摘要:

Contents 3663 3669 3675 3683 3693 370 1 3709 3717 3725 3737 Normal and Abnormal Electron Spin Resonance Spectra of Low-spin Cobalt(r1) IN,]-Macrocyclic Complexes. A Means of Breaking the Co-C Bond in B12 Co-enzyme M. Green, J. Daniels and L. M. Engelhardt The Interaction between Superoxide Dismutase and Doxorubicin. An Electron Spin Resonance Approach V. Malatesta, F. Morazzoni, L. Pellicciari-Bollini and R. Scotti Biomolecular Dynamics and Electron Spin Resonance Spectra of Copper Complexes of Antitumour Agents in Solution. Part 2.-Rifamycins R. Basosi, R. Pogni, E. Tiezzi, W. E. Antholine and L. C. Moscinsky An Electron Spin Resonance Investigation of the Nature of the Complexes formed between Copper(I1) and Glycylhistidine D. B. McPhail and B. A. Goodman A Vibronic Coupling Approach for the Interpretation of the g-Value Temperature Dependence in Type-I Copper Proteins M.Bacci and S. Cannistr aro The Electron Spin Resonance Spectrum of Al[C,H,] in Hydrocarbon Matrices J. A. Howard, B. Mile, J. S. Tse and H. Morris N; and (CN); Spin-Lattice Relaxation in KCN Crystals H. J. Kalinowski and L. C. Scavarda do Carmo Single-crystal Proton ENDOR of the SO, Centre in y-Irradiated Sulphamic Acid N. M. Atherton, C. Oliva, E. J. Oliver and D. M. Wylie Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice 1,. Part 1.-The 0- Radicals Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice I,. Part 2.-The HO, Radicals J. Bednarek and A. Plonka J. Bednarek and A. PlonkaContents 3663 3669 3675 3683 3693 370 1 3709 3717 3725 3737 Normal and Abnormal Electron Spin Resonance Spectra of Low-spin Cobalt(r1) IN,]-Macrocyclic Complexes.A Means of Breaking the Co-C Bond in B12 Co-enzyme M. Green, J. Daniels and L. M. Engelhardt The Interaction between Superoxide Dismutase and Doxorubicin. An Electron Spin Resonance Approach V. Malatesta, F. Morazzoni, L. Pellicciari-Bollini and R. Scotti Biomolecular Dynamics and Electron Spin Resonance Spectra of Copper Complexes of Antitumour Agents in Solution. Part 2.-Rifamycins R. Basosi, R. Pogni, E. Tiezzi, W. E. Antholine and L. C. Moscinsky An Electron Spin Resonance Investigation of the Nature of the Complexes formed between Copper(I1) and Glycylhistidine D. B. McPhail and B. A. Goodman A Vibronic Coupling Approach for the Interpretation of the g-Value Temperature Dependence in Type-I Copper Proteins M. Bacci and S. Cannistr aro The Electron Spin Resonance Spectrum of Al[C,H,] in Hydrocarbon Matrices J. A. Howard, B. Mile, J. S. Tse and H. Morris N; and (CN); Spin-Lattice Relaxation in KCN Crystals H. J. Kalinowski and L. C. Scavarda do Carmo Single-crystal Proton ENDOR of the SO, Centre in y-Irradiated Sulphamic Acid N. M. Atherton, C. Oliva, E. J. Oliver and D. M. Wylie Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice 1,. Part 1.-The 0- Radicals Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice I,. Part 2.-The HO, Radicals J. Bednarek and A. Plonka J. Bednarek and A. Plonka

ISSN:0300-9599    

DOI:10.1039/F198783FX001

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

Electrochemistry Group Workshop on Electrochemical Techniques and Instruments To be held at the University of Warwick on 6-7 January 1988 Further information from Dr P. N. Bartlett, Department of Chemistry, University of Warwick, Coventry CV4 7AL Surface Reactivity and Catalysis Group with the Process Technology Group and the Institute of Chemical Engineers Opportunities for Innovation in the Application of Catalysis To be held at Queen Mary College, London on 6-7 January 1988 Further information from Professor J. Pritchard, Queen Mary College, London Division with the Institute of Mathematics and its Applications Mathematical Modelling of Semiconductor Devices and Processes To be held at the University of Loughborough on 7-8 January 1988 Further information from the Institute of Mathematics, Maitland House, Warrior Square, Southend-on-Sea SS1 2JY Division London Symposium: Modern Electrochemical Systems To be held at Imperial College, London on 12 January 1988 Further information from Mrs Y.A. Fish, Royal Society of Chemistry, Burlington House, London W1V OBN Polymer Physics Group with the 3Ps Group Plastics, Packaging and Printing To be held at the Institute of Physics, 47 Belgrave Square, London on 18 February 1988 Further information from Dr M. Richardson, National Physical Laboratory, Teddington, Middlesex l w 1 1 OLW Theoretical Chemistry Group Postgraduate Students’ Meeting To be held at University College, London on 2 March 1988 Further information from Dr G. Doggett, Department of Chemistry, University of York, York Colloid and Interface Science Group with The Society of Chemical Industry and British Radio frequency Spectroscopy Group Spectroscopy in Colloid Science To be held at the University of Bristol on 5-7 April 1988 Further information from Dr R. Buscall, ICI Corporate Colloid Science Group, PO Box 11, The Heath, Runcorn WA7 40E Annual Congress: Division with Electrochemistry Group Solid State Materials in Electrochemistry To be held at the University of Kent, Canterbury on 12-15 April 1988 Further information from Dr J.F. Gibson, Royal Society of Chemistry, Burlington House, London W1V OBN Electrochemistry Group with The Society of Chemical Industry Electrolytic Bubbles To be held at Imperial College, London on 31 May 1988 Further information from Professor W. J. Albery, Department of Chemistry, Imperial College of Science and Technology, South Kensington, London SW7 2AY Electrochemistry Group with The Society of Chemical Industry Chlorine Symposium To be held at the Tara Hotel, London on 1-3 June 1988 Further information from Dr S.P. Tyfield, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GLI 3 9BP Gas Kinetics Group Xth International Symposium on Gas Kinetics To be held at University College, Swansea on 24-29 July 1988 Further information from Dr G. Hancock, Physical Chemistry Laboratory, South Parks Road, Oxford OX1 302 (xiii)Electrochemistry Group Workshop on Electrochemical Techniques and Instruments To be held at the University of Warwick on 6-7 January 1988 Further information from Dr P.N. Bartlett, Department of Chemistry, University of Warwick, Coventry CV4 7AL Surface Reactivity and Catalysis Group with the Process Technology Group and the Institute of Chemical Engineers Opportunities for Innovation in the Application of Catalysis To be held at Queen Mary College, London on 6-7 January 1988 Further information from Professor J. Pritchard, Queen Mary College, London Division with the Institute of Mathematics and its Applications Mathematical Modelling of Semiconductor Devices and Processes To be held at the University of Loughborough on 7-8 January 1988 Further information from the Institute of Mathematics, Maitland House, Warrior Square, Southend-on-Sea SS1 2JY Division London Symposium: Modern Electrochemical Systems To be held at Imperial College, London on 12 January 1988 Further information from Mrs Y.A. Fish, Royal Society of Chemistry, Burlington House, London W1V OBN Polymer Physics Group with the 3Ps Group Plastics, Packaging and Printing To be held at the Institute of Physics, 47 Belgrave Square, London on 18 February 1988 Further information from Dr M. Richardson, National Physical Laboratory, Teddington, Middlesex l w 1 1 OLW Theoretical Chemistry Group Postgraduate Students’ Meeting To be held at University College, London on 2 March 1988 Further information from Dr G. Doggett, Department of Chemistry, University of York, York Colloid and Interface Science Group with The Society of Chemical Industry and British Radio frequency Spectroscopy Group Spectroscopy in Colloid Science To be held at the University of Bristol on 5-7 April 1988 Further information from Dr R.Buscall, ICI Corporate Colloid Science Group, PO Box 11, The Heath, Runcorn WA7 40E Annual Congress: Division with Electrochemistry Group Solid State Materials in Electrochemistry To be held at the University of Kent, Canterbury on 12-15 April 1988 Further information from Dr J. F. Gibson, Royal Society of Chemistry, Burlington House, London W1V OBN Electrochemistry Group with The Society of Chemical Industry Electrolytic Bubbles To be held at Imperial College, London on 31 May 1988 Further information from Professor W. J. Albery, Department of Chemistry, Imperial College of Science and Technology, South Kensington, London SW7 2AY Electrochemistry Group with The Society of Chemical Industry Chlorine Symposium To be held at the Tara Hotel, London on 1-3 June 1988 Further information from Dr S. P. Tyfield, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GLI 3 9BP Gas Kinetics Group Xth International Symposium on Gas Kinetics To be held at University College, Swansea on 24-29 July 1988 Further information from Dr G. Hancock, Physical Chemistry Laboratory, South Parks Road, Oxford OX1 302 (xiii)

ISSN:0300-9599    

DOI:10.1039/F198783BX003

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1987, 83, 13-27 hv Magnetic Resonance of Ultrafast Chemical Reactions L \ < l o p s B2'P-(QFe) ca. 10ns \ ca. 200 ps 3*BzP(QFe) B2'P(QFe)- cu. 1 ms J Examples from Photosynthesis J. R. Norris, C. P. Lin and D. E. Budil Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A . and Department of Chemistry, University of Chicago, Chicago, Illinois 60637, U.S.A. Using e.s.e. (electron spin-echo), RYDMR (reaction-yield-detected mag- netic resonance) and e.s.r. (electron spin resonance) in protein single crystals, we have investigated the cation, the triplet and the initial radical pair associated with the process of photoinduced charge separation in photo- synthesis. The initial charge separation of bacterial photosynthesis occurs in a few picoseconds within in a reaction-centre protein containing eight electron-transfer components, namely four bacterial chlorophylls, two bac- teriopheophytins and two quinones.Two bacteriochlorophylls, BChl,, form the primary donor and one bacteriopheophytin serves as the primary acceptor. E.s.e. determination of the anisotropic 15N hyperfine interactions have shown that the primary donor cation resides symmetrically in two of the four BChls in the form of a special pair of bacterial chlorophylls, BChlz. In direct contrast, our e.s.r. studies on the triplet state of the primary donor in single crystals of reaction centres suggest that the triplet state resides asymmetrically in BChl,, where the triplet is highly asymmetrical in R. viridis but is much more symmetrical (C,) in R.sphaeroides. RYDMR studies of the initial radical pair formed indicate that the extra bacteriochlorophyll molecule, BChl,, that is between the special pair donor cation and the primary acceptor bacteriopheophytin anion is not involved in a discrete electron-transfer step in bacterial reaction centres. By combining the results of our magnetic-resonance experiments with X-ray structural information, the following description emerges: (1) the ground-state, primary donor is a supermolecule dimer with approximately C, symmetry; (2) the bridging Bchl, molecule probably functions as a superexchange site for rapid transfer of electrons from the primary donor to the primary acceptor but with negligible back reaction; (3) the special pair, BChl,, is lower in energy than the bridging molecule, BChl,, such that the initial radical pair is formed via super-exchange between the distant (10 A edge-to-edge) special pair BChl, and the bacteriopheophytin.14 Magnetic Resonance of Ultrafast Chemical Reactions The initial act of photosynthesis is the formation of a radical pair BiP-.The purpose of this paper is to characterize this initial radical pair using magnetic techniques. The primary donor (B,) is composed of a pair of bacteriochlorophylls.l Since B, is part of the radical pair, the nature of B, is involved in the present investigation. Whether B, acts as a monomer or a dimer is important to any characterization of the initial radical pair. The initial acceptor (P), a bacteriopheophytin,2 is an accepted monomer anion in the initial radical-pair state.QFe is a quinone-iron complex3 that serves as a secondary electron acceptor. In addition, a bacteriochlorophyll (not shown here) may be a bridging molecule between B, and P.4 Such a bridging intermediate may be pertinent to the radical-pair state. The electron transfer from *B, to P to form the initial radical pair takes place in < 6 P S . ~ The secondary electron acceptors permit additional electron transfer to proceed from P- to Q in ca. 200 P S . ~ ~ If Q is removed or previously saturated by electrons 'in the dark' then charge annihilation and recombination occurs on a 10 ns timescale5 to form a triplet 3*B2. Otherwise the fast forward reaction to form Q- proceeds. As a consequence of the relatively slow back reactions the efficiency of the primary reaction in the photo-oxidation of BZ exceeds 98% .8 y An important feature of this radical-pair reaction, which distinguishes it from ordinary in vitro liquid solution reactions, is that it takes place embedded in a protein matrix, essentially in the solid state. Consequently, the participant molecules are maintained in a fixed spatial relationship with continuous electronic interactions among them. The various states shown in the above scheme are often described in terms of discrete loci of electronic charge or excitation energy, by analogy with conventional chemical reactions. The constant proximity of the reactants and strong interactions among them, however, suggest that such a treatment is at best incomplete.A refined description of the primary processes of photosynthesis should include the delocalization of states such as lB?, 3B2*, Bl, and even the radical pair BZP-, over the many components of the reaction centre. Time-resolved magnetic resonance techniques such as electron spin-echo (e.s.e.) and reaction-yield-detected magnetic resonance (RYDMR) provide powerful probes into the nature of the various states of bacterial photosynthesis. In turn, electron spin resonance (e.s.r.) of single crystals of reaction centres provide a means of correlating the properties of these states with the molecular structure of the reaction-centre chromo- phores which has recently been established by X-ray crystallography. lo In fact, much of our current understanding of the primary events of photosynthesis is derived from time-resolved optical spectroscopy and magnetic resonance spectroscopy of reaction centres in solution.Although such techniques afford a detailed description of kinetic events in photosynthesis, only limited structural information is available without the use of single crystals of reaction centres. On the other hand, X-ray diffraction studies on single crystals provide a detailed structural description of the core electrons of the reaction-centre componentslO but no kinetic information. The availability of single crystals of reaction centres for magnetic-resonance studies11-13 provides a powerful and highly accurate means of assigning observed 'working' valence states of the reaction centre unambiguously to its structural features.Although the ground-state special pair model for the primary donor of bacterial photosynthesis has now been confirmed by the X-ray structural data,lo additional work is necessary to establish whether the working states are dimeric in nature. The initial derivation of the special pair model was based on a symmetrical dimer of chlorophyll or bacteriochlorophyll.' The structure of the special pair as presently revealed from the X-ray study is a symmetrical dimer with approximate C, symmetry.lO In this paper we discuss e.s.e. modulation data on 15N-enriched cation of the special-pair donor which confirms the symmetrical nature of the primary-donor cation. In addition to, and in contrast with these data, we also present e.s.r. evidence that the triplet state of the special pair and/or its immediate protein environment deviates substantially and significantly from C, symmetry, and thus has monomer-like properties.J.R . Norris, C. P. Lin and D. E. Budil 15 Finally, we report studies designed to measure the singlet-triplet energy gap, ZJ, in the initial radical pair. This experimental evidence depends on measuring the relative triplet yield (or decay rate of the radical pair) as a function of magnetic-field strength and temperature. In one series of experiments the magnetic field is a static field of strength zero to several hundred gauss. In the other case the magnetic field is a microwave-induced magnetic field in the rotating frame ranging from zero to a few tens of gauss as in a RYDMR experiment.The direct measurement of J places severe limits on the type of intermediates, if any, that might precede the initial radical pair. Experiment a1 Reaction centres of R . sphaeroides (in some cases depleted of quinone) and R. viridis were prepared in the standard manner. R. rubrum, similar to R . sphaeroides in its e.s.r. properties, was studied as chromatophores. The e.s.r., e.s.e. and RYDMR spectrometers have been described previously. The e.s.e. studies of cations were performed on light irradiated samples in which the quinones were not removed or reduced, producing a long-lived cation. The triplets were investigated using light-modulated e.s.r. spectroscopy in single crystals of R. viridis in which the quinone was already in a reduced state in the dark, These single-crystal e.s.r.data have been published previously, but have not been compared with the molecular coordinates of the bacteriochlorophylls as determined by X-ray diffraction. Results and Discussion E.S.E. 15N Anisotropic Hyperfine Coupling Constants for the Primary Donor Cation Although the X-ray structure for the reaction centre of Rhodopseudomonas uiridislO proves the dimeric nature of the ground-state special pair, the first question to be resolved is whether the cation of the special pair behaves like a monomer or like a dimer. The four nitrogens near the centre of the chlorophyll macrocycle can serve as an e.s.e. probe of the cation unpaired electron spin distribution and thus determine the nature of the special pair with respect to the cation state.l* Recent ENDOR studies15 found an average reduction of two in isotropic coupling constants for 15N BChla'+ us.P865+ in R26 R. sphaeroides. However, reduction factors of about five were reported for the anisotropic coupling constants for the same systems.15 The reduction factor of two supports a symmetrical dimer whereas the reduction factor of five does not. Here we argue that the anisotropic 15N coupling constants are the more rigorous parameters for probing the monomeric us. dimeric nature of the primary donor, and thus the reduction factor of five is a serious discrepancy for the cation special pair. The 15N anisotropic hyperfine interaction measures primarily only the nitrogen spin density. In contrast, the isotropic hyperfine interaction of 15N of chlorophylls measures the spin density of the adjacent carbon atoms as well as the nitrogen. Thus the anisotropic interaction is the parameter of choice.To measure this anisotropic quantity we use e.s.e. envelope modulation spectroscopy16~ l7 on 15N-enriched R. rubrum chromatophores. Our e.s.e. results support a set of anisotropic 15N hyperfine couplings for P86Y which have an average reduction factor closer to two, as required by the special pair model. Several stimulated echo modulation curves taken at various z are shown in fig. 1 and 2. The hyperfine coupling constants obtained by a non-linear least-squares fitting procedure of the ratios of different modulation curves are in table 1. In analysing the data we have assumed that the modulation arises from four pairs of approximately parallel, axially symmetric 15N hyperfine tensors.The four isotropic coupling constants are constrained to match the liquid-solution ENDOR mea~urernents,~~ while the four anisotropic coupling constants ( A II components of the traceless, axially symmetric tensors) are varied until the best fit is achieved by the least-squares procedure. A , , and16 Magnetic Resonance of Ultrafast Chemical Reactions c I I 1 I I I I 1 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8 tlw I I 1 I I 1 I I 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8 tlW I Fig. 1. (a) Stimulated echo modulation from the cation of bacteriochlorophyll a. Traces from top to bottom were taken with z setting from 0.28 to 0.56 ps. (6) Calculated modulation using the above t settings and the hyperfine coupling constants (table 1 ) from e.s.e.Each calculated trace is multiplied with an exponential decay constant of 4 ps. The time axis is z+ T. A , (where A , = - A , , / 2 ) denote components of the traceless dipolar hyperfine tensor A . The fitting procedure is analogous to extracting hyperfine coupling constants by fitting powder spectra in the frequency domain, except that the problem of data truncation inherent in all e.s.e. experiments is considerably less severe in the time domain. For more detail, see ref. (18) and (19). The average reduction factor for the four anisotropic coupling constants (relative to BChla+) is 2.3. We therefore conclude that the cation state is a symmetrical dimer special pair in R . rubrum. In addition we argue that the special pair is a symmetrical dimer cation in the radical pair state in R .rubrum and R . sphaeroides .J . R. Norris, C. P . Lin and D . E. Budif 17 I I I 1 1 1 1 1 1.0 2.0 3.0 4.0 5.0 6.0 7.0 a tlP 0.b 1.b 2.b 3.b 4.b 5.b 6.b 7.b 8 tlPS Fig. 2. (a) Stimulated echo modulation from the primary donor cation of R. rubrum. Traces from top to bottom were taken with z setting from 0.28 to 0.56 ps. (b) Calculated modulation using the above z settings and the hyperfine coupling constants (table 1) from e.s.e. Each calculated trace is multiplied with an exponential decay constant of 4 pus. The time axis is z+ T. Single-crystal E.S.R. Studies of Reaction-centre Triplets The initial X-ray structure shows a local, approximate, C, symmetry for the ground state of the electron-transfer components of the photosynthetic reaction centre of R.uiridis including the special pair. A rigorous test of the actual ‘working’ symmetry based on the triplet state of the donor is simple and straightforward. By working symmetry we mean the symmetry exhibited in states other than the ground-state core electrons probed by the X-ray technique, such as the cation state, the lowest excited triplet state or lowest excited singlet.18 Magnetic Resonance of Ultrafast Chemical Reactions Table 1. 15N hyperfine coupling constants for BChla'+ and P865'+ of R. rubrum as determined by e.s.e. modulation BChla'+ P865'+ aiso Aila aiso A I /MHz /MHz /MHz /MHz RFb 3.16 2.43 1.0Y 0.57 4.26 3.41 2.61 1.61 1.34 1.95 4.10 2.67 2.22 1.77 1.51 4.45 2.91 2.60 1.96 1.48 a A l l denotes the largest element of the traceless and axially symmetric dipolar tensor.R , is defined as the ratio A lI(BChla'+)/A ,,(P865'+). Isotrophic coupling constants in this column are taken from liquid-solution ENDOR measurements.* The triplet-state method used to probe working symmetry involves comparing the principal directions for the zero-field splitting tensor of the triplet state (in a single crysta1)llY l2 to the direction of the C, axis as determined by X-ray diffraction.1° If the triplet state resides symmetrically in a special pair of bacteriochlorophylls with C, symmetry then one of the three triplet axes, either x, y or z , must lie along, or approximately along, the two-fold rotation axis as defined by the C, symmetry. Of course the two remaining triplet-state axes must then be perpendicular to the C, axis. Since the X-ray crystal structure has not yet been refined, the C, axis direction is not expected to be perfect.Also the symmetry of the immediate protein environment as yet has not been reported by X-ray diffraction techniques. For the purposes of spectroscopic observations, the environment is as important as the geometry of the bacteriochlorophyll molecules in the symmetry/asymmetry question. The orientation of the zero-field axes of the molecular triplet in single crystals of both R . viridis and R. sphaeroides has been detennined.ll9 l2 E.s.r. analysis of single crystals in the triplet state show that no zero-field triplet axis is approximately parallel to the X-ray determined C, axis. To the contrary, the total angle between the C, and the triplet z-axis is closer to 45" than O 0 ., O The reported zero-field axes belong to one of the eight possible triplet orientations in the unit cell; therefore, we compared each site to the reported direction of the C, axis.l* Table 2 shows the directional angles of the C, axis with the zero-field axes for each of the eight orientations of the P890 triplet in the unit cell. Based on this tabulation no zero-field triplet axis is even approximately parallel (or antiparallel) to the C, axis at any orientation. The closest approach is 27" (considering only a single direction cosine), much larger than possible errors in the direction of the triplet or the C, axes. Thus, the triplet state of the P890 special pair deviates significantly from C, symmetry, despite the highly symmetric structure indicated by the X-ray data.Such asymmetry probably reflects an asymmetry in the protein environment immediately surrounding the triplet state of the special pair donor. The observed triplet axes may be more closely related to those of a monomeric triplet excited state residing on one of the BChl b molecules. In order to make this comparison, approximate zero-field axes for each BChl were derived using the atomic coordinates from the crystal structure kindly provided by Deisenhofer and Michel. The z axis wasJ . R. Norris, C . P . Lin and D. E. Budil 19 Table 2. Directional angles (in degrees) of the C, axis in the molecular coordinates for the eight triplet orientations in the unit cell of R. viridis site X Y z 27 138 100 142 118 71 54 69 116 48 148 128 68 127 39 47 95 89 60 94 37 137 78 129 taken as the normal vector to a plane fitted by least-squares to the coordinates of the members of the conjugated ring; the y direction was assigned to the projection of the ni trogen-t o-ni trogen vector on to the plane.Table 3 gives the direction angles of the measured triplet axes with the assumed monomer axes of the special pair BChls for the eight crystal sites. Only one BChl of the special pair in the unit cell coincides appreciably with the experimental axes (L subunit, site l), but the agreement for this case is excellent: the three measured axes are all within 5" of the assumed monomer axes. We are using the standard nomenclature of L (low molecular weight) and M (medium molecular weight) to denote the two protein subunits that contain the chemically active pigment molecules and quinones.The L protein is the only photoactive subunit. The P890 triplet appears to reside primarily on the L-subunit side of the dimer, which is the side closer to the menaquinone site revealed in the X-ray data. Fig. 3 depicts the experimental axes and the assumed monomer axes relative to the molecular framework of the special pair. The P890 triplet is not simply a BChl b monomer triplet, since its zero-field splitting parameters D and E are smaller than the observed values for in vitro BChl b monomer triplets. One way to reduce D and E is to redistribute the electrons by including charge-transfer character in the P890 triplet. We approximate D = -61 G for a pure charge-transfer triplet [BChl(L) + BChl(M) -3 by distributing the unpaired spins over the atomic coordinates according calculated spin densities for BChl b ions2' Ca.23% charge-transfer character is required to reduce D from the monomeric value of 227 G to the in vivo value of 165 G. Thus by including triplet-state charge transfer within the special pair the directions of the triplet zero-field tensor directions agree with the three measured directions within ca. 3", as shown in table 4. Dimers of organic molecules often display dimeric optical properties for the lowest excited singlet state but monomeric properties for the triplet state.22 In these cases the triplet state is 'self-trapping' and as a result resides on both 'monomer' halves of the dimer but not at the same time.The rationale for a given state behaving as monomer or as dimer is based primarily on the differences in spin angular momentum for the state involved. For example, the spin angular momentum restrictions for sharing a state or excitation between two monomers is rather severe for triplets. However, for singlets or cations spin restrictions are much less important. Thus the triplet state provides a severe test of the symmetry of the special pair and its local environment. We emphasize that the triplet of R . viridis correlates only with one half of the special pair. If the triplet state itself were responsible for the asymmetry, i.e. the triplet were self-trapping, then the triplet would sometimes be found on the M side as well as the L side. Since we find the triplet axes to correlate only with the L side, we conclude that the observed asymmetry20 Magnetic Resonance of Ultrafast Chemical Reactions Fig.3. Bacteriochlorophyll special pair in R. viridis crystal. The long arrows locate the L- bacteriochlorophyll 'monomer' x, y and z. The short arrows are the observed x, y and z axes of the triplet state. By allowing 23 % charge transfer between the L-bacteriochlorophyll of the special pair and the M-bacteriochlorophyll the 'monomer' arrows and the observed arrows all coincide within 3". of triplet distribution probably reflects an asymmetry in the protein environment immediately surrounding the special pair. Such an asymmetry in the protein environment of the special pair also suggests that the cation of the special pair of R .viridis is probably asymmetrical. The observed e.s.r. linewidth of the cation (ca. 11.5 G ) is too broad to indicate a symmetrical dimer and too narrow to indicate a monomer. Thus the cation e.s.r. linewidth also suggests an asymmetrical spin distribution between the two chlorophylls of the special pair of R. viridis. However, the much narrower line widths of ca. 9.5 G for the cation of R. sphaeroides or R . rubrum suggests that the cation state in these organisms is much more symmetrical, in agreement with the anisotropic 15N hyperfine coupling constants. We take the triplet evidence to support the view that the exact nature of the special pair ranges from a highly symmetrical dimer to an asymmetrical dimer. Even in the asymmetrical case the special pair is acting essentially as a supermolecule, since the triplet zero-field constants are reduced relative to a monomer of bacteriochlorophyll.In addition the triplet asymmetry suggests that the pigments of the M protein subunit, except for the special pair, are not very important to the characterization of the initial radical pair. Static and Timedependent Magnetic-field Effects in Bacterial Photosynthesis The effect of an applied magnetic field upon the quantum yield of triplets produced in photosynthetic reaction centres of bacteria has been widely studied to gain insight intoJ . R . Norris, C . P . Lin and D. E. Budil 21 the dynamics and energetics which govern the primary stages of photoinduced charge separation. Of particular interest are the very weak electronic exchange interactions which can be measured by magnetic-field effects, for these may be related to the rates of electron transfer within the reaction centre.To date, magnetic exchange in the primary radical pair has been determined indirectly by a parametric fit to the observed low field dependen~e,~-,~ or resonant microwave power dependence26-28 of the triplet yield, or by fitting the measured electron spin polarization in Fe-depleted reaction centres. 29 We report here the first direct measurement of isotropic electron exchange in quinone- depleted reaction centres from the bacterium R. sphaeroides R-26. Triplets arise in the reaction centre by charge recombination between the primary acceptor anion (P.-) and the primary donor cation (B;+) when electron transfer to a secondary acceptor is blocked.The mechanism for 3B2 formation is analogous to the nuclear hyperfine-induced intersystem crossing found in reactions of radicals in solution;30* 31 however, since the photosynthetic reaction takes place embedded in the reaction-centre protein, electron exchange is not modulated by diffusion of the radicals as it is in liquid solution. Effects of static external magnetic fields on 3B, yield are usually explained in terms of the energy levels of the spin states of the Bi+P'- ion pair. The radical pair is born in the singlet state, which is non-stationary because of nuclear hyperfine terms which mix it with the triplet states. At zero field, for sufficiently small singlet-triplet energy gaps, transitions can occur to all three of the triplet sublevels.When a static magnetic field is applied, two of the triplet states are removed by the Zeeman energy, and transitions occur only to one of the sublevels, resulting in a reduction of the 3B2 yield measured upon charge recombination. An isotropic exchange interaction in the radical pair gives a singlet-triplet energy gap E,- Et = 2J, which results in a singlet-triplet level crossing at an applied external field gPBo = 124. At this point, radical-pair intersystem crossing is most efficient, and therefore an initial increase in triplet yield from B, = 0 to B, = 124/gP could be expected if isotropic exchange is present. A similar effect occurs when a microwave field is applied in resonance with the energy differences in the radical pair.The analogy between the static and resonant magnetic field experiments becomes clear when the radical pair spin states are viewed in the reference frame rotating at the microwave frequency, with z defined along the direction of the microwave field B,. The magnitude of the B, field is varied from zero to a few tens of gauss by adjusting the power of the microwaves. Since the B, field is stationary in this reference frame, the experiment is entirely analogous to turning on a static field in the laboratory frame, although the effective Hamiltonian of the system is changed by the transformation into the rotating-z frame. Again a maximum in relative triplet yield should occur when B, = 2J,279 28 in the absence of significant homogeneous or inhomo- geneous broadening terms.An exchange resonance does appear in the static field effect on reactions between radicals in solution which are linked to each other by a methylene chain, as observed either optically32 or by the CIDNP effect.33 Some theoretical calculations of the field effect on reaction centres predict an observable peak under certain c i r c ~ m ~ t a n c e ~ . ~ * - ~ ~ No resonance has yet been reported in experiments on quinone-depleted reaction centres from several laboratories at 0 0C23 and room temperat~re.~~ Some workers have concluded from this that 2J is zero or negligible in the radical ~air;,~g 35 however, most recent model calculations include J as a parameter to fit the observed triplet-yield field dependence. When the temperature is dropped below 0 "C, a peak does appear at lBol = 14 G in the low-field profile of reaction-centre triplet yield, as shown in fig.4. The resonance is observable at the absorption of 3B, at 420 nm, the ground-state B, absorption band at 870 nm, and in plots of radical-pair lifetime us. 38 Although the amplitude of the peak relative to zero field varies slightly among different samples, the resonance has22 Magnetic Resonance of Ultrafast Chemical Reactions Table 3. Overlap matrices for the measured x, y and z directions of the triplet state2 and the molecular coordinate x, y and z directions determined from X-ray diffraction data of R. viridis'O BCMP BCLP BCMP BCLP e.s.r. axis X Y Z X Y Z X Y Z X Y Z X Y z X Y z X Y z X Y z 165 100 101 102 15 81 80 79 165 130 87 40 98 172 92 41 97 50 109 32 115 20 76 104 96 119 151 77 31 63 164 83 75 81 120 32 3 87 90 51 80 139 118 99 30 93 5 87 75 165 90 91 169 101 90 93 3 137 101 131 28 97 62 64 68 145 12 93 79 164 106 93 69 155 103 85 8 97 106 27 69 146 101 122 101 83 13 93 69 158 78 37 56 91 148 122 72 154 72 154 94 64 152 104 67 36 85 125 67 127 45 118 61 138 61 65 41 120 33 101 83 149 60 91 138 132 33 65 111 25 97 114 139 116 60 103 110 156 66 60 40 131 59 123 0.040 0.055 !4 8 d- n 0.030 c, 0 3 -2 8 2 Q 0.02s W B' 0.020 -50 I I 1 1 50 100 150 200 magnetic field/(; Fig.4. Triplet yield in quinone-depleted reaction centres from R. sphaeroides R-26 as a function of applied external magnetic field near 0 G. Triplet absorbance was measured by a least-squares fit of an asymptote to radical pair decay measured at 420 nm up to ca.180 ns after excitation with 600 nm light. Conditions: 42 pmol dm-3 reaction centres in buffer containing 60% ethylene glycol, 0.01% LDAO in a 0.25 mm path at 243 K. appeared consistently at lBol = 14 G in several different reaction-centre preparations, independent of the detergent (LDAO or BRIJ-58) and low-temperature glass (glycerol, sucrose or ethylene glycol) used. The position of the resonance is also independent of the direction of polarization of excitation light at 600 nm relative to the field direction (data not shown). We therefore ascribe the peak to an isotropic electron exchange interaction 124 = 14 G in the primary radical pair in the bacterial reaction centre.J. R. Norris, C.P. Lin and D. E. Budil 23 1.10 1.05 5 9" 0 1 r4 d Y m g 0.95 E .- t o - 0.90 0.85 0 \ * I I I I I 1 5 10 15 20 25 30 B , / G Fig. 5. Relative triplet yield as a function of microwave B, with triplet yield determined at 420 nm. Conditions: 80 pmol dmP3 reaction centres in buffer containing 0.01 % LDAO in 0.1 mm path at room temperature. Line: Theoretical triplet yield calculated according to ref. (25) with z, = 25 ns (singlet radical-pair lifetime) and rt = 2 ns (triplet radical-pair lifetime). The (24 from the static-field study agrees closely with the value of 16 G recently estimated from initial RYDMR spectroscopy of the radical pair.26 Fig. 5 shows a room-temperature field study taken in the rotating frame which exhibits a resonance at B, = 13 2 G, again demonstrating the excellent agreement of RYDMR, static magnetic-field measurements and theoretical calculations of the microwave field effect.27 For this experiment the B, field was carefully calibrated against applied power by measuring the duration of 7r/2 and 71 pulses in our e.s.e. spectrometer. It is noteworthy that the J resonance appears in the rotating-z spectrum but is obscure in the static-field profile at room temperature. As Lersch and Michel-Beyerle have rioted,28 a major difference between the high- and low-field spin Hamiltonians is the presence of electronic magnetic dipolar terms mixing the m, = 1 triplet levels at low field, which are neglected in the high-field approximation used to describe RYDMR. This provides some indication that the dipolar interaction D in the primary radical pair may not be neglected.However, the obscurity of the resonance in the static-field dependence and the relatively high temperature at which it appears permits strict limits to be placed on D and the kinetic parameters of the radical pair. The temperature at which the peaks are first discernible in the static-field study varies with detection method. The signal-to-noise ratio is generally best in the lifetime spectrum, since the fitting of a single exponential to the observed radical pair decay acts as a noise filter. Fig. 6 illustrates the appearance of the peaks in the lifetime spectrum of the radical pair with decreasing temperature; they are faintly visible at temperatures as high as 0 "C, and reach their maximum amplitude by ca.- 30 "C. The peaks persist at 14 G down to ca. 130 K, at which point they are no longer visible above the noise because of the diminishing relative field effect as the triplet yield approaches unity. If the uncertainty broadening of the triplet levels is to be < 14 G, the decay processes must have characteristic times z > 2 ns below - 30 0C.39 The limits which may be placed on the magnetic dipolar interaction D depend upon the relative signs of D and J . Making the reasonable assumption that D < 0 for a radical pair, if 2J > 0 (singlet higher in24 Magnetic Resonance of Ultrafast Chemical Reactions Table 4. Overlap matrices comparing charge-transfer triplet states in the special pair of R. viridisa calculated axis fraction e.s.r. charge axis x Y z transfer DIG E/G 0.00 227 59 X 3.4 86.6 90.0 y 93.4 4.8 86.7 z 89.8 93.3 3.3 X 4.1 86.0 91.0 y 93.9 4.7 87.3 z 88.8 92.6 2.8 X 4.7 85.7 91.7 y 94.3 4.8 87.8 z 88.1 92.1 2.8 X 4.8 85.6 91.9 y 94.4 4.8 87.9 z 87.9 91.9 2.8 0.15 187 51 0.23 166 46 0.25 160 45 a The L-bacteriochlorophyll ‘monomer’ contains most of the triplet state.The M-bacteriochlorophyll ‘monomer’ of the special pair participates in the triplet state of the primary donor via a small amount of charge transfer. energy) then 12D/31 should be < 124 for the singlet state to lie outside the dipolar width, i.e. ID1 < 21 G. In the case 2 J < 0, the appearance of the J-resonance suggests [Dl < 42 G, although this limit is not as accurate. An estimate of the sign of J is available from model calculations of the RYDMR spectrum of the radical pair,26 which indicate that the singlet radical pair state is higher in energy.Recent calculations by Hoff and H ~ r e ~ ~ of electron spin polarization in Fe-depleted reaction centres support this assignment. Hoff and Hore actually assert that their determination of the sign of J conflicts with the earlier RYDMR result; however, these authors did not take into account the difference between ref. (26) and (29) in sign convention for the exchange interaction. In fact, the two results are consistent, indicating a ferromagnetic (singlet higher) exchange in the radical pair. If these determinations are correct, they would indicate the more severe restriction on the value of D in the radical pair, which is not consistent with the values for D obtained from RYDMR26 and some other field-effect 3 4 9 36 However, such a limit is compatible with the value for D in the radical pair in reaction centres of R.viridis which has been calculated using the atomic coordinates of the B, and P radicals recently available from X-ray diffraction A significant aspect of the measured exchange term is that it is far too small to account for the rapid rates of forward electron transfer in the reaction centre.41 One possible explanation for this discrepancy is that 2J arises from a kinetic hopping between a ‘distant’ radical pair with no exchange and a ‘close’ radical pair with significant electronic interaction^.^' This model is consistent with a two-step charge separation, with the electron first transferred to the intermediate BChl and then to P.42 Alternatively, the primary charge separation might be accomplished via a superexchange 44 in this case the radical pair would be better described as a coherent rather than a kineticJ.R. Norris, C. P. Lin and D. E. Budil 30 - 25 - 2 ; 20- \ n .ii 1 W - 15 - 25 222 K U 2 K 251 K 272 K 296 K 10 : I I I 1 I 1 -100 -50 0 50 100 150 200 magnetic field/(; Fig. 6. Radical-pair lifetime as a function of applied static magnetic field and temperature. Lifetimes were determined by least-squares fit of a single exponential to radical-pair decay measured at 420 nm. Conditions the same as for fig. 4. combination of electronic configurations. Such a mechanism can reconcile a small spin exchange with fast electron transfer because the charge-transfer terms contributing to the radical pair singlet-triplet energy gap enter into the calculation at much higher order for superexchange than for direct The ability to measure spin exchange in the radical pair directly may provide a sensitive test of proposed mechanisms for primary charge separation in photosynthesis.The observed temperature independence of 124 suggests that spin exchange does not arise from a kinetic mechanism; our results are more consistent with a description of the BZP- radical pair as a coherent combination of supermolecular electronic configuration^.^^ Summary In conclusion, we wish to emphasize that the relatively high temperature at which the J-resonance can be detected permits a substantial refinement of the parameters used to characterize electronic interactions'in the reaction centre at physiological temperatures.In addition, the possibility of measuring J directly provides an important experimental means of testing proposed mechanisms for the origin of magnetic exchange in the primary radical pair. A more complete treatment of the temperature dependence of magnetic exchange in bacterial reaction centres is in preparation. Because of the small J value and the high magnetic resolution (of the order of J) of these radical pair experiments, we can rule out any significant kinetic contribution to spin exchange in the radical-pair state. In other words, these magnetic-field investigations26 Magnetic Resonance of Ultrafast Chemical Reactions of the initial radical pair suggest that the extra bacteriochlorophyll molecule, BChl,, that is between the special-pair donor cation and the primary acceptor bacteriopheophytin anion is not involved in a discrete electron-transfer step in bacterial reaction centres.By combining the results of our magnetic-resonance experiments with X-ray structural information, the following description emerges: (1) the ground state, the cation state and the triplet state of the primary donor are each a supermolecule dimer with approximately Cz symmetry; (2) the bridging BChl, molecule probably functions as a superexchange site for rapid transfer of electrons from the primary donor to the primary acceptor but with negligible back reaction; (3) the special pair, BChl,, is lower in energy than the bridging molecule, BChl,, such that the initial radical pair is formed via super exchange between the distant (10 A edge-to-edge) special pair BChl, and the bacteriopheophytin and (4) the triplet is a sensitive probe of the asymmetry of the reaction centre.We thank Dr J. Deisenhofer and Dr H. Michel for providing X-ray data coordinates from R . viridis. We also thank Mr S. Kolaczkowski for help in measuring the relative triplet yield as a function of B,. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract W-31-109-Eng-38. References 1 J. R. Norris, R. A. Uphaus, H. L. Crespi and J. J. Katz, Proc. Nut1 Acad. Sci. USA, 1971, 68, 625. 2 J. Fajer, D. C. Brune, M. S. Davis, A. Forman and L.D. Spaulding, Proc. Natl Acad. Sci. USA, 1975, 3 G. Feher, M. Y. Okamura and J. D. McElroy, Biochim. Biophys. Acta, 1972,267,222. 4 C. Kirmaier, D. Holten and W. W. Parson, FEBS Lett., 1985, 185, 76. 5 W. W. Parson, R. K. Clayton and R. J. Cogdell, Biochim. Biophys. Acta, 1975, 387, 265. 6 K. Kaufmann, P. L. Dutton, T. L. Netzel, J. S. Leigh and P. M. Rentzepis, Science, 1975, 188, 1301. 7 M. B. Rockley, M. W. Windsor, R. J. Cogdell and W. W. Parson, Proc. Natl Acad. Sci. USA, 1975,72, 8 C. A. Wraight and R. K. Clayton, Biochim. Biophys. Acta, 1974, 333, 246. 9 C. A. Wraight, J. S. Leigh, P. L. Dutton and R. K. Clayton, Biochim. Biophys. Acta, 1974, 333,401. 72, 4956. 225 1. 10 J. Deisenhofer, 0. Kepp, K. Miki, R. Huber and H. Michel, J. Mol. Biol., 1984, 180, 385.1 1 P. Gast, M. R. Wasielewski, M. Schiffer and J. R. Norris, Nature (London), 1983, 305, 451. 12 P. Gast and J. R. Norris, FEBS Lett., 1984, 177, 277. 13 J. P. Allen, R. Theiler and G. Feher, Biophys. J., 1985, 47, 4a. 14 P. J. O’Malley and G. T. Babcock, Advances in Photosynthesis Research, ed. C. Sybesma (Martinus 15 W. Lubitz, R. A. Isaacson, E. C. Abresch and G. Feher, Proc. Natl Acad. Sci. USA, 1985, 81, 7792. 16 L. G. Rowan, E. L. Hahn and W. B. Mims, Phys. Rev. A, 1965, 137, 61. 17 W. B. Mims, Phys. Rev. B, 1972, 5, 2409. 18 C. P. Lin and J. R. Norris, to be published. 19 J. Tang, C. P. Lin and J. R. Norris, J. Chem. Phys., 1985, 83, 4917. 20 J. R. Norris, D. E. Budil, H. L. Crespi, M. K. Bowman, P. Gast, C. P. Lin, C. H. Chang and M. Schiffer, Antennas and Reaction Centers of Photosynthetic Bacteria, ed.M. E. Michel-Beyerle (Springer- Verlag, Berlin, 1985). 21 M. S. Davis, A. Forman, L. K. Hanson, J. P. Thornber and J. Fajer, J. Chem. Phys., 1979, 83, 3325. 22 J. B. Birk, Photophysics of Aromatic Molecules (Wiley Interscience, New York, 1970). 23 A. Ogrodnik, H. W. Kruger, H. Orthuber, R. Haberkorn and M. E. Michel-Beyerle, Biophys. J., 1982, 24 C. E. D. Chidsey, C. Kirmaier, D. Holten and S. G. Boxer, Biochim. Biophys. Acta, 1984, 766, 424. 25 J. Tang and J. R. Norris, Chem. Phys. Lett., 1982,92, 136. 26 J. R. Norris, M. K. Bowman, D. E. Budil, J. Tang, C. A. Wraight and G. L. Closs, Proc. Natl Acad. 27 J. Tang and J. R. Norris, Chem. Phys. Lett., 1983, 94, 77. 28 W. Lersch and M. E. Michel-Beyerle, Chem. Phys., 1983,78, 115. 29 A. J. Hoff and P. J. Hore, Chem. Phys. Lett., 1984, 108, 104. 30 R. Haberkorn and M. E. Michel-Beyerle, FEBS Lett., 1977, 75, 5 . 31 G. L. Closs, J. Am. Chem. SOC., 1969,91, 4552. Nijhoff, The Hague, 1984), vol. I, pp. 697-700. 39, 91. Sci. USA, 1982,79, 5532.J. R. Norris, C. P. Lin and D. E. Budil 27 32 A. Weller, H. Staerk and R. Treichel, Faraday Discuss. Chem. SOC., 1984, 78, 279. 33 G. L. Closs, Chemically Induced Magnetic Polarization, ed. L. T . MUUS, P. W. Atkins and K. A. 34 M. G. Roelofs, C. E. D. Chidsey and S. G. Boxer, Chem. Phys. Lett., 1982, 87, 582. 35 H.-J. Werner, K. Schulten and A. Weller, Biochim. Biophys. Acta, 1978, 502, 255. 36 A. J. Hoff, Q. Rev. Biophys., 1981, 14, 599. 37 M. R. Wasielewski, C. H. Bock, M. K. Bowman and J. R. Norris, J. Am. Chem. Sac., 1983,105,2903. 38 M. R. Wasielewski, C. H. Bock, M. K. Bowman and J. R. Norris, Nature (London), 1983, 303, 520. 39 M. E. Michel-Beyerle, H. Scheer, H. Seidlitz, D. Tempus and R. Haberkorn, FEBS Lett., 1979,100,9. 40 A. Ogrodnik, W. Lersch, M. E. Michel-Beyerle, J. Deisenhofer and H. Michel, Antennas and Reaction Centers of Photosynthetic Bacteria, ed. M. E. Michel-Beyerle (Springer-Verlag, Berlin, 1985). 41 R. Haberkorn, M. E. Michel-Beyerle and R. A. Marcus, Proc. Nut1 Acad. Sci. USA, 1979, 76, 4185. 42 V. A. Shuvalov and W. W. Parson, Proc. Nut1 Acad. Sci. USA, 1981,78, 957. 43 J. R. Miller and R. V. Beitz, J. Chem. Phys., 1981, 74, 6746. 44 A. M. Kuznetsov and J. Ulstrup, J. Chem. Phys., 1981,75,2047. 45 J . Yamashita and J. Kondo, Phys. Rev., 1958, 109, 730. 46 F. Keffer and T. Oguchi, Phys. Rev., 1959, 115, 1428. 47 P. Bertrand, Chem. Phys. Lett., 1985, 113, 104. McLauchlan (Reidel, Boston, 1977). Paper 6/ 1000; Received 22nd May, 1986

ISSN:0300-9599    

DOI:10.1039/F19878300013

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1987, 83, 29-35 Two-dimensional Transient Electron Spin Resonance Spectroscopy Keith A. McLauchlan" and David G. Stevens Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ A two-dimensional field-time transient electron spin resonance spectrum is reported of the cyclohexyl- 1-01 radical. The spectrum shows linewidth alternation effects due to a ring-flip process. The radical is polarized and its spectrum displays symmetric E/A characteristics at early times, showing the radicals to originate in the reaction of a triplet state, but these invert to an A/E pattern in time. At longer times still the spectrum exhibits excess emission. This represents a fourth case of radicals whose spectra show these time-dependen t patterns. Using the continuous-wave (c.w.) and spin-echo methods developed in recent years, the high-resolution electron spin resonance (e.s.r.) spectra of transient free radicals can be obtained at times as short as 30 nslf after the radicals are created.When observed soon after radical formation most spectra exhibit electron spin polarization, which originates in chemically induced dynamic electron polarization (CIDEP) proce~ses.~ The two CIDEP mechanisms, the triplet mechanism (TM) and the radical pair mechanism (RPM), produce characteristic intensity distributions which allow the multiplicity of the radical precursor to be deduced. The radicals are identified via the positions of the lines in the spectra, these being unaffected by the CIDEP processes. As time elapses the initial intensity patterns change owing to a variety of physical and chemical processes and the observation of this time-dependence yields further information an the system, Most obviously, the initial polarized intensities diminish in absolute magnitude as a result of spin-lattice relaxation; in C.W.experiments the rate is enhanced by microwave-driven transitions. Often, as the radicals which were created together in geminate cages separate, the identity of the radical pairs formed by random diffusion (F pairs) differs from that of the original pair. These yield a different polarized intensity distribution which continues to be generated whilst reactive radicals persist in the solution and which discloses the identities of these radicals. Even where the type of radical does not change in time, there is growing evidence that for a given pair of radicals, the polarization pattern produced in a triplet geminate pair differs from that observed at later times in an F pair; this is contrary to accepted polarization theory.If the primary radical reacts within its lifetime to create a secondary species, this too may be identified. As a final example, it has been discovered that in C.W. experiments the development of the spectra of spin-polarized radicals in time is unusually sensitive to electron and proton exchange To observe such phenomena as these, the spectrum must be monitored for long periods of time following radical creation. Here C.W. methods have considerable advantages over the spin-echo one7 in that the intensity of a spin-echo falls rapidly with time (at the spin-spin relaxation rate) after radical creation, and also the measurable signal (the echo itself) is of very short duration. With C.W.methods a continuous signal may be sampled over a long period of time, and the signal height falls in time at an effective spin-lattice relaxation rate; in our experience b T, in solutions of normal viscosity. 2930 Two-dimensional Transient E.S.R. The C.W. methods, like the spin-echo one, are sampling techniques. Since it is not possible to sweep through a spectrum in a short time (cf. 30 ns) the spectrum is obtained by creating the radicals repetitively as the magnetic field is varied either continuously or in discrete steps. In the former case a sample, over a chosen period of time, is taken with a box-car averager and the continuous output from this as the field is swept constitutes the spectrum;8 it is important to realize that only that information pertinent to the chosen time period is recorded.In the latter, the entire decay curve is stored in a dedicated microcomputer and the period for which the sample is required is selected afterwards. In the time-integration spectroscopy (TIS) technique9 the signal is digitally summed over the chosen period and this sum is entered into an address which corresponds to a specific value of the magnetic field. As operated until recently, the contents of the store have then been destroyed before the field advances to its next value and the process is repeated. A curious feature of the TIS experiment has consequently been that all the information to define the time-behaviour of the system completely has been obtained, but most of this has been rejected subsequently.To obtain the TIS spectrum at a different time after radical creation has necessitated repeating the whole experiment, involving thousands more photolysis flashes. Moreover where the spectrum varies rapidly in time important behaviour may be missed by an unfortunate choice of sample periods. It is consequently better to store all the information obtained at each field value and to plot its intensity versus time and field on a three-dimensional surface. Following n.m.r. usage we define this as a two-dimensional plot since there are two independent experimental variables. Method The experiment has been described in a recent publication7 and is based upon our existing TIS apparatus to which a 10 Mb Winchester disc data store has been added.The radicals are created in U.V. pulses at 308 nm from a Lambda Physik EMG 103 MSC excimer laser; since our first paper an output control system has been added to ensure that the mean pulse energy (of ca. 50 mJ per pulse) does not vary during data acquisition, a period of ca. 2 h in the experiments reported here. The deoxygenated sample flows continuously through the quartz irradiation cell to avoid sample depletion. At each field position a chosen number (128) of transient decays are recorded and signal-averaged by the dedicated microprocessor described previously. The data are then output, together with information on the field position and a scaling factor, to allow them to be compared with subsequent data, to a PDP 1 1 /23 microcomputer where they are stored on the 10 Mb disc.The flash repetition rate is limited to 20 Hz by data- handling considerations. After data transfer, the data and add buffers of the micro- processor are cleared, the field is stepped and data collection recommences on the next laser pulse. The cycle is repeated until all the field addresses have been filled. The experi- ment is controlled by extensive software. In the spectra shown below 512 field points have been used and at each a decay curve has been obtained which consists of 2048 channels of 16-bit data, a total of ca. 2 Mbytes of information. This was transferred from the PDP 11 /23 to a Norsk Data ND-520 minicomputer for data manipulation. Whenever a laser beam strikes a tuned e.s.r.cavity it produces a spurious long-lived signal (even off-resonance) which originates in photoelectron emission and sample and cavity heating. At constant laser power the signal is reproducible at all field positions. It has consequently been subtracted from each decay curve used to construct the surface shown below. In addition pre-trigger subtraction has been employed to ensure that only pulse-correlated information is displayed.K . A . McLauchlan and D. G . Stevens 31 Fig. 1. A field-time-intensity surface of photolytically generated cyclohexyl- 1-01 radicals. The spectrum is spin polarized by the ST, RPM process to yield an E/A pattern at early times after radical creation, which changes to an A/E one later in time. Attention is also drawn to the marked alternating linewidth effect, which originates in the modulation of j? couplings by ring flipping.Results and Discussion Fig. 1 shows the two-dimensional field-time spectrum of the cyclohexyl-1-01 radical produced by photolysis of cyclohexanone in cyclohexanol. Its signal-to-noise ratio is sufficient that the normal spectrum at a specific time can be obtained simply by taking a point time sample (corresponding to a 10 ns sample period) across the field span (fig. 2). We stress that the surface and the derived figures display direct measurements of the magnetization in the y-direction of the rotating frame, and not its integral as in our TIS experiments. With weaker signals, time-integration can of course be applied to the contents of the data store, without the need for further experimentation, to yield TIS spectra.This can be invaluable too in studies of the variation of the absolute signal height with time, where equal integration periods are essential to compare signal heights at different times. In normal TIS experiments this integration window is often varied with time after radical creation to optimise the signal-to-noise ratio. As is obvious from the figures, the spectrum at earliest time is polarized and displays the emission at low field/absorption at high field (E/A) characteristics of a radical produced by reaction of the triplet state of the precursor ketone. This is consistent with a previous continuous-irradiation C.W.study of CIDEP in this radical. lo The reaction is consequently The spectrum when first observed is completely symmetrical in its intensities and exhibits the characteristics of the action of a pure ST, RPM polarization mechanism between identical radicals. The outermost lines (which would be weakest in the unpolarized 2 FAR I32 Two-dimensional Transient E.S.R. (a) absorption f I t II field 161 - 20G Fig. 2. Spectra obtained by taking a single point sample, at each of two chosen times after radical creation, from the surface shown in fig. 1. (a) At 2.5 ps, where the spectrum shows a symmetric E/A pattern and (b) at 75.0 ps, where the intensities have inverted to yield an A/E pattern. The spectra have been normalized to the same overall amplitude. spectrum) are strongly accentuated, whilst the central line (the strongest in the unpolarized situation) is of low intensity.Its finite width in this spectrum removes some of its intensity away from the exact centre of the spectrum and yields small signals in emission at low field and in absorption at high field. The appearance of the other lines is discussed below. Rather unusually, there is no contribution from the TM to the observed polarization. This is significant for previous studies of (CH3),cOH11 and (CH3),CHl2? l3 radicals displayed overall similar polarization characteristics to those discussed here, but with excess absorption at early times; it now appears that this originated in the TM process and that the early time behaviours of these systems are consistent with theory.At longer times after the flash [fig. 2(b) and 31 the nature of the electron spin polarization changes completely and, as with all other RPM-polarized radicals emanating from reactions of triplet states which have been monitored for a sufficiently long time, an absorption/emission (A/E) pattern is observed. This occurs with the radicals mentioned above besides (CH3)3c radicals.’* This is not as expected on simple theoretical grounds if this polarization arises in the encounters of freely diffusing radicals to form ‘F pairs’. As before in between the initial E/A and later A/E patterns the signals have low absolute intensities as the two opposite phases of polarization tend to cancel, whilst relaxation diminishes the large initial signal.In this region a completely absorptive spectrum is observed (but cannot be discerned clearly in fig. 1). The origin of the inversion of the phase of polarization with time remains incompletely understood. It is apparent that spin polarization continues to be generated at long timesK. A . McLauchlan and D . G. Stevens 33 - field absorption I c 1 emission 2OG Fig. 3. A time integration spectroscopy (TIS) spectrum of the cyclohexyl-1-01 radical obtained with sampling and summation between 160.0 ps and 172.5 ps after radical creation. It exhibits a basic A/E pattern, but shows excess emission. after the flash that creates the radicals since the A/E signal continues to grow in for some time, and the timescale suggests F pairs as its only reasonable source. Independent experiments seem to have confirmed this directly for (CH3)3c radicals,14 although other evidence has been submitted which apparently shows that radicals from F pairs observed at early times do yield the expected E/A pattern.15 The growing generality of the phenomenon makes it difficult to sustain the only model so far suggested to explain it, which was based upon a variation in the sign of the electron exchange interaction with inter-radical separation together with a difference in trajectories between identical radicals forming geminate and F pairs.16 It may be that electron or nuclear relaxation effects should be considered.However, a further advantage of the two-dimensional display is to allow a clear indication of the variation of the signal heights with time right across the spectrum. This allows an apparently facile comparison of the absolute magnitudes of initial and F pair polarizations which will yield further insight into this problem. To model such behaviour requires the calculation of the whole surface and, although it is fully defined through the Bloch equations, a considerable computational effort is required; it is already underway.At longer times, the polarization behaviour of the radicals has been investigated using our normal TIS technique (fig. 3). As with the radicals mentioned above, the spectrum then exhibits excess emission. The timescale of the observation suggests that the origin of the effect may be chemical. It is possible that the relative concentration of oxygenated radicals, for example peroxy radicals formed by oxygen-scavenging of primary radicals in incompletely deoxygenated solutions, becomes significant in time and that the nature of the majority radical pair alters. The oxygenated radical is not observed, possibly because of line broadening due to rapid relaxation, but an ordinary ST, RPM process with such a radical would yield excess emission in the cyclohexyl- 1-01 radical, as a result of the difference in g values between the two.Experimental attempts to confirm this have so far failed, and the phenomenon may well be an experimental artifact. For a radical with few hyperfine lines in its spectrum the display of the field- and time-dependence given by the surface in fig. 1 gives a very clear indication of its identity 2-234 Two-dimensional Transient E.S.R.absorption . . _ .. emission - 4 I I - 20 G field absorption a ._I - Fig. 4. A contour plot constructed from the surface shown in fig. 1. The peak positions are identified by a concentration of contours, with the magnetic field plotted horizontally, and the time vertically. In this black and white version of a colour-coded original information on the phases of the signals has been lost although reference to the previous figures allows them to be recognized. The contours have been drawn at equal intensity intervals with a grain chosen to illustrate the smallest peaks. In this way each line observed soon after the flash is seen to pass through near zero intensity to re-emerge later in time (actually in the opposite phase). and behaviour.Unfortunately as the complexity of the spectrum increases such a plot requires many more field points to define the peaks adequately and the time required to record the surface increases. At the same time the peaks become difficult to distinguish in this type of plot. It becomes convenient to use a different mode of display, the contour diagram. That corresponding to the surface shown in fig. 1 is given in fig. 4. In it the field is plotted horizontally, and the time vertically, and the peaks, which on the surface would project into (for emission) or out of (for absorption) the plane, are indicated by contours of equal height drawn through them. In the original the contours are colour-coded, with different colours used for emission and absorption, but this is not reproduced here.This type of display gives a very clean and clear picture but must be interpreted with caution, particularly with respect to the times at which the various lines apparently attain zero intensity. Even in spectra in which all the lines have identical effective relaxation times, this null point occurs at different instants owing to the different initial magnitudes of the various lines. These depend, of course, upon the CIDEP process occurring. The apparent null point also depends upon the contour density. It remains to discuss the striking linewidth alternation visible in these spectra. This originates in an interconversion between degenerate conformers of the radicals, with modulation of the couplings to protons B to the radical centre. The ring flip is not sufficiently fast to cause complete averaging of the positions of lines which vary in the process.Previous workers,17* * who studied the radical produced by continuous irradiation of a solution of di-t-butyl peroxide in cyclohexanol, were able to stop the interconversion by cooling to 183 K and the individual couplings to two pairs of identical /3 protons were measured. From the values obtained (35.5 and 10.3 G) it was deduced that the radical existed in a twisted-chair conformation at this temperature. AtK. A . McLauchIan and D . G. Stevens 35 higher temperatures the interconversion probably involves both twisted-chair and normal-chair conformations. In none of the previous observations of the radical has the linewidth alternation been observed as clearly at room temperature as in our experiments.To some extent this may reflect the different solutions used but it also owes something to the increase in intensities of the lines in spin-polarized radicals as compared with equilibrated ones. In this preliminary communication we have sought to introduce some of the important features of the polarized spectrum of the cyclohexyl- 1-01 radical, although our main purpose has been to demonstrate the two-dimensional e.s.r. method. We hope to have shown its great potential in studying a range of chemical and physical processes. A detailed study of the polarization behaviour in this and similar radicals, and of the interconversion kinetics, is in progress both experimentally and theoretically. D. G. S. is grateful to the S.E.R.C. for financial support. References 1 A. D. Trifunac and J. R. Norris, Chem. Phys. Lett., 1978,59, 140. 2 K. A. McLauchlan and D. G. Stevens, unpublished work. 3 A recent review is by C. D. Buckley and K. A. McLauchlan, Mol. Phys., 1985, 54, 1. 4 S. Basu, K. A. McLauchlan and A. J.,D. Ritchie, Chem. Phys. Lett., 1984, 105, 447. 5 K. A. McLauchlan and A. J. D. Ritchie, Mol. Phys., 1985, 56, 141. 6 K. A. McLauchlan and A. J. D. Ritchie, Mol. Phys., 1985, 56, 1357. 7 K. A. McLauchlan and D. G. Stevens, Mol. Phys., 1986, 57, 223. 8 e.g. A. D. Trifunac, M. C. Thurnauer and J. R. Norris, Chem. Phys. Lett., 1978,57, 471. 9 S. Basu, K. A. McLauchlan and G. R. Sealy, J. Phys. E, 1983, 16, 767. 10 P. B. Ayscough, T. H. English, G. Lambert and A. J. Elliott, Chem. Phys. Lett., 1975, 34, 557. 11 S. Basu, A. I. Grant and K. A. McLauchlan, Chem. Phys. Lett., 1983,94, 517. 12 A. I. Grant and K. A. McLauchlan, Chem. Phys. Lett., 1983, 108, 120. 13 K. A. McLauchlan and D. G. Stevens, J . Mugn. Reson., 1985,63,473. 14 I. Carmichael and H. Paul, Chem. Phys. Lett., 1979,67, 519. 15 M. C. Thurnauer, T-M. Chiu and A. D. Trifunac, Chem. Phys. Lett., 1985, 116, 543. 16 A. I. Grant, N. J. B. Green, P. J. Hore and K. A. McLauchlan, Chem. Phys. Lett., 1984, 110, 280. 17 C. Corvaja, G. Giacometti and M. Brustolon, 2. Phys. Chem. (Frankfurt am Muin), 1972,82, 272. 18 C. Corvaja, G. Giacometti and G. Sartori, J. Chem. SOC., Furuduy Trans. 2, 1974,70, 709. Paper 6/823; Received 28th April, 1986

ISSN:0300-9599    

DOI:10.1039/F19878300029

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I , 1987,83, 37-42 A Single-crystal ENDOR Study of y-Irradiated Pyridoxine Hydrochloride Neil M. Atherton* and Wendy A. Crossland Department of Chemistry, The University, Shefield S3 7HF Hyperfine coupling tensors for five protons have been determined from room-temperature ENDOR measurements on a single crystal of y-irradiated pyridoxine hydrochloride. Their interpretation supports the identification of the radical made by Masiakowski, Krzyminiewski and Pietrzak from analysis of the e.s.r. spectra using resolution enhancement. The identification of the radical formed on y-irradiation of pyridoxine hydrochloride is of interest because of the molecule's close relation to vitamin B,. In a recent study Masiakowski, Kryzyminiewski and Pietrzak used a resolution enhancement technique to enable the e.s.r.spectra of an irradiated single crystal to be ana1ysed.l They were able to distinguish six hyperfine couplings, and concluded that the radical had the structure I, formed by loss of hydrogen atom from c6 (cf. fig. 1). It seemed desirable to confirm the conclusions of the resolution enhancement technique using ENDOR spectroscopy. In this paper we report the results of a room-temperature ENDOR study of an irradiated crystal : five proton hyperfine tensors have been determined and their interpretation does indeed confirm that the trapped radical has the structure I. CHOH I I Experimental Well formed single crystals of pyridoxine hydrochloride (Aldrich) were grown from aqueous solutions at room temperature.The crystal structure has been accurately determined from neutron diffraction by Bacon and Plant,2 and the axes of our crystal were kindly determined for us by Dr N. A. Bailey of this department. The crystals are triclinic, space group p i , the two molecules in the unit cell being related through a centre of symmetry. The crystals grow as tablets and the morphology is such that a* is perpendicular to the well defined face, so a* was taken as a reference axis for our measurements. Our b*c* reference axes thus lie in the bc plane. After overnight irradiation in a ,OCo y-ray source, ca. 250 Ci, a crystal was glued to an accurately machined Perspex cube of 3 mm side. Thus by rotating the cube about the three fourfold axes perpendicular to its faces three orthogonal planes could be explored, the b*c* plane being located accurately but with the relation between the b*c* and bc axes determined by the mounting.E.s.r. and ENDOR spectra were taken every 10" during rotation about each of the 3738 ENDOR Study of y-Irradiated Pyridoxine Hydrochloride Fig. 1. Molecular structure of the pyridoxine hydrochloride cation and atomic numbering of ref. (2). three orthogonal axes using a Bruker ER 200D e.s.r. spectrometer with a Bruker ENDOR unit (100 W r.f.) and an Aspect 2000 computer. Good-quality proton ENDOR was obtained without difficulty. The crystal was not cooled, so the temperature of measurement was a little way above room temperature on account of r.f. heating. The e.s.r. spectra are broad doublets for many orientations, and for some show more complex, but very poorly resolved, hyperfine features.In general the ENDOR response was taken at the maximum of the e.s.r. absorption but the setting was not critical. The ENDOR spectra show a strong narrow feature which arises from distant protons and the centre of this was taken as an internal calibration of the free proton frequency. The spectra were taken from scans in the range 0.1-50 MHz. This limited the digital resolution to 50 kHz, which is adequate for our present purpose whose principal objective is the identification of the radical. Derived hyperfine parameters are quoted to 0.1 MHz, which is more than sufficient to account for mounting errors arising from non- orthogonality of the reference axes. The ENDOR transition frequencies were analysed assuming that the hyperfine interaction could be treated to first order and that the anisotropy of g could be neglected.Thus the orientation dependence of each transition frequency, v, was fitted to3 (1) v 2 * - V ~ + ( 1 / 4 ) ( t l ’ A . A . t l } T V ~ , - ( t l . A . t t } 2 where the upper and lower signs refer to the M, = +f and M , = -; electron spin states, respectively, vN is the free proton frequency, A the hyperfine tensor, and a unit vector defining the orientation of the applied field in the a*b*c* axis system. Transition frequencies were linearly corrected to a constant free-proton frequency of 15 MHz before fitting to eqn (1) using a least-squares procedure. An example of the fit for the b*c* plane is shown in fig. 2. Elements of A were then obtained from the difference between the parameters for the pairs of curves corresponding to each proton. Results and Discussion Couplings to five protons, which we initially label A-E, have been completely charac- terised.Their resolution into isotropic and principal dipolar components, and the orientation of the principal axes in a*b*c*, are shown in table 1 . Smaller proton couplings can be tracked to some extent but have not been analysed for the present. The spectra also show a number of weak features whose intensities are markedly orientation dependent; many of these lie at frequencies which are close to sums and differences ofN . M. Atherton and W. A . Crossland 39 18 17 16 15 14 13 0 ’2 g 11 % 10 2 9 ---. .* + k CI : 8 ; 7 6 5 4 3 2 1 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 4 0 42 ENDOR frequency/MHz Fig.2. Angular dependence of the ENDOR frequencies in the b*c* plane. The curves are least-squares fitted as described in the text. Table 1. Proton coupling tensors principal values/MHz direction cosines proton isotropic dipolar in a*b*c* A -40.3 18.5 0.803, - 0.553, - 0.222 3.0 -0.158, -0.557, 0.812 -21.6 0.575, 0.619, 0.535 B - 13.7 9.6 0.807, 0.333, 0.487 -9.4 -0.523, 0.787, 0.328 -0.2 -0.274, -0.519, 0.810 C 7.8 - 1.2 0.853, 0.206, 0.478 3.7 -0.332, 0.923, 0.193 -2.5 -0.403, -0.324, 0.886 D 9.6 - 2.4 0.126, 0.715, -0.688 3.7 -0.527, 0.636, 0.564 - 1.3 0.841, 0.291, 0.457 E 2.4 - 5.4 0.946. - 0.122. 0.302 0.6 -0.087, 0.799, 0.596 4.7 -0.314, -0.589, 0.745 those of strong transitions and we take them to be ‘forbidden’ transitions arising from simultaneous flips by two protons.No 14N ENDOR has been detected in these high-temperature measurements. Its value is of interest, the more so because ENDOR would reveal the quadrupole coupling, and we intend to pursue a low-temperature study. So far as the comparison between ENDOR and resolution enhancement is concerned40 ENDOR Study of y-Irradiated Pyridoxine Hydrochloride the latter appears to have the edge for this nucleus if one is restricted to room temperature. The pattern of hyperfine coupling for an a-proton, i.e. a proton in a ,C-H fragment whose C atom is part of a conjugated system, is well known:4 if the isotropic coupling constant is - 2a then the principal dipolar components are about (+a, 0, - a ) with the positive component lying along the C-H bond and the near-zero component along the axis of the 2p orbital on the C atom.A similar pattern is expected for an >NH fragment. The couplings to protons A and B conform to this pattern so these are clearly a-protons. The angle between the positive dipolar components is 69" and that between the small components is 8". This is consistent with the radical having structure I with proton A attached to C6, and lying close to the bisector of the H5C6H6 angle of the parent molecule, and proton B being the N-H proton, Hlo. However, these relative orientations would also hold for structure 11, which is the radical resulting from the loss of a hydrogen atom from C,. \ CHOH CH3 I1 It is useful to note that both I and I1 are iso-n-electronic with the benzyl radical. Now one readily sees that if the radical has structure I1 then proton B must be H,, the ring proton attached to C,.The choice between the two structures has to be settled by relating the orientations of the tensor components to crystal axes and hence to bond directions in the molecule. In our measurements the best-defined crystal axis is a*, which is perpendicular to the bc plane, and consideration of the crystal structure2 shows that a clear test is provided by the angles between the N-H,, and C,-H2 bonds and this axis. From the crystal structure we calculate the angles to be 37.5 and 89.5", respectively, while for the positive dipolar component of proton B the angle is 36". This essentially confirms that the radical is correctly assigned as I.The N-HI, centre is well removed from the site of the radiation damage, so one would expect no change of geometry on forming the radical and can confidently take the orientations of the tensor axes for proton B as defining the orientation of the ring in our axis system. Our results for proton A indicate that the plane of the O,C,H, group is not quite coincident with that of the ring, a point already remarked by Masiakowski et al. and discussed by them.' The distortion is not a small rotation about c3c6. Assignment of the remaining three proton tensors can be made by using the comparison with benzyl. We thus expect significant spin population at C, and C, and small, probably negative, spin population at C , and C,.The hydroxy proton at C , lies close to the plane of the ring in the undamaged crystal,2 and if this geometry is preserved in the radical the spin density at it should arise almost exclusively from spin polarisation and be small. On the other hand, in the CH,OH group at C,, O3 lies close to the plane of the ring so protons H, and H, should bear relatively high-spin densities from hyperconjugation. We assign protons C and D to these two positions. The isotropic couplings to such P-protons are often described by4 a = B,+B, cos20 (2) where 8 is the dihedral angle between the plane containing the axis of thep-orbital and the C-CH bond and that containing the C-C-H atoms. The contribution from spinN . M. Atherton and W. A . Crossland 41 polarisation is given by B,: it is much smaller than B, and often neglected.If this approximation is made the experimental isotropic couplings would indicate cos2 I!?~Jcos~ I!?,, = 1.25. This is not pleasing agreement with the value 1.09 which we calculate from the neutron diffraction data:, taken at their face value the observed couplings imply that in the radical H, and H, are much less symmetrically disposed with respect to the plane of the ring than they are in the undamaged crystal. However, this analysis is severely oversimplified. The protons in question must be virtually as close to the substantial spin population on C, as they are to the smaller one on C,, and this must be reflected in the couplings, whose analysis is thus not trivial. This consideration also complicates the interpretation of the dipolar couplings to these protons, in particular any geometrical information one might hope to obtain from the orientations of the principal components. The full theoretical analysis of the observed couplings thus offers an interesting problem.The magnitudes of the dipolar components for proton E do not have the characteristic pattern for an a-proton, but nonetheless we assign the proton as H,, attached to the ring carbon C,. From the analogy with benzyl we expect small (negative) spin population on this ring atom and under these circumstances the much larger spin populations on C, and N should play a crucial role in determining the dipolar coupling to the proton on the intervening carbon atom. The effect has been well understood since the definitive study by Heller and Cole of the HOOCaCHCHCH-COOH radical in irradiated glutaconic acid., For a planar C-C(H)-C fragment symmetry demands that one principal component of the coupling should be perpendicular to the plane: for proton E the 4.7 MHz dipolar component is 6" from the intermediate component for proton B, the N-H proton.This is very strong support for this assignment. The different spin populations on N and C, cause the in-plane components to be skewed with respect to the C5-H2 bond. The protons of the methyl group substituted at ring atom C, are expected to have small hyperfine couplings if our analogy with benzyl is valid. They could well be responsible for the small couplings which we have observed but not analysed, as mentioned previously.Analysis of these couplings might be facilitated by studying a crystal grown from D,O, and this will be pursued. A more serious feature of the present analysis is our failure to observe a signal from the hydroxy proton at the C(H)OH radical centre. It is not obvious that its ENDOR transition probability should be so dramatically less than those of the other protons. If the radical had ionised the resulting neutral radical might be formally isoelectronic with the radical anion of benzaldehyde so one might take a qualitative discussion of the spin density using this anion as a prototype. in fact this would not affect the general conclusions in any way: the pattern of spin distribution in the benzaldehyde is quite similar to that in benzyl,; for example, there are negative spin populations at the meta positions and the aldehydic proton has the largest coupling.To conclude, we can say that, despite a number of unanswered questions which may arise because we have deliberately restricted ourselves to room temperature in order to make a fair comparison with the Polish work,l this ENDOR study essentially confirms the identification of the trapped radical. This is a pleasing result, for resolution enhancement facilities are probably more easily obtainable than those for ENDOR and so may become more generally available. We thank the S.E.R.C. for funding most of the purchase of the ENDOR spectrometer, Croydon Education Authority for undergraduate support for W. A. C., Mr Alan Hall and Mr Brian Watson for the construction of crystal mountings and goniometers, and Mrs Jean Stevenson for general technical assistance.42 ENDOR Study of y-Irradiated Pyridoxine Hydrochloride References 1 J. T. Masiakowski, R. Krzyminiewski and J. Peitrzak, Chem. Phys. Lett., 1985, 116, 387. 2 G. E. Bacon and J. S. Plant, Acta Crystallogr., Sect. B, 1980, 36, 1130. 3 See, e.g., N. M. Atherton and A. J. Horsewill, Mol. Phys., 1979, 37, 1349. 4 See, e.g., N. M. Atherton, Electron Spin Resonance (Ellis Horwood, Chichester, 1973). 5 C. Heller and T. Cole, J . Chem. Phys., 1962, 37, 243. 6 N. Steinberger and G. K. Fraenkel, J . Chem. Phys., 1964,40, 723. Paper 6/999; Received 22nd May 1986

ISSN:0300-9599    

DOI:10.1039/F19878300037

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1987,83,43-49 The Use of Electron Spin Resonance and ENDOR and TRIPLE Resonance Methods for Structural Elucidation Isomeric 10,lO-Diphenylphenanthren-9( 1 OH)-ones Bernard0 J. Herold,* Maria J. Romiio and Jose' M. A. Empis Laboratdrio de Quimica Orgbnica, lnstituto Superior Tkcnico, Au. Rovisco Pais, 1096 Lisboa Codex, Portugal Jeffrey C. Evans and Christopher C. Rowlands Department of Chemistry, University College Cardig, P.O. Box 78, Cardig CFl 1 XL, Wales A series of substituted 10,lO-diphenylphenanthren-9( lOH)-ones has been reduced in ethereal solvents, and the resulting paramagnetic solutions have been examined by multiple resonance techniques. The values and relative signs of the various coupling constants proved useful and unambiguous in ascertaining the position of ring substitution in the starting material. These methods have therefore proved to be an alternative, if rather specialized means to determine which of several isomeric compounds is present.lO,lO-Diphenylphenanthren-9( 10H)-one [DPP, structure (3)] is the main product of the reaction of fluorenone (1) with diphenylchloromethyllithium (2) in tetrahydrofuran (THF) solution at low temperatures?! C H 6 5, /L' C6H5 (2) 0 A similar reaction starting with either 2- or hubstituted fluorenones could result in (a) either the 7-substituted DPP (5) and or the 2-substituted RPP (4), (b) the 3-substituted DPP (8) or the 6-substituted DPP (9), respectively (see below). 0 (4) (5) (6) 4, 5, 6: (a) X = C1 (b) X = OCH, (c) X = N(CH,), (d) X = F (e) X = CH, 4344 E.S.R.Resonance Methods for Structural Elucidation .x 0 (7) 7, 8, 9: (a) X = OCH, (b) X = F (c) X = CH, Predominance of isomers (6) and (9) over (5) and (8), respectively, means that substituent X will always maintain a through-resonance interaction with the carbonyl of the starting materials. The other possibility [i.e. predominance of (5) and (8)] means that such an interaction might arise between the substituent and the diphenylmethyl carbon. The full description of these reactions, as well as the yields of the various ketones (5, 6, 8 and 9)2 and their structures, previously determined by X-ray diffraction2, and by multinuclear n.m.r. spectroscopic techniques,2$ are given elsewhere. The structural similarity between the radical anion of DPP and that of an 'ortho- biphenylylketyl model ' (lo), enables the approximate electron spin distribution to be easily predictable.Hence, the spin distribution obtained from ENDOR and triple resonance experiments has been used to distinguish between isomerically substituted DPPS. Thus the conjugated system (10) can be considered as an odd-alternant radical bearing an anionic 0x0 substituent in position 9. In such a system, carbon atoms at even-numbered ring positions are expected to bear large positive spin densities, and those at odd-numbered ring positions, rather smaller and negative spin den~ities.~ Comparison of the substituted DPPs (5) and (6) with system (10) shows that in (5) the substituent occupies a low spin position, whereas in (6) it occupies a high spin position. The same reasoning shows that a low-spin-density carbon atom is substituted in the ketyl of (8), and a high-spin one in the ketyl of (9).Also, it has been established that substitution does not dramatically alter the electron spin distribution6$ in similar compounds. Structural elucidation therefore can in this case be obtained by determining whether the substituent is on a high positive or a low negative spin density carbon atom. Experimental 10,lO-Diphenylphenanthren-9( 1 OH)-one, (DPP) (3), was prepared as described earlier1 and purified to constant melting point. 7-Chloro-DPP (5a), 7-methoxy-DPP (5b) and 7-N,N-dimethylamino-DPP (5c) were obtained from the corresponding 2-substituted fluorenones (4a, 4b and 4c) and purified as describedB.J . Herold et al. 45 3-Fluoro-DPP (8b) and 6-fluoro-DPP (9b) resulted in the same way from (7b), 3-methoxy-DPP (8a) from (7a), 7-methyl-DPP (5e) and 2-methyl-DPP (6e) from (k), 3-methyl-DPP (&) from (7c), and 7-fluoro-DPP (6d) from (4); as above, the preparation, purification and identification of these compounds is described elsewhere.2 All other starting materials were used as commercially available or purified as previously Room-temperature electrolytic reductions of dimethoxyethane solutions of the various DPPs were carried out under vacuum. 0.1 mol dm-3 tetrabutylammonium perchlorate was used as a supporting electrolyte. The paramagnetic solutions obtained were stable for > 3 h at 200 K. E.s.r., ENDOR and triple resonance spectra were recorded at ca.200 K using a Varian E 109 spectrometer interfaced with a Bruker ENDOR/triple resonance unit. Computational work was carried out using a VAX/VMS system, and standard Huckel-McLachlan and linear regression analysis programs. Results 10,lO-Diphenylphanthren-9( 10H)-oneg (DPP) DPP (3) was reduced by a sodium film in dry oxygen-free tetrahydrofuran (THF) at 200 K. The brown solution had an e.s.r. spectrum comprising a featureless broad triplet (total width 1.5 1 mT). The ENDOR spectrum gave eight proton coupling constants and one sodium coupling constant [see fig. 1 (a)]. Their relative signs were determined by general triple resonance [see fig. 1 (b)]. The coupling constants are listed in table 1. These results are in agreement with others previously reported by Franco et aL9 Variable-temperature ENDOR and special triple resonance spectra [see fig.1 (c)] show that the smallest proton coupling is a multiple absorption, and that its optimum ENDOR temperature is higher than that observed for the other protons. An estimate of the multiplicity of this coupling was obtained by subtracting from the measured e.s.r. spectral width the width attributable to all the other coupling constants. In this way a multiplicity of 10 (k 1) was found for this hyperfine coupling. Electrolytic reduction of DME solutions of DPP gave a broad featureless e.s.r. spectrum. From this an ENDOR measurement gave seven coupling constants, one of which could be attributed to more than one proton. These values are similar to those obtained from the sodium reduced ketyl system.Substituted DPPs ENDOR, general and special triple resonance measurements on sodium reduced THF solutions of (5a), (5b), (5c) and (8a) gave the results listed in table 1. The measurements were made at 200 K immediately the solution became coloured. Complete sets of data were not obtained for all the other substituted DPPs, but the existing data were sufficient for st ruc t ure-elucidation purposes as described. Fluorine coupling constants were detected in the spectra obtained from various isomeric fluorinated DPPs. One of these gave an e.s.r. spectrum with a total width of 2.3 mT, and no apparent l9F ENDOR coupling, whereas the ENDOR spectrum of its isomer shows an unmistakable value of a(F) = 1.49 MHz. The compounds producing these results proved to be, respectively, (9b) and (8b); a(F) = 1.06 mT was measured from the e.s.r.spectrum of the ketyl of the former compounds. The ENDOR spectrum of the radical anion of (5d) shows a(F) = 8.01 MHz. For the various methylated DPPs, special triple and ENDOR spectra obtained at46 E.S.R. Resonance Methods for Structural Elucidation ( b ) 11 I MI 11 /I d d n a 5 MHz I ’ I 23 MHz I 0.1 MHz I 8 MHz Fig. 1. (a) ENDOR spectrum of a solution of sodium-reduced DPP in THF at 200 K. (b) General triple resonance spectrum obtained from DPP ketyl solution as above; the pump frequency was 13.013 MHz. (c) Special triple resonance spectrum obtained from DPP ketyl solution as above; the centre frequency was 13.875 MHz, starting at +O. 1 MHz. various temperatures easily pinpointed each isomer and confirmed the values of the methyl-proton coupling constants.These were determined to be 3.65 MHz for (5e), 2.49 MHz for (6e) and 0.328 MHz for (8c). Discussion Hiickel-McLachlan calculations of carbon spin densitiesg were carried out for the radical anions in table 1 using a set of literature values for the various resonance and Coulomb integral Calculations were performed on ‘substituted (10) model systems’, as described above, and no attempt was made to account for the electronic effects of the diphenylmethyl bridging between positions 9 and 10. As a consequence of thisTable 1. Hyperfine couplings obtained from ENDOR and general triple resonance spectra of some 10,lO-diphenylphenanthren-9( 10H)-one ketyl solutions, at 200 K ke tyl substituent/generation a,/MHz a,,/MHz 3'- none/electrolysis in DME - 15.02 -8.77 3.70 - 3.24 1 .86" 1.86" 0.86 O.3Ob - Sb'-Na+ 7-OCH3/THF -14.86 -10.56 5.36 -2.51 - - 1.47 0.72 0.29 1.26 3'-Na+ none/THF -15.54 -9.66 3.78 - 3.00 2.34 -1.80 0.90 O.3Ob 0.86 Sa'-Na+ 7Cl/THF -16.26 -10.08 3.37 - 2.63 - - 1.62 0.72 0.32 1.23 5c'-Na+ 7-N(CH3),/THF -14.23 -10.86 3.32 - 2.46 - - 1.44 0.72 0.29 1.14 8a'-Na+ 3-OCH3/THF -15.44 -9.67 3.77 - 2.77 2.22" 2.22" 0.85 0.28 0.95 a Double peak, as revealed by special triple resonance; the fact that no detectable general triple effect is observed is taken to mean the peak is due to two coupling constants of equal magnitude and opposite signs.Multiple peak (see text). Table 2. Calculated proton hyperfine couplings in (MHz), and linear least mean squares parameters for various lO,lO-diphenylphenanthren-9( 10H)-one ketyls position subs ti tuent ke t yl 6 8 5 2 7 4 QFH" rb SC ~~ - 3'-d - 14.52 - 8.07 4.86 -4.13 1.635 -3.10 - 7 1.32 0.990 75 > 99.9 - 3'-Na+e - 15.12 - 8.66 5.16 -4.12 1.966 -3.11 -74.01 0.988 55 > 99.9 7-C1 5a'-Na+e - 15.25 -9.57 5.04 - 4.23 - - 3.21 -75.24 0.980 63 > 99.0 7-OCH3 5b'-NaPe -13.40 -11.06 4.75 - 3.66 - - 2.68 -77.06 0.983 39 > 99.0 7-N(CH3), 5c'-Na+f - 14.57 - 11.37 5.07 - 3.92 - - 2.92 -76.27 0.978 57 > 99.0 3-OCH3 8a'-Na+e - 15.03 - 8.67 5.03 - 3.75 1.91 - 3.48 -72.91 0.990 21 > 99.9 a Q& is McConnell's constant, in MHz, determined by the least-mean-squares method for each case.r is the correlation coefficient. s is the significance of the correlation by Fischer's test, percentagewise.Huckel-McLachlan m.0. parameters used were, in units of pee:' 6, = 0.1; ~ 1 4 = 1.15; y,, = Y , , ~ , = 1.3; y11,12 = 0.8; y9,14 = 1.6. Same as above but 614 = 1.2, other parameters as in ref. (7). f Same as above but dC(C--N(CH3)2) = - 0.25, an arbitrary non-optimized value. P 2348 E.S.R. Resonance Methods for Structural Elucidation simplification, small equal calculated spin densities appear at positions 1 and 3. However, because of the approximate nature of the calculations, we prefer not to speculate as to the effect of this bridging group, even at the cost of disregarding some potentially useful data in the analysis that follows. In each case, the two smaller proton couplings were ignored. The others were compared with the calculated carbon spin densities using h e a r least-mean-squares analysis, which yielded best McConnell’s Q values and hence ‘ calculated ’ coupling constants.Table 2 lists these ‘calculated’ proton coupling constants and best Q values, in MHz, together with pertinent regression analysis data. These show excellent signifi- cance levels. One can point out that for the radical anions of compounds (5a, b and c ) listed in table 1 the triple resonance results would clearly have sufficed for structural elucidation, because a positive proton coupling, of ca. 2.3 MHz, is missing. This unmistakably indicates that substitution occurs at a low negative spin density carbon atom, i.e. at the 7 position and not 2. The remaining radical anion of (8a) (table 2) can similarly be proved to be a 3-substituted-DPP, because if it were the 6-substituted isomer the - 15.44 MHz coupling would not be present.Although incomplete, the data relative to the other compounds can easily be interpreted as compatible with what might be expected. Thus the fluorinated DPP ketyls, with a(F) values of ca. 1.060 mT, 1.49 MHz and 8.01 MHz, are, respectively, the 6-fluoro (9b), 3-fluoro (8b) and 7-fluoro (5b) ketyls. It is known that substitution of H for F in alternant systems can result in a ratio a(F)/a(H) = 2.1° This would confirm the hyperfine coupling constants for ring positions 7, 3 and 6. The same can be said for the methylated DPPs, where special triple resonance pinpoints the methyl-proton couplings of 3.65 MHz in the 2-methyl-DPP (6e) and 2.49 MHz in the 7-methyl-DPP (5c) ketyls.These results are in accordance with the fact that methyl-proton coupling constants closely match in magnitude the single proton coupling observed for the same position in the unsubstituted compound. The ca. 0.3 MHz absorption which is common to all these ENDOR spectra is assigned to the diphenylmethyl protons. This assignment not only yields a correct e.s.r. spectral simulation but is also comparable to results obtained by other authorsll in situations where 0-n: electron spin density transfer occurs. We believe that by confirming the earlier X-ray2V3 and n.m.r.2*4 data, these results show that ENDOR and triple resonance techniques can be of use in isomer structural elucidation. The scope of application is actually quite broad; in fact, the basic requirements for this rather simple approach to chemical structure elucidation are that aromatic or conjugated substrates be involved in rearrangement reactions whereby an ambiguity in migratory aptitudes might arise.Although admittedly specialized, the methods described are reasonably direct and, barring secondary chemical reactions, unambiguous. We thank M. L. Franco for helpful suggestions concerning the probable non-planarity of the DPP radical anion. Financial support for this work is gratefully acknowledged from NATO, for a cooperative research grant, and from ‘Instituto Nacional de Investigaqgo Cientifica’, through ‘ Centro de Processos Quimicos da Universidade Tecnica de Lisboa’. A grant from the S.E.R.C. towards the purchase of an ENDOR spectrometer by University College Cardiff is also gratefully acknowledged. References 1 J. M. A. Empis, M. L. T. M. B. Franco, B. J. Herold and J. J. R. P. Queiroga, Tetrahedron Lett., 1975, 2 B. J. Herold, M. J. Romgo, C. Krueger and R. Mynott, presented in part at 7 O Encontro Anual da 47, 4153. Suciedade Portuguesa de Quimica, Lisbon, July 1984, p. PCl1, to be published.B. J . Herold et al. 49 3 M. J. Romiio and C . Krueger, Acta Crystaflogr., Sect. C., in press. 4 B. J. Herold, R. Mynott and M. J. Romiio, to be published. 5 N. M. Atherton, in Electron Spin Resonance (Ellis Horwood, Chichester, 1973). 6 J. M. A. Empis and B. J. Herold, J . Chem. Soc., Perkin Trans. 2, 1986, 425. 7 B. J. Herold, J. M. A. Empis, J. C. Evans and C. C. Rowlands, J . Chem. Soc., Perkin Trans. 2, 1986, 8 W. Schlenk and E. Bergmann, Ann. Chem., 1928,463, 209. 9 M. L. Franco, B. J. Herold, C. C. Rowlands and J. C. Evans, presented in part at the 5 O Encontro Anuaf 431. da Sociedade Portuguesa de Quimica, Oporto, p. C29.58, (1982). 10 P. H. H. Fischer and J. P. Colpa, 2. Naiurforsch., Teil A, 1969, 24, 1980. 1 1 R. Biehl, K. P. Dinse, K. Moebius, M. Plato, H. Kurreck and U. Mennenga, Tetrahedron, 1973, 29, 363. Paper 61998; Received 22nd May, 1986

ISSN:0300-9599    

DOI:10.1039/F19878300043

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I, 1987,83, 51-55 Electron Spin Resonance, ENDOR and TRIPLE Resonance of some 9,lO-Anthraquinone and 9,lO-Anthraquinol Radicals in Solution Mikko Vuolle" and Reijo Makela Department of Chemistry, University of Jyvaskyla, Kyllikinkatu 1-3, SF-40100 Jyvaskyla, Finland E.s.r., ENDOR and TRIPLE resonance spectra have been recorded for 2-methylanthraquinone, anthraquinone-2-sulphonate and anthraquinone- 2,6-disulphonate radical anions, and in a strongly acidic medium for 2-methylanthraquinol, anthraquinol-2-sulphonate and anthraquinol-2,6- disulphonate radical cations. The hyperfine coupling constants (h.f.s.) and g factors are given. The ENDOR spectra show there to be more h.f.s. than were detected earlier by e.s.r. spectroscopy. The spectra of the di-deutero radicals of the anthraquinols and the assignment of h.f.s.are discussed. Quinones are perhaps the most widely studied organic redox system. Semiquinones are relatively stable free radicals in alkaline media, formed by one-electron reduction of quinones or by one-electron oxidation of quinols. In 1961 Vincow and Fraenkell examined the 9,lO-anthrasemiquinone radical anion by e.s.r., and in 1962 Bolton et aZ., examined the 9,lO-anthraquinol radical cation in strong acidic media. Many mono- and di-substituted 9,lO-anthrasemiquinone radical anions have been examined by e.s.r., but not many radical Only a few ENDOR and TRIPLE spectra of anthrasemiquinone radical anions and semiquinol radical cations have been published. Assignment of the splitting constants of aromatic protons requires comparison with data for substituted radicals having the same basic anthraquinone structure.We have measured the e.s.r. and ENDOR spectra of the radical anions and cations of eight compounds, but publish here the spectra of only three compounds. Most of the splitting constants of semiquinone radical anions included in the tables were originally assigned by the authors. However, some of these are incomplete and we are currently revising all assignments in the light of the better-resolved ENDOR and TRIPLE spectra. Quinones play an important role in biological processes and anthracycline anticancer drugs contain both quinone and hydroquinone structures. Experimental Materials 2-Methyl-9,lO-anthraquinone 97 % and 9,l O-anthraquinone-2,6-disodiumsulphonate (m.p.> 325 "C) from Aldrich and 9,1O-anthraquinone-2-sodiumsulphonate (puriss. p.a.) from Fluka were used without further purification. Other chemicals were Na,S,O, (Merck, lab.), D,SO, (Merck, 96-98 % , deuteration degree not less than 99% ), CF,SO,H (Fluka, purum), FS0,H (Fluka, techn.) S0,ClF (Cationics Inc.), KO, (Fluka, pract.), DMSO (Merck, scintillation grade), 18-crown-6 (Fluka, purum), linolic acid (Merck, p.a.), TCNE (Aldrich, 98%) and NH, (Merck, 99.9%). 5152 9,lO-Anthraquinone and 9,l Q-Anthraquinol Radicals Equipment E.s.r. spectra were recorded on a Varian E-9 spectrometer with a field-frequency lock, a Varian variable-temperature control unit, a Takeda Riken Industry Co TR 5211 microwave counter, a Varian E 500 gauss meter and an Apple I1 computer.ENDOR and TRIPLE spectra were measured with a Bruker ER 200 D-SRC spectrometer (laboratory-made ENDOR coil). U.V. illumination was provided by an Airam HQU 300 W mercury lamp. Sample Preparation Samples were prepared in two different ways, by a high-vacuum technique or using a laboratory-developed flow system. Semiquinone radicals were generated by Na,S,O, reduction in 0.1-0.05 mol dm-, NaOH ethanolic water solution, where the ethanol concentration varied from 10 to 90% .g DMSO was added to a mixture of the appropriate quinone, KO, and 18-crown-6, and the sample was degassed before use with two freeze-pumpthaw cycles on a vacuum system. Ammonia was distilled under a nitrogen atmosphere into an e.s.r. ampoule containing a very small piece of alkali metal and a quinone.The system was maintained over a cold bath at -85 "C and the sample was sealed under high vacuum. Radical cations were prepared by dissolving the corresponding quinone in H,SO,, D,SO,, FS0,H or CF,SO,H and the samples were sealed under high vacuum. Radical cations were also produced by dissolving the parent compound in a mixture of D,SO, and SO,ClF, FS0,H and D,SO, or FS0,H and S0,ClF. U.v.-irradiation of solutions, if necessary, was carried out using a mercury lamp outside the e.s.r. cavity. Some of the samples were irradiated overnight because of the slow formation of semiquinones. Results and Discussion After irradiation of 9,lO-anthraquinone by U.V. light in a mixture of TCNE and ethanolic NaOH aqueous solution (80 : 20), the general TRIPLE spectrum was observed.The spectrum shows the coupling constants of anthrasemiquinone anion radical to be of the same sign, in contrast to the INDO calculation reported in table 1. Analysis of the e.s.r. spectrum recorded from 9,lO-anthraquinone dissolved in a mixture of D,SO, and CF,SO,H shows hydroxy protons, a = 0.1 17 mT (ENDOR), to be exchanged for deuterons a = 0.018 mT. Fig. 1 shows the ENDOR and TRIPLE spectra of' the 2-methylanthrasemiquinone radical anion produced by reduction of 2-methylanthraquinone with sodium dithionite in alkaline ethanol-water solution. A careful comparison of the e.s.r., ENDOR and special TRIPLE spectra gives the ENDOR coupling constants shown in table 1. The methyl protons and one ring proton are magnetically equivalent, as noted earlier by Brumby., Except for the coupling constants of protons 4 and 5, values obtained from the ENDOR spectrum are different from Brumby's e.s.r.values. According to the general TRIPLE experiment, the coupling constants of the methyl protons are of opposite sign to those of the ring protons. 2-Me thylanthraquinol cation radical was generated by dissolving 2-met hylan t hra- quinone in CF,SO,H; the e.s.r., ENDOR and TRIPLE spectra are shown in fig. 2 and coupling constants in table 2. The general TRIPLE spectrum shows that the smallest coupling constant has an opposite sign and may belong therefore to the methyl protons. Attempts were made to generate the deuterated radical cation of 2-methylanthraquinol, but a resolved spectrum has not yet been obtained.The ENDOR coupling constants of the semiquinone anion radical of 2,6-disulphonateM . Vuolle and R. Makela 53 Table 1. Hyperfine splitting constants (mT) of semiquinone anions 2-met hylanthra- anthraquinone- anthraquinone- semiquinone 2,6-disulphonate 2-sulphonate 9,lO-anthraquinone ENDOR e.s.r.a ENDOR e.s.r.b ENDOR e.s.r.b ENDOR INDOC 0.106 0.105 0.094d 0.09P 0.094 0.095 0.052 0.052 0.052 0.052 0.074 0.073 0.086 0.090 0.063 0.061 g = 2.004 04e 0.025 0.039 0.109 0.123 0.033 0.040 0.025 0.039 0.109 0.123 0.033 0.040 g = 2.004 41" - __ - - 0.083 0.075 0.097 0.08 - - 0.056 -0.02 0.122 0.124 0.028 0.028 0.059 0.053 0.094 0.097 0.083 0.079 0.046 0.053 - - - - - - - - - - _. - g = 2.004 08e g = 2.004 02e a Ref. (4). Ref. (7). Ref. (10). CH, group.Our e.s.r. value. m 12 13 m 14 15 m 16 MHz Fig. 1. (a) ENDOR and (b) general TRIPLE spectra of the semiquinone radical anion produced by reduction of 2-methylanthraquinone with Na,S,O,. produced under the reducing conditions of a sodium-ammonia system are listed in table 1 . The general TRIPLE spectrum shows them to be of the same sign for all protons. When anthraquinone-2,6-disodiumsulphonate was dissolved in CF,SO,H the ENDOR and TRIPLE spectra shown in fig. 3 were observed. According to the general TRIPLE spectrum, all coupling constants have the same sign. The highest temperature for the ENDOR measurement was 40 "C using CF,SO,H as solvent; the e.s.r. spectrum was measured at 50 "C and reveals the necessity of higher temperature for good resolution.54 9,1O-Anthraquinone and 9,1O-Anthraquinol Radicals I .I II” ‘ I I 8 m 8 8 8 8 10 12 14 16 18MHz 8 8 8 8 8 1 0 1 2 3 4 5 MHz Fig. 2. (a) E.s.r., (b) ENDOR, (c) special TRIPLE and (d) TRIPLE spectra of the radical cation produced by dissolving 2-methylanthraquinone in CF,SO,H. Table 2. Hyperfine splitting constants (mT) of anthraquinol cations (ENDOR) anthraquinol- 2-sulphonate ~ anthraquinol-2,6- SO,ClF/ D,SO,/ 9,lO-anthraquinol 2-methylanthraquinol disulphonate e.s.r. FS0,H FS0,H d2504 0.378 0.157 0.158 0.184 0.183 0.158 0.363 0.145 0.150 0.161 0.165 0.099 0.268 0.111 0.113 0.144 0.156 0.018 (D) 0.235 0.115 - 0.131 g = 2.003 13 0.093 0.02 1 0.1 12 0.1 10 g = 2.003 08 0.196 g = 2.003 16 0.089 0.088 g = 2.003 90 (deuterated) 0.009 g = 2.002 83 The coupling constants of anthrasemiquinone-2-sulphonate anion radical, which all have the same sign, are shown in table 1.According to the special TRIPLE spectrum, there are two protons with the same coupling constant, 0.053 G. Table 2 gives the coupling constants of cation radicals generated by dissolving anthraquinone-2-sulphonate in a mixture of S0,ClF and FS0,H or D,SO, and FS0,H. The e.s.r. spectra are shown in fig. 4.M. Vuolle and R . Makela 55 " I = 8 rn 8 1 1 12 13 14 15 16MHZ PF 1 (c ) Fig. 3. (a) E.s.r., (b) ENDOR and (c) general TRIPLE spectra of the radical cation produced by dissolving anthraquinone-2,6-disodiumsulphonate in CF,SO,H. Fig. 4. The e.s.r. spectra of the radical cation produced by dissolving anthraquinone-2- sulphonate (a) in a mixture of S0,ClF and FSO,H, (b) D,SO, and FS0,H. References 1 G. Vincow and G. K. Fraenkel, J. Chem. Phys., 1961,34, 1333. 2 J. R. Bolton, A. Carrington and J. dos Santosveiga, Mol. Phys., 1962, 5, 465. 3 P. J. Baugh, G. 0. Phillips and J. C. Arthur Jr, J . Phys. Chem., 1966,70, 3061. 4 S. Brumby, J. Magn. Reson., 1979, 34, 317. 5 J. A. Pedersen, Handbook of EPR Spectra from Natural and Synrhetic Quinones and Quinols (CRC 6 J. A. Pedersen, J . Magn. Reson., 1984, 60, 136. 7 N . J. F. Dodd and T. Mukherjee, Biochem. Pharm., 1984,33, 379. 8 J. A. Pedersen and R. H. Thomson, J . Magn. Reson., 1981,43, 373. 9 S. I. Bailey, Chem. Austr., 1983, 50, 202. Press, Boca Raton, Florida, 1984). 10 J. A. Pople and D. L. Beveridge, Approximate Molecular Orbital Theory (McGraw-Hill, New York, 1970), p. 138. Paper 61853; Received 1st May, 1986

ISSN:0300-9599    

DOI:10.1039/F19878300051

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1987, 83, 57-68 Electron Nuclear Double Resonance of S = 1/2 Defects in a Single Crystal of the Morpholinium-TCNQ 1 : 1 Complex Anna Lisa Maniero, Ornella Priolisi and Carlo Corvaja* Department of Physical Chemistry, University of Padova, Padova, Italy Single crystals of the rnorpholinium-TCNQ 1 : 1 complex give e.s.r. spectra showing a central resonance at g = 2 in addition to the lines of thermally excited triplet excitons. ENDOR spectra recorded by saturating the central line show that it is due to isolated TCNQ anions which are produced at two different crystal defects. The hyperfine tensors of the TCNQ anion obtained from ENDOR give a spin density distribution in the solid much different from that in solution. The polarization of the spin distribution is consistent with a rearrangement of the morpholinium cations around the anion in the defect.The 1 : 1 morpholinium (M) salt of the TCNQ radical anion crystallizes with the TCNQ molecules stacked in columns with alternate large and small distances between them.l The morpholinium cations occupy lattice positions between the stacks. In such a crystal structure the ‘molecular unit’ in the crystallographic unit cell is a (TCNQ)2- dimer having a singlet ground state and a thermally accessible triplet state. The triplet excitation migrates through the crystal and constitutes a mobile triplet exciton whose e.s.r. lines are very narrow (200 mG), any hyperfine interaction being averaged out by the fast motion. Besides triplet exciton lines, the e.s.r.spectra of the M-TCNQ salt show lines due to spin-1/2 species which are the result of crystal defects or impurities. Much speculation has appeared in the 1iteratu1-e~~~ as to the nature of this species and its temperature behaviour.2v 4 7 Little can be said from the only analysis of the e.s.r. spectrum, since no hyperfine structure is resolved, and the small anisotropy of the g factor is difficult to correlate witb the molecular structure. In this work we have used ENDOR spectroscopy6 to investigate the nature of the spin-1/2 defect in M-TCNQ. Owing to the inherently high spectral resolution of ENDOR we were able to measure several hyperfine couplings of this species which in the e.s.r. spectrum give rise only to an inhomogeneous broadening. We have determined by ENDOR the complete hyperfine tensors (isotropic and anisotropic parts) of eight protons which were found present in two different defects both containing TCNQ.This knowledge allows us a detailed insight of the structure and of the electron spin distribution of the paramagnetic species. A knowledge of the spin distribution is also of interest in relation to another important aspect. The fine-structure parameters D and E of the triplet exciton spin hamiltonian DS; + E(S: - S i ) are often calculated by taking the (TCNQ); dimer geometry as found by X-ray diffraction studies and an appropriate spin density distribution.’ The latter is obtained by assuming it to be the same as for the TCNQ- anion in liquid solution. The discrepancy between calculated values (always larger than experimental values) was attributed to delocalization of the triplet state over more than two molecules*~ or by assuming a triplet species formed by the more distant TCNQ neighbours instead of the near neighbours.1° Flandrois and Boissonade7 proposed that the discrepancy had to be attributed to a different spin density distribution in the solid.They showed that there is a quite large variation of calculated zero field splitting parameters on different choices 5758 ENDOR Spectra of the Morpholinium-TCNQ Complex of the spin density distribution. Their proposal is relevant in relation to another problem, the determination of the charge-transfer character of the charge-transfer complexes in their excited triplet states from the measured hyperfine coupling constants.ll7 l2 The charge-transfer character is usually obtained from the ratio of the hyperfine coupling of the complex to that of the uncomplexed molecule.This procedure is correct only when there is no spin redistribution in the solid owing to the charge interaction. Finally, there is another need for accurate data concerning the spin densities and isotropic coupling in crystals of TCNQ salts. Their knowledge is a prerequisite for obtaining information on the exciton motion and its dimensionality through measure- ments of nuclear spin relaxation times, T.l3 Experimental Single crystals of morpholinium-TCNQ complexes were obtained by the method of Melby et a1.14 Reddish-purple prisms of the 1 : 1 complex were grown from acetonitrile solutions with molar ratios of TCNQ to morpholinium iodide between 0.3 and 0.4.The solutions were prepared with acetonitrile that had previously been distilled under nitrogen and kept in a nitrogen atmosphere, with due care being taken to minimize the presence of water and oxygen.l59 l6 TCNQ was repeatedly vacuum-sublimed while the morpholinium iodide, prepared from morpholine and HI, was purified by recrystallization from ethanol. M-TCNQ crystallizes in the triclinic system, space group P I , with two molecules in the unit cell of dimensions a = 9.54 A, b = 8.78 A, c = 10.60 A, a = 98.4", j? = 122.9", y = 93.5O.l Single crystals of M-TCNQ were mounted inside a small Plexiglas cube to facilitate their mounting on a gonionieter rod along three orthogonal axes. E.s.r. and ENDOR spectra were recorded at 300 K for magnetic-field directions in the three orthogonal planes.The spectrometer was a Bruker ER 200 D X-band spectrometer equipped with an ENDOR accessory and an EN1 300 W r.f. amplifier. TRIPLE resonance17 experiments were performed using a Systron Donner 5000 A r.f. sweeper to supply the additional r.f. field at the ENDOR transition frequencies. The r.f. frequency was measured by a Hewlett-Packard 5342A frequency counter. Results The e.s.r. spectra of the M-TCNQ complex have already been discussed, as well as the variation of intensity and linewidth with The room-temperature e.s.r. spectra show a pair of motionally narrowed lines due to thermally activated triplet excitons. The magnitudes of the zero-field splitting parameter, D and E, determined from the angular dependence of the line separation, agree with the previously reported values.2T In addition to the triplet lines a central resonance is present composed of two components, a broad one (AH from 3 to 12 G depending on orientation) and a narrow one (AH = 0.8 G).Both have a thermally activated intensity.2 ENDOR experiments were performed by setting the magnetic field at the centre of the central e.s.r. resonance where both components are saturated, even though the ENDOR spectra are due to the broad component only. In fact, the same ENDOR spectrum, although weaker, was obtained by setting the magnetic field at the wings of the broad resonance where the narrow one does not contribute to the e.s.r. intensity. A typical ENDOR spectrum in the range 10-20 MHz in the region of the protons' absorption is shown in fig.1. It consists of eight lines whose frequency dependence on the crystal orientation with respect to the magnetic field was fitted to the equation18 v2 + - 1 + cos2 # + B , sin2 # - 2C+ sin # cos 41. - - - -A . L. Maniero, 0. Priolisi and C. Corvaja 59 Fig. 1. ENDOR spectra of an M-TCNQ single crystal for arbitrary orientations of the magnetic field. The spectra are due to protons (1&20 MHz) and to nitrogen nuclei (4-9 MHz). The spectra were recorded by saturating the broad central line of the e.s.r. spectrum. A + , - - B , and C+ - are related to the hyperfine tensor elements by the equations where the indexes i andj (i, j = x, y , z ) refer to a particular plane where the magnetic-field direction is rotated.vH is the free proton frequency at the given magnetic field intensity. Fig. 2 shows the variation of the ENDOR frequency with the crystal orientation. Eight tensors were determined whose principal values and directions are reported in table 1. In addition to the abovementioned ENDOR lines, the spectra show other lines with smaller (ca. 1 MHz) hyperfine separation. These are due to protons not directly bonded to the paramagnetic centre, and their analysis has not yet been made in detail. Moreover, many other lines were recorded with frequencies ranging from 2 to 10 MHz (see fig. 1). Their intensity is strongly dependent on the crystal orientation. These resonances are to be attributed to the hyperfine interaction of the unpaired electron with the N nuclei of the CN groups.Discussion Models of Spin-1/2 Defects The central resonance line in the e.s.r. spectrum of M-TCNQ is typical of many TCNQ The concentration of the species giving rise to this anomalous resonance appears to be a characteristic of the salt and not of the preparative method.lg Thus the central resonance is not to be simply ascribed to a doublet-state chemical impurity, since the60 ENDOR Spectra of the Morpholinium-TCNQ Complex Fig. 2. Frequencies of the proton ENDOR lines for magnetic field orientation in three orthogonal planes. Only half the experimental points are shown in the figure. Table 1. Principal values and directions of protons in the I and I1 defects in M-TCNQ salt proton . Ia Ib Ic Id IIa IIb IIC IId isotropic value/MHz - 6.29 - 5.35 -2.10 - 2.20 -5.14 - 5.29 - 3.00 - 2.54 anisotropic principal values/MHz direction cosines x Y -7 4.05 -2.58 - 1.47 3.63 - 2.00 - 1.63 2.52 - 0.88 - 1.64 2.62 - 0.92 - 1.70 3.53 - 2.00 - 1.53 3.67 -2.19 - 1.48 2.92 - 1.31 - 1.61 2.69 - 1.01 - 1.68 0.1646 0.7477 0.2582 0.6820 0.0640 0.6006 0.2035 0.6175 0.1073 0.6984 0.2884 0.601 5 0.0808 - 0.6028 0.7938 0.22 10 - 0.6540 0.7235 - 0.6433 - 0.6843 - 0.7970 - 0.7598 - 0.7076 -0.7450 0.6266 - 0.5879 -0.5 173 - 0.8338 - 0.2005 - 0.5 1 44 0.6752 0.4777 0.5620 0.1947 0.5148 0.6226 -0.8349 - 0.602 1 - 0.4999 -0.8364 - 0.2205 -0.5018 0.6598 0.6293 0.4 107 0.28 12 0.5036 -0.8169 -0.7618 -0.3180 - 0.5644 0.4880 - 0.7034 - 0.5 169 - 0.7348 0.3695 0.5687 0.51 14 0.6203 0.5948 -0.7752 -0.3869 - 0.4994 0.466 1 - 0.7679 -0.4395 -0.7471 0.4905 0.4486 0.5328 0.7023 0.472 1A .L. Maniero, 0. Priolisi and C. Corvaja 61 latter would be expected to be present in a concentration strongly dependent on the preparation of the crystals. This observation favours the hypothesis that the paramagnetic species are intrinsic crystal defects. Also, the fact that the intensity of the line increases with increasing temperature2* and that this variation is reversible, points to the presence of intrinsic crystal defects. Buckman et al. measured the variation of the e.s.r. linewidth with the crystal orientation and they found the same linewidth anisotropy as for the triplet excitons.20 Since the linewidth of the latter ones is due to unresolved hyperfine structure the authors suggested that the central e.s.r.line derived from a species, possibly TCNQ-, with the same orientation as the TCNQ, dimers. Several models have been proposed in order to describe the defects. Their validity is examined in the following and checked on ihe basis of the ENDOR results. (1) The defects consist of isolated TCNQ, dimers with a single unpaired electron and unit negative charge. These species could be formed from disproportionation according to the following equation :5 2[TCNQ];- e [TCNQ]; + TCNQ2- + TCNQ-. In this hypothesis the obvious question arises as to the role of the two individual molecules in the dimers : are both molecules equivalent, or in other words is the unpaired electron evenly shared by the two? If not, how much is the electron spin transfer from one molecule to the other? (2) The defects consist of isolated TCNQ- molecules. In addition to the above equation, TCNQ- radicals could arise as solitons.These may be formed in the alternating chain of large and small distances between the TCNQ planes as discussed later in more detail. (3) Another model which was proposed for the species giving the central resonance is that of Wannier-Mott excitons, elementary excitations with loosely bonded electrons and holes.21 The linewidth would be due to unresolved electron-electron dipolar interactions. The ENDOR experiments immediately rule out this model, since they show that the cause of the e.s.r. linewidth is a hyperfine interaction with a limited number of nuclei. (4) Quite recently it was shown that the powder spectra of weakly perturbed triplet states, such as the triplet excitons in the complex (&-AsCH,t)(TCNQ);, are dominated by a sharp central line which is derived from the effects of Heisenberg spin exchange.22 The authors also suggested that in single crystals of other materials the central line could have the same origin and be due to the presence of amorphous regions in the crystals.This interpretation is discarded by the ENDOR experiments for the same reason as in point (3). Models (1) and (2) remain for discussion in the following section. The Structure of the Spin-1/2 Defects The ENDOR spectra of M-TCNQ single crystals in the proton region, recorded by saturating the central resonance line for an arbitrary orientation of the magnetic field, consist of eight pairs of lines due to the hyperfine interaction of eight different protons.Although by using ENDOR one cannot determine the number of nuclei giving rise to a particular resonance line, we exclude the assumption that more than one proton contributes to each ENDOR line on the basis of the determined values of the spin densities, as we will show later. The eight protons do not belong to the same paramagnetic species, as we showed by general TRIPLE res0nance.l' This experiment consists of irradiating the spin system with three radiation fields. One saturates an e.s.r. transition and at the same time irradiates the sample with r.f. radiation at the frequency of an ENDOR line and then observes the effect of a variable-frequency third r.f. field. The latter has an effect only when its frequency matches the value corresponding to62 END0 R Spectra of the Morpholinium-TCNQ Complex Table 2.Experimental spin densities" on C-H bonded carbon of TCNQ- ion radical and of I and I1 defects H H H H TCNQ- I I1 ref. (24) proton positionb Pi Pi Pi ~~ . -~ a 2 0.0948 0.0775 0.0662 b 6 0.0806 0.0797 0.0662 C 5 0.03 17 0.0452 0.0662 d 3 0.033 1 0.0383 0.0662 ZPii 0.2402 0.2407 0.2648 a From e.s.r. and ENDOR hyperfine data, with pi = laHl/lQ&I, taking Q& = -23.7 G = 66.36 M H z . ~ ~ For attribution, see text. another ENDOR line of a nucleus coupled to the same unpaired electron. In fact only in this case does the third r.f. field induce transitions in the same manifold of levels. In this way we were able to distinguish two sets of four protons.The pumping of one ENDOR transition has an effect only on the ENDOR lines of the protons in the same set. The hyperfine tensors obtained from an analysis of the variation of the ENDOR frequencies with the crystal orientation and reported in table 1 are accordingly collected in two different sets (I and 11). The ENDOR lines of the protons of set I1 are always (at all crystal orientations) lower in intensity by a factor of ca. Q than those of set I. Although the ENDOR line intensity is not in general a direct measure of the concentration of the species involved, as the two defects are expected to have similar relaxation properties, this indicates that the ratio of the concentrations of the two species is ca. 3 : 1 . Table 1 shows that all proton tensors have a principal axis (the last given in table 1) along the same direction.This one turns out to be almost parallel to the principal axis of the triplet exciton electron-electron dipolar interaction corresponding to the largest eigenvalue. This is perpendicular to the planes of the TCNQ molecules. The above observations support the idea that both defects I and I1 consist of TCNQ- molecules with the same orientation as those in the dimer stack. Further support for this comes from the consistency of the following analysis and from the fact that nitrogen ENDOR lines were also recorded. The isotropic hyperfine couplings of ring protons are related to the 71.-carbon spin density by the McConnell equation, a = Qp.23 With the assumption of a Q value of 67 M H z , ~ ~ the spin densities reported in table 2 are obtained.For comparison the values for a free TCNQ anion in solution are also reported in the same table. If one considers the total spin density on the four ring positions one obtains for both I and I1 almost the same value as for the free TCNQ anion. The assumption of a defect structure containing two equivalent TCNQ molecules connected by an inversion centre, and that the ENDOR lines are due to the couplings of pairs of magnetically equivalent protons, would give an unrealistically high total spinA . L. Maniero, 0. Priolisi and C . Corvaja 63 density on the eight (four for each molecule) 2, 3, 5, 6 positions. Moreover, if the ENDOR lines were assigned to pairs of protons the e.s.r. linewidth would be larger than the measured one.In this way we exclude the symmetric dimer model and come to the conclusion that the defects giving rise to the broad e.s.r. line are isolated TCNQ anions; if another (or more) TCNQ molecule is involved in the formation of defects I and I1 the spin density transfer from one molecule to the other(s) should be very small. Spin Density Distribution Inspection of table 2 shows that the spin distribution is strongly polarized. Before coming to the possible cause of this polarization we first examine how the different couplings and spin densities are to be attributed to the single positions. This is done on the basis of the anisotropic part of the hyperfine tensors. The anisotropic hyperfine tensor of protons in a n-radical can be computed from a known spin density distribution.The tensor elements have contributions from the 'local' spin density on the carbon atom bonded to the considered proton, as well as 'non-local' contributions coming from spin densities on the other n-centres. McConnell and Strathdee have given the expressions far the dipolar interaction of a proton with the unpaired spin density in a Slater p - orbital. 25 For the local contributions better results are obtained by taking Ti = 35.0 MHz, qj = 5.3 MHz and Tkk = -40.4 MHz, where i and j are, respectively, two perpendicular axes along the C-H bond and along the p-orbital, with k completing the orthogonal set.26 Comparing the principal values and the directions of the calculated anisotropic tensors with the experimental values of table 1, one has a test of the goodness of the assumed spin distribution and of the correctness of the attribution of the couplings to the individual positions.A first guess for the attribution is based on the fact that at least for the positions bearing the largest spin density we expect the anisotropic tensor to be to a large extent dominated by the local contribution and therefore to have one principal axis approximately along the C-H bond. This principal direction should correspond to the largest eigenvalue. This consideration and the data of table 1 allow us to assign protons a and b to positions 2 and 3 (case A) or 2 and 6 (case B), excluding the other possibilities (case C) (fig. 3) since the corresponding principal directions are not parallel, making an angle of ca.150" (or 30"). On the other hand, these principal directions are almost parallel (within 5") to the corresponding directions of the protons c and d. This is true for both defects I and 11. We have performed calculations of the dipolar tensor elements for the two cases A arid B by taking the spin density at the carbon centres bonded to a proton as given by the isotropic coupling constant and the McConnell equation. For the spin densities on the other centres we had to rely on the values calculated for the TCNQ anion by semi-empirical methods24 (set 1) or by an ab initio method27 (set 2) (table 3). In order to facilitate a discussion of the results of the calculations, we denote by ti the principal directions of the dipolar tensor of the proton i corresponding to the largest principal value.For a 7~ free radical with unit spin density on the carbon atom, as mentioned before, t corresponds to the C-H bond direction. When the spin density is also present on other carbon atoms the 5 direction is rotated away from the CH bond; this is why the experimental tensors have directions making angles other than 0 and 60" (120") as is the case for the C-H bond directions. Both choices of spin distrib~tions,~~? 27 when applied to model A, give dipolar tensors with reasonable principal values, but the angles are far from the experimental ones. In particular, the calculated angle between rz and c5 and that between r3 and (close to zero according to the experiments) are too large. Apparently there is no way to improve the situation 3 FAR 164 ENDOR Spectra of the Morpholiniurn-TCNQ Complex A B C Fig.3. Different models of spin density distributions for TCNQ- in M-TCNQ according to the possible attributions of the proton tensors. The dimensions of the circles indicate the magnitudes of the spin density on the carbon atoms. by a slight change in the spin density distribution. Conversely, model B gives satisfactory results not only for the principal values but also for the angles between the principal axes with both spin density distributions. We note that the largest coupling tensors were best accounted for by the spin densities of set 1, while the smallest ones were best accounted for by the other set of spin densities, which involves a larger spin density on C4. An improvement is therefore obtained by allowing a change in the spin density distribution, and the best agreement is obtained by using set 3 shown in table 3.The principal values of the proton dipolar tensors are reported in table 4, while the angles between the principal directions of the tensors are given in table 5. Formation of TCNQ- Defects in M-TCNQ Crystals Isolated TCNQ- radicals in the M-TCNQ crystal may arise as solitons in the alternating chain of large and small distances in the TCNQ stack, in a manner similar to that in a polyacetylene polymer chain.28 The model is schematically shown in fig. 4. It is worth mentioning that in an isolated polyacetylene chain the ground state is degenerate, and this fact allows for a certain freedom of movement of the soliton.In the case of a stack of charged TCNQ molecules there is a strong interaction with the surrounding cations not symmetrically placed around the TCNQ anion (TCNQ is not placed at an inversion point in the crystal lattice).l This interaction removes the degeneracy of the ordering of the short and large distances and freezes the motion.A . L. Maniero, 0. Priolisi and C. Corvaja 65 Table 3. Spin densities on the n-centres of the TCNQ- radical ~ set 3" atom set l a set 2b I I1 set dd 1 0.0494 2 0.0539 3 0.0539 4 0.0494 5 0.0539 6 0.0539 7 0.2245 8 0.2245 9 0.0126 10 0.0126 11 0.0 126 12 0.0126 13 0.0465 14 0.0465 15 0.0465 16 0.0465 0.132 0.061 0.061 0.132 0.061 0.061 0.180 0.180 0.002 0.002 0.002 0.002 0.03 1 0.03 1 0.03 1 0.03 1 0.048 0.094 0.033 0.108 0.032 0.08 1 0.218 0.190 0.0 12 0.002 0.012 0.002 0.045 0.039 0.045 0.039 0.048 0.077 0.038 0.108 0.045 0.080 0.218 0.190 0.0 12 0.002 0.012 0.002 0.045 0.039 0.045 0.039 0.064 0.126 0.001 0.132 0.006 0.087 0.281 0.075 0.029 0.00 1 0.035 0.00 1 0.06 1 0.0 19 0.063 0.0 19 a Ref. (24).agreement with experimental tensors (see tables 4 and 5). densities calculated with McClelland method. Ref. (27). Spin densities which give the best Spin Table 4. Calculated principal values of proton dipolar tensor in the I and I1 defects in M-TCNQ salt proton T/MHz proton T/MHz Ia 4.11 IIa - 2.65 - 1.46 Ib 3.71 IIb -2.16 - 1.55 Ic 2.68 IIc -0.52 -2.16 Id 2.71 IId -0.51 -2.20 3.58 - 1.98 - 1.60 3.69 - 2.09 - 1.60 3.08 - 1.01 - 2.07 2.88 -0.77 -2.11 Furthermore, the cations are expected to rearrange their position around the defect. It is not surprising that two different arrangements, I and 11, can arise because of the low symmetry of the crystal lattice.The polarization of the spin distribution reflects the strength of the electrostatic cation interaction. This is more effective for defect I than for 11. The e.s.r. measurements2 have shown that the defect concentration is almost constant for temperatures <250 K and increases with increasing temperature in the range 250-350 K. The same behaviour is shown by the ENDOR lines intensity. At 425 K a phase transition occurs with a sudden decrease in e.s.r. intensity. Note that above the 3-266 ENDOR Spectra of the Morpholinium-TCNQ Complex Table 5. Matrix of the angles (in ") between the < principal directions of the calculated dipolar tensors (values obtained from the experimental data are in parentheses) defect I <, 131 (148) 6(7) 46(28) t b 42 (25) 3 (3) t C 140 (158) defect I1 5, 131 (148) 3(3) 43 (26) 5 6 45 (29) 6(6) t c 140 (157) t defect \ Fig.4. Models which show a defect of solitonic nature in trans-polyacetylene (a) and in a chain of TCNQ molecules stacked in dimeric units (b). transition temperature the e.s.r. spectra retain almost the same characteristics as for the low-temperature phase. In particular, triplet exciton lines are observed in both phases with very similar zero field splitting parameters.2 Analogous behaviour cannot be observed by ENDOR because the ENDOR lines at temperatures ~ 3 2 0 K are broader and become undetectable above 350 K.No further information is available on the nature of the phase transition; however, the abovementioned observations indicate that only minor changes in the crystal properties should be involved. This allows us to suggest that little energy is needed to create a defect in the crystal lattice, or regions with the molecules arranged as they are in the phase that is stable at high temperature. In this way another hypothesis can be formulated as to the origin of the isolated TCNQ- radicals which is in line with the temperature behaviour of the concentration. The defects may be attributed to TCNQ- radicals present at the boundary between the regular lattice and the defective region.A . L. Maniero, 0. Priolisi and C . Corvaja 67 Polarization of the Spin Density Distribution The polarization of the spin density in the TCNQ- anion radicals, which is caused by the electrostatic interaction of the cations, resembles those of alkali-metal radical ion pairs in solutions of low dielectric constant.When the radical anion contains strongly charged groups, as occurs for example in semiquinones, the alkali-metal cation is situated close to the negative charge and polarizes the spin distribution, making the latter asymmetric. Calculations of the latter have been carried out with success following a model proposed by M~Clelland.~~ Some recent results on the K+TCNQ- salt in tetrahydrofuran and dimethoxyethane as solvent seem to indicate that TCNQ- also forms asymmetric ion pairs.3o This would be demonstrated by a linewidth alternation in the e.s.r.spectra observed for temperatures lower than 249 K in DME, and produced by the K+ cation undergoing a jumping process between two equivalent potential minima. However, we observed only two very narrow lines (Av = 50 kHz) separated by 3.97 MHz (1.417 G) in the ENDOR spectrum of K+TCNQ- in liquid DME for temperatures (T = 190 K) well below that indicated as the onset of linewidth alternation. This fact means that the proposal of an asymmetric structure for TCNQ- in solution deserves further attention. According to an X-ray diffraction study, the TCNQ anion in M-TCNQ has four neighbouring M+ cations. Calculations of the spin distribution were performed by the McClelland method by considering the limiting situation where two cations close to the same C(CN), group of the TCNQ anion are absent.The results are reported in table 3 (set 4). Note that the spin densities have the same trend as those we found the best when calculating the dipolar tensor. Finally, we should mention that up to now there is no information on the nature of the species giving rise to the narrow resonance accompanying the broad one. Conclusions ENDOR spectra obtained by saturating the central broad resonance line of the M-TCNQ crystal e.s.r. spectrum show that this line is due to two different spin-1/2 defects with unresolved hyperfine structure. The defects consist of isolated TCNQ- molecules with a strongly polarized and asymmetric spin distribution, thus confirming the possibility of a different spin distribution in the solid with respect to the solution.This was invoked to explain the zero field splitting parameters of thermally excited triplet excitons in TCNQ salts. The defects are suggested to arise at the boundary regions between zones of the crystal lattice with different molecular arrangements. We thank Prof. Renato Bozio of this department for many helpful discussions. This work was supported by the Italian National Research Council (CNR) through its Centro di Studio sugli Stati Molecolari Radicalici ed Eccitati and in part by the Minister0 della Pubblica Istruzione. References T. Sundaresan and S. C. Wallwork, Acta Crystallogr., Sect. B, 1972, 28, 3507. 3. C. Bailey and D. B. Chesnut, J . Chem. Phys., 1969,51, 51 18. D. B. Chesnut and W. D. Phillips, J. Chem. Phys., 1961, 35, 1793. S.Nespurek, J. Pilar, P. Schmidt, M. Sorm, U. Langer, A. Graja and E. Sopa, Chem. Phys., 1986,101, 81. S. K. Hoffmann, P. J. Corvan, P. Singh, C. N. Sethulekshmi, R. M. Metzger and W. E. Hatfield, J . Am. Chem. Soc., 1983, 105, 4608. L. Kevan and L. D. Kispert, Electron Spin Double Resonance Spectroscopy (John Wiley, New York, 1976). S. Flandrois and J. Boissonade, Chem. Phys. Lett., 1978, 58, 596.ENDOR Spectra of the Morpholinium-TCNQ Complex 8 T. Hibma, P. Dupuis and J. Kommandeur, Chem. Phys. Lett., 1972, 15, 17. 9 T. Hibma, G. A. Sawatzky and J. Kommandeur, Chem. Phys. Lett., 1973, 23, 21. 10 A. J. Silverstein and Z. G. Soos, Chem. Phys. Lett., 1976, 39, 525. 11 N. S. Dalal, D. Haarer, J. Bargon and H. Mohwald, Chem. Phys. Letf., 1976, 40, 326. 12 C. Corvaja, A. L. Maniero and L. Pasimeni, Chem. Phys., 1985, 100, 265. 13 J. Avalos, F. Devreux, M. Guglielmi and M. Nechtshein, Mol. Phys., 1978, 36, 669. 14 L. R. Melby, R. J. Harder, W. R. Hertler, W. Mahlor, R. E. Benson and W. E. Mockel, J. Am. Chem. 15 L. Komorowsky and G. Malackowicz, J. Phys. C3, 1983,44, 1207. 16 A. Cehak, A. Chyla, M. Radomska and R. Radomsky, Mol. Cryst. Liq. Cryst., 1985, 120, 327. 17 R. Biehl, M. Plato and K. Moebius, J. Chem. Phys., 1975, 63, 3515. 18 A. Colligiani, C. Pinzino, M. Brustolon and C. Corvaja, J. Magn. Reson., 1978, 32, 419. 19 R. G. Kepler, J. Chem. Phys., 1963,39, 3528. 20 T. Buckman, 0. Griffith and H. M. McConnell, J. Chem. Phys., 1965,43, 2907. 21 Z. G. Soos, J. Chem. Phys., 1967,46,4284. 22 D. B. Chesnut and D. C. Meinholtz, J. Chem. Phys., 1985, 83, 5495. 23 H. M. McConnell and D. B. Chesnut, J. Chem. Phys., 1958, 28, 107. 24 P. H. Rieger and G. K. Fraenkel, J. Chem. Phys., 1962,37, 2795. 25 H. M. McConnell and J. Strathdee, Mol. Phys., 1959, 2, 129. 26 R. H. Clarke and C. A. Hutchinson Jr, J. Chem. Phys., 1971, 54, 2962. 27 H. Johansen, Int. J. Quantum Chem., 1975,9,459. 28 M. A. Collins, Adv. Chem. Phys., 1983, 53, 225. 29 B. J. McClelland, Trans. Faraday Soc., 1961, 57, 1458. 30 R. D. Rataiczak, M. Thomas Jones, J. R. Reeder and D. J. Sandman, Mol. Phys., 1985,56, 65. SOC., 1962,84, 3374. Paper 61849; Received 1st May, 1986

ISSN:0300-9599    

DOI:10.1039/F19878300057

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I , 1987,83, 69-75 An Electron Nuclear Double Resonance Study in a Glassy Matrix of Nitroxide Radicals with Delocalized Spin Density Marina Brustolon," Anna Lisa Maniero and Ulderico Segre Dipartimento di Chimica Fisica, Universita di Padova, Via Loredan 2, 35131 Padova, Italy Lucedio Greci Dipartimento delle Scienze dei Materiali e della Terra, Universita di Ancona, Via Brecce Bianche, 60131 Ancona, Italy Indolinone (l), benzimidazole (2) and quinoline (3) nitroxide radicals have the nitroxide function in a conjugated position with respect to the 71 system. Their electron nuclear double resonance (ENDOR) spectra in a [2H,]toluene glassy matrix have been detected and the principal values and the orientation of the proton hyperfine coupling tensors have been obtained by comparison with computer-simulated spectra.Typical features of ENDOR spectroscopy in frozen solution, and in particular its more general feasibility with respect to use in solution, are discussed. Proton electron nuclear double resonance (ENDOR) of organic radicals in solution is a useful tool for the determination of isotropic hyperfine coupling c0nstants.l However, in many cases the application of ENDOR spectroscopy in solution is severely limited by the low sensitivity of the technique, the ENDOR signal being in the best cases some fraction of the e.s.r. one. Moreover, to obtain the optimum ENDOR signal it is necessary to find the appropriate arrangement for a number of experimental conditions (e.g. solvent viscosity, concentration, intensity of the time-dependent fields), so that it is difficult to use this technique in solution as routinely as e.s.r.On the other hand, organic radicals trapped in solids can be routinely studied by the ENDOR technique, owing to its more favourable spin relaxation properties2 Many radicals giving very weak or undetectable ENDOR spectra in liquid solution can give strong signals when the solvent is frozen as a glass. In a previous paper it was shown that from proton ENDOR spectra of organic radicals in a polycrystalline or glassy matrix one can easily obtain the principal values of the hyperfine coupling tensors of the protons, together with information on the principal directions. The radicals studied were aliphatic nitroxide radicals, showing ENDOR spectra of methyl and methylene proton^.^ In this paper an ENDOR investigation in frozen solution is reported on organic radicals with delocalized spin density.The radicals studied ( l a d , 2, 3) have the nitroxide function in a conjugated position with respect to the molecular n system.* The hyperfine coupling tensors of all protons in the a position with respect to the carbon atoms bearing unpaired spin density (a-protons) have been measured, and in some cases the relative orientations of the nitrogen and proton hyperfine tensors have been obtained by computer simulation of the ENDOR spectra. Theory The spin hamiltonian for a nitroxide radical is given by 6970 Electron Nuclear Double Resonance Study where I , and A , are the nitrogen spin and the hyperfine (h.f.) tensor, and Ii and Ai are the spin and the h.f.tensor of the ith proton. The shape of the polycrystalline e.s.r. spectrum is given by the superposition of the lineshapes at different orientations, i.e. where B,(R) and p,.(Q) are, respectively, the resonance field and the probability of the rth transition for a radical at orientation n, andflx) is a shape function. The values of B, and pr can be computed for nitroxide radicals according to standard first-order expressions.' The nitrogen h.f. tensor A , gives the largest anisotropic interaction of the hamiltonian (l), so that it is convenient to choose the A , principal axes as the molecular frame, with the z axis along the nitrogen p-orbital direction. Since the tensor A , is quite anisotropic, it is possible to obtain selectively oriented ENDOR spectra of radicals embedded in glassy disordered samples.2 Simulated e.s.r.absorption spectra for radical (1 a) are shown in fig. 1 (a). They have been calculated by taking the magnetic field along the principal directions of the A , tensor. Note that, when Bo is parallel to z, the spectrum extends over a much wider range, and the three hyperfine components are well separated. The conventional first-derivative e.s.r. spectrum for a disordered sample is shown in fig. 1 (b). Absorption in the low- and high-field zones is mainly due to molecules with Bo nearly parallel to the z axis. It is therefore possible in principle to saturate these molecules selectively by taking the magnetic field equal to the low or high value: crystal-like ENDOR spectra can therefore be observed, provided that the saturation is not transferred by spin diffusion to molecules having different orientations.On the other hand, it is possible to saturate only the probes with Bo lying in the (x,y) plane by setting the magnetic field at the value B,, z 3410 G, as can be seen from fig. 1. The ENDOR signal for the ith proton is therefore given by3 where v is the radiofrequency, vi+(n) is the proton frequency corresponding to M, = f 1/2, g(x) is a shape function and w(n) is the orientation-dependent intensity factor given by a Gaussian distribution: w ( n ) = 2 exp { - [B- B,(R)]2/2a2}. T (4) The distribution width CT is taken to be equal to the e.p.r. inhomogeneous linewidth, (i = 2.G. Since each proton contributes additively to the ENDOR signal, the total lineshape is obtained as I(v) = z [z,+(v)+ri-(v)].( 5 ) i Hyperfine enhancement effects5 have not been included in the simulations, since the radiofrequency field used in the experiments is strong enough to saturate the nuclear transitions.6 The ENDOR spectra obtained for a model system of a proton with the hyperfine interaction principal axes parallel to the A , principal axes are reported in fig. 2. These calculations show the dependence of the lineshapes upon the parameter (i when the value of the magnetic field is chosen to pick up molecules with zllBo [fig. 2(a)] or molecules with z l B o [fig. 2(b)]. The positions of the ENDOR peaks are in any case determined by the principal values of the Ai tensor. Therefore, on picking out the three pairs of lines corresponding to the same proton, the proton hyperfine tensor Ai should be extracted straightforwardly from this type of ENDOR spectrum, and some infor- mation on its orientation should also be obtained.M .Brustolon et al. 71 3410 3 4 4 0 BIG Fig. 1. Computer simulated e.s.r. spectra for radical (1 a). The magnetic interaction values are as follows: g = (2.008, 2.005, 2.002); A,/MHz = (1 1, 1 1 , 65); for Ar see table 1. (a) Oriented ‘ single-crystal ’ spectra, with B,, aligned along the three principal axes; (b) ‘powder ’ first-derivative spectrum. , I I , 1 I I I I 1 -3 - 2 -1 0 1 2 3 - 3 -2 -1 0 1 2 3 ( Y -vH )/MHz Fig. 2. Computer-simulated ENDOR spectra for a model proton with non-axial hyperfine interaction in a nitroxide radical: g and A , are the same as in fig.1, A,/MHz = (2, 4, 6). (a) B = 3440.G, (b) B = 3410.G. a/G = (i) 2, (ii) 10 and (iii) 40.72 Electron Nuclear Double Resonance Study Experimental Nitroxides (l),' (2)* and (3)9 were prepared as described in the literature. The concen- tration of each sample was ca. mol drnp3. E.s.r. and ENDOR spectra were recorded with a Bruker ER 200 spectrometer equipped with a 300 W radiofrequency amplifier. (la): R2 = Ph, R5 = R' = H (lb): R2 = Me, RS = R7 = H Ph 1 I a Ph Ph I (lc): R2 = Ph, RS = H, R7 = OBuf (Id): R2 = Ph, Rs = OBut, R7 = H 0 (3) Results In fig. 3 the e.s.r. and ENDOR spectra for radical (1 a) in frozen [2H,]toluene are reported as an example. ENDOR spectra of comparable intensity were detected for the other radicals.We tried without success to detect ENDOR signals for the same radicals in [2H,]toluene fluid solution, at different concentrations and on varying all the relevant experimental parameters. When the magnetic field is set on positions A and B of the e.s.r. spectrum, markedly different ENDOR spectra are obtained. According to the previous discussion, spectrum A [in fig. 3 (a)] is given by the pairs of lines due to the z components of the proton tensors A , ; spectrum B [in fig. 3(b)] results instead from the x, y components and has twice as many lines as spectrum A. The ENDOR spectra for the radicals studied here are all given by a pattern centred at the free-proton frequency. Some general considerations help in obtaining the proton hyperfine tensors reported in table 1 from these spectra: (i) a pair of lines can be attributed to the lowest, intermediate or highest principal values (A,,A,,A,) on the basis of their lineshapes (see fig.2 ) ; (ii) if the isotropic hyperfine splitting Aiso is known, the (4) relations hip can be exploited; (iii) the dipolar tensor for an a-proton is generally determined mainly by the spin density on the bonded carbon atom, and therefore Tr A = ( A , + A , + A,)/3 = Aiso A,:A,:A, z 1 : 2 : 3 . (7) The hyperfine tensors obtained are reported in table 1. Discussion Radicals (1 a-d) The isotropic splittings for protons 4-7 are known from the e.s.r. spectra in CHCl,:' Ais0(5, 7) = 8.5 MHz and AiS0(4, 6) = 2.8 MHz for (1 a); similar values are obtained for (1 b). The four radicals have similar hyperfine tensors, and the traces of the hyperfine tensors for proton pairs 5,7 and 4,6 are in good agreement with the isotropic splittings obtainedM .Brustolon et al. 73 I . . , , , * . b , . ~ V V . 0 2 4 6 0 2 4 6 Fig. 3. E.s.r. (insert) and ENDOR spectra for radical (la) in [2H,]toluene at T = 105 K. (a) and (b) ENDOR obtained at positions A and B of the e.s.r. spectrum, respectively. Upper and lower traces correspond, respectively, to experimental and simulated ENDOR spectra. The simulated ENDOR spectra have been computed with the same parameters as in fig. 1 and CT = 2. G. The ENDOR lines near vH are due to the phenyl protons, which have not been included in the simulations. (v-vH)/MHz (v-vH)/MHz Table 1. Proton hyperfine tensor principal values = A j - & z= TrAa - radical proton Tj/MHz A/MHz l a 4, 6 1.7 5 4.9 7 2.5 l b 4, 6 1.7 5 4.9 7 2.5 l c 4, 6 1.7 5 5.0 I d 4, 6 1.5 7 2.5 2 4 1.1 5 6.1 7 3.6 3 3 2.0 5, 7 1.6 6 4.9 8 2.9 -0.2 0.1 -0.5 - 0.2 0 -0.5 -0.3 0.2 -0.1 -0.7 0.1 0.2 - 1.8 0.1 -0.1 0.5 0.6 - 1.5 - 5.0 -2.1 - 1.5 -4.9 - 2.0 - 1.4 - 5.2 - 1.4 - 1.8 - 1.2 -6.3 - 1.8 -2.1 - 1.5 - 5.4 - 3.5 2.8 - 8.9 - 8.3 2.9 -9.0 - 8.4 2.6 2.8 2.3 - 8.9 - 8.'2 - 10.7 - 9.8 - 3.9 2.9 - 8.9 - 8.0 a The signs are based on the spin density distribution assumed for the calculations.from the e.s.r. spectrum. The principal values for protons 5 and 7 are markedly different owing to the stronger dipolar interaction of proton 7 with the spin density on the N-0 group. Note that the relationships (7) do not hold for the principal values of proton 7, whereas they hold approximately for protons 4, 6 and 5 , which interact mainly with the spin density on the corresponding carbon atom.74 Electron Nuclear Double Resonance Study Table 2.Calculated proton tensorsa in radical (1 4 proton T/MHz direction cosinesb 4 1.50 0.42 - 1.92 5 4.40 - 0.20 - 4.20 6 1.70 0.40 -2.10 7 3.93 - 1.78 -2.15 0.3453 0 0.7073 0 0.7068 0.5585 0 0.7783 0 - 0.9385 0.8294 - 0.6279 0.9385 0.3453 0 0.7068 0 0.8294 0.5585 0 -0.7073 -0.6279 -0.7783 0 0 0 1 0 1 0 0 0 1 0 0 1 a The spin density distribution used in the calculation is as follows: pN = po = 0.310, psa = p7& = p3 = po = 0.050 (see text). In the molecular frame (x along the N-0 bond, z along the nitrogen p-orbital). p4 = p6 = - 0.044, p5 = 0.139, p7 = 0.130, In fig.3(a) the features due to protons 4, 6, 5 and 7 are indicated. They correspond, respectively, to the lowest (A4,61 = 1.3 MHz), the intermediate (A,, = 8.9 MHz) and the maximum (A7h = 10.4 MHz) principal values of the corresponding tensors. Since only these features appear for the protons, the conclusion must be drawn that the z axis of the nitrogen 2p orbital is nearly parallel to the principal directions corresponding to A4,61, A,, and A7h, as discussed in the previous section. To support our interpretation, we have performed a McConnell-StrathdeelO calculation for protons 4-7 in radical (1 a) by using a planar molecular geometry as determined by the crystal struct~re.~ The spin densities for nitrogen and oxygen have been assumed to be equal, and have been estimated by comparing the nitrogen isotropic splitting for radical ( l a ) with the corresponding average value (43 MHz) for nitroxide radicals with localized spin density on the N-0 gr0up.l' The spin densities for protons 4-7 have been obtained semiempirically from the hyperfine splitting values by making use of the McConnell relation with Q = -64 MHz.Negative spin densities have been taken on C(4) and C(6), in agreement with INDO calculation^.^ The residual spin density has been considered as equally distributed on the 'blind' positions C(3), C(3a), C(7a) and O(C-0). The calculated principal values and principal directions are reported in table 2. No attempt has been made to optimize the calculated values on adjusting the spin density distribution or the semiempirical parameters.The calculated values are in reasonable agreement with the experimental ones, and the principal directions corresponding to A4, 61, A,, and A7h are parallel to the z axis, as found experimentally. Radicals (2) and (3) The isotropic splittings for these two radicals are in good agreement with those obtained by e.s.r. spectroscopy.s Protons 5 and 7 in (2) have similar Aiso values being ortho and para with respect to the N-0 group, but they have different dipolar tensors owing to their different positions with respect to the same group. The same observation holds for protons 6 and 8 in (3). The considerations which have been made for radicals (1) areM. Brustolon et al. 75 also relevant for these species, since the same trend is found for the coupling tensors, as is reported in table 1.Some general considerations are worth mentioning. The detection of six ENDOR lines for each nucleus, instead of two as in liquid solution, complicates the analysis of the spectrum. On the other hand, more information is obtained from rigid-matrix spectra, since the dipolar tensor depends not only on the local spin density, as AiSO, but also on the relative position of the proton with respect to the other atoms bearing some spin density. Therefore an ENDOR investigation in frozen solution should be tried even when the ENDOR spectrum in fluid solution is obtainable, as an aid in the attribution of the hyperfine parameters to the different protons, and in order to obtain information on the total spin distribution on the radical.Conclusions We have shown that the determination of the dipolar tensors for a-protons in aromatic radicals in frozen solution by the ENDOR technique can be made relatively easily. ENDOR investigations in frozen solutions therefore appear to be a useful alternative technique to ENDOR in fluid solutions when the latter is difficult to perform. Moreover, the information obtainable in frozen solution on the dipolar hyperfine coupling of the protons can be exploited in determining the position of the proton in the radical, together with the spin distribution on the conjugated system. This work was supported in part by the C.N.R. through its Centro Studi sugli Stati Molecolari Radicalici ed Eccitati, and in part by the Minister0 della Pubblica Istruzione. References 1 N. M. Atherton, Electron Spin Resonance (Ellis Horwood, Chichester, 1973). 2 L. Kevan and P. A. Narayana, in Multiple Electron Spin Resonance, ed. M. M. Dorio and J. H. Freed 3 M. Brustolon, A. L. Maniero and U. Segre, Mol. Phys., 1985, 55, 713. 4 R. Benassi, F. Taddei, L. Greci, L. Marchetti, G. D. Andretti, G. Bocelli and P. Sgarabotto, J . Chem. 5 D. H. Whiffen, Mol. Phys., 1966, 10, 595. 6 J. H. Freed, J. Chem. Phys., 1965,43, 2312. 7 C. Berti, M. Colonna, L. Greci and L. Marchetti, Tetrahedron, 1975, 31, 1745; L. Greci, Tetrahedron, 8 C. Berti, M. Colonna, L. Greci and L. Marchetti, J. Heterocycl. Chem., 1979, 16, 17. 9 C. Berti, M. Colonna, L. Greci and L. Marchetti, Tetrahedron, 1976, 32, 2147. (Plenum, New York, 1979). SOC., Perkin Trans. 2, 1980, 786. 1982, 38, 2435. 10 H. M. McConnell and J. Strathdee, Mol. Phys., 1959, 2, 129. 1 1 L. J. Berliner, Spin Labelling (Academic Press, New York, 1976). Paper 6/328; Received 17th February, 1986

ISSN:0300-9599    

DOI:10.1039/F19878300069

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1987,83, 77-83 Direct Observation of a 1,4-Hydrogen Shift in Vinyl Radicals derived from the Reaction of Alkynes with Thiyl Radicals Bruce C. Gilbert" and David J. Parry Department of Chemistry, University of York, Heslington, York YO1 5DD Loris Grossi* Istituto di Chimica Organica, Universita, Viale Risorgimento 4, Bologna, Italy E.s.r. experiments demonstrate that in vinyl radicals generated from thiyl radicals and alkynes, intermolecular abstraction of a thiol hydrogen (k z lo7 dm3 mol-l s-l) is in competition with lphydrogen shifts. A rapid 1,4-shift (k M lo5 s-l) is shown to occur in cases where the resulting radical is stabilized by the presence of a-sulphur and a-carboxy substituents, whereas in other examples a 1,hhift predominates. The occurrence of intramolecular hydrogen shifts from carbon to the heteroatom in alkoxyl and aminyl radicals is well documented, but relatively little is known about analogous carbon-to-carbon shifts (except for the reactions of phenyl radicals).' Our interest in the rearrangements of vinyl radicals2 and, in particular, in the conversion of 'CH2CH2SMe into 'CH,SCH,CH, (apparently uia a novel 1,4-shift), led us to investigate the fate of related sulphur-containing vinyl radicals using e.s.r.spectroscopy and a continuous flow system. Experimental E.s.r. spectra were recorded on a Varian E-104 spectrometer equipped with an X-band klystron and 100 kHz modulation. Splitting constants and g-values were measured by comparison with an aqueous solution of Fremy's salt [a(N) = 1.309 mT,4 g = 2.00S5].Relative radical concentrations were determined by spectrum simulation using a program supplied by Dr M. F. Chiu. A mixing chamber was employed which allows simultaneous mixing of three reagent streams ca. 40 ms before passage through the cavity of the e.s.r. spectrometer. The flow was maintained using a Watson-Marlowe 502 peristaltic pump positioned on the inlet tubing. The solutions used were as follows: stream (i) contained titanium(II1) chloride (0.008 mol dm-,) and concentrated sulphuric acid, stream (ii) contained hydrogen peroxide (0.03 mol dm-,) and stream (iii) con- tained the substrate (at a concentration up to ca. 0.5 mol drn-,) together with butynedioic acid or propynoic acid (up to ca. 0.1 mol drn-,). pH measurements were made using a Pye-Unicam PW9410 pH meter with the electrode inserted into the effluent stream.All solutions were deoxygenated both before and during use by purging with oxygen- free nitrogen. Flash-photolysis experiments were carried out on an Applied Photophysics kinetic spectrometer coupled with an excimer laser excitation source (A 308 nm), monitored by a pulsed xenon flash lamp; collecting and plotting of data was achieved via a 4500 Biomation digital oscilloscope interfaced to an Apple microcomputer. The kinetic simulation program, executed on a DEC-10 computer, was kindly provided by Prof. D. J. Waddington. The chemicals employed were all commercial samples and used as supplied. 7778 Direct Observation of a 1,4-Hydrogen Shift in Vinyl Radicals Results and Discussion E.S.R.Results Radicals were generated continuously in the cavity of the spectrometer with an aqueous flow system in which the reaction between TiIII and hydrogen peroxide was utilized to generate the hydroxyl radical:2 in the third stream was included the thiol and alkyne (normally butynedioic acid, typically l 0-2- mol dm-3), with the former in sufficient concentration (b mol dm-3) to ensure that reaction with 'OH generated the appropriate thiyl radical [k('OH + RSH) is typicallys > lo9 dm3 mol-l s-l].* When relatively high concentrations of thiol (ca. 0.03 mol dm-3) were employed, the spectra detected in most cases showed relatively strong signals with high g-values (ca. 2.0057) and low /I- and y-proton splittings characteristic of a$-di-sulphur-substituted radicals (see table 1 and fig.1):' these are evidently formed via a sequence in which the thiyl radical adds to the triple bond, the resultant vinyl radical (1) undergoes rapid intermolecular hydrogen abstraction (from more thiol) and the alkene formed traps a further thiyl radical to give (2a) or (2b) [see schemes 1 and 2 and ref. (7)l.t However, when lower concentrations of thiol were employed, different spectra were detected which reveal other fates for the intermediate vinyl radicals (see fig. 1). For example, generation of 'SCHR1R2 (R2 = C02H, R1 = H, Me) from 2-mercaptoethanoic acid and 2-mercaptopropanoic acid, respectively, in the presence of butynedioic acid led to the detection of strong signals from radicals which, on the basis of their g-values and a- and /I-splittings, are assigned to (3, R2 = R3 = R4 = C02H, R1 = H, Me), in which the radical centre has both carboxy and thioalkyl substituents; a long-range coupling to the alkenyl proton is also revealed.In similar experiments with propynoic acid and 2-mercaptoethanoic acid the expected extra long-range proton splitting was also observed. Signals from radicals (3) were first accompanied by, then replaced by, those from (2a) as [RSH] was increased. These results clearly establish that a rapid 1,4-shift can compete effectively with intermolecular abstraction (scheme I). The novel and intramolecular process shown by adducts from 2-mercaptoethanoic and 2-mercaptopropanoic acid is evidently assisted by the presence not only of the sulphur substituent but also the electron-withdrawing carboxylate group (which leads to the production of a ' merostabilized ' or 'capto-dative ' radicalg), since thiyl radicals lacking this substituent behaved in contrasting fashion.For example, with EtS' at low concentrations of thiol (< 5 x mol dm-3) two signals were detected, depending on the concentration of alkyne. For relatively low concentrations of alkyne (1 0-3 mol dm-3) the spectrum comprised a broad-lined doublet (a = 0.21 mT, g = 2.0097), characteristic of a sulphinyl radical (RSO*),10 which was replaced at higher [alkyne] by a sharper spectrum comprising a large doublet (a = 3.52 mT, g = 2.0049). Our assignment of the spectra to (5) and (6), respectively, (see scheme 2 ) is based in part on the observation of identical radicals in the corresponding reactions of a variety of thiyl radicals RS' (R = CH,CH,OH, CH2CH2C02H, Pr); further support for the correct identification of (6) derives from the parameters exhibited by some radical adducts of thiophen and its derivatives" [see e.g. (7)] and from the appearance of the expected extra (@-proton splitting and g-shift when propynoic acid was employed [cf.e.g. (S)]. We propose that for these substrates a facile 1,5-shift is followed by rapid loss of the thiyl-like radical (4), which can be readily oxidized by hydrogen peroxide to the corresponding sulphinyl radical (cf. the behaviour of other thiyl radicals under these conditions)1° or add to a second molecule of alkyne [the sequence of bond formation * Concentrations quoted are those after mixing.t The reluctance of radicals (2a, R3 = R4 = C0,H) and of analogous adducts of RS' and maleic acid to undergo fragmentation is in marked contrast to the extremely rapid scission of (J) sulphur-substituted radicals lacking the carboxylate substituents.*Table 1. E.s.r. parameters for radicals formed by reaction of thiyl radicals with alkynesa hyperfine splittings/mTd thiyl radical alkyne* [RSHIc radicals a(a-H) a@H) a (other) ge *b 'SCH,CO,H B P 'SCHMeC0,H B (X = H, OH, CO,H 'SCH,CH,X B or CH,) P 'SCH(CO,H)CH,CO,H B P (k:h i k:h low ( high low high k:h high low (3, R1 = H, R2 = R3 = R4 = CO,H) (2a, R1 = H, R2 = R3 = R4 = CO,H) (3, R' = R3 = H, R2 = R4 = CO,H) -f (3, R' = CH,, R2 = R3 = R4 = CO,H) (2a, R' = CH,, R2 = R3 = R4 = C02H) (5, R3 = R4 = C0,H) (6, R3 = R4 = CO,H) (2b, R3 = R4 = C02H) (6, R3 = H, R4 = CO,H) -f (3, R' = CH,CO,H, R2 = R3 = R4 = CO,H) (5, R3 = R4 = C0,H) l(6, R3 = R4 = C0,H) (2a, R' = CH,CO,H, R2 = R3 = R4 = C02H) (3, R1 = R4 = C02H, R3 = H, R2 = CH,CO,H) (6, R3 = H, R4 = CO,H) 1.44 - 1.46 - - 0.21 - - 1.70 - 0.21 - - 1.70 - 0.34 - 1.69 (3) 0.28 3.52 - ca.0.3 4.34 0.96 (2) 3.52 - ca. 0.3 0.92 (2) 4.34 0.22 0.28 (2) 0.24 0.10 0.28 ( 0.10 - - ca. 0.35 (2) - 0.07 - - ca. 0.3 0.25 (0.18 - 2.0050 9 2.0056 9 2.0051 ? 2 2.0050 2.0057 t 2.0097 2.0049 9 2.0057 < 2.0041 2.0048 2.0097 9 2.0049 $. 2.0057 2.0051 2.0041 Q P a Typically pH ca. 1. B = butynedioic acid, P = propynoic acid. High: typically 3 x lop2 mol drn-,; low: 3 x mol dm-,. fO.O1 mT. fO.0001. f Complex signals, including radicals of type (2).4 \o80 Direct Observation of a I,4-Hydrogen Shgt in Vinyl Radicals R3 R4 \ I c =c. / R3 \ /R4 S /c=c\H 'CHR'R2 kadd 1 'SCHR1R2 H / \ R'R'CHS Scheme 1. in (6) remains to be established]. At higher concentrations of thiol (ca. 0.03 mol dm-3) intermolecular reaction of the intermediate vinyl radicals evidently occurs, as indicated by the detection in increased concentrations of radicals of type (2b) (see table 1) togetter, in some cases, with complex signals attributed to the corresponding hydroxyl-radical adducts of the alkenes thus formed. Reaction of mercaptosuccinic acid led to the detection of radicals of type (2) (at high [thiol]) and (3), as well as (5) and (6) (at lower [thiol]), indicating that 1,4- and 1,Sshifts occur for this substrate at a comparable rate.Kinetic Analysis From simple steady-state analysis of the competition between the 1,4-shift and inter- molecular abstraction we conclude that kabs = 1O2k1,, dm3 mol-l. Flash photolysis of an aqueous solution of the disulphide [SCH,CH,OHJ, (5 x lo-, mol dm-3) and maleic acid (1.4 x mol dm-3) at room temperature led to the pseudo-first-order disappearance of the weak absorption12 from the appropriate thiyl radical (A = 330 nm), from which the rate constant for addition was determined13 as ca. 5 x lo7 dm3 mol-l s-l. In similar experiments with butynedioic acid, absorption from a product obscured the decay: the rate constant for addition is expected to be similar to that found for maleic acid [see e.g. ref. (14)]. We have employed a kinetic simulation program in an attempt to match the variation in the steady-state concentrations of (2a), (2b) and (3) (as measured by e.s.r.) with [RSH]; it has been previously pointed out that a pseudo-steady state is achieved in the cavity in the TilI1-H,O, system and that such analysis is a~pr0priate.l~ We haveB.C . Gilbert, D. J . Parry and L. Grossi 0 X 81 t Fig. 1. E.s.r. spectra obtained during the oxidation of 2-mercaptopropanoic acid (2 x mol dm-3) at pH ca. 1. 9 2.0057 mol dm-3) with 'OH in the presence of butynedioic acid (3 x (3, R' = Me, RZ = R3 = R4 = CO,H) X HozC \ foZH S '=c\H \dMeCOz H HOZC /COZH R4 = C02H) 0 'C-C-SCHMeCOZH (2a, R' = Me, R2 = R3 = \ H0, CCHMeS 'H used parameters as follows: k('OH+RSH), 5 x lo9 dm3 mol-1 s - ~ ; ~ k(Ti111+H202), 2 x lo3 dm3 mol-1 s-l [ref.(16)] ; k(RS' + alkyne or alkene), 5 x lo7 dm-3 mol-1 s-l; all radical-radical termination rates, lo9 dm3 mol-1 s-l; and a time between mixing and observation of 0.04 s (measured using a spectrophotometric method and the Fe3+-thio- cyanate reaction17). The observed behaviour of those substrates which exhibit a 1,4-shift is reproduced with absolute values of kabs and kl,4 of ca. lo7 dm3 mol-l s-l and ca. lo5 s-l, respectively. In the rather more complex behaviour exhibited by those substrates which undergo a 1,5-shift, the rate of rearrangement also appears to be ca. lo5 s-l (and within the range reported for comparable shifts in vinyl radicals with simple alkyl chair@). Conclusions The rapidity of the intramolecular 1 $shifts in the vinyl radicals described here presumably reflects a contribution to the exothermicity of reaction associated with the formation of a C(sp2)-H bond at the expense of a C(sp3)-H bond (ca.40 kJ rn0l-l).l9 The apparent facility of the corresponding 1,4-shift in certain cases appears similarly to reflect this, together with the extra stability associated with production of a radical in which the unpaired electron is delocalized onto sulphur and the carboxy group: that the effect of the latter is crucial (with calculated contribution to the stabilization energy of ca. 40 kJ mo1-1)20 is indicated by the lack of a corresponding abstraction reaction82 Direct Observation of a 1,4-Hydrogen Shgt in Vinyl Radicals 'SCH2CHzX kadd 1 R'C*CR4 IR4 L c . R3 / \CH2CH2X S R\ /R4 / ,c=c \ S H C H , - ~ H X -CHI -CHX I S' H (4) 1 \ /R4 '0s /" =c\H R3 ( 5 ) R3 \ /R4 ,c = c, S H CHzCH2X \ kadd I 'SCH,CH2X R3 S ' 'H \.IR4 C - C - SCHz CHZ X \ CHZCHZX (2b) Scheme 2. 2.60 0.225 2.90 0.20 H HO2C H 0 k 7 $ ) . 0 3 H 1.69 +-I H H g = 2.005 1 g = 2.0038 (k < lo4 s-l) in the absence of the carboxylic acid group. Consideration of the likely geometry of the intermediate vinyl radicals and inspection of models show that the relatively long C-S bonds (expected21 to be ca. 0.18 nm) and small LCSC (ca. looo) allow a distance of closest approach of the radical centre and the &hydrogen (for 1,4-shift) of ca. 0.16 nm. We suggest that similar favourable geometric considerations, as well as the radical stabilization conferred by an a-sulphur substituent play an equally important role in bringing about the rapid 1,4-shifts observed for Q-sulphur-substituted alkyl radical^.^ We thank the S.E.R.C.for a studentship (for D. J.P.) and NATO for a research grant (567/84) to L.G.B. C. Gilbert, D. J . Parry and L. Grossi 83 References 1 A. L. J. Beckwith and K. U. Ingold, in Rearrangements in Ground and Excited States, ed. P. de Mayo 2 J. Foxall, B. C. Gilbert, H. Kazarians-Moghaddam, R. 0. C. Norman, W. T. Dixon and G. H. 3 L. Lunazzi, G. Placucci and L. Grossi, J. Chem. SOC., Perkin Trans. 2, 1981, 703; Tetrahedron, 1983, 4 R. J. Faber and G. K. Fraenkel, J. Chem. Phys., 1967,47,2462. 5 J. Q. Adams, S. W. Nicksic and J. R. Thomas, J. Chem. Phys., 1966,46, 654. 6 See e.g. Farhataziz and A. B.Ross, Selected Specific Rates of Reactions of Transients from Water in Aqueous Solution. III. Hydroxyi Radicals and Perhydroxyi Radicals and their Radical Ions (National Standard Reference Data Series, National Bureau of Standards, Washington, D.C., 1977). 7 T. Kawamura, M. Ushio, T. Fujimoto and T. Yonezawa, J. Am. Chem. SOC., 1971,93, 908. 8 See e.g. Y. Ueno, T. Miyano and M. Okawara, Tetrahedron Lett., 1982, 23, 443; P. S. Skell and 9 H. G. Viehe, Z. Janousek, R. Merenyi and L. Stella, Acc. Chem. Res., 1985, 18, 148. (Academic Press, London, 1980), vol. 1, p. 162. Williams, J. Chem. Soc., Perkin Trans. 2, 1980, 273. 39, 159. R. G. Allen, J. Am. Chem. SOC., 1960, 82, 151 1. 10 B. C. Gilbert, H. A. H. Laue, R. 0. C. Norman and R. C. Sealy, J. Chem. SOC., Perkin Trans. 2, 1975, 11 B. C. Gilbert, R. 0. C. Norman and P. S. Williams, J. Chem. SOC., Perkin Trans. 2, 1981, 207. 12 G. H. Morine and R. R. Kuntz, Photochem. Photobioi., 1981, 33, 1; K. Schafer, M. Bonifacic and 13 B. C. Gilbert, R. N. Perutz, C. J. Scarratt and B. P. H. Smith, unpublished results. 14 C. Sivertz, J. Phys. Chem., 1959, 63, 34; 0. Ito, K. Nogami and M. Matsuda, J. Phys. Chem., 1981, 15 G. Czapski, J. Phys. Chem., 1971,75, 2957. 16 M. Fitchett, B. C. Gilbert and M. Jeff, fhiios. Trans. R. SOC. London, Ser. B, 1985, 311, 517. 17 J. F. Below, R. E. Connick and C. P. Coppel, J. Am. Chem. SOC., 1958, 80, 2951. 18 B. C. Gilbert and D. J. Parry, J. Chem. SOC., Perkin Trans. 2, 1986, 1345. 19 S. W. Benson, Thermochemical Kinetics (Wiley, New York, 1968). 20 W. Lung-min and H. Fischer, Helv. Chim. Acta, 1983, 66, 138. 21 A. Streitwieser and C. H. Heathcock, Introduction to Organic Chemistry (Macmillan, New York, 2nd 892. K-D. Asmus, J. Phys. Chem., 1978, 82, 2777 and references therein. 85, 1365; E. I. Heiba and R. M. Dessau, J. Org. Chem., 1967,32, 3837. edn, 1981). Paper 618 1 5 ; Received 28th April, 1986

ISSN:0300-9599    

DOI:10.1039/F19878300077

出版商:RSC    

年代:1987

数据来源: RSC