摘要:

Quarterly Reviews No1 VoI22 1968 Hyponitrites By M. N. Hughes Equivalence of Nuclei in High-resolution Nuclear Magnetic Resonance Spectroscopy By M. van Gorkom and G. E. Hall The Tungsten Bronzes and Related Compounds By P. G. Dickens and M. S. Whittingham Electron Spin Resonance Studies of the Triplet State By Colin Thomson The Application of High Resolution Mass Spectroscopy to Organic Chemistry By G. W. A. Milne Page 1 14 30 45 75 The Chemical Society London Quarterly Reviews contains articles by recognised authorities on selected topics from general physical inorganic and organic chemistry. The Journal and Annual Reports interest primarily the research worker Quarterly Reviews is designed for a wider range of readers. It is intended that each review article shall be of interest to chemists generally and not only to workers in the particular field being reviewed. The submission of reviews for publication is welcomed but intending authors are advised to write in the first place to the Editor The Chemical Society Burlington House Piccadilly London W. 1. Such preliminary communications should be accompanied by an outline of the ground to be covered (about two quarto pages) rather than by the completed manuscript. Price to non-fellows E4 10s. Od. per annum @ Copyright reserved by The Chemical Society 1968 Published by The Chemical Society Burlington House London. hinted in England by The Thanet Press Margate.

ISSN:0009-2681    

DOI:10.1039/QR96822FP001

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

Equivalence of Nuclei in High-resolution Nuclear Magnetic Resonance Spectroscopy By M. van Gorkom and G. E. Hall UNILEVER RESEARCH LABORATORY VLAARDINGEN NETHERLANDS UNILEVER RESEARCH LABORATORY SHARNBROOR BEDFORD Since free rotation about a single bond is generally easy two identical atoms or groups attached to a common atom (e.g. the protons of a CH group) are often assumed to be indistinguishable. In practice they may be chemically or magnetic- ally distinguishable. A proper three-dimensional consideration of molecular structure allows an understanding of otherwise unexpected non-equiva1ence.l The aim of this Review is to reduce the confusion surrounding the concept of equivalence of nuclei. The principles involved are of value to organic and inorganic chemists in appreciating stereochemical subtleties and in interpreting even the simplest high-resolution nuclear magnetic resonance (n.m.r.) spectra in terms of chemical structures; to the physical chemist investigating barriers to chemical changes or the interaction forces between groups; and to the biochemist requiring to distinguish two similar groups and trying to understand the niceties of enzymic action.1 Definitions A. Chemical Non-equivalence.-Two atoms of the same isotopic species in a molecule are chemically non-equivalent if there is no molecular symmetry axis of rotation relating the two atoms. Citric acid (1) has been shown to react enzymically in such a way that the two indicated carboxyl groups are distinguishable. The ‘three-point attachment’ theory which attributes the enzyme with special features was suggested2 as an explanation of this observation.These features are however not a unique3 y x - c - x F H02C.CH2-y-CH;.C02H (1) 2 (2) C0,H b&!! H,‘ OH (4) HB K. Mislow and M. Raban in ‘Topics in Stereochemistry’ ed. N. L. Allinger and E. L. Eliel John Wiley New York vol. I ch. I. 8 A. G. Ogston Nature 1948 162 963. * G. Popjiik and J. W. Cornforth Biochem. I. 1966,101 553. 14 van Gorkom and Hall condition for a stereospecific reaction. Conside#s5 a molecule Cxxyz (2). Looking from the ‘left-hand’ x group towards the rest of the molecule the clockwise sequence is y-x-z. From the right-hand x group the clockwise sequence is y-z-x. The x-groups are enantiotopicl because they are situated in enantiomeric environments! Consequently the approach of an asymmetric reagent will be affected by the direction of approach and the x groups may react at different rates.With a highly asymmetric reagent such as an enzyme the differentiation may be complete. Further consideration shows5 that for two otherwise identical groups to be chemically non-equivalent (distinguishable) the relevant criterion is one of rotational symmetry. Chemically non-equivalent atoms require an optically active reagent for their chemical differentiation only if the molecule has a rotation-reflection axis such as a plane of symmetry relating these atoms as in (2). B. Isochronous Nuclei.-Nuclei which experience equal magnetic shielding have identical chemical shifts; such nuclei are termed’ isochronous. Chemically equivalent nuclei are isochronous but the reverse is not necessarily true.For example the two x nuclei of (2) are isochronous since a plane of symmetry relates them. C. Magnetic Non-equivalence.-Two atoms (or groups of atoms) are magnetic- ally equivalent if they are isochronous and if the constants (J) for the coupling to any other atom are identical. In catechol(3) the nuclei labelled B are chemic- ally shifted (non-isochronous) from those labelled A. They are magnetically non-equivalent in the chemical-shift sense. Also the coupling between A and B is through three bonds whilst between A’ and B it is through four bonds; the coupling constants must therefore be expected to be different. Consequently A and A’ are magnetically non-equivalent in the spin-coupling sense (this is an example of isochronous magnetic n~n-equivalence~). The aromatic ring proton spin system is described in the usual convention* as AA’BB’.On the other hand the atoms labelled B in resorcinol (4) are isochronous and do have identical values for the coupling constants with either nuclei C or A. They are therefore magnetically equivalent and this spin system is described as AB,C. This analysis ignores any coupling between the ring protons and the hydroxyl protons. D. Accidental Magnetic Equivalence.-The above arguments are based simply on considerations of symmetry without recourse to experimental data. If in (3) by some quirk of fate it had transpired that J(AB) =J(A’B) and J(AB’) =J(A’B’) P. Schwartz and H. E. Carter Proc. Nat. Acad. Sci. U.S.A. 1954,40,499. 6 H. Hirschmann J. Biol. Chem. 1960 235 2762. K. Mislow M. A. W. Glass H. B. Hopps E. Simon and G.H. WahI J. Amer. Chem. SOC. 1964 86 1710. (a) A. Abragam ‘The Principles of Nuclear Magnetism’ Oxford University Press 1961 p. 480; (b) E. I. Snyder J. Amer. Chem. Soc. 1963,85,2624. 8 E.g. J. W. Emsley J. Feeney and L. H. Sutcliffe ‘High Resolution Nuclear Magnetic Resonance Spectroscopy’ Pergamon Press Oxford 1965 p. 283. 15 Nuclei in High-resolution Nuclear Magnetic Resonance Spectroscopy then A and A' would be said to be accidentally magnetically equivalent. Two atoms may appear to be equivalent if the operating conditions are insufficient to resolve the appropriate peaks. For example the acetylenic and methyl protons in propyneQ are accidentally isochronous at 60 Mc./sec. and produce a one-line spectrum. E. Time-averaged Equivalence.-If a configurational or conformational change occurs rapidlylo compared with the inherent time scale of the n.m.r.experiment then two atoms may become equivalent by averaging if each spends the same time in each particular environment. Table 1 The possible combinations of chemical and magnetic equivalence Relationship of the two CE IS ME Fluorine nuclei in 1,l -difluoroallene CE IS MNE Fluorine nuclei in 1,l -difluoroethylene CNE IS ME Protons in chlorofluoro- methane CNE IS MNE Protons in tetraethyl-lead CNE NIS MNE aand/?Protonsin (groups of) nuclei Example pyridine Spectrum type A2X2 AA'XX' A2X A J 2 X A A' B B'C Ref. a a b C d C(N)E = chemical (non-)equivalence (N)IS = (non-)isochronous M(N)E f magnetic (non-)equivalence a H. M. McConnell A. C. McLean and C. A. Reilly J. Chem. Phys. 1955,23 1152; Ref. 8; C E. B.Baker J . Chem. Phys. 1957,26,960; W. G. Schneider H. J. Bernstein and J. A. Pople Ann. New York Acad. Sci. 1958 70 806. Table 1 lists all the possible combinations of chemical and magnetic equival- ence with examples. Further examples are given both later and elsewhere.",18 2 Temperature-independent Magnetic Non-equivalence Any factor which locks two nuclei in different magnetic environments will cause them to be magnetically non-equivalent. In the isotopically unusual form (5) of trans-1 ,Zdichloroethylene the protons exhibitlS magnetic non-equivalence which since there is no reaction causing exchange should be temperature- N. s. Bhacca L. F. Johnson and J. N. Shoolery 'N.M.R. Spectra Catalog' National Press New York 1962 vol. 1 Spectrum 16. lo J. E. Anderson Quart. Rev. 1965 19 426.l1 F. A. Bovey Pure Appl. Chem. 1966,12 525. l2 M. L. Martin and G. J. Martin Bull. Soc. chlm. France 1966,2117. l3 A. D. Cohen N. Sheppard and J. J. Turner Proc. Chem. Soc. 1958 118. 16 van Gorkom and Hall independent. Similarly the two methylenedioxy rings of the tetradehydro-otobain (6) have sharp proton n.m.r. lines.14 The plane of the molecule evidently bisects the <HCH angle of the two methylene groups and the two protons in each Cl \’3 12/ HA ,c=c\ Cl HA’ (5 1 ,A-a A-a ’NH -7H-5- Me 0 a’c ‘A w ‘a-A’ (9). A=S-alanyl residue o =R-alanyl residue group are equivalent. This is not so for otobain (7) itself; the methylenedioxy- group on ring c shows a singlet but the two protons of the other methylenedioxy ring (attached to ring A) exhibit non-equivalence as an AB quartet.It is also interesting that the protons at 2’ 5’ and 6’ of otobain are accidentally magnetic- ally equivalent and have a single proton n.m.r. signal. The corresponding protons of the dehydro-compound (6) are non-equivalent. Diastereoisomers may also have sufficiently different magnetic environments at particular atoms for distinguishable spectra to be observed. As an example the proton n.m.r. spectra of ~-[Co(en),-~-ala]Cl and ~-[Co(en),-~-ala]Cl are different. These complexes involve cobalt(m) ethylenediamine (en) and alanine (ala). On the other hand the n.m.r. spectra of ~-[Co(en),-c-ala]CI and ~-[Co(en),-~-ala]Cl should be identicaP5 because they are enantiomeric com- pounds ( i e . mirror images). Cycloenantiomers16 must also have the same n.m.r. spectra. The cyclohexapeptides (8) and (9) are mirror images of each other; although they have the same distribution of chirall’ (i.e.asymmetric) centres they differ by virtue of the ring direction which is indicated by the arrows. Such compounds are termed cycloenantiomers. As expected their n.m.r. spectra are identical and their optical rotations are equal and opposite. On the other hand each methyl group of either compound ‘sees’ a different sequence of R- and S-chiral centres from those ‘seen’ by the other five and the proton spectrum contains six methyl doublets. Cyclodiastereoisomers,la which have an identical l4 T. Gilchrist R. Hodges and A. L. Porte J . Chem. SOC. 1962 1780. l6 D. A. Buckingham S. F. Mason A. M. Sargeson and K. R. Turnbull Inorg. Chem. 1966 5 1649. l6 V. Prelog and H. Gerlach Helv.Chim. Acta 1964 47 2288; H. Gerlach J. A. Owtschin- nikow and V. Prelog ibid. p. 2294. l7 R. S. Cahn C. Ingold and V. Prelog Angew. Chem. Internut. Edn. 1966,5,385. 17 Nuclei in High-resolution Nuclear Magnetic Resonance Spectroscopy sequence of chiral centres and differ only in the ring direction but are not mirror images are expected to give distinguishable n.m.r. spectra. 3 Magnetic Non-equivalence with Both Temperature-dependent and -independent The substituted ethanes form the basis of the following discussion not only for historical reasons; the principles involved can be clearly demonstrated and then used for the understanding of related instances of magnetic non-equivalence. (i) Ethanes. In 1957 the 30 Mc./sec. room-temperature fluorine-19 n.m.r. spectrum of CF2Br.CHBr.C,H was reported.ls It was clear that the fluorine atoms are magnetically non-equivalent on two counts; they are chemically shifted and they couple with the vicinal proton to different extents.The fluorine spectrum of CF,Br.CBrCl on the other hand is down to -3O”c a singlel9 sharp line. It is thus unlikely20 that restricted rotation is the cause of the observed non-equivalence the origin of which can be understood in the following way. Consider a compound of the general formula HxYZC.CHAHBR. In general the energy barriers to rotation will be sufficiently small that at ordinary tempera- tures free rotation occurs. The eclipsed forms21 correspond to potential maxima Contributions. The ‘Ethane Type’ have very short residence times and may be ignored. There are then three dis- tinguishable rotational forms represented2 as (10)-(12).The environment of HA in (1 1) is such that it is ‘opposite’ Z and has Y and R to one side and HB and Hx to the other. Magnetic anisotropy in the bonds can23 cause significant differences in magnetic shielding. In none of the conformers (10)-(12) does HB have exactly that same environment. In (10) HB is opposite Z but it has R and Hx to one side and Y and HA to the other. Even if HA and HB spent equal times in each of the three possible conformations the average environments can never be exactly identical and a chemical shift can therefore be expected. This pos- sibility has not been recognised by some a ~ t h o r s . ~ ~ ~ (It should be noted that either Y or 2 may be the same as RCH,. in which case the two RCH,. groups are chemically and the two protons within each group magnetically non- J.J. Drysdale and W. D. Phillips J . Amer. Chem. SOC. 1957 79 319. lS P. M. Nair and J. D. Roberts J . Amer. Chem. SOC. 1957,79,4565. *O E. 0. Bishop Ann. Reports 1961 58,67. a1 D. H. R. Barton and R. C. Cookson Quart. Rev. 1956,10,44. a8 M. S. Newman ‘Steric Effects in Organic Chemistry’ John Wiley New York 1956. 24 H. Finegold J. Amer. Chem. SOC. 1960,82,2641; R. Freymann Compt. rend. 1965 261 2637. L. M. Jackman and N. S. Bowman J. Amer. Chem. SOC. 1966,88 5565. K. Deutsch and I. Deutsch Ann. Physik 1965 16 30. 18 van Gorkom and Hall equivalent. If neither Y nor Z is RCH,. then optical isomerism arises.) The non-equivalence within one group arises by its interaction with another of low symmetry (in this case a carbon atom with either three or four different sub- stituents).Although magnetic non-equivalence always arises by virtue of some form of low symmetry in the molecule the term intrinsic asymmetry26 is reserved for this special case. Two atoms (or groups) such as HA and HB which reside in diastereomeric environments and cannot be interchanged by symmetry operations are said to be diastereotopic? Since populations of excited vibrational and solvation states probably influence chemical shifts and coupling constants only slightly the intrinsic asymmetry effect is generally assumed to be temperature-independent. The temperature- dependent effect which results from unequal populations (i.e. residence times) of the conformers must however be allowed for. As we have already seen the environments of HA and HB in any one of the conformers (1 0)-( 12) are different.Consequently if the populations are unequal a weighted average must be computed. Since the populations will vary with temperature there results a temperature-dependent contribution. At ordinary temperatures this is normally more important than the intrinsic asymmetry effect. Various simplified mathe- matical have been given. In one method a least-mean-square analysis allows an estimation of the rotamer populations and of all the coupling constants. These calculations have been made for several and in the case of protons the gauche and trans coupling constants (about 2 and I6 c./sec. respectively) compare favourably with those obtainedz8 by the analysis of the carbon-13 satellite proton spectra. All methods agree that as the tempera- ture is raised the chemical shift differences should approach a limiting value owing to the intrinsic asymmetry.At infinite temperature the populations of all three rotamers will be equal and so this limiting value should be the average of the chemical shifts within each rotamer. For the geminal fluorine atoms in CF,Br.CFBrCl the limiting value was found29 to be 0.11 p.p.m. At very low temperatures the three possible (f)-rotamers can be frozen out:* and. the fluorine-19 spectrum is the superposition of the spectra of these three forms. Analysis of these spectra gives the chemical shift between the geminal fluorine atoms for each rotamer and their average (0.09 ~.p.m.)~l agrees well with the above figure. An interesting compound is the phthalide (13) for which the measured temperature-independent chemical shift (0.73 p.p.m.) between the methyl groups is said32 to be due to intrinsic asymmetry.A temperature-independent spectrum 8* G. M. Whitesides F. Kaplan K. Nagarajan and J. D. Roberts Proc. Nat. Acad. Sci. 27 J. N. Shoolery and B. Crawford J. Mol. Spectroscopy 1957,1,270; J. A. Pople Mol. Phys, 1958 1 3. 2a N. Sheppard and J. J. Turner Proc. Roy. Soc. 1959 A 252 506. as H. S. Gutowsky J. Chem. Phys. 1962,37,2196; H. S . Gutowsky G. G. Belford and P. E. McMahon ibid. 1962 36 3353. so R. A. Newmark and C. H. Sederholm J. Chem. Phys. 1963,39 3131; 1965,43,602. *l M. Raban Tetrahedron Letters 1966 3105. u.s.A. i962,4a 1112. G. C. Bnunlik R. L. Baumgarten and A. I. Kosak Nature 1965,201 388. 19 Nuclei in High-resolution Nuclear Magnetic Resonance Spectroscopy Y A Y A R-Y-O-y-Ph Ph-$-0-7-R (14) HB (15) 0 can actually be observed2' in three circumstances (1) if one rotamer has a much lower energy than the others; (2) if all possible rotamers have almost equal energies; (3) if the energy barriers between the rotamers are high.Some interest- ing spectral types are shown in Table 2. Table 2 Some possible spectral types for substituted ethanes Proton spectrum type Fast rotation Fast rotation populationsb populationsa Substituted ethane Slow rotationa unequal equal * CHS-CXYZ ABCC A3 A3 * CHZU-CHXY 3 x ABCd ABC CH2U-CXYZ 3xABd AB * ABC AB * * CHUV-CHXY 3 x ABd AB AB a Not temperature-dependent; Three over- lapping spectra; * Indicate possible asymmetric carbon atoms. Enantiomers of each formula give identical spectra.Table 2 is based upon that given by J. A. Pople Mol. Phys. 1958 1 3. Temperature-dependent ; C Not yet reported; In the case of longer substituted chains many conformational isomers become possible. An example is 2,3,5-tricyano-2,3,5-trimethylhexane all five methyl groups of which can be magnetically non-equivalent. More interestingly though the chemical shift between the methylene protons increases33 in many solvents on increase in temperature. This presumably means that a low temperature favours a conformation in which these protons experience almost identical shieldings. It is now possible to generalise the conditions which must be satisfied in order that magnetic non-equivalence may be observed (a) There must be no molecular symmetry operation which relates the nuclei concerned but not those nuclei to which they are spin-coupled.The plane of symmetry which relates A and A' 88 P. Smith and J. J. McLeskey Canad. J. Chem. 1965 43,2418. 20 van Gorkom and Hall in catechol(3) also relates B and B’ to which the protons A are coupled; A and A’ are thus magnetically non-equivalent. (6) Any molecular motions which are occurring rapidly compared with the n.m.r. time scale must not both correspond to such a symmetry operation and allow the nuclei to reside in the same environ- ments for comparable times. (c) There must be a field gradient between the nuclei. In other words the previous two conditions having been satisfied if the (average) environments are insufficiently dissimilar then no magnetic non- equivalence will yet be observed. The effect leading to satisfying condition (c) appears to be transmitted mainly spatially.The number of bonds is in itself not significant and the interacting sites may be quite far apart and not necessarily involve carbon atoms. In other words non-equivalence of geminal nuclei can always be expected if there is present somewhere in the molecule a carbon atom with three different substituents. A selection of examples25 is given in the following sections. (ii) Oxygen-containing compounds. The two interacting groups may be separated by a bivalent atom such as oxygenM as in (14) and (15). The difficulties encoun- tered in studying the various conformations of such compounds are considerably increased by the oxygen; the ether linkage has been insufficiently studied in this respect% for the possible conformers confidently to be predicted.The compounds (14) and (15) were however studied to help determine the manner in which an asymmetric centre exerts its influence. In (14) the chemical shift between A and B appears to increase the larger the group R which is a saturated alkyl chain; in the compounds (15) this is not so. Steric size at the chiral centre therefore plays some part. Roberts and his co-workers3Qs3B considered that the shielding differ- ences between A and B would result from two general effects. These are electronic differences in the two carbon-hydrogen bonds concerned and differences in shielding by more distant parts of the molecule and solvent. Now the protons in the methylene groups of diethyl s~lphoxide~~ and diethyl ~ u l p h i t e ~ ~ exhibit magnetic non-equivalence The constants for the coupling between the methylene carbon-13 and each of the protons are said% to be equal in these cases.But for acetaldehyde diethyl acetal there are two distinct39 such coupling constants which are a sensitive indication of the bond character. They may therefore provide a new criterion for non-equivalence since they should be39 largely unaffected by the magnetic shielding contributions which complicate the interpretation of chemical shifts. They therefore probably reflect the first of the two effects considered by Roberts and his co-workers and indicate that there are electronic differences in the two carbon-hydrogen bonds concerned. Of the various factors considered possibly to affect shielding differences (the G. M. Whitesides D. Holtz and J. D. Roberts J.Amer. Chem. SOC. 1964 86 2628. S. C. Abrahams Quart. Rev. 1956 10 407. 36 G. M. Whitesides J. J. Grocki D. Holtz H. Steinberg and J. D. Roberts J. Amer. Chem. SOC. 1965 87 1058. 37 K. Mislow M. M. Green P. Laur J. T. Melillo T. Simmons and A. L. Ternay J. Amer. Chem. SOC. 1965 87 1958. 3* F. Kaplan and J. D. Roberts J. Amer. Chem. SOC. 1961 83 4666. 39 L. S. Rattot L. Mandel and J. H. Goldstein J. Arner. Chern. SOC. 1967 89,2253. 21 Nuclei in High-resolution Nuclear Magnetic Resonance Spectroscopy second effect) interactions with the solvent are significant. Chemical shift differences therefore form an unreliable measure of conformational equilibria.' It has been shown*O that for substituted ethanes the energy differences between the rotamers are not constant but are a function of both the dielectric constant and the temperature of the medium.A change of solvent may therefore affect the observed non-equivalence by altering conformer populations. However the ring current41 of the aromatic ring attached to the methylene group in (14) was considered the most important factor. This would mean that the chiral centre affects the conformation of the benzyl group in such a way that the two protons are in different positions relative to the aromatic ring. The experimental data do not suffice to decide whether the effects discussed are altering conformer popula- tions or the intrinsic asymmetry contributione or both. Various families of optically inactive compounds may contain nuclei which meet the conditions for observable magnetic non-equivalence. Diethyl acetals sucN12 as (1 6) and triglycerides such as triacetin (1 7) have a molecular symmetry plane and consequently are optically inactive.However this plane does not bisect the connecting line of the geminal protons which are magnetically non- Me A I HO Me B Ph\ a / O-CHAHBMe/ CHAHgO. COMe O.CHgHe,Me Me-CH CH I .O*COMe ,,CH- C-0-Ce (18) (16) CHA. He; 0-COMe (17) ,OMe ' Me equivalent. This is manifested by the relative complexity of the AA'BB'X type proton spectrum of the glyceryl moiety of triacetin (Figure). Similar magnetic non-equivalence can be induced across an ester bond; e.g. the two methyl groups of the isopropyl ester (18) area non-equivalent. (iii) Nitrogen-containing compounds. The asymmetry effect in a molecule can be transmitted across a nitrogen atom similarly to the way it is transmitted across an oxygen.The methylene protons of (19) areu non-equivalent probably for this reason. However non-equivalence may be caused by the nitrogen atom itself. One cause may be a sufficiently slow inversion of a trigonal nitrogen atom. The two methylene protons of the substituted hydroxylamine (20) are non- equivalent at low temperatures and this can be explained on the assumption of 40 R. J. Abraham L. Cavalli and K. G. R. Pachler MoZ. Php. 1966 11,471 ; R. Freeman and N. S. Bhacca J Chem. Phys. 1966,453795. 41 H. P. Figeys Tetrahedron Letters 1966,4625; J. I. Musher J. Chem. Phys. 1965,43,4081. 43 P. R. Shafer D. R. Davis M. Vogel K. Nagarajan and J. D. Roberts Proc. Nat. Acad. Sci. U.S.A. 1961 41,49. 43 N. S. Bowman D. E. Rice and B. R. Switzer J. Amer.Chem. SOC. 1965,87,4477; C. van der Vlies Rec. Trav. chim. 1965 84 1289. 44 T. H. Siddall J. Phys. Chem. 1966 70,2249. 22 van Gorkom and Hall The 100 McJsec. spectrum of the glycerylprotons of triacetin. a non-planar nitrogen atom as in the conformers (21)-(23). The energy barrier to umbrella inversion was found45 to vary inversely with the dielectric constant of the medium. X - Y Y x v - Y X Y Y A (24) R (25) (26) 46 D. L. Griffith and J. D. Roberts J. Amer. Chem. SOC. 1965,87,4089. 23 Nuclei in High-resolution Nuclear Magnetic Resonance Spectroscopy However although a non-planar nitrogen atom probably introduces a greater asymmetry a planar nitrogen (or a nitrogen atom pseudo-planar owing to rapid inversion) may introduce sufficient asymmetry for observable magnetic non- equivalence.There is confusion in the l i t e r a t ~ r e ~ ~ ~ ~ ’ as to the conformations in which a compound containing a planar trisubstituted atom exists. However by arguments similar to those used to demonstrate that the average environments of A and B in (10)-(12) are not identical it may be seen that A and B in the conformers (24)-(26) have differing average environments. The compound (27) in which the methylene protons are48 magnetically non-equivalent may be an example of such a situation. It appears that before invoking a slow nitrogen inversion on the basis of magnetic non-equivalence some independent evidence ought to be obtained. (iv) Phosphorus-containing compounds. Other multivalent atoms may introduce the requisite degree of low symmetry into a molecule.For example the tetraco- ordinated approximately tetrahedral phosphorus atom can induce magnetic non-equivalence. Proton n.m.r. spectra49 of (28) and its analogues show that the two methyl groups are non-equivalent. In compounds (29) if R = 2-propyl the two methyl groups may similarly44 be non-equivalent even if R2 = R1 [since the phosphorus atom has three different groups attached to it; see Section 3 (i)]. If R2 # R1 then the two R groups can also be distinguishable as there is then no symmetry plane relating these two groups on each phosphorus. A trico-ordinated phosphorus atom may50 also introduce magnetic non- equivalence in a molecule. (v) Allenes biphenyls and other aromatic compounds. The examples discussed so far have been concerned with compounds in which the required low order of symmetry is related to a chiral centre.Other types of chirality1’ may cause 46 Ref. 21 p. 50; J. A. Elvidge in ‘Nuclear Magnetic Resonance for Organic Chemists’ ed. D. W. Mathieson Academic Press London 1967 p. 39; I. S. Showell Progr. Chem. Fats and Other Lipids 1965 8 275. 47 E. L. EIiel ‘Stereochemistry of Carbon Compounds’ McGraw-Hill New York 1962 p. 155. 40 A. H. Lewin J. Lipowitz and T. Cohen Tetrahedron Letters 1965 1241. 4g T. H. Siddall and C. A. Prohaska J . Amer. Chem. Soc. 1962 84 2502 3467. T. H. Siddall C. A. Prohaska and W. E. Shuler Nature 1961 190,903. 24 van Gorkom and Hall magnetic non-equivalence. The allenes (30) serve as examples of axial chirality the methylene protons of the ethyl group being5l non-equivalent. The (hydroxy) methylene protons of the biphenyl (31) are5% similarly non-equivalent.If the stable biphenyl conformers occur with the aromatic rings at right-angles to one another then such magnetic non-equivalence can only be observed if rotation around the bond between the rings is slow since a rotation through 180” ex- changes the environments of the two protons in the methylene groups. This rotation is the process by which an optically active biphenyl would be racemised. Consequently a temperature study of the spectra of a compound such as (31) can yield information on the racemisation process52 without recourse to optical resolution. The protons of the unsubstituted ring in monoacetyl-ferrocene mthenocene and -0smocene produce a sharps singlet. This is due to the fast rotation of the five-membered ring as is typicalM for metallocenes.However non-equivalence of the methylene protons in the side-chain of the ferrocene derivative (32) is55 observed. This observation is presumably related to the non-equivalence of the methylene protons of NN-dimethylben~ylamines~~ with ortho or meta sub- Me / Ph a) R=*CO-CH \CH,OH stituents lacking symmetry. A similar example is the non-equivalence of the two methyl groups of an isopropyl residue5’ attached to a highly substituted naph- thalene nucleus. The exact cause of such magnetic non-equivalence has not been established but may be explicable in terms of conformers such as (24)-(26) in which X-Y would represent the asymmetrically substituted aromatic nucleus. However not only interspatial effects but also the influenceM of asymmetry induced in the molecular electronic system might be important.(vi) Vicinal atoms. The previous examples have been concerned with demon- strating that some low order of symmetry may cause two geminal (groups of) nuclei to be magnetically non-equivalent. Similar effects can be observed with two vicinal groups. One example of this is the non-equivalence of protons A and B in (33a) and their equivalence in (33b).58 52 M. L. Martin R. Mantione and C. J. Martin Tetrahedron Letters 1965 3185. sz (a) W. L. Meyer and R. B. Meyer J . Arner. Chern. SOC. 1963,85,2170; (b) D. M. Hall and T. M. Poole J. Chem. SUC. (B) 1966 1034. 5s M. D. Rausch and V. Mark J . Org. Chem. 1963 28 3225. 64 M. Rosenblum and R. B. Woodward J. Amer. Chem. SOC. 1958 80 5443. 65 P. Smith J. J. McLeskey and D. W. Slocum J . Org. Chern.1965 30,4356. 58 J. C. Randall J. J. McLeskey P. Smith and M. E. Hobbs J. Amer. Chem. SOC. 1964 86 3229. 67 F. Conti C. H. Eugster and W. von Philipsborn Helv. Chirn. Acta 1966 49,2267. 68 S. R. Johns and J. A. Lamberton Chern. Comrn. 1965,458. 25 Nuclei in High-resolution Nuclear Magnetic Resonance Spectroscopy 4 Temperaturedependent Magnetic Non-equivalence in other than Ethane-type Any process which effectively exchanges the environments of two nuclei will cause time-averaged equivalence of the nuclei if the process is fast on the n.m.r. time scale. If the rate of the process is temperature-dependent and comparable with the n.m.r. time scale (which is a function of the magnetic field strength employed) the spectra and any magnetic non-equivalence observed will be a function of temperature.Molecules A. Intramolecular Processes.-Owing to the partial double bond between and hence the restricted rotation around the carbonyl-nitrogen bond of NN- dimethylformamide (34) two methyl signals may be observed. These correspond M e o w (35) OMe to methyls cis and trans to the carbonyl group. As the temperature is raised the ratelO~~~ of the relevant rotation increases the spectra alter and finally no non-equivalence is observed. The phenanthrene derivative (35) exists as two distinct molecular species which giveeo overlapping spectra. These species one of which has the methyl group cis to the carbonyl and the other the methyl trans are interconverting by rotation. There is thus an important difference between this example and dimethylformamide in which non-equivalence of the two methyl groups occurs within the single molecular species.Valence isomerisation can cause similar effects. The proton spectrume1 of bullvalene one form of which is shown in (36) at low temperatures consists of two bands corresponding to the six olefinic and the four allylic protons. As the temperature is raised the Cope rearrangements become easier and at room temperature only a single resonance close to the weighted average of the other two is observed. Organometallic compounds also undergo processes which can affect the appearance of the n.m.r. spectra. An X-ray crystallographic analysis sB L. W. Reeves in ‘Advances in Physical Organic Chemistry’ ed. V. Gold Academic Press London 1965 vol. 3 p. 196; W. D. Phillips Ann. New York Acad. Sci. 1958 70 817. O0 S. R.Johns J. A. Lamberton and A. A. Sioumis Chem. Comm. 1966 480. G. Schroder Angew. Chem. 1963 75,722. 26 van Gorkom and Hall of (37) has showns2 the structure to be that given; the second C,H group is present as a normal a-bonded 2,4-cyclopentadienyl group. In solution at room temperature the proton spectrum consists of two singlets. The five protons of this second C,H group are evidently all equivalent. A study of this resonance as a function of temperature has led to the conclusion that a rapid intramolecular reorientation process occurs possibly by repeated 1,2-shifts. Such organometallic compounds ('sterically non-rigid') are thus phenomenologically related to the 'fluxional' structures typified by bullvalene. Similar averaging processes have also been postulateda to occur in trisallylrhodium to account for some other- wise fortuitous magnetic equivalences.B. Intermolecular Processes.-If a molecule containing two or more equivalent nuclei interacts with the solvent or with some third material magnetic non- equivalence may arise either by conversion into a new species or by conforma- tional changes. Clearly the reverse may also occur. Since the extent of interaction is ordinarily a function of temperature the spectra observed will change with temperature. (i) Solvent eflects. There has been considerable interest recently in inducing non-equivalence by the use of a suitable solvent. The magnetic anisotropy of aromatic compounds such as benzene p ~ r i d i n e ~ ~ and quinolines5 is normally exploited for this purpose. The technique has been frequently employed in the steroid field.gp Orientation occursse in the collision complex and so different parts of the molecule may experience different shielding effects.For example the C(12) axial and equatorial protons in 2p-epoxy-5a-androstan-1 l-one have practically identical chemical shifts in deuteriochloroform but in benzenes4 solution are non-equivalent. The method has been extended to distinguish two enantiomers. Racemic 2,2,2-trifluoro-l-phenylethanol (38) gives two overlapping spectras7 in an optically active base. The two types of collision complex formed between the base and the two forms of the alcohol are diastereomeric and consequently 6a M. J. Bennett F. A. Cotton A. Davison J. W. Faller S. J. Lippard and S. M. Morehouse J. Amer. Chem. SOC. 1966 88 4371 ; P. von R. Schleyer J.J. Harper G. L. Dunn V. J. Dipasquo and J. R. E. Hoover J . Amer. Chem. SOC. 1967,89 698. 68 J. K. Becconsall and S. O'Brien Chem. Comm. 1966 720. 84 N. S. Bhacca and D. H. Williams 'Applications of N M R Spectroscopy in Organic Chemistry' Holden-Day San Francisco 1964 ch. 7. 65 A. P. Tulloch J. Amer. Oil Chemists' SOC. 1966 43 670. 68 J. Ronayne and D. H. Williams Chem. Comm. 1966,712. 67 W. H. Pirkle J. Amer. Chem. SOC. 1966,88 1837; J. C. Jochens G. Taigel and A. Selinger Tetrahedron Letters 1967 1901. 27 Nuclei in High-resolution NucZear Magnetic Resonance Spectroscopy the trifluoromethyl groups are in different environments. This means that the presence of optical isomers can be established without optical resolution and the optical purity checked by the relative intensity of the overlapping spectra.Normally high-resolution n.m.r. experiments are deliberately obtained under conditions in which both the solute and solvent molecules are tumbling rapidly. In this way direct dipoledipole interactions are averaged. However a new field may be opened up if liquid crystalssa are used as the anisotropic solvent. Such nematic phases as pp’-di-n-hexyloxyazoxybenzene cause largescale ordering of the solute molecules andsg additional lines due to intramolecular dipoledipole interactions are observed. (ii) Reaction with a third material. If the solute reacts with some other compound in the solution the molecular symmetry may be altered. For example the methylene protons of the benzylamines are generally equivalent. However in the presence of trifluoroacetic acid the protonated species (e.g.39) is formed,7o and the protons A and B are magnetically non-equivalent. Again diethyl sulphide gives a normal 1 3 3 1 quartet for the methylene protons but it reacts with borane to give an addition compound (40) of reduced symrnetry7l and the methylene protons A and B of this adduct are magnetically non-equivalent. If dimerisation of a compound occurs in solution72 similar spectral changes may be observed. As an example the meso protons of certain72 porphyrin tetramethyl esters give the expected singlet signals in dilute solution but in concentrated solutions extra signals owing to dimerisation. (iii) Exchange reactions. The copper-63 resonance of a mixture of the copper(1) and copper(I1) ions in water consists73 of a single peak. This is due to a rapid electron-transfer reaction between the two types of ion which then appear to be equivalent.On the other hand the rate of exchange between bulk water mole- cules and water molecules bound to cobalt(r1) ions may be slow at certain temperatures. In this case two signals may be observed for the oxygen-17 reson- ance of the water molecules74 and the relative areas of the resonances give an indication of the hydration number of the cobalt(r1) ion. 5 Interpretation of Spectra Before concluding it is worth drawing attention to some pitfalls that can be met in certain circumstances. The concept of magnetic equivalence is not absolute but is briefly an observational property. In this way it is a somewhat negative 68 G. W. Gray ‘Molecular Structure and the Properties of Liquid Crystals’ Academic Press London 1962; D.Chapman Science Journal 1965 Oct. 1 32. 60 A. Saupe 2. Naturforsch. 1964 19a 161; S. Meiboom and L. C. Snyder J . Amer. Chem. SOC. 1967,89,1038; R. A. Bernheh and B. J. Lavery ibid. p. 1279; J. 1. Musher J. Chem. Phys. 1967 46 1537. 70 W. F. Reynolds and T. Schaefer Canad. J. Chem. 1964 42 21 19. 71 T. D. Coyle and F. G. A. Stone J. Amer. Chem. SOC. 1961 83,4138. 72 R. J. Abraham P. A. Burbidge A. H. Jackson and D. B. Macdonald J. Chem. SOC. (B) 1966,620. 73 H. M. McConnell and H. E. Weaver J. Chem. Phys. 1956,25,307. 74 J. A. Jackson J. F. Lemons and H. Taube J . Chern. Phys. 1960,32,553; 1. R. Lantzke and D. W. Watts Austral. J. Chem. 1967 20 173. 28 van Gorkom and Hall feature since its validity may depend on the experimental conditions such as the operating magnetic field strength and homogeneity.As an example the aromatic protons of toluene and cumene give an unsplit singlet at 60 Mc./sec. but well- resolved75 analysable resonance lines at 200 Mc./sec. It is therefore dangerous to rely on negative n.m.r. evidence; apparent equivalence must not be interpreted as proving the nuclei involved to be identical. It is also dangerous to assume that measured splittings necessarily correspond to the coupling constants involved since the first-order interpretation rules76 may only be applied to groups of magnetically equivalent nuclei. The ABX system has been fully disc~ssed'~ in this respect. Faiiure of first-order rules is sometimes manifested by virtual coupling.78 When in three sets of nuclei the first set is coupled to the second which is also coupled to the third the spectrum of the first set of nuclei may appear more complicated owing to virtual coupling with the third set even if the real coupling is zero.6 Conclusion The magnetic non-equivalence which has been the main concern of this Review arises from quite subtle differences in (average) environment or conformer populations. The existence of these differences does not aecessarily lead to observable non-equivalence. So far most workers have attempted to accentuate such potential differences by means of a magnetically anisotropic group such as cyan0 or phenyl. Any resulting magnetic non-equivalence can normally be qualitatively explained on topological grounds after observation but quantitative prediction is virtually impossible. More research is required regarding inter- action between groups.An awareness of the possibility of otherwise unexpected magnetic non-equivalence is important in spectral interpretation. When two atoms (or groups) are distinguishable in an n.m.r. spectrum they may sometimes be differentiated chemically. For example the methylene protons of methyl benzyl sulph~xide~~ are magnetically non-equivalent and on reaction with sodium deuteroxide the resonances due to the lower-field proton disappear more rapidly than those of the higher-field proton. The fact that the two protons of a methylene group in biologically important materials such as triglycerides can be distinguished may assist in showing enzymic preference for one particular proton. 76 F. A. Bovey F. P. Hood E. Pier and H. E. Weaver J. Amer. Chem. SOC. 1965,87,2060. 76 E. D. Becker J . Chem. Educ. 1965,42 591. 77 J. D. Roberts 'An Introduction to the Analysis of Spin-Spin Splitting in High Resolution Nuclear Magnetic Resonance Spectra' W. A. Benjamin New York 1962 p. 71. 78 J. I. Musher and E. J. Corey Tetrahedron 1962,18 791; D. L. Hooper N. Sheppard and C. M. Woodman J. Chem. Phys. 1966,45 398. 70 A. Rauk E. B u d R. Y. Moir and S. Wolfe J. Amer. Chem. SOC. 1965,87 5498. 29

ISSN:0009-2681    

DOI:10.1039/QR9682200014

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

The Tungsten Bronzes and Related Compounds By P. G. Dickens and M. S. Whittingham INORGANIC CHEMISTRY LABORATORY OXFORD 1 Introduction In 1824 Wohler? in passing dry hydrogen over heated acid sodium tungstate observed the formation of golden yellow crystals of metallic appearance. His was the first account of the formation of a tungsten bronze a name originating from the metallic lustre characteristic of these compounds. Tungsten bronzes are well defined non-stoicheiometric compounds of general formula M,WO where M is some other metal most commonly an alkali and x is a variable <l. The large variety of metal species M which are known to participate in tungsten bronze formation is shown in Table 1 ; it is probable that the discovery of optimum preparative conditions will enable this list to be extended further.Table 1 Elements known to form tungsten bronzes shown in bold type H Li Be B C N 0 F Ne Na Mg A l S i P S C I A K Ca Sc Ti V Cr Mn Fe Cr Ni Cu Z n Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re 0 s Ir Pt Au Hg T1 Pb Bi Po At Rd Fr Ra Ac Th Pa U Ce Pr Nd Pm Sm Eu Gd Tb Dy H o Er Tm Yb Lu For a considerable time the tungsten bronzes were thought to be unique but in recent years analogous compounds of molybdenum,2 vanadium niobium4 and titanium6 have been prepared and found to have similar properties. The term 'bronze' is now applied to a ternary metal oxide of general formula M',M",O where (i) M" is a transition metal (ii) M",O is its highest binary oxide (iii) M' is some other metal (iv) x is a variable falling in the range 0 < x < 1.Such a compound has the following characteristic properties (a) it possesses high F. Wohler Ann. Physik 1824 2 350. R. P. Ozerov Russian J . Inorg. Chem. 1959 4,476. D. Ridgley and R. Ward J. Amer. Chem. SOC. 1955,77 6132. M. Kestigian and R. Ward J. Amer. Chem. SOC. 1955 77 6199; S. Anderson and A. D. 5. A. Wold W. Kunnmann R. J. Amott and A. Ferretti Inorg. Chem. 1964 3 545. Wadsley Acfa Cryst. 1962 15 201. 30 Dickens and Whittingham electrical conductivity either metallic or semi-conducting ; (6) it is intensely coloured and in crystalline form shows metallic lustre; (c) it is chemically inert ; (d) sequences of solid phases occur through variation of x with definite ana sometimes wide ranges of homogeneity. Although in some ways the bronzes constitute a unique class of non-stoicheio- metric compounds they show resemblances to other apparently unrelated types of inorganic system.Thus in the structural principles of their lattice architecture they resemble the silicates and the tungstosilicates in the wide ranges of homo- geneity of successive phases they resemble alloys and in the typical freeelectron behaviour underlying their optical and electrical properties they recall solutions of alkali metals in liquid ammonia. From a thermodynamic standpoint they are most simply regarded as solutions of the Metal M' in a matrix of the host oxide M",O,. In this Review most emphasis will be placed on the sodium tungsten bronzes since not only has this system been studied much more extensively than others but the crystal structures adopted are relatively simple and the sequences of solid phases which occur cover the largest continuous range in the variable x.2 Preparative Methods Three basic methods have been used for the preparation of bronzes. A. Vapour-phase Reaction.-As an example may be given the reaction Crystals of the bronze are deposited on a cold finger projecting into the reac- tion vessel. This method is only suitable where the metal M is appreciably volatile at high temperatures and can be manipulated fairly easily at room temperature (this precludes the use of the alkali metals). Good single crystals of TI,WO have been made6 by this procedure. B. Electrolytic Reduction.-Tungsten bronzes can be prepared by an electrolytic reaction in which molten mixtures of tungstate and tungstic oxide are de- composed with platinum or tungsten electrodes.Crystals grow at the cathode and oxygen is liberated at the anode. This is the most successful method for the growth of large single crystals. However optimum experimental conditions can be very difficult to find and careful control of the melt temperature can be crucial. Molybdenum bronzes have been made by this technique. C. Solid-state Reaction.-This is the most versatile method. The finely ground reagents are heated in vacuo and react according to the equation 850" 3 - 2x WO + - Na,WO + - 2 3 W __+ Na WO, x 13 M. J. Sienko J . Amer. Chem. Soc. 1959 81 5556. 31 2 The Tungsten Bronzes and Related Compounds For the tungsten bronzes the highest value of x which can be obtained in this way is ca. 0.8. To avoid the need to break down the very stable tungstate lattice an alternative reaction has been used:' 900" in argon xBaC1 + xW02 + WO + Ba,WO + xW02CI A further extension of the basic method is to make use of high pressures.8 The reaction is then carried out in a platinum container in a hydraulic press at pressures in the range 60-65 kbars.By this means several hitherto unknown phases in the tungsten and molybdenum systems have recently been prepared e.g. cubic K,WO,. 3 Crystal Structures A. Tungsten Bronzes.-The structures of the tungsten bronzes and of the related oxides of tungsten were determined by Hagg and Magneli9 using X-ray methods. There are three general features of the crystal structures of the tungsten bronzes which emerge (i) as the value of x in M,WO decreases so does the symmetry of the structure; (ii) the particular structure adopted is controlled to a consider- able degree by the ionic radius of M; (iii) all the structures are based on the linking together of WO octahedra by the sharing of comers.The limiting structure of 'NaWO,' is that of perovskite (Figure 1). The unit Figure 1 Perovskite structure of NaWO 0 Na; 8 W; 0 0 cell has a tungsten atom at the centre of a cube octahedrally surrounded by six oxygen atoms at the face centres; there are eight 'interstitial' sites at the cube corners occupied by sodium atoms. The structure of WO is a distorted version of the ReO structure (Figure 2) in which tungsten atoms are slightly off-centre in adjacent unit cells such that the W-W distances are alternately long and short. For Na,W03 in the approximate composition range 0.30 < x < 0.95 a cubic structure is found which is intermediate between the hypothetical NaWO and undistorted WO structures and in which a fraction 1 - x of sodium atoms 7 L.E. Conroy and T. Yokokowa Inorg. Chem. 1965,4,994. ST. A. Bither J. L. Gillson and H. S. Young Inorg. Chem. 1966 5 1559. G. Hiigg and A. Magneli Rev. Pure Appl. Chem. (Australia) 1954 4 235. 32 Dickens and Whittingham Figure 2 ReO structure 8 Re; 0 0 are missing from the cube corners of the NaWO unit cell. As the sodium content of the bronze decreases the high (cubic) symmetry of the lattice is lowered and the structure passes through two tetragonal phases (I and 11) (the nomenclature used here for the tetragonal phases is that of Hagg and Magneli) to the mono- clinically distorted phase of pure WO,.At very low sodium contents (x < 0.05) anorthorhombic structure may also be observed; an analogous structure is found for pure WO in the temperature range 320-720". At slightly higher x values the tetragonal structure tetragonal 11 is formed. The relationship between these structures in terms of the basic cube is shown in Figure 3. a. t I (ort ho) I I I I I I I I I I I I I i I ! I I I Figure 3 Relationship between the cubic tetragonal ZI and orthorhombic phases 33 The Tungsten Bronzes and Related Compounds Figure 4 001 Projections of tetragonal Z and cubic structures ,WO octahedra; 0 alkali metal; - - - - boundary of unit cell (a) Tetragonal I structure (6) Cubic structure 34 Dickens and Whittingham Above x = 0.1 the tetragonal I structure occurs. A projection of this complex structure is shown in Figure 4a.It can be regarded as being built up of three- four- and five-membered rings of WO dctahedra. In this arrangement each unit cell contains interstitial holes of two types which can be occupied by sodium atoms; four holes per unit cell are each surrounded by eight tungsten ions situated at the corners of a cube and eight holes per unit cell are each surrounded by ten tungsten ions in the form of a pentagonal prism. For comparative purposes the perovskite structure of NaWO is shown in an analogous projection in Figure 46. In this case only four-membered rings are present. The maximum alkali-metal content of the tetragonal I phase would correspond to x = 0.6 on the assump- tion that only the larger holes in the lattice can be so occupied.For the sodium tungsten bronzes the observed homogeneity range of this phase is Nao,28W03- Na,.38W03. However for the potassium tungsten bronzes it is K0.40W03- K0.57W03. The complete set of phase relationships for Na,W03 as determined by Ribnick Post and BankslO is shown in Figure 5. Cubic Tctragonal I T I X Figure 5 Phase diagram for NasWO 800 400 04 0.2 0-3 0-4 0 . 5 06 0.7 The phase transformations observed in this system resemble those occurring in many alloy systems. They all show a lowering of the transition temperature with increasing solute (i.e. Na) concentration the appearance of two phases between the regions of homogeneity and a progression from lower to higher crystal symmetry as the temperature increases. A further crystal structure which is found with other alkali-metal tungsten bronzes is the one of hexagonal symmetry shown in Figure 6.Here the WO octahedra are formed into a six-membered ring. The maximum alkali-metal lo A. S . Ribnick B. Post and E. Banks 'Advances in Chemistry' Series No. 39 h e r . Chem. SOC. 1963. 35 The Tungsten Bronzes and Related Compounds X 1-00 0.80 0.60 0.40 0.20 0 content is 0.3 and K Rb and Cs all form tungsten bronzes with x values close to this. The relationship between the nature of M and the range of structures adopted in M,WO is summarised in'Figure 7 (for the alkali-metal tungsten bronzes). The radii of the inscribed spheres of the cavities available for occupation by M in the foregoing structures are cubic 0.96 A tetragonal I 0.96 and 1.29 A hexagonal 1-63 A. As can be seen from Figure 7 the smaller cations Li+ (0.60 A) ' Figure 6 Li Na K 1 rn ? 11 cell Figure 7 Relationship between crystal structure and composition for the alkali-metal tungsten bronze,s ,cubic; tetr.I; ,tetr. 11; hexagonal 36 Dickens and Whittingham and Na+ (0.95 A) tend to adopt the cubic structure whereas K+ (1.33 A) Rb+ (1.48 A) and Cs+ (1.69 A) form hexagonal bronzes. Although there is no reason to suppose that the crystal structures described here are not valid it should be stressed that the conventional X-ray methods employed are insensitive to the actual sodium and oxygen atom positions since the scattering process is domin- ated by the much heavier tungsten atoms. However a neutron diffraction studyll of Na,,,,WO revealed that the sodium atoms were indeed at the lattice sites assumed for the cubic structure (Figure 1) and moreover formed an ordered (rather than random) sub-lattice.B. Other Bronzes.-The bronzes formed by elements other than tungsten have more complex structures and only some of their more general features are out- lined here. Those of molybdenum12 reflect the greater complexity found generally in the chemistry of oxy-compounds of molybdenum relative to that of tungsten. The potassium molybdenum bronzes consist of MOO units forming infinite sheets held together by potassium ions. Wide ranges of homogeneity do not occur and two compounds of definite composition are known Ko.&oO,* which is red and K,,28Mo03 which is blue. A sodium molybdenum bronze Nao,,Mo,Ol exists and has a distorted perovskite structure. The alkali-metal vanadium bronzes M,V205 and M1+,V03 differ from those of tungsten in that the nature of M appears to have no effect on the structure adopted.In M,V205 the alkali-metal atoms reside in tunnels in the V20 matrix and in M1+,V03 between layers in the VO structure. These and related structures have been reviewed in detail recently by Wad~1ey.l~ Recently a number of bronzes of pre- viously unknown structures have been prepared by the high-pressure technique.8 Some typical examples are Nao,,5Mo03-cubic perovskite; Ko,,MoO,-cubic perovskite Ko,5MoO tetragonal I (isostructural with KO ,WO& A hexagonal sodium tungsten bronze has also been prepared. 4 Electrical Properties1* Considerable interest has been aroused by the unusual electrical conductivities of the bronzes. Single crystals of alkali-metal tungsten bronzes with x > 0-25 exhibit metallic conductivity that is the specific resistance is very low and increases (linearly) with temperature.Room-temperature resistivities and their thermal coefficients are shown in Table 2. The observed constancy of the thermal coefficient of resistance suggests a common origin for the charge-carrier scatter- l1 M. Atoji and R. E. Rundle J. Chem. Phys. 1960 32 627. l2 J. Graham and A. D. Wadsley Acta Cryst. 1966 20 93; N. C. Stephenson ibid. p. 59; N. C. Stephenson and A. D. Wadsley ibid. 1965,19,241. l3 A. D. Wadsley “on-Stoichiometric Compounds’ ed. L. Mandelcorn Academic Press New York 1964. l4 H. R. Shanks P. H. Sidles and G. C. Danielson ‘Advances in Chemistry’ Series No. 39 Amer. Chem. SOC. 1963. *Note added in proof The analytical composition of the red potassium bronze is that due to G.H. Bouchard J. Perlstein and M. J. Sienko Inorg. Chem 1967 6 1682 and differs slightly from that given in ref. 2. 37 The Tungsten Bronzes and Related Coinpornids Table 2 Resistivity (p) and temperature coefficient of resistivity at 25" for some tungsten bronzes Compound p(ohm-cm.) T10.20w03 6.0 x 10-3 Ba0-12W03 1.5 x 10-4 Tm0.1w03 5.0 x 10-4 Rb0.32w03 6.3 x 10-5 Li0.38w03 1-26 x 10-4 K040W03 3-82 x 10-5 Na0-49W03 1-05 x 10-4 Na,.33Bao.loW03 2.4 X Re03 6.7 X 2-3 x 10-3 8.0 x 10-4 4.7 x 10-3 1.1 x 10-3 4.5 x 10-3 1.4 x 10-3 1.1 x 10-3 - Free electron concentration per mole 0.20 0-24 0.30 0.32 0.38 0.40 0.49 0.53 1-00 ing process which might most reasonably be associated with the lattice vibrations of the common W03 matrix. That the charge carriers are free electrons is con- firmed by measurement of (a) the Hall effect (an e.m.f.generated in a sample when a magnetic field is applied at right angles to the direction of a current pass- ing through it); (6) the Seebeck effect (an e.m.f. generated when a temperature gradient is applied across the sample). Detailed consideration of these effects reveals that there is one free electron per metal atom in the host lattice and that the carrier mobility is comparable with that of free electrons in the conduction band of a typical metal. A plot of conductivity against alkali-metal content x over the full cubic range of the alkali-metal tungsten bronzes extrapolates to zero conductivity at x = 0.25. This suggests that a different mechanism of conduction is operative below x = 0.25.For a single crystal of a sodium tungsten bronze of composition Na,.,,,W03 semiconductor-type behaviour has been established;15 that is resistivity decreases with increasing temperature according to a relationship log pcc 1/T. Similar behaviour was also found for Lio.09,W03. In both cases an activation energy for the conduction process of ca. 0.02 ev was recorded. Conductivity measurements on homogeneous crystals are sparse for other types of bronze but Sienko and Sohn16 showed that Nao.,,V2O5 be- haved as a semiconductor in the range 77-500"~. Cubic niobium4 and titanium5 bronzes appear to be metallic conductors. The potassium molybdenum bronzes2 are interesting in that the blue K0.28M~03 is metallic at room temperature whereas the red Ko.&oO3 is a semiconductor.At lower temperatures the blue bronze undergoes a metallic-semiconductor transition. 5 Magnetic Properties A. Magnetic Susceptibility.-The magnetic susceptibilities of single crystals of the cubic sodium tungsten bronzes have been measured17 and weak temperature- l5 W. McNeill and L. E. Conroy J . Chem. Phys. 1962,36,87. lG M. J. Sienko and J. B. Soh J . Chem. Phys. 1966,44 1369. l7 J. D. GrZiner H. R. Shanks and D. C. Wallace J . Chem. Phys. 1962,36,772. 38 Dickens and Whittingham independent paramagnetism found such as occurs for example in sodium metal itself. Measurements on powder samples of other alkali-metal tungsten bronzes in the metallic range reveal similar behaviour. The band theory of metals predicts that if the charge carriers are treated as quasi-free electrons the electronic con- tribution to the magnetic susceptibility per unit volume will be where rn = electron rest mass m* = effective mass p0 = Bohr magneton n = carrier density h = Planck's constant and Xe = electronic susceptibility per unit volume.A reasonable fit to the data of Greiner Shanks and Wallace for the sodium tungsten bronzes can be obtained if rn* is taken as 1.6rn, and the closeness of the effective mass to the electronic mass substantiates the general correctness of the band model for these compounds. However the variation of Xe with the Na content as predicted by the use of (1) is of the form Xe E X * ; this is not in good agreement with the measured variation which suggests a relation of the form XeCc x . The significance of this result will be emphasised later in the discussion of the electronic structures of these compounds.For the semi- conductor Na,.,,V,O a susceptibility an order of magnitude greater was reported by Sienko and Sohn.ls In contrast to the behaviour of the metallic sodium tungsten bronzes the temperature-dependence in this case was that of a typical paramagnetic material possessing localised unpaired electrons i.e. Xe 1 cc IT. B. Nuclear Magnetic Resonance.-In many metals a large chemical shift of the n.m.r. signal to lower magnetic fields is observed relative to the signal of the same nucleus in some non-metallic environment. This is known as the 'Knight' shift and the magnitude of the effect is directly proportional to the electron density of the conduction electrons at the nucleus. Electrons in s type orbitals or bands can contribute to the shift whereas those in p or d orbitals cannot.StudieP on the alkali-metal tungsten bronzes reveal very small or zero Knight shifts for both the alkali metal and tungsten nuclei. The s orbitals of the alkali metal cannot therefore participate in the conduction band whereas the 5d (but not 6s) orbitals of the tungsten may do so. The line-widths of the alkali-metal resonances in the tungsten bronzes are ca. 1 OE. Calculations of the line-width due to nuclear dipolar interactions (van Vleck) are in fair agreement with this value. Lithium and sodium vanadium bronzes again show the absence of a Knight shift for the alkali-metal nucleus. In the case of Li,V,O a significant line narrowing was foundlg between 7 7 " ~ and room temperature for the Li resonance suggesting the onset of some diffusional motion of the lithium ions R.G. Barnes R. A. Hultsch and W. H. Jones Bull. Amer. Phys. SOC. 1959 4 166; A. Narath and D. C. Wallace Phys. Rev. 1962,127,724; W . H. Jones E. A. Garbaty and R. G. Barnes J . Chem. Phys. 1962,36,494; A. T. Fromhold and A. Narath Phys. Rev. 1964,136 A 487. lo J. Gendell R. M. Cotts and M. J. Sienko J . Chem. Phys. 1962 37 220. 39 The Tungsten Bronzes and Related Compounds through the lattice. There was no evidence for a similar motion of the larger sodium ions in the corresponding sodium vanadium bronze. C. Electron Sgin Resonance.-The conduction electrons in a metal do not usually give rise to a well-defined e.s.r. signal since the short spin-spin relaxation time causes a massive broadening of the absorption region.No e.s.r. signal has been reported for the sodium tungsten bronzes. However well-resolved signals have been found for the semiconducting M,V20,1a and M,MOO,~~ bronzes. The g value of 1.96 reported for both Li,V,O and Na,V,O ( x = ca. 0.33) is consistent with the g values found for V4+ centres in other vanadium compounds and the measured intensities indicate the presence of one V4+ for each alkali- metal atom in the bronze. This evidence is consistent with the electrical conductivity and magnetic susceptibility data presented previously for these com- pounds. The g value found for the red potassium molybdenum bronze ( g = 1-97) agrees with that found for oxygen-deficient MOO and may be identified with the presence of Mo5+ centres. Again a localised set of electron states is suggested for this semiconducting material in which an electron transfer has occurred Mo + Mas+ 3 M+ + Mo5+ 6 Spectra The intense colours exhibited by the bronzes are one of their most characteristic features.For the sodium tungsten bronzes the sequence shown in Figure 8 is observed. 0-2 0.4 0.6 0-8 wo* NaWO Figure 8 Quantitative measurements of the absorption spectra of these compounds are practically impossible to obtain however on account of the extremely high extinction coefficients involved. Reflectance spectra of pellets of the sodium tungsten bronzes were recorded by Brown and Banks21 in which more than 95 % of the incident light was absorbed. A single structureless and very broad absorp- tion band was found in the range 3000-12000 A the maximum of which moved to lower wavelengths with increasing sodium content.There is a likelihood that the formation of a superficial WO layer2 may interfere with the intrinsic bronze spectra recorded by this method and detailed conclusions concerning the elec- tronic structures of these compounds cannot be drawn from the measurements. 2o P. G. Dickens and D. J. Neild Trans. Furuduy SOC. (in the press). a1 B. W. Brown and E. Banks J. Amer. Chem. SOC. 1954,76,963. a2 Personal communication from Dr. D. W. Lynch Iowa State University. 40 Dickens and Whittingharn 7 Electronic Structures of the Sodium Tungsten Bronzes Straumanis2 regarded the tungsten bronzes as solutions of W-0 in NaWVO, i.e. as compounds containing W in two valency states. Such a formulation implying as it does isolated spin states is incompatible with the observed small temperature-independent paramagnetism of the cubic alkali-metal tungsten bronzes.Moreover the electrical transport data demand the presence of nearly free electrons as current carriers. There is now little doubt that the tungsten bronzes are best considered as solutions of M in a W03 matrix in which the alkali metal is ionised and the nearly free electrons are located in a delocalised conduction band. There is controversy however about which atomic orbitals are the constituents of the conduction band. There are two distinct viewpoints here one theory (Mackintosh:* F u ~ h s ~ ~ ) considers the conduction band to be composed of overlapping alkali-metal orbitals the other (Sienko,26 Good- enough2’) supposes that it is mainly the tungsten orbitals which are involved.To examine the consequences of these two approaches it is convenient to con- sider a possible energy-level diagram for cubic Na,WO,. Suppose that the atomic arrangement is as shown in Figure 1 (cubic perovskite). Each oxygen atom can form sp hybrids directed towards neighbouring tungsten atoms. The central tungsten atom with 6 4 6p and 5d (e,) orbitals can combine with the 6a-type orbitals of the oxygen atoms directed towards it in a way exactly comparable with the a-bond formation encountered in an octahedral metal complex. If the whole lattice rather than an individual unit cell is considered the discrete energy levels of the (T and a* molecular orbitals so formed broaden into bands (Figure 9). In the Sienko model the tungsten 5d (t2g) orbitals combine to form a a+ band while the remaining oxygen p orbitals remain as discrete non-bonding levels ( p ~ in Figure 9).In the Goodenough rehnement of this model half the oxygenp orbitals (ofT symmetry) mix with the W,5d ( t 2 3 orbitals to convert the previously non-bonding W,tzo orbitals into a bonding and anti- bonding combination. In both schemes the conduction band is made up pre- dominantly of W,5d (tzg) orbitals. In the Sienko-Goodenough model electrons are donated from the sodium atoms into the conduction band. That a conduc- tion based on metal d orbitals is feasible is supported by the ob~ervation~~ that ReO, which is isoelectronic with NaWO and isostructural with cubic WO, has high metallic conductivity. The model accounts very well for the absence of a Knight shift from the n.m.r. spectra since d orbitals have nodes at the parent nuclei and provide very small electron densities at the alkali-metal positions.In the alternative theory of Mackintosh the energy band formed by the over- lap of sodium atomic orbitals is assumed to lie below that formed by the tungsten 5d (t2J orbitals. The conduction band which is the lowest incomplete energy band is accordingly assumed to be constructed from sodium atomic orbitals 23 M. E. Straumanis J. Amer. Chem. Soc. 1949 71 679. 24 A. R. Mackintosh J . Chem. Phys. 1963 38 1991. 2s R. Fuchs J . Chem. Phys. 1965,42 3781. 26 M. J. Sienko ‘Advances in Chemistry’ Series No. 39 h e r . Chem. SOC. 1963. 27 J. B. Goodenough Bull. SOC. chim. France 1965 1200; A.Ferretti D. B. Rogers and J. B. Goodenough J. Phys. and Chem. Solids 1965,26,2007. 4 / The Tungsten Bronzes and Related Compounds W 6+ 302' Sienko model Goodenough model Figure 9 Energy diagram for WO The u and IT bands consist of bonding orbitals the PIT+ energy level and a+ band are non- bonding and the u* and IT* are antibonding.( ) signifies the orbital degeneracy per molecule alone. Clearly these cannot be sodium 3s orbitals in view of the Knight-shift evidence and it is postulated that the band arises from the overlap of 3p orbitals which can achieve good mutual overlap but at the same time are better able than are the 3s to avoid the filled WO orbitals. The Na-Na distance in the bronze (3.78 A) is only a little larger than in sodium metal (3.72 A). This model can explain the increasing symmetry of the bronze lattice with increasing sodium content since the cubic structure provides maximum mutual overlap of the sodium 3p orbitals.It also provides an explanation of the changeover from metallic conductivity to semiconductivity at low x values since for some com- position in the region of x = ca. 0.25 there would be insufficient sodium atoms present for the formation of infinite linkages through the crystal and the band structure would break down in favour of isolated levels. Neither of these features is adequately explained by the Sienko-Goodenough model. However it appears that the measured spin-lattice relaxation time of the Na nucleus is too long to be compatible with a picture of the conduction band based on sodium atoms; in addition there is also no very good a priori ground for believing that the Na,3s orbitals should not contribute to such a conduction band.Both the foregoing theories assume a uniform distribution of alkali-metal atoms throughout the bronze lattice and the normal type of conduction band 42 Dickens and Whittingham of an ideal metal in which the Fermi level will rise with increasing alkali-metal concentration. Fuchs has pointed out that such a picture cannot give a quanti- tative account of the observed magnetic susceptibility and electronic specific heat data which suggest a Fermi level virtually independent of x. It is to over- come this difficulty that Fuchs has suggested a model for the sodium tungsten bronzes in which the sodium atoms occur in clusters and for which the focal conduction electron density (based on Na orbitals) is independent of x. The metal-semiconductor transition is explained in the same spirit as in the Mac- kintosh theory as being the critical concentration at which sodium atoms cannot connect throughout the lattice.The magnetic susceptibilities can be accounted for as well as the observation that the spin-lattice relaxation time of the 25Na nucleus is independent of x since in both cases the local electron density for the clusters is independent of the overall composition. Direct experimental evidence for the existence of clusters is so far lacking however. A weakness of the Mackintosh-Fuchs approach is that it is applicable speci- fically only to the alkali-metal tungsten bronzes whereas the common electronic behaviour of tungsten bronzes containing other metals as M and also that of the lower oxides of tungsten themselves suggests that a more general conduction mechanism is operative involving the parent WO lattice.In this respect the Sienko-Goodenough theory appears to be more versatile and applicable to other highly conducting transition-metal oxides (e.g. CrOd as well as to the bronzes. In any event further measurements possibly in Mossbauer studies which could define more precisely the electron density at the tungsten nuclei in the bronzes are needed to distinguish between the rival theories. 8 Other Properties The tungsten bronzes are insoluble in water and very resistant towards acids. Niobium and titanium bronzes behave similarly. The tungsten bronzes are readily oxidised to tungstates in the presence of alkalis 4NaW0 + 40H+ + 0 = 4W042- + 2H20 They are capable of reducing awoniacal silver nitrate to silver and this reaction may be employed for their quantitative analysis.Other strong electron acceptors such as I (or WO,) can degrade the bronzes NaZWO3 in a controllable manner2 to compounds closer in composition to WO,. Electron donors such as molecular hydrogen at high temperatures cause the formation of compounds closer in composition to NaW0,.28 The chemical inertness of the tungsten bronzes may be associated with the high energy of activation for diffusion of the alkali metal in the oxide matrix (51-8 kcal./mole for Na in Na,.,8W0&29 The vanadium bronzes are far more active chemically being attacked by mineral acids as well as alkalis. The molybdenum compounds are dissolved by aqua regia. In contrast to the general chemical inertness of the alkali-metal tungsten 28 M. S.Whittingham D.Phil. Thesis Oxford 1967. J. F. Smith and G. C. Danielson J . Chem. Phys. 1954,22,266. 43 The Tungsten Bronzes and Related Conipounds bronzes the ‘hydrogen bronzes’30 H,WO prepared by the wet reduction of tungstic acid are extremely reactive. They are slowly attacked by air and rapidly and quantitatively oxidised by hot dichromate solution. These compounds which are deep blue have been shown by both30 X-ray and neutron diffraction methods to be structurally related to the sodium tungsten bronzes. The tungsten and vanadium bronzes have been investigated as heterogeneous catalysts in relation to their special electronic properties. Although they are generally poor catalysts a marked change in activity was observed31 for the alkali-metal tungsten bronzes on passing through the semiconductor-metallic conductor transition the activity pattern being largely independent of the nature of the alkali metal. 30 0. Glemser and C. Naumann 2. anorg. Chem. 1951 265 288; P. G. Dickens and R. J. Hurditch Nature 1967 215 1266. *l P. G. Dickens and M. S. Whittingham Trans. Faraday SOC. 1965 61 1226. 44

ISSN:0009-2681    

DOI:10.1039/QR9682200030

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

Electron Spin Resonance Studies of the Triplet State By Colin Thomson DEPARTMENT OF CHEMISTRY UNIVERSITY OF ST ANDREWS ST ANDREWS FIFE SCOTLAND 1 Introduction Electron spin resonance (e.s.r.) spectroscopy has proved to be of immense value to the Chemist in the study of the electronic structure and properties of transition- metal ions1 and free radicalsY2 and one might expect this technique also to be of use in the study of the ground or excited triplet states of molecules. Owing however to experimental difficulties it was 1958 before Hutchison and Mangum3 first detected the naphthalene triplet state by e.s.r. but since that time it has become apparent that e.s.r. can yield very detailed information concerning the electronic structure of the triplet state information which supplements that obtained by conventional spectroscopic techniques.The experimental study of photoexcited triplet states by optical means has been thoroughly reviewed and the present Review is concerned with the study of organic triplet states by e.s.r. Of more recent origin is the study of molecules with a triplet ground state. Such states have considerable importance in organic chemistry and e.s.r. is a valuable tool in their study. The electronic spin interactions within the triplet state are examples of 'weak interaction^'^ and their measurement and theoretical calculation constitute sensitive tests of approximate molecular wave functions. 2 The Phosphorescence of Aromatic Molecules Many aromatic molecules dissolved in rigid glassy solvents at low temperatures exhibit phosphorescence upon irradiation with ultraviolet light.* The lifetime T~ varies from a few milliseconds to ca.30 sec. and this phenomenon was attributed by Lewis and Kasha6 to the radiative decay from the lowest excited triplet state of the molecule (7') to the ground state So. The triplet state is popu- lated via a radiationless transition from the lowest excited singlet state S, into which the molecule reverts following the initial excitation. The situation is sum- marised in Figure 1. The triplet state is paramagnetic as Lewis Calvin and Kasha showed by susceptibility measurements,' and should therefore be detectable by e.s.r. After considerable effort Hutchison and Mangum3 succeeded in observing the naphtha- A. Carrington and H. C. Longuet-Higgins Quart. Rev. 1960 14 427. C . A. Hutchison jun. and B.W. Mangum J . Chem. Phys. 1958,29,952; 1961,34,908. S . K. Lower and M. A. El-Sayed Chem. Rev. 1966 66 199. M. Karplus Rev. Mod. Phys. 1960 32,455. G. N. Lewis and M. Kasha J. Amer. Chem. SOC. 1944,66,2100. a A. Carrington Quart. Rev. 1963,17 67; J. R. Morton Chem. Rev. 1964,64.453. ' G. N. Lewis M. Calvin and M. Kasha J. Chem. Phys 1949,17 804. 45 Electron Shin Resonance Studies of the Triplet State ’s ‘S \ I I i l l 3 r 7; 3 0 Figure 1 Excitation and decay of aromatic triplet states. Dashed lines denote non-radiative processes. Transitions denoted kr and kp are the radiative fluorescence and phosphorescence transitions with typical lifetimes of ca. sec. and several seconds respectively lene triplet in 1958. The interpretation of the e.s.r. observations necessitates an understanding of the quantum mechanical description of the magnetic interac- tions between the two unpaired electrons and an external magnetic field to which we now turn.3 Quantum Mechanical Descriptfon of the Triplet State The triplet-state wave function and energy levels are the eigenfunctions and eigenvalues of the total molecular Hamiltonian operator Jf which can be written as a sum of a spin-independent part So and a spin-dependent part Sg. The usual electronic problem of quantum chemistry is that of finding those wave functions #k,g,m which are simultaneously eigenfunctions of So and of the total spin operator s2 and its component 3,. z=z,+z (1) For a triplet state S = S + S = 1 and m can take the values -l,O,l. In phenomena involving the interaction of the electron and nuclear spins with external electromagnetic fields Sa has to be included with the results that the eigenfunctions +k,g,m are no longer eigenfunctions of S.3 causes mixing be- tween singlets and triplet states and lifts the triplet spin degeneracy and the eigenvalues and eigenfunctions must be found by diagonalisation of the matrix of 2 using the #k,s,m as basis functions. Ss is a sum of two parts the spin-orbit interaction Sa0 and the dipolar spin- spin interaction Zag. These interactions have the explicit form8 H. F. Hameka ‘Advanced Quantum Chemistry’ Addison-Wesley New York 1965. 46 Thomson 2 yeso = c (si. A ~ ) i where (3) and 1 3 In these equations the summation is over all the electrons Hi and Fi are the magnetic and electric field strengths at the position of electron i due both to external fields and to the nuclei and the other symbols have their usual meaning.Since SS 2Po the effect of Zg on the zeroth-order eigenfunctions and energies t,bk,s,m and Ek can be found by perturbation theory if the zeroth order functions $k,s,m are known. In the triplet state the eigenfunctions of So 3$k,s,m can be written as products of a spatial part and the appropriate spin function. The spin functions for a single electron are a and /3 and are eigenfunctions of s with eigenvalues m of +&h and - 8h respectively and of g2 with eigenvalue S(S + 1)h2 - $a2. In the case of two unpaired electrons coupled to give S = 1 the appropriate spin functions are where we have written the value of 2s + 1 as superscript and the m value as subscript. The usual Dirac notation for these states is given on the right-hand side.The three triplet functions with a common spatial part combined with the above spin functions are degenerate in the absence of Zs and correspond to pure spin states. Since the Pauli principle states that the total wave function must be antisymmetric the above symmetric spin functions must be multiplied by an antisymmetric spatial part. The perturbation 2Pso causes a mixing between singlet and triplet states and therefore a relaxation of the selection rules for T -+ So and S1 -+ T transitions.* X, on the other hand doesnot mix singlets and triplets but both Zso a n d x lift the degeneracy of the triplet state. From the point of view of e.s.r. Sss is the most important perturbation* and we henceforth discuss this term alone referring to the small effects of Sso in Section 15C.Although singlets and triplets are mixed by S it is meaningful to describe the states as predominantly singlet or triplet. *This is true for organic triplet states but in other systems such as transition-metal ions spin- orbit effects can be comparable with or greater than spin-spin effects. 47 Electron Spin Resonance Studies of the Triplet State 4 The Effixt of an External Magnetic Field For an atom with spherical symmetry a magnetic field lifts the triplet degeneracy so that the possible energy levels in the absence of Zss and Zso are the eigen- functions and eigenvalues of 2 =gps,< (7) as in the free-radical case but where s can have eigenvalues 0 f l i.e. Transitions between the triplet sub-levels are subject to the selection rule Am = 1 and occur at values of the magnetic field H which satisfied the resonance condition 3 hv =gpHz (9) where Y is the microwave frequency.Therefore resonance occurs at the same field value as for free radicals. The transition corresponding to Am = 2 is strictly forbidden because of the high symmetry. However in the case of mole- cules when Zss is included the reduced symmetry results in quite a different splitting which we shall examine in detail in Section 6. 5 Semi-classical Picture of Spinspin Interaction@ The two unpaired electrons with spin S = 8 each have magnetic moment $ = g/%? and the interaction SSs is equivalent to the classical interaction between two magnetic moments p1 and p2 with interaction energy r12 is the distance between the magnetic moments and 4 is the angle between the direction of the magnetic moments and the vector K2.The angular brackets denote an average over the distribution of the two electrons. For an atom <cos* 4> = &and therefore E, = 0 and the triplet degeneracy remains. In the molecular case we no longer have spherical symmetry but only one or more symmetry axes. In the absence of an external field the interaction energy between the spins will be different in different directions the spin will be coupled to the molecular framework and this is equivalent to quantisation of the spin along the molecular axes due to a local magnetic field within the molecule. This field is of the order of 2000 gauss and the energy levels of the system are then determined by this molecular field. In planar aromatic molecules the z-axis perpendicular to the aromatic plane is always a symmetry axis and may be a three- or six-fold symmetry axis in molecules like coronene.In this case the x and y axes are equivalent and quantisa- tion of spins can occur along the z-axis or in the x-y plane. The former situation will be doubly degenerate since the situations with mz = f l will be physically J. H. van der Waals and M. S. de Groot J. Chim. phys. 1964 1643. 48 Thomson indistinguishable but quantisation in the plane (corresponding to in = 0) will have a different energy. In this case the spin-spin interaction will result in one doubly degenerate level and one single level (Figure 2a). Z mZ t @ \ +* - 1 - f 0.13 cm-l t i I \ - 0 mx=O f- my=O .C mZ=o 0028 0.086 Figure 2 Zero-field splitting of the triplet spin sub-levels by dipolar interaction [Reproduced with permission from J.H. van der Waals and M. S. de Groot J. Chim. phys. 1964 16431 (a) Triphenylene (b) Naphthalene In molecules of lower symmetry one can show that the triplet degeneracy is lifted completely and there are three levels even in zero-field corresponding to quantisation of the spin along one of the three symmetry axes of the molecule (Figure 26).9 One might expect that the e.s.r. signal will give information con- cerning the symmetry of the triplet state. If an external magnetic field H is applied which is small compared with zs8 the spins will still be predominantly coupled to th,e molecular axes but if H is verylarge the spins will be quan- tised along H,. In the usual e.s.r. experiment H = ca. 3000 G and is thus a competition between the molecular axes and the external field for the spin quantisation.The result\is that the triplet energy levels are strongly dependent on the angle between H and the molecular symmetry a~es.39~1~~ For an assembly of randomly oriented molecules there will be a wide range of possible energy levels and the resonance fields will occur over several hundred gauss for transitions at constant microwave frequency. Thus the resonance will be highly anisotropic and the resulting broad lines difficult to detect. This fact was responsible for the earlier failures in the search for triplet state e.s.r. signals. lo C. A. Hutchison jun. Rec. Chem. Progr. 1963,24 105. 49 Electron Spin Resonance Studies of the Trrlplet State 6 Quantum Mechanical Treatment of Triplet State Energy Levels in the Presence of H and Xss In the presence of Zso and an external field the Hamiltonian becomeslo 3 ye = pGo .g . s -+ zss = p ~ . g . s + S . 5 . s (1 1) where since 3 and H are vectors the interaction may be written in tensor form and g and F are the g and spin-spin interaction tensors. However the tensors can always be transformed to an axis system in which they are diagonal. Further- more the g-tensor is usually nearly isotropic and in this case and henceforth we denote its isotropic value by g and the diagonal elements of the tensor by qi in which case 2 = g / G O . S + TxZSz2 + TvYSv2 + TzzSz2 (12) 2 and the terms gpS. H and ZSs are comparable in magnitude. The zero-field splitting of the levels is due to Zss and the splitting depends on the orientation of the spins and on their spatial distribution.The zero-field energy levels and wavefunctions are found by diagonalisation of the matrix of Hss with respect to the zeroth order basis functions 350 35 f 1. It is convenient to rearrange Xss so that spatial and spin parts are separated. This is a straightforward expansion and if the axes are chosen so as to diagonalise the tensor F one can show thatll Since the total wave function is written as a product the principal values of the tensor in the situation where 3 8 s is diagonalised will involve integrals of the antisymmetric spatial part of the wave function with the above co-ordinate operators. Van Vleck and McLachlan12 have shown that the matrix elements of ZSs are equivalent to those of the spin Hamiltonian operator Hs = D(Sz2 - $3') + E(gZ2 - $2) where s, sY and 3 are the components of the total spin = s + 3,.This form is the most useful for describing experimental results since these are the result of transitions between spin levels and the separations between these levels enables one to measure the zero-field splitting (Z.F.S.) parameters D and E.9910 The zero-field splitting of the levels is easily found by use of eqn. (14) with H = 0. In molecules with a three- or six-fold symmetry axis E = 0 and the eigenvalues of 2' = D ( S ~ - gS2) are l1 S. A. Boorstein and M. Gouteman J . Chem. Phys. 1963 39,2443. J. H. van Vleck Rev. Mod. Phys. 1951,23,213; A. D. McLachlan Mol. Phys. 1963,6,441. 50 Thornson in agreement with the semi-classical picture of Section 5. For E # 0 there are off-diagonal elements of iF8 between the basis functions 11 > lo> and I - 1 > and the 3 x 3 matrix is then (16) and there are three non-degenerate levels with El = QD + E E = 60 - E E = -$D (17) -1 This splitting is illustrated in FiguLe 2b.The effect of an external field H is shown in Figure 3 for the c a z of H along two of the three principal axes of the naphthalene molecule. For H parallel to they,-axis the energy-level diagram is shown in heavy lines in Figure 3 and for H parallel to z the energy levels are H (gauss) Figure 3 Energies of the triplet state sub-levels of naphthalene as a function of the external field Ho 2 Heavy lines Ho parallel to y-axis A - Light lines Arrows correspond to e.s.r. transitions with v' = 9650 Mc./sec. Ho parallel to z-axis denoted by the light lines.1° This behaviour is predicted from the above spin Hamiltonian and this accurately describes the experimental results in single crystals.In the limit of very strong fields the wave functions of the states are the pure spin functions but at 3000 G the 11 > and I - 1 > states are strongly mixed. The quantum of radiofrequency energy is indicated in the diagram and it is clear that for am = 1 transitions (the high field region) one obtains two possible transitions at very different resonance fields and these fields are different for each of the three orientations above. Thus the e.s.r. spectrum is highly anisotropic and the exact field strengths depend critically on the angle between H and the molecular axes. 51 Electron Spin Resonance Studies of the Trblet State However in the molecular case van der Waals and de Groot13 have shown that there is a finite probability for observing Am = 2 (low field) transitions with an intensity of ca.1 to 2 % of the Am = 1 transitions and these transitions are much more isotropic than the high-field transitions as is clear from Figure 3. 7 E.S.R. Spectra in Randomly Oriented Samples For glassy samples where the triplets are randomly oriented each triplet state has its molecular axis system at different angles 8 4 to the direction of H (z-direction in the laboratory frame; Figure 4). The energy levels are determined by 8 and 4 and are obtained by expanding the expression for gps . H and dia- gonalising 3 = gps. H + A?, with the triplet basis functions. The general analysis by Kottis and Lefebvre following earlier work by van der Waals can be summarised as follow^.^^^^^ z 4 Figure 4 Reference systems Oxyz in the laboratory OXYZ in the molecule.ON is the inter- section between the xOy and XOY planes OM the projection of Oz in the XOY plane. U is a unitary vector in the XOZ plane along the direction of the oscillating field H is the static magnetic field 01 the angle between the two fields [Reproduced with permission from P. Kottis and R. Lefebvre J . Chem. Phys. 1963,39,393] If we use the basis functions (these are more convenient for reasons of sym- metry) and rewrite l3 J. H. van der Waals and M. S. de Groot Mol. Phys. 1959,2,333; 1960,3,190. l4 Ph. Kottis and R. Lefebvre J. Chem. Phys. 1963,39,393; 1964,41 379. 52 Thornson where X Y and Z are the principal values of the coupling tensor then X = QD + E ; Y = QD- E; z= -#D X + Y + Z = O In this case the energy levels are the solutions of the cubic equation E3 - E[(gflHo)2 - ( X Y + XZ + Yz)] + (g/3H,,)2 [Xsin2 8 cos2 4 + Ysin2 8 sin24 + Zcos2 8] - XYZ = 0 (21) The condition for resonance is that two of the roots be separated by 6 = hv and this condition is x sin2 8 + Y sin2 8 sin2 4 + z cos2 B = XYz(g/3H,)-2 'f 4(XY + XZ + Y2)p f(8 4) = FW, 6) 3-* [(gfiHo)-2 (a2 + XY + XZ + Yz) - 11 [(4g/%f0)~ - S2 - (22) which can be written (23) For a given molecule the solution can be illustrated graphically and is given in Figure 5 for the case of naphthalene3 which has values of X = 0.0197 cm.-l Y = 0-0471 cm.-l 2 = -0.0669 cm.? g = 2-0030.The function F(H, 8) is plotted with these values and if a line parallel to the Ho axis is drawn at an ordinate equal to the value off(8 4) the abscissae give the allowed resonance fields.It is clear that the observed behaviour is predicted by the construction. There are in general three resonance fields for every 8,+; two at high field (Am = 1) and one at low field (&n = 2) and the low-field one is very much less aniso- tropic. The largest value off(8 4) is 2 and thus the largest resonance field possible in the low-field region occurs at H,1 = 1648 for ClOH8 whereas the minimum field Hm occurs when F(H, 6) ceases to be real i.e. H = (2gfl)-1 [62 + 4(XY + xz + Yz)] = (2gB-l [S2 - 4[D2/3 + E 2 ] ] i For Cl&€, the low-field anisotropy is ca. 121 G and a peak can be seen at this value in random samples. The largest signals occur when the resonance field is stationary i.e.dH = 0 where and this occurs (1) when F'(Ho 8) -KO which is the case for H = H, and (2) when df = 0 which is the case for H aIong one of the three principal axes of the molecule. These are the CANONICAL ORIENTATIONS and for these values there are again three possible fields which we label H2H;H; in the Am = 2 region and HZ2H$H and Hz3Hv3 H in the Am = 1 region. 53 I I I _all-------------------------------*-----.-v==.- Figure 5 The function F& 8) for naphthalene at 9279 Mc.lsec. The axis system used by Kottis and Lefebvre has the x and y axes interchanged from those used in Figure 3 [Reproduced with permission from P. Kottis and R. Lefebvre J. Chern. Phys. 1963,39,393] Thomson It has been shown that when one averages the resonance fields over all the molecules the distribution of resonance fields is that shown in Figure 6.14 There are thus maxima (peaks) in random samples at the canonical fields and H, the latter being the most intense.For molecules of higher symmetry Hzl and H,l 1500 ibQ0 woo Figure 6 Distribution of resonance fields in the Amz = 2 region for naphthalene at v = 9219 Mc. /see. [Reproduced with permission from P. Kottis and R. Lefebvre J . Chem. Phys. 1963,39,393] 55 Electron Spin Resonance Studies of the Triplet State merge into a single peak and H disappears hence the structure of the spectra in random samples will enable one to deduce the triplet symmetry. The Am = 1 region gives similar structure from the canonical peaks in random samples and it is possible to relate the resonance peak positions to X Y and Z and hence obtain the Z.F.S.(zero field splitting) parameters in randomly oriented samples. The gross features of the spectra of random samples can also be predicted from the experimental results on single crystals. 8 Experimental Methods in the Study of Triplet States A. Single Crystals,-For the study of oriented triplet states,3J0 the molecule is incorporated into a material with which it will form a substitutional solid solution and a single crystal grown. The concentration of guest molecules should be ca. 10-2-10-3~ (pure crystals of the host do not show phosphorescence owing to rapid exciton transfer effects). If the crystal is a true substitutional solid solution the guest molecules will be in known orientations in the lattice if the crystal structure of the host is known. The crystals are mounted so that they can be rotated about their principal axes or the magnet can be rotated and the crystal heLd fixed.In this way the variation of the resonance with the angle between H and the molecular axes can be determined. Crystals are cooled to 7 7 " ~ and irradiated with ultraviolet light in the singlet absorption band after suitable focusing of the light through ports in the side of the cavity. The characteristic phosphorescence decay time can be measured from the decay of the e.s.r. signal after the radiation is cut off. With mixed crystals energy transfer effects can be studied. The disadvantage is that suitable host crystals for the molecule of interest are difficult to find. B. Rigid Glasses.-The zero-field splitting parameters can fortunately be obtained from the use of random sample^.^,^^ In this technique the molecule is dissolved in a solvent which freezes to a clear transparent glass at 77"~.The most commonly used is ether-isopentane-alcohol (E.P.A.) but many other sol- vents have been described by Smith Smith and M~G1ynn.l~ E.P.A. is relatively stable and easily prepared but most glasses are not stable over a wide tempera- ture range. C. Plastics.-The incorporation of the organic molecule into a plastic16 has the advantage of stability over a wide temperature range and in this case the molecule is dissolved in monomer the polymerisation is carried out and the sample irradiated. Variations in lifetimes and Z.F.S. parameters with temperature can then be determined. D. The Cavity Arrangements.-The observation of the e.s.r. signals in single crystals can be carried out with the conventional e.s.r.system with the radio- F. Smith J. Smith and s. P. McGlynn Rev. Sci. Insfr. 1962 33 1367. l6 C. Thornson J . Chern. Phys. 1964 41 1. 56 Thomson 4 frequency field Hrf perpendicular to H,. This arrangement also can be used to observe the Am 1 transitionsin random samples. However in the special arrangement with H parallel to Ho additional features of the spectra and the canonical peaks at low field are more readily seen and this is to be preferred to the conventional arrangement for studies of = 2 lines in random samples since D and E can be obtained directly. For Hrf perpendicular to H,, low-field measurements give H which is related to D* = (D2 + 3E2)* and D and E cannot be determined separately. The experimental results in single-crystal studies are usually fitted to the spin Hamiltonian so that the experimental and predicted spectra agree.For random samples computer simulation of the spectra is carried out. 9 Experimental Investigations of TripIet States of Aromatic Molecules by E.S.R. The triplet states of aromatic molecules can be divided into two types? In aromatic hydrocarbons the lowest unfilled and highest filled M.O.’s are n-type and the lowest triplet state is always of the (n -v*) type in which an elktron is promoted to an excited T* orbital. With the introduction of heteroatoms with lone pairs there is also the other possibility that the lowest triplet state is an (n 3 n*) type with the electron promoted from a lone-pair non-bonding orbital n on the heteroatom. The nature of the lowest triplet state depends on the molecule; in general (n -T*) states have short (< 0-1 sec.) phosphorescence lifetimes whereas (r 3 n*) have lifetimes > 0.1 sec.Only (n 3 n*) triplet states have been detected by e.s.r. and in this section we consider only this type of triplet. A. E.S.R. of Aromatic Molecules in Single Crystals.-The full power of the e.s.r. method is available for the case of oriented molecules in single crystals and in this case the observed spectra allow one to determine the g-tensor components and those of the spin-spin interaction tensor. The principal values of the latter X,Y,Z are related to D and E (eqn. 20). Despite the limitations discussed above a large number of aromatic molecules have been studied in the four host crystals durene biphenyl fluorene and benzophenone.(i) Aromatic hydrocarbons. The pioneering work of Hutchison and Mangum and subsequent studies by their &t0up,17 remain the classic experiments on oriented triplet states. The results are given in Table 1. Naphthalene and its deuterated derivatives have been the most extensively studied in durene biphenyl and fluorene. The observed spectra in these matrices show that matrix effects on D and E are small. The results accurately fit the spin Hamiltonian and show that the host molecules are well oriented and that spin-spin interaction is the domin- ant perturbation. In the case of naphthalene17 or phenanthrene in biphenyl and pyrene in l7 N. Hirota C. A. Hutchison jun. and P. Palmer J. Chern. Phys. 1964 40 3717; R. W. Brandon R. E. Gerkin and C. A. Hutchison jun.ibid. 1962,37,447. 57 Electron Spin Resonance Studies of the Triplet State Table 1 E.s.r. results in single crystals aromatic compounds Molecule Naphthalene [2H,]Naphthalene Phenanthrene Pyrene Quindxaline Quinoline Isoquinoline Phenoxazine Host crystal Durene Biphenyl Durene Biphenyl Fluorene Durene Durene Durene Biphenyl D/hc (cm.-l) +O-10119 + 0.0994 + 0.0992 1 f0-1010 f0-10043 f00810 f0.0806 &O* 1007 f0-1030 f0-1004 f 0.1247 E/hc (cm.-l) -0.0141 1 -0.0154 - 0.01 548 0.01 34 f0.04658 f0.0182 f0.0182 0.01 82 0.01 62 r0.0117 f0-0119 Ref. 3 10 17 10 17 10 17 18 b a 19 19 48 J. S. Vincent and A. H. Maki J. Chem. Phys. 1963,39,3088; b 0. H. GrifFith J . Phys. Chem. 1965 69 1429. biphenyl,l* four resonance lines (Am = 1) are found for each orientation since in these systems there are two inequivalent lattice sites for the C1& molecules in which the planes of two such molecules are mutually perpendicular.The simple two-line pattern discussed above has so far not been found in practice because of the more common feature of inequivalent lattice sites. In the cases of quinoline and isoquinoline eight peaks are found for each orientation,lg because in this case the principal axes of the molecular spin-spin tensor do not coincide with the durene axes so there are two magnetically inequivalent orientations of quinoline in each of the two lattice sites in durene. The effect of temperature on the D and E values is small but measurable. The decay of the phosphorescence and that of the e.s.r. signal as measured by the lifetimes T~ and T ~ R are found to be exponential and the two sets of values are in good agreement.For instance C,,H in durene has T~~ = 2.1 f 0.1 sec. compared with T~ = 2.6 + 0-2 sec. The substitution of deuterium in the mole- cule increases both T~ and the signal intensity and T~ approaches the true radia- tive lifetime which is often much larger than the observed decay time. The single-crystal results at 7 7 " ~ give only the relative signs of D and E and l8 S. W. Charles P. H. H. Fischer and C. A. McDowell Mul. Phys. 1965 9 517. l9 J. S. Vincent and A. H. Maki J . Chem. Phys. 1965,42 865. 58 Thomson measurements at 4 ' ~ and 1 ' ~ are necessary to obtain the absolute signs.2o These studies have furnished very detailed information and there is hope that other host crystals will be found which will extend the applicability of the method.B. E.S.R. of Aromatic Molecules in Rigid Glasses.-The study of triplets in glasses greatly extended the utikty of the method. The earlier workers observed Am = 2 transitions a,nd with Hrf pqpendicular to H, only Hmin was observed. However the use of Hrf parallel to H does result in characteristic structure in this region from which D and E can be determined. The use of fully deuterated molecules has also helped in detecting these weak peaks since line-broadening is less. The observation of Am = 1 transitions has superseded this work and D and E have now been measured for a large variety of molecules (Table 2). Table 2 E.s.r. results in glasses and plastics aromatic hydrocarbons Molecule Benzene Djhc (cm.-l) E/hc (cm.-l) 0.1 56 0 0.1 59 0 Naphthalene [2H,]Naphthalene Anthracene [2H ,]An t hracene Phenanthrene [2H ,]Phenan t hrene Pyrene 0.10046 - 0.01 536 0.0724 0.0081 - - 0.100 0.047 - I 0.1050 0.046 Chrysene - - 0.095 10.025 [ D*/hc (cm.-l) Ref.0.1 56 13 0.1 59 22 0.1048 13 0.1 049 22 0.1063 16 0.1008 a 0.077 22 0.0737 a 0.1335 22 0.129 23 0.1 29 b 0.1336 16 0.1321 C 0.093 23 0.0929 22 0-1052 22 0.103 b 0-104 23 2o A. W. Hornig and J. S. Hyde Mol. Phys. 1963 6 33. 59 Electron Spin Resonance Studies of the Triplet State Table 2-continued Molecule 1 ,ZBenzanthracene 1,2 :5,6-Dibenzan- thracene 1 ,2-Benzopyrene 3,CBenzopyrene Triphenylene Coronene 1,12-Benzoperylene Biphenyl Terphenyl Fluorene Fluoroant hene Toluene Acenap ht hene Trip t ycene Decacylene 60 D/hc (cm.-l) E/hc (cm.-l) D*/hc (cm.-l) Ref. 0.079 0.090 0.090 - 0.1353 0-1 34 0.1360 0.1 338 0.096 0.097 1 0.0983 0.093 - 0.1092 - - - 0.1075 - - - - - - 0.135 (77°K) 0 0.057 0 0.083 0.100 0.098 0.0758 0.1353 0.1 34 0.1360 0.1338 0.096 0.0971 0.0983 0.093 0.071 8 0- 1 094 0.1 130 0.1111 0.0961 0- 1092 0.1 096 0.1088 0-076 0-08 17 0-171 0.1029 0.1 35 0.057 23 23 23 22 22 13 16 a 13 22 16 b 16 d 22 16 22 d 22 16 b 16 21 16 21 41 Thomson Table %continued Molecule D/hc (cm-l) E/hc (cm.-l) D*/hc (cm.-l) Ref.[ DecacyleneI2- 0.021 0 0.021 41 1,3,5 Triphenyl- benzene 0.111 0 0.1 11 13 [TriphenylbenzeneI2- 0.042 0 0.042 41 a E. Wasserman L. C. Snyder and W. A. Yager J . Chem. Phys. 1964 41 1763 ; b G. von Foerster Z . Nuturforsch. 1963 189,620; M. S. de Groot and J. H. van der Waals Physica 1963 29 1128; d S. Siege1 and H. Judeikis J . Phys.Chem. 1966 70 2201. (i) Aromatic hydrocarbons. The appearance of Am = 2 peaks with Hrf parallel to H reflects the symmetry of the state and in molecules of DBh symmetry two peaks are observed and this has been found for coronene triphenylene and triphenylbenzene (Figure 7)>3 Benzene does not give the two-peak pattern 11 77°K 1646 - V /t 40 I K 1 7 I I I I I I I I I I I I I 1 1500 1550 1600 1650 1400 1450 1500 1550 GAUSS 1492 I Figure 7 AmZ = 2 Transitions in glassy samples [Reproduced with permission from J. H. van der Waals and M. S. de Groot Mol. Phys. 1960 3 1901 (a) Naphthalene (b) Triphenylene showing conclusively that the lowest triplet state no longer has hexagonal symmetry.21 This surprising result had been suggested in early phosphorescence studies and the e.s.r. results showed this fact unequivocally.The spectrum of benzene and the methylbenzenes and the decay times T~~ 21 M. S. de Groot and J. H. van der Waals Mol. Phys. 1963,6,545; M. S . de Groot I. A. M. Hesselmann and J. H. van der Waals ibid. 1965 10,91. 61 Electron Spin Resonance Studies of the Trblet State are very temperature-dependent. These effects have been interpreted in terms of the non-equivalence of the possible conformational isomers and these effects could hardly have been investigated except by e.s.r.21 For those molecules which have been studied in glasses and in single crystals observed values of D and E and D* are in good agreement. The line-widths have been correlated with the carbon-hydrogen ratios and indicate that hyperfine broadening predominates.22 Hyperfine interactions are not usually resolved in glasses.Measurements on the Qm = 1 peaks give D and E directly and have been carried out for a large number of hydrocarbons most recently by Brinen and O r l ~ f f . ~ ~ A typical spectrum (deuteriophenanthrene) is shown in Figure 8. (ii) Aromatic heterocycles and substituted hydro~arbons.~~ Generally speaking (Table l) substituents affect E most and D least but the effects are small and usually can be attributed to increased delocalisation. However no systematic study has yet been reported and correlations with spectroscopic evidence might be fruitful. C. E.S.R. of Aromatic Molecules in Plastics.-The D and E values obtained by this t e c h n i q ~ e ~ ~ ~ ~ are in good agreement with the work in single crystals and glasses; slight differences are believed to be due to a stronger matrix effect than is found in the other matrices.Line-widths are always larger in plastics. D has been studied as a function of temperature for coronene and triphenylene16 but the interpretation given of the slight decrease with T is not unambiguous. The triplet-state lifetimes vary with temperature but also with the nature of host and the previous history of the sample. Further work is needed on these systems. 10 Hyperhe Interactions in Triplet States Additional structural information is obtained from hyperfine splittings which can be observed in single crystals. The origin of hyperfine interactions (h.f.i.) both anisotropic and isotropic is now well understood from studies of radicaIs and has been reviewed.2 The unpaired spins in triplets give rise to h.f.i.in a similar manner but unlike the case of free radicals in solution it is possible to determine the anisotropic h.f.i. if the triplets are oriented. To a very good approximation the isotropic and anisotropic splittings of a proton in an aromatic C,-H band are proportional to the spin density pa on the adjacent carbon 2 where a 18 and y are direction cosines of H with respect to (1) the C-H bond direction (a axis) (2) the normal to the plane (b axis) and (3) the c axis per- 22 B. Smaller J . Chem. Phys. 1962 37 1578. 23 J. S. Brinen and M. K. Orloff J . Chem. Phys. 1966 45 4741; S. Siege1 and H. S. Judeikis J . Phys. Chem. 1966 70 2201. 24 See refs. 16 21 and 22. 25 N. Trublin R. Santus and M. Ptak Comp. rend. 1965 260 1134. 62 Thomson 8 N M N I X I 00 0 cu M I X v) v) 3 Q (3 0 0 9 3 3 ? 3 3 3 3 3 z 3 n c 3 E 63 Electron Spin Resonance Studies of the Triplet State pendicular to a and b.A By and C are the isotropic plus anisotropic h.f.i. per unit spin density on the contiguous carbon atom in the axes a b and c respec- tively. Measurements of the hyperfine splittings give Aa Bu and Ca since the isotropic splitting Ai for C-H is ca. -63 Mc./sec. The results show that Aa = 30 Bu = -61 and Ca = -92 Mc./sec. The spectra can be rationalised in terms of these values. The most detailed measurements have been made on naphthalene deuterionaphthalene and phenanthrene. We can regard the naphthalene molecule as a collection of eight C-H frag- ments (Figure 9) of which 3ere are two types which are structurally inequivalent the a and 18 fragments.If H is applied along the x (long) axis of the molecules -61.5 A -30.0 Figure 9 Hyperfine interactions in naphthalene in terms of the interaction with a and fl C-H fragments [Reproduced with permission from C. A. Hutchison jun. Rec. Chem. Progr. 1963 24 1051 then this direction is parallel to the c axis of the 01 protons (which have the largest spin density on the adjacent carbon atom) but at the same time H, is close to the a axis for the fragments. Since these directions are those for largest h.f.i. for a protons and smallest h.f.i. for fi protons the hyperfine pattern is one of five lines from the four a-protons (aaH) each split into five from the smaller /%splitting (apH). The five-he pattern is observed at 7 7 " ~ bUtgpH is unresolved though some additional structure is observed at 4 " ~ .For H parallel to the z axis dl protons will appear equivalent and only a single broad line is observed. For H parallel to the y axis a single peak with some structure is observed as is predicted from the values of A" BQ and CQ for the protons involved. Detailed analysis of the h.f.i. gives the spin densities on the carbon atoms and these can be correlated with theoretical calculations and the results confirm the symmetry of the triplet state. 64 Thornson No hyperfine structure has been observed for oriented pyrene since here the spin densities are low and there are several similar coupling constants but it has been observed in oriented quinoxaline quinoline and isoquin~line~~ where similar information has been obtained. 11 E.S.R. Spectra of Ground-state Triplets There are a number of interesting molecules26~27~2* which have a triplet ground state (as opposed to the triplet excited state of an aromatic molecule) and in recent years the chemistry of carbenes and nitrenes have shown that derivatives of methylene CH2 and nitrene NH react as though they were in a triplet ground state and this has been confirmed by e.s.r.We define a ground-state triplet as one which gives an e.s.r. spectrum which fits the usual spin Hamiltonian with S = 1 but whose e.s.r. signal is stable at 7 7 " ~ for several hours in contrast to excited states which decay in seconds. The ground-state triplet states of substituted methylenes have been studied both in rigid glasses and in single crystals,29 and are produced by photolysis of the particular precursor diazo-compound at 77 OK i.e.Am = 1 and a m = 2 transitions can be observed although not all possible transitions are always detected. The triplets are stable to ca. 1 3 0 " ~ and in con- trast to aromatic triplets D is usually >Om1 cm.-l. The origin of this large value is discussed in Section 16C. A tabulation of some of the resuIts representing the various types is given in Table 3 and considerable structural information has been obtained. Diphenyl- methylene is almost linear but with mutually perpendicular phenyl rings since E #0.30 Phenylmethylene has been studied but not :CH itself. The e.s.r. results for pentaphenylcyclopentadienyl confirm the prediction of a triplet ground state by molecular orbital theory.31 The derivatives of NH the nitrenes have also been extensively studied and are similar to the methylene~.~~ The observed Z.F.S.correlate with the crr-electron spin densities in phenyl-substituted nitrenes,= since only one of the two r orbitals z6 W. Kirmse 'Carbene Chemistry' Academic Press New York 1964. 27 A. M. Trozzolo R. W. Murray and E. Wassermann J . Amer. Chem. SOC. 1962,84,4990. 28 R. W. Murray A. M. Trozzolo E. Wasserman and R. M. R. Cramer J. Amer. Chem. SOC. 1962 84 3213. ps R. N. Brandon G. L. Closs C. E. Davoust C. A. Hutchison jun. B. E. Kohler and R. Silbey J. Chem. Phys. 1965 43 2006. 30 E. Wasserman A. M. Trozzolo W. A. Yager and R. W. Murray J. Chem. Phys. 1964 40 2408. 81 R. Breslow H. W. Chang and W. A. Yager J . Amer. Chem. SOC. 1963 85 2033. 82 G. Smolinsky E. Wasserman and W. A. Yager J . Amer. Chem. SOC. 1962 84 3220; E.Wasserman G. Smolinsky and W. A. Yager ibid. 1964 86 3166. G. Smolinsky L. C. Snyder and E. Wasserman Rev. Mod. Phys. 1963 35 576. 65 Electron Spin Resonance Studies of the Triplet State Table 3 E.s.r. results for ground-state triplets Molecule Structure Dlhc (cm.-') E/hc (cm.-l) Ref. CARBENES Diphenylmethylene Ph-C-Ph 0.401 0.4050 Phenylmet hylene Ph-C-H 0.518 Perfluoroalkylmethylenes R-CH 0.7 Cy anome t h y lene H-C-CEN 0.889 ./.A H . 1,NaPhthYl- & & 0.4555 methylenes 0.4347 DICARBENES p-Phenylene-bis (phenyl methylene) pht=(=Jt~h - 0.0521 NITRENES Nitrene Cyanoni trene 2-Naphthylnitrene 4-Ni trophenylni trene n-Propylnitrene MISCELLANEOUS Diazomethylene Dicyanomet hylene Fluoren ylidene C ycl open t adien y 1 i dene Indenylidene Propargylene NH 1-86 NCN 1-544 PrLN 1 -607 CNN 1.153 NCCCN 1-002 0.4078 0 0-4089 OrJl 0.3777 H-C-C CH 0.6276 0.01 8 0.01 86 0.024 0.02 0.04 0 0.0202 0.0208 0.002 0 0.002 0 0 0.0034 0.002 0.002 0.0283 0.01 20 0.01 60 0 27 28 a 27 b 37 36 36 C d e 67 67 32 e e f 27 34 34 38 Q R.W. Brandon G . Closs and C. A. Hutchison jun. J. Chem. Phys. 1962 37 1878; E. Wasserman L. Barash and W. A. Yager J . Amer. Chem. SOC. 1965 87 4974; CA. M. Trozzolo R. W. Murray G. Smolinsky W. A. Yager and E. Wassennan J . Amer. Chem. SOC. 1963 85 2526; 6 R. N. Dixon Canad. J . Chem. 1959 37 1171; E. Wasserman L. Barash and W. A. Yager J . Amer. Chem. SOC. 1965 87 2075; f E. Wasserman A. M. Trozzolo W. A. Yager and R. W. Murray J . Chem. Phys. 1964,40,2408. 66 Thomson involved conjugates with the ring. The other 2p7r orbital on the nitrogen is in the molecular plane.A rather different type are the triplets of fluorenylidene (l) cyclopenta- dienylidene (2) and indenylidene (3),= the first of which has also been studied in single crystals. D values of ca. 0.4 cm.-l are observed owing to the occurrence of one un- paired electron in a 2p7~ orbital and the other in a 0 orbital on the same centre. The bonds to the bivalent carbon are bent and this has been found to be the case in the methylene derivatives from 13C studies.35 In the case of naphthyl- methylenes two distinct geometrical isomers are observed from a single diazo- precursor thus confirming the non-linear characterF6 Several more complicated ground-state triplets such as ~yanomethylenes,3~ propargylene derivatives:* the species CNN NCN and NCCCN,39 and several dicarbenes and dinitrenes where the spins are ca.6 A apart,4O have been studied. Finally aromatic hydrocarbons with three- or six-fold symmetry axes may form dinegative ions with triplet ground states and triphenylene and decacyclene dianions are examples of this type with low values of D owing to stronger electron correlation.4l 12 Studies of Energy Transfer Of considerable importance is the use of e.s.r. to study the problems of energy transfer from excited triplet states. This problem has been studied for many years by optical methods but once more additional information is obtained by use of e.s.r. The effect of excitation transfer within the triplet state is typified by the examples of benzene the methylbenzenes and the molecules triptycene and tribenzotriptycene; in the last the three aromatic rings are not conjugated with one another.2f As mentioned in Section 9B (i) the benzene spectrum indicates that the molecule no longer has hexagonal symmetry.This implies that there are two configurations I and I1 with two different bond lengths and different energies in 34E. Wasserman L. Barash A. M. Trozzolo R. W. Murray and W. A. Yager J. Amer. Chem. SOC. 1964,86,2304. 3b E. Wasserman J. Chem. Phys. 1965 42 3739. 36 A. M. Trozzolo E. Wasserman and W. A. Yager J. Amer. Chem. SOC. 1965,87,129. s7 K. A. Bernheim R. J. Kempf P. W. Humer and P. S. Skell J. Chem. Phys. 1964,41,1156. 38 R. A. Bernheim R. J. Kempf J. V. Gramas and P. S. Skell J . Chem. Phys. 1965 43 196. 39 E. Wasserman L. Barash and W. A. Yager J. Amer. Chem. SOC. 1965 87 2075. 40 A. M. Trozzolo R.W. Murray G. Smolinsky W. A. Yager and E. Wasserman J. Amer. Chem. SOC. 1963 85,2526. 41 R. E. Jesse P. Biloen R. Prins J. D. W. van Voorst and G. J. Hoijtink Mol. Phys. 1963 6 633. 67 Electron Spin Resonance Studies of the Triplet State each of which there are two equivalent conformations. Vibrations of the right symmetry can cause the system to interconvert between the Type I conforma- tions via Type 11. A theoretical study substantiates this model.21 The rate of this process compared with characteristic e.s.r. times determines the spectral appear- ance and accounts for the temperature-dependence. For a very fast inter- conversion rate the time-average conformation will have D,* symmetry and in this case one can show that the resonance peak broadens and moves to higher fields as is observed when the temperature is raised.The converse is found for decreasing temperature. The methylbenzenes in which the equivalence of the conformational isomers is destroyed exhibit similar behaviour. Triptycene and tribenzotriptycene21 are similar except now the individual conformations are replaced by excitations in individual n-systems. For rapid excitation transfer once again Dgh symmetry is approached and the observed temperature-dependence gives the rate of transfer of triplet excitation. At ~ O " K the excitation is mainly localised but it is delocalised at 7 7 " ~ . The second triplet state energy-transfer mechanism is intermolecular. This was first observed by Farmer Gardner and McDowell bye.s.r.,42 who showed that the benzophenone triplet transfers energy to a naphthalene molecule giving the triplet of the latter in a rigid glass.No benzophenone triplet resonance is observed since this is an (n 3n*) state. Much more detailed investigations on the mechanism of energy transfer in single crystals have since been carried out by Hutchison's which have shed some light on triplet-transfer and triplet-triplet annihilation processes and have shown that the transfer proceeds via the crystalline host lattice and that complex-formation does not occur. Smaller Avery and R e m k ~ ~ ~ have studied similar problems in glasses of different viscosity and have shown that exchange mechanisms dominate at high 7 but diffusion processes at low 7. Similar work has been done by Siegel and his collaborator^.^^ Finally it has been shown that energy transfer from the triplet state to the solvent produces free radicals at a rate proportional to the rate of decay of the triplet state and it is believed to involve a highly excited triplet state.47 The influence of deuteration of triplet state lifetimes has been extensively studied by Hir~ta.*~ 13 The EIectronic Structure of Triplet States The correlation of the observed Z.F.S.parameters with the molecular electronic structure is perhaps the most important aspect of this work since these weak p2 J. B. Farmer C. L. Gardner and C. A. McDowell J. Chern. Phys. 1961,34 1058. N. Hirota and C. A. Hutchison jun. J . Chem. Phys. 1964 42 2869. 44N. Hirota J . Chem. Phys. 1965 43 3354. O5 B. Smaller E. C. Avery and J. R. Remko J. Chew. Phys. 1965 43 922. authors. 47 B. Smaller Nature 1962 195 593.p8 N. Hirota J. Chem. Phys. 1967 46 1561. S. Siegel and H. Judeikis J. Chem. Phys. 1965 42 3060 and many earlier papers by these 68 Thomson interactions constitute a very sensitive test of the molecular wave fun~tion.~ Since the triplet wave function must have an antisymmetric spatial part this means that the unpaired electrons must occupy different spatial orbitals i.e. they are kept apart by Pauli's principle and approximate wave functions can be used to describe the state with some confidence in this case.49 For a 2Nr-electron aromatic molecule the ground-state wave function can be approximated by the single Slater determinant based upon doubly occupied M.O.'s #1 ...... +NO The lowest triplet state is described in terms of one-electron excitations from configuration being M.O.+i to an antibonding M.O. +k (k = N + 1 ..... 2N) the lowest energy 3@. (N-+ N + 1) = b&&&. . .+N+N+~ I (28) for the state with Mz = +1 (we restrict our discussion to this component). The exact triplet wave function in general will be a superposition of all possible contigurations of the appropriate symmetry i.e. 'y/ = cc Ctk @(i 3 k) i k # i with the dominant term 3@(N 3 N + 1) particularly if the +i are open-shell S.C.F. orbitals.50 More complicated wave functions such as the Unrestricted Hartree-Fock51 functions do not appear to be as useful for evaluating the zero-field splittings. The ground-state triplet states have similar descriptions where however the unpaired electrons may now occupy different orthogonal orbitals on the same centre one of which can conjugate with aromatic rings.It should be emphasised that these orbital descriptions are all approximate but a similar analysis52 holds for exact wave functions in the calculation of Z.F.S. parameters. 14 Theoretical Calculation of D and E The theoretical calculation of D and E involves the evaluation of the expectation value of ZS8 (eqn. 5) with the triplet-state wave function. The evaluation of the spin-spin interaction energy can be shown to give the following expressions for D and E 48 J. C. Slater 'Quantum Theory of Molecules and Solids' McGraw-Hill New York 1963 vol. 1. 61 G. G. Hall and A. T. Amos Adv. Atomic and Mol. Phys. 1965,1,1; A. T. Amos and L. C. Snyder J. Chem. Phys. 1965,43,2146. 62 R. McWeeny J . Chem. Phys. 1961,34,399; R. McWeeny and Y . M i m o Proc. Roy. Soc. 1960 A 259,554; R.McWeeny J. Chem. Phys. 1965,42 1717. C. C. J. Roothaan Rev. Mod. Phys. 1960,32 179. 69 Electron Spin Resonance Studies of the Tr&let State where the expectation value is over the antisymmetric spatial part of the wave function. We shall not go into a detailed discussion of the evaluation of this expectation value in this Review (for further details consult refs. 51 53 54-57) but shall indicate the problems involved and the significance of the results. It is clear from eqn. (29) that the evaluation of D and E will involve knowledge of the coefficients Ci in the configuration interaction description and the integrals involving the particular configurations. The former is a straightforward problem in quantum chemistry and for the latter one can show that these integrals reduce to molecular integrals over M.O.’s of the form {ab cd) = JJa(l)b(2) 6 [c(l)d(2) - ~ ( 2 ) d ( l ) ] d ~ ~ d ~ ~ where the operators 6 are those of eqn.(31). The numerators of the operators measure the anisotropy in the two-electron distribution functions and the de- nominators reflect its overall size. If the M.O.’s are expressed in L.C.A.O. form i.e. 2N a = CaiXi (33) i= 1 then the problem reduces to evaluation of products of the coefficients ai and the atomic integrals (ij kZ) where now i,j,k,Z refer to 2pn atomic orbitals. Until quite recently only approximate values of the latter were available but recent work by Karplus and his co-workers% has furnished accurate values of these integrals. The integrals may involve n atomic orbitals on two three or four centres for ( 7 r - 7 ~ ~ ) states but for methylenes and nitrenes there are also one-centre integrals .15 Comparison of Experimental and Theoretical Values of D and E A. Aromatic Hydrocarbons.-Early work was based on a two-configuration approximation5* to the triplet state with wave function 3YLa = sin e@(N -f N + 1) + cos 8@(N - 1 -+ N + 2) (34) a31. Shavitt and M. Karplus J . Chem. Phys. 1965 43 398; C. W. Kern and M. Karplus ibid. 1965 43 415; M. Godfrey C. W. Kern and M. Karplus ibid. 1965 44 4459. 64 J. H. van der Waals and M. S. de Groot Mol. Phys. 1964 8 301. 6s Y. N. Chiu J . Chem. Phys. 1963 39 2763 2749. 66 C. Thomson Mol. Phys. 1966 11 197. 67 J. S. Brinen and M. K. Orloff J . Chem. Phys. 1966 45 4747. N. S. Ham and K. Ruedenberg J. Chem. Phys. 1956,25 13. 70 Thornson 8 is the mixing parameter determined in ref.58 by energy minimisation but in Goutermann’s earlier it was treated as a parameter and the optimum value found by comparison with experiment. These early calculations used approximate values of the two-centre integrals and Hiickel or Hoffmanso M.O.’s and neglected three- or four-centre integrals. The results in Table 4 Columns 1 and 2 show that the results were satisfactory for the smaller mole- cules. Similar calculations were carried out by chi^.^^ A more serious criticism of this approach is that the best 8 values differ from those values which give the best triplet energy58 and so these calculations do not really test the quality of the triplet wave function. For higher accuracy the triplet-state wave function must be obtained by minimisation of the energy as was shown by van der Waals and de Groot5* and Thom~on.~~ These authors used similar approximations for the integrals but used rather different wave functions.Van der Waals’s calculations and later work by Brinen and Orloff5’ were based on Open Shell S.C.F.-Pariser-Parr-type wave functions including a semi- empirical treatment of the 0 electrons but employing the zero-differential over- lap approximation. Thorn~on~~ on the other hand used Hummel and Ruedenberg’ssl wave func- tions which give very good agreement for triplet-state energy levels. These include overlap between nearest neighbours (T.B. M.) or nearest and next- nearest neighbours (I.R.M.) and employ well-tested methods of evaluating electron repulsion and core integrals. Both sets of calculations used extensive configuration interaction which is necessary for accurate work (Table 4 Columns 3,4; 6,7).This work is currently being extended by use of the accurate integral values of K a r p l ~ s . ~ ~ ~ ~ Agreement with experiment was very good particular improvements being apparent for the peri-condensed molecules. However the importance of the influence of (T electrons54 and more accurate integral values53 need studying in order to obtain values of D and E to within a few per cent. of experimental values. Nevertheless the calculations using properly energy minimised wave functions do appear to constitute a good test of the wave functions used.G3 B. Aromatic Heterocycles and Other Molecules.-Very little work has been carried out on other than hydrocarbons particularly with respect to the use of accurate integrals.Boorstein and G~uterman~~ have carried out similar calcula- tions to those of van der Waals on quinolines and quinoxalines using more accurate (but still not exact) integrals but more work is needed in this area. An estimate of (nlr*)-state Z.F.S. parameters has been made by Sternli~ht.~~ See ref. 11 for bibliography of earlier calculations in this approximation. R. L. Hummel and K. Ruedenberg J . Phys. Chem. 1962 66 2334. 6o R. Hoffman J . Chem. Phys. 1963 39 1397. 62 C. Thomson unpublished work. 6s See however T. J. Dougherty T. Vladimiroff and S. T. Epstein J . Chem. Phys. 1966,45 1803. 64 S. A. Boorstein and M. Gouterman J . Chem. Phys. 1965,42 3070. 66 H. Sternlicht J . Chem. Phys. 1963 38 2316. 71 Table 4 Theoretical values of D and E for some aromatic hydrocarbons 1 2 -3 4 5 6 7 Molecule D E D E D E D E D E D E D E I - - - Benzene (Dab) 0.1519 0 0.1519 0 0-179 0 0.15333 0 - - - - 0.157 - - - Naphthalenet 0.1003 -0.0133 0.0958 -0.0211 0.111 -0.028 0.099 -0-024 0.045 0.005 0.088 -0.011 0-100 -0.011 - - 0.097 -0.022 - - 0.1002 -0.0146 - - Anthracene 0.0727 -0.0148 0.0721 -0.0166 0.076 -0.012 0.071 -0.009 0.034 0.012 0.049 -0.002 0.074 0.001 0.077 -0.011 - - I - - 0.084 -0.041 - - 0.069 0.037 0.080 0.039 Pyrene - - - Phenanthrene 0.0731 0.0269 0.0851 0.0304 0.115 0.056 0-100 0.048 0.063 0-082 0.082 0-059 0.092 0.051 0.116 0.039 1,12-Benzperylene - - - - - - - - - 0.042 0.001 0.063 -0.001 Coronene 0.0522 0 0.0608 0 - - - - - - - - - - - - 0.0546 0 - - - - 0.134 0 Triphenylene 0.0697 0 0.0810 0 - - - - - - - - 0.0693 0 - *Approximations used 1.Reference 1 1 All two-centre integrals included. Single Gaussian approximation in integral evaluation. Two-configuration wave function (eqn. 34) based on Hiickel orbitals. 2. Reference 11 As for 2*but with Hoffmann orbitals. 3. Reference 54 Pariser-Parr approximation in Open-Shell SCF-M.O. calculation including C.I. with all singly and some doubly excited configura- tions; integrals by direct quadrature. 4. References 54 and 57 As for 4 but only singly excited configurations included; semi-empirical adjustment of nearest-neighbour ( X i X j ; X r X j ) to take o-effects into account. 5 . Reference 5 1 Unrestricted Hartree-Fock method; single Gaussian approximations in integral evaluation Pariser-Pam parameters. 6. Reference 56 Ruedenberg TBM wave functions; single Gaussian approximation to integrals; overlap between nearest neighbours.7. Reference 56 As in 6 but wave functions (IRM) included nearest and next-nearest neighbour overlap integrals. ?The values of D and E for naphthalene using Pariser's eight-configuration wave function and accurate values of all two- three- and four-centre dipolar integrals are D = 0.1081 E = -0.0093 (ref. 53). % P Y Thomson C. Ground-state Triplet States.-The calculation of the Z.F.S. parameters in the substituted methylenes and nitrenes has been considered in detail by Higuchi in a series of papers!e These calculations typified by the work on substituted methylenes have shown that the one-centre terms are the most important and for reasonable values of the integrals the calculated D and E are in fair agree- ment with experiment.D and E mainly depend on the spin density at the methy- lene carbon.% The effect of bond angle on D and E shows that the -C- bond is bent.6s The exact explanation of the observed angle must however await more detailed calculations including in-plane a-electrons. Similar calculations on nitrene derivatives have been made?' The use of ab-initio wave functions will be of great importance in this type of work and Loundsburyss has calculated D for NH using one centre ab-initio SCF wave functions with encouraging results. Finally the influence of spin-orbit interaction has been investigated (eqns. 3 and 4) which was neglected in all the above discussion^.^^ This is a second-order effect and it appears likely that for methylenes the contribution to D is ca.10% of the spin-spin interaction. It is expected to be less important in delocalised systems but quantitative calculations are needed. 16 Miscellaneous Remarks The Triplet State in Solution.-Very few studies have been carried out on triplet states in solution since (1) lifetimes are usually too short or (2) the Z.F.S. aniso- tropy results in lines too broad to observe. Recently however Lemaire and his co-workers have described'O some biradicals which give well-resolved triplet state spectra in solution. The molecules consist of two nitroxide radical fragments linked via a saturated chain. The e.s.r. spectra depend on the magnitude of the exchange integral J coupling the two unpaired electrons. For J < nN the spectrum is a triplet charac- teristic of two independent nitroxide groups but for J B aN five lines are observed separated by aN12.Each electron interacts with both nitrogen nuclei. For J = ca. a, line-width alternation occws and Lemaire et al. have described biradicals in which J varies from one extreme to the other. By using liquid 66 J. Higuchi J . Chem. PAYS. 1963 38 1237; 1963 39 1847; 3455; 1964 41 2084. 42 54. *O S. H. Glarum J. Chem. Phys. 1963 39 3141; S. J. Fogel and H. F. Hameka ibid. 1965 42 132. 70 R. M. Dupeyre H. Lemaire and A. Rassat J. Amer. Chem. SOC. 1965,87,3771; R. Briere R. M. Dupeyre H. Lemaire C. Morat A. Rassat and P. Rey Bull. SUC. chim. France 1965 11 3290. J. A. R Coope J. B. Farmer C. L. Gardner and C. A. McDowell J . Chem. Phys. 1965 J. B. Loundsbury J. Chem. Phys. 1965 42 1549. 73 Electron Spin Resonance Studies of the Triplet State crystals,71 where the molecules are partly oriented these workers have been able to observe further splitting of the lines owing to the zero-field splitting in the triplet state.Extensions of this type of work to other systems should be most interesting. Polarised Light Studies.-Oriented triplet states can be produced by use of polarised light and the influence of the polarised light on the canonical peak intensities has given interesting information on the polarisation of the exciting singlet.72 17 Conclusions I have tried to show that e.s.r. has provided a wealth of important information concerning the electronic structure of the triplet state together with information of energy-transfer processes and the factors influencing such processes. The detailed information available emphasise the power of the e.s.r. technique and present the theoretical chemist with data which test the available theories of the electronic structure of such states. I thank Professors D. Kivelson M. A. El-Sayed and Y-N. Chiu for many fruitful discussions on this topic. 'l H. R. Falle G. R. Luckhurst H. Lemaire Y. Marechal A. Rassat P. Rey MoI. Phys. 1966 11 49. 72 M. A. El-Sayed and S. Siegel J . Chem. Phys. 1966,44 1416; M. Lhoste P. Hang and M. Ptak ibid. p. 645 654; G. P. Rabold and L. H. Piette Photochem. und Photobiol. 1966 5 733. 74

ISSN:0009-2681    

DOI:10.1039/QR9682200045

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

The Application of High Resolution Mass Spectroscopy to Organic Chemistry By G. W. A. Milne LABORATORY OF METABOLISM NATIONAL HEART INSTITUTE NATIONAL INSTITUTES OF HEALTH BETHESDA MARYLAND 20014 U.S.A. 1 Introduction The fundamental principles of mass spectrometry were established at the turn of the century in a series of classical experiments by inter alios Wien,l and Thomson.2 These workers demonstrated that a beam of positive ions could be deflected by electrical or magnetic fields and that the angle of deflection of any given ion varied with a number of parameters one of which is the mass of the ion. Th0mson~9~ designed a mass spectrograph in which he demonstrated that neon gave not one but two lines since it consists of two stable isotopes differing in mass. An improved mass spectrograph was subsequently developed by Aston4 and was used to carry out similar isotope analyses on a variety of element^.^ At about this time Dempster6 published details of his mass spectrometer in which the analysed ions were focused at a point rather than in a plane as in mass spectrographs.In such a system detection of the ions may be accomplished electrically rather than photographically and this variation with its inherent greater sensitivity has come to assume considerable significance in high resolu- tion mass spectrometry. Thus mass spectrometry was effectively dichotomous at birth and in terms of instrumentation the subsequent development of deflection spectrometers has been in two similar but rarely intersecting directions. The most sophisticated instruments now available are direct descendants of Aston’s mass spectrograph or of Dempster’s mass spectrometer and in the area of high resolution mass spectrometry to which this Review will be restricted no funda- mental change in either of these designs has enjoyed great success.The first area of organic chemistry in which mass spectrometry was employed was hydrocarbon analysis,’ a field in which sample handling problems are minimal the compounds generally being relatively stable and volatile. From this point the development of the role of mass spectrometry in organic chemistry was largely arrested pending the handling of two distinct problems (i) that of efficiently vaporising and ionising non-volatile or unstable compounds; (ii) that W. Wien Ann. Physik 1898 65 440. J. J. Thomson Phil. Mag. 1911 21 225.J. J. Thomson ‘Rays of Positive Electricity’ Longmans Green and Co. London 1913. F. W. Aston Phil. Mag. 1919 38 707. F. W. Aston ‘Isotopes’ Edward Arnold London 1922 1924. A. J. Dempster Phys. Rev. 1918 11 316. H. W. Washburn H. F. Wiley and S. M. Rock Ind. and Eng. Chem. (Anal.) 1943,15 541. The Application of High Resolution Mass Spectroscopy to Organic Chemistry of recognising and extracting the wealth of information in mass spectra. Most sample handling systems that are useful with organic compounds have evolved from the premise that the sample should be admitted to the ionising region in the vapour phase with a vapour pressure no greater than about 10” mm. of mercury. At such pressures the mean free path of a particle is about 5 metres and ion-molecule collisions may be ignored.Early mass spectrometers were designed with gas inlet systemsa which during the 1940’s were modified to accommodate volatile organic 1iquids.O In 1955 the technique of direct introduc- tion of the sample into the ionising region was applied.lO*ll Direct introduction of the sample via a vacuum lock12 is now routine and most commercially avail- able machines have such an inlet system which is probably the most generally useful system for the organic chemist. Using one or another of these various sample handling systems the organic chemist will be able to obtain mass spectra on the majority of organic compounds. Those which fail to vaporise unchanged however cannot be handled in this way and create a special problem which is discussed later (v. infra). Techniques by which information can be extracted from mass spectra are in their infancy but it is already abundantly clear thanks to the pioneering efforts of Beynon,13J4 Biemann,15 Djerassi,ls McLaffertyl’ and others that mass spectro- metry has enormous potential as an analytical tool in organic chemistry.There is already available a considerable amount of empirical knowledge of fragmenta- tion mechanisms intelligent application of which has been shown in many instances18 to be of great assistance in problems of structure determination. Since the demonstration by Beynod* that with instruments of higher resolv- ing power the mass of an ion may be measured sufficiently accurately to permit the deduction of its atomic composition a great deal of activity and expenditure has been applied to the field of high resolution mass spectrometry.Mass spectro- meters have been improved to the point where such accuracy can be obtained routinely. Computer acquisition and processing of the large quantity of data in any one high resolution spectrum has become fairly common and an exciting J. H. Beynon ‘Mass Spectrometry and its Applications to Organic Chemistry’ Elsevier Ref. 8 p. 161. J. L. Courtney and J. S. Shannon Tetrahedron Letters 1963 13. J. H. Beynon ‘Advances in Mass Spectrometry’ ed. J. Waldron Pergamon Press London 1959 p. 328. lP J. H. Beynon ‘Advances in Mass Spectrometry’ ed. M. Elliott Pergamon Press London 1961 p. 216. See also ref. 8. l6 K. Biemann ‘Mass Spectrometry. Organic Chemical Applications’ McGraw-Hill New York 1962. 16 H. Budzikiewicz C. Djerassi and D. H. Williams ‘Interpretation of Mass Spectra of Organic Compounds’ Holden-Day San Francisco 1964.1’ ‘Mass Spectrometry of Organic Ions’ ed. F. W. McLafferty Academic Press New York 1963. 18 See for example K. Biemann in ‘Progress in the Chemistry of Organic Natural Products’ ed. L. Zechmeister XXIV 1966. Publishing Co. London 1960 p. 147. lo P. Bradt and F. L. Mohler Analyt. Chem. 1955 27 875. l1 P. de Mayo and R. I. Reed Chem. and Ind. 1956 1481. 76 Milne new field computer-assisted interpretation of mass spectra is now being actively explored. This Review which is not intended to be comprehensive will be concerned with the more important results of these endeavours. A number of important areas such as the various methods of ionisation will not be considered in depth and for details of this sort the reader is referred to the more exhaustive survey by McLafferty and Pin~e1ik.l~ The purpose of this review is to consider the present position of high resolution mass spectrometry vis-&vis organic chemistry in the light of past results and future prospects.2 Sample Handling Systems A large number of gas inlet systems have been designed,s most of which use a molecular leak in a pressure reducing system and many of which employ mercury as a cut-off phase. These are therefore of somewhat limited value for organic chemists whose samples are often less volatile than mercury. Replacement2* of mercury by gallium extends the temperature range of many of these systems to beyond 300" and allows the design of an inlet system that may be used for a large number of non-polar organic molecules.21 For general use with such compounds the glass-metal system first described by Beynon21 is extremely useful.A simpler version of this system has subsequently been designed22 but is not however commercially available as is its forerunner. An enamelled all- metal system has also been developed.23 Temperature controlled direct introduction of the sample into the ion source was first reported some twelve years a g ~ ~ ~ ~ but all these devices were in- convenient because the mass spectrometer could not be kept under vacuum whilst a sample was admitted. This difficulty was overcome by the incorporation of a vacuum 10ck12s25s26 and the resulting direct insertion probes which are now commercially available can be used with all thermally stable compounds which do not represent extremes in volatility.It has been pointed out moreover26 that for compounds that are moderately thermally unstable the direct probe is superior to the older inlet systems in that the sample is in effect flash-evaporated and is not heated protractedly. For very volatile samples a cryogenically cooled probe has been de~igned.~' A similar simpler probe has been developed and used at -100"28 but no commercial models of such systems are available. In F. W. McLafferty and J. Pinzelik Analyt. Chem. 1966 38 350R. See also ref. 39. 2o M. J. O"ei1 and T. P. Wier Analyt. Chem. 1951 23 830; M. J. O'Neal 'Applied Mass Spectrometry' The Institute of Petroleum London 1954 p. 27. 21 Ref. 8 p. 184. 22 V. J. Caldecourt Analyt. Chem. 1955 27 1670. 23 C. Brunee 2. Instrumentenk. 1965 73 16. 24 R.I. Reed J . Chem. SOC. 1958 3432. 26 H. C. Hill and R. I. Reed J. Sci. Instr. 1963 40 259; G. A. Junk and H. J. Svec Analyt. Chem. 1965,37 1629. 26 G. L. Kearns Analyt. Chem. 1964 36 1402; M. Barber T. 0. Merren and W. Kelly Tetrahedron Letters 1964 1063. 27 H. A. McGee Jun. NASA Doc. N6323401 1963. W. F. Haddon E. M. Chait and F. W. McLafferty Annlyt. Chem. 1966,38 1968. 77 The Application of High Resolution Mass Spectroscopy to Organic Chemistry A simple labelling technique evolves from the fact that so-called ‘active hydrogens’ may be replaced by deuterium if the sample is admitted into the ion source with D,O either via a probe29 or an indirect inlet system.30 Since Gohlke’s first successful attempt31 to pass the effluent from a gas chroma- togram into the source of a mass spectrometer much work has been done on the dual problems of (i) reducing the recording time of the mass spectrometer to the 1-10 seconds available during the emergence of a gas chromatography peak and (ii) removing the carrier gas used in the chromatography.Electronic im- provements such that magnet scanning at high resolution can be done in under 10 seconds per decade have been made and instruments with this capability are available commercially. The efficient removal of carrier gas without loss of solute has proved to be considerably more difficult however and none of the various separators that have been designed is very efficient although the enormous sensitivity of the mass spectrometer ameliorates this problem to some extent. Three commercially available separators are the Biemann-Watson which is in effect a molecular sieve the Ryhage molecular jet ~eparator:~ which removes the molecules of carrier gas as they emerge from a jet with much lower momentum than solute molecules and the Llewellyn semi- permeable membrane separator= which permits the passage of organic molecules but not of carrier gas.The use of porous Teflon as a separator has been de- and such a device has been critically compared36 to the Biemann- Watson separator. Capillary columns may be used without a separator3’ and a general discussion of some of the problems encountered in coupled gas chroma- tography-mass spectrometry has The majority of organic compounds can be handled in one or another of the inlet systems described above but some highly polar compounds cannot be vaporised without decomposition.The most promising approach to such com- pounds is the formation of volatile derivatives such as trimethylsilyl ethers and esters. Compounds which vaporise satisfactorily but fail to give a molecular ion also cause difficulties. The use of lower electron voltages sometimes helps in such cases and the use of field emission rather than electron bombardment holds great promise in spite of its lower sensitivity. But at present the common approach to this problem is to tolerate it and extract the remaining available information. 29 See ref. 15 p. 231. 8o J. S. Shannon Austral. J . Chern. 1962 15 265. 31 R. S. Gohlke Analyt. Chern. 1959 31 535. 32 J. T. Watson and K. Biemann Analyt. Chem. 1964 36 1135; 1965,37 844. 33 R. Ryhage Analyt. Chem. 1964,36,759; Arkiv Kemi 1966,26 305.34 P. M. Llewellyn and D. P. Littlejohn Pittsburgh Conf. Anal. Chem. App. Spectroscopy Feb. 1966. 36 S. R. Lipsky C. G. Horvath and W. J. McMurray Analyt. Chem. 1966 38 1585. 36 M. A. Grayson and C. J. Wolf Analyt. Chem. 1967 39 1438. 37 D. Henneberg Analyt. Chem. 1966 38 495. See also ref. 47. 38 W. H. McFaddon and E. A. Day Analyt. Chern. 1964,36,2362. 78 Milne 3 Instrumentation Two commercial high resolution mass spectrometers suitable for use with organic compounds are the CEC 21 -1 10B (Consolidated Electrodynamics Corporation Pasadena California) and the AEI MS 902 (Associated Electrical Industries Ltd. Manchester England). Of the other commercially available high resolution spectrometers none will be considered here because they are all recent additions to the field and are geometrically similar to either the 21-llOB or the MS 902.In addition relatively little information is available upon the performance of these other machines when coupled with automatic data-handling systems. Ions are produced in both the 21-llOB and the MS 902 by electron bombard- ment in sources which are basically similar. The ions are then extracted out of the source by an accelerating potential commonly SkV and focused first in an electrostatic sector and then in a magnetic sector. The resolved ion beams each one homogeneous with respect to mass to charge ratio,* then enter a detector system. ELECTROSTATIC ANALYZER SOURCE ISOLATING VALVE SOLID SAMPLE PROBE ADJUSTABLE SLIT ONITOR COLLECTER PUMPING PUMPING SYSTEM SYSTEM TO AMPLlFlE AND RECORD ECTER I T PLlER Fig.1 Schematic diagram of the MS 902. Nier-JohnJon geometry. The MS 902 is related to Dempster’s original spectrometere in that it employs the Nier-Johnson geometry shown in Figure 1. In this system the analysed ion beams are focused in a complex conic surface. At the focal point of this system is placed an exit slit behind which is the first dynode of an electron multiplier. The magnetic field is then scanned usually from high field (high mass) down- wards and as one mass after another enters the multiplier the output signal is amplified and recorded in an oscillographic recorder. The mass of a particular ion is related therefore to the time elapsed from the start of a scan when it appears and if the times for standard ions are known accurately the precise mass of the unknown ion may be calculated.MAGNETIC AN A LY 2 ER * The only ions considered in this Review are singly charged. Reference will be made therefore to their ‘mass’ rather than to their ‘mass to charge ratio’. 79 The Application of High Resolution Mass Spectroscopy to Organic Chemistry In contrast to this the Mattauch-Herzog geometry of the 21-110B shown in Figure 2 ensures that the analysed ions are focused in a plane in which con- ventionally a photoplate is placed. This ‘spectrometer’ is therefore really a ‘spectrograph‘ as was Aston’s original design? An image is registered on the plate by each resolved ion in a position which depends upon the square root of the mass. The final density of the line is proportional to the abundance of the ion. No scanning of the magnetic field is required; the precise mass of an ion can be obtained by accurately measuring its position on the photoplate with respect to standard ions.The 21-1 10B is also equipped with an electrical detector system based upon an electron multiplier which is placed in the focal plane at the far end of the photoplate (see Figure 2). photoplate and permits the recording of magnetic field much as with the MS 902. SOLID SAMPLE PROBE This represents an a mass spectrum 1kS INLET -& k O 0 J E C T SLIT alternative to the by scanning the i t \ PUMPING SYSTEM \ ELECTROSTATIC SECTOR \ MULTIPLIER Fig. 2 Schematic diagram of the 21-1 JOB. Mattauch-Herzog geometry. A comparison of these two spectrometers is less fruitful than a comparison of the two methods of recording high resolution mass spectra i.e. the photo- plate method and the scanning procedure.The static nature of photoplate recording is very convenient because automatic data acquisition from a photo- plate is relatively easy requiring only a series of accurate linear measurements and photodensitometer readings. Scanning a magnetic field rapidly and repro- ducibly is difficult and the circuitry required is complex. A further serious tech- nical problem is created by the requirement that the time centroid of each peak be measured with an accuracy of 1 part in lo5. The 21-llOB is harder to focus39 and since it cannot avail itself of an ampli- fication system for photoplate recording it is inherently less sensitive in this mode towards a given number of ions than is a scanning spectrometer which ss R. I. Reed Quart. Rev. 1966 20 527.80 Milne can easily detect a single ion. The photoplate method of recording moreover does not lend itself to abundance measurements whilst the output from an electrical detection system is effectively a plot of mass versus abundance. A complete spectrum at a resolving power of 1 part in lo4 can be recorded by either method in about 10 seconds thus permitting the analysis of effluent peaks from a gas chromatogram. Peak matching (v. infra) is possible with both machines but in this respect the MS 902 has a superior performance. Finally the MS 902 shows ‘metastable’ ions considerably more readily than does the 21-1 10B although this is of no moment in high resolution work when such ions are rarely observed in either spectrometer. In summary then whilst it is pointless to argue that one spectrometer is superior to the other it is fair to say that electrical detection by virtue of its much higher sensitivity and accuracy is quite superior to photoplate recording whose ultimate and serious limitation is the size of the grain in the photoplate emulsion.Handling data from magnetic scans is difficult but as will be seen has been accomplished with both machines and will surely prove to be the future method of choice for the recording of high resolution mass spectra. 4 Resolving Power Two peaks of equal height are said to be just resolved if the valley between them is 10% of the peak height and the resolving power of the system at this point is defined as the difference in the masses of the two ions e.g. 1 part in 5000 or 200 ppm. This ‘10% valley definition’ of resolving power is not the onIy defini- tion in use but enjoys wider currency than others such as ‘1 % valley’ and will be used here.Instruments with which a resolving power of 1 part in 5000 cannot easily be achieved are generally incapable of mass measurement to the accuracy required for formula assignment to ions from any but the simplest of organic compounds and will not be considered here. A resolving power of 1 part in 10,OOO is often assumed for the purposes of discussion to be a prerequisite for mass measurement of an accuracy sufficient to permit formula assignment but this is a misleading generalisation in two senses. It is perhaps more useful to remember the rule derived from experience that depending upon the technique used the accuracy of mass measurement may be expected to be between one and two orders of magnitude greater than the resolving power being used.Thus a resolving power of 1 part in 10,OOO should allow mass measurement with an accuracy of perhaps 1 part in 200,000 or 5 ppm. The other aspect of the problem is that the accuracy required depends only on the problem hhand. If it is not known what elements are present in the sample then the highest feasible accuracy should be sought but where this information is available or as will be discussed later (v. infra) can be obtained from the spectrum itself it is pointless to use a resolving power that is vastly too high for the problem at hand. As a trivial example the only two possible ions of mass 140 (ignoring 13C- and 2H-containing ions) in the mass spectrum of a hydrocarbon are C,,H,,,+ 81 The Application of High Resolution Mass Spectroscopy to Organic Chemistry (m/e 140.1565)* and C,,H,+ (m/e 140.0626).These will be completely separated with a resolving power of 671 ppm (1 part in 1492) and the mass of either could be easily measured accurately enough to permit a choice between the two at a resolving power of 1 part in 1000. In fact this particular problem is so easy that it may be done by manually measuring the position of the peak on the oscillo- graph Hydrocarbons represent the simplest case and as the number of elements involved increases the accuracy of mass measurement required to permit an unequivocal assignment of formula increases. The vast majority of organic compounds however contain no elements other than C H 0 and N. The ions representing simple combinations of these ele- ments that will be closest together (again ignoring minor isotopes) will be separated by 0.0126 mass units which is the difference in mass between CH and N.At mass 500 these should appear as separate peaks at a resolving power of 25 ppm (1 part in 40,000) and should be quite easily mass measured at a resolving power of 1 part in 10,000. Although they are somewhat less common doublets that are more difficult to resolve do occur e.g. C2H20 and N differ by only 0.00135 mass units. Introduction of sulphur halogens or carbon-13 into the consideration creates a requirement for even greater accuracy and with currently available instruments the spectroscopist soon finds himself between the Scylla of high resolution and the Charybdis of adequate sensitivity.In practice the presence of sulphur chlorine or bromine can be immediately discerned from their characteristic isotopic distributions whilst iodine is distinguished by its large negative mass defect. Separation of the 13CJ2CH and 1H2-2H doublets is generally considered to be beyond the capabilities of present instruments requiring as it does a resolving power of 1 part of 25,OGO at mass 100. Currently most instruments are used at a resolving power of about 1 part in 10,000. Although both the 21-1 10B and the MS 902 are capable of three times that this figure represents a compromise between sensitivity and resolution. It is here that the field of high resolution mass spectrometry begins to become distinct from that of low resolution mass spectrometry. Mass spectrometers of low resolving power are of great value in a variety of connections such as trace analysis reaction kinetics isotope labelling and structural studies but it is in this latter field that high resolution mass spectrometry has unparalleled potential.Assignment of an unequivocal formula to every ion type in the spectrum and subsequent reassembly of all the fragments to obtain a unique structure for the few micrograms of sample compound is perhaps a breathtaking feat but as will be seen it has actually been done in a few cases. 5 Peak Matching In principle a mass spectrometer could be calibrated and all the variables controlled so that the accurate mass of an ion could be calculated immediately from its position on a photoplate or the time of its appearance during a magnetic * Based upon the 12C = 12WOOOO scale adopted by I.U.P.A.C.40 B. H. Johnson and T. Aczel Analyf. Chern. 1967,39 682. 82 Milne scan. There are however too many variables to deal with and adequate control of some of these is very difficult. The usual technique therefore is to calibrate continuously with standard ions and measure the mass of the unknown ion with reference to one or two standard ions. This requires that the sample and the reference compound be admitted simultaneously. Most laboratories use as a standard compound either perfluorokerosene (PFK) or heptacosafluorotri- butylamine. The latter is a homogeneous compound which is sufficiently volatile to be admitted via a gas inlet system whilst the former is normally supplied as a mixture of hornologues which requires a heated inlet system.PFK has the great advantage however of giving a standard ion at least every 12 mass units whilst in the spectrum of heptacosafluorotributylamine there are some large gaps between abundant ions and there are no ions above m/e 614. The only simple method of mass measurement with an accuracy of 10 ppm is the peak matching technique developed by Quisenberry Scolman and Nier.41 With the accelerating voltage and the electrostatic analyser voltage kept constant the magnetic field is adjusted so as to bring into focus a standard ion lower in mass but as close as possible to the unknown ion. A small auxiliary magnet is then swept repetitively over a very small mass range about this ion e.g. for the standard ion C,F,+ (99.99362) from 99.996 to 99.990 and the output displayed on an oscilloscope.The accelerating and electrostatic analyser voltages are then switched back and forth on alternate sweeps to lower values which can be adjusted until the unknown peak is superimposed upon the standard peak. When this condition is fulfilled the ratio of the higher accelerating potential to the lower accelerating potential can be read directly and is equal to the ratio of the unknown mass to the standard mass. Thus if the unknown ion is super- imposed upon the C,F,+ standard ion at a ratio of 1.020150 the unknown mass can be immediately calculated to be 102.0085 a number which in practice has six significant figures and is therefore accurate to 1 mmu below m/e 1OOO. Since the accelerating potential can only be decreased the magnetic field must be adjusted to focus the lower mass ion and it is convenient to make this the standard ion.The maximum ratio that could be used with Nier’s originalsystem was 1.10 i.e. the unknown mass had to be within 10% of the standard mass. A voltage trimming device has subsequently been de~eloped,~ for the MS 902 and extends this range to 2-00 although for other reasons the useful range is about 1.70. Under these circumstances heptacosafluorotributylamine proves to be an adequate internal standard and is often used with the peak matching system of the MS 902. With the 21-llOB however the peak matching system has a maximum ratio of 1.10 and since it employs a meter as opposed to an oscillo- scope it is quite difficult to use. Whilst peak matching will probably be overwhelmed by automatic data pro- cessing techniques in the fight for survival its present position is one of great importance.It still is the quickest and simplest way to measure accurately the 41 K. S. Quisenberry T. T. Scolman and A. E. Nier Phys. Rev. 1956 102 1071. 43 H. M. Fales R. Binks M. Elliott and R. Freeman Proceedings of the ASTM Committee E-14 Dallas 1966. 83 The Application of High Resolution Mass Spectroscopy to Orgunic Chemistry mass of a limited number of ions in the spectrum such as the molecular ion and major fragment ions. With the MS 902 a dexterous and resilient operator can peak match one ion per minute with this technique. This is perhaps 10-100 times slower than the current automatic data processing systems discussed in the following section but is inestimably cheaper. It may be noted parenthetically that Ryhage& has adapted the peak matching system to a single focusing mass spectrometer and reported surprisingly accurate mass measurements.6 Automatic Data Processing Data processing techniques are inescapable in high resolution mass spectro- metry for two reasons. Firstly the sheer amount of data in just one high resolu- tion mass spectrum is too much to collect manually by peak matching and secondly the assignment of formulae to ions of known mass whilst it can be done in individual cases with the help of tables44 is for a large number of ions a problem tailor-made for a high speed digital computer. The automatic retrieval of data from a photoplate is relatively easy and was first achieved by Biemann’s group in 1964.45 The equipment necessary is basically a travelling microscope equipped with a photodensitometer.The microscope scans the photoplate and measures the density of any image at intervals of 0.25 microns. This density reading is converted to an electrical signal by a photo- multiplier and becomes a y-coordinate which together with its corresponding x-co-ordinate (the distance from the beginning of the spectrum) is fed to a high speed analogue-to-digital converter. The resulting digital data are discarded unless the y-co-ordinate is greater than a preset ‘threshold’ value i.e. a peak is being observed. If this is the case the data are transferred to magnetic tape. In this way tape is not wasted recording all the data points between peaks. It now is a relatively simple job for a digital computer to calculate the centroid of each peak recognise the standard peaks calculate the exact masses of the unknown peaks separate the two and discard the former.The results of these operations are accurate mass versus intensity data for the unknown ions in the spectrum. To raise raw data from the MS 902 to this level is somewhat more difficult but has been a~complished.4~~~~ For fast scans the amplifier system must have a bandpass of at least 10 kc and it is primarily this that distinguishes the MS 902 from its predecessor the MS 9. The output signal from a scan is recorded 48 R. Ryhage Proceedings of the ASTM Committee E-14 Denver 1967. 44 No completely satisfactory tables exist or could exist. The most popular J. H. Beynon and A. E. Williams ‘Mass and Abundance Tables for Use in Mass Spectrometry’ Elsevier Publishing Co. London 1963 contain only C H 0 and N combinations whilst an expensive alternative D.D. Tunnicliff P. A. Wadsworth and D. 0. Schissler ‘16-Element Mass and Abundance Table with Supplement’ 4 vols. Shell Development Co. Emeryville California 1965 places an upper limit of 10 on the number of carbon atoms considered. 46 K. Biemann P. Bommer and D. M. Desiderio Tetrahedron Leffers 1964 1725; K. Bie- mann Proceedings of the ASTM Committee E-14 St. Louis 1965. A. J. Campbell J. S. Halliday B. N. Green T. 0. Merron and J. G. Murray Proceedings of the ASTM Committee E-14 St. Louis 1965; C. Merritt P. Issenberg M. L. Bazinet B. N. Green T. 0. Merron and J. G . Murray AnaZyt. Chem. 1965 37 1037. W. J. McMurray B. N. Green and S. R. Lipsky Analyt. Chem. 1966 38 1194. a4 Milne directly on an FM tape and this analogue tape is then digitised.Analogue-to- digital conversion in this case is an operation in which the computer measures and records the output voltage at given time intervals-optimally 10-15 micro- seconds in a total scan of 10 seconds which even with a tape-slowdown factor of 32 requires a high-speed digitising system. The time centroid of each peak is then calculated and the resulting digital information (voltage versus time) corre- sponds to the intensity-distance data derived from a photoplate. In this latter case however distance is related simply to the square root of the mass whilst the time-mass relationship in the quasi-exponential magnetic scan is more complex. In practice it has been demonstrated that given three known ions within a mass range of not more than 24 mass units the mass of any other ion appearing within 12 mass units of either end of this range may be calculated sufficiently accurately.This may not encompass an unknown ion but if PFK is the internal reference it must include a standard ion which will be recognised as such and so the range of 24 mass units may be shifted 12 mass units to lower mass and the search for an unknown ion resumed. As before a ‘threshold’ level eliminates the large distance between peaks standard ions are discarded and the resultant mass-intensity digital data axe collected on magnetic tape. The final stage in the production of an element map is nothing more than a digital computer problem. Each mass must be correlated with a unique elemental formula. So-called ELCOMP programmes have been written which accomplish this in a variety of ways.The simplest and probably most common of these computes the mass of every possible combination of C H 0 and N and any other elements that are requested. If possible upper and lower limits are placed on the number of atoms of each element present. If these are not known rough limits can be estimated from the molecular ion. Each computed mass is compared to the observed mass and if the difference between them exceeds a preset error typically 1-3 mmu the formula is rejected and the next one is calculated. A self- consistency check may be applied to settle difficult choices such as that between say C,,H3,05N+ and C,,H3gO,+ which may be tentatively resolved in favour of the latter if N is absent in all other ions. Various other programmes have been written to cope with this problem48 and some very ingenious solutions have been proposed.One method49 is to store all the calculated masses in the computer’s memory or ‘core’ and simply search for the correct one. A slightly different and quite promising approach50 requires the storage of C 0 N combinations in core and the combination of each of these with the appropriate number of hydrogens to arrive at the correct answer. Calculation of the formula from the mass defect of the ion i.e. the difference between the observed mass and the nearest lower integer has been attempted5I and an ingenious method based upon a mass scale where CH = 14~000000 has been reported.52 The programme of 4.3 D. D. Tunnicliff P. A. Wadsworth and D. 0. Schissler Analyt. Chem. 1965,37 543 and references cited therein.49 A. L. Burlingame private communication 1966. 61 J. Van Katwijk Appl. Spectroscopy 1964 18 102. sa E. Kendrick Analyt. Chem. 1963,35,2146. E. Gilbert unpublished work. 85 The Application of H&h Resolution Mass Spectroscopy to Organic Chemistry choice depends upon the size and speed of the computer available the goal being to deal with each mass as rapidly as possible certainly before the next mass on the tape arrives. This is fairly simple as the tape can be slowed or even halted but in the on-line work to be discussed where the data are processed as they are produced this refuge is absent and the fastest available systems prove to be barely fast enough. As has already been mentioned assignment is facilitated considerably by the knowledge of the elements present in the molecule.One way to secure this in- formation is by a careful study of the ions below m/e 100. It appears to be a fairly accurate assumption53 that all elements of atomic weight less than 100 will appear in some ion below m/e 100 and since the accuracy of mass measurement required for assignment of formulae to these low masses is considerably relaxed this promises to provide a simple technique for qualitative analysis. Armed with these data the computer will be far more able to make the difficult choices that arise at higher masses. The search for a satisfactory internal standard is being continued by many groups in two divergent directions. Improvement of the computational methods should in principle lead to a system in which perhaps ten reference ions each 100 mass units apart would be sufficient for mass measurement.Alternatively with the rather inefficient software (programmes) presently available accuracy could be vastly improved if standard ions were present at every mass unit as would be the case if a straight chain hydrocarbon were to be used as a reference compound. This could however lead to the obscuring of hydrocarbon peaks from the sample under investigation and whilst deuterocarbons have been considered they are relatively inaccessible and in any case fail to solve the problem because of the small separation of the 1H2-2H doublet. A major advance that has appeared as fallout from the splutter of program- ming has been the appearance of bar-graph plotting routines. The input data for these are usually the mass against abundance figures and the output is a neat bar-graph suitable for filing or publication.This is generally done off-line but has been done on-line54 in which case the scan speed of the spectrometer is badly reduced to the slow speed of the plotter. Venkataraghavan McLafferty and Amy55 have reported some computer calculations based upon photoplate data in which overlapping bands are mathematically resolved. Such a deconvolution is successful in increasing the effective resolution by probably more than 300 % a result which should promote great activity in this direction. When the computation techniques are rapid enough to deal with a whole spectrum of perhaps several hundred ions in less than a minute the real pos- sibility exists that tape recording may be abandoned and the analogue signal from the mass spectrometer fed directly into the computer.Such ‘on-line’ tech- niques have been developed to handle the data from a magnetic scan or from a 63 E. Gilbert H. M. Fales and G. W. A. Milne unpublished work. 64 R. N. Stillwell Analyt. Chem. 1966 38 940. 55 R. Venkataraghavan F. W. McLafferty and J. W. Amy Analyt. Chem. 1967 39 178. 86 Milne photoplate reader although rather more effort appears to have been devoted to the former.5s Quite apart from being an impressive tour de force such a system possesses some real advantages. The problem of tape flutter is eliminated; a great deal of valuable computer time is saved; the whole system becomes integral and the interesting vista of feed-back control is opened up. This is clearly one area where the incredible gap between programming ignorance and computer capa- bilities is being closed a little.Having arrived at a self-consistent listing of ions in terms of their elemental composition and abundance the immediate problem is how best to present this. Biemann's element map45 was the first attempt to solve this problem and in some senses has not subsequently been improved upon. A good feature of this element map as may be seen from Figure 3 is that all the information is presented concisely yet fairly lucidly. The main criticism that has been levelled at the Biemann element map is that being digital in nature it is not communicative as is a graphic display such as a bar graph. In an attempt to cope with this difficulty Venkataraghavan and McLafferty5' have devised a three-dimensional display as shown in Figure 4 and met precisely the opposite problem.All the abundances may be seen at a glance but the elemental compositions cannot. Yet another presentation has been devised58 in which a complete bar graph is drawn for every combination of heteroatoms as is shown in Figure 5. This is in some ways the clearest presentation of the three; abundances and compositions are fairly easily arrived at but the transfer of one's attention from one graph to another as a heteroatom is lost in what is after all a common enough type of fragmentation is disconcerting no less so than is the thought of such a representation for a compound containing six nitrogen atoms and six oxygen atoms. The conclusion suggested by a study of all these Daedalian efforts is that the information has not perhaps at this stage been processed sufficiently for human digestion and that the computer must be allowed to take the problem further.How this can be done is discussed in the next section. 7 Computer Assisted Interpretation of Spectra The computer handling of element maps is a branch of mass spectrometry whose age is better measured in months than in years. With the exception of one spectacular and heartening success the effort so far invested in this problem has failed to yield any substantial dividends and the purpose of this section will be to discuss approaches and methods. Much of this material is unpublished and as is the case with all unpublished work referred to in this Review the reader is referred to future issues of Analytical Chemistry where the majority of the papers are expected to appear.Even a casual glance at an element map will usually yield some structural 66 W. J. McMurray S. R. Lipsky and B. N. Green; C. Merritt P. Issenberg and M. L. Bazinet Proceedings of the ASTM Committee E-14 Denver 1967; A. L. Burlingame D. H. Smith and R. W. Olsen Anulyt. Chem. 1968 40 13. 68 A. L. Burlingame and D. H. Smith private communication. R. Venkataraghavan and F. W. McLafferty Anulyt. Chem. 1967,39,278. 87 The Application of H&h Reso Iurion Mass Spectroscopy to Organic Chemistry DEOXY DlHY DRO-Nb-METHY LAJMALINE CH CHO CHN CHNO CHN2 CHN2O 94 6/8 O**** 98 5/8 1 * * * * * * 95 7/11 0 * * 6/10-0 * * * 103 8/7-O*** 106 71'8 0 * * 107 7/9-0 * 108 7/10-0 * * 110 115 9/7-O*** 7/12-0 * * * * 117 817 O** 118 8/8 0" 120 8/10 1 * * 122 8/12 2 * 123 8/13 2 * 124 7 / 1 0 0 * * * 126 7/12-0* * 127 10/7-0** 129 9 n 1 * 130 3 / 8 O * * * 131 9 / g 4 * * * * 132 9/10-1 * * * 142 10/8-0 * 143 10/9-0 * * 144 145 152 154 1 0/20-0 * * 156 11 /lo-0 * 157 11/11-0*** 158 11/12 O*** 10/8-0** 159 101'9-0 * 160 1 * * o/lo-0" 167 12/9-0 * 168 10/18 0 * * 170 12/12 0 * * 173 11/11 o * * 181 182 13/12.0**** 11/20 O****** 10/10-0 * * * * 1 OA 1.0* * * 1 O / w O * * * * 13A 1-0 * * * 12/11 2 * * 183 197 13/13 O * * * * 21 3 14/l7-0*** 269 17/21 1 * Fig.3 Element map devised by Biemann et al. The first column shows the nominal mass; the second all the ions containing only C and H; the third those containing C H and one 0 etc. The entry for each ion represents the number of C and H the deviation in mmu from the theore- tical mass and the relative intensity on a logarithmic scale represented by 1-9 asterisks.(Reproduced by permission from Pure Appl. Chem. 1964 9 104.) 326 21/30-o* information. For example it may perhaps be seen at once that the molecular ion loses CH and COOH and these groups may therefore be assumed to be in the molecule. But this type of information could be extracted albeit with less certainty from a low resolution spectrum and to treat an element map in this way is to ignore most of the information it contains. A serious consideration of every ion in the element map might on the other hand be expected to reveal 88 T,z I Methyl Propionate. Fig. 4 Element map devised by Venkataraghavan and Mchfferty. Each major division in the X-axis represents the number of carbon atoms and each minor division a whole number of rings and double bonds.The Z-axis represents the heteroatom content and the Y-axis the abundance. (Reproduced by permission of F. W. McLaflerty.) Fig. 5 Element map devised by Burlingame and Smith. Each plot gives only ions with a specific heteroatom content. Each major division on the X-axis represents a CHg unit and is divided into 14 sub-divisions each corresponding to one hydrogen or one mass unit. (Reproduced by permission from J. h e r . Chem. SOC. 1967 89 3232.) 89 The Application of Hkh Resolution Mass Spectroscopy to Organic Chemistry Y-NHCHRl- -CO- -NH.CHR,- -CO- +BIB I +BB,-- 1 -A,-+ t A + - - - -NHCHRn- -CO- -OCH t B n + -An-+ 50 K. Heyns and H. F. Grutzmacher Tetrahedron Letters 1963 1761 ; Ann. Chem. 1963,669 189; N. S. Wulfson V. A. Puchkov B V. Rozinov A.M. Zyakoon M. M. Shemyakin Yu. A. Ovchinnikov A. A. Kiryushkin and V. T. Ivanov Tetrahedron Letters 1965 2793. 6o M. Barber P. Jolles E. Vilkas and E. Lederer Biochem. Biophys. Res. Cornm. 1965 18 469. 61 M. Senn R. Venkataraghavan and F. W. McLafferty J . Amer. Chem. SOC. 1966,88,5593. 62 K. Biemann C. Cone B. R. Webster and G. P. Arsenault J . Amer. Chem. SOC. 1966,88 5598. 90 Milne the whole process is repeated until the molecular ion is found. This approach which with minor variations is employed by both groups has a number of corollaries. Side-chain fragmentation independent of the main A-B cleavages can create dilemmas. For example the rearrangement of and loss of C3H6 from valine would result in its being indistinguishable from glycine but when this happens it seems never to be an exclusive process and the usual result is that both glycine and valine are indicated as the next amino acid in which case the choice of the latter is elementary.The peptide must be derivatised at both the N-terminal and C-terminal ends in order to be sufficiently volatile to give a mass spectrum. The C-terminal derivative is usually the methyl ester whilst the best of the N-terminal derivatives (Y) that have been studied62 are acetyl trideuterio- acetyl trifluoroacetyl and carbobenzoxy. No peptides of more than ten amino acids have been investigated in this way and the results suggest that the upper limit of molecular weight must be in this range the limitation being volatility. These methods are of course absolutely specific to peptides which in terms of fragmentation at least are remarkably uncomplicated considering their mole- cular weight.The general problem of devising the software necessary to handle any molecule or even any class of molecule other than peptides has not been solved but work has been begun in this area. One approach63 has been to cause a computer to go automatically through the interpretive operations that an experienced spectroscopist would apply to a high resolution spectrum such as ring and double bond calculations identification of heteroatoms in terms of functional groups and searching for homologous series of ions. Another very interesting device64 is that of the computer dialogue technique in which the spectroscopist can communicate with the computer via a keyboard programme it with a set of fragmentation rules for the type of compound in question and then make the computer apply these rules and so use the spectrum to arrive at the structure of the compound in question.A ‘pathfinder’ type of programme has been writtens5 in which the comguter considers each ion in turn and searches back up the element map listing until it finds the ‘first possible parent ion’ which must contain at least as great a number of atoms of each element as the daughter ion from which it must differ by a ‘permissible’ fragment such as H but not a ‘non-permissible’ fragment such as H2 or H,. Thus C10H220+ could constitute a parent for C1,H2f but not for C,OH2402+ or for CloH1,O+. In this way an at least partially valid fragmentation map can be assembled and a great deal of structural information is sometimes uncovered.Programmes have been writtens6 which enable a computer to determine the structures of aliphatic hydrocarbons and fatty acids from their mass spectra by 83 R. Venkataraghavan and F. W. McLafferty Proceedings of the ASTM Committee E-14 Denver 1967. G4 A. Mandelbaum P. V. Fennessey and K. Biemann Proceedings of the ASTM Committee E-14 Denver 1967; K. Biemann and P. V. Fennessey Chimia (Swim) 1967 21 226. 65 H. M. Fales E. Gilbert P. G. Gordon R. J. Highet and G . W. A. Milne unpublished work. 6G B. Pettersson and R. Ryhage Arlciv Kemi 1967 26 293; Analyt. Chem. 1967 39 790. 91 The Application of High Resolution Mass Spectroscopy to Orgunic Chemistry the application of simple empirical rules of fragmentation. They appear to give incorrect answers very rarely but give no answer at all in about 20 % of the cases studied.Some very significant work has recently been reported by Lederberg6’ from the Stanford Artificial Intelligence Project. This group has devised a programme which generates all the possible structural isomers corresponding to a molecular formula. It can furthermore avoid generating those isomers which contain chemical absurdities such as -NH-O-O-H and can arrange the remainder in order of plausibility. Thus given a high resolution mass spectrum in which the molecular ion is identified this list is constructed and ‘heuristically’ searched (i.e. the search is not comprehensive as the time required for this would be prohibitive but is conducted in intelligently selected areas) for a structure con- sistent with the mass spectrum. This so-called DENDRAL 64 programme is written in LISP a computer language far better suited to the representation and manipulation of chemical structures than is the more commonly used FORTRAN and the success attending this departure from convention is educational.The programmes so far completed cannot handle cyclic structures but this problem is said to be now conceptually solved. A very desirable feature of any data processing system would be access to information that would permit the definitive conclusion that A+ - B++ C does occur with the molecule in question. Metastable transitions do provide this information and their automatic recording and interpretation is possible,ss but they are observable only at low resolving power thus a second spectrum is required. Moreover the appearance of a metastable ion is a matter of chance depending as it does upon a number of variables such as the quite unpredictable rates of fragmentation reactions.Mainly for this last reason metastable ions constitute only ancillary evidence and an interpretive system built around them would be severely defective. Of far greater promise is the technique developed by Jenningse9 who has taken advantage of the fact that if the process MI+- M2+ + M3 occurs in the field-free region between the two sectors of the MS 902 the daughter ion will have been accelerated as MI+ but will be magnetically deflected as Mz+. With a simple change in the circuitry of the instrument it is possible to study a daughter ion M2+ and identify its parent unequivocally. An interesting out- come of these studies has been the discovery that a daughter ion usually has several parents.This method has been applied recently to a problem in peptide sequencing70 and the technique is now semi-automatic. 67 J. Lederberg NASA Doc. CR-57029 1964; CR-68898 1965; CR-68899 1966. Proc. Nut. Acud. Sci. U.S.A. 1965 53 134. See also Stanford Artificial Intelligence Project Memos 49 and 54. 68 R. E. Rhodes M. Barber and R. L. Anderson Analyt. Chern. 1966,38,48; N. R. Mancuso S. Tsunakawa and K. Biemann Ibid. 1966 38 1775. 69 K. R. Jennings Chem. Comm. 1966 283. See also M. Barber K. R. Jennings and R. E. Rhodes 2. Naturforsch. 1967,22a 15. 70 M. Barber W. A. Wolstenholme and K. R. Jennings Nature 1967 214 664. 92 Milne 8 summary High resolution mass spectrometry is now second only to nuclear magnetic resonance spectroscopy in the organic chemist’s armamentarium and it seems fairly clear its expense notwithstanding that it is only a matter of time before it assumes first place.Its overwhelming advantage over other techniques is its prodigious sensitivity rivalled only by that of the scintillation counter. The amount of information per microgram of sample provided by the mass spectro- meter permits entire research projects to be carried out at the sub-milligram level and the impact of such a technique in areas of applied organic chemistry such as biochemistry is large and obvious. As a field high resolution mass spectrometry embraces many disciplines ranging from mathematics to biochemistry and perhaps because of this is an area in which iconoclasm is often attended by success as has been amply demon- strated in the recent past. The resultant absence of dogma is invigorating and invests the whole field with tremendous promise for the future. I thank Dr J. Daly Dr H. M. Fales Dr P. G. Gordon and Dr R. J. Highet for many helpful discussions and suggestions. 93

ISSN:0009-2681    

DOI:10.1039/QR9682200075

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

INDEXES Volume 20 1966 Contains the cumulative indexes for the volumes 1 to 20 The indexes in this issue cover volumes 21 & 22 INDEXES Volume 20 1966 Contains the cumulative indexes for the volumes 1 to 20 The indexes in this issue cover volumes 21 & 22 Index INDEX OF AUTHORS Abraham E. P. 21 231 Anbar M. 22 579 Ashby E. C. 21,259 Barefield E. K. 22 457 Boyd D. R. 22,95 Bransden B. H. 21,474 Brocklehurst B. 22 147 Bruce J. M. 21,405 Buckingham A. D. 21 Buncel E. 22 123 Burgess J. 22 276 Busch D. H. 22,457 Cadogan J. I. G. 22 Chambers D. B. 22,317 Cherry R. J. 22 160 Clive D. 22 435 Cookson R. C. 22,423 Cox R. A. 22,499 Dale B. W. 22 527 Davidson R. S. 21 249 Dickens P. G. 21 30 Evans U. R. 21,29 Fensham P. J. 21 507 Gill G. B. 22 338 195 222 Glockling F. 22 317 van Gorkom M.22,14 Hall G. E. 22 14 Howe A. T. 21,507 Hughes M. N. 22 1 Jefferson A. 22 391 Jones J. H. 22 302 Kerr J. A, 22 549 Lambert J. D. 21 67 Lee J. B. 21,429 Light J. R. C. 22 317 Lloyd A. C. 22,549 Luckhurst G. R. 22 Luz Z. 21,458 McKervey M. A. 22,95 McLennan D. J. 21 Milne G. W. A. 22,75 Muetterties E. L. 21 Nelson S. M. 22 457 Norris A. R. 22 123 Orr B. J. 21 195 Parker W. 21 331 Pelletier S. W. 21 525 179 490 109 INDEX OF TITLES Alkaloids the chemistry of the C2,,- diterpene 21,525 Amino-acid and peptide derivatives mass spectra of 22 302 Aromatic nitro-compounds inter- action of with bases 22 123 Arthropod defensive substances the chemistry of 21 287 Biogenesis sesquiterpene 21 33 1 Biological pigments semiconduction and photoconduction of 22 160 Carbanion mechanism of olefin- forming elimination 21,490 Cephalosporin C Group 21,231 Chemistry of arthropod defensive substance 21,287 Claisen rearrangement molecular re- arrangements related to the 22 391 Conformational analysis heterocyclic 21 364 Co-ordination number molecular polyhedra of high 21,109 Asymmetric synthesis 22,95 Penzer G.R. 21 43 Pope M. T. 22,527 Ramage R. 21 331 Radda G. K. 21,43 Riddell F. G. 21,364 Roberts J. S. 21 331 Robinson D. L. 21,314 Ruff I. 22 199 Russell K. E. 22 123 Salmond W. G. 22,253 Scheinmann F. 22 391 Silver B. L. 21,458 Sklarz B. 21 3 Symons M. C. R. 22 Theobald D. W. 21 Thompson C. 22,45 Uff B. C. 21 429 Waley S. G. 21 379 Walker D. C. 21 79 Weatherston J. 21 287 Wehry E. L. 21 213 Wittingham M. S. 22 Wright C. M. 21 109 276 314 30 Crystals liquid as solvents in nuclear magnetic resonance 22 179 Decomposition reactions of radicals 22,549 Diterpene alkaloids the chemistry of the C20 21 525 Electron the hydrated 21,79 Electrons react ions of hydrated with inorganic compounds 22,579 Electron - t ransfer theory of thermal reactions in solution 22 199 Electron spin resonance chemical applications of oxygen-17 nuclear and 21,458 Electron spin resonance of the triplet state 22,45 Electronic properties of binary com- pounds of the first-row transition metals 21 507 Electrophilic oxygen organic reactions involving 21,429 Elementary particles 21,474 601 Index Enzyme action mechanism of 21,379 Energy transfer vibration-vibration in gaseous collisions 21 67 Free radicals hydrogen abstraction in the liquid phase by 21 249 Grignard reagents.Compositions and mechanisms of reaction 21 259 Heterocyclic conformational analysis 21 364 Hydrated electrons reactions of with Hydrogen abstraction in the liquid 21 249 Inert gases the reactions of ions and excited atoms of the 22 147 Inorganic compounds reactions of hydrated electrons with 22 579 Ions and excited atoms of the inert gases the reactions of 22 147 Ion-solvent and ion-ion interactions by magnetic resonance techniques study of 22,276 Iron cobalt and nickel complexes having anomalous magnetic mo- ments 22,457 Isoalloxazines (Flavins) the chemistry and biological function of 21 43 Isopoly-vanadates -niobates and -tantalates 22 527 Light-induced reactions of quinones 21,405 Liquid phase hydrogen abstraction in the by free radicals 21,249 Macromolecular structure and pro- perties of ribonucleic acids 22,499 Magnetic moments iron cobalt and nickel complexes having 22 457 Magnetic resonance techniques study of ion solvent and ion-ion inter- actions by 22,276 Mass spectra of amino-acid and pep t ide derivatives 22 302 Mass spectra of organometallic corn- pounds 22 317 Mass spectroscopy application of Molecular hyperpolarisabilities 21 195 Molecular rearrangements related to the Claisen rearrangement 22 391 inorganic compounds 22,579 Hyponitrites 22 1 phase by free radicals high resolution 22,75 Nuclear and electronic spin resonance chemical applications of oxygen-17 21,458 Nuclear magnetic resonance equi- valence of nuclei in high resolution 22 14 Nuclear magnetic resonance liquid Niobates isopoly-vanadates and -t antalates 22 527 Olefin-forming elimination carbanion mechanism of 21,493 Organic chemistry of periodates 21 3 Organic reactions involving electro- philic oxygen 21,429 Organo-metallic compounds mass spectra of 22 317 Oxygen organic reactions involving elec tr op h ilic 21,429 Oxygen- 17 nuclear and electron spin resonance chemical applications of 21,458 Periodates organic chemistry of 21 3 Phosphorus reagents reduction of nitro- and nitroso-compounds by tervalen t 22,222 Photochemical behaviour of transition- metal complexes 21,213 Photochemistry of some allylic com- pounds 22,423 Quinones light-induced reactions of 21,213 Radicals decomposition reactions of 22,549 Reduction of nitro- and nitroso- compounds by tervalent phosphorus reagents 22,222 RNA macromolecular structure and properties of 22,499 Rusting the mechanism of 21 29 Solvents liquid crystals as in nuclear magnetic resonance 22 179 Sulphur .heterocycles valence shell expansion in 22,253 Tantalates isopoly-vanadates -nio- tates and 22 527 Tetracyclines chemistry of 22 435 Theory of thermal electron-transfer crystals as solvents in 22,179 Synthesis asymmetric 22,95 reactions in solution 22 199 602 Index TILDEN LECTURE.Photochemistry of some allylic compounds 22 423 Transition-metal complexes photo- chemical behaviour of 21 213 Transition metals electronic pro- perties of binary compounds of the first-row 21,507 Tungsten bronzes and related com- pounds 22 30 Vibration-vibration energy transfer in gaseous collisions 21 67 Valence-shell expansion in sulphur heterocycles 22,253 Vanadates iso-poly- -niobates and -t antalates 22 527 Woodward-Hoffmann orbital sym- metry rules to concerted organic reactions application of 22 338 603

ISSN:0009-2681    

DOI:10.1039/QR9682200599

出版商:RSC    

年代:1968

数据来源: RSC