cyclopropane rings are not cleaved even when an excess of diboron tetrachloride is used.lo6 bCH + B2X4 -wCHzBX2 BXZ (20) C>-CH=CH~ C>-CH(BX,)-CH,-BX~ + B~X.,--* (21) A break-through has been achieved in the chemistry of the-boron monochlorides. B. Singararn and J. R. Schwier J. Orgunomerullic Chem. 1978 156 C1. '04 M. C. R. Syrnons and J. E. Drake J. Chem. Res. (S) 1978 122. lo' W. Haubold A. Gemmler and U. Kraatz Z. Nuturforsch. 1978 33b 140. W. Haubold and K.Stanzl Chem. Ber. 1978 111,2108. The Typical Elements Addition of 9 equivalents of S02C1 to (NBu4),B9H9 at -78 "C results in good yields of (NBu4),B9C19; more interesting however is the fact that a large excess of S02C12 (ca.20 equivalents) produces B&19 in 3040%yield. It appears likely that B9H9 or B9H9- C1 are first formed by oxidation of the corresponding anions and then these are chlorinated to give the observed B&19 because the BgC1g2- ion does not react with S0,Cl,.'07 Degradation of BloCllo and BllClll with chlorine produces B&19 in high yield; with hydrogen bromine and iodine mixed species B9C19-,X, are formed.'" Heterocyclic Boron Derivatives.-Oxidation of the complexes (22; R = Me or Ph) prepared from Co(C5H,BR) and [Fe(CO),(C,H,)], results in a novel ring contrac- tion producing Fe(C,H,R)(C,H,)' and boric acid.On attempted Friedel-Crafts acetylation of (22) a rather unique ring-member substitution occurs to give after hydrolysis Fe(C,H5Me)(CSH5)+ and RB(OH),; when carried out on Fe(C5H5BMe) the product was either Fe(C6HsMe)(C5H5BMe)' and/or Fe(C6H5Me) depending on the reaction conditions.109a (22) The addition of sodium cyanide to CO(C,H,BR)~ R = Me; Ph in acetonitrile results in the formation of the sodium borinates from which the first isolable Main Group borabenzene derivatives T1(C5H5BR) may be prepared by addition of thallium(1) The recently described metallo-p-diketone (22)reacts with all the boron trihalides at -35 "C to form metallo analogues of the well-known P-diketone complexes of BF,:'" Me Me /C -q F-9 (Co)4Re ,,H X=F+Jw (CO),Re ,BX, BX3 C-0 c-0 Me Me (22) N-Alkyl amino-acids react with BF3 to eliminate hydrogen fluoride and form cyclic (a-amino-carboxylato-N)boronderivatives" ' \ +BF R = CH2Ph; R' =Pr' Bu' Bus CH2COOH R = CH2CH =CHPh; R' =CH2Ph.lo' R. M. Kabbani and E. H. Wong J.C.S. Chem. Comm. 1978,462. 'OB S. B. Awad D. W. Prest and A. G. Massey J. Inorg. Nuclear Chem. 1978,40 395. '09 (a)G. E. Herberich and K. Carsten J. Orgunomerullic Chem. 1978,144 C1; (b)G. E. Herberich H. J. Becker and C. Engelke J. Organometallic Chem. 1978,153 265. C. M. Lukehart and L. T. Warfield Inorg. Chem. 1978,17,201. ''' J. Halstr~m,E. Nebelin and E. J. Pedersen J. Chem. Res. (S) 1978 80.F. A. Hart A. G. Massey P. G. Harrison and J. H. Holloway When diphenylborinic acid anhydride and N-methylacethydroxamic acid are mixed in absolute ethanol crystals of diphenylboron N-methylacethydroxamate (27) begin to precipitate after a few minutes; the 2,1,3,4-B02CN ring in (24) is essentially planar. Ph Ph Ph Ph \/ \/ B ?'-'p e) /C=N+ C-N \ /\ Me Me Me Me (244 (24b) Although it is difficult to give an exact formal description of (24) it may be best regarded as a 3 :1 hybrid of the resonance forms a and b.*12 A variety of BSiO-rings have been made from phenylboronic acid using reactions such as113 OH CISiMe 0-SiMe, / \ +Et3N Phi 'NMe + PhB\ /NMe -Et3NHC \ / OH CISiMez 0-SiMe? Me Me si%i,2 /OH C1-SiMe +Et,N PhB + 1 0 I CI-SiMe -Et3HNC1 PhB ,SiMe, \OH 0-SiMe Similar condensation reactions between phenylboronic acid and a,o-diols HO(CH2),0H n =5 6 10 or 12 give rise114a to equilibrium mixtures of oli-gomeric ringsfO-(CH2) -OB(Ph)Q.Tetrazadiborines are formed when 1,2-dimethylhydrazine is treated with either B (SMe)3 or B (NMe2) -XB Me Me N-N BX + MeNHNHMe 'BX X=SMe NMez \/ N-N Me Me The groups on boron may be substituted by a variety of reagents but these have to be chosen carefully to reduce the possibility of ring cleavage. For example although extensive cleavage occurs with boron tribromide bromine substitution can be effected by dimethylboron Both B and N organo-substituted tetra-zadiborines may be prepared directly by suitable choice of reactants114c 11* S.J. Rettig J. Trotter W. Kliegel and D. Nanninga Cunud. J. Chem. 1978 56 1676. U. Wannagat and G. Eisele 2.Nuturforsch. 1978 33b 475. 'I4 (a)U. W. Gerwarth Mukromol. Chem. 1978,179 1497; (b)H. Noth and W. Winterstein Chem. Ber. 1978 111 2469; (c) D. Nolle H. Noth and W. Winterstein Chem. Ber. 1978 111 2465. The Typical Elements -RB R' R' N-N RB(SMe)2 + R'NHNHRI /\ ,BR N-N R' R1 Reaction of 1,3,4,5-tetraethyl-2-methyl-l,3-diborolenewith cyclopenta-dienylnickel carbonyl dimer leads to the formation of the nickelocene analogue (25) From (25) it is possible to make triple-decker sandwich complexes in high yield using to so-called 'stacking' reaction technique Et Et n (25) + C5H5Co(CO) -(C,H,)NiEtBQBEtCo(C,H,i Me (26) Me Et Et n (25) + [C,H,Ni(CO)] -+ (C,H,)NiEtBOBEtNi(C,HS) v Me (27) The deep green air-stable complex (27) is paramagnetic (with 33 valence electrons); reduction gives the diamagnetic ion (27)- while treatment with silver tetrafluoro- borate produces the blue (27)'BF4- which is paramagnetic with two unpaired electrons being isoelectronic with (26)."' The di-iodothiadiborolene (28) has been prepared by first adding boron tri-iodide across hex-3-yne and then treating the cis-trans mixture of 3-diiodoboryl-4-iodo-3-hexene so formed with (IBS)3 Et Et Et Et c=c \ (IBS) \ EtCGCEt + BI -* /C=C / * RB ,BR S ',B \I (28; R = I) The two iodine atoms in (28)can be readily substituted by Me C1 Br OEt SMe and NMe2.'16 When (28; R=Me) is irradiated with either Cr(CO) or Mo(CO) in tetrahydrofuran complexes (28)M(C0)4 and (28),M(CO) are formed in which the thiadiborolene is thought to be acting as a penta-hapto ligand."' Like diborolene W.Siebert J. Edwin and M. Bochmann Angew. Chem. Infernal. Edn. 1978 17 868. W. Siebert R. Full J. Edwin and K. Kinberger Chem. Ber. 1978 111 823. 'I7 K. Kinberger and W. Siebert Chem. Ber. 1978 111 356. F. A. Hart A. G. Massey P. G. Harrison and J. H. Holloway thiadiborolene is also able to bond simultaneously to two metals so that triple- and even tetra- decker sandwich compounds can be built up:"' (28; R = Me)+ Mn2(CO)lo+ [C5H5Fe(CO)J2 -+ (C0)3Mn(28)FeC5H5 (C0)3Mn(28)FeC5H5 + C6H6+ A1CI3 + [C6H6Fe(28)Mn(C0)3]'AlC14-(CO)3Mn(28)Fe(28)Mn(CO) i heat In contrast the heterocyclic ring of benzodiazaboroles does not function as a ligand to the Cr(C0)3 moiety in (29),the bonding being exclusively via the benzo A novel rearrangement to yield benzoxazaboroles benzothiazaboroles and benzoselenazaboroles occurs when diborane reacts with benzoxazole or its sulphur and selenium ana10gues.l~~~ E = 0,S Se; (i) $B2H6;(ii) detected by n.m.r.Ring expansion by two atoms occurs when 1,3,2-diazaboracycloalkanesare treated with either phenyl isocyanate or phenyl isothiocyanate'20 e.g. Et Aniline bis (difluoroborondimethylglyoximato)nickel(II)forms as deep scarlet needles when aniline and Ni(dmgBF2)2 are mixed in warm dichloromethane. The dimeric nature of the planar parent Ni(dmgBF2)2 with a cis conformation of the BF2 groups is retained upon reaction with aniline.The nickel atoms are each surrounded by four nitrogen atoms ofthe closely planar macrocycle and an aniline is co-ordinated in an axial position to form an irregular square pyramid. The two formula units of the dimer (Ni-Ni = 3.654A) are non-equivalent the Ni-N(ani1ine) distances 2.724 'I8 W. Siebert C. Bohle C. Kriiger and Y.-H. Tsay Angew. Chem. Infernut.Edn. 1978,17 527. 'I9 (a)R.Gotze and H. Noth J. Orgunomefulfic Chem. 1978,145,151; (b)K. K. Knapp P. C. Keller and J. V. Rund J.C.S. Chem. Comm. 1978,971. 12') U. W. Gerwarth and K. D. Muller J. Organomerullic Chem. 1978 145 1. The Typical Elements and 2.602 A differ significantly and the two nickel atoms are displaced unequally out of the equatorial co-ordination plane by 0.1 18 and 0.138 A.121 The seven-membered borepin ring of spiro(!?H-borepino[2,3-6:7,6-b']dithio-phene-9,2'-[ 1,3,2]oxazaborolidine} (30) is in a boat configuration; the five- membered oxazaborolidine ring in a half chain conformation is practically at right angles to the borepin ring giving the boron atom essentially tetrahedral co-ordination.Neither thiophene ring shows a significant deviation from planarity.'" The vibrational spectrum of B-trimethylborazine has been recorded from 200 to 4000cm-' and assignment of fundamentals proposed with the help of a study of selected isotopically labelled species. Although extensive mixing of vibrations was observed it is evident that methylation of the borazine ring has virtually no influence on the electronic nature of the heter~cycle.'~~ Reaction of the bifunctional 2,4- dichloro-1,3,5,6-tetramethylborazinewith a mixture of dimethylformamide and dimethylamine leads to low yields of a macrocyclic polyborazine in which six borazine rings are linked by oxygen bridges.lZ4 Boron-nitrogen Compounds.-The layer form of boron nitride reacts with S206F2to form'25 a deep blue electrically conducting solid of approximate composition (BN)4+S03F-.A close study has been made of the thermal decomposition of ammonia-borane; a vigorous decomposition with rapid hydrogen formation begins at 120 "C and is followed by a slower gas evolution between 145 and 200 "C. The composition of the white residue is approximately BNH2.' after heating to 170 "C and BNH0.8-l.2 after heating to 200 "C.The infrared spectrum of a sample pyrolysed for a short period at 500-600 "C resembled that of boron nitride although an NH band at 3450 cm-' was still present.1260 When ammonia is passed through molten ammonia-borane at 125 "C diaminoborane (NH2)2BH is formed in 10-17% yield. In the liquid state diaminoborane decomposes to a glassy solid but it is stable in liquid ammonia and may be kept in the gas phase at room temperature for several days. The short B-N bonds 1.42 A suggest substantial 7r-bond character in the NBN framework.'26b 121 R. S. Vagg and E. C. Watton Actu Cryst. 1978 B34 2715. 122 I. Cynkier and H. Hope Actu Cryst. 1978 B34 2990.123 K. E.Blick E. B. Bradlry K. Niedenzu M. Takasuka T. Totani and H. Watanabe 2.anorg. Chem. 1978,442 183. 124 A.Meller and H.-J. Fiiilgrabe Chem. Ber. 1978,111,819. 125 N. Bartlett R. N. Biagioni B. W. McQuillan A. S. Robertson and A. C. Thompson J.C.S. Chem. Comm. 1978,200. 126 (a)M. G. Hu R. A. Geanangel and W. W. Wendlandt Thermochim. Actu 1978 23 249; (b) T. S. Briggs W. D. Gwinn W. L. Jolly and L. R. Thorne J. Amer. Chem. SOC.,1978,100,7762. F. A. Hart A. G. Massey P. G. Harrison and J. H. Holloway A microwave structural investigation of the weak adduct tri-methylamine-trimethylboraneshows that the B-N bond (1.698k0.01A) is the longest of any BX3 adduct with trimethylamine; the B-C bond length of 1.69k 0.04 8 also appears to be the longest yet reported.Steric effects are considered to be responsible at least in part for the decrease in stability of the trimethylborane adduct relative to that of boron triflu~ride.~" An apparently general synthesis of amine-cyanoboranes involves refluxing sodium cyanotrihydroborate with the corresponding amine hydrochlorides in tetra- hydrofuran.128 Ammonia-cyanoborane can be prepared by the amine replacement reaction C6HSNHZBH2CN+NH3 + H3NBH2CN+C6HSNH? The B-N and B-C bond lengths in this adduct are 1.577 and 1.589A respec-tive~~.*'~ Aminoboranes which show no evidence of decomposition when heated at 150-200 "C in an inert atmosphere have been made via the reaction NR2 NR2 / +2NH3 / (Me3Si)'N-B ____* (Me3Si)'N-B R = Me Et Pr' -NH4CI \ \C NH2 The bis(trimethylsily1)amino group is twisted about 90" out of the plane defined by the boron and three nitrogen atoms.Hence the boron is sterically protected from nucleophilic attack which presumably explains the unusual thermal stability of these compounds since the necessary intermolecular interaction is prevented.I3' The bis(di-isopropy1amino)amino boranes (Pr2'N)2BNRR' possess helically chiral configurations which are stereochemically rigid at low temperatures. N.m.r. line shape analysis indicates that these molecules enantiomerize by correlated BN rotations through transition states in which the substituents attached to two of the nitrogen atoms are in the plane of the BN3 unit while those on the third nitrogen are perpendicular to this plane.131a Phosphorus atoms act as the donor sites in complexes such as (CO)SMR2PB(NR')2 [(CO)SM]2(PEt2)2BNEt2 cis-M(C0)4(PR2)2BNR and c~~-M(CO)~(PR~)~B~(NM~~)~ where M is chromium molybdenum or tungsten.13' N.m.r. studies of scrambling reactions between B(NEt,),/PCl and B(NEt&/XPC13 X =0 S show that the mixed species are more favoured than would have been the case if a completely random mechanism was involved. In the B(NEt,),/PCl system the chlorine substituent displayed a preferential affinity for phosphorus over boron while the reverse was true in the systems involving four-co- ordinated phosphorus. It was assumed that for the former system the more effective P. M. Kuznesof and R. L. Kuczkowski Znorg. Chem. 1978 17 2308.'** P. Wisian-Neilson M. K. Das and B. F. Spielvogel Inorg. Chem. 1978 17 2327. A. T. McPhail K. D. Onan B. F. Spielvogel and P. Wisian-Neilson J. Chem. Res. (S),1978 205. 130 D. M. Graham J. R. Bowser C. G. Moreland R. H. Neilson and R. L. Wells Znorg. Chem. 1978,17 2028. 13' (a)K. K. Curry and J. W. Gilje J. Amer. Chem. Soc. 1978 100 1442; (b)H. Noth and S. N. Sze Z. Nuturforsch. 1978 33b 1313. The Typical Elements rr-donor ligand NEt, would be bonded to the better acceptor B; in four-co- ordinated phosphorus system the presence of oxygen and sulphur on the phosphorus centre could enhance its effective electronegativity thus perhaps increasing its acceptor ability to such a degree that it became superior to boron in this N-Substituted trimethylsilylamides react with 2-chloro-1,3-dimethyl-l,3,2-diazaborolidine chloro(dimethylamino)phenylborane tris(diethy1amino)borane and B-chloropentamethylborazine to give monomeric amido boranes and borazines in nearly quantitative yields:'33 H2 H2 c-c II 0 R2 Me MeN ,NMe R'=Me R2 =Me R'-C-N / B R' = Ph R2= Me II + c,By-iH2-\ \ I RI R' =Me R2= Ph SiMe N-cH R2/N\ C/ Me II 0 0 The phosphiniminoboranes prepared via the reaction Ph3P=NSiMe3 + R2BX + Ph3P=NBR2 R = F C1 Br Bun Ph X = F C1 Br are monomeric in the gas phase; in solution the dihalogeno compounds are asso- ciated probably having a dimeric Reactions of methyl bis(methy1thio)borane with various hydrazines have been described; 1,1-dimethylhydrazine gives the hydrazinoboranes MeB(SMe)NHNMe2 and MeB(NHNMe2)2.Symmetrical dimethylhydrazine reacts rapidly to produce the bis(bory1hydrazino)methylborane(31) or hexamethyltetrazadiborine (32) depend-ing on the reaction ratios Me BSMe Me / Me Me N-N / N-N / Me \ MeNHNHMe + MeB(SMeI2 -* MeB \ MeB BMe ,Me \I N-N N-N Me \ Me Me BSMe Me (32) 13* J.-P. Costes G. Crcs and J.-P. Laurent J. Inorg. Nuclear Chem. 1978 40 829. 133 W. Maringgele and A. Meller Chem. Ber. 1978,111,538; A. Meller W. Maringgele and K.-D. Kablau Z. anorg. Chem. 1978,445 122. 134 W. Maringgele A. Meller H. Noth and R. Schroen 2.Naturforsch. 1978 33b 673. 198 F. A. Hart A. G. Massey P. G. Harrison and J. H. Holloway Only the five-membered triazadiborolidine ring derivative (33) could be isolated with m~nomethylhydrazine'~~ Me H N-N MeB BMe N' I /N\ H Me The reactive N-H bonds in Me,BNHNMe and Me2BNHSiMe3 can be metallated by methyl-lithium the intermediate salt [Me,BNHR]Li being detectable by n.m.r.136 Berates.-Boron atoms lie at the centres of equilateral triangles of oxygens in the isolated BO groups of NdGa3(B03)4 and the gallium atoms are octahedrally co-ordinated by oxygen.137 In synthetic manganese kurchatovite CaMn(B205) two triangular B03 units are linked through a common oxygen atom to form the B20s4- groups.138 The polymeric repeating B305 unit in LiB,O6 has two planar three-co- ordinated and one tetrahedral four-co-ordinated boron atoms; the compound is made by reaction of LizO with water-free boric A redetermination of the structure of borax by single crystal neutron diffraction fully confirms the conclusions of an earlier X-ray study and also provides accurate locations of the hydrogen atoms; each stoicheiometric anion B4072- incorporates two water molecules to form the B405(OH)42- gro~ping.'~' The synthetic borate Na2[B406(OH)2] has B-0 chains with the repeat unit B406(OH)22- in which there are two triangles and two tetrahedra condensed in a double ring arrangement.Sodium atoms are linked to oxygen atoms from three different chains so that a three-dimensional structure further strengthened by hydrogen-bonding is formed. 14' An isolated polyanion B506(OH),- is present in NaB506(0H)4 and is formed by one tetrahedron and four triangles in the shape of a double hexagonal ring; the two rings are almost perpendicular to each other (34).14 H 0 \ /OH /B-o\...O-B 0 B. /o \ '0-B, B-0 OH 0' H 135 D. Nolie and H. Noth Chem. Ber. 1978 111,469. 136 H.Fusstetter and H. Noth Chem. Ber. 1978 111 3596. 137 E.L. Belokoneva L. I. Al'shinskaya M. A. Simonov N. I. Leonyuk T. I. Timchenko and N. V. Belov Zhur. strukt. Khim. 1978,19,382. 13' 0.V. Yakibovich M. A. Simonov and N. V. Belov Doklady Akad. Nauk S.S.S.R.,1978 238 98. 139 H. Konig and R. Hoppe 2.anorg. Chem. 1978,439 71. H. A. Levy and G. C. Lisensky Acta Cryst. 1978 B34 3502. 141 S. Menchetti and C. Sabelli Acta Cryst. 1978 B34 1080. 142 S. Menchetti and C. Sabelli Acta Crysf. 1978 B34 45. The Typical Elements 199 Several quaternary ammonium salts -of the B506(OH),- anion have also been described.143 A refinement of the structure of the industrial bleaching agent disodium tetra- hydroxo-di-p-peroxo-diboratehexahydrate Na2[B2(02)2(OH)4],6H20 confirms the original work. The compound contains centrosymmetric cyclic B2(02)2(OH)42- ions in which the boron atoms are joined by two peroxo bridges and their tetrahedral co-ordination is completed by terminal OH groups. 144 Selected Organometallic Topics.-The barrier to internal rotation in phenylboron dichloride is 13.6f0.8 kJ mol-I as deduced from the microwave spectrum; this is in line with the value for phenylboron difluoride and considerably less than the previous estimate of 188 kJ mol-' calculated from infrared data.',' Diphenylborane made in situ by either the reaction of methoxydiphenylborane with LiAlH or addition of boron trifluoride etherate to Ph2BH,pyridine reacts readily with olefins to give high yields of alkyldiphenylboranes; the latter method of generation is suitable for the hydroboration of olefins containing functional groups sensitive to lithium tetrahydroaluminate.The synthesised alkyldiphenylboranes react with methylvinylketone to give good yields of the corresponding 4-alkyl-2- butanones whereas only one alkyl group of a trialkylborane can be transferred 146 Ill RBR +:c=c-c=o + H2° + R-C-C-C=O+R:BOH I I Ill R' = Ph or alkyl The spectral changes that occur on solidification of tris(perfluoroviny1)borane indicate the presence of two conformers in the gas and liquid phases neither of which is planar as might have been expected from the structure of trivinylborane.Even though the fluorine atoms on adjacent C2F3 groups are separated by more than twice their van der Waals radii the twist about the B-C bonds must relieve unfavoured steric and/or electronic interactions. The 'twisted' C3conformation is a geometry which severely limits the amount of p,,-p, overlap that is possible in this molecule. It is concluded that there is little iit any multiple bond character in the B-C bonds.',' Similarly there is apparently little interaction between the diene .rr-system and the electron deficient boron in 1-methyl- 1,4-dihydroborabenzene. 14* Bis-cyclopentadienylmercury reacts with dimethylboron chloride at -78 "C to form 5-C,H,BMe2 which is fluxional down to -90 "C;above -15 "C a 1,5 hydrogen shift occurs to give a mixture of the two 'vinyl' isomers (C,H,),Hg + Me,BCI -78"c_ above BMe 143 G.Heller and B. Bichowski 2.Nuturforsch. 1978 33b 20. 144 M. A. A. de C. T. Carronodo and A. C. Skapski Actu Cryst. 1978 B34,3551. 145 W. Caminate and D. G. Lister J.C.S. Furaduy 11,1978 896. 146 P. Jacob J. Organometallic Chem. 1978,156 101. 141 J. D. Odom E. J. Stampf J. R. Durig V. F. Kalasinksy and S. Riethmiller J. Phys. Chem. 1978 82 308. 14' S. A. Sullivan H. Sandford J. L. Beauchamp and A. J. Ashe J. Amer. Chem. SOC.,1978 100 3737. F. A. Hart A. G.Massey P. G. Harrison and J. H. Holloway Pentamethylcyclopentadienyldimethylboranecannot undergo a hydrogen shift and consequently is stable as the 5-isomer at room temperature whilst remaining fluxional down to -90 0C.149 Me Me,C,Li + Me,BCI -+ Me@&BMe2 Me a-Methoxyvinyllithium adds to trialkylboranes forming the Li'[R3BC(OMe)=CH2]-salts which are stable at -80 "C; on warming to room temperature however an alkyl group migration occurs to give Li'[R2B(OMe)CR=CH2]-.150 High yields of substituted allenes have been obtained by reacting alkynyltrimethylstannanes with trialkylboranes:151 RiB\ ,SnMe R:B + Me,SnCrCR' 4 [R.;B--CGCR2Sn'Me,] -C=C R' /\ R2 Reagents i THF or hexane at 0 "C; ii Me3SnC=CR3 at 80-120 "C 2 Aluminium Oxygen plays a dual role in the corrosion of aluminium in oxygen-saturated water.It takes part in the oxygen reduction reaction so providing the cathodic part of a corrosion cell and it will also repair the passivating surface oxide film. The repair may be impeded by the presence of anions in solution thus nitrite iodide bromide and chloride increasingly impede the repair reaction whereas sulphate and nitrate have no influence at all. Chloride ion is known to adsorb onto the surface; following adsorption a thermodynamic equilibrium is set up between the surface oxide and aluminium chloride and depending on the state of this equilibrium the surface will remain passive or will undergo pitting corrosion. Below a critical surface chloride concentration the corrosion may repa~sivate.~~~ The infrared stretching frequencies of the diatomic molecules AlH and AID isolated in an argon matrix at 14 K are 1593 and 1159 cm-' re~pectively.'~~ A study of the decomposition of aluminium sulphate has shown that the 'anhy- drous' salt made by usually accepted methods contains about 0.03 mol of water per mol of sulphate ion.The solid product when the decomposition is carried out at 149 H. D. Johnson T. W. Hartford and C. W. Spangler J.C.S. Chem. Comm. 1978 242. A. B. Levy S. J. Schwartz N. Wilson and B. Christie J. Organometallic Chem. 1978 156,123. ''' B. Wrackmeyer and R. Zentgraf J.C.S. Chem. Comm. 1978,402. "* R. T. Lowson Austral. J. Chem. 1978,31 943. 153 R.B. Wright J. K. Bates and D. M. Gruen Znorg. Chem. 1978 17 2275. The Typical Elements 201 600 "C is an amorphous form of aluminium oxide which has a very high surface area ca.165 m2 g-'.15 There has been considerable activity this year in the field of aluminium chloride and chloroaluminate melt chemistry. Silica glass is attacked by anhydrous alu- minium chloride at temperatures above 300 "C to produce silicon tetrachloride; the solid product is either AlOCl or A1203depending on the temperature. When a mixture of aluminium and aluminium chloride is heated to approximately 350 "C in a silica tube silica 'foil' is deposited on the walls and is assumed to arise from reduction of silicon tetrachloride-formed initially by attack of the tube-by gaseous A12Cl A single-crystal Raman and infrared spectral study has been carried out on A1Cl3,6H2O; v1 for A106 is placed at 524 ~rn-'.'~~ The highly hygroscopic potassium tetrachloroaluminate contains discrete but slightly distorted AlC1,- anions in which the Al-Cl distances vary from 2.119.to 2.140 A.157 Ammonium tetrachloroaluminate which is isostructural with ammonium perchlorate has similar Al-Cl bond lengths to those of the alkali tetrachloroaluminates (average 2.117 Although solid acetyl chloride-alu- minium chloride is ionic with AlCl,- ions a 27Aln.m.r. study shows that the complex has a donor-acceptor structure in dichloromethane The infrared spec- trum of the tetrachloroaluminate anion in chloroaluminate melts has been recorded for the first time.'60 Fused chloroaluminate melts of composition (Na,K)Al,Cl are essentially ionic M+A12C17-. The amount of free aluminium chloride is small and the equilibrium 2AIC14-+Al2Cl6 S 2A12C17-is reached rapidly.16' Similarly the Raman spectra of molten aluminium chloride/ 1-butylpyridinium chloride mixtures varying in molar composition from 0.75 :1.O to 2.0 :1.O indicate the presence of AlC1,- and A12C17- ions. The equilibrium constant for the above reaction was found to be significantly larger than that for alkali metal chloride-aluminium chloride melts. Aluminium chloride/alkylpyridinium chloride mixtures have relatively low liquidus temperatures and thus make useful aprotic molten salt media of varying acidity for studying the reactions of many organic or thermally unstable compounds. 162 A mass spectrometric investigation of the system CuC1(s)/Al2Cl6(g) established that Cu3A1Cl6 and Cu2A12C18 are the principal gas complexes.The complexes are assumed to have cubic-type structures similar to that of CU,C~,'~~ e.g. (35). The cyclic structure (36) for the gaseous complex CuA12C18 formed when CuCl and A12C16 are heated together at 250-300°C in a sealed tube has been 154 G. F. Knutsen and A. W. Searcy J. Electrochem. SOC. 1978 125 327. 155 H.Schafer 2.anorg. Chem. 1978 445 129. D. M. Adam and D. J. Hills J.C.S. Dalton 1978 782. 157 G. Mairesse P. Barbier and J.-P.Wignacourt Acra Crysr. 1978 B34 1328. G. Mairesse P. Barbier J.-P. Wignacourt A. Rubbens and F. Wallart Canad. J. Chem. 1978,56,764. 159 J. Wilinski and R. J. Kurland J. Amer. Chem. SOC., 1978 100 2233. I6O N.R. Smyrl G. Mamantov and L. E. McCurry J.Znorg. Nuclear Chem. 1978,40 1489. 161U. Anders and J. A. Plambeck J. Inorg. Nuclear Chem. 1978,40 387. 162 R. J. Gale B. Gilbert and R. A. Osteryoung Znorg. Chem. 1978 17 2728. 163 H. Schafer and H. Rabeneck 2.anorg. Chem. 1978 443 28. 16* C. W. Schlapfer and C. Rohrbasser Inorg. Chem. 1978 17 1623. F. A. Hart A. G. Massey P. G. Harrison and J. H. Holloway ~ha1lenged.l~' At higher temperatures CUA12Cls loses aluminium chloride to form CuAlCl which probably has the structure (37).165 A DZhstructure having palladium as the central atom is suggested for the gaseous complex PdA12Cls formed in the heated AlzC16/PdClz Two complexes EuAl3Cll1 and EuAl,Cl,, are thought to exist in the vapour above liquid mixtures of aluminium chloride and europium(I1) chloride.It is calculated that the concen- tration of Eu" which can be maintained in the gas phase at 700 "C is increased by a factor of 10'2.5by the presence of the aluminium trichloride.'66 From a study of the Raman spectrum of elemental sulphur dissolved in either molten aluminium tri- chloride or chloroaluminate melts it is concluded that the sulphur dissolves as Ss.'67 Melts of sodium chloride and methylaluminium dichloride have been shown to have a high specific conductivity 0.19 ohm-' cm-' at 180"C. Electrolysis of these melts using sacrificial anodes such as tin and mercury leads to fair yields of the metal methyls.168 Dibromo- and di-iodo-dimethylaminoaluminiumhave been prepared by the action of the corresponding mercury(I1) halide on dimethylaminoalane H2AINMe2+HgX2 + Hg +Hz+X2AlNMe2 The molecules exist as dimers with a planar four-membered ring of alternating aluminium and nitrogen atoms.169 Aluminium tri-iodide reacts with iodine azide in benzene yielding di-iodoaluminium azide I,AlN,; treatment of this with liquid bromine gives BrzA1N3.170 The mixed complexes LiMg(AlH4)3 and LiMg3(AlH4)7 together with Mg(AIH,)z,EtzO have been prepared from magnesium chloride and lithium tetra- hydroaluminate in ether s01ution.l~' t-Butylalcoholysis of MAlH at room temperature for M=Li Na or K and at 135 "C for M=Cs produces the tris(t- butoxy)aluminates M[HAl(OBu'),] the anion HAI(OBU')~- being tetrameric.The stability towards further alcoholysis increases from Li to Cs.17' Reaction of the alkaline earth metals with tris(isopropoxy)aluminium in boiling isopropanol in the presence of HgC12 as a catalyst gives the double alkoxides {M[A1(OPri)4]2}n where n is 1 for M=Mg and n is 2 for M=Ca Sr or Ba.165 G. N. Papatheodorou and M. A. Capote J. Chem. Phys. 1978,69 2067. 166 M. Sorlie and H. A. Oye J. Znorg. Nuclear Chem. 1978,40,493. 167 R. Huglen F. W. Poulsen G. Mamantov R. Marassi and G. M. Begun Znorg. Nuclear Chem. Letters 1978 14 167. A. von Rumohr and W. Sundermeyer Z. anorg. Chem. 1978,443 37. 169 A. Ahmed W. Schwarz and H. Hess 2.Naturforsch. 1978 33b 43. 170 K. Dehnicke and N. Kriiger Z. anorg. Chem. 1978,444 71. ''I B. M. Bulychev K. N. Semenenko and K. B. Bitsoev Koord. Khim. 1978,4,374. 17* M. I. Karpovskaya N. Ya. Turova G.A. Kirakosyan V. P. Tarasov and N. I. Kozlova Koord. Khim. 1978 4 907. The Typical Elements 203 Treatment of the double alkoxides with an excess of either t-butylalcohol or t-amylalcohol in boiling benzene leads to partial alcoholysis of isopropoxy groups and the formation of either M[A1(OPri)(OR),I2 (M=Mg Ca Sr) or Ba[Al(OPri)2(0R)2]2 which are volatile and monomeric.173 The volatile double isopropoxide of tin and aluminium S~[AI(OPT~)~]~ made by treating Na[Al(OPr'),] with tin tetrachloride is monomeric in boiling benzene. 174 Three main bands corresponding to orbital binding energies of approximately 10 13.5 and 27 eV occur in the X-ray photoelectron spectrum of spinel MgA1204. The two bands of lowest energy are mostly oxygen 2p in character whilst the band at 27 eV is almost exclusively oxygen 2s.Aluminium 3s and 3p character are found in both of the least tightly bound orbitals but somewhat more is present in the 13.5 eV band than that at 10 eV; this feature is reversed for the magnesium 3p orbitals and reflects the different bonding situation of the aluminium and magnesium atoms in the spinel Pentacalcium trialuminate 5Ca0,3A1203 better known to cement chemists as unstable C5A3 has a structure which consists of alternating twisted sheets of distorted AlO tetrahedra and layers of calcium atoms which lie approximately perpendicular to the [OOl] plane; the A104 tetrahedra are linked through corners to form a network of five-membered rings.176 Previous doubts concerning the degree of hydration of crystalline potassium trioxalatoaluminate have been dispelled.The structure is comprised of discrete trioxalatoaluminate anions potassium cations and three water molecules per formula unit. 177 The hygroscopic crystals of A1(H2PO4), produced by evaporation of viscous solutions of aluminium in concentrated phosphoric acid possess a structure made up of isolated AlO octahedra stacked with corner-sharing 02P(OH) tetrahedra to form columns i[Al(H2P04)3] parallel to the crystal c axis. The columns are linked together by hydrogen-bonds each OH group acting simul- taneously as a proton donor and a~cept0r.l~~ The crystal structure of aluminium dihydrogenphosphate-monohydrogenphosphate-monohydrate(acid aluminium phosphate) contains macromolecular units ~[A1(HzP04)(HP04)(H20)],, consisting of A105(H20) octahedra which share vertices with PO,(OH) and P03(OH) tetra- hedra; neighbouring layers are held together by hydrogen In the basic sulphate AlZ(OH),SO4,7H2O (aluminite) an important structural feature is the presence of the complex ion [A~,(OH)~(HZ~)~]~' consisting of four edge-sharing A106 octahedra; these ions are connected to each other to make chains running in the a direction and are linked by hydrogen-bonds to SO tetrahedra.In the asymmetric unit there are seven water molecules four of which lie among the chains and tetrahedra as free water and three are co-ordinated together with OH to the aluminium atoms as ligand water. Hence the crystal chemical formula is best written i.e.[A12(OH)4(H20)3]S04,4H20a hydrated sulphate of a hydroxoaluminium complex.I8O 173 R.C. Mehrotra S. Goel A. B. Goel R. B. King and K. C. Nainan Znorg. Chim. Acta 1978,29 131. R.C. Mehrotra A. K. Rai andN. C. Jain J. Znorg. Nuclear Chem. 1978,40 349. 17' D. E. Haycock C. J. Nicholls D. S. Urch M. J. Webber and G. Wiech J.C.S.Dalton 1978 1785. 176 M. G. Vincent and J. W. Jeffery Acta Cryst. 1978 B34 1422. 177 D. Taylor Austral. J. Chem. 1978 31 1455. 178 R. Kniep and M. Steffen Angew. Chem. Internat. Edn. 1978 17 272. 179 R. Kniep D. Mootz and A. Wilrns 2. Naturforsch. 1978 33b 1047. C. Sabelli and R.T. Ferroni Acru Cryst. 1978 B34 2407. 204 F. A. Hart A. G.Massey P. G.Harrison and J. H. Holloway Two reports describe the synthesis of compounds containing the first known Al-Ge bonds.Tris(trimethylgermy1)aluminium has been isolated as a tetrahydro- furan complex from the reaction THF Hg[GeMe3I2+2A1 -A1[GeMe3l3,THF5MeA1[GeMe3I2 pentane It is said to have 'astonishing thermal stability' although being spontaneously inflammable in air. Lithium triorganogermyltrihydroaluminates,which are stable under an inert atmosphere are intermediates in the LiA1H4-reduction of hexa- organodigermanes. 182 3 Gallium Using a diamond anvil cell the crystal structures of the high pressure Ga" and Ga"' forms of gallium have been studied. Gal' has a body-centred cubic structure with twelve atoms in the centred cell and each atom having eight nearest neighbours at 2.78 A (2.6 GPa 313 K); Ga'" has a body-centred tetragonal cell with two atoms in the cell each atom having four nearest neighbours at 2.81 8 and eight second neighbours at 2.99 A (2.8 GPa 298 K).lg3 Gallium trichloride and tribromide form 1:1 molecular complexes with a variety of pyridine-N-~xides."~ The 1 1 trimethylphosphine and trimethylarsine adducts of gallium trichloride adopt eclipsed rather than the expected staggered con-formations in the solid state.The 35Cl NQR frequencies of PMe3GaC13 and PEt3GaC13 are among the lowest known for gallium trichloride adducts indicating that phosphines are extremely good donors towards GaC13. lS5 The absolute infrared spectral intensities of v3 and v, and the Raman intensity of v1 (relative to vl of CCl,) have been measured for the tetrahedral MX,-anions M = Ga In or T1.lS6 Tetra- methylstibonium tetrachlorogallate possess a structure containing isolated slightly distorted cationic and anionic tetrahedra with averaged Sb-C and Ga-Cl bond lengths of 2.126 and 2.172 A re~pectively."~The stoicheiometric reaction between gallium metal and gallium trichloride tribromide or tri-iodide in benzene at 60 "C provides a facile synthesis for the lower halides Ga,X,.An excess of gallium with the tri-iodide produces Ga416 which is formulated as (Ga+)2Ga2162-; the bromide Ga&6 cannot be made by this method as it decomposes to Ga2Br4 and Ga in benzene.lS8 67 Ga is used clinically as a tumour scanning agent but it has been found that inorganic phosphate inhibits gallium uptake by L1210 leukemic cells.A 71Ga and 31 P n.m.r. study has shown the gallium-phosphoric acid system to be complicated; as many as four distinct species are present of which two were identified as GaH3POd3+ in1 L. Rosch and W. Erb Angew. Chem. Znternat. Edn. 1978,17 604. 182 N.Duffant J. Dunogues R. Calas J. Gervol P. Riviere J. Satge and A. Cazes J. Organomefallic Chem. 1978,149,57. 183 L.Bosio J. Chem. Phys. 1978,68 1221. 184 J. R. Masaguer A. Sanchez J. S. Casas J. Sordo and A. Castineiras J. Znorg. Nuclear Chem. 1978,40 355; A. Sanchez J. S. Casas J. Sordo and J. R. Masaguer J. Znorg. Nuclear Chem. 1978,40,357. 185 J. C. Carter G. Jugie R. Enjalbert and J. Galy Znorg. Chem. 1978 17 1248. I86 S.P.Andrews P. E. R. Badger P. L. Goggin N. W. Hurst and A. J. M.Rattray J. Chem. Rex (S),1978 94. 187 H.-D. Hausen H. Binder and W. Schwarz 2.Naturforsch. 1978,33b 567. 188 J. C. Beamish M. Wilkinson and I. J. Worrall Znorg. Chem. 1978 17 2026. The Typical Elements and GaH2P0,2' complexes the others being assumed to be gallium-phosphoric acid polymers. 189 The two terdentate anionic ligands (38)190 and (39)191 form octahedral M(ligand) complexes with Mn Fe Co Ni Cu and Zn; the Ni(38) complex exists in mer and fac forms the gallium atoms in both isomers having distorted tetrahedral co-or- dination with OGaN angles of 90.3-93.2" and CGaC angles of 122.0-123.3°.'90 A number of organometallic compounds have been made using (39) uiz. (39)-Mn(C0)3 (39)Mo(CO),NO (39)Mo(CO),7 3-C4H7 and (39)Mo(C0),7 3-C3H5 ; a structure determination on the latter confirmed the tridentate nature of the gallium ligand.'" 'CH JH (38) Lithium gallate of stoicheiometry LiGa02,6H20 is correctly formulated as [Li(H20),]'[Ga02,2H20]- and not as containing the tetrahydroxogallate anion.The GaO tetrahedron is very distorted the OGaO angles varying from 71" to 127.7°.'92 The selenogallate T1' GaSe crystallizes with a layer structure. The main features of the layers are large corner-linked Ga4Selo tetrahedra which themselves are formed from four corner-linked GaSe4 tetrahedra. The thallium atoms are situated on straight lines parallel to the edges of the Ga4Se, groups and are surrounded by six selenium atoms in the form of a trigonal prism.193 4 Indium The vapour pressure of indium measured over the temperature range 1165 -1344 K via the rate of weight loss of zirconia Knudsen cells can be represented by log p = -~lY9O6 +5.08(f0.02) atmospheres.T This gives the enthalpy of vaporization at 298 K as 57.3 f0.3 Kcal mol-' which is slightly lower than earlier ~a1ues.l~~ In indium trifluoride trihydrate the indium atoms are octahedrally co-ordinated by four fluorines and two oxygens. Adjacent octahedra share their apical fluorine atoms to form infinite chains parallel to the c axis whilst in the equatorial plane there are two fluorine atoms and two water molecules distributed in disorder around the indium; the third water molecule is lattice water.195 Two types of indium occur in lS9 R. E. Lenkinski C. H. F. Chang and J.D. Glickson J. Amer. Chem. Soc. 1978 100 5383. 190 K. S. Chong S. J. Rettig A. Storr and J. Trotter Canad. J. Chem. 1978 56 1212. 19' K. R. Breakell S. J. Rettig D. L. Singbeil A. Storr and J. Trotter Canad. J. Chem. 1978 56 2099. 192 C. Caranoni G. Pepe and L. Capella Acta Cryst. 1978 B34 741. 193 D. Miiller and H. Hahn 2.anorg. Chem. 1978 438 258. 194 J. Valderram-N and K. T. Jacob J. Inorg. Nuclear Chem. 1978,40,993. 195 P. Bukovec and V. Kaucic Znorg. Nuclear Chem. Letters 1978 14 79. 206 F. A. Hart A. G.Massey P. G.Harrison and J. H. Holloway crystalline Rb21n3Fll ;two indium atoms per 'In3F11' unit have regular pentagonal bipyramidal co-ordination with In-F distances in the range 2.10-2.13 A the other indium having very deformed octahedral co-ordination.Parallel sheets formed by edge- and corner-sharing pentagonal bipyramids are joined together by infinite and parallel chains of corner-sharing octahedra. The large tunnels in the three- dimensional net-work thus produced hold the rubidium ions. 196 The electrolytic behaviour of potassium chloride in water-dimethyl sulphoxide is practically identical with its behaviour in water up to 50% by weight of DMSO. Thus Walden's rule is followed very closely implying that the only effect on the speeds of the ions is that of viscosity. The behaviour of indium trichloride is very different. The conductance increases rapidly with dilution in the dilute range and is time depen- dent. It appears probable that the ionization of indium trichloride takes place in steps and that in dilute solutions progressive hydrolysis takes place the hydrolysis products precipitating out of solution.19' Solid magnesium calcium and manganese dichlorides react with gaseous indium trichloride above 500 "C to form MIn,Cl,(g) and MInC15(g) the former being the dominant species between 500 and 530 "C; with copper(I1) chloride however only CuInCl,(g) could be dete~ted.'~~.'~~ It is concluded from a study of their photoelectron spectra that gaseous InCl and TlCl monomers are covalent molecules in which both the ns and the np electrons are involved in bonding; thus it would be incorrect to conceive of the metal s electrons as being inert non-bonding pairs. The spectra also confirmed the view that the inner d shells of indium and thallium should be regarded as part of their atomic cores.'99 Indium has slightly deformed octahedral co-ordinations in both NH41n(S04)2,- 4H2O2O0 and NH41n(Se04)2,4H20.201 Two vertices of both the SO or Se0 tetrahedra are shared and the remaining octahedral positions are completed by two trans water molecules.The low temperature form of In2Se3 has two types of indium present one with trigonal bipyramidal and the other with tetrahedral co-ordination.202 A second form of In2TeS has been isolated from the reaction of stoicheiometric amounts of tellurium and indium at 550 "C. The material is composed of separate planar sheets of atoms stacked perpendicular to the c axis; the sheets which are about 7 8,thick consist of chains of four-membered In2Te2 rings in which each indium is tetrahedrally co- ordinated average In-Te distance 2.8498,.The rings alternate with and are cross-linked by groups of three tellurium atoms which on an ionic formulation are Te32-polyanions. These polyanions are linked by bonds of intermediate strength to form continuous chains of tellurium atoms running across the ~heets."~ In view of the strong phosphorus-sulphur bonding in InPS4 (average P-S bond length 2.043 A) it is suggested that the compound be classified as a thiophosphate. J.-C. Champarnaud-Mesjard and B. Frit Acta Cryst. 1978 B34 736. A. N. Campbell Canad. J. Chem. 1978 56 355. F. Dienstbach and F. Emmenegger J. Inorg. Nuclear Chem. 1978,40 1299. 199 R. G. Egdell and A. F. Orchard J.C.S. Faraday 11 1978 1179.N. N. Mukhtarova R. K. Rastsvetaeva and V. V. Ilyukhin Doklady Akad. Nauk S.S.S.R. 1978,239 322. '01 E. N. Soldatov E. A. Kuzinin and N. V. Kadoshnikova Doklady Akad. NaukS.S.S.R. 1978,240,362. 'O' A. Likforman D. CarrC and R. Hillel Acta Cryst. 1978 B34 1. '03 P. D. Walton H. H. Sutherland and J. H. C. Hogg Acta Cryst.,1978 B34 41. 19' 19' 19' The Typical Elements 207 The structure is built up of a puckered cubic close-packed arrangement of sulphur atoms both the indium and phosphorus atoms being tetrahedrally co-ordinated by sulphur; d(1n-S) = 2.480 A. The sulphur atoms in In4(P2S6)3 are also cubic close packed but with the indium atoms and P2 groups occupying the octahedral holes; average In-S distance = 2.657 A.204 5 Thallium The reaction between thallium(II1) and hydrogen peroxide Tl"' + H202+ TI' + O2+ 2H' has been followed iodimetrically and by edta titration; it was found to be independent of thallium(1) ions but was inhibited by sulphate and chloride ions as well as monomers and radical scavengers.The mechanism could be explained at least partly by a one electron reaction of mono-and di-(hydrogen-peroxo)thallium(IIr) species with hydrogen peroxide2" e.g. T1(02H)'++ H202-D T1++ O2+ H202+ H+. Thallium(1) sulphite made by passing a stream of sulphur dioxide through a saturated solution of thallium(1) carbonate under an inert atmosphere possesses a structure containing two different co-ordination positions for the thallium average T1-0 distances were 3.0118 and 2.797 A close to the sum of the ionic radii for T1' and 0'-.The difference Fourier map revealed that the sulphur was apparently surrounded by four equivalent oxygens showing a disordered structure in which the sulphite ion could be considered a free rotator.206 Discrete molecules of T1(NO3),(H20), linked by hydrogen-bonding occur in crystalline thallium(II1) nitrate trihydrate. The thallium atom is chelated by three bidentate nitrate groups (Tl-0 = 2.299 2.637 A) and by three water molecules (Tl-0 = 2.293 A) giving rise to tricapped trigonal prismatic co-ordination in which the three equatorial distances are significantly longer (2.64A) than the distances to the corners of the prism (2.29A). The nitrate ion is distorted from D, symmetry by the different bonding environments of the three oxygen^.^^^ In thallium(II1) triacetate the thal- lium atom is chelated by three acetate groups (Tl-0 = 2.26-2.34 A) and forms two further bonds (2.57A) to adjacent molecules along the c axis thus linking the molecules into chains."* A convenient synthesis for TlOBr which is isomorphous with InOBr is to treat thallium(1) carbonate with liquid bromine; if bromine vapour is used the products are T1203 TlzBr4 and T1Br.'09 The new oxothallate(II1) Na5T104 has been made as a yellow solid by heating a mixture of sodium oxide and thallium(II1) oxide (ratio 5.4 1)in a silver cylinder at 620 "Cfor 7 days.Thermal decomposition yields first the yellow Na,TIO and then red NaT1_02.2'0 Thallium(1) sulphide and germanium 204 R.Diehl and C.-D. Carpentier Actu Cryst. 1978 B34 1097. 2os Z. Boti I. Horvath Z. Szil and L. J. Csanyi J.C.S. Dalton 1978 1012. 206 Y. Oddon G. Pepe J.-R. Vignalou and A. Tranquard J. Chem. Res. (S),1978,250. '07 R. Faggiani and I. D. Brown Acru Cryst. 1978 B34 1675. 208 R. Faggiani and I. D. Brown Actu Cryst.,1978 B34 2845. 209 J. R. Giinter 2. unorg. Chem. 1978,438,203. *lo R. Hoppe and D. Fink Z. anorg. Chem. 1978,443 193. F. A.Hart A. G. Massey P. G. Harrison and J. H. Holloway disulphide when heated in a 1:1 ratio at 500 "C give the thiogermanate Tl,Ge,S the structure of which is characterized by Ge2S64- anions held together by T1' cations. The anions consist of two GeS4 tetrahedra sharing an edge and the cations are surrounded by either seven or eight sulphur atoms at distances ranging from 3.024 A to 3.734 A.211 When T1' ions are trapped in the polar cavity of the polyether dibenzo-18-crown- 6 a strong interaction appears to occur between the metal ion and the aromatic rings of the ether as judged by fluorescence quenching and n.m.r.studies.212

ISSN:0308-6003    

DOI:10.1039/PR9787500165

出版商:RSC    

年代:1978

数据来源: RSC