摘要:

Date 1962 1962 I963 1963 1964 1964 1965 1965 1966 1966 1967 1967 1968 1968 1969 1969 1970 1970 1971 197 I 1972 1972 1973 1973 I974 I974 1975 I975 1976 1977 1977 1977 1978 1978 1979 1979 1980 1980 1981 1981 1982 1982 1983 1983 I984 FARADAY DISCUSSIONS OF THE CHEMICAL SOCIETY Subject Inelastic Collisions of Atoms and Simple Molecules High Resolution Nulcear Magnetic Resonance The Structure of Electronically Excited Species in the Gas Phase Fundamental Processes in Radiation Chemistry Chemical Reactions in the Atmosphere Dislocations in Solids The Kinetics of Proton Transfer Processes Intermolecular Forces The Role of the Adsorbed State in Heterogeneous Catalysis Colloid Stability in Aqueous and Non-aqueous Media The Structure and Properties of Liquids Molecular Dynamics of the Chemical Reactions of Gases Electrode Reactions of Organic Compounds Homogeneous Catalysis with Special Reference to Hydrogenation and Bonding in Metallo-organic Compounds Motions in Molecular Crystals Polymer Solutions The Vitreous State Electrical Conduction in Organic Solids Surface Chemistry of Oxides Reactions of Small Molecules in Excited States The Photoelectron Spectroscopy of Molecules Molecular Beam Scattering Intermediates in Electrochemical Reactions Gels and Gelling Processes Photo-effects in Adsorbed Species Physical Adsorption in Condensed Phases Electron Spectroscopy of Solids and Surfaces Precipitation Potential Energy Surfaces Radiation Effects in Liquids and Solids Ion-Ion and Ion-Solvent Interactions Colloid Stability Structure and Motion in Molecular Liquids Kinetics of State Selected Species Organization of Macromolecules in the Condensed Phase Phase Transitions in Molecular Solids Photoelectrochemistry High Resolution Spectroscopy Selectivity in Heterogeneous Catalysis Van der Waals Molecules Electron and Proton Transfer Intramolecular Kinetics Concentrated Colloidal Dispersions Interfacial Kinetics in Solution Oxidation 35 1 Volume 33* 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65* 66 67 68 69 70 71 72 73 74 75 76 77 * Not available; for current information on prices, etc., of available volumes, please contact the Marketing Oficer, Royal Society of Chemistry, Burlington House, London Wl V OBN stating whether or not you are a member of the Society.Date 1962 1962 I963 1963 1964 1964 1965 1965 1966 1966 1967 1967 1968 1968 1969 1969 1970 1970 1971 197 I 1972 1972 1973 1973 I974 I974 1975 I975 1976 1977 1977 1977 1978 1978 1979 1979 1980 1980 1981 1981 1982 1982 1983 1983 I984 FARADAY DISCUSSIONS OF THE CHEMICAL SOCIETY Subject Inelastic Collisions of Atoms and Simple Molecules High Resolution Nulcear Magnetic Resonance The Structure of Electronically Excited Species in the Gas Phase Fundamental Processes in Radiation Chemistry Chemical Reactions in the Atmosphere Dislocations in Solids The Kinetics of Proton Transfer Processes Intermolecular Forces The Role of the Adsorbed State in Heterogeneous Catalysis Colloid Stability in Aqueous and Non-aqueous Media The Structure and Properties of Liquids Molecular Dynamics of the Chemical Reactions of Gases Electrode Reactions of Organic Compounds Homogeneous Catalysis with Special Reference to Hydrogenation and Bonding in Metallo-organic Compounds Motions in Molecular Crystals Polymer Solutions The Vitreous State Electrical Conduction in Organic Solids Surface Chemistry of Oxides Reactions of Small Molecules in Excited States The Photoelectron Spectroscopy of Molecules Molecular Beam Scattering Intermediates in Electrochemical Reactions Gels and Gelling Processes Photo-effects in Adsorbed Species Physical Adsorption in Condensed Phases Electron Spectroscopy of Solids and Surfaces Precipitation Potential Energy Surfaces Radiation Effects in Liquids and Solids Ion-Ion and Ion-Solvent Interactions Colloid Stability Structure and Motion in Molecular Liquids Kinetics of State Selected Species Organization of Macromolecules in the Condensed Phase Phase Transitions in Molecular Solids Photoelectrochemistry High Resolution Spectroscopy Selectivity in Heterogeneous Catalysis Van der Waals Molecules Electron and Proton Transfer Intramolecular Kinetics Concentrated Colloidal Dispersions Interfacial Kinetics in Solution Oxidation 35 1 Volume 33* 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65* 66 67 68 69 70 71 72 73 74 75 76 77 * Not available; for current information on prices, etc., of available volumes, please contact the Marketing Oficer, Royal Society of Chemistry, Burlington House, London Wl V OBN stating whether or not you are a member of the Society.

ISSN:0301-7249    

DOI:10.1039/DC98478FX001

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

h Introductory Lecture From Trapped Radicals to Transients BY MARTYN C. R. SYMONS Department of Chemistry, The University, Leicester LEI 7RH AND KEITH A. MCLAUCHLAN Physical Chemistry Laboratory, South Parks Road, Oxford OX 1 342 Received 6 th September, 1984 This overview is divided into two sections, one concerned primarily with radicals stabilised by matrix-isolation procedures and the other with the detection of transients. In the former, attention is given to e.s.r. studies of radical cations, with particular emphasis on results for alkane cations and the remarkable range of structures revealed by the spectra. In our consideration of transient radicals major attention is given to the electron-spin requirements of radical combination and the way in which interconversion between the spin states of radical pairs is often controlled by proton hyperfine coupling.This leads us to consider CIDEP and CIDNP effects. In addition, the uses of ODMR techniques in the study of transient radicals is outlined. The subject of this meeting, radicals in condensed phases, encompasses a very wide range of structural and mechanistic chemistry, of fundamental physical proces- ses in liquids and solids and of photosynthesis and biology. Rather than attempt a global coverage, the organisers have sought to select those new aspects which seem particularly meet for attention at this Discussion. Many of the topics covered complement correspondingly exciting studies of radicals and radical ions in the gas phase, but no single Discussion could readily embrace both fields.In this introduc- tory paper we attempt to present an explanatory background to the major themes covered, whilst highlighting some aspects and showing how the contributed papers are relevant to the overall scheme. Our own main interests conveniently differ, one of us being involved with the study of radical ions in rigid matrices and the other with transient radicals. It is with the former that we start. MATRIX-ISOLATED RADICALS RADICAL CATIONS The use of e.s.r. spectroscopy to study radical cations has arrived late in their physical and chemical history. Indeed, it is curious that radical anions, which have been much less studied by other techniques, were quite extensively studied by e.s.r. spectroscopy long before the systematic study of cations.' 78 FROM TRAPPED RADICALS TO TRANSIENTS Table 1.Some radical cations prepared by photoionization in rare-gas mat rice^^'^ CO+, N:, H20+, NH;, CH:, H2CO+, C201 Two different procedures used to obtain matrix-isolated cations have proven to be complementary. In one, gas-phase photoionization is used to generate the cations prior to, or during, deposition in a rare-gas matrix.*” This proves to be an excellent method for generating cations of small molecules (table 1 ) ; these have so far not been prepared by the second method. The latter, a radiolysis technique, was originally developed by Hami114*’ for optical studies and was applied to e.s.r. studies by Shida el aL6 Its basis is summarized in scheme 1 below with the widely used CFC1, as solvent, although other freon solvents and SF, and CCI, have also been used effectively.Hamill’s use of n-butyl chloride is not satisfactory for e.s.r. studies because of the intense free-spin signals from alkyl radicals. (The effectiveness of this procedure underlines a fact that radiation chemists often stress, but which is still not widely accepted, that the use of ionizing radiation is a useful technique for inducing electron transfer.) Reaction (1) summarizes the initial ionization. The ejected electrons are rapidly and irrevers- ibly trapped by the solvent in reaction (2), but fortunately the resulting radicals yield broad, background, e.s.r. spectra at low temperatures. In contrast, the cations are mobile via electron transfer [reaction (3)] and readily react with solute molecules (S), provided these have ionization potentials lower than those of the solvent molecules (ca. 11.8 eV for CFCI,).h w CFC1, _I* CFCl;++ e- (1) (2) (3) CFCl;++S - CFCl,+-S+ (4) CFCl,+e- -+ (CFC1,)- -+ ‘CFCl,+Cl- CFCl;++ CFCl, % CFC1, + CFCl;+ Scheme 1 It is interesting to speculate whether process (4) is as specific as it seems. If so, are the ‘solvent complex’ species, S+---Cl-C( F)Cl,, considered in depth by Snow and William~,~ sometimes intermediates in reaction (4) or are they formed sub- sequently, as postulated by Iwasaki et aLs The radiolysis method, which has been successful in the preparation of many types of organic radical cations (table 2), has been reviewed recently.’ There is no overlap between tables 1 and 2, partly because the ionization potentials of the parent molecules used for generating the radicals in table 1 are too high for the second procedure and partly because some of these cations are able to transfer protons to the matrix molecules despite their low basicity.ALKANE CATIONS Since the initial discovery that alkane cations can be prepared in halogenocarbon matrices and that well defined e.s.r. spectra can be obtained,*’ there have been several studies of these fascinating species. The most extensive is the work of Iwasaki and his colleagues, some of which is reviewed and extended in this volume.”M. C. R. SYMONS AND K. A. McLAUCHLAN Table 2. Classes of radical cations in freon and related matrices saturated hydrocarbon cations alkene and diene cations aromatic and substituted aromatic cations chloro-, bromo- and iodo-alkane cations ether, acetal and diether cations aldehyde, ketone and ester cations dialkyl sulphide and disulphide cations: Me,Se+ amine, diamine and amide cations nitroalkane cations phosphine and phosphate cations alkyl silane, germane and stannane cations R,Si-SiR; and R,Sn-SnRl pyridine and substituted pyridine cations 5-membered heteroaromatic cations vinyl cations 9 It has long been considered that u bonds are localized whereas n bonds are not.However, delocalization within the (T framework is clearly demonstrated by the e.s.r. spectra of alkane cations, be they linear, branched or cyclic. In general, theory reproduces these results satisfactorily, as is illustrated by the paper of Lunell and Huang,l2 but there is usually a choice of several low-lying levels for the SOMO (semi-occupied molecular orbital) and often a Jahn-Teller distortion must occur; the actual SOMO selected by a given cation is an issue so subtle that different matrices (and, indeed, different theoreticians) may favour different structures.Usually marked distortions are involved,'0y11 but which distortion is selected is not easy to predict. It is satisfying that for 'CH: the distortion to a CZu structure, with two strongly coupled protons, is reproduced by ab initio theory. and the isoelectronic 'BH4 ALKENE CATION Alkene cations have also been prepared in halogenocarbon matrices, the most challenging result being that for C2H:, which stron ly implies a twisted ground state.I6 To what extent the alkyl-substituted cationj5 are also twisted is not yet known.However, for the stabilized cyclic-diene cations this cannot be an important issue, as is discussed by Davies." Apart from the achievement of preparing these cations in the liquid phase, this work is interesting in that it represents a return to the use of protic acids as media for preparing and stabilizing radical cations, a method which was developed many years HETEROATOM CATIONS In marked contrast to the results for alkane cations, the SOMO for organic cations containing heteroatoms is usually quite firmly located on these atoms; delocalization into the alkyl groups is relatively small although often greater than is found with alkyl radicals. Quite high spin densities have been detected on S protons in the cations of ketones7 for which the SOMO is, in a formal sense, localized on the carbonyl oxygen (I).It seems that, once in the alkyl part,10 FROM TRAPPED RADICALS TO TRANSIENTS CT delocalization prevails and the results might be expected to mimic those obtained with the alkane cations. R \ 0' R'C=b The structure of ester cations is a matter of controversy, as is stressed by Sevilla et d 2 0 There is no argument about a species formed from methyl formate in CFC13, which shows a large coupling to chlorine, all agreeing that the SOMO on the ester unit is similar to that for ketone cations (I). Although radicals formed irreversibly from this were originally thought to be the 7~ cations, convincing evidence has been presented8 that they are, in fact, rearranged radicals (11) of the type clearly formed from higher esters.The system reported here by Kemp and coworkers2' also seems to involve incipient radical cations since the efficiency of quenching of the fluorescence of uranyl ions follows the ionization potentials of the quenchers. Indeed, for R2S quenchers there is e.s.r. evidence for the formation of species identified as (R2S);" 'dimer' cations and, possibly, the parent cations. This tendency to form 'dimers', which is clearly linked to the solvent interactions discussed by Snow and William~,~ is very marked for 'heavy' heteroatom cations such as RCl'++ (RCl);+, R2S'++ (R2S);', R,P+ + (R3P)2+ etc., but is rare for first-row cations. A possible chemical reason for this is that first-row cations such as R20'+ prefer to extract hydrogen from R20 although calculations imply that CT* dimers such as R,O-*ORl are less stable than the parent In general, radiolysis initially yields radical cations and electrons [reaction ( 1 )], the latter usually giving rise to radical anions.In the examples given above, attention is focused on the radical cations. However, in most studies both centres are initially formed and they can occasionally be detected together. This happens with irradiated DNA, as exemplified by the work of Huttermann et It is extraordinary that irradiation of this complex system gives only two primary centres, G+ and T-. Even so, spectral analysis is difficult because the spectra are poorly defined and completely overlap. The use of oriented fibres helps in the task of unravelling these spectra.MEDIA INVOLVEMENT Before turning to techniques which probe the formation of transient species at short time intervals, we consider a group of papers which focus attention on various aspects of the medium rather than the radicals themselves. These are bridged by a paper7 in which the solvent effect is so specific that interaction of only one atom of one solvent molecule is ~bserved.~ Many interesting questions are raised by this and similar studies, and the issue of when such complexes form is made even more interesting by the observation that sometimes a unique interaction with fluorine is observed rather than with chlorine.25M. C. R. SYMONS AND K. A. McLAUCHLAN 11 Henglein and coworkers have developed techniques for probing and utilizing the ability of colloidal TiOz particles to store electrons or holes.26 These stores can then be used to oxidize or reduce compounds in solution.Their main techniques are flash photolysis coupled with optical and conductimetric measurements, so in a sense this study acts as a bridge between our two major themes. Kevan and his group,27 on the other hand, use the now traditional method of spin labelling to study the interface in microemulsions but using a sophisticated modification of the electron spin-echo technique to obtain useful information about the environment. In some respects, the conclusions drawn from the completely different study by Siebrand et d2* complement much of Kevan's pioneering work on trapped electrons and various radicals in low-temperature glasses.29 Sorting out the complex kinetic profile for the loss of trapped methyl radicals in methanol glasses in this way is an important development and opens up a range of possible experiments.The concept of a range of trapping sites in which the effective reactivity of the medium varies is certainly attractive in its simplicity. Perhaps ENDOR and electron spin-echo studies of matrix effects on CH3 radicals in CD30D would help to define these systems further. As Albery and Jones electrolysis can be used as an important tool to prepare interesting radicals for e.s.r. or other studies. Their concern, however, is the converse, namely to use e.s.r. spectroscopy to probe reactions at electrodes. The major advance in their work is their ability to incorporate laminar-flow electrodes in the e.s.r.cavity without unduly lowering the cavity Q. This gives them enough sensitivity to obtain spin assays of radicals formed by electrons or holes injected into surface coatings, and there is good agreement between the electrical and e.s.r. measurements. In extensive matrix-e.s.r. studies many workers have established that electron capture by alkyl halides (RX) is dissociative, giving R'+X-.' However, weak, diffuse binding is predicted in the gas phase. This problem has been probed by Clark using 20 helium atoms to reproduce a matrix, and the effect is dramatic.31 Thus molecules such as MeCN or MeCl, which merely flirt with electrons in the gas phase, are correctly predicted to interact to give well defined anions (MeCN-) or dissociative electron capture (Me' + C1-) in this condensed phase.In a sense this is theory coming to the aid of theory, since it justifies the use of approximations which actually provide the correct answers. Although this study relates to recent e.s.r. studies, the phenomenon of enhanced electron localization by a surrounding medium has long been known. Thus, for example, the p's' excited state of chloride ions is purely dissociative in the gas phase but the SOMO is localized in condensed phases. It also relates to the problem of localized electrons in condensed media. We now turn our attention to the new developments in the study of transient radicals in condensed phases. TRANSIENT RADICALS In the past ten years the study of transient free radicals and their reactions in solution has undergone a profound change.New methods have become available for the study of the earliest stages of reaction and for the unequivocal identification of the radicals, and in particular the spin requirements of radical combination reactions have been recognized as having great significance. In retrospect it seems astonishing that until quite recently radicals were regarded simply as reactive species whose unpaired electrons caused them to combine with low activation energy often12 FROM TRAPPED RADICALS TO TRANSIENTS at diff usion-controlled rates. The fundamental spin requirements known for so long to be necessary for chemical bonding were not considered in the kinetic context. Much of the advance in our understanding of radical reactions and many of the newer concepts are the results of fundamentally new experimental methods.The emphasis now is not on the traditional detection of radicals and the study of their reaction kinetics, but rather on the fundamental processes of their formation and on the details of their reaction. Major bridges have been built between kinetics and the theory of liquids, and molecular dynamics and spin dynamics are at last being considered simultaneously. Techniques now exist whereby a given, e.g. photochemically initiated, reaction can be followed continuously from the instant of light absorption to the formation of stable products, with each intermediate identified and each reaction established. This involves none of the guesswork associated with the interpretation of rate laws or the analysis of reaction products. Several independent experiments are necessary and in fact the complete history has probably not been elucidated yet for any single reaction.Historically the identification of the electronically excited states, formed on light absorption, which react to give radical products has been the province of the flash-photolysis method together with U.V. detection. The later technique of reson- ance-Raman spectroscopy also takes advantage of the sensitivity associated with optical detection methods, whilst yielding greater spectral resolution, and has been used in radiolysis studies This same information is now available from the magnetic-resonance phenomena, chemically induced dynamic electron and nuclear polarization (CIDEP and CIDNP) and from studies of the effects of magnetic and microwave fields on radical recombination reactions.These yield additional infor- mation associated with molecular dynamics and probe the fundamentals of reaction processes rather directly. THE RADICAL PAIR The immediate product of the reaction of an excited state, or a thermally dissociated ground state, is often a pair of free radicals. The concept of this radical pair, which is formed with the unpaired electrons on the radicals spin-correlated as a result of reaction with conservation of spin alignment, is basic to many of the new experiments. It is shown diagramatically in fig. 1, together with its energy levels. If, for example, the radicals are created by reaction of a triplet precursor, they are prevented by their electron-spin orientation from reacting immediately.They consequently separate by diffusion. During this period the radical pair undergoes triplet-singlet (T-S) mixing before the radicals might re-encounter each other as a result of their random motion. The mixing is the consequence of the electrons on the two radicals experiencing different local magnetic fields, normally due to different hyperfine interactions but in the presence of an external magnetic field due to different Zeeman interactions as well. In the latter case, at high field the radical pair may exist in a singlet or any of three triplet states (To, T*,); as the radicals separate the S and To states become degenerate and mix, but in normal liquids no mixing occurs with the non-degenerate T,l states.For a typical radical pair in a 1 tesla external magnetic field, complete S-To interchange takes ca. lo-* s , which establishes the timescale on which triplet geminate recombinations can occur. The rate at which a given pair attains a singlet configuration, and may react on re- encounter, consequently depends in part upon the hyperfine states in the radicals. Pairs with high local magnetic-field differences react first, leaving the others with aM. C. R. SYMONS AND K. A. McLAUCHLAN 13 TRIPLET 4- WENCHER - 00 ENERGY MIXING W L DIFFUSION I MICROWAVE 1 NUCLEAR AND ELECTRON SPIN - POLARIZED - DISTANCE BETWEEN RADICALS Fig. 1. Representation of the formation and subsequent reaction of a radical pair inside an applied magnetic field. Its crucial characteristic is a change in the multiplicity of the initially formed pair under the influence of local magnetic interactions which mix the S and To states.This affects the reaction probability if the radicals re-encounter and is also the origin of CIDNP and some CIDEP effects. S-To mixing becomes possible only as the levels approach degeneracy when the radicals diffuse apart and the electron exchange interaction tends to zero. The variation in the energies of the radical-pair states as the radicals separate and subsequently re-encounter is shown in the lower part of the diagram. Indicated are the microwave transitions which can be caused so as to affect the radical-combination probability at re-encounter in the ODMR experiment (see text). greater chance of diffusing out of the geminate cage before reaction.In this way radicals in some hyperfine states contribute to the geminate product whilst their complements are found in the escape products. This causes nuclear spin dis- equilibrium in the products and is the basis of CIDNP. The related electron spin resonance phenomenon, CIDEP, is observed in the free radicals which escape geminate recombination, and shares a similar origin in one of its forms (the radical-pair mechanism). Now, however, reaction on re- encounter is not a requisite. Rather, following S-To mixing the pair wavefunctions suffer a phase shift, due to the radicals experiencing the short-range electron exchange interaction, on re-encounter; this causes more electron spin of one type to accrue on one radical than on the other, with the sorting being into specific hyperfine states.CIDEP also arises in an independent process via the reaction of a polarized triplet molecule formed by state-selective intersystem crossing from an excited singlet state. This type (called the triplet mechanism) yields quite different characteristic intensities in the observed spectra than does the former. Spectra observed shortly after radical creation often show the effects of both processes. FIELD EFFECTS ON RADICAL PAIRS Since S-To interconversion is a magnetic phenomenon its rate should change if the applied magnetic field is changed. This in turn implies that the reaction14 FROM TRAPPED RADICALS TO TRANSIENTS probability of the geminate pair should change. This is the basis for expecting that the rate of a chemical reaction involving the combination of two radicals could depend upon an external magnetic In typical pairs of organic radicals the difference in the g factor is small (ca.0.001), and the Zeeman effect dominates the hyperfine effect only at comparatively large fields (ca. 1 T). At lower field strengths the hyperfine fields are most significant and indeed are responsible for reaction in the absence of an applied field. At zero field the triplet energy levels are split by electron-dipolar interactions and at low fields the states are intermediate between these zero-field states and the high-field triplet states (although either limiting set constitutes a suitable basis set for describing them). At low fields mixing is allowed consequently between all the levels, and the simple S-To description given above is no longer valid.A result is that resonances are observed in the effects of external fields on reactions when these fields become of the order of the hyperfine Since the latter exist only when the radical pair contains a magnetic isotope, this is known as an isotope effect ; it has been used to separate magnetic and non-magnetic isotopes.38339 Mixing can occur only between near-degenerate states, which implies that the electron exchange interaction has a profound influence, particularly through its distance dependence. This has been investigated in an elegant experiment involving radical-ion centres separated by a polymethylene chain of variable length.35 The same interaction plays a seminal role in transferring the ability to react from one radical-electron pairing to another in radiation spur reaction^.^' The radical-reaction probability within the geminate pair may also be changed by subjecting the system to resonant microwave radiation. This is done conveniently at X-band, at which the high-field description of the triplet radical-pair states holds.In principle the experiment is straightforward: since only the To state can mix with S so as to allow the radicals to react on re-encounter, the reaction rate depends upon the population of the To state. This can be changed by effecting To - T,, transitions which lie in the microwave region of the spectrum (fig. 1). Indeed for two radicals, neither of which contains a magnetic nucleus, the transitions at low microwave powers lie at the g values of the two isolated radical^.^' This provides a means for the positive identification of radicals on the 10 ns timescale.However, for reasons of sensitivity most experiments are performed at much higher microwave field strengths, and here the theoretical description is more complex. Not only does the microwave field itself mix the triplet levels but also, owing to S-To mixing, the singlet state is not pure and transitions occur between the mixed state and the triplet states. At the highest fields used none of the states is stationary in the usual spectroscopic sense. Although a qualitative description is difficult, the problem is reasonably straightforward theoretically and the observed effects of microwave fields, as the applied static magnetic field is varied, can be analysed to yield radical identities and the detailed molecular and spin dynamics which are In the presence of the microwave radiation used to pump these transitions normal electron spin resonance methods cannot be used to monitor its effect.Detection is consequently by opticaI methods in the new technique of optically detected magnetic resonance (ODMR), which is the fastest magnetic-resonance technique. Both have been used, an advantage of the latter being its generality and ability to monitor any of several short-lived species. It has been applied to great effect to study paramagnetic intermediates of very short lifetime in the photosynthetic processes in besides more conventional chemical radical ion pairs.The fluorescence studies are extraordinarily sensitive and have been applied to radical ion pairs at very low concentration besides fundamental studies of electrons and holes in hydrocarbon and optical-absorptionM. C. R. SYMONS AND K. A. McLAUCHLAN 15 MEDIA EFFECTS Throughout the above description of radical-pair processes the importance of the re-encounter of radicals in the correct electron-spin configurations for reaction has been emphasized. The re-encounter probability depends intimately on the diffusion which occurs, and also the extent of S-T mixing depends upon the lifetime of the radical pair. Much current interest consequently exists in prolonging this lifetime, for example by performing reactions in micellar solutions. These affect reaction rates in the absence of an applied field but also some of the most convincing reports of magnetic-field effects have been obtained using them.45946 They are of value too in facilitating the application of microwave radiation whilst the pair exists.47 Polarization observations are affected also, as S-T, mixing becomes possible at curve crossings which are no longer bypassed as quickly as in normal solutions .48 LATER REACTION STAGES Many of the processes which occur within the first 10 ns after the formation of a pair of free radicals are of a fundamental physical nature.Chemical processes involving triplet radical pairs are in general slower, although some reactions are sufficiently fast to cause radical substitution in the geminate pair between the time the pair is created and the re-encounter of the radicals; these affect all of the radical-pair experiments mentioned above and can be studied through them.Some- times comparatively long-lived exciplexes arise in the initial stages and lead, for example, to enhanced magnetic-field effects.49 This previously largely neglected period of a radical’s history is now being studied intensively. The radicals which can be observed directly in experiment are those which escape combination in the geminate cage. Although conventional U.V. techniques can be used to detect these radicals, poor resolution in solution often prevents positive identification. Here again the use of time-resolved resonance-Raman methods improves the information content of the spectra. Also, the time-resolved electron spin resonance method usually gives positive radical identification on the ca.0.5 ps timescale from the well resolved hyperfine structure of the spectra. At these short times the radicals almost invariably display CIDEP effects which, as described above, have the prehistory of the radical and of its molecular precursors enshrined within them. CIDEP effects also arise at later times in the encounters of radicals which diffuse randomly in solution, having escaped geminate combination. These allow direct identification of the pair which encounter at any time to yield a novel type of kinetic information. In general, too, the observation of CIDEP allows direct study of the reactions that radicals undergo to yield secondary radicals. Having identified the radicals it remains to establish their precise reaction pathways to products.This is where the CIDNP technique is pre-eminent, although it too can yield the identities of the radicals and the multiplicities of their molecular precursors. If the n.m.r. spectrum of a system undergoing free-radical reactions is observed whilst reaction proceeds, anomalous transition intensities are observed whose interpretation allows the detailed reaction mechanism to be deduced, besides distinguishing the cage and escape products. The intensities are affected severely by differential relaxation, and the most satisfactory way of obtaining undistorted information is to use flash-photolysis methods, with fast detection using a Fourier- transform spectrometer. These also allow greater time resolution than more conven- tional CIDNP studies and the detection of more transient species.CIDNP is unique not only in the information it yields on reaction pathways but also in its unequivocal16 FROM TRAPPED RADICALS TO TRANSIENTS identification of the primary products of a reaction. An important recent extension of the technique has been to incorporate it with two-dimensional n.m.r. method^.^' Whilst the magnetic and microwave techniques described above are all-powerful by their nature in investigating phenomena which originate in the innate spin dynamics of the system, other new methods make important contributions in related areas. The use of isotopes has long formed a part of the armoury of the kineticist, and in its newest form promises to be particularly valuable in investigating reactions involving hydrogen atoms. Thus when muons are stopped by matter a species, muonium, is produced which consists of a light nucleus, about one-ninth the rest mass of the proton, together with a single orbiting electron.It has a reduced mass very close to that of the hydrogen atom, which makes it a much more realistic probe of hydrogen-atom reactions than the much lighter species positr~nium.~' The nucleus also has a magnetic moment approximately three times that of the proton, and magnetic-resonance methods predominate in studies of the species. CONCLUSIONS In this brief and incomplete introduction to the state of studies of radicals in condensed phases we have attempted to discuss what seem to us the major themes of this Discussion.Some of the contributions do not fit easily into these and have been under-emphasized; the fact that the subjects they concern are represented in this volume signifies that we find them no less interesting or exciting than the ones we have highlighted. It is in the nature of a developing subject that not all can be collected together without artificiality. Similarly we are aware of those areas notably of laser experiments with both specific molecular excitation and/or with very high time resolution in transient studies, which are not represented. Nevertheless we hope to have indicated why studies of matrix-isolated radical ions and medium effects and transient studies of systems which involve radical pairs are of great and growing current interest.M. C. R. Symons, Pure Appl. Chem., 1981, 53, 223. L. B. Knight and J. Steadman, J. Chem. Phys., 1982, 77, 1750. W. H. Hamill, in Radical Cations, ed. L. Kevan and B. Webster (Wiley Interscience, New York, 1968), p. 321. T. Shida and W. H. Hamill, J. Chem. Phys., 1965,44, 2369. T. Kato and T. Shida, J. Am. Chem. SOC., 1979, 101, 6869. L. D. Snow and F. Williams, Furaday Discuss. Chem. SOC., 1984, 78, 57. M. Iwasaki, H. Muto, K. Toriyama and K. Nunome, Chem. Phys. Lett., 1981, 105, 586. M. C. R. Symons, Chem. SOC. Rev., 1984, in press. ' B. W. Keelam and L. Andrews, J. Am. Chem. SOC., 1981, 103, 829. l o M. C. R. Symons and I. G. Smith, J. Chem. Research ( S ) , 1979, 382. I ' M. Iwasaki, K. Toriyama and K. Nunome, Faruduy Discuss. Chem.Soc., 1984, 78,19. S. Lunell, M. B. Huang and A. Lund, Furaday Discuss. Chem. SOC., 1984, 78, 35. L. B. Knight, J. Steadman, D. Feller and E. R. Davidson, J. Am. Chem. SOC., 1984, in press. I4 T. A. Claxton, T. Chen, M. C. R. Symons and C. Glidewell, Furuduy Discuss. Chem. SOC., 1984, 78, 121. I s T. Shida, Y. Egawa and H. Kubodera, J. Chem. Phys., 1980, 73, 5963. Y. Nagata, M. Shiotani and J. Sohma, reported at the 22nd Japanese ESR Symposium, 1983. J. L. Courtneidge, A. G. Davies, P. S. Gregory and S. N. Yazdi, Furuduy Discuss. Chem. SOC., 1984, 78, 49. 12 13 16 17 l 8 T. R. Tuttle, S. Ward and S. Weissman, J. Chem. Phys., 1956, 26, 963. l9 A. Carrington, F. Dravnicks and M. C. R. Symons, J. Chem. SOC., 1959, 947. M. D. Sevilla, D. Becker, C. L. Sevilla, K. Plante and S.Swarts, Faruduy Discuss. Chem. SOC., 1984, 78, 71. 20M. C. R. SYMONS AND K. A. McLAUCHLAN 17 21 H. B. Ambroz, K. R. Butter and T. J. Kemp, Furuday Discuss. Chem. SOC., 1984, 78, 107. 22 M. C. R. Symons and B. W. Wren, J. Chem. SOC., Perkin Trans. 2, 1984, 5 1 1. 23 C. Glidewell, J. Chem. SOC., Perkin Trans. 2, 1983, 1285. 24 J. Huttermann, K. Voit, H. Oloff, W. Kohnlein, A. Graslund and A. Rupprecht, Faraduy Discuss. 2s A, Hasegawa and M. C. R. Symons, J. Chem. Soc., Furaday Trans. I , 1983, 79, 93. 26 D. Bahnemann, A. Henglein and L. Spanhel, Furuday Discuss. Chem. SOC., 1084,78, 151. 27 R. Maldonado, E. Szajdzinska-Pietek, L. Kevan and R. R. M. Jones, Furuday Discuss. Chem. 28 T. Doba, K. U. Ingold, W. Siebrand and T. A. Wildman, Furuday Discuss.Chem. SOC., 1984,78, 29 D. F. Feng and L. Kevan, Chem. Rev., 1980, 80, 1. 30 W. J. Albery and C. C. Jones, Furaduy Discuss. Chem. SOC., 1984, 78, 193. 3 1 T. Clark, Furuday Discuss. Chem. SOC., 1984, 78, 203. 32 R. Wilbrandt, Faruduy Discuss. Chem. SOC., 1984, 78, 213. 33 R. Kaptein, in Chemically Induced Magnetic Polarization, ed. L. T. MUUS, P. W. Atkins, K. A. 34 C. D. Buckley, A. I. Grant, K. A. McLauchlan and A. J. D. Ritchie, Furuday Discuss. Chem. Soc., 35 A. Weller, H. Staerk and R. Treichel, Faruday Discuss. Chem. SOC., 1984, 78, 271. 36 R. Z. Sagdeev, K. M. Salikhov and Yu. N. Molin, Russ. Chem. Rev., 1977,46, 297. G. P. Zientara and J. H. Freed, J. Chem. Phys., 1979, 70, 2587. 38 A. V. Podoplelov, T. V. Leshina, R. Z. Sagdeev, Yu. N. Molin and V. I. Goldarskii, JETP Lett., 1979, 29, 380. N. J. Turro, M.-F. Chow, C.-J. Chung, G. C. Weed and B. Kraeutler, J. Am. Chem. SOC., 1980, 102, 4845. Chem. SOC., 1984, 78, 135. SOC., 1984, 78, 165. 175. McLauchlan and J. B. Pedersen (Reidel, Dordrecht, 1977). 1984, 78, 257. 39 39 40 B. Brocklehurst, Faruday Discuss. Chem. SOC., 1984, 78, 303. 41 A. B. Doktorov, 0. A. Anisimov, A. I. Burshtein and Yu. N. Molin, Chem. Phys., 1982, 71, 1. 42 M. R. Wasielewski, J. R. Norris and M. K. Bowman, Faruduy Discuss. Chem. SOC., 1984,78, 279. 43 Yu. N. Molin, 0. A. Anisimov, V. I. Melekhov and S. N. Smirnov, Furaday Discuss. Chem. SOC., 44 J. P. Smith, S. M. Lefkowitz and A. D. Trifunac, J. Phys. Chem., 1982, 86, 4347. 45 N. J. Turro and B. Kraeutler, J. Am. Chem. Soc., 1978, 100, 7432. 46 H. Hayashi, Y. Sakaguchi and S. Nagakura, Chem. Lett., 1980, 1149. 47 A. I. Grant, K. A. McLauchlan and S. G. Nattrass, to be published. 48 A. D. Trifunac and D. J. Nelson, Chem. Phys. Lett., 1977, 46, 346. 49 T. Ulrich, V. E. Steiner and R. E. Foll, J. Phys. Chem., 1983, 87, 1873. 50 R. M. Scheek, S. Stob, R. Boelens, K. Dijkstra and R. Kaptein, Furaduy Discuss. Chem. SOC., 5 1 P. W. Percival, J. C. Brodovitch and K. E. Newman, Furuduy Discuss. Chem. SOC., 1984,78,315. 1984, 78, 289. 1984, 78, 245.

ISSN:0301-7249    

DOI:10.1039/DC9847800007

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1984, 78, 19-33 Electron Spin Resonance Studies of Structures and Reactions of Radical Cations of a Series of Cycloalkanes in Low-temperature Matrices BY MACHIO IWASAKI," KAZUMI TORIYAMA AND KEICHI NUNOME Government Industrial Research Institute, Nagoya, Hirate, Kita, Nagoya 462, Japan Received 9th May, 1984 Radical cations of a series of cycloalkanes (c3-cg) produced radiolytically at 4.2 K in a variety of halogenated matrices have been characterized by e.s.r. spectroscopy as 0-delocalized radicals, in which the unpaired electron occupies the molecular orbital lying in the molecular (equatorial) plane. Thus the protons in the equatorial C-H bonds participating in the MO give rise to large hyperfine couplings as compared with the axial protons. Among these, radical cations of C3, C4, C6 and C8 are Jahn-Teller (J.T.) active and exhibit static distortions at 4.2 K, giving non-equivalent proton couplings. Regardless of the matrices used, the cation distorts to the same direction along the J.T.-active ring-deformation mode giving essentially the same geometry and singly occupied molecular orbital (SOMO).The distorted structure is dynamically averaged at elevated temperatures, giving equivalent couplings. The tem- perature changes of the spectra of c-C3Hl and c-C6HT2 are accounted for by a three-site jumping (or tunneling) model using a modified Bloch treatment. The site-jumping process may be pseudorotation in the J.T. potential trough. However, the onset temperature of the jumping process and thus the activation energy differ from matrix to matrix, so that the observed distortion may be a matrix-assisted static J.T.distortion. The results may be also accounted for in terms of a static distortion due to matrix effects and an averaging of the matrix field by reorientation of the cations around the normal to the molecular plane. However, the latter explanation seems less probable because the onset of the averaging process of c-C6HT2 at temperatures as low as S4.2 K does not appear to be ascribable to molecular reorientation. Most of these cations undergo deprotonation to form cycloalkyl radicals in SF6 and in CFC12CF,Cl at b 100 K. In comparison with T- and n-cation radicals, little is known about the organic a-cation radicals formed by loss of an electron from a-bonding orbitals. Among these, radical cations of simple alkanes must be of the most fundamental importance.Previously we have successfully observed the e.s.r. spectra of the radical cations of a series of linear and branched alkanes produced radiolytically in low-temperature matrices and have characterized their structures and reactions for the first time.'" In the present work these studies are extended to radical cations of cycloalkanes and the results are compared with those of alkanes studied previously. Of particular interest are radical cations of highly symmetrical alkanes having their unpaired electrons in degenerate orbitals; these provide us with a class of Jahn-Teller (J.T.)-active species. In previous studies we have shown that J.T.-active radical cations such as C2H6+,'33 CHMe:,'73 CMe:,'*3 c-C,H,',~ C-C,H,',~ c-C6H ;'* and C6H6+ l o exhibit static distortions at 4.2 K, although most of them give dynami- cally averaged structures at 77 K.Direct experimental information on the ground- state geometry and on the unpaired-electron orbital have been obtained from the e.s.r. spectra at 4.2 K. Although the preliminary results have been briefly r e p ~ r t e d , ~ - ~ further studies on the dynamical behaviour and its matrix dependence are reported 1920 E.S.R STUDY OF CYCLOALKANE RADICAL CATIONS here together with an overall view of the structural characteristics of cycloalkane radical cations. In addition to these results of structural interest, the reactions of cycloalkane radical cations and their matrix dependence are briefly described.The radiolytic production of radical cations in a CFCl, matrix was first reported by Shida and coworkers;" we have further shown that SF,, 1,3 chlorofluorocarbons such as CFC12CF2C12,3 and the perfluorocarbons2>' can be used as matrices, in which radical cations of a number of prototype alkanes are stabilized at 4.2 K. A variety of chlorofluorocarbons such as CF3CC13 l 2 and the perfluorcycloalkanes l 3 have since been proved useful at 77 K by other groups. In the present study irradiation of these matrices was carried out at 4.2 K, since the spectra obtained from the samdes irradiated at this temperature often exhibit irrevirsible changes upon warming to 77 K. 1-3,8,9,14 Although some of the cycloalkane radical cations (Cl, C,' and C z ) produced at 77 K have been studied previously by other group^,^^'^^ neither the ground-state geometry nor the singly occupied molecular orbital has been determined for c-C3H,' and c-C6HT2, and the observed and calculated coupling constants for c-CsHTo produced at 77 K were not in agreement.EXPERIMENTAL c-C3H6 and SF6 were obtained from Takachiho Kogyo and the other cycloalkanes and CFCl, from Tokyokasei Kogyo. Other chlorofluorocarbons and perfluorocycloalkanes were from P.C.R., except for CFClzCFzCl which was from Daikin Kogyo. The method of sample preparation and the apparatus were essentially the same as those described in our previous papers.'-'' A frozen solution containing a dilute solute (0.1 - 1 .O mol% ) of the cycloalkane was irradiated by X-rays at 4.2 K and by 6oCo y-rays at 77 K.The e.s.r. spectra were measured at 4.2 K by Varian E-12 and Bruker ER-200 D spectrometers using a home-made liquid- helium cryostat, and the temperature dependence of the spectra below 77 K was measured with an Oxford ESR-9 variable-temperature system, after careful calibration of the specimen temperature. RESULTS AND DISCUSSION STRUCTURES OF CYCLOALKANE RADICAL CATIONS E.S.R. SPECTRA AND STRUCTURES AT 4.2 K To elucidate detailed structures we have studied a series of cycloalkane radical cations from C3 to C8 produced at 4.2 K, among which C3, C4, c6 and C, are J.T.-active. c-C,Hl produced in CFC12CF2CI at 4.2 K exhibits a 3 ~ 5 - l i n e spectrum [fig. I(a)] with coupling constants of 11.0 G (2H) and -24.0 G (4H), giving evidence for Q h --+ C,,distortion from a regular to an obtuse triangle with the unpaired electron in the a , orbital rather than to ar? acute triangle with it in the 61 0rbita1.~ The isotropic coupling constants obtained from INDO calculations gave resonable agreement with the observed results, as shown in the parentheses in fig.I ( a ) . Shown in fig. 2(a) is a spectrum observed immediately after irradiation at 4.2 K in CFC13. The slight difference in the spectral features from those given in our preliminary report' is due to a partial orientation of the frozen sample previously used." In comparison with that observed in CFCI,CF,CI [fig. I ( a ) ] the spectrum is well resolved, exhibiting g and hyperfine anisotropy. The spectrum [fig. 2( 6)] simulated with the parameters listed in table 1 reveals the essential features of the observed85 GI102.1) 4G (17.9) 50 G r Fig.1. E.s.r. spectra of radical cations of a series of cycloalkanes produced and observed at 4.2 K, their static distortions and the unpaired electron orbitals. ( a ) c-C,H:in CFCl2CF2C1, (b) c-C,HZ in CFCl,, ( c ) c-CSHIo in CFCl2CF2C1, ( d ) c-C,HT, in CFCl,, (e) c-C7Ht4 in CFC12CF2C1 and (f) c-C8HT6 in CFC13. The assignments of the observed coupling constants and comparison with the INDO values (in parentheses) are also given. The spectrum of c-C8HT6 in CFC1, was observed at 4.2 K after annealing at 77 K, because of the low yield of the cation at 4.2 K and its increase upon warming to 77 K. * Ref. (16). z P M22 E.S.R STUDY OF CYCLOALKANE RADICAL CATIONS CFCI, 2 1 20 - 19 21 8 I 17 16 i I /I \ b\i .A A 10 20 30 1 0 3 ~ 1 T Fig. 2. E.s.r. spectra, their temperature changes and simulations for c-C,H; produced in both CFCl, and SF, at 4.2 K. ( a ) Observed at 4.2 K in CFCl,; ( b ) anisotropic simulation for ( a ) using the parameters listed in table 1 ; ( c ) observed at 18 K in CFCl,; ( d ) observed at 27 K in CFCl,; ( e ) simulation for ( d ) with T - ~ = 0.67 x lo8 s-'; (f) observed at 77 K in CFCl,; ( g ) observed at 140 K in CFCl,; ( h ) observed at 4.2 K in SF,; (i) observed at 12 K in SF,; ( j ) simulation for ( i ) with T - ~ = 1.4 X lo8 s-'; ( k ) observed at 37 K in SF,. The spectra ( e ) and ( j ) are simulated by the modified Bloch treatment given in the text. The inset shows Arrhenius plots of the jumping rate ( T - I ) .0, c-C,Hz in CF,CCl,; 0, c-C,H; in CF,ClCF,Cl; 0, c-C,Hl in SF,; A, c-C,H; in CFCl,; 0, c-C,HT, in CFC1, (low-temperature dynamics); ., c-C,HT2 in CFC1, (high-temperature dynamics) (see text). In ( a ) C.C. denotes colour centres. spectrum, although a further refinement may be desirable to reproduce the observed small substructures. The hyperfine anisotropy characteristic of the a -protons attached to C2 and C3 supports our interpretation in terms of the a , SOMO (table c-C,Hl, observed in CFC13 immediately after irradiation at 4.2 K, exhibits a 3 x3 line spectrum [fig. l(b)] with coupling constants of 49 G (2H) and 14 G (2H), giving evidence for the D2d - C2, distortion from a puckered square to a puckered rhombus with the unpaired electron in the b2 orbital, rather than the D2 distortion to a rectangle with the b3 orbital as previously reported.8 c-C,HT, produced at 4.2 K in CFCl2CF2C1 exhibits a 3-line spectrum [fig.l(c)] with 25 G(2H), suggesting the C, form with the unpaired electron in the u N orbital, in agreement with the previous prediction at 77 K.12915316 1).M. IWASAKI, K. TORIYAMA AND K. NUNOME 23 Table 1. Hyperfine and g-tensors used for the simulation of the spectra of c-C3Hl and c-C6HT2 observed in CFC1, at 4.2 K direction cosines" principal values/ G X Y Z c-C3Hl -4.5 0.47 1 0.240 0.843 -12.4 -0.483 0.876 0.000 -18.1 0.739 0.407 -0.538 -1 1.7 27.8 0.000 0.892 -0.452 22.8 1 .ooo 0.000 0.000 21.4 0.000 0.452 0.892 24.0 2.0032 1 .ooo 0.000 0.000 2.0040 0.000 1 .ooo 0.000 2.0060 0.000 0.000 1 .ooo 2.0044 C-C,H T2 88.7' 0.000 0.957 0.289 83.2 85.0 (68.0)' 32.2 34.0 (45.0)' 17.4 0.930 -0.037 0.366 12.0 13.8 (18.0)' 2.0 109 0.000 0.000 1 .ooo 2.0046 2.0067 (2.0067)' 1 to the All axis 37.6 -0.930 -0.037 0.366 1 to the All axis 1 to the All axis 1 to the 811 axis a Given for one of the magnetically non-equivalent protons.The others can be obtained by symmetry operations. The z-axis is perpendicular to the molecular (equatorial) plane and the y-axis along C1 in the plane. The estimation of the anisotropic term is based on the calculations of the dipolar interactions by the two-centred integrals for all the spin densities on each carbon atoms. ' The isotropic values after annealing at 77 K. The same anisotropic terms wepe tenta- tively assumed for the simulation given in fig.2(d). c-C,HT2 produced in CFC13 at 4.2 K exhibits a 3 x 3 X 3-line spectrum [fig. 1 (d) and 4(a)] with coupling constants of 85 G(2H), 34 G(2H) and 14 G(2H). The anisotropic components determined by the spectral simulations are given in table 1. From the dynamical behaviour and from the INDO calculations, the D3d -+ CZh distortion from a regular to elongated chair with the unpaired electron in the ag orbital is suggested rather than that to a compressed chair with the bg orbital.' The non-equivalence of the two smaller couplings is attributable to the matrix effect.' c-C,HT, produced in CFCl2CF2C1 at 4.2 K exhibits a 3-line spectrum [fig. I ( e)] with a coupling constant of 81 G(2H) with substructures of 17 G(2H) and 14 G(2H).24 E.S.R STUDY OF CYCLOALKANE RADICAL CATIONS The spectrum can be interpreted in terms of a C, twisted-chair form17 with the a orbital rather than of a C, chait form with the a" orbital based on the INDO calculation given in fig.1 ( e ) . The cations exhibit a I Wine spectrum with 16 G( 14H) at 113 K in c-C6FIICF3.'* At 4.2 K c-CgHT6 in CFC13 exhibits a 19-line spectrum [fig. l ( f ) ] with coupling constants of 89 G(2H), 44 G(4H) and 22 G(2H), giving evidence for the D4d - CZu distortion. The unpaired electron is probably in the b, orbiial rather than in b , . Although the trial INDO calculations do not yet give satisfactory agreement, the compression along the orbital axis by the out-of-phase e2 deformationi9 shown in fig. l ( f ) gave a closer set of coupling values to those observed than that caused by the in-phase e2 deformation.The spectral change observed at 77 K is accounted for by the partial average of the equatorial (89 G ) and axial (22 G) proton couplings by ring-puckering with the linewidth alternation. Details will be given elsewhere.20 These cations from c-C,Hl to c-C,HT6 possess essentially the same structures in other matrices, as shown in table 2. CHARACTERIZATION OF CYCLOALKANE RADICAL CATIONS As is shown in fig. I , the unpaired electron in these cycloalkane radical cations is delocalized over the a-molecular orbital lying in the equatorial plane of the puckered ring, except in the case of c-C,H,'. As a result, the equatorial C-€3 bonds preferentially participate in the molecular orbital, giving larger positive coupling constants in comparison with the axial C-H protons.The situation is similar to that of the in-plane terminal protons in linear alkane radical cations, as is described in our previous papers. The planar c-C3Hl obviously gives an equivalent coupling constant for the CH2 protons. The protons in the axial C-H bonds and in the apical CH2 group, which are perpendicular to the unpaired-electron orbital, exhibit an a -coupling by spin polariz- ation and generally give a relatively small coupling constant because of the relatively low density of the unpaired electron at each carbon atom in such a-delocalized radicals. In c-C3H,' the a-protons are attached to the basal CHI groups. The absence of a large unpaired-electron density at the two basal carbon atoms in c-C5HTo is due to the symmetry requirement for the u" orbital, in which the molecular orbital must be antisymmetric with respect to the symmetry plane.If the c-C7HT4 has a C, chair form, the unpaired electron cannot be delocalized over the two basal carbon atoms as in c-C,HTo. In the C2 twisted-chair form, however, the absence of mirror symmetry permits the delocalization of the unpaired electron to the basal carbon atoms. The J.T.-active cycloalkane radical cations studied in this work exhibit static distortion at 4.2 K, which is expected from deformation along the J.T.-active ring- deformation mode. The observed coupling constants are also consistent with the INDO values (fig. 1) calculated for the deformation along the J.T.-active ring- deformation mode given in table 2. The dynamically averaged coupling constants observed at elevated temperatures (table 2) are nearly equal to the average of the coupling values observed for the distorted structure at 4.2 K.In comparison with usual carbon-centred radicals, the J.T.-active cycloalkane radical cations studied show a tendency to give a relatively large positive g-shift, as is shown in tables 1 and 2. The g,,, values estimated from the simulation of the anisotropic spectra are 2.0060 and 2.0109 for c-C,Hl and c-C6HT2, respectively, the g,,, axis being perpendicular to the molecular (equatorial) plane. In these J.T.-active u-radical cations with the electron configuration . -(a,)'( a,,)', the excitation of an electron from ax to the half-filled a,, orbital by L, perpendicular to the molecularTable 2.Geometrical and electronic structures and e.s.r. parameters of radical cations of a series of cycloalkanes (C3-C,) produced at 4.2 K hyperfine averaged F radical gf splitting/Gg hyperfine other matrices cations matrix J.T. distortion SOMO at 4.2 K at 4.2 K position splitting/G examinedk e' c-C3Hz a CFC12CF2Cl C,, __* CZv obtuse a, 2.0040 triangle b2 c-C4Hl CFC1, D,, - CZv rhombus b, 2.0045 c-C,HT-, CFCl2CF2C1 - cs u" 2.0036 e6, C-C~HTZ CFC13 D3, .__* C2, elongated ag 2.0066 chair c-C,HT4 CFCl,CF,Cl - twisted a 2.0042 chair e2 C-CgHT6 CFCI3 D4d __* C2: compressed b2 2.0059 24.0 (2H) - 1 1.0 (4H) 49 (2H) 14 (2H) 25 (2H) 85 (2H) 34 (2H) 14 (2H) 81 (2H) 17 (2H) 14 (2H) 89 (2H) crowne (2.0068)" 44 (4H) 22 (2H) SF6, CFC13, CF3CC13 C F2CI C F2 C 1 SF,, CFCl2CF2CI ~~ ~ _ _ _ ~~~~ a CJ ref.(7); ' Cf. ref. (8); Cf. ref. (9); e2 out-of-plane deformation''; Structures and e.s.r. parameters are essentially the same; ' the value in SF,; " the value in c-C6Fl,CF3; ' I at 77 K. compression along the orbital axis; crossover point of the isotropic value; " one of the equivalent positions; 'abbreviations e and a denote equatorial and axial; from ref. first-derivative spectrum; (15);26 E.S.R STUDY OF CYCLOALKANE RADICAL CATIONS (equatorial) plane must give a large positive g-shift, because all the (p,,~L,~p,) components at each carbon atom have the same sign contributing to the positive g-shift without cancellation, although contributions from the overlap with adjacent atomic orbitals and those from other levels must not be neglected. For this reason, J.T.-active a-radical cations may have a higher possibility of exhibiting a larger positive g-shift in comparison with J.T.-active n-radical cations, in which L, cannot mix the two J.T.or matrix split levels, giving a relatively small g-shift.21 DYNAMICAL BEHAVIOUR AND MATRIX EFFECTS c-C,H,' The spectra observed in CF,CC13 at elevated temperatures give better resolution, although the spectrum at 4.2 K [fig. 3(a)3 is essentially the same as that in CFCI3 [fig. 2( a)] except for the slightly poor resolution of the anisotropic substructure. The e.s.r. parameters (including anisotropy) in CF3CCI3 are almost exactly the same as those observed in CFC13. With increasing temperature the onset of a dynamical process becomes appreciable at ca.65 K, giving rise to an increase in the intensity of the central line accompanied by a decrease in those of the outer lines. However, the spectral positions of the outer lines persist in a fairly wide temperature range during broadening, and a sudden coalescence of the outer lines to a single one takes place over a narrow temperature range (102-1 13 K). This coalescence is due to a dynamic average of positive and negative hyperfine couplings arising from p- and a-protons, respectively, giving a coupling constants as small as (24.0-1 1.7 x 2)/3 = 0.2 G. The observed spectral changes are typical of a dynamical process involving site-jumping (or tunneling) and cannot be expected from the change in Boltzmann populations with temperature of the two near-degenerate states often assumed in the solution e.s.r.of J.T.-active species." The spectral changes can be accounted for by modified Bloch treatment22 for jumping between three sites corresponding to the three equivalent obtuse-triangle structures. Since the spectral anisotropy is reduced with increasing temperature, the isotropic spectral simuiations were perfor- med neglecting the residual anisotropy. As shown in fig. 3, the essential features of the observed spectral changes are reproduced. From the change in jumping rate with temperature shown in the inset of fig. 2, the epparent activation energy, E,, of this dynamic process is found to be 680 cal mol-', with a relatively low frequency factor ( A = 3.4 x lo9 s-') as shown in table 3.The change in the spectra of CFC13 with temperature (fig. 2) is considerably different from that in CF3CC13, although the distorted structure at 4.2 K is essentially the same. In CFC13 the onset of a decrease in the outer-line intensities starts at ca. 20 K. However, the increase in intensity of the central line is not as remarkable as in CF3CC13. The broad and complicated feature at the centre indicates that the anisotropy due to the non-coaxial orthorhombic hyperfine tensors of the four a-protons is not reduced efficiently with the onset of three-site jumping in CFC13. This may suggest that the efficient reduction of the anisotropy in CF3CC13 is partly due to dynamics other than three-site jumping. A plausible explanation may be the onset of in-plane oscillations of the small ring.Leibling and M ~ C o n n e l ~ ~ reported a similar observation for cyclopentadienyl radicals in a single crystal of naphthalene at low temperatures. If this is the case, the onset of three-site jumping in CFC13 preceeds that of in-piane oscillation, so that the larger residual anisotropy makes the spectral features broader than in CF3CC13. Indeed, the tensor average of the three distorted structures indicates that the residual anisotropic tensor elements areM. IWASAKI. K. TORIYAMA AND K. NUNOME 102 K i 27 113 K Fig. 3. Temperature change of the e.s.r. spectra and their simulation for c-C3Hz produced at 4.2 K in CF3CC13. ( a ) Observed at 4.2 K; the stick diagram indicates the isotropic line positions. ( b ) 48, (c) 64, ( d ) 77, ( e ) 95, ( f ) 102 and (8) 1 13 K.The simulations were made by the modified Bloch treatment for equivalent three-site jumping (see text) with 7 - l = ( h ) 0.44, ( i ) 0.67, ( j ) 1.0 and ( k ) 1.7 x lo8 sf'. Table 3. The apparent activation energies and the frequency factors for the averaging processes in distorted c-C3H,f and c-C6HT2 cations matrices EJcal mol-l A/ lo9 s-l c-C3Hz CF3CCl3 680 3.4 C F2ClC F2C 1 540 2.1 C FC13 (50)" (0.14)" F6 34 0.62 1 70b 3.2 240' 0.22 " The values may be less reliable in comparison with others (see text); ' averaging of a , and a2 in the low-temperature region (see text) ; averaging of a3 and ( a , + a2)/2 in the high-temperature region (see text).28 E.S.R STUDY OF CYCLOALKANE RADICAL CATIONS Fig. 4. E.s.r.spectra, their temperature changes and simulations for c-C6HT, produced at 4.2 Kin CFC13. ( a ) Observed at 4.2 K immediately after irradiation; ( b ) anisotropic simulation for ( a ) using parameters listed in table 1 ; (c) at 4.2 K after annealing at 77 K; ( d ) anisotropic simulation for (c) using the parameters listed in table I ; ( e ) 18, (f) 24, (8) 37, ( h ) 58 and ( i ) 140 K. The stick diagrams indicate the change in the line positions due to the averaging of the coupling constants. The simulations were made by the modified Bloch treatment (see text) for low-temperature changes with ( j ) 7;' = 0.33 x 10' s-' and ( k ) 7;' = 0.67 x 10' s-', and for the high-temperature changes with (Z) 7;' = 0.1 1 x 1 O8 s-I. is as large as 2.3, -1.8 and -0.5 G and are reduced to 1.7, -0.9 and -0.9 G by the in-plane oscillations.The spectra calculated by the Bloch treatment using a broader linewidth reveal an essential feature of the spectral changes in CFCI,, as shown by a typical example in fig. 2. The Arrhenius plot consists of two straight lines with E, = 50 and 810 cal mol-', corresponding to the low-(S77 K) and high-(a77 K) temperature changes, respectively. The high-temperature dynamics may be associ- ated with other motions which reduce the residual anisotropy, so that the results obtained from the high-temperature region were disregarded. However, the value of 50 cal mol-' for the low-temperature dynamics obtained from such isotropic simulations may not be so reliable. The spectrum observed in SF6 (fig. 2) exhibits broadening due to the dynamic process even at 4.2 K, and coalescence to a single line is complete at ca.37 K, giving E, = 34 cal inol-' (inset of fig. 2 and table 3). The temperature change of the spectra in CF2ClCF2Cl is similar to that observed in CF,CCI, giving E, = 540 cal rnol-' (inset of fig. 2 and table 3). The spectrum observed in CFC12CF2CI at 4.2 K [fig. l(a)] exhibits signs of the onset of dynamics even at 4.2 K. However, a partialM. IWASAKI, K. TORIYAMA AND K. NUNOME 29 conversion of the cation to a cyclopropyl radical upon warming to 77 K does not allow us to estimate the activation energy. These results indicate that the dynamic behaviour differs from matrix to matrix. Nevertheless, the cation exhibits essentially the same hyperfine couplings and g-values at 4.2 K regardless of the choice of matrix.C-C,H T2 Shown in fig. 4( a ) is the spectrum observed at 4.2 K immediately after irradiation at 4.2 K. After the sample had been annealed at 77 K, the spectrum was remeasured at 4.2 K and it exhibits an irreversible change in the hyperfine coupling constants while retaining the overall spectral width [fig. 4( c ) ] . The three coupling constants, a , , a2 and a3 comprising the 3 ~3 ~ 3 - l i n e spectrum change from 85, 34 and 14 G to 68, 45 and 18 G, respectively. Once this change occurs, the spectrum exhibits a reversible temperature change. As shown in fig. 4, with increasing temperature the averaging of a , and a2 occurs in the temperature range 18-37 K, whilst a3 shows a little change. The averaging of a3 with a , and a2 becomes prominent at ca.30 K, when the averaging of a , and a2 is almost complete. The equalization of the three coupling constants is accomplished at ca. 140 K. During these changes, the overall spectral width remains unchanged. Fig. 5( a ) gives the variation with temperature of these three coupling constants plotted against the largest coupling a , . Note that the variation of both a2 and a3 is linear, including the values observed at 4.2 K before thermal annealing at 77 K. The results indicate that the irreversible change induced by thermal annealing is also related to the dynamics, which averages the coupling constants at elevated temperatures, and that the cause of the change is ascribable to an irreversible change in the environment.In addition, the non- equivalence of a2 and a3 at 4.2 K is ascribable to the partial averaging of a , and a2 by zero-point vibrations. We have further confirmed that the spectrum observed at 1.5 K is essentially the same as that at 4.2 K. The preferential averaging of a , and a2 may be explained by the non-equivalent-site jumping model, as previously described.’ Although the qualitative features of the observed spectra were repro- duced by non-equivalent-site jumping, it was not feasible to obtain satisfactory agreement to estimate the activation energy. So, by assuming simple equivalent two-site jumping corresponding to the exchange of a , and a2, the approximate activation energy was estimated to be 170 cal mol-I, whereas that of the averaging of a3 with a , and a2 is estimated to be 240 cal mol-’ assuming jumping between the three equivalent sites corresponding to a3, ( a , +a2)/2 and ( a , +a2)/2 (see fig.4, inset of fig. 2 and table 3). The 7-line spectrum of 44 G(6H) at 140 K indicates that ring inversion does not occur at 140 K, suggesting that the six-membered ring of c-C6HT2 is rigid in com- parison with other cycloalkane radical cations, in which the ring puckering averages the axial and equatorial proton couplings at elevated temperatures, making all the protons equivalent.8” 2,15318924 The spectral features of c-C6H T2 are also matrix-dependent reflecting differences in the extent of the dynamic-averaging process at the same temperature. The spectrum observed in CF3CC13 at 4.2 K [fig. 5 ( 6 ) ] gives similar sets of coupling constants to those observed in CFC13 at 4.2 K immediately after irradiation at 4.2 K, as shown by the filled circles in fig.5 ( a ) . On the other hand, the spectra observed at 4.2 K in CFCl2CF2C1 [fig. 5 ( c ) ] as well as in C-C6Flo(CF3)2 [fig. 5( d ) ] and c-C6F12 [fig. 5 ( e ) ] show that the averaging of a , and a2 is almost complete even at 4.2 K as are plotted as the half-filled circles in fig. 5(a). The results indicate that the structure of c-C6HT2 is essentially the same regardless of the matrices examined.30 E.S.R STUDY OF CYCLOALKANE RADICAL CATIONS &ALL I C F3CC13 -ILl IY I CFCI,CF,CI v 1\11 I I I1 II 1 C-CcF., Fig. 5. ( a ) Correlation of the temperature changes of the three coupling constants a,, a2 and a3 in c-C6HT2 produced at 4.2 K in CFCI,.The matrix dependence of the coupling constants is also given in the figure. 0, 4.2 K in CFC13 immediately after irradiation at 4.2 K. 0 , temperature change observed in CFC13 after annealing at 77 K. From the decreasing order of a, the observation temperatures are 4.2, 12, 18, 30, 36,48, 58,77, 105 and 140 K. 0, 4.2 K in CF3CC1,; a, 4.2K in CFC1,CF2Cl. Essentially the same values were observed in c- C6F,,(CF,), and in c-C,F,,. ( b ) - ( e ) E.s.r. spectra observed at 4.2 K immediately after irradiation at 4.2 K, ( b ) in CF3CC13, ( c ) in CFCl2CF2C1, (d) in c-C,F,,(CF,), and (e) in c-C6Fl2. The stick diagrams indicate the isotropic line positions. MATRIX EFFECTS IN J.T.-ACTIVE RADICAL CATIONS The most important finding is that the geometrical and electronic structures of c-C,H,' and c-C,HTz are not greatly affected by the choice of matrix, although dynamic behaviour is seriously affected by the choice of environment.The results suggest that the distorted structure observed may be essentially intrinsic to these cations. The first-order J.T. distortion energy may be sufficiently high to determine the symmetry of the distorted structure regardless of the matrix. However, the three-fold J.T. potential surface in the trough, which is determined by second-order effects such as anharmonicity and the quadratic must be sensitive to the choice of environment.26 If the observed three-site jumping process is pseudorotation around the trough, the matrix dependence of the apparent activation energy suggests that the potential barrier at the saddle point corresponding to the acute triangle for c-C3H,' and the compressed chair for c-C6HT2 is affected by interaction with the environment.M. IWASAKI, K.TORIYAMA AND K. NUNOME 31 Alternatively the results might be accounted for in terms of distortion due to matrix effects and their averaging by reorientation around the normal to the molecular (equatorial) plane. However, it seems unrealistic to assume that the onset of reorientation of c-C6HT2 starts at temperatures as low as ~ 4 . 2 K. In addition, if this is the case, the e.s.r. parameters for the distorted structure might differ from matrix to matrix. suggest that there is a trend for the onset temperature of the dynamics (and thus the activation energy) to be lower in SF6 than in other halogenocarbon matrices.This may suggest that both the bond dipole moment and the polarizability might cause the matrix effects, since the dipole moments of these halogenocarbons are relatively small. Note also that any variation in the concentrations of the cations caused by irradiation dose, thermal decay, solute concentration etc. does not affect the e.s.r. parameters or their dynamic behaviour. This suggests that the distributions of the counter-anions and other ionic species do not seem to be a decisive factor. The rigidity of the matrices and the packing conditions of the solute cations in the matrices may be an important factor, since the sign preference of the distortion coordinate (* Q) may be controlled by these factors and thus vary the potential energy at the saddle point.The irreversible change observed for c-C,H T2 after annealing at 77 K might be attributable to the change in packing conditions due to matrix relaxation. Although details of these matrix effects are not clear, the observed distortion is considered to be a matrix-assisted static J.T. distortion. Nevertheless, it is remarkable that the direction of the distortion coordinate is not altered by the matrix effects, giving essentially the same geometry and SOMO. It is suggested that interactions with a matrix stabilize a given distorted structure, which is intrinsic of species. Both these results and those obtained from other REACTIONS OF CYCLOALKANE RADICAL CATIONS The reactions of cycloalkane radical cations were mainly studied in CFC12CF2CI, SF, and CFC13 at solute concentrations of 0.1-1.0 mol %.The reactions observed above 100 K in CFCl2CF2C1 and in SF6 are almost exclusively deprotonation to give cycloalkyl radicals. Typical examples of the spectral changes due to deproton- ation are shown in fig. 6. The results are similar to the deprotonation of linear and branched alkane radical cations in these matrices. Deprotonation is thought to take place from the equatorial C-H bond, in which the highest unpaired electron density is populated, although it is not possible to confirm this by a site preference of deprotonation as is observed in linear and branched alkane radical cations. Although details will be given el~ewhere,’~ it is noteworthy that c-C,Hl in CFC13 gives but- 1 -ene radical cations by ring opening following 1,3-hydrogen shift on exposure to visible light.27 The but- 1 -ene radical cation further isomerizes into the but-2-ene radical cation by prolonged exposure, as was previously observed for CFC1, by Shida et aZ.28 As previously reported, l4 radical cations of propane and their methyl derivatives undergo thermally induced elimination of H2 or CH4, forming olefinic cation radicals in CFC13. However, we did not observe such elimination reactions from cycloalkane radical cations: they simply decayed upon warming.On the other hand, Shida’s group reported that cycloalkyl radicals are formed together with the radical cations in CFC1, immediately after irradiation at 77 K, when the solute concentration is as high as 10% .15 Tabata and LundI2 have reported that radical cations of cyclopentane and cyclohexane undergo photoinduced elimination of H2, forming cycloalkene radical cations in CF3CC13, although they also did not observe thermally induced elimination reactions in this matrix.32 E.S.R STUDY OF CYCLOALKANE RADICAL CATIONS ( h ) CFCI,CF,CI - Fig.6. Typical examples of the e.s.r. spectral changes due to deprotonation of cycloalkane radical cations produced at 4.2 K. ( a ) c-C3Hl -+ c-C3H, in SF,; ( 6 ) c-C,H;,-+ c-C,H,, in CFCI,CF,CI; ( c ) c-C7HT4 -+ c-C7HI3 in CFCI2CF2Cl. CONCLUDING REMARKS The potential barrier in the trough may be so small without the aid of matrix effects that dynamic averaging by a zero-point energy may make it difficult to deduce direct information on the geometric and electronic structure of the J.T.-distorted radical cations of cycloalkanes.However, the matrix-assisted static distortion makes it possible to obtain direct experimental information on both the ground-state geometry and that of the SOMO, which are believed to be essentially intrinsic to these fundamental J.T.-active cations. The information obtained from the e.s.r. analyses may be of significance regardless of whether the static distortion is caused by J.T. or matrix effects. The results may shed light not only on further experimental studies in related fields but also on theoretical studies. Radom et have recently compared their a6 initio calculations with our experimental results of some prototype alkane radical cations. Such a comparison seems useful for the future refinement of a6 initio calculations.M. Iwasaki, K. Toriyamaand K. Nunome, J. Am. Chem. SOC., 1981, 103, 3591. K. Toriyama, K. Nunome and M. Iwasaki, J. Phys. Chem., 1981, 85, 2149. K. Toriyama, K. Nunome and M. Iwasaki, J. Chem. Phys., 1982, 77, 5891. M. Iwasaki, K. Toriyama and K. Nunome, Radiat. Phys. Chem., 1983, 21, 147. K. Nunome, K. Toriyama and M. Iwasaki, J. Chem. Phys., 1983, 79, 2499. K. Nunome, K. Toriyama and M. Iwasaki, Chem. Phys. Lett., 1984, 105, 414. ’ M. Iwasaki, K. Toriyama and K. Numone, J. Chem. SOC., Chem. Commun., 1983, 202. K. Ushida, T. Shida, M. Iwasaki, K. Toriyama and K. Nunome, J. Am. Chem. SOC., 1983,105,5496. K. Toriyama, K. Nunome and M. Iwasaki, J. Chem. Soc., Chem. Commun., 1984, 143. M. Iwasaki, K. Toriyama and K. Nunome, J. Chem. SOC., Chem. Commun., 1983, 320. 10M. IWASAKI, K. TORIYAMA AND K. NUNOME 33 I ' T. Shida and K. Kato, Chem. Phys. Lett., 1979, 68, 106. M. Tabata and A. Lund, Chem. Phys., 1983, 75, 379. l 3 M. Shiotani, Y. Nagata, M. Tasaki, and J. Sohma, J. Phys. Chem., 1983, 87, 1170. l4 M. Iwasaki, H. Muto, K. Toriyama and K. Nunome, Chem. Phys. Lett., 1984, 105, 586. l 5 T. Shida and Y. Takemura, Radiat. Phys. Chem., 1983,21,157; K. Ohta, H. Nakatsuji, H. Kubodera and T. Shida, Chem. Phys., 1983, 76, 271. M. B. Huang, S. Lunell and A. Lund, Chem. Phys. Lett., 1983, 99, 201. J. B. Hendrickson, J. Am. Chem. SOC., 1961, 83, 4537; D. F. Bocian, H. M. Pickett, T. C. Rounds and H. L. Strauss, J. Am. Chem. SOC., 1975, 97, 687. I s K. Toriyama, K. Nunome and M. Iwasaki, 22nd ESR Symposium, Matsuyama, Nov. 1983, abstract 12A08, p. 142. l9 A. D. Liehr, J. Phys. Chem., 1963, 67, 389. 'O K. Toriyama, K. Nunome and M. Iwasaki, unpublished work. '' See for example, M. K. Carter and G. Vincow, J. Chem. Phys., 1967, 47, 292. '' I. Miyagawa and K. Itoh, J. Chem. Phys., 1962, 36, 2157. 23 G. R. Liebling and L. M. McConnell, J. Chem. Phys., 1965,42, 3931. K. Toriyama, K. Nunome, M. Iwasaki, K. Ushida and T. Shida, 48th Annual Meeting of Chem. SOC. Jpn, Sapporo, Oct. 1983, abstract I, 2A03, p. 3; to be published. 25 G. Herzberg Molecular Spectra and Molecular Structure. III. Electronic Spectra and Electronic Structure of Polyatomic Molecules (Van Nostrand, New York, 1966), p. 45; M. D. Sturge, Solid State Phys., 1967, 20, 9 1. 26 M. S. de Groot, I. A. M. Hesselmann and J. H. van der Waals, Mol. Phys., 1969, 16, 45; J. H. van der Waals, A. M. D. Berghuis and M. S. de Groot, Mol. Phys., 1971, 21, 497. K. Toriyama, K. Nunome and M. Iwasaki, 26th Symposium on Radiat. Chem., Osaka, Sept. 1983, abstract B204, p. 1 14. 16 24 27 2g T. Shida, Y. Egawa and H. Kubodera, J. Chem. Phys., 1980, 73, 5963. 29 W. J. Bouma, D. Popponger and L. Radom, Zsr. J. Chem., 1983, 23, 21.

ISSN:0301-7249    

DOI:10.1039/DC9847800019

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1984, 78, 35-47 Equilibrium Geometries and Hyperfine Interactions in Propane and Cyclopropane Cations BY STEN LUNELL" AND MING BAO HUANG Department of Quantum Chemistry, Uppsala University, Box 5 18, S-75 1 20 Uppsala, Sweden AND ANDERS LUND The Studsvik Science Research Laboratory, S-6 1 1 82 Nykoping, Sweden Received 26th March, 1984 The equilibrium geometries of the propane cation in its three lowest electronic states, ' B , , 'B2 and ' A , (C2u symmetry assumed), have been calculated by the UHF method within the ab initio MO-LCAO-SCF approximation. The 2 B , state is predicted to be lowest in energy and the 2B2 and 2A, states to be of almost the same energy, 12 kcal mol-' above the ' B , state. The isotropic hyperfine coupling constants have been calculated and compared with experi- mental data.The calculations confirm the e.s.r. identification of the 2B, state, obtained in an SF, matrix. The previous experimental assignment, 2B2, for the state obtained in Freon matrices was found to be ambiguous and an assignment to the ' A , state can not be ruled out. Dipolar hyperfine coupling constants have been calculated for the ' B , , 2B2 and ' A , states of the propane cation and for the 2A, ground state of the cyclopropane cation. In the latter case, comparison with experimental data has been made with reasonable agreement. The equilibrium geometry agrees well with previous calculations. Complementary experi- mental data for the propane and cyclopropane cations are reported. Positive ions of saturated hydrocarbons have recently been observed and charac- terized by electron spin resonance (e.s.r.) spectroscopy.14 In these studies the cations were generated by ionizing irradiation at low temperature of the hydrocarbon contained in a halogenated matrix.Under these conditions the cations are stably trapped, permitting their e.s.r. spectra to be recorded. These studies have shed new light on the electronic structure of the cations of hydrocarbons and particularly detailed results have been obtained for the cations of ethane and ~ r o p a n e . " ~ The assignments of the spectra have been based on intuitive arguments or, more recently, on semi-empirical molecular-orbital calculations. Ab initio results are available for the ethane ~ a t i o n , ~ but such calculations on the larger ions in the n-alkane series seem to be missing.Very recently, e.s.r. data for some cycloalkane cations were reported and com- pared with the results of ab initio calculation^.^-^ It was found that ab initio MO-LCAO calculations are able to confirm and refine the experimentally obtained geometry and spin distribution of these systems. A similar study of the cations of normal hydrocarbons therefore seems to be necessary. 23 years ago the equilibrium geometry of propane was successfully studied by means of its microwave spectrum," which indicated that it has C2v symmetry with the CH3 groups staggered with respect to the CH2 group. As with both methane and ethane, the three highest-lying occupied molecular orbitals (46,, 2b2 and 6a,) are energetically very close to one a n ~ t h e r .~ This makes it non-trivial to predict the symmetry of the ground state of the cation. Ionization of the highest occupied 3536 PROPANE AND CYCLOPROPANE CATIONS orbital will not necessarily give the most stable doublet state, especially in view of the rather large geometry changes that can occur upon ionization. In the e.s.r. experiments on the n-propane cation reported by Toriyama et a2.: two different spectra were obtained, depending on the matrix, which were assigned to two different states, 2Bl and 2B2. Their theoretical analysis is, however, simplified and based only on INDO calculations. Toriyama et aL4 observed that a change of ground state from ,B2 to *B1 could be produced in their calculations by a slight tilting of the C3 axes of the CH3 groups.It is likely that this result is more or less a coincidence, since more relevant parameters such as C-C bond lengths and the C-C-C angle were left unchanged. A detailed theoretical study is therefore required. The main purpose of this work is to investigate the three lowest states of the propane cation, ’ B I , ,B, and * A , , using a6 initio MO-LCAO UHF calculations with reasonably large basis sets, in particular with respect to equilibrium geometries, relative energies and spin properties. These data are discussed together with avail- able e.s.r. spectra4 and some new e.s.r. data which are briefly reported. In addition, we report some theoretical and experimental results for the cyclopropane cation which supplement previous work by Ohta et aL7 and by Iwasaki et aL9 THEORY Energies and wavefunctions were calculated by the unrestricted Hartree-Fock (UHF) method within the a6 initio MO-LCAO-SCF approximation.Two different basis sets were used in the calculations, a 4-31G basis*’ and a larger basis of double-zeta quality, namely Dunning’sI2 (9s5p/4s) basis contracted to (4s2p/2s), with a scale factor of 1.25 for the hydrogen exponents. The 4-31G basis was used in the geometry optimizations of the 2Bl, 2B2 and ,A, states of C3Hl and the ,A, and 2B1 states of C3HL using the program MONSTER- GAUSS.I3 We have assumed that the cations have C,, symmetry, and that C3Hl has both CH3 groups staggered with respect to the CH, group. In all five cases the independent parameters were optimized and the notation used is shown in fig.l ( a ) and ( b ) . The optimized geometries, obtained in the 4-31G basis, were then used in the final calculations of wavefunctions and energies in the larger basis, for which the program MOLECULE'^ was used. Isotropic and anisotropic hyperfine coupling constants for the hydrogen atoms were calculated only in the double-zeta basis. The isotropic coupling constants were calculated both with and without previous annihilation of the quartet contamination in the total wavefunctions. In the former case, a program based on the theory of Amos and Snyder” was used. The anisotropic constants were calculated directly from the UHF function only, since previous experience with systems similar to the present ones, e.g. the methyl and ethyl radicals,I6 has shown that the proton dipolar couplings are normally relatively little affected by quartet annihilation.EXPERIMENTAL In earlier work^^,',^ cation radicals of propane and cyclopropane were produced by y- or X-irradiation of the solutes contained in a frozen matrix of SF6, CFC1, or CFC12CF2CI and the e.s.r. spectra were recorded at 4 and 77 K. In the present work, the cations were observed after X-irradiation at 77 K of propane and cyclopropane in a CF,CCI3 matrix. The temperature dependence of the e.s.r. spectra was investigated between 4 and 77 K using a liquid-helium flow cryostat manufactured by Oxford Instruments. The temperature depen- dence between 77 and 140 K was studied using a variable-temperature Dewar manufacturedS. LUNELL, M. B.HUANG AND A. LUND 37 " 2 Fig. 1. ( a ) Geometry and labelling used for the C3H6+ ion (C2v symmetry assumed). ( b ) Geometry and labelling used for the C3Hl ion (C2v symmetry assumed; the H,-C-H3 and H , -C-H, planes are perpendicular to the H2-C-C-C-H2 plane). by Varian. The spectra were recorded at X-band frequencies (9.3 GHz) on a Varian E9 spectrometer using 100 kHz magnetic field modulation. EXPERIMENTAL RESULTS CYCLOPROPANE CATION Experimental data have previously been reported by Shida et aL7 for a CFCl, matrix at 77 K and by Iwasaki et aL9 for a CFC12CF,C1 matrix at 4 K. At 4 K a spectrum 100 G wide with partially resolved hyperfine structure was obtained. The spectrum has been interpreted using an anisotropic g-factor, anisotropic hyperfine couplings to four a-hydrogen atoms and nearly isotropic couplings to two p- hydrogen atoms.The hyperfine data of ref. (9) are included in tables 2 and 3 (uide infra). In the present work, e.s.r. spectra of the cyclopropane cation were obtained in a CF3CC13 matrix over the temperature range 4-77 K. The spectrum recorded at 10 K, fig. 2(a), has the same general shape as that obtained at 4 K by Iwasaki et ~ 1 . ~ We find for the isotropic part of the couplings a , = 22 G (2H) and a2 = - 1 1.5 G (4H), in close agreement with the values reported previously. Thus the matrix does38 PROPANE AND CYCLOPROPANE CATIONS Fig. 2. E.s.r. spectra of the cyclopropane cation in a CF3CC13 matrix recorded at ( a ) 1 1 , ( b ) 41 and ( c ) 60K. not significantly affect the magnitudes of the hyperfine couplings of the cyclopropane cation.As the temperature was raised the e.s.r. spectrum changed reversibly, fig. 2( 6) and (c), and at 60 K a single line 4 G wide became prominent. A spectrum resembling that in fig. 2(c) was also observed by Shida et u1.’ at 77 K. PROPANE CATION Experimental data at low temperatures (4 and 77 K) have been reported by Toriyama et aL4 In this case the isotropic hyperfine coupling constants depend on the matrix used. In the SF6 matrix the resolved hyperfine structure is due to the atoms labelled H2 on each methyl group [fig. l(b)] with u2 = 98.0 G (2H) at 4 K. In the CFC12CF2C1 matrix the hyperfine couplings are a , = 105.5 G (2H) and u3 = 52.5 G (4H). The assignments were made with the help of deuterium-labelled compounds. The results show quite unambiguously that the cation is in the 2BI state in the SF, matrix.The spectrum observed in the CFC12CF2C1 matrix was assigned to the 2B2S . LUNELL, M. B. HUANG AND A. LUND 39 state by Iwasaki and coworker^,^ who used INDO calculations. Although this assignment may be correct, it is by no means obvious that it is the only possible one, since it is known that the INDO method sometimes gives the wrong symmetry for the ground state of radi~a1s.l~ In fact, the assignment 2A1 can not be excluded on the basis of available experimental and theoretical data, as will be discussed below. In the present work, experimental e.s.r. data have been obtained for the propane cation in the CF,CC13 matrix at temperatures between 77 and 140 K. At 77 K the hyperfine couplings are similar to those reported in ref.(4) for the CFCl2CF2C1 matrix, namely a, = 110 G (2H) and a3 = 50 G (4H). Thus the cation is in the 2B2 or 2A, state in the CF3CC13 matrix. At 140 K the spectrum consists of a triplet with a, = 100 G (2H) and an additional septet splitting of 23 G (6 H). The spectral change between 77 and 140 K is reversible. This suggests the onset of rotation about the CI-C2 and C2-C3 bonds at 140K making the H, and H3 atoms magnetically equivalent. THEORETICAL RESULTS CYCLOPROPANE CATION EQUI LI BRI UM GEOMETRY AND ISOTROPIC HY PERF1 NE COUPLING CONSTANTS A careful theoretical study of the cyclopropane cation was recently published by Nakatsuji and coworker^.^ In particular, the geometric stability of the cation was thoroughly analysed using the Jahn-Teller theorem, as were the isotropic hyperfine coupling constants for the state of lowest energy, i.e.the *A, state. We will therefore limit the present discussion primarily to the anisotropic coupling constants, which were not considered by Nakatsuji and coworkers. Since certain differences exist with respect to both the geometry determination (different basis sets) and the calculation of the isotropic coupling constants (basis sets as well as theoretical model), we compare our present results with those of ref. (7) in tables 1 and 2 for the two lowest states, 2A1 and ,B2.* As is seen in table 1, the differences between the 4-3 1G and STO-4G geometries are rather small. Both basis sets predict a flattening 2f the equilateral triangle in the 2A1 state, accompanied by a slight shortening (0.02 A) of the C, -C2 and CI -C3 bonds, and a narrowing of the base of the triangle in the 2B2 state, accompanied by an elongation of the C,-C, and C,-C, bonds.The actual bond lengths differ between the two basis sets (0.02-0.04 A), whereas most bond angles are similar. For the isotropic hyperfine coupling constants (table 2) there is acceptable qualitative agreement between the present results after quartet annihilation (a UHFAA) and the results of the so called pseudo-orbital (PO) theory of ref. (7), and, in the case of the 2A, state, with experiment. Note, however, that significantly better quantitative agreement is obtained by following the suggestion by Snyder and Amos" to estimate the isotropic coupling constant from the quantity a(3a + aUHF).This gives the values a, = 17.8 G and a2 = - 12.3 G for the 2A1 state, and a, = - 19.3 G and a2=6.3 G for the 'B2 state, in excellent agreement with ref. (7) and (9). DIPOLAR COUPLING CONSTANTS The calculated (UHF) dipolar coupling constants for the cyclopropane cation are given in table 3 for the four equivalent a-hydrogen atoms [H2 in fig. l(a)] and * T h e symmetry designations in C,, symmetry are to some extent a matter of choice. We here follow ref. (7); in ref. (9), this state is labelled ' B , .40 PROPANE AND CYCLOPROPANE CATIONS Table 1. Optimiztd geometries and total energies for the ' A , and 'B2 states of C3Hl (distances in A, angles in degrees and energies in a.u.; for notation see fig. 1) 2A, 2B2 c3 H6 4-3 1 G" STO-4Gh 4-3 1 G" STO-4Gb 4-3 1 G" exptl' W C , -Cd 1.483 1 SO4 40 81.7 75.6 R(C,-H,) 1.075 1.088 R (C2 - H2) 1.072 1.092 Q 113.7 116.5 Y 109.7 P 120.9 121.0 4-3 1G energy double-zeta energy -1 16.573 59 - 1 16.705 43 ~ ~ ~~~~ 1.721 1.683 1 SO3 I .524 1.069 1.090 1.072 1.07 1.072 1.094 48 .O 50.1 60.0 60.0 120.6 119.6 113.7 120 108.1 108.5 120.6 -1 16.563 76 - 1 16.695 02 a This work.Ref. (7). Ref. (18). Table 2. Isotropic hyperfine coupling constants aH (in G) for the different hydrogen atoms of C3Hl S ( S + 1) isotropic coupling constants before after state annihilation annihilation HI H2 2A, 0.781 53 0.750 3 1 QUHF 24.9 -25.7 PO theory" 16.5 -12.3 ~ U H F A A 15.5 -7.9 exptl 21h 22' -12.5b -1 1.5' 2B2 0.756 28 0.750 03 QUHF -38.9 5.8 QUHFAA -12.8 6.5 PO theorya -19.9 7.1 a Pseudo-orbital theory, ref.(7). CF2C1CFCl2 matrix, ref. (9). CF3CC13 matrix, this work. the two P-hydrogen atoms (H,). The experimental data reported by Iwasaki et aZ.' are included for comparison. For the a-hydrogen atoms we note satisfactory agreement between theory and experiment, with respect to both the principal values and the directions of the principal axes of the dipolar tensor. For the P-hydrogen atoms, the agreement is at first sight considerably worse. There are, however, two possible reasons for the discrepancies. The first is that the anisotropy is very small, both in absolute terms and in comparison with the isotropic coupling constant (only ca. 15%). In the case of the a-hydrogen atoms, the dipolar coupling is much larger and ca.70% of the isotropic value. An accurate experimental determination is therefore much more difficult for the P-hydrogen atoms, especially of the direction cosines. The second reason is that the results of INDO calculations were used to extract the anisotropic component of the hyperfine coupling,' and this may have introduced errors which were not present in the raw data. In fact, we have simulated the experimentalS. LUNELL, M. B. HUANG AND A. LUND 41 Table 3. Calculated (UHF) dipolar coupling constants in the cyclopropane cation (*A ,) direction cosines" principal nucleus value/G X Y z H2 +9.5 -0.9 -8.6 exptl +8.2 -1.3 -6.9 H , -1.7 + 0.3 +1.4 exptlb +3.2 -1.0 -2.2 0.337 -0.75 1 0.567 0.578 -0.478 0.66 1 0.0 1 .o 0.0 0.0 0.0 1 .o 0.846 -0.022 -0.533 0.794 0.142 -0.592 -0.999 0.0 0.038 0.686 0.728 0.0 - 0.413 0.660 0.628 0.188 0.868 0.46 1 0.038 0.0 0.999 -0.728 0.686 0.0 " The direction cosines are given for one of the equivalent protons and those for the others can be obtained by symmetry operations; coordinate system as in fig.l(a). Ref. (9), CFC13 matrix. spectrum obtained by Iwasaki et u Z . , ~ using our theoretical data from table 3, with a fit which is about as good as with the data of ref. (9). The theoretical results must therefore be considered to be satisfactory. PROPANE CATION EQUILIBRIUM GEOMETRIES AND ENERGIES OF THE 2 B ~ , 2B2 AND 2 A ~ STATES The 4-31G optimized geometry parameters of the 2 B I , 2B, and 2A1 states are given in table 4 [for notation see fig. l(b)], together with the 4-31G optimized parameters of neutral propane.Also included in table 4 are the total energies obtained at the optimized geometries using the 4-31G and double-zeta bases. Table 4 shows that in both the 4-31G and the double-zeta bases the state of lowest energy is the 2Bl state. In the 4-31G basis the minima of the 2B2 and *Al energy surfaces lie 11.5 and 12.0 kcal mol-I respectively, higher than the 2B1 energy minimum, and in the double-zeta basis the corresponding values are almost the same (12.0 and 1 1.7 kcal mol-I). The 2B2 and 2AI states are hence placed at almost exactly the same energy above the 2Bl state, even though they have very different equilibrium geometries, especially for the carbon framework ( cJ: table 4). A complete calculation of the full potential-energy surfaces for all three states would be both tedious and not very useful.In fig. 3 we show the relative energies of the 2B,, 2B2 and 2A1 states at the three different optimized geometries of table 4. Note that at the optimum geometry for one state the other two states lie rather far (240 kcal mol-') above this lowest state. In addition to the relative energies of the three states, fig. 3 also shows the slopes of the different energy surfaces at the considered geometries. Table 4 and fig. 3 show that the equilibrium geometry of the propane cation is shifted from that of neutral propane in different ways for the different states. These geometry changes can be understood from the shapes of the singly occupied42 PROPANE AND CYCLOPROPANE CATIONS Table 4.4-31G optimizaed geometries and total energies for the 2 B , , 'B2 and * A , states of C3Hg (distances in A, angles in degrees and energies in a.u.; for notation see fig. 1) 4-3 1 G energy" +117.0 double-zeta energy" + I 17.0 1.646 1.075 1.095 I .073 96.1 112.5 93.9 112.2 1 1 1.8 -0.7 17 97 (0) (0) -0.849 95 I .480 1.177 1.077 I .088 121.5 70.3 113.5 109.0 109.7 -0.699 65 (48. I ) -0.830 75 (50.2) 1.600 1.092 1.078 1.080 130.5 129.2 1 1 1.8 109.9 105.8 -0.698 89 - (50.2) (49.0) -0.831 29 1.530 1.085 1.083 1.084 112.6 106.4 I 1 1.3 107.8 11 1.0 1.093 81 " The values within parentheses give the energies relative to the energy of the ' B , state in the same basis (in kJ mol-'). -116.5500 - 1 1 6.6000 c : v q-116.6500 -116.7000 90.0 2 B2 A , 2 B1 2 100.0 110.0 120.0 130.0 140.0 g P / O Fig.3. Relative energies of the 'B,, 'B, and ' A , states of C3Hl at the optimized geometries for each state (4-3 1G basis). The signs u , \ and / indicate that the energy derivatives with respect to q are zero, negative and positive, re2pectively. R = ( a ) 1.646, (b) 1.480 and (c) 1.600 A. molecular orbitals (SOMO), which can be clearly identified in all three states (fig. 4). As a general rule, single occupancy of a certain molecular orbital, instead of double, changes the bonding situation such that the bond length increases when the MO is bonding and decreases where it is antibonding ( i e . both bonding andS. LUNELL, M. B. HUANG AND A. LUND f 43 Fig. 4. Schematic representations of the three highest occupied MO in neutral C3Hs: ( a ) 2 bZ, ( b ) 4b, and ( c ) 6 ~ 1 .antibonding effects become weaker). Thus the antibonding character of the B2 orbital between tbe pT orbitals on the carbon atoms results in a shortened C-C bond ( R = 1.480 A) when it is singly occupied in the 2B2 state. The 46, orbital, on the other hand, displays bonding between the mid-carbon and the end-carbons and potential antibonding between the two end-carbons. m i n it is singly occupied in the 2B1 state, the C-C bonds are lengthened ( R = 1.646 A) and the C-C-C angle becomes smaller ( 4 = 96.1 "). The bonding situation is thus almost identical to that found in the cyclopentane ~ a t i o n . ~ ' ~ Finally, the 6 ~ 1 orbital is bonding between all three carbons for a bent geometry. Single occupancy weakens this bonding, leading to longer C-C bonds ( R = 1.601 A) and a larger C-C-C angle ( 4 = 130.5 "). A final point of interest is the changes in the C-H bond lengths.One can see from table 4 that the 2B1 state, on one hand, and the 2B2 and 2A, states, on the other, behave differently in this respect. In the 2B, state, the bonds to the in-plane end-hydrogens (H2) are lengthened, whereas the other C-H bonds become shorter. For both the 2B2 and 2A1 states, however, the bond lengthening occurs for the mid-carbon hydrogens (Hl). This difference, which can be seen to be consistent with the shape of the orbitals in fig. 4, is important in connection with the possible decomposition of the cations by deprotonation at higher temperatures (vide infru). ISOTROPIC COUPLING CONSTANTS The calculated isotropic hyperfine coupling constants at the different protons in C3Hl are given in table 5 for the three different states, under the headings uUHF44 PROPANE AND CYCLOPROPANE CATIONS Table 5.Isotropic hyperfine coupling constants aH (in G) for the different hydrogen atoms of C3HZ S ( S + 1) before after isotropic coupling constants state annihilation annihilation HI H2 H3 2B, 0.763 86 0.750 12 aUHF a u H FAA 2B2 0.757 48 0.750 04 ~ U H F 2A, 0.759 17 0.750 06 ~ U H F ~ U H F A A a u H FAA exptl SF6 matrixu CFC12CFzCl matrixU CFC13 matrix" CF3CC13 matrix' -16.3 -5.4 206.9 181.4 84.4 69.7 105.5 100 110 65.1 55.3 -0.1 -0.03 0.1 4.3 98.0 - 14.9 -4.9 40.2 26.6 18.1 16.6 52.5 52 50 Ref. (4). ' This work. (without quartet annihilation) and aUHFAA (after annihilation).Considering first the 'B1 state, one can see that the largest coupling is obtained for the two in-plane end-carbon hydrogens (H2), whereas the couplings for H I and H3 are one order of magnitude smaller. This is consistent with the form of the singly occupied molecular orbital in fig. 4. In the 2B2 and 2A, states, however, the in-plane hydrogens have practically vanishing coupling constants and instead the mid-carbon hydrogens (HI) show large isotropic couplings, with non-negligible values for the four out-of-plane hydrogens (H3). Again, this is in agreement with the SOMO in fig. 4, which shows that the delocalized unpaired spin is responsible for most of the isotropic coupling and that spin polarization is of minor importance. One can compare the calculated coupling constants with the experimental values obtained by Iwasaki and c o ~ o r k e r s , ~ which are also included in table 5.An interesting feature of the experiments is that two different types of spectra were obtained, one in the SF6 matrix and another, quite different, in the CFCl2CF2C1 and CFC13 matrices. In the first case a coupling constant of 98.0 G is obtained for the two in-plane end-carbon hydrogens ( H2), with the remaining couplings being too small to be resolved. A comparison with the theoretical results in table 5 shows that the cation must be in the 2 B , state, even though the quantitative agreement between theory and experiment clearly leaves room for improvement. In the CFCl2CF2C1 matrix, on the other hand, a coupling constant of 105.5 G was obtained for the mid-carbon hydrogens ( H , ) and 52.5 G for the four out-of-plane end-carbon hydrogens (H3).In this case the assignment is less obvious. Iwasaki and coworkers4 assigned this spectrum to the 2B2 state, guided by INDO calculations, and the possibility of a 2A1 assignment was not even considered. However, the observed hyperfine structure is consistent with both the 2bz and the 6 ~ 1 orbitals being SOMO (cf. fig-4). Table 5 shows that HI has the largest coupling constant and H3 the second largest in both the 2B2 and the 2A, states, in complete agreement withS. LUNELL, M. B. HUANG AND A. LUND 45 Table 6. Calculated (UHF) dipolar coupling constants in the states ' B , , ' B , and ' A , of the propane cation direction cosines' principal nucleus value/G X Y z 9.40 -7.67 - 1.73 7.60 -6.19 - 1.41 7.90 -6.22 - 1.68 12.9 1 -7.42 -5.49 2.43 -1.38 - 1.05 2.70 - 1.69 -1.01 12.25 -7.20 -5.05 7.43 -4.35 -3.08 4.5 1 -4.15 -0.36 0.0 0.0 1 .o 0.464 0.0 -0.886 0.190 0.170 0.967 0.0 1 .o 0.0 0.967 0.255 0.0 0.532 0.209 -0.82 1 0.0 1 .o 0.0 0.944 0.0 -0.33 1 0.087 0.496 0.864 0.78 1 0.624 0.0 0.0 1 .o 0.0 -0.755 -0.605 0.255 0.174 0.0 0.985 0.0 0.0 1 .o -0.297 0.954 0.050 1 .o 0.0 0.003 0.0 1 .o 0.0 -0.903 -0.328 0.279 -0.624 0.78 1 0.0 0.886 0.0 0.464 -0.628 0.778 -0.0 13 -0.985 0.0 0.174 -0.255 0.967 0.0 -0.793 -0.2 17 -0.569 -0.003 0.0 1 .o 0.33 1 0.0 0.944 -0.42 1 0.804 -0.41 9 The direction cosines are given for one of the equivalent protons and those for the others can be obtained by symmetry operations; coordinate system as in fig.l(b). experiment. We have also shown (vide supra) that there are no energetic reasons to prefer the 'B2 state over the *AI state (or vice versa). A quantitative comparison between the theoretical and experimental values for the H, coupling constants shows that the 'B2 value is 80% too large, while the ' A , value is 30% too small. The discrepancy between theory and experiment is thus considerable, but if any preference must be shown the values support assignment to 2Al rather than ' B 2 . As additional support for this choice one could consider the fact that the calculated coupling constant is too low by approximately the same factor as in the *BI state. However, there is no guarantee that basis-set effects and other errors are regular enough to allow safe extrapolation from one atom to another and from one electronic state to another.Calculations using more accurate methods and basis sets would therefore help in this assignment. Such calculations have been started.46 PROPANE AND CYCLOPROPANE CATIONS DIPOLAR COUPLING CONSTANTS No anisotropic hyperfine coupling constants were reported in ref. (4), nor were they possible to extract from the present measurements. We have nevertheless calculated the relevant proton dipolar couplings for the three states of interest (table 6). As can be seen the largest anisotropy is found for the HI atoms in all the states considered. Although the anisotropy of H, is almost identical in magnitude in the 2B2 and 2A1 states, its directions are different, which in principle should provide the possibility of distinguishing between these states.Presently available experi- mental spectra do not, however, allow a distinction on this ground. DISCUSSION Although the existence of matrix effects in e.s.r. spectra is well known and extensively studied, the case of propane is unusual in that these effects are large enough to produce cations in different electronic states in different matrices. The explanation of the difference between e.g. the SF6 and CFCl2CF2C1 matrices presents interesting problems, since in principle both steric and electronic explanations are possible. A steric explanation would be based on the fact, shown in fig. 3 , that the ground state of the cation is ' B , for some geometries, 2B2 for some geometries and 2AI for some geometries.It is also not unreasonable to assume that the geometry of the cation can be affected by the steric properties (molecular shape, packing etc.) of the matrix. An electronic explanation, on the other hand, assumes a direct dependence of the electronic state of the cation on the electronic properties (ioniz- ation potential, polarizability etc.) of the matrix. The geometry of the cation then becomes a consequence of its electronic state (cf: fig. 3). Whichever explanation one attempts, however, correct knowledge of the electronic state of the cation is a prerequisite for any meaningful discussion. Although this knowledge is satisfactory for the SF6 matrix, our present results show that the situation is less clear for the CFCl2CF2C1 matrix.Additional experimental evidence has been collected by means of deuteration and by considering the decomposition of the cations at higher temperature^.^ The deuteration experiments give definitive confirmation that the largest hyperfine coup- ings occur at the in-plane end-carbon hydrogens (H,) in the SF6 matrix and at the mid-carbon hydrogens (H,) in the other matrix. From the decomposition reaction one can see that the cations are deprotonated at an end carbon in the SF6 matrix and at the mid carbon in the CFCl2CF2C1 m a t r i ~ . ~ Both these experiments thus confirm the 2B, assignment for the SF6 matrix but, unfortunately, neither of them discriminates between the ,B2 and 2A, assignments in the other matrix. Deproton- ation, for instance, is expected to occur when the C-H bonds have been lengthened, but this singles out the C-Hi bonds in both the 2B2 and the *Ai states (cJ table 4).The experimental spectra of the cyclopropane cation at low temperatures [ref. (9) and fig. 21 seem to be affected by g-factor anisotropy in the CFC13, CFCl,CF,Cl and CF3CC13 matrices. The anisotropy (gl = 2.0023, g, = 2.0039 and g , = 2.0060)9 is larger than has previously been reported for hydrocarbon cation radicals.20321 Sevilla and coworkers22 recently observed that the e.s.r. spectrum of the methyl formate cation had marked g-factor anisotropy and showed chlorine hyperfine structure in the CFC13 matrix. Iwasaki et aL9 observed hyperfine lines caused by the CFC13 matrix in the cation spectrum of cyclopropane. These facts show that the cation and the matrix can interact.It is possible that the g-factor anisotropy of the cyclopropane cation noticed for the CFC13, CFC1,CF2C1 and CF3CC13 matrices is caused by an ion-matrix interaction.S. LUNELL, M. B. HUANG A N D A. LUND 47 According to Sevilla and coworkers, a strong interaction occurs when the differ- ence between the ionization potentials of the matrix and the solute molecules is small, which is indeed the case for both the cyclopropane and the propane cations in the CFC13, CFC12CF2C1 and CF3CC13 matrices. The difference is larger for the SF6 matrix, which implies that the propane cation is less affected by the matrix. This is consistent with the fact that the 2 B , ground state, predicted for an isolated ion, is obtained in the SF6 matrix and not in the other matrices.However, further work, both theoretical and experimental, is needed in order to understand the detailed mechanism of the matrix-solute interaction in these systems. This work was supported by the Swedish Natural Science Research Council (NFR). ’ M. C. R. Symons and I. G. Smith, J. Chem. Res. (S); 1979, 382. * J. T. Wang and Ff. Williams, J. Phys. Chem., 1980,84, 3156. K. Toriyama, K. Nunome and M. Iwasaki, J. Phys. Chem., 1981, 85, 2149. K. Toriyama, K. Nunome and M. Iwasaki, J. Chem. Phys., 1982, 77, 5891. A. Richartz, R. J. Buenker and S. D. Peyerimhoff, Chem. Phys., 1978,31,187 and references therein. M. Tabata and A. Lund, Chem. Phys., 1983, 75, 379. K. Ohta, H. Nakatsuji, H. Kubodera and T. Shida, Chem. Phys., 1983, 76, 271. M. B. Huang, S. Lunell and A. Lund, Chem. Phys. Lett., 1983, 99, 201. M. Iwasaki, K. Toriyama and K. Nunome, J. Chem. SOC., Chem. Commun., 1983, 202. l o D. R. Lide Jr, J. Chem. Phys., 1960, 33, 1514. I ’ R. Ditchfield, W. J. Hehre and J. A. Pople, J. Chem. Phys., 1971, 54, 724. T. H. Dunning, J. Chem. Phys., 1970, 53, 2823. Program MONSTERGAUSS, M. R. Peterson and R. A. Poitier (University of Toronto, Ontario, Canada, 1980). This program incorporates the integral and SCF routines from G A U S S I A N ~ ~ , J. S. Binkley et al., QCPE 368 (Chemistry Department, University of Indiana, Bloomington, Indiana). The analytic force gradients used in the geometry optimization are calculated with the FORCE subroutine, H. B. Schlegel, Ph. D. Thesis (Queen’s University, Kingston, Ontario, Canada). J. Almlof, USIP-Report 72-09 (University of Stockholm, 1972). T. Amos and L. C. Snyder,oJ. Chem. Phys., 1964, 41, 1773. J. Almlof, A. Lund and K-A. Thuomas, Chem. Phys., 1975, 7, 465. See, for example, A. Lund, P-0. Samskog, L. Eberson and S. Lunell, J. Phys. Chem., 1982,86,2458. Landolt- Bornstein, Structural Data for Free Polyatomic Molecules, ed. K-H. Hellwege and A. M. Hellwege (Springer, Berlin, 1976), new series, vol. II/7. L. C. Snyder and T. Amos, J. Am. Chem. SOC., 1964, 86, 1647. G. S. Owen and G. Vincow, J. Chem. Phys., 1972, 54, 368. T. Komatsu and A. Lund, J. Phys. Chem., 1972, 76, 1727. 12 13 14 15 16 17 19 20 21 22 D. Becker, K. PIante and M. D. Sevilla, J. Phys. Chem., 1983, 87, 1648.

ISSN:0301-7249    

DOI:10.1039/DC9847800035

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

Faraday Discuss. Chem. Soc., 1984, 78,49-55 Homolysis of Conjugated Carbanions and Carbenium Ions BY JOHN L. COURTNEIDGE, ALWYN G. DAVIES," PETER S. GREGORY AND SAFIEH N. YAZDI Chemistry Department, University College London, 20 Gordon Street, London WC 1 H OAJ Received 6th June, 1984 Methods are reported by which fulvalene and pentalene radical anions and cyclobutadiene, cyclopentadiene and buta- 1,3-diene radical cations can be prepared in fluid solution and studied by e.s.r. spectroscopy. These preparations illustrate the general principles that radical anions and cations can be regarded as the conjugate bases and acids, respectively, of neutral radicals and that bonds to metals and to hydrogen often show common properties. Most of the small hydrocarbon radicals that can be generated and studied by e.s.r.spectroscopy in fluid solution have hitherto been electrically neutral (e.g. methyl, vinyl and allyl), and the simplest common hydrocarbon radical ions were those of the arenes. This picture is changing rapidly with the development and application of methods for generating radical ions, particularly radical cations, of simple unsaturated hydrocarbons. We review here briefly some of our relevant work on neutral radicals, then present recent results of e.s.r. studies of the preparation of the radical anions and cations of simple T systems, particularly the radical cations of 1,3-dienes. PENTADIENYL AND CYCLOPENTADIENYL RADICALS Pentadienyl radicals can be generated by standard methods such as the abstrac- tion of hydrogen from a penta- 1,3- or penta- 1,4-diene, of halogen from a pentadienyl halide, or by the ring-opening of a cyclobutenylmethyl radical.' Cyclopentadienyl radicals are less easy to generate, as t-butoxyl radicals react with cyclopentadiene by addition rather than by hydrogen abstraction.It became possible to study these cyclopentadienyl radicals in detail only when it was found2 that the cyclopentadienyl derivatives of many metals were photosensitive, and irradiation of these compounds in solution, in the cavity of an e.s.r. spectometer, gave strong spectra of the corresponding cyclopentadienyl radicals (1): R (1) M = e.g. Li, $Hg, Bu3Sq3 Ph3Pb: Me2CpTi,' C ~ Z r c l ~ . ~ R = e.g. 'H, 2H, a l k ~ l , ~ Me,Si, C13Si, Me,Ge, Me3Sn.7 The magnitude of the I3C hyperfine coupling shows that these are wdelocalised radical^,^ and Huckel molecular-orbital theory has been used to interpret the sub- stituent effects on the e.s.r.spectra in terms of the electronic effects of the substituent 4950 CONJUGATED CARBANIONS AND CARBENIUM IONS group^.^,^-^ As we are dealing with neutral radicals in non-polar solvents, the effects of solvation are probably not important. No radicals are observed when cyclopentadiene itself is irradiated with ultraviolet light, but penta-alkylation of the ring confers a remarkable photosensitivity. If pentamethylcyclopentadiene is irradiated with ultraviolet light, unfiltered or filtered through Pyrex glass, a strong spectrum of the pentamethylcyclopentadienyl radical (2) is observed, a( 15H) = 6.35 G, and dihydrogen is evolved: This reaction exemplifies the principle that, in the same molecular environment, hydrogen often shows the same behaviour as a metal." Just as reaction (1) gives rise to the formation of a metallic radical, reaction (2) appears to involve the liberation of a free hydrogen atom from the ring, since in ethene solvent it can be trapped to form the ethyl radical.' I Experiments involving deuterium labelling show that, in forming dihydrogen, this hydrogen atom derives its partner not from the ring but from a methyl group." This reaction warrants a thorough photochemical study.FULVALENE AND PENTALENE RADICAL IONS This photolysis of cyclopentadienylmetallic compounds can be exploited in preparing radical anions of 5-annulene systems where the parent annulene itself cannot be isolated.Thus fulvalene polymerises in solution at concentrations > 0.00 1 mol dm-3. Dihydrofulvalene (3), however, is stable, and gives a stable disodium derivative. In line with reaction (l), if this is irradiated with ultraviolet light it shows the e.s.r. spectrum of the fulvalene radical anion (4), a(2H) = 1.55, a(2H) = 3.70 G:12 2Na' Na' (3) (4) Similarly, pentalene cannot be isolated, but dihydropentalene ( 5 ) can be prepared and converted into its dilithio derivative, and this, again on irradiation, generates the e.s.r. spectrum of the radical ion (6), a(2H) = 0.83, a(4H) = 7.73 G:13J . L. COURTNEIDGE, A. G. DAVIES, P. S . GREGORY AND S . N. YAZDI 51 CYCLOPENTADIENE RADICAL CATIONS Reactions (3) and (4) emphasise that protic radicals are formally related to the corresponding radical anions as acid and conjugate base [reaction (91, although in fact, in these reactions, the proton transfer preceded the homolysis: HB' $ B'-+H+ ( 5 ) Conversely, protic radical cations can be regarded as the conjugate acids of neutral radicals [reaction (6)]: A'+H+ s AH'+ (6) We anticipated therefore that the photolysis of precursors of cyclopentadienyl radicals under acid conditions should generate the corresponding cyclopentadiene radical cations, and indeed photolysis by unfiltered or Pyrex-filtered U.V. light of pentamethylcyclopentadiene in trifluoroacetic acid shows a strong spectrum of the corresponding pentamethylcyclopentadiene radical cation (7), a( Me) = 0.8, a( 1 H) = 1.6, a(2Me) = 4.0, a(2Me) = 15.0 G.I4 The hyperfine coupling constants of the methyl groups on the diene unit agree with the values calculated from simple Huckel theory, and the hydrogen atom on the ring, lying as it does in the nodal plane of the SOMO of the diene, is only weakly coupled: .. R R ' 'R When R = H, the reaction might be thought to proceed by formation of the pentamethylcyclopentadienyl radical [reaction (2)] (as does indeed occur in acetic acid solution), followed by rapid protonation of the neutral radical to give the radical cation. However, hexamethylcyclopentadiene, which is photostable in neutral solvents, shows a strong spectrum of the hexamethylcyclopentadiene radical cation (7, R = Me) when it is photolysed in trifluoroacetic acid (fig. 1). The reaction therefore appears, at least in this latter case, to involve the photolysis of a carbenium ion, as represented in reaction (7). We do not know yet whether a hydrogen atom is formed, or whether dihydrogen is liberated, to complete the analogy with reaction (2).CYCLOBUTADIENE RADICAL CATIONS The tetra-t-butylcyclobutadiene radical cation was prepared by treating tetra-t- butyltetrahedrane or tetra-t-butylcyclobutadiene with aluminium ch10ride.l~ Hogeveen provided a more generally useful route to these radical cations when he showed that the cyclobutadiene-AlC1, u complex (8), which is formed when but-2- yne is treated with aluminium chloride, shows the spectrum of the tetramethyl- cyclobutadiene radical cation (9) under U.V. irradiation, a( 12H) = 8.70 G.16 This route to an annulene radical by the homolysis of a ring-metal bond may be regarded as the cationic equivalent of reaction (1).It is not necessary to isolate the intermediate (8), and the whole reaction can be carried out in the e.s.r. tube.52 CONJUGATED CARBANIONS AND CARBENIUM IONS Fig. 1. E.s.r. spectrum of the hexamethylcyclopentadiene radical cation (7; R = Me), obtained by photolysis with U.V. light of hexamethylcyclopentadiene in trifluoroacetic acid at - 15 "C. Me AlCI, 2MeEMe CH,C.,b AICI, #"'* MeHMe Me Me Me Me (8) (9) By this means, using mixed alkynes, R + R', or mixtures of simple alkynes, R = R and R' = R', or of simple and mixed alkynes, cyclobutadienes carrying mixed alkyl groups (10-12) can be prepared: l7 R' )qR' R R R' )qR R R' R' )qR R R The e.s.r.hyperfine coupling constants can then be interpreted in terms of the electronic effect of the substituents, as was done for cyclopentadienyl radicals. The order of electron-releasing power by different alkyl groups appears to be different in the two series of compounds; the same sequence would not necessarily be expected, but with the radical ions the situation may be complicated by strong solvation e ~ ~ e c t s . ' ~ In view of the success of reaction (7) for generating cyclopentadiene radical cations, it seemed possible that protic acids might be able to replace the Lewis acid in reaction (8) to provide a new route to cyclobutadiene radical cations, and we find that if a solution of di-t-butylethyne or of di- 1 -adamantylethyne in trifluoroacetic acid is photolysed, strong spectra of the corresponding tetra-t-alkylcyclobutadieneJ. L.COURTNEIDGE, A. G. DAVIES, P. S. GREGORY AND S. N. YAZDI 53 n Fig. 2. E.s.r. spectrum of the tetra- 1 -adamantylcyclobutadiene radical cation (14, R = 1-adamantyl), obtained by photolysis with U.V. light filtered through Pyrex of a solution of di-1-adamantylethyne in trifluoroacetic acid at 8 "C; ca. 20% v/v of dichloromethane was added to promote solubility. radical cations (14) are observed. The spectrum of the tetra- 1 -adamantyl- cyclobutadiene radical cation is shown in fig. 2. The sequence of the various steps of protonation, homolysis and dimerisation has not yet been established, but a number of alkynes have been shown to be converted into cyclobutenyl cations (13) under super-acid conditions," and it seems likely that we are observing again the photohomolysis of a carbenium ion [cf: reaction (S)]: As in reaction (7), we do not yet know the fate of the hydrogen fragment, but again hydrogen is reproducing the behaviour of a metal, in this case aluminium.The radicals prepared by using protic acids [reaction (9)] are much longer lived than those prepared using aluminium chloride [reaction ( 8)].16,17,19 A wide variety of cyclobutenyl cations can be prepared by different methods,20 and it appears likely that their photolysis may provide a route to cyclobutadiene radical cations which have hitherto been inaccessible.54 CONJUGATED CARBANIONS AND CARBENIUM IONS BUTA-1,3-DIENE RADICAL CATIONS When an alkyne is treated with aluminium chloride in dichloromethane, no e.s.r.spectrum is usually observed until the mixture is photolysed. Sometimes, however, a spectrum can be observed before photolysis; for example, di-t-butylethyne may display a strong spectrum showing hyperfine coupling by two methyl groups of one type, and two pairs of another, a(2Me) = 4.2, a(2Me) = 10.55, a(2Me) = 10.70 G. The hyperfine coupling constants correlate with those calculated for the hexamethyl- butadiene radical cation, and indeed this diene generates the same spectrum when it is treated with aluminium chloride2’ or with mercury( 11) trifluoracetate in trifluoroacetic acid (Kochi’s reagent22). An alternative route to the same 1,3-diene radical cation is the photolysis of a solution of the alkyne itself with Kochi’s reagent:23 The detailed mechanism of the reaction involving aluminium chloride is not known, but Olah has shown that di-t-butylethyne rearranges to hexamethylbutadiene under super-acid conditions.18 One possibility therefore is that adventitious hydro- lysis of A1C13 produces a trace of HA1C14, which induces a double nucleophilic rearrangement.CONCLUSION We have shown here that a variety of carbenium ions RH+ (or RM’) can be induced to undergo homolysis to give the corresponding radical cations Re+. This suggests the exciting possibility that the many further types of carbenium ions which have been identified in acid or super-acid solution24 may provide an entree to new families of radical cations. We are pleased to acknowledge the support of the S.E.R.C.I 2 3 4 5 6 7 8 9 10 II 12 K. U. Ingold, B. Maillard and J. C . Walton, J. Chem. SOC., Perkin Trans. 2, 1981, 970; A. G. Davies, D. Griller, K. U. Ingold, D. A. Lindsay and J. C. Walton, J. Chem. SOC., Perkin Trans. 2, 198 1 , 633. A. G. Davies and M-W. Tse, J. Chem. Soc., Chem. Commun., 1978, 353. P. J. Barker, A. G. Davies and M-W. Tse, J. Chem. SOC., Perkin Trans. 2, 1980, 941. A. G. Davies, J. A-A. Hawari, C. Gaffney and P. G. Harrison, J. Chem. SOC., Perkin Trans. 2, 1982, 631. P. B. Brindley, A. G. Davies and J. A-A. Hawari, J. Organomet. Chem., 1983, 250, 247. J. M. Atkinson, P. B. Brindley, A. G. Davies and J. A-A. Hawari, J. Organomet. Chem., 1984,264, 253. P. J. Barker, A. G. Davies, R. Henriquez and J-Y. Nedelec, J. Chem. SOC., Perkin Trans. 2, 1982, 745.A. G. Davies, E. Lusztyk and J. Lusztyk, J. Chem. Soc., Perkin Trans. 2, 1982, 729. A. G. Davies, J. P. Goddard, E. Lusztyk and J. Lusztyk, J. Chem. SOC., Perkin Trans. 2, 1982,737. C. Eaborn, Organosilicon Compounds (Butterworths, London, 1969), p. 125; A. G. Davies, J. Organomet. Chem., 1982, 239, 87. A. G. Davies, E. Lusztyk, J. Lusztyk, V. P. J. Marti, R. J. H. Clark and M. J. Stead, J. Chem. SOC., Perkin Trans. 2, 1983, 669. A. G. Davies, J. R. M. Giles and J. Lusztyk, J. Chem. SOC., Perkin Trans. 2, 1981, 747.J . L. COURTNEIDGE, A. G. DAVIES, P. S. GREGORY AND S. N. YAZDI 55 l 3 D. Wilhelm, J. L. Courtneidge, T. Clark and A. G. Davies, J. Chem. SOC., Chem. Commun., 1984, 810. J. L. Courtneidge, A. G. Davies and S. N. Yazdi, J. Chem. SOC., Chem. Commun., 1984, 570. Q. B. Broxterman, H. Hogeveen and D. M. Kok, Tetrahedron Lett., 1981, 22, 173. J. L. Courtneidge, A. G. Davies, E. Lusztyk and J. Lusztyk, J. Chem. SOC., Perkin Trans. 2, 1984, 155 and unpublished work. 14 l 5 H. Bock, B. Roth and G. Maier, Angew. Chem., Znt. Ed. Engl., 1980, 19, 209. 16 17 '' G. A. Olah and H. Mayr, J. Am. Chem. Soc., 1976, 98, 7333. l 9 Q. B. Broxterman, H. Hogeveen and R. F. Kingma, Tetrahedron Lett., 1984, 25, 2043. 2o G. Olah, J. S. Staral, R. J. Spear and G. Liang, J. Am. Chem. SOC., 1975, 97, 5489. J. L. Courtneidge and A. G. Davies, J. Chem. SOC., Chem. Commun., 1984, 136. 22 W. Lau, J. C. Huffman and J. K. Kochi, J. Am. Chem. SOC., 1982, 104, 5515. 23 W. Chan, unpublished work. 24 Carbonium Ions, ed. G. A. Olah and P. von R. Schleyer ( Wiley-Interscience, New York, 1968-1976), vol. I-v.

ISSN:0301-7249    

DOI:10.1039/DC9847800049

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

Famday Discuss. Chem. SOC., 1984, 78, 57-69 Spin Delocalization in Radical Cations of Oxygen-containing Organic Compounds as Revealed by Long-range Hyperfine Interactions and Solvent Effects BY LARRY D. SNOW AND FFRANCON WILLIAMS* Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996- 1600, U.S.A. Received 10th May, 1984 E.s.r. evidence is presented for long-range ' H hyperfine interactions and thermally revers- ible solvent effects involving mainly the radical cations of aldehydes and ketones in Freon matrices. Long-range couplings to S protons are found to be surprisingly large in the cations of acyclic, cyclic and polycyclic carbonyl compounds. The conformational requirements for these large couplings are discussed in terms of facile spin transmission via a trans arrangement of C-C and C--H u bonds, the assumption of this trans effect providing a rigorous and consistent analysis of the experimental results for both non-rigid and rigid molecular geometries.The matrix perturbation observed at low temperatures is believed to arise from a weak complex formed between the radical cation and the solvent, the hyperfine interaction occurring mainly with a single chlorine nucleus. The reversible loss of the e.s.r. substructure associated with this complex at elevated temperatures is attributed to motional averaging of the chlorine hyperfine anisotropy rather than to dissociation of the complex. ~ During the past few years solid-state e.s.r. studies have provided a wealth of information regarding the structure of radical cations derived from simple organic molecules.This broad advance was triggered by the discovery of a simple method for the generation of these cations by y-irrradiation of dilute solutions of the parent molecules in Freon matrices,14 these solvents functioning as efficient positive hole carriers to the solute by virtue of their high ionization potentials. The other desirable properties of Freons for this application are their high electron attachment rates and their ability to provide a chemically inert environment for the highly reactive solute radical cations. Also of great importance is the fact that the e.s.r. spectra of the cations are frequently well resolved in these matrices. Using this technique, the radical cations of several oxygen-containing organic compounds have been charac- terized, including those of simple ethers,576 a c e t a l ~ , ' ~ ~ aldehydes,"*' ketones,I2 esters of carboxylic a ~ i d s , ' ~ , ' ~ epoxide~'~-'' and vinyl monomers.'* Except where molecular rearrangements are i n ~ o l v e d ~ .' ~ , ' ~ the aforesaid cations are either 7 ~ ~ - ~ or a10-14 oxygen-centred radicals showing appreciable hyperfine couplings to p hydrogens as a result of efficient hyperconjugation. Another manifes- tation of the tendency for some of these radical cations to undergo spin delocalization comes from the e.s.r. observation of significant matrix hyperfine interactions. In the case of the methyl formate ~ a t i o n ' ~ " ~ this solvent interaction is very strong and has been interpreted in terms of the formation of a (T* radical, the hole being shared with a filled chlorine lone-pair orbital in a Freon molecule.Here we are concerned with 'weak' hyperfine interactions as evidenced by thermally reversible solvent effects and coupling to remote ( y and 6) hydrogens, with examples of such effects drawn primarily from radical cations of carbonyl 5758 HYPERFINE INTERACTIONS AND SOLVENT EFFECTS compounds. Our preliminary reports' ',I2 have already commented on the surpris- ingly large magnitude of the hyperfine couplings which are involved, given the secondary character of these effects. Here we present additional results that fully confirm the generality of these interactions, particularly in regard to long-range proton hyperfine couplings which are interpreted according to previous empirical and theoretical generalization^.'^-*' Such experimental studies should eventually contribute to a greater understanding of the mechanisms of orbital interactions through bonds and through space.'2923 EXPERIMENTAL Dilute solutions (0.1-1 mol%) of the solutes in various Freons were prepared in Spectrosil or Suprasil e.s.r.tubes (3 mm i d . ) on a vacuum line. Acetaldehyde, propionaldehyde, cyclo- hexanone, adamantan-2-one and bicyclo[3.3. I lnonan-9-one were obtained from the Aldrich Chemical Co. The various deuterated compounds were supplied by Merck, Sharp and Dohme Isotopes and purified by trap-to-trap distillation on the vacuum line immediately before use. The Freon solvents included trichlorofluoromethane and 1 , 1 , 1 -trichlorotrifluoroethane, both from Aldrich.The sample tubes were irradiated at 77 K by exposure to y-rays from a 6oCo source (Gammacell 200, Atomic Energy of Canada Ltd) for doses up to 0.5 Mrad (5 x lo3 J kg-'). Subsequently, the sample tubes were transferred to a variable-temperature Dewar insert mounted in the cavity of an e.s.r. spectrometer (Bruker ER 200 D SRC), the ESR-900 cryostat (Oxford Instruments) being used to obtain temperatures below 77 K. Spectra were recorded over a wide range of temperature as the sample was progressively annealed up to the softening point of the particular matrix. Additional e.s.r. spectra were recorded during the course of the work with a Varian V-4502- I5 instrument. The microwave frequency was measured with a Systron-Donner counter (model 6054 B) and magnetic-field markers were recorded with an n.m.r.gaussmeter (Walker model G-502 or Bruker microprocessor-controlled model ER 035 M). RESULTS ACETALDEHYDE A N D PROPIONALDEHYDE RADICAL CATIONS We have previously reported e.s.r. studies on the acetaldehyde cations CH,CHO', CH3CDO', CD3CHO' and CD,CDO'.'' These isotopic experiments proved that there is no resolved hyperfine coupling to the hydrogens of the methyl group in the acetaldehyde cation, the interactions being confined to the aldehydic hydrogen and the Freon matrix. As shown in fig. 1, the spectrum of CD,CHO' in CFC13 at 50 K possesses a large doublet splitting of 136 G together with a highly anisotropic substructure arising from the matrix interaction. Although this substructure is lost reversibly at ca.120 K, the large coupling to the aldehydic hydrogen remains virtually unaltered.' ' It was therefore of interest to carry out a similar e.s.r. study of the propional- dehyde radical cation, and the salient results are presented in fig. 2 and 3. In contrast to the single reversible change observed for the acetaldehyde cation, the e.s.r. spectrum of the propionaldehyde cation in CFC1, undergoes a series of reversible changes between 100 and 160 K. As shown in fig. 2( c), the spectrum of CH3CH2CHO' at 40 K is distorted by anisotropic features which are similar to those observed in the low-temperature spectra of the acetaldehyde cations and previously assigned to a chlorine interaction.' ' Also paralleling the behaviour of the acetaldehyde cations is the reversible disappearance of these extraneous peaks on warming to 120 K, as indicated by comparison of the CH3CH2CHO+ spectra inL.D. SNOW AND F. WILLIAMS 59 9433.4 MHz Fig. 1. First-derivative e.s.r. spectrum of the CD3CH0 radical cation recorded at 50 K. The cation was generated by y-irradiation of a 0.6mol0/0 solution of the parent compound in CFC13 at 77 K for a dose of 0.5 Mrad. fig. 2 ( a ) and ( c ) . Instead of changing to a simple doublet, however, the spectrum of CH3CH2CHO+ at 120 K becomes a doublet of doublets. That the smaller doublet splitting arises from a methyl rather than a methylene hydrogen is established by the identical spectrum of CH3CD2CHO+ in fig. 2(b). Further changes are observed in the e.s.r. spectra of CH3CH2CHOf and CH3CD2CHO+ at higher temperatures, as shown in fig.3. At 140K the smaller doublet splitting of 12.5 G is no longer resolved [fig. 3(c)], but additional structure becomes evident just before the softening point of the CFC13 matrix (ca. 160 K) is reached. Although not fully resolved, the spectra of CH,CD2CHO+ [fig. 3(a)] and CH3CH2CHO+ [fig. 3( b ) ] at this limiting temperature reveal that a quartet substruc- ture [A(3H) =: 4.6 GI has replaced the sharp 12.5 G doublet in the corresponding spectra at 120 K (fig. 2). Since this change is reversible it can be attributed to the onset of methyl-group rotation about the C(2)-C(3) bond. Thus deuterium labelling and studies of motional effects show that coupling to the single non-aldehydic hydrogen in the 120 K spectrum of the propionaldehyde cation can definitely be assigned to a methyl hydrogen.This situation in which a methyl group adopts a preferred conformation at low temperature such that only one of its hydrogens is strongly coupled to the electron spin has also been observed for the isobutyraldehyde and 2,4-dimethylpentan-3-one radical cations.12 Here, non-aldehydic hyperfine couplings to two and four equivalent 6 hydrogens, respec- tively, were detected, consistent with the presence of two locked methyl groups in each isopropyl group. At this point we digress from the main theme to note that the high-temperature e.s.r. spectra from CH3CD2CH0 and CH3CH2CH0 solutions in fig. 3 contain fairly strong signals in the centre which are readily identified as coming from the radicals CH3CDCH0 and CH3CHCH0 or their conjugate acids formed by protonation of the carbonyl group.The hyperfine pattern for CH,CDCHO (or its conjugate acid) indicated by the stick diagram in fig. 3(a) is particularly clear, the couplings A(3H) = 22.1 G and A( D) = 3.2 G being characteristic of this type of carbon-centred radical. If these radicals originate by the loss (or transfer) of a proton from the60 HYPERFINE INTERACTIONS AND SOLVENT EFFECTS T=120K T = 4 0 K 9 2 7 3 . 5 MHz I I 9 4 2 8 . 8 MHz Fig. 2. First-derivative e.s.r. spectra of the’ radical cations of CH,CH2CH0 [ ( a ) and (c)] and CH3CD2CH0 ( b ) at the temperatures shown. The spectra were recorded after exposure of CFC13 solutions of the parent compounds (0.5 mol %) to y-irradiation at 77 K for a dose of 0.5 Mrad in each case.L.D. SNOW AND F. WILLIAMS 61 L I I 9273.3 MHz r;l T=140K i v 9272.9 MHz 0 v) c) d c) 0 r- Fig. 3. First-derivative e.s.r. spectra of the radical cations of CH3CD2CH0 ( a ) and CH,CH,CHO [( b) and (c)] recorded at higher temperatures than in fig. 2. The central regions of the spectra ( a ) and ( b ) also include signals from the radicals CH,CDCHO and CH,CHCHO, respectively (see text).62 HYPERFINE INTERACTIONS AND SOLVENT EFFECTS Hz I Fig. 4. First-derivative e.s.r. spectra of the ( a ) cyclohexanone and ( b ) [2,2,6,6-2H,]cyclo- hexanone radical cations at 130K. In each case the cation was generated by y-irradiation at 77 K of a 1 mol% solution of the parent compound in CFC13 for a dose of 0.5 Mrad. corresponding radical cation, the results imply that deprotonation occurs mainly from the methylene group.However, kinetic proof of this step is lacking. CYCLOHEXANONE RADICAL CATION Because of their greater conformational rigidity, ring structures are intrinsically more suitable than acyclic compounds for the detailed exploration of angular- dependent long-range couplings.'' As a first step cyclohexanone was used to test the above finding that the strongly coupled non-aldehydic hydrogens in carbonyl radical cations reside in 6 rather than y positions. Accordingly, experiments were carried out on 2,2,6,6-tetradeuterocyclohexanone as well as the fully protiated compound, and the e.s.r. results are shown in fig. 4. The spectra of the two radical cations are seen to have identical 1 : 2 : 1 triplet patterns except for slightly narrower linewidths from the partially deuterated species, verifying that the two strongly coupled hydrogens [A(2H) = 27.5 GI are not in the y positions.Although these results do not exclude the possibility that the two E hydrogens are responsible for the hyperfine interaction, the experiments described in the following section make this assignment very unlikely, as do the empirical rules mentioned later in the Discussion. Therefore, by elimination, the strongly coupled hydrogens must be those in either the two equatorial or the two axial 6 positions.L. D. SNOW AND F. WILLIAMS 63 jli Fig. 5. First-derivative e.s.r. spectrum of the adamantan-2-one radical cation at 145 K. The cation was generated by y-irradiation of a 0.1 mol% solution of the parent compound in CFCl, at 77 K for a dose of 0.4 Mrad.9267.5 M H z (3 (3 I \ I r"' P Q, (D €9 €9 Fig. 6. First-derivative e.s.r. spectrum of the bicyclo[3.3. I]-nonan-9-one radical cation at 144 K. The cation was generated by y-irradiation of a 0.1 mol% solution of the parent compound in CFC1, at 77 K for a dose of 0.4 Mrad.64 HYPERFINE INTERACTIONS AND SOLVENT EFFECTS Table 1. E.s.r. parameters for radical cations of various aldehydes and ketones in the CFCI, matrix radical cation T / K g hyperfine splitting/G CH,CHO+ CH,CDO+ CD3CHO+ CD3CHO+ CH3CH2CHO+ CH3CH2CHO+ CH3CD2CHO+ ( CH3)2CHCHO+ 2,4-dimethylpentan-3-one+ cyclohexanone+ [2,2,6,6-2H,]cyclohexanonef adamantan-2-onet bicyclo[3.3. 1]nonan-9-onef I58 140 50 88 40 120 158 120 159 120 88 88 88 145 144 2.0048 2.0054 CQ.2.0050 ca. 2.0050 ca. 2.0050 2.0048 2.0048 2.0048 2.0048 2.0044 2.0033 2.0032 2.0032 2.0025 2.0025 1363 1 HP) 20.7( 1 DP) 136(1H,), 19(135C1) 135(1H,), 12.5(1H8), 15( 135C1) 1354 IH,), 1 2 3 1H8) 135.1(1Hp), 4.4(3H8) 135.3(1HP), 12.5(1Hg) 135.3( lHP), 4.8(3H8) 120.3( lH,), 20.4(2H8) 15.2(4H8) 27.5(2H8) 27.5(2H8) 22.9(4H8),7.2(2H,) 1 3 6 ( i ~ , ) , 2 1 ( 1 ~ ~ c 1 ) 22.3(4H8),6.9(2HY) RADICAL CATIONS OF ADAMANTAN-2-ONE AND BICYCLO[3.3.1]NONAN-9-ONE In addition to having rigid polycyclic character, these subject compounds possess C,, symmetry. Consequently, they offer an excellent opportunity for probing the couplings to remote hydrogens in a cyclohexanone-like system. In fact, the e.s.r. spectra of their radical cations in fig.5 and 6 possess very similar hyperfine patterns and each can be analysed into a 1 : 4: 6: 4: 1 quintet of 1 : 2: I triplets, the only difference being that the spectrum of the more rigid adamantan-2-one radical cation (fig. 5 ) has better resolution. Also, the e.s.r. parameters of the two cations, summar- ized together with data for other radical cations in table 1, are almost identical allowing for the appreciable error associated with the analysis of broad-line patterns. Given the nearly identical e.s.r. results for these two cations, the assignment problem is greatly simplified. Using the individual cyclohexanone-like rings to define the axial (a) and equatorial (e) positions, each cation has sets of 2HYe, 4Hse, 4Hs, and 2H,,. However, the B2 symmetry of the SOMO allows us to eliminate 2H,, from consideration because these hydrogens lie in the nodal plane of this orbital, i.e.the mirror plane perpendicular to the R=C=O plane (see Discussion). The triplet splitting of ca. 7 G can therefore be assigned to 2H,, while the ca. 22 G quintet splitting must be due to either 4H6, or 4Hs,. This is consistent with the analysis of the results for the cyclohexanone cation, the 27.5 G triplet splitting being assigned to the 2Hs, or 2Hs,. R DISCUSSION REVERSIBLE MATRIX EFFECTS The present study provides further evidence for thermally reversible solvent effects on the e.s.r. spectra of certain aldehyde radical cations in Freon matrices, these effects being characterized by the development of asymmetric substructures in the low-temperature spectra.Although these anisotropic features are difficult toL. D. SNOW AND F. WILLIAMS 65 analyse in detail,' ' there is general agreement that the quartet-like substructure in the doublet e.s.r. spectrum of the acetaldehyde cation (fig. 1) arises mainly from an anisotropic hyperfine coupling to a single chlorine nucleus.' This coupling shows a small but significant negative temperature dependence, All ( 35Cl) decreasing from 21.4 G at 50 K to 18.7 G at 88 K." For the propionaldehyde cation, a tentative analysis of the spectrum in fig. 2( c) gives an estimate of ca. 15 G for the corresponding C1 coupling at 40 K, indicating that the interaction is weaker than for CH3CH0. It has been suggested that the magnitude of the matrix interaction should be a function of the difference between the ionization potentials of the solute and solvent molecules, a smaller difference giving rise to a stronger intera~tion.'~"~ Our results are in accord with this hypothesis, the ionization potentials of CFCl,, CH3CH0 and CH,CH2CH0 being 11.78," 10.2624" and 9.85 eV,24" respectively.Also, the fact that no matrix interaction can be discerned in the e.s.r. spectrum of the isobutyraldehyde cation between 82 and 120 K (table 1) is consistent with the even lower solute ionization potential (9.74 eVZ4') in this case. Another significant feature of the results is that for both CH3CHO' and CH3CH2CHO+ the reversible loss of the chlorine hyperfine structure is accompanied by virtually no change in the large ( 135-1 36 G) isotropic H coupling to the aldehydic hydrogen.This would be a most remarkable finding if the collapse of the 35Cl hyperfine interaction were to result from the thermal dissociation of a weak (T bond between the cation and a CFC1, m01ecule.'~~~'~ The formation of (T* adduct radicals of this type is well known in y-irradiated solids,25 but in our experience the dissociation of such adducts is always accompanied by an alteration in the spin- density distribution within the radical part. It might also be expected that bond scission would lead to sudden changes in the spectrum, but this behaviour is not observed. Indeed, a careful examination of the propionaldehyde cation spectra in fig. 2 reveals that weak anisotropic features are still present at 120 K, albeit with loss of fine structure, suggesting a gradual rather than a sudden loss of the chlorine hyperfine structure with increasing tem- perature.The same conclusion can be drawn from a study of the published e.s.r. spectra for various deuterium-labelled acetaldehyde cations.' ' Alternatively, the reversible temperature effect can be attributed to a modulation of highly anisotropic couplings brought about by the intermolecular motion. Assum- ing the isotropic component of the 35Cl hyperfine interaction is small, this would explain the absence of resolved structure at high temperature. The marked insensitiv- ity of the isotropic aldehydic hydrogen coupling to the loss of 35Cl hyperfine structure is also understandable in terms of this description, since the actual solvent interaction would probably be unaffected by motional considerations.Thus it seems more reasonable to us that the reversible loss of the substructure with temperature results not from a dissociation of the cation-solvent complex but from a motional averaging of the chlorine hyperfine anisotropy associated with this complex.' ' Irrespective of the mechanism responsible for the loss of the solvent hyperfine structure at high temperature, it is generally agreed that the aldehyde-cation-CFC1, interaction is a relatively weak e f f e ~ t , ' ~ ~ ~ ~ ' ~ ' ~ especially in comparison with that observed for the methyl formate ~ a t i o n . ' ~ " ~ Accordingly, we previously described this weak solvent perturbation as a 'superhyperfine' interaction. " We feel that this description is an appropriate one because the e.s.r.spectra of some other cations are also observed to possess hyperfine splittings which do not originate from interaction with the magnetic nuclei of the cation. For example, the quintet e.s.r. spectrum of the ring-opened cation CH20CHl from ethylene oxide shows a curious fine structure in CFC13 which persists up to 150 K, whereas a different substructure 3566 HYPERFINE INTERACTIONS AND SOLVENT EFFECTS is observed in CF3CC13 below 140 K.16 However, the e.s.r. parameters of the main quintet pattern remain sensibly constant in the two matrices.I6 Evidently the effect is highly specific for the cation-matrix-temperature combination, but the structure of the cation does not seem to be significantly affected by these weak solvent perturbations.Despite the weak nature of the solvent interactions described here, it is possible that such complexes may play a significant role in radical cation chemistry.26 As mentioned above, e.s.r. studies show that the radical cation formed from ethylene oxide in the solid state is the ring-opened (2A2)2-oxatrimethylene cation with a symmetrical ( C2v) planar structure similar to that of the isoelectronic ally1 radical. Since a large symmetry-imposed barrier of 25-30 kcal mol-' was originally predic- ted27 for ring opening from the 2B1 ground state of the isolated ethylene oxide it seemed reasonable to consider the alternative of a symmetry-allowed reaction pathway from a complex between the Freon solvent and the 2A1 excited state of the cation.However, recent higher-level calculations by Clark2* indicate that the barrier to the 'forbidden' ring opening is only 3.7 kcal mol-I and may disappear completely at even higher levels. Therefore there is no compelling need to invoke a solvent effect to explain the formation of the ring-opened cation. LONG-RANGE PROTON HYPERFINE INTERACTIONS &HYDROGEN COUPLINGS. A common feature of our results is the detection of a surprisingly large hyperfine coupling to one or more 6 hydrogens in carbonyl radical cations at low temperatures. Considering the acyclic cations first, this long-range coupling is observed to one, two and four equivalent hydrogens in the cations of propionaldehyde, isobutyraldehyde and 2,4-dimethylpentan-3-one, respectively. The results clearly indicate that each of the strongly coupled hydrogens is associated with a stereospecific position in a locked methyl group, and this has been confirmed by studies of the temperature dependence of the e.s.r.spectra. Thus we showed previously that the well resolved quintet spectrum of the 2,4-dimethylpentan-3-one cation is observed only at low temperatures (ca. 100 K),l2 as expected when the equivalence of the S hydrogens requires a preferred conformation involving all four methyl groups. Similarly, the present study of the propionaldehyde cation establishes that the loss of the 6-hydrogen interaction is clearly accompanied by the onset of methyl group rotation. In accordance with many previous empirical and theoretical studies of long-range 'H coupling^,'^-^^ we adopt the simple rule that the 6-proton coupling in these cations is maximised when the interaction can be relayed from the major site of spin density at the carbonyl group uia a trans (W-plan) arrangement of C-C and C-H (T Specifically, the preferred conformations of the methyl groups in the acyclic cations are chosen such that the dihedral angle C,C,C,H, between the C,C,C, and CyCSHG planes is always 180".Although the adoption of this rule leads to a simple and self-consistent explanation of the results, it does not allow us to predict the overall conformation of the acyclic cations. For example, two possible conformations (1) and (2) of the propionaldehyde cation are shown below, these being the cis and trans rotamers corresponding to values of 0 and 180" for the dihedral angle OCBC,C, between the OC,C, and C,C,C, planes.In fact, the requirement that one of the S hydrogens in the methyl group occupies the trans position relative to the C,-C, bond places no restriction on the value of the dihedral angle obtained by rotation of this bond, at least when only one methyl group is present in the 6 position as for the propionaldehyde cation.L. D. SNOW AND F. WILLIAMS 67 In contrast to the propionaldehyde cation, the C,, symmetry of the 2,4-dimethyl- pentan-3-one cation implied by the equivalence of the four 6 hydrogens allows only two possible conformations to be achieved by rotation about the C,-C. bonds. These correspond to OC,CyCs dihedral angles of either *60 or f 120" for the two 6 methyls in each isopropyl group, the y hydrogen residing in the trans or cis position, respectively, with respect to the oxygen of the carbonyl group. The skeletal form of the latter conformation is very similar to that of cyclohexanone (3), especially as regards the OC,C,C, dihedral angle.leads to the assignment of the two strongly coupled S hydrogens in the cyclohexanone cation to those in the equatorial positions, as indicated in structure (3). The coupling of 27.5 G to these hydrogens is the largest we have observed for &hydrogen splittings (table 1) and almost certainly reflects the perfect trans arrangement associated with the chair conformation of the cyclo- hexanone ring, the dihedral angles C,C,C,H,, being close to 180". The use of the trans Similarly, the four strongly coupled S hydrogens in the cations of adamantan-2- one (4) and bicyclo[3.3.l]nonan-9-one ( 5 ) are assigned to those in the equatorial positions as defined by the individual cyclohexanone-like rings of these polycyclic compounds.The smaller Hs, coupling of 22.5G in these polycyclic cations is probably attributable to the fact that four protons are now involved instead of two as for the cyclohexanone cation, leading to an overall increase in the delocalisation of the spin from the carbonyl group. The same type of effect is well known in alkyl radicals where the &proton hyperfine splitting decreases upon progressive methyl substitution at the cy carbon for the series of ethyl, isopropyl and t-butyl radicals.2968 HYPERFINE INTERACTIONS AND SOLVENT EFFECTS The B2 SOMO of the adamantan-2-one cation3’ (6) illustrates how the spin is delocalized into the four equatorial 6 hydrogens, the mechanism involving ‘homohyperconjugation’ from u spin density induced into the C,-C, bond at the two C, positions.Clearly, the trans or alignment effect should be important for this kind of spin transmission, just as it is with the cos20 rule for the more common 7~ hyperconjugation. Y-HYDROGEN COUPLINGS. It remains to consider why resolved y-hydrogen split- tings of ca. 7 G are apparently observed for the polycyclic cations but not for the acyclic or cyclohexanone cations. Small spin populations at the y hydrogens are consistent with the symmetry of the SOMO in (6), so we suggest that the absence of hydrogen splittings in the cyclohexanone and acyclic cations is related to the lesser rigidity of the y hydrogens in these latter systems.Even for the fairly rigid cyclohexanone ring, the carbonyl-containing region of the ring is apparently more flexible than cyclohexane3’ so that the two equatorial y hydrogens can flex out of the OC,C, plane. This is impossible in the polycyclic molecules where a perfect symmetry of the two y hydrogens is maintained.L. D. SNOW AND F. WILLIAMS 69 We acknowledge helpful discussions and useful correspondence with Professors T. Clark, E. Haselbach, S. F. Nelsen, L. Radom and M. D. Sevilla. We also thank Prof. Nelsen for his continuing interest which has led to ongoing collaborative work on various aspects of the problems discussed here. This research was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S.Department of Energy. ( a ) T. Shida and T. Kato, Chem. Phys. Lett., 1979, 68, 106; ( b ) T. Shida, Y. Egawa, H. Kubodera and T. Kato, J. Chem. Phys., 1980, 73, 5963. M. C. R. Symons and I. G. Smith, J. Chem. Res. ( S ) , 1979, 382. J. T. Wang and F. Williams, J. Phys. Chem., 1980, 84, 3156; see also A. Grimison and G.A. Simpson, J. Phys. Chem., 1968, 72, 1176. K. Toriyama, K. Nunome and M. Iwasaki, J. Phys. Chem., 1981,85, 2149; J. Chem. Phys., 1982, 77, 5891. H. Kubodera, T. Shida and K. Shimokoshi, J. Phys. Chem., 1981, 85, 2583. J. T. Wang and F. Williams, J. Am. Chem. SOC., 1981, 103, 6994. L. D. Snow, J. T. Wang and F. Williams, J. Am. Chem. SOC., 1982, 104, 2062. M. C. R. Symons and B.W. Wren, J. Chem. SOC., Chem. Commun., 1982, 817. K. Ushida and T. Shida, J. Am. Chem. SOC., 1982, 104, 7332. l o ( a ) M. C. R. Symons and P. J. Boon, Chem. Phys. Lett., 1982, 89, 516; ( b ) Chem. Phys. Lett., 1983, 100, 203. I ' L. D. Snow and F. Williams, Chem. Phys. Lett., 1983, 100, 198. L. D. Snow and F. Williams, J. Chem. Soc., Chem. Commun., 1983, 1090. I 3 ( a ) D. Becker, K. Plante and M. D. Sevilla, J. Phys. Chem., 1983, 87, 1648; ( 6 ) M. D. Sevilla, D. Becker, C . L. Sevilla and S. Swartz, J. Phys. Chem., 1984, 88, 1701. G. W. Eastland, D. N. R. Rao, J. Rideout, M. C. R. Symons and A. Hasegawa, J. Chem. Res. ( S ) , 1983, 258. 12 14 l 5 M. C . R. Symons and B. W. Wren, Tetrahedron Lett., 1983, 24, 2315. l 6 L. D. Snow, J.T. Wang and F. Williams, Chem. Phys. Lett., 1983, 100, 193. I n M. Tabata and A. Lund, 2. Naturforsch., Teil A , 1983, 38, 687. l 9 ( a ) G. A. Russell, in Radical Ions, ed. E. T. Kaiser and L. Kevan (Wiley-Interscience, New York, 1968), p. 87; ( b ) F. W. King, Chem. Rev., 1976, 76, 157. 2o ( a ) Y. Ellinger, A. Rassat, R. Subra and G. Berthier, J. Am. Chem. SOC., 1973,95,2373; J. Chem. Phys., 1975, 62, 1; (b) Y. Ellinger, R. Subra, B. Levy, P. Millie and G. Berthier, J. Chem. Phys., 1975, 62, 10. K. U. Ingold and J. C. Walton, J. Am. Chem. SOC., 1982, 104, 616. Hoffmann, Acc. Chem. Res., 1971, 4, 1. (a) K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki and S. Iwata, Handbook of He ZPhotoelectron Spectra of Fundamental Organic Molecules (Japan Scientific Societies Press, Tokyo, 198 l), pp. 144-146; ( b ) K. Watanabe, T. Nakayama and J. Mottl, J. Quant. Spectrosc. Radiat. Transfer, 1962, 2, 369. 25 ( a ) F. Williams and E. D. Sprague, Acc. Chem. Res., 1982, 15, 408; ( b ) E. D. Sprague and F. Williams, J. Chem. Phys., 1971, 54, 5425. 26 c f , P. S. Skell and J. C. Day, Acc. Chem. Res., 1978, 11, 381. 27 W. J. Bouma, J. K. MacLeod and L. Radom, J. Am. Chem. SOC., 1979, 101, 5540; see also W. J. Bouma, D. Poppinger, S. Saebo, J. K. MacLeod and L. Radom, Chem. Phys. Left., 1984,104, 198. T. Clark, J. Chem. SOC., Chem. Commun., 1984, 666. R. W. Fessenden and R. H. Schuler, J. Chem. Phys., 1963,39, 2147. T. Bally, S. Nitsche and E. Haselbach, Helu. Chim. Acta, 1984, 67, 86. 17 22 ( a ) R. Hoffmann, A. Imamura, and W.J. Hehre, J. Am. Chem. SOC., 1968, 90, 1499; ( b ) R. 23 M. N. Paddon-Row, Acc. Chem. Res., 1982, 15, 245. 24 28 29 30 B. Kovac and L. Klasinc, Croat. Chem. Acta, 1978, 51, 5 5 . 3' R. J. Abraham, D. J. Chadwick and F. Sancassan, Tetrahedron, 1981, 37, 1081.

ISSN:0301-7249    

DOI:10.1039/DC9847800057

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

Faraday Discuss. Chem. Soc., 1984, 78, 71-81 An Electron Spin Resonance Investigation of Ester Cation Radicals at Low Temperatures Proton Transfer and Rearrangement Reactions B Y MICHAEL D. SEVILLA,* DAVID BECKER, CYNTHIA L. SEVILLA, KEVIN PLANTE AND STEVEN SWARTS Department of Chemistry, Oakland University, Rochester, Michigan 48063, U.S.A. Received 30th April, 1984 The cation radicals of a series of esters have been produced by y-irradiation of CFCI3 matrices containing the ester at 77 K. In previous work cations of methyl and ethyl esters were investigated. In this work we report results for larger esters. The cations of these esters are found to undergo immediate internal proton-transfer reactions involving specific sites on one of the alkyl substituents. For example, deuteration studies show that proton transfer occurs from the terminal methyl group in propyl formate to produce -OCH,CH,CH;, whereas in propyl acetate -OCH2CHCH3 is produced.The proton lost from these groups is assumed to add to an oxygen on the ester functional group. In the case of propyl acetate and esters with branched side chains we find that fragmentation reactions follow the proton transfer. In the cases of t-butyl acetate and isobutyl formate the fragmentation process occurs at 77 K and results in the isobutylene cation. Neopentyl formate gives evidence for the cation radical, an RCH; radical and the fragmentation radical cation as the sample is annealed. These results show that ester cation radicals are highly reactive even at low temperatures where proton-transfer and fragmentation reactions are found.Since the recent development of techniques to produce isolated cation radicals of small organic molecules in y-irradiated freon matrices by resonant charge transfer at low temperatures, considerable interest has developed in these species.'-' It is now becoming evident that certain of these cation radicals possess unique properties which are not observed in neutral free radicals or anion radicals. One of these features is the propensity of the cation radical to form electron-deficient inter- molecular bonds which could not be formed in the parent molecule.879 An example of this type of bonding involving a solute and solvent is the unexpectedly strong electron-deficient bond to the CFC13 matrix formed by methyl formate radical cation and characterized in our work.' Formation of complexes of this type depends on two principal factors.' First Id the difference in ionization potential between solute and solvent should be small.Covalent bonding is favoured for a small change in ionization potential whereas charge transfer occurs for a large change. The second factor which aids complex formation is a localized spin-density distribution in the solute cation.' ' Both theoreti- cal and experimental studies by other workers gives added support for these factors.10-'2 In recent work we have investigated small methyl and ethyl ester cation radicals in CFC13.9 In this paper we report results for esters of higher molecular weight in the same matrix. We find that cations of these larger esters undergo intramolecular proton-transfer reactions and fragmentation reactions often immediately at 77 K.7172 REACTIONS OF ESTER CATION RADICALS Table 1. Hyperfine couplings for ester proton transfer and fragmentation radicals hyperfine parent ester mechanismn radical splittings/G’ T/K‘,d n-propyl formate 1, 1 -C2H2lpropy1 r2H,]formate n-propyl acetate 1 9 1 -E2H21ProPYl acetate n-propyl propionate 1 7 1 -[2H21ProPYl propionate isopropyl formate t-butyl acetate isobutyl acetate isobutyl formate neopentyl formate [2H3]methyl isobutyrate [2H,]methyl-2,2- dimethyl propanoate PT PT PT Frg PT PT A PT PT Frg Frg Frg Frg PT Rot HS or A PT H(CO;H)CH~CH~CH; D(COTH)CH2CH2CH; CH3(COlH)CH2CHCH3 CH,CHCH; CH3(COzH)CD2CHCH3 C2H=,(C0iH)CH2CHCH, CH3CHC02C3H7 C2H5( CO; H)CD2CHCH3 H(COTH)C(CH3), (CH3)2C=CH;+ (CH3)2C=CH;+ HC02CH,C( CH3);+ (CH3)2C=CHf CD3( HOTC)CHCH,CH; CD3( H01C)CHCH;CH2 C D3 ( HO l C ) C (CH3)Z CD3(HOTC)C(CH3)2CH; (CH,),C=CH;+ RCH; 21(2aH), 43(1PH), 120 21(2aH), 43(lPH), 120 23( 1 aH), 26(3PH), 77 14.7 (4aH),4(1aH) 77 23(laH), 26.5(3PH) 77 W P H ) 8(lPH) 10(lPH), 5UPW 23( 1 aH), 27(3PH), 77 23(laH), 24(3PH) 160 23(laH), 27(3PH) 77 10(1PH), 4(lPH) 2 1.2(6P H) 14.5 (6/3H, 2aH) 14.8 (6PH, 2aH) 15.8 (6PH, 2 a H ) 33 (3H) 23 ( 2 a H ) 14.8 (6PH, 2H) 23 (2aH) 77 77 77 128 77 100 150 77 23 (2aH, 1PH) 120 21.7 (6PH) 130 23 (2aH) 77 PT (proton transfer), Frg (fragmentation), HS (hydrogen shift), A (abstraction).The spectra arising from a -proton couplings show well resolved anisotropic structure often atypical of normal tensors.We report only an estimate of the isotropic part of the coupling. Tem- peratures reported are those at which the radical first appear. The g values of these radical species were in the range 2.0025-2.0035. These results combined with our previous results on the reactions of ester anionsI3 help to elucidate the effect of radiation on this class of molecules. EXPERIMENTAL Samples were prepared from commercially available compounds and checked for purity by gas-liquid chromatography. Esters deuterated at various positions were found to be essential to the interpretation of e.s.r. spectra of the ester radicals and were prepared in our laboratory using standard preparative methods. Gas-liquid chromatography was used for separation and purification of the deuterated esters and n.m.r.spectra were employed to confirm deuterium substitution when any doubt was present. Samples of esters in CFCl, were irradiated in Spectrosil quartz tubes at 77 K for doses of 0.2 Mrad. A Varian Century e.s.r. spectrometer with an E-4531 dual cavity was employed. Hyperfine splittings and g values were measured with respect to Fremy’s salt with A, = 13.09 G and g = 2.0056.M . D. SEVILLA, D. BECKER, C. L. SEVILLA, K. PLANTE AND S. SWARTS 73 RESULTS AND DISCUSSION The results of the y-irradiation of dilute (0.2-1 mol "/o) samples of a series of esters in CFC1, at 77 K are summarized in table 1. Below are given details of these results for the individual esters. PROPYL ESTERS The ester cations of isopropyl formate, n-propyl formate, n-propyl acetate and n-propyl propionate usually show evidence for deprotonation or proton transfer immediately at 77 K. Work at very low concentrations (0.1 mol %) gave results similar to those at higher concentrations (1 mol "/o).This is evidence either for internal proton transfer or for proton transfer to the matrix, not only transfer to another solute molecule. Results which will be presented and discussed below suggest that the transfer is intramolecular in nature. Thus the radicals are believed to remain cationic in nature. The spectrum for isopropyl formate at 77 K shows a well resolved septet from 6 equivalent protons. In this case proton transfer occurs from the secondary carbon. (See table 1 for the structure and splitting.) The e.s.r. spectra of propyl formate (1 mol "/o) in CFC13 at 77 K are poorly resolved but improve in resolution on warming.The spectrum at 145 K is shown in fig. l(A). Fig. 1(B) shows the spectrum of the deuterated analogue DC02CD2CH2CH3 at 155 K; this spectrum is virtually identical to that in fig. l(A). Fig. 1(C), the spectrum of the deuterated analogue at low temperature (128 K), is presented as it clearly shows the anisotropic structure expected of a -CH; group. The spectra in fig. 1 and couplings (table 1) are believed consistent only with radicals (IA) and (IB): The spectra for propyl acetate and the deuterated analogue (CH3C02CD2CH2CH3) are presented in fig. 2(A) and (B). In this case deuteration results in the loss of two proton couplings (table 1) and causes a marked effect on the spectrum.It is clear that in this case proton transfer has occurred from the secondary carbon to give radical (11) rather than from the terminal methyl group, as occurred in propyl formate. The proton couplings lost on deuterium substitution are unusually small for @-protons. We believe this may be an effect of the charge on an ester oxygen. Warming samples to 138 K results in the quintet spectrum shown in fig. 2(C). This spectrum arises from four protons at 14.7 G and one proton at 4 G and is characteristic of the neutral ally1 radical ( III).14715 This radical probably results from fragmentation of the proton-transfer species (11) as indicated in reaction (1). Proton transfer to the ether oxygen is also possible. Note that annealing samples containing the deuterated compound did not result in any further reaction.This is an example of the McLafferty rearrangement often reported in mass spectroscopy as a mechanism for the gas-phase fragmentation of molecular cations.74 REACTIONS OF ESTER CATION RADICALS I 145K I 155K Fig. 1. First-derivative e.s.r. spectra found after y-irradiation at 77 K of dilute CFC13 solutions of (A) n-propyl formate and (B) 1,l -[*H,]propyl [2H2]formate and annealing to temperatures shown in the figure. The spectra are assigned to radicals (IA) (A) and (IB) (B). (C) shows the deuterated compound at a lower temperature where the anisotropy in the CH; group is apparent. The three reference marks are separated by 13.09 G. The centre mark is at g = 2.0056. The magnetic field increases from left to right.Results found (table 1) for propyl propionate and its deuterated analogue (CH3CH2CO2CD2CH2CH3) give evidence for a proton-transfer reaction identical to that found for propyl acetate. However, in this case annealing samples to 150 K resulted in the fatty acid a-carbon radical and not the ally1 radical. ISOBUTYL ACETATE, ISOBUTYL FORMATE, f-BUTYL ACETATE At 77 K the spectra for the isobutyl acetate and t-butyl acetate samples are essentially identical. The spectra show a nine-line spectrum due to eight nearly equivalent protons with couplings of 15 G. The spectra are shown in fig. 3(A) and (B) at higher temperatures where resolution was improved. The spectrum of theM . D. SEVILLA, D. BECKER, C. L. SEVILLA, K. PLANTE A N D S. SWARTS 75 V J Fig. 2.(A) E.s.r. spectrum found after y-irradiation at 77 K of a 0.5 mol YO CFC13 solution of n-propyl acetate. This spectrum is assigned to radical (11). (B) Spectrum found for 2% 1,l-[*H,]propyl acetate in CFC1,. Notice the loss of two proton couplings as expected for the deuterated analogue of radical (11). Spectra with narrower lines and less apparent anisotropy were found at lower concentrations. (C) Spectrum of the ally1 radical found after annealing the sample in (A) to 138 K. isobutyl formate sample [fig. 3(C)] at low temperature is the same as that of isobutyl acetate and t-butyl acetate, except that additional superhyperfine couplings are present. Such couplings have been observed for the acetaldehyde cation in CFCl3,I6 and are good evidence for weak intermolecular bonding to the matrix.On warming to 138 K, however, this additional structure disappears and a nine-line spectrum with a separation of 15.8 G is found [fig. 3(D)]. These four spectra are probably from the same radical, and for the reasons outlined below we assign it to the isobutylene cation (IV) formed from the original ester by a fragmentation pr0ce~s.l~ The reaction for t-butyl acetate proceeds, starting with the radical cation formed by resonant energy hole transfer, as shown by dH I H\ + ,CHB H H L ~ A C (2) 1 1 CH3 H, + H,c'c ~c,C.o/C( H' I CH3 " p o H 'C Hg (Iv) H76 REACTIONS OF ESTER CATION RADICALS n Fig. 3. E.s.r. spectra found after y-irradiation at 77 K of dilute CFCI, solutions of (A) isobutyl acetate, (B) t-butyl acetate and (C) and (D) isobutyl formate.All spectra are attributed to the isobutylene cation radical (11). The fine structure in (C) is irreversibly lost on warming and may be a matrix superhyperfine interaction. For isobutyl formate and isobutyl acetate the reaction must proceed by transfer of the proton at the tertiary carbon instead of a methyl-group proton. This assignment of-the spectra in fig. 3 to the isobutylene cation is confirmed by comparison with the results of Iwasaki et ai.; who prepared this radical in an unequivocal fashion from isobutylene; they also observed the radical as a result of the fragmentation of n e ~ p e n t a n e . ~ ’ ~ This previous work reports a 15 G average splitting for the eight protons, in agreement with our observations. NEOPENTYL FORMATE The results of y-irradiation of neopentyl formate (0.1 mol O h ) in CFC13 afford a detailed description of the process by which the cation radical fragments.The results in fig. 4 give evidence for the original cation, an intermediate and one of the final fragmentation products. The spectrum in fig. 4(A) shows three equivalent protons with 33 G coupling and is assigned to the neopentyl formate radical cation (V). We view this cation as one in which the transferred hole is localized largely on the neopentyl group. The nature of the hyperfine coupling, in which only three of the possible eleven protons exhibit any observable coupling, is explained by direct analogy with the a-radical cation of neopentane described by Iwasaki et aL4 These workers formed the neopentyl radical cation in an unequivocal fashion from neopentane, and found that only three of the twelve protons on the radical displayed observable hyperfine interactions.As Iwasaki et al. note, this coupling pattern is consistent with a a-radical in a specific conf~rmation.~-~ The neopentyl cation radical had three protons with 41 G coupling, while our neopentyl formate cation radical has three protons with 33 G coupling. This difference may be due to theM . D. SEVILLA, D. BECKER, C . L. SEVILLA, K. PLANTE AND S. SWARTS 77 V Fig. 4. E.s.r. spectra found after y-irradiation of dilute CFC1, solutions of neopentyl formate and annealing to various temperatures shown in the figure. (A) The neopentyl formate cation radical (V). (B) Radical (VI). (C) The isobutylene cation radical (IV).fact that in the neopentyl formate cation radical some of the spin density is localized in the -C02- group. Warming of the cation radical of neopentyl formate to 100K results in the spectrum shown in fig. 4( B). This spectrum shows the anisotropic structure expected for a RCH; radical, in which the CH; is not rotating. The two protons have a 23 G isotropic or average coupling. In addition there is further structure which suggests a small coupling. This RCH; radical (VI) may result from either intramolecular proton transfer from a methyl group to an ester oxygen or C-C bond cleavage to produce HC0,CH;. Experiments are underway to make this distinction. The fine structure in fig. 4( B) suggests further coupling; however, anisotropic computer simulations’* employing the expected anisotropy for the CH2 proton couplings (33.5, 23 and 9.8 G), appropriate g tensor and direction cosines reported for a similar radical” as well as 5 G nuclear Zeeman terms reproduce the hyperfine structure in the wings quite well [fig.5( A)]. The central components can be simulated by assuming an inequivalence in the two proton-coupling tensors. The simulation in fig. 5(B) assumes a 5 G difference in the intermediate 23 G couplings (21 and 26 G). This latter simulation is in excellent agreement with experiment. However, we believe that deuteration experiments are necessary to conclude definitively whether or not an additional small coupling is actually present. Further warming of the neopentyl formate proton-transfer cation results in the spectrum in fig.4(C). The spectrum is from eight protons at 15 G and is that of the78 REACTIONS OF ESTER CATION RADICALS Fig. 5. Anisotropic computer simulations of the e.s.r. spectrum arising from a CH; radical employing parameters described more fully in the text. (A) This simulation assumes equivalent proton hyperfine coupling tensors (33.5, 23 and 9.8 G). (B) This simulation assumes a 5 G difference in the intermediate value of the hyperfine coupling tensor, i.e. 21 and 26 G. Compare the simulation in (B) with the experimental spectrum in fig. 4 (B). The marks are separated by 5 G. isobutylene cation (IV). This final radical likely arises from the fragmentation reaction shown below: This overall reaction has been proposed from mass-spectroscopic studies of neopentyl esters.I7 Deuterium-substitution experiments in these previous studies showed the methyl group in the product methyl acetate originated from the methylene group, and thus methyl-group transfer was not involved.'" METHYL ISOBUTYRATE At 77 K the spectrum found for samples of [*H,]methyl isobutyrate [(CH3)2CHC02CD3] shows the anisotropic structure and couplings typical of aM .D. SEVILLA, D. BECKER, C. L. SEVILLA, K. PLANTE AND S. SWARTS 79 Fig. 6. E.s.r spectra found after y-irradiation of CFC13 solutions of ['HJmethyl isobutyrate at 77 K and annealing to various temperatures shown in the figure: (A) 2% solution and (B) 0.2% solution. The concentration has a marked effect on the linewidth but had no affect on the subsequent chemistry.The spectra in (A) and (B) are assigned to (VIIA). (C) Spectrum assigned to (VIIB) and (VIII). (D) Spectrum predominantly from the final radical (VIII). RCH; radical [fig. 6(A) and (B) and table I]. The spectra found for samples of high solute concentration (2 mol O h ) , shown in fig. 6(A), are considerably broadened relative to those at lower concentrations [fig. 6(B)]; however, we found that the concentration did not effect the subsequent chemistry. Upon warming to 100 K, a 1 : 3: 3 : 1 quartet with 23 G splitting appeared [fig. 6(C)], which at 160 K was converted into the pattern shown in fig. 6(D). This septet arises from six equivalent protons with a coupling of 2 1.7 G. The mechanism which explains these spectral changes is given below: (VIIA) (VIIB) (VIII) At 77 K the CH; group in (VIIA) is considered to be held, so that the single @-proton is in the nodal plane of the p-orbital containing the unpaired electron and hence little coupling to the @-proton is observed.At 100 K we suggest that a rotation of the CH2 group occurs so that the proton coupling becomes near that of the protons in the CH; group resulting in the quartet which we assign to (VIIB). Continued warming to 130 K probably results in a hydrogen-atom transfer to yield the stable tertiary radical (VIII). The protonated compound gave similar results except that the twisted radical (VIIB) was not observed.80 REACTIONS OF ESTER CATION RADICALS [2H3]METHYL-2,2-DIMETHYLPROPANOATE (METHYL TRIMETHYL ACETATE) At 77 K a spectrum consistent with an RCH; radical (a triplet with anisotropic structure) is found (table 1).This indicates that intramolecular proton transfer to the carbonyl oxygen has probably occurred. No further reaction was noted, although the anisotropic structure was reversibly lost on warming to 130 K, resulting in an isotropic 1 : 2 : 1 pattern with 23 G spacing. CONCLUSIONS The most common reaction of the ester radical cations observed in this work is the proton-transfer reaction. This reaction occurs with very low concentrations of ester, indicating the transfer is not to another ester molecule and is consequently either intramolecular or to the freon matrix, CFC13. Proton transfer to the matrix has been suggested by Iwasaki et al. for alkane cations in other solvents; however, in CFC13 they observe CH4 or H2 elimination rather than proton t r a n ~ f e r .~ These results combined with our concentration study strongly suggest that the proton transfer we observe is internal. The proton may transfer to either the carbonyl or ether oxygen. Other workers, for example, have proposed proton transfer to the oxygen in n-propyl ethers7 Proton-transfer reactions involving both oxygens have been suggested to explain fragmentation patterns found in mass-spectrometric work on e s t e r ~ . ' ~ We feel, as is commonly proposed in mass-spectrometric results, that ring formation may greatly favour one site over another. These proton-transfer radicals are unstable intermedi- ates. For certain esters the proton-transfer step is the first step in the well known McLaff erty rearrangement reaction proposed from mass-spectroscopic experiments on aldehydes, ketones and Our results for propyl acetate, in which the intermediate was observed, indicate that the McLaff erty rearrangement for the cation of this compound is at least not a concerted process and proceeds in discrete steps.The fact that we observe the McLafferty rearrangement at 77 K or a slightly higher temperature is evidence that the bond-cleavage and reformation processes involved have little activation energy. Normally such unimolecular bond-cleavage reactions would require much higher temperatures in order to proceed. The fact that these reactions take place at such low temperatures implies that internal bonding in these cations may provide a potential-energy surface to reformation of the molecular structure which lacks a significant barrier.Iwasaki et al. have observed loss of CH, and H2 from alkane cations at low temperatures as well.4 Both the alkanes and esters have relatively high ionization potentials. We believe that the ionization potential of the parent compound provides a measure of the driving force for the reformation in the cation radicals of these compounds. We thank the Office of Health and Environmental Research of the U.S. Depart- ment of Energy and the U.S. Department of Agriculture for support of this research. Note added in prooJ: Very recently Iwasaki et reported work in CFCl3 on two of the 13 molecular structures investigated in this paper, i.e. isopropyl formate and t-butyl acetate. The authors conclude that the McLafferty rearrangement occurs in the cations of these structures as we have.Remarkably they find that t-butyl acetate cation rearranges to formM . D. SEVILLA, D. BECKER, C. L. SEVILLA, K. PLANTE A N D S. SWARTS 81 the isobutene cation even at 4 K . Although these results agree with ours, Iwasaki et al. disagree with our earlier identification of methyl and ethyl formate cation radicals as the original oxygen-centred radical cations in CFC13 at 77 K.8,9 They report, correctly we now believe, that these radicals are cations in which internal hydrogen transfer has occurred. I J. T. Wang and F. Williams, J. Phys. Chem., 1980, 84, 3156. L. D. Snow, J. T. Wang and F. Williams, J. Am. Chem. SOC., 1982, 104, 2062. H. Kubodera, T. Shida and K. Shimokoshi, J. Phys. Chem., 1981, 85, 2583. M. Iwasaki, K. Toriyama and K. Nunome, Radial. Phys. Chem., 1983, 21, 147, ' K. Toriyama, K. Nunome and M. Iwasaki, J. Chem. Phys., 1982, 77, 5891. ' M. C. R. Symons, Annu. Rep. R. Soc. Chem., Sect. C , 1981, 78, 151. ' M. C. R. Symons and B. W. Wren, J. Chem. SOC., Perkin Trans. 2, 1984, 5 11. D. Becker, K. Plante and M. D. Sevilla, J. Phys. Chem., 1983, 87, 1648. M. D. Sevilla, D. Becker, C. L. Sevilla and S . Swarts, J. Phys. Chem., 1984, 1701. l o T. Clark, J. Comput. Chem., 1982, 3, 112. I ' M. Mautner, J. Phys. Chem., 1980, 84, 2724. l 2 G. W. Eastland, D. N. Ramakrishna Rao, J. Rideout, M. C. R. Symons and A. Hasegawa, J. Chem. l 3 M. D. Sevilla, K. M. Morehouse and S. Swarts, J. Phys. Chem., 1981, 85, 923. I s J. E. Wertz and J. R. Bolton, Electron Spin Resonance (McGraw-Hill, New York, 1972), p. 120. l 6 L. D. Snow and F. Williams, Chem. Phys. Lett., 1983, 100, 198. I' R. Lefebre and J. Maruani, J. Chem. Phys., 1965, 42, 1480. l9 W. A. Bernhard, T. L. Horning and K. R. Mercer, J. Phys. Chem., 1984, 88, 1317. Res. ( S ) , 1983, 258. M. D. Sevilla and R. A. Holroyd, J. Phys. Chem., 1970, 74, 2459. 14 F. W. McLafferty, Interpretation of Mass Spectra (W. A. Benjamin, New York, 1967), pp. 120-138. 17 W. H. McFadden, K. L. Stevens, S. Meyerson, G. J. Karabatsas and C. E. Orzech Jr, J. Phys. Chem., 1965, 69, 1742. 20 2' M. Iwasaki, H. Muto, K. Toriyama and K. Nunome, Chem. Phvs. Lett., 1984, 105, 587; 593.

ISSN:0301-7249    

DOI:10.1039/DC9847800071

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

GENERAL DISCUSSION Dr S . Lunell (Uniuersity of Uppsala, Sweden) said: I would like to make a comment in connection with Dr Iwasaki's results for the cyclohexane cation. We have recently completed a series of theoretical calculations on different states of the c-C,Ht2 cation, using the a6 initio MO-LCAO-UHF method.' Our results for the symmetric-chair conformation ( C 2 h symmetry) do confirm that the elongated- chair (2A,) is slightly more stable (ca. 1 kcal mol-') than the compressed-chair ('B,) conformation, as suggested by Dr Iwasaki and collaborators. However, a further unsymmetrical distortion to C, symmetry, according to fig. 1, produces a 2A" ground state with an energy ca. 7 kcal mol-' lower than both of the abovementioned states for C2h symmetry. For this state the calculations predict an e.s.r.spectrum consisting of three sets of triplets with coupling constants 60 G(2H), 20 G(2H) and 9.4 G(2H). Apart from a uniform scale factor of ca. 1.5, these values are in excellent agreement with the experimental values 85 G(2H), 34 G(2H) and 14 G(2H) reported by Iwasaki et al. This suggests another possible explanation of the low-temperature e.s.r. spectrum of c-C,HT2 without changing the numerical values of the coupling constants reported by these authors, namely that the ion has an unsymmetric-chair conformation with C, rather than C 2 h symmetry, and that the largest set of couplings should be assigned to H1 and H2, the next largest to H3 and H4, and the smallest set of resolved couplings to H9 and H l l , which, although not symmetry-related, are predicted to have almost identical coupling constants by the calculations.I wonder if Dr Iwasaki would like to comment on these results? ' S. Lunell, M. B. Huang, 0. Claesson and A. Lund, J. Chem. Phys., to be published. 9 10 Fig. 1. Unsymmetrical distorted chair with C, symmetry.84 C E NERAL DISCUSSION Dr M. Iwasaki (Nagoya, Japan) said: Dr Lunell has assumed the C, distortion for c-C,Hf, for energy optimization to obtain three sets of coupling constants. However, the e, ring-deformation mode, which leads to the C, distortion, is not Jahn-Teller active for the e, electronic orbital of c-C6HT2 with D3d symmetry. Thus, such a deformation cannot be assumed because there is no matrix element for vibronic interaction. In addition, the C,$ distortion cannot explain the observed temperature change of the three coupling constants reported in the first paper.Dr S. Lunell (University of Uppsala, Sweden) replied: It is correct that e, is not a Jahn-Teller-active mode for the e, orbitals in D3d symmetry. However, once the D3d symmetry has been broken by one of the Jahn-Teller-active modes, the Jahn- Teller theorem no longer applies, and in principle any type of further distortion is possible. Thus in this respect the distorted-chair conformation of C, symmetry is perfectly allowed. Therefore, if the temperature dependence uniquely confirms a ’A, assignment, this implies that the cyclohexane cation exhibits a change of ground state owing to matrix interactions in these matrices, in the same way as the propane cation does in e.g.CFC13 or CF3CC13, which is itself an interesting result. Dr M. Iwasaki (Nagoya, Japan) said: It is hard to believe that c-C6Ht2 with C2, symmetry distorts to the asymmetric C, form by intramolecular interactions. With the exception of c-C,HT2, all of the Jahn-Teller-active cations studied so far exhibit the static distortion expected from the Jahn-Teller-active vibrational mode [ref. (3) of our paper]. Furthermore, in order to explain the averaging of the two sets of coupling constants (85 and 34 G) by C, distortion one has to assume a jumping between the two equivalent distorted structures which interchanges (HI, H2) with (H3, H4) and (€-I9, H l l ) with (Hlo, HI’). The dynamic process involving such interchanges cannot explain the observed temperature change. Because of no resolvable extra coupling from Hlo and HI2, the interchange of (H9, HI1) with (Hlo, HI2) must result in a decrease of the coupling to (H9, H I 1 ) with an increase of the coupling to (Hlo, HI2).However, this is not observed. Furthermore, it seems difficult to explain the continuity of the changes in the three coupling constants by the model without a dynamic average. For such a dynamical process the potential barrier at the symmetrical structure must be very high and equal to the distortion energy of the C, form. On the other hand, the dynamic process in the Jahn-Teller potential minimum is a two-dimensional oscillation. The radial oscillation must have a very high potential maximum at the symmetrical position, whereas the tangential oscillation can have a low potential barrier resulting in the pseudo-rotation at quite low temperature. Prof.M. Sevilla (Oakland University, Michigan) said: The static Jahn-Teller distortions of the cycloalkane cations described in Dr Iwasaki’s work produce sites of high charge localization in certain instances. Could it be that some form of matrix interaction with these sites tends to stabilize one state over another? Dr M. Iwasaki (Nagoya, Japan) replied: It is hard to believe that matrix interaction with a specific site having a high charge density preferentially stabilizes one state over another regardless of matrix. In addition to the variation of the matrix molecule, the distribution and the relative orientations of the surrounding molecules must be different from matrix to matrix, so that it is difficult to adduce that one of the two states is stabilized by a specific matrix interaction.For this reason we have suggested that matrix interaction assists stabilization of a givenGENERAL D I SC U SS ION 85 ground state, which is determined by the intramolecular interaction and is intrinsic to radical cations. We may have to consider solvation of matrix molecules similar to that of a self-digging trapped electron. One might suppose that complexing with a matrix molecule at some specific site having high charge density may play a role in stabilizing one state over another. However, the unpaired electron of radical cations of alkanes delocalizes over the C-C and C-H bonding orbitals. Even when the unpaired electron is more confined to one of the C-C bonds, it is a a-bonding orbital.Thus the ability for complex formation with a matrix is thought to be lower than for the T- and n-cation radicals, in which the unpaired electron orbital is directed outside the molecular frame of the cations. The complexing of cations with the matrix may be of less importance in alkane radical cations. Prof. L. Kevan (University of Houston, Texas) asked: Is there much predictive understanding of the differing dynamic behaviours between matrices? Dr Iwasaki suggests that the activation energies may be related to matrix polarity, but the low value for CFC1, does not seem to fit in with this. What about the role of asymmetry of the matrix molecule or of the matrix sites? Dr M. Iwasaki (Nagoya, Japan) replied: The temperature of the onset of dynamic averaging is generally lower in SF, in comparison with that in other halogenocarbon matrices, suggesting that the activation energy is lower in non-polar solvents.However, since the dipole moments of C-Cl and C-F bonds are not very different, halogenocarbons such as CFCI, and CFC12CF2C1 also have relatively small dipole moments. For example, the dipole moment of CFC1, is only 0.45 D. The total matrix polarity may not be a decisive factor, but a difference between the bond polarizabilities of C-F and C-C1 may be much more important. Since maxima and minima in the Jahn-Teller potential trough are determined by the second-order effect such as anharmonicity and a quadratic term, the contact of the radical cations with a cavity wall may be another important factor in determining the potential barrier to pseudorotation.Thus tight or loose contact with the surrounding matrix molecule may be related to the difference in the activation energy. In this sense the size of the matrix cavity may be an important factor. In general, the Jahn-Teller radical cations exhibit more rigid structure immediately after irradiation at 4 K. However, once the sample is annealed at 77 K, they exhibit a larger zero-point motional effect, suggesting that irreversible matrix relaxations give more space to cations. The molecular symmetry may play a role through interactions at the cavity wall of the substitutional site with a solute molecule. However, not only the spherical SF, molecule but also asymmetrical CFCI2CF2Cl give generally low activation energies. CFC13 and CF3CC13 form rather rigid matrices in which the onset tempcrature of the dynamics is usually higher than in SF, and We have also compared glassy and crystalline media.However, no decisive difference has been found. At present, the details of the matrix interaction are not known. However, what is most striking is that the sign of the distortion coordinate is independent of matrix. Recently, we have examined the effect of environment on benzene cations adsorbed on silica gels and on a variety of synthetic zeolites. Although the activation energy of the dynamic motion is lower in these environments than that in halogenocarbon matrices, the sign of the distortion coordinate is independent of environment, i.e.the compressed quinoidal structure is always the ground state. C FC12C F2Cl.86 GENERAL DISCUSSION Dr N. Klassen (N.R. C., Ottawa, Canada) said: In several hydrocarbon liquids, including cyclohexane, the cation is found to possess an unusually high mobility. Does Dr Iwasaki have any evidence that the initial cation of cyclohexane is different from other alkane cations, for example, is it more stable? Dr M. Iwasaki (Nagoya, Japan) replied: The cyclohexane radical cation does not seem to be especially stable'in halogenocarbon matrices under our experimental conditions. It is converted into the cyclohexyl radical in CFC12CF2CI. Experiments on the addition of electron scavengers to alkanes indicate that the alkyl radicals must be formed from primary cationic species.' In addition, pure n-alkanes (n-C,oH22-n-C19H,0) irradiated at 4 K indicate that the chain-end alkyl radicals are selectively formed regardless of the chain length.2 These results suggest that the primary event in alkyl-radical formation from the cationic species takes place at the chain end, and this is just what is expected from the unpaired electron distribution in the linear alkane radical cations with an extended structure in the crystal [ref.( 3 ) and (4) in our paper]. Taking hydrogen-atom formation in irradiated alkanes into c~nsideration,'~~ it is suggested that the unrelaxed primary radical cation decomposes into R+ and H: (RH+)* -+ RT+H (1) Rr+e- + RI (2) (3) In reactions (1)-(3) R, is the chain end, RII is the penaltimate alkyl radical and R,,, the interior alkyl radical.Another reaction path which has to be considered is the ion-molecule reactions of unrelaxed alkane radical cations, followed by a charge recombination to form H:3 followed by the neutralization reaction to form a chain-end alkyl radical: and by the hydrogen-abstraction reaction to form various types of alkyl radicals: H+RH --* R,, RI,, R,,,. (RH+)*+RH -+ R,+RH; (4) RH;+e- -+ RH+H ( 5 ) ( 6 ) H+ RH -+ R,, RII, R,,,. The suppression of radical-pair formation at 4 K by the addition'of a small amount of electron scavenger' indicates that R+ and RH; are mobile since hydrogen abstraction by the inmobilized hydrogen atom at 4 K would form radical pairs if R+ and RH; were not mobile before the neutralization reactions (2) and ( 5 ) .So, there are three candidates for the mobile positive charge in irradiated cyclohexane, namely unrelaxed RH+, R+ and RH;. K. Toriyama, H. Muto, K. Nunome, M. Fukaya and M. Iwasaki, Radiat. Phys. Chem., 198 1,18,104 1. ' M. Iwasaki, H. Muto, M. Fukaya, K. Toriyama and K. Nunome, 26th Symp. Radiat. Chem. (Osaka, Sept. 28, 1983, Abstract B207), p. 118. M. Iwasaki, K. Toriyama, H. Muto, K. Nunome and M. Fukaya, Radiat. Phys. Chem., 1981,17,304. Dr N. Klassen (N.R.C., Ottawa, Canada) said: A reaction of alkane cations which seems more and more likely to me is proton transfer to an alkane molecule: RH++RH + R*+RHl. Although reactions such as this do not seem to take place in the gas phase, published proton affinities suggest that many such reactions are exothermic.GENERAL DISCUSSION 87 Dr M.Iwasaki (Nagoya, Japan) replied: The fate of a detached proton cannot be detected because it does not form a paramagnetic species. In any case, the deprotonation reaction is bimolecular because of the solute concentration depen- dence. There are three possible mechanisms which can explain the concomitant decrease in cation radicals with an increase in alkyl radicals. The first is the ion-molecule reaction: RH+i + RH --+ Re? + RH;. (1) Although gas-phase energetics are unfavourable for this mechanism, in the solid phase there is a possibility that the potential-energy surface is different from that in the gas phase, so that this mechanism cannot be excluded. It is consistent with selective deprotonations at the C-H bond having the highest spin density.The second is the charge-neutralization reaction: RH+i+X- ---* R"/'+HX. (2) This mechanism can also explain selective deprotonation, because X- will attack the C-H bond with high positive-charge density. The third mechanism is positive-hole migration from the cation to X- forming X atoms followed by hydrogen abstraction by the X atom from RH. However, this mechanism is less probable because selective deprotonation cannot be explained. Now, the reactions of alkane radical cations are dependent upon matrix, and the deprotonation reaction is typical of SF6 and CFCl2CF2C1. Shida's group have reported that cycloalkane radical cations undergo deprotonation in CFC1, when the solute concentration is as high as 10% [ref. (15) in our paper].However, unimolecular detachment of H2 or CH4 is more common in CFC1, at low solute concentrations. Prof. M. C. R. Symons ( University of Leicester) (communicated) In our original study of the (Me,C-CMe,)+ cation we postulated that the SOMO was considerably confined to the central C-C u bond, which was therefore stretched and weakened, with some degree of flattening at the two Me3C- groups.' This has been confirmed by the extensive studies of Iwasaki and coworkers.2 For example, for the series of cations (Me$ H)+, (Me,C CH,)+ and (Me,C CMe3)+ the representation given appears to be reasonable. The cation (Me,C * H)+ has a 'H coupling of 251 G, showing the very large spin density in the unique C-H orbital. Probably the extent of flattening of the Me3C- units decreases in this series for steric reasons. ' I.G. Smith and M. C. R. Symons, J. Chem. Res. ( S ) , 1979, 382. * K. Nunome, K. Toriyama and M. Iwasaki, J. Chem. Phys., 1983,79, 2499. Dr M. Iwasaki and Dr K. Toriyama (Nagoya, Japan) (communicated) We totally agree with Prof. Symons' comment that the structure of branched-alkane radical cations are characterized in line with HME'.' Note that C2H6+ and other linear- alkane radical cations, on the other hand, are not characterized as an extension of HME+ as we have discussed in our earlier work.* In these cations the unpaired electron is rather delocalized over the in-plane u molecular orbital. ' M. C. R. Symons and I. G. Smith, J. Chem. Rex ( S ) , 1979, 382. ' K. Toriyama, K. Nunome and M. Iwasaki, J. Chem. Phys., 1982, 77, 5891.Dr M. Iwasaki and Dr K. Toriyama (Nagoya, Japan) (communicated) In his presentation at the Discussion, Dr Lunell pointed out the following two problems in relation to our paper at this Discussion and to ref. (3) therein: (1) the disagreement of the principal directions of the &proton coupling tensor calculated by their ab88 GENERAL DISCUSSION initio spin density for c-C3H; with the values used in our spectral simulation and (2) the possibility of 2A, propane cation radicals. ( 1 ) The ab initio directions seem to be incorrect because the two p protons possess essentially coaxial tensors which cannot be expected from the geometry. The directions of the intermediate and minimum elements are quite sensitive to the spin distribution, especially on the C( 1 ) atom.In our preliminary communication [ref. (7) of our paper at this Discussion] a small spin density on the C ( l ) atom is taken into consideration, whereas it was assumed to be zero in the present paper. The difference arises only in the extent of the deviation from the axial symmetry of the P-proton couplir,g tensor. In principle it is difficult to estimate a small deviation from axial symmetry with high reliability either experimentally or by theoretical calculation. Thus the comparison has little meaning. (2) We have, of course, considered the possibility of 2AI. However, we thought that it may give a smaller set of coupling constants as compared with a large experimental value because of a smaller overlap of H I , and C,, orbitals.Indeed, calculated values presented in Dr Lunell’s paper support our prediction, although their ab inftio values for both ,B2 and *A1 do not reproduce the experimental ratio of 2: 1 for the central and non-central CH,-proton couplings. Our INDO values gave better agreement in this respect [ref. (3) of our paper]. According to our comparisons over more than 35 alkane radical cations, the INDO values usually show reasonable agreement with experimental values. We stress that our experi- mental data made such an assessment possible. Of course, our data must be a guide for the refinement of more sophisticated ab initio molecular-orbital theory of (T- electron systems. Dr S . Lunell ( University of Uppsala, Sweden) (communicated) (1) The disagreement that Dr Iwasaki refers to is with the principal directions of the P-proton coupling tensor given in his preliminary communication [ref.(7) of his paper], which indeed differ significantly from ours. Dr Iwasaki’s revised results, published in these proceedings, which were not available to us when our paper was written, are in contrast rather close to our ab initio results, so that I do not think that any real controversy remains on this point. The puzzling coaxiality of the ab initio directions for the two P-proton coupling tensors might well be an artefact because of the very small magnitude of the coupling constants, which makes them sensitive even to small inaccuracies in the wavefunction. (2) I quite agree that experimental data should always constitute a guide for the development and refinement of theoretical methods.However, this applies to the primary data and not necessarily to their interpretation, which often relies on theoretical models or methods of much lower reliability than the experimental work. I should perhaps also add, although this is well known, that good agreement with experiment in itself does not guarantee the quality of a parametrized theory, since deficiencies and approximations in the theory may be masked by a suitable choice of parameter values. This applies to INDO as well as to any other semiempirical theory . Dr M. Iwasaki and Dr K. Toriyama (Nagoya, Japan) (communicated) We address Dr Lunell: In your answer, you still persist in comparing your ab initio directions with ours presented in our preliminary and Faraday papers, even though you admit the peculiarity of the ab initio directions.It is hard to believe that your ab initio results have sufficient reliability to judge which is closer to the true one, becauseGENERAL DISCUSSION 89 your ab initio directions show unrealistic coaxiality for the two P-proton coupling tensors. As is stated in our paper, further refinement is needed for our spectral simulations to reproduce the fine details of the substructures. Before we reproduce the observed substructures, we cannot decide which direction is closer to the true one. In the present stage, the comparison of the ab initio directions with our tentative ones is meaningless. Prof. F. Williams (Knoxville, Tennessee) said: In addition to the ring-closed cyclopropane radical cation described in the papers by Iwasaki et al.and Lunell et al., we have recently obtained experimental evidence for the ring-opened 'CH2CH2CH2+ species.' This isomer is formed from the cyclopropane radical cation in a thermal reaction which occurs at 80-84 K in the CFCl2CF2C1 matrix. The e.s.r. spectrum is well resolved at 108 IS and the parameters [a(2H,) = 22.4 G, a(2Hp) = 30.2 G, g = 2.00281 can be assigned to the trimethylene radical cation in which the two terminal CH2 groups are perpendicular to each other. Essentially, the electron spin is confined to one end of the molecule, the radical centre adopting a bisected conformation very similar to that of the n-propyl radical. We have similarly observed that the ring-closed form of the 1 , 1,2,2-tetramethyl- cyclopropane radical cation undergoes an irreversible monorotatory ring opening above 110 K in CFCl2CF2C1 to give an orthogonal ring-opened structure.2 In this case, however, the radical centre adopts an eclipsed conformation as expected for an a,&-dimethyl-substituted alkyl r a d i ~ a l .~ As far as we are aware, these observations are novel insofar as both the ring-closed and ring-opened forms of a radical cation have been characterized by e.s.r. spectros- copy. Note that these ring-opened radical cations do not isomerize by 172-hydrogen shifts to give olefin cations under these conditions. This conclusion is reinforced by the fact that attempts to generate the propylene radical cation4 directly from the olefin led only to allyl radicals, presumably by proton transfer from the propylene cation, at 108 K in the CFC12CF2C1 matrix, these allyl radicals being absent in the corresponding cyclopropane experiments.Moreover, the e.s.r. spectrum and hyper- fine parameters previously assigned to the propylene radical cation4 are quite different from those reported here for the trimethylene radical cation.' ' X-Z. Qin and F. Williams, Chem. Phys. Lett., 1984, 112, 79. * X-Z. Qin, L. D. Snow and F. Williams, J. Am. Chem. SOC., 1984, 106, 7640. K. S. Chen, D. Y. H. Tang, L. K. Montgomery and J. K. Kochi, J. Am. Chem. SOC., 1974,96,2201. K. Toriyama, K. Nunome and M. Iwasaki, Chem. Phys. Lett., 1984, 107,86 and references therein. Dr M. Iwasaki and Dr K. Toriyama (Nagoya, Japan) said: We have observed exactly the same spectral change and have interpreted the results by two models, one is the asymmetrical ring-opened structure, which is similar to that Prof.Williams has suggested, and the other is the twisted propylene radical cation, which may be formed by ring opening followed by intramolecular hydrogen shift. However, it is difficult to decide which is the case, so we have not published the result nor described the situation in our paper. Ring opening followed by intramolecular hydrogen transfer is also observed for c-C,Hi [see our paper and ref. (24) therein]. In this case the spectral change can be interpreted unequivocally.90 GENERAL DISCUSSION Dr M. R. Wasielewski (A.N.L., IZZinois) said: In the case of the radical cation of propane Dr Lunell proposes that C-H bond stretching may be responsible for the observed hyperfine splittings.I thus have two questions. ( I ) Has anyone used site-selective I3C enrichment to measure whether the C(1) or C(2) carbon atom shows enhanced hyperfine splitting? (2) The theoretical values of the I3C hyperfine interaction should be obtainable by default from Dr Lunell's calculations. What values of the "C hyperfine interaction do his calculations predict? Dr S . Lunell ( University of Uppsala, Sweden) replied: Let me first make a small clarification of my remark about the C-H bond stretching vis-u-vis observed proton hyperfine splittings. The point I wanted to make was that both the hyperfine splittings and the C--H bond-length variations can be understood from the shape and nodal pattern of the singly occupied molecular orbital in the different states of the propane cation.The C--H bond stretching does also affect the proton hyperfine splitting directly, but this effect is sufficiently small that it can be overlooked in the present context. In answer to Dr Wasielewski's questions, I do not know of any e.s.r. work on C-enriched propane to date. The calculated 13C splittings (table 1 ), however, indicate that such an experiment would be very illuminating, since the Cjl) and C(2) splittings are sufficiently different in the different electronic states to provide an independent check on the assignments obtained by other methods. We will, indeed, look into the possibilities of performing such an experiment in the near future. 13 Table 1. Isotropic I3C hyperfine coupling constants (in G) for the different carbon atoms in C,H: 2 ~ i aUHF -23.2 13.2 HFA A -7.2 11.3 -3.5 -14.0 -0.9 -4.7 2Ai aUHF -20.9 8.9 aUH FAA - 14.2 7.7 Dr W.Siebrand (N.R.C., Ottawa, Canada) said: My comment concerns the paper of Dr Lunell and also that of Dr Iwasaki. Dr Lunell calculates that the lowest three states in the propane cation are close in energy and argues on the basis of observed e.s.r. spectra that their ordering may be solvent-dependent. In cases such as this it is not immediately obvious that the Born-Oppenheimer approximation applies. It is possible that these states are subject to strong non-adiabatic mixing. Thus although this cation is not subject to a Jahn-Teller effect, similar to that discussed by Dr Iwasaki, it may be subject to a pseudo-Jahn-Teller effect if the coupling between two electronic states is larger than their separation.In my opinion this aspect deserves further investigation.GENERAL DISCUSSION 91 The breakdown of the Born-Oppenheimer approximation is of course manifest in the Jahn-Teller-active cation radicals studied by Dr Iwasaki. However, I am puzzled by his remark that the observed e.s.r. spectrum of the cyclopropane radical cation cannot be explained in terms of a thermal distribution over different levels but requires a model in which the electron jumps between different minima. I see no contradiction between these two pictures. In the Jahn-Teller system the lower vibronic levels represent states that are partially delocalized, their degree of localiz- ation depending on the depth of the trigonal minima induced by quadratic electronic- vibrational couplings.In principle, the e.s.r. spectrum can at all temperatures be expressed in terms of these eigenstates. It should be realized, however, that these states are not electronic but vibronic, since the Born-Oppenheimer approximation does not apply here. Dr M. Iwasaki (Nagoya, Japan) replied: (1) In Jahn-Teller-active species, generally, the energy difference Vo of the two distorted structures having an opposite sign of the distortion coordinate is believed to be very small as compared with the zero-point energy if the cations are free. However, in our case Vo is considerably higher than the zero-point level and is affected by the environment, so that a relatively high value of Vo is supposed to be due to a matrix interaction.In this sense we termed our case a ‘matrix-assisted Jahn-Teller distortion’. What is most surprising is that the direction of the distortion coordinate is determined uniquely regardless of environment. Thus we suggest that a matrix interaction assists in the stabilization of a given distorted structure, which is determined by the intramolecular interaction. In this sense the crossing of the level ordering observed for the propane radical cation is unusual. However, this is only one exceptional case among more than 35 alkane radical cations studied so far. The nature of the matrix interaction which causes this level crossing is not clear, and is not evident if one has to consider the propane radical cation as a pseudo-Jahn-Teller species.(2) Strictly speaking, the problem must be treated quantum-mechanically by solving Mathieu’s equation. However, I have treated the problem rather classically because our experimental results show that a relatively high potential barrier as compared with kT results in a localized oscillator in each minimum in the trough at 4.2 K. There seems to be misunderstanding concerning our statement that the temperature dependence of the spectrum cannot be explained by the change of the Boltzmann populations of the two near-degenerate states, as is often assumed in solution e.s.r. studies of Jahn-Teller-active species. The near-degenerate states mean 2Al and 2Bl for c-C,H; for example, the energy difference ( A E ) of which is usually thought to be very small.In such a case the method customarily used in solution e.s.r. is as follows: The temperature change of the coupling constant is assumed to be expressed by a(2Al)+ a(’&) exp( - A E / k T ) 1 +exp( - A E / k T ) a( T ) = where the *A, ground state is assumed and a stands for the coupling constant. In such a case, the hyperfine coupling constant shows a gradual change with increasing temperature. However, the observed spectra exhibit a sudden change, which is typical of jumping phenomena between the three sites, corresponding to the three Jahn-Teller potential minima. Of course, we have to take the zero-point oscillational92 GENERAL DISCUSSION effect for a low potential barrier, i.e. the mixing of the 2Al and 2Bl states. However, we have neglected such an effect, because strictly speaking we have to solve Mathieu's equation for the low-barrier case.Instead, we have handled the problem phenomenologically. This is also the way customarily used. Dr S. Lunell (University of Uppsala, Sweden) also replied to Dr Siebrand. You bring up a very important problem and, in principle, I fully agree with you that the strength of the vibronic interactions should be investigated before one can make any safe statements about geometry or electronic state of any molecular system. Since we have not done such a study, I really should not try to answer your comment. I do believe, however, that one can get a rough feeling for the importance of these interactions in the present case from rather qualitative considerations.At the equilibrium geometry of the neutral molecule, it is clear that one will have a Jahn-Teller-like situation upon vertical ionization, because of the extremely small spacing between the 4 b l , 2b2 and 6a, orbitals, which all crowd into an interval of ca. 0.2eV. The lines in the photoelectron spectrum of C3H81 indeed show a structure very similar to the Jahn-Teller-active eg levels of C2H6. For the e.s.r. spectra, however, I do not think that vibronic effects need to be quite as important, for the following reason. Assume that we are interested in the vibronic mixing between a certain electronic state, e.g. the 'B2 state, and the remaining states. Following Fulton and Gouterman,' we can in the harmonic approximation express the total wavefunction for the system approximately as Here q and Q are of the electronic state of interest the electronic and nuclear coordinates, $k( q, Qo) the eigenfunctions Hamiltonian for the equilibrium nuclear configuration Qo of the and the functions &( Q ) determine the extent of vibronic mixing between the different electronic states.Fulton and Gouterman showed that under certain sim- plifying assumptions, vibronic coupling between two electronic states (c-( q, Qo) and &( q, Qo) can be ignored if I v , k < Q > i < < I w/ - WkI (3) where and Q is in the range of nuclear vibrations. It is naturally not a trivial task to estimate the magnitude of yk(Q) without performing the actual calculations, but Herzberg' suggests as a rule of thumb that the possibility of vibronic mixing should be considered whcnever I - Wk( < ca.I eV. From fig. 3 of our paper one can see that, at the equilibrium geometry of the 282 state, the other two states are ca. 3 eV higher in energy which, according to this suggestion, should make it unnecessary to consider vibronic effects for this state. The situation for the other states is similar.GENERAL DISCUSSION 93 It is, of course, possible that a more careful theoretical treatment will change these conclusions. There are also certainly points on the energy hypersurfaces where the different states are much closer and even cross. The 2B2 and 2A1 surfaces should, for example, cross at a C-C distance intermediate between their equilibrium values 1.480 and 1.600 A. Judging from the same figure, however, it seems likely that the barrier separating these two minima should be rather high, which again would make a vibronic mixing between them less plausible.' He I Photoelectron Spectra of Organic Compounds (Monograph Series of the Research Institute of Applied Electricity, Hokkaido University, 1978), no. 25. R. L. Fulton and M. Gouterman, J. Chem. Phys., 1961, 35, 1059. G. Herzberg, Molecular Spectra and Molecular Structure (Van Nostrand, Princeton, 1966), vol. 111. Dr S. F. J. Cox (Rutherford Appleton Laboratory) said: In reply to various queries from the floor about the freedom of methyl groups to rotate within molecules, let me confirm that there is considerable variation between different materials. The barrier heights to rotation may be deduced from magnetic-resonance or neutron- scattering measurements (usually of the proton TI minimum or of the tunnel-splitting frequency at low temperature) and these are found to vary over at least three orders of magnitude.' The materials for which these barrier heights have been determined are for the most part diamagnetic and this enormous variety is usually interpreted simply in terms of different degrees of steric hindrance in the immediate vicinity of the methyl group.My question is to what extent the barrier may be altered in the paramagnetic derivative of each material, as a result of conjugation or hyperconjugation. Le., to what extent does electron release from a C-H bond into the singly occupied orbital (I) affect the barrier to rotation, and is the effect likely to be dominant, or only a small perturbation? An illustrative example is given below.It may be that the answer depends on whether the species is ionic or neutral. Thus Prof. Symons has demonstrated that methyl groups can be essentially frozen in radical cations (11) where the positive charge favours this electron release.2 On the other hand, in radicals formed by radiolytic ejection of hydrogen the tunnel splitting is always found to be larger, so that the barrier to rotation must be smaller, than in the original molecule. CH3-C (molecule) CH3-C (radical) ,sp2 orbital 400 K only barrier 200 MHz splitting (111) __* sp3 orbital 2000 K barrier 100 kHz splitting The numerical values in (111) are for methyl malonic acid, which exhibits this be haviou r .3 An elucidation of this question would be particularly valuable in interpreting the additional barriers to rotation which appear upon substitution of the methyl protons, for instance with heavier deuterons or lighter (positive) muons.4 Is this94 GENERAL DISCUSS ION isotope effect uniquely steric or dynamic in origin or is there, as Prof.Symons has ~uggested,~ an additional electronic contribution? ' S. Clough, A. Heidemann, A. J. Horsewill, J. D. Lewis and M. N. J. Paley, J. Phys. C, 1981,14, L525. M. C. R. Symons, personal communication. S. Clough, personal communication. M. J. Ramos, 0. McKenna, B. C. Webster and E. Roduner, J. Chem. Soc., Faraday Trans. 1, 1984, 80, 267. M. C. R. Symons, Hyperfine Interactions, 1984, 17-19, 793. Dr E. Roduner (University of Zurich, Switzerland) commented: I would like to respond on the questions posed by Dr Cox on barriers to internal rotation of methyl groups in radicals and on their isotope dependence.Apparently, the results for methyl rotation in diamagnetic molecules are usually understood well in terms of a potential barrier with threefold symmetry. In a radical, CA2X-CB,, all odd terms in the Fourier series expansion of the potential are zero as long as the radical centre is planar and the leading term is of twofold symmetry. The barriers are therefore not directly comparable. Furthermore, for X = A the twofold and fourfold terms drop out, and the leading term is of sixfold symmetry,' obviously associated with a very small barrier. Let us consider the ethyl radical and its isotopic derivatives (A, B, X = hydrogen isotopes Mu, H, D).An ab initio calculation by. Pacansky and Dupuis2 yields a sixfold potential of 630 J mol-' (75 K) for CH,-CH,. Calculations are performed for different rigid geometries and exclude dynamic effects. The resulting energy hypersurface is conventionally termed the electronic energy. Within the Born- Oppenheimer (B.O.) approximation it is the same for all isotopic species. The effects of internal rotation and vibration lead to isotope-dependent average nuclear coordin- ates and thus to different positions on this surface. Although isotopic substitution affects the average distribution of electron density the origin of the effect is dynamic. Experimental values for twofold barriers are obtained from the analysis of the temperature dependence of p- hyperfine coupling constants, assuming that they are a function of internal rotation only.For CHD2-CD2 V, = 385 J mol-' (46 K) was reported,' for CMuH,-CH, V, = 2963 J mol-' (356 K) was found.3 These drastic isotope dependences were explained as dynamic-steric effects. The lighter isotopes have higher zero-point vibrational amplitudes and appear bulkier (higher van der Waals radii). Furthermore, owing to anharmonicity, the average C-X bond length will be slightly larger for the lighter isotope. This certainly facilitates what Dr Cox calls electron release from the C-X bond into the singly occupied orbital. It is reflected in the isotopic dependence of the constants in the cos2 8 law.3 It should not be subsumed under electronic effects.Of course, the B.O. surface is always an approximation which leaves the option of an electronic effect. However, this was shown to be reasonably small, even for molecules containing MU.^ ' R. W. Fessenden, J. Chim. Phys., 1964, 1570. ' J. Pacansky and M. Dupuis, J. Am. Chem. SOC., 1982, 104, 415. M. J. Ramos, D. McKenna, B. C. Webster and E. Roduner, J. Chem. SOC., Faraday Trans. 1, 1984, 80, 267. D. McKenna and B. C. Webster, J. Chem. Soc., Faraday Trans. 2, 1984, 80, 589. Prof. M. C. R. Symons ( University of Leicester) (communicated) Dr Rpduner has raised the interesting question of the source of the isotope effect in R,C-CH,Mu radicals. There are two factors involved, one being the preferred orientation, which tries to maximise u-T overlap between the 2p, oribital on carbon and the C-Mu bond, and the other is any residual isotope effect once this has been allowed for.GENERAL DISCUSSION 95 The marked experimental enhancements detected are normally explained in terms of the ‘steric’ factor which makes Mu ‘larger’ than H, as mentioned by Dr Roduner.I have recently suggested that this may be less important than what I consider to be an ‘electronic effect’,’ despite the fact that Roduner says that the phenomenon ‘should not be subsumed under electronic effects’. We have moved forward in that there is now agreement that the phenomenon may exist, namely that hyperconjuga- tive electron release may be more efficient from C-Mu than from C-H bonds. This will contribute to both effects mentioned above. However, if this is the case, then it is, surely, ‘electronic’, since it involves electrons.We should also consider the possibility that, when in the preferred conformation, the muon itself moves as well as the electrons, as in (I), so as to increase the extent of the interaction. I M. C. R. Symons, in Muon Spin Rotation and Associated Problems, Part ZI, ed. T. Yamazaki and K. Nagamine (J. C. Baltzer AG, Basel, 1984), p. 771. Prof. T. J. Kemp ( University of Warwick) said: Concerning the doubts expressed by one questioner about the role of the protonated form of cyclopentadiene as the photoactive species leading to the cyclopentadiene radical cation, is it not the case that the presence of these precursor species in CF3C02H has been confirmed unequivocally by n.m.r.spectroscopy? Prof. A. G. Davies (University College, London) replied: Yes, the formation of the protonated species from most of the methylated cyclopentadienes has been established by n.m.r. and U.V. spectroscopy but usually in stronger acids such as H,SO, and FS03H [e.g. ref. (l)]. ’ N. C. Deno, H. G. Rickley, N. Friedman, J. D. Hodge, J. J. Houser and C. U. Piltman, J. Am. Chem. SOC., 1963,85,2991; N. C. Deno, N. Friedman, J. D. Hodge and J. J. Houser, J. Am. Chem. SOC., 1963, 85, 2995; N. C. Deno, J. Bollinger, N. Friedman, K. Hofer, J. D. Hodge and J. J. Houser, J. Am. Chem. SOC., 1963, 85, 2998. Prof. W. J. Albery (Imperial College, London) said: First, can Prof. Davies give us any estimate of the quantum efficiency of his systems? Secondly, it may be possible to follow the production of H2 in situ using the membrane electrode developed by Dr A.Mills while he was at the Royal Institution. Prof. A. G. Davies (University College, London) replied: (1) As yet, we cannot give any quantative estimate of the quantum efficiency. We have measured the rate of decay of the Me5C; radical and have found it to be diffusion controlled. Its e.s.r. spectrum, however, is more intense than we normally observe for such a short-lived radical, and the most we can say is that the quantum efficiency for the formation of Me&; from Me,C,H is greater than that of most of the reactions which we have studied. The radical cations are relatively very persistent. The high intensities of the spectra reflect the low rate of decay rather than rate of96 GENERAL DISCUSS ION formation, and we have no idea of the quantum efficiency.Although we believe we know the bare bones of the mechanisms involved, alternative mechanisms cannot be ruled out, and the systems warrant a thorough photochemical study. (2) We have used laser Raman spectroscopy to analyse the dihydrogen ( H2, HD and D2) evolved from isotopically labelled pentamethylcyclopentadiene.' This is a very convenient method, which can be carried out on the sealed e.s.r. tube. Mills' electrochemical method might be useful for following the kinetics of the reaction. ' A. G. Davies, E. Lusztyk, J. Lusztyk, V. P. J. Marti, R. J. H. Clark and M. J . Stead, J. Chern. Soc., Perkin Trans. 2, 1983, 669. Prof. M. C . R. Symons (University of Leicester) said: We now direct attention to Prof.Williams' paper. ( 1 ) With respect to your qualitative view of spin delocalisation onto S protons, this receives considerable support from our study of ( '3CH3)2CO'f cations. Our estimated spin density on l3C is only slightly less than that onto 'H for H,CO'+ cations, showing that, as you suggest, delocalisation into the u frame by normal hyperconjugation is extensive.' (2) The problem of the reversible loss of chlorine hyperfine coupling for CH,CHO+ and CH,CH,CHO+ cations is most interesting. Our suggestion was that this is due to a reversible breaking of the O-.-Cl v bond, postulated for these complexes,* whilst you favour the idea of a rotational averaging. Obviously, the former RCHOf-C1CFCl2 S RCHO" + CFC13 ( 1 ) will result in loss of chlorine coupling, but you suggest it should also result in a change in the e.s.r.parameters for the RCHO" group. On the other hand, the isotropic coupling for RCHO" should not alter if the complex rotates as a whole unit, whilst the chlorine coupling could average to near zero provided A, is negative and ca. half the value of All. Just this situation is observed for R'X- adducts (X=Cl, Br, I), where the isotropic coupling is very small, and A , (hal) must be n e g a t i ~ e . ~ - ~ However, it seems to me that such a rotation must involve the whole unit [RCHO+-- .G1CFCl2], which is perhaps surprising. Since the large proton coupling is averaged, the RCHO' unit must rotate, but if that involved the RCHO+ unit alone, that would require bond breakage.If the specific CFC13 molecule were able to rotate on its own, this again implies reversible bond-breakage. It is conceiv- able that both units librate so extensively that much of the anisotropy is lost, but this still requires that A, (Cl) be negative. If A, is negative, the estimated spin density on chlorine is ca. 19%, which is quite high. I have previously shown that for genuine u* complexes there is a good correlation between the s character (estimated from AiSJ and the p character (estimated from 2B).' Using a percentage s character of 0 and percentage p character of 19, the correlation is not obeyed (as indeed is the case for alkyl-radical halide-ion adducts). However, if A , is ca. 9.5 G and positive, we get a percentage s character of ca. 0.5 and a percentage p character of ca.9.5. These results fit nicely onto the line which correlates the results for many u* complexes. Regarding your case that for bond-breaking a sudden change should be observed, I am not sure that this is correct. What I expect, and what seems to me to be observed, is that, for dissociation, both species should be observed together over a range of temperatures. Given some broadening of the Cl hyperfine features caused by librational effects, that seems to be what both of us observe [Prof. Williams' fig. 2 and ref. (S)].GENERAL DISCUSSION 97 If A , is positive, as we suggest, the extent of delocalisation is about half that required on your interpretation. Thus the bond is weaker and reversible dissociation is more probable. Furthermore, for such a weak interaction I see no reason why there should be any major change in A('H) for RCHO' cations.Certainly for strongly bonded complexes, such as RS-SR- or R2S'SR2+, there are major changes, but can this argument be carried through to very weak complexes with < 10% delocalisation? It would be nice to have this issue settled since, if our hypothesis is correct, we can attempt to estimate the bond strength for these interesting complexes. P. J. Boon, L. Ham's, M. T. O h , J. L. Wyatt and M. C. R. Symons, Chem. Phys. Lett., 1984, 106, 408. M. C. R. Symons and P. J. Boon, Chem. Phys. Lett., 1983, 100, 203. M. C. R. Symons and I. G. Smith, J. Chem. Soc., Perkin Trans. 2, 1979, 1362. M. C . R. Symons and I. G. Smith, J. Chem. SOC., Perkzn Trans. 2, 1981, 1180.I. G. Smith and M. C. R. Symons, J. Chem. Soc., Faruday Trans. 1, 1985,81, 1095. M. C. R. Symons and I. G. Smith, J. Chem. SOC., Faraday Trans. I , 1981, 77, 2701. P. J. Boon, M. C. R. Symons, K. Ushida and T. Shida, J. Chem. Soc., Perkin Trans. 2, 1984, 1213. ' M. C. R. Symons, Chem. Phys. Lett., 1980, 72, 559. Prof. F. Williams (Knoxville, Tennessee) remarked: Although the results presen- ted in our paper establish that the strongly coupled hydrogens in the cyclohexanone and adamantan-2-one radical cations are in the S positions, it was pointed out that the assignment to the equatorial rather than the axial hydrogens depends on the adoption of the 'trans rule'. To the best of our knowledge, however, this rule has never been proved rigorously, at least in e.s.r.studies. It is therefore of interest to report that additional work carried out since submitting the manuscript has verified this rule through the direct proof of the 'H hyperfine coupling assignments for the adamantan-2-one cation. These studies were made possible through the synthesis of the stereospecifically labelled monodeuteroadamantan-2-ones 1 and 2 in high isotopic purity by Prof. S. F. Nelsen and his group at Wisconsin. 1 2 First, the e.s.r. spectrum of 1" shown in the upper half of fig. 2 demonstrates that the introduction of a deuterium atom into one of the two y positions of the adamantan-2-one radical cation has no effect on the main quintet pattern [A(4H) = 22.5 GI but changes the substructure from a triplet to a doublet [A( 1H) = 7.0 GI.This establishes that the 7 G triplet substructure in the spectrum of the fully protiated cation (fig. 5 of our paper) originates from coupling to the 2y hydrogens, as proposed. Secondly, the contrasting effect of deuterium substitution into one of the equatorial S positions is revealed by the e.s.r. spectrum of 2" shown in the lower half of fig. 2. The main pattern is now seen to consist of a quartet [A(3H) = 22.4 GI instead of a quintet, proving that the strongly coupled hydrogens are indeed in the 6 equatorial positions. The loss of resolution in the substructure is also explained by the additional 3.5 G coupling to the one deuterium ( I = 1) nucleus. These results clearly establish the 'trans rule' and will be reported in more detail elsewhere.' ' S. F. Nelsen, D.Kapp, L. D. Snow and F. Williams, to be published.98 GENERAL DISCUSSION 9273.0 MHz 9271.3 M H z 0 m r h 0 Fig. 2. First-derivative e.s.r. spectra of the monodeuteroadamantan-2-one radical cations 1" (upper spectrum) and 2*+ (lower spectrum) at 140K. The cations were generated by y- irradiation of solid solutions of the parent compound in CFC13 at 77 K. Dr M. R. Wasielewski (A.N.L., Illinois) said: Prof. Williams' long-range hyperfine splittings nicely illustrate the ability of the all-trans a-bond framework to transmit spin density over many bonds with good efficiency. As we know, this problem is related to that of the electronic coupling present between an electron donor-acceptor pair spaced by rigid hydrocarbon spacers. Since his 6 splittings are so large, has he tried, or does he contemplate trying, to measure longer-range splittings in e.g.a 2-keto decalin system or a 3-keto steroid molecule? Prof. A. Weller ( Max-Planck-lnstitut, Gottingen, West Germany) added: Would Prof. Williams agree that the strong equatorial hyperfine coupling constant aHh involves through-bond orbital interaction rather than through space? Prof. F. Williams (Knoxville, Tennessee) said: Yes. A through-bond orbital interaction is strongly supported by the stereospecificity of the 6- hydrogen coupling in the adamantan-2-one system.' Moreover, the coupling to the y hydrogens is about a factor of three smaller than that to the more distant equatorial 6 hydrogens, a result which is clearly incompatible with a through-space interaction.' F. Williams, Furuday Discuss. Chem. SOC., 1984, 78, 97; S. F. Nelsen, D. Kapp, L. D. Snow and F. Williams, to be published. Prof. W. Bernhard (University of Rochester, New York) said: Prof. Williams' analysis of long-range hyperfine interactions will most likely be helpful in solving a problem that has been puzzling me for some time. There are a fair number ofGENERAL DISCUSSION 99 alkoxy radicals that have been characterized using single-crystal e.s.r./ ENDOR. These studies provided the first measurements of the unusually large P - hyperfine couplings in oxy-centred radicals. However, in many of the alkoxy radicals there are additional weak hyperfine couplings that have been carefully measured using ENDOR.' Although these weak hyperfine interactions have been ascribed to y hydrogens, I have found it difficult to work out a self-consistent explanation based only on y-hyperfine interactions. From Prof.Williams' analysis it is now clear that we should go back and consider the rather likely possibility that at least some of these additional hyperfine couplings are due to hydrogens in S positions. ' H. C. Box, E. E. Budzinski and G. Potienko, J. Chem. Phys., 1980, 73, 2052. Dr M. Iwasaki and Dr K. Toriyama (Nagoya, Japan) said: Prof. Williams has observed extremely large &proton couplings in a variety of aldehyde and ketone radical cations. Although these range from 12.5 to 27.5 G, cyclic ketone radical cations seem to give a larger coupling than that of aliphatic aldehyde cations. Now, if one considers a spin density of ca.0.25 on the P-carbon atom, the conventional pB, cos2 8 rule for P-proton coupling with B2 = 60 G can give a 8-proton coupling of ca. 15 G, if 8 = 0 is assumed. The B2 value of neutral alkyl radicals is 58 G. The largest coupling of 27.5 G corresponds to B, = 110 G assuming 8 = 0. This value is fairly close to our B2 value of 120 G determined for a series of branched alkane cations [ref. ( 5 ) in our paper]. So, a large 6-proton coupling may not be surprising and the origin of the long-range coupling is in the high-spin density on the P-carbon atom, as Prof. Williams called it relay of spin transfer. Now the conformation with respect to the C,-C, bond in the cyclohexane radical cation, which gives the largest &proton coupling of 27.5 G, is gauche rather than trans.In the case of methyl-substituted butane radical cations, the INDO calculations show that the gauche methyl proton gives a considerably lower value than that of the trans methyl proton [ref. ( 5 ) in our paper]. Compared with this result, the higher coupling value in the cyclic-ketone radical cations is puzzling. Prof. F. Williams (Knoxville, Tennessee) said: We very much appreciate the comments and suggestions by Dr Wasielewski and Prof. Weller. In answer to the question, our work has not progressed beyond the systems described in our paper and in my supplementary remarks. As pointed out, through-bond orbital interactions can contribute significantly to the efficiency of long-range electron transfer between donor and acceptor groups separated by a rigid hydrocarbon framework, the impor- tant parameter being the electronic coupling J.In view of the recent elegant work on the rates of intramolecular electron transfer in radical anions possessing two rr-electron groups linked by steroidal spacers,' we are particularly interested in the investigation of long-range hyperfine interactions in related skeletal systems of the type suggested by Dr Wasielewski. One cautionary note should be added, however, in regard to the possibility of observing longer-range splittings. While it is certainly true that the &hydrogen splittings reported in table 1 of our paper are large enough (12.5-27.5 G) to suggest that coupling to even more remote hydrogens might be detectable in favourable circumstances, such expectations must be tempered by the fact that splittings of < 4 G are not generally resolved in these freon matrices.I am glad to learn from Prof. Bernhard that our analysis of long-range hyperfine interactions may be of some assistance in clarifying some of the assignments for the weakly coupled protons in alkoxyl radicals. Clearly it would be helpful to have a quantitative treatment of long-range hyperfine interactions comparable to that100 GENERAL DISCUSS I 0 N developed some time ago by Prof. Bernhard for P-proton couplings in oxygen- centred radicals,2 and the comments by Dr Iwasaki and Dr Toriyama are pertinent on this point. Using a pB2 cos2 8 relation, their first comment quantifies our proposal that spin delocalisation from the carbonyl group into the sigma-bonded carbon frame, thus producing p = 0.25 at our C,, allows spin d b i t y to be relayed to the S, hydrogens via the trans or alignment effect.The close connection between our results for cyclic ketone cations and Dr Iwasaki's work on branched alkane cations [ref. (5) in his paper] became apparent to us after submitting our manuscript, and we regard the similar B2 values for the two systems as highly supportive of the model for spin transmission. For the acyclic aldehydes and ketones, the couplings to the trans S hydrogens are smaller than those for the rigid cyclic ketones by a factor of ca. 2 [table 1 of our paper]. This apparent discrepancy could be due to torsional oscillation in the non-rigid systems resulting in a value of (cos2 8) of < 1. In response to the last comment by Dr Iwasaki and Dr Toriyama, there seems no reason to believe that the gauche conformation for the cyclohexanone cation about the C,-Cy bond should have any significant effect on spin delocalisation from the carbonyl group into this bond, and hence on the magnitude of the 8-hydrogen splittings.The conformation about the C,-Cy bond can be varied in an acyclic system, and MNDO calculations for the propionaldehyde radical cation3 show only a small dependence for the 6-hydrogen splittings on the OC,C,C, dihedral angle. Irrespective of the conformation about the C, -C, bond, however, the calculations show that the &hydrogen splitting is extremely sensitive to the C,C,C,H, dihedral angle, the maximum Hs splitting being obtained in each case when this dihedral angle is 180', as expected for the trans effect.Similarly in the methyl-substituted butane radical cations, the trans (8 = 180') and gauche (8 = 60") methyl-proton splittings are determined by the C2C3CM,H, dihedral angle, the spin density now being in the C2-C3 bond. L. T. Calcaterra, G. L. Closs and J. R. Miller, J. Am. Chem. Soc., 1983, 105, 670; 1984, 106, 3047. W. A. Bernhard, D. M. Close, J. Hiittermann and H. Zehner, J. Chem. Phys., 1977, 67, 1211. S. F. Nelsen, personal communication. Dr W. Siebrand (N.R.C., Ottawa, Canada) said: In his paper Prof. Williams presents convincing evidence that the methyl groups in cations of simple aldehydes start to rotate (on the timescale of the experiment) above 120 K. In our work on tunnelling in a methanol glass, to be presented later during this Discussion, we see evidence for rotation down to 15 K or lower (on, admittedly, a much longer timescale).Other estimates can be found in the literature; they cover a wide range of values. Is there any serious discrepancy between these results? Prof. F. Williams (KnoxviZZe, Tennessee) said: In our experience, the onset temperature of methyl-group rotation in radicals is extremely variable and has to do with structural factors which are not always easy to predict. Therefore, I see no serious underlying discrepancy between Dr Siebrand's results and ours. This is a general question on which I am sure others in the audience may also wish to comment. Dr M. Iwasaki (Nagoya, Japan) said: It is well known that the hindering potential barrier to the rotation of CH3 group is very low when the radical carbon atom has an sp2 planar structure, whereas it becomes higher when the radical site becomes non-planar. For example, the hindering potential barrier in CH3CH2 is essentially zero, whereas it is 2.2 kcal mol-' in a bent CH3CF2.'GENERAL DISCUSSION 101 In radical cations of alkanes, in which the radical carbon atom has bent structure, the hindering potential barrier is expected to be relatively high.’ K. S. Chen and J. K. Kochi, J. Am. Chem. SOC., 1974 96, 794. Dr M. Iwasaki (Nagoya, Japan) said: I address my remarks to Prof. Sevilla. I would like to make a comment on the rearrangement of radical cations, more specifically the intramolecular proton transfer in ester radical cations.In principle, gas-phase mass spectroscopy cannot give direct evidence for intramolecular rearrangement because there is no change in mass. However, e.s.r. spectroscopy can give direct evidence. Sevilla et al. have assumed intramolecular proton transfer based on solute concentration dependence. On the other hand, we have obtained direct e.s.r. evidence for intramolecular proton transfer from the ester alkyl group to the carbonyl oxygen atom. In addition, the out-of-plane conformation of the transferred proton is also determined by anisotropic spectral simulations [ref. (21) in Prof. Sevilla’s paper]. It is to be mentioned that the transfer to the ether oxygen atom is also excluded by the simulation. Prof. M. C . R. Symons (University of Leicester) said to Prof.Sevilla. In our own work on ester cations, which has run parallel with yours,’ we also supposed that the species derived from the ( HC02Me)+-C1CFC12 adduct on annealing was the rr cation, but, like you, we are inclined to agree that the new results of Iwasaki et aL2 are better interpreted in terms of the proton-transfer cation (I). I would like to stress that we were both misled by the fact that theoretical estimates of the ‘H coupling for the rr cation agree remarkably well with the experimental results now assigned to (I). I was also misled by the clear e.s.r. evidence for a non-rearranged cation obtained from various a m i d e ~ . ~ These cations are very similar to the ester n cations, except that the spin-density-on the -NR2 group is greater than that on the -OR group, whilst that on the carbonyl oxygen is corre- spondingly reduced.Problems remain, however. If, as is now ~uggested,~ the first-formed cation at 4 K is indeed the n cation rather than the a cation, why should the structure change to the a-cation adduct (11) on annealing, and why should this be formed uniquely on irradiation at 77 K? If this is correct, then it sheds interesting light on the charge-transfer mechanism for cation formation in this matrix. It also implies that the energies of the n and n cations must be very close. However, it is noteworthy that A(’H) for the unique proton for this complex is only ca. 17 G. Possibly this large reduction relative to the value for the aldehyde cation complexes (ca. 140 G) arises in part because the structure is not actually ‘planar’, as is implied in (11), but twisted, so that the orbital on the ester group is neither pure a nor pure n.102 GENERAL DISCUSSION I would like to stress that these ester cations and the lactone cations that our two groups are studying jointly, represent perhaps the most recalcitrant and contrary species that I have yet encountered in this field.Genuine ester cations which have neither undergone rearrangement nor formed bonds to solvent molecules seem to be rare indeed, in marked contrast with almost any other system so far studied. ' D. N. R. Rao, J. Rideout and M. C . R. Symons, J. Chem. SOC., Perkin Trans. 2, 1984, 1221. ' M. Iwasaki, H. Muto, K. Toriyama and K. Nunome, Chem. Phys. Lett., 1984, 105, 413. D. N. R. Rao and M.C. R. Symons, Chem. Phys. Lett., 1982,93, 495. Prof. M. D. Sevilla (Oakland University, Michigan) replied: There is an alterna- tive possibility to the discrepancy found between the 4 and 77 K results for methyl formate. Dr Iwasaki reports the cation at 4 K and suggests the lack of rotational motion at this low temperature does not allow for the u*-complex formation. Dr Iwasaki has shown that the uncomplexed cation is unstable at 77 K and undergoes proton transfer to produce 'CH20C(OH+)H. The e.s.r. parameters for this proton- transfer radical are very close to those reported by Dr Iwasaki at 4 K for the cation. As a consequence the 4 K species may also be a proton-transfer radical not the original cation. It is true that molecular-orbital calculations suggest that the n cation and the proton-transfer species should have similar couplings so this problem may be a difficult one to resolve.A possible solution may be 4 K experiments on deuterated methyl formate (CD,OCHO) which does not undergo deuteron transfer even at 77 K. Dr M. Iwasaki (Nagoya, Japan) said: I turn first to Prof. Sevilla. The evidence for the oxygen n cation of methyl formate is weak as compared with the firm evidence for the carbon-centred r-radical cations formed by McLafferty rearrangement. However, at 4 K methyl and ethyl formate radical cations exhibit essentially the same three-line spectra, the feature of which is more like arising from the two equivalent &proton couplings. Even if the spectra at 4 K could be attribu- table to the carbon-centred T radical having the two Q protons, the resemblance of the spectra obtained from methyl and ethyl formate may not be explained unless we assume that CH, group of ethyl formate once shifts to the carbonyl group to give a three-line spectrum and then CH2 returns to the radical carbon atom: O+ 0-CH, OH 0-CH2 +/ + / .CH3 + H-C --* H-C-CH2 // \ / \ H-C \ / 0-CH, O-CH2 If this sort of rearrangement can occur, the 0, ester cation might not exist. However, I would like to point out that the SOMO of this sort of molecule is well known to exhibit a u-T crossing by a slight change of the bond angle and other geometries. Prof. Sevilla has also shown this by INDO calculations and we have confirmed it. I turn now to Prof. Symons. Methyl formate in CFCl, gives a u* complex with a matrix C1 atom when irradiated at 77 K, whereas the complexing is suppressed when irradiated at 4 K.This seems to suggest that the local molecular reorientation is required for the cation to form a u* complex. Originally an electron may be ejected from the in-plane oxygen u orbital forming 0, cation radicals. However, it changes into more stable oxygen-centred n- cation radicals because the competitive u* formation is sup- pressed by retardation of molecular reorientation at 4 K, whereas at 77 K the 0,GENERAL DISCUSSION 103 cation radical can complex with CFC1, because the local molecular reorientation becomes possible at 77 K. The yield of 0, cation radicals at 4 K is relatively small, and upon warming to 77 K the u" complex is formed, with a decrease of the remaining matrix cation radicals.This also indicates that the complexing is possible when positive charge transfer to the solute occurs at elevated temperature. On the other hand, we have observed that complexing of acetaldehyde radical cations cannot be suppressed at 4 K (unpublished work). This is because CH,CHO'+ cannot be converted into an oxygen T cation. Thus we may have to take the two factors into consideration at 4 K: one is the retardation of the change of the local geometry, including the deformation of the radical cation itself, and the other is the change of the unpaired electron orbital from u to T. Prof. M. C. R. Symons ( University ofLeicester) (communicated) Whilst I accept the mechanism for ester cation formation proposed by Dr Iwasaki, and also the reservations made by Prof.Sevilla about the 4 K results, I think the point is not entirely answered. The following equations for the formation of ester (E) cations summarise the problem as I envisage it: (CFCl,)+ + E ---* (FClZCCl-. -E)+ (1) (CFCl,)++E -+ CFCl,+E; (2) (3) (4) (CFCI,) + E + CFC1, + EZ E: + CFC13 --* (FC12CC1- * *E)+ (where EfR is the rearranged final product). Although reaction (3) is possible, there is no direct evidence for it since EZ has not been detected. The solvent cr complex can be formed directly, as in reaction ( 1 ), provided the correct relative orientations can be achieved, rather than via charge transfer followed by adduct formation, as in reactions (3) and (4). Similarly, the T cation, E:, can be formed directly, as in reaction (2), there being no need to postulate reactions (3) and (5).Unfortunately, the irreversible step (6) apparently occurs so readily that it seriously interferes with attempts to learn more about this reaction. Dr W. Siebrand (N.R.C., Ottawa, Canada) said: The proton transfer in the ester cations which Prof. Sevilla described resembles the transfer between keto and enol tautomers which is known to proceed by hydrogen tunnelling.'.2 This, combined with the deuterium effect he observes, suggests that the present transfer also proceeds by tunnelling. Has this been confirmed? ' K-H. Grellmann, H. Weller and E. Tauer, Chem. Phys. Let?., 1983, 95, 195. W. Siebrand, T. A. Wildman and M. Z. Zgierski, J. Am. Chem. SOC., 1984, 106, 4089. Prof. M. D. Sevilla (Oakland University, Michigan) said: We have recently found that the (T* complex formed between [2H3]methyl formate and CFC1, is far more stable than that formed with methyl formate itself and it undergoes a different chemistry on dissociation.While methyl formate undergoes reaction ( 1):I04 GENERAL DISCUSSION X +- // 0' II OH+ II ( CT* complex) the deuterated compound undergoes loss of the formyl proton: X +- ,/ Of 0 where X = CFC1, and S represents a solute molecule. Williams' in other systems and are associated with tunnelling. Such 'all or none' isotope effects have been reported previously by Wang and I J. T. Wang and F. Williams, J. Am. Chem. SOC., 1972, 94, 2930. Dr M. Iwasaki (Nagoya, Japan) added: We have observed intramolecular proton transfer in a variety of ester radical cations even below 77 K and in some cases it takes place even at 4 K .The reaction at such low temperatures suggests that the tunnelling process may be involved. In addition, the steric factor is important for the occurrence of this proton transfer at low temperature, suggesting that the reaction rate depends upon the tunnelling distance. Prof. M. C. R. Symons ( University of Leicester) (communicated) Regarding the solvent adducts exhibiting hyperfine coupling to chlorine nuclei observed for certain aldehyde and ester cations, we now have several examples of such interaction. For example, RCl" and RBr" cations show this coupling, although RI" cations do not.''2 Also, as has also been observed by Williams and his coworkers, (MeO),PO'+ cations show a large coupling to ~ h l o r i n e .~ We have also sometimes observed a species having coupling to two equivalent chlorine nuclei [A11(35Cl) = 101 GI, which may be due to two solvent molecules sharing a 'hole'. [Neb. The isostructural Cl, ion has an identical Ai,(35C1) coupling.] Furthermore, the cation of Me,C-CN, which probably has an ionisation potential slightly greater than that of CFC13, actually forms a complex having A11(35C1) of 125 G, which is greater than that expected for 50% electron sharing4 We suggest that this adduct has structure (I), since the isotropic coupling to 14N (ca. 70 G) is very large. Me,C-C_N-Cl I c / I \ CI C1 F ' G. W. Eastland, D. N. R. Rao, J. Rideout, M. C. R. Symons and A. Hasegawa, J. Chem. Rex ( S ) , ' G. W. Eastland, S.P. Maj, M. C. R. Symons, A. Hasegawa, C . Glidewell, M. Hayashi and 1983, 258. T. Wakabayashi, J. Chem. SOC., Perkin Trans. 2, 1984, 1439. G. D. G. McConnachie and M. C. R. Symons, J. Chem. Rex ( S ) , 1985, 54. J. Rideout and M. C. R. Symons, J. Chem. Rex ( S ) , 1984, 268.GENERAL DISCUSSION 105 Prof. F. Williams (Knoxville, Tennessee) said: I should like to report e.s.r. evidence for a strong interaction between the trimethyl phosphate radical cation and the CFC13 matrix' which resembles Prof. Sevilla's results reported some time ago on the u* complex between the methyl formate cation and CFC13.* The e.s.r. spectrum of the trimethyl phosphate radical cation in CFC13 is characterised by a large anisotropic chlorine hyperfine interaction [All(35C1) = 85 GI and a 3'P doublet splitting of ca.26 G, the chlorine coupling being very close to that (84.4 G) observed for the methyl formate complex.2 Since the ionisation potentials of trimethyl phophate3 and methyl formate4 are both equal to 10.81 eV, the similarity in the chlorine hyperfine couplings supports the idea that strong cation-solvent interactions are facilitated when the ionisation potential of the solute is within ca. 1 eV of that for the solvent (ionisation potential of CFC13 = 11.78 ev).' Localisation of the spin density has also been suggested as a favourable factor contributing to complex formatioq2 and it is noteworthy that the unperturbed SOMO of these two cations satisfy this criterion. Thus in the methyl formate radical cation, the unpaired electron is largely localised in a 2p, orbital on the carbonyl oxygen, while in the trimethyl phosphate radical cation the unpaired electron is similarly in a non-bonding 2p orbital on the unique oxygen.It is also interesting that the u* complex between the trimethyl phosphate cation and CFC13 undergoes an irreversible thermal or photoinduced dissociation.' This dissociation is accompanied by formation of the (MeO),P+( 0H)OCH; radical formed by hydrogen-atom transfer from one of the methyl groups to the unique oxygen,' a rearrangement which again is similar to that recently described for the dissociation of the methyl formate cation-CFC1, u* ~ o m p l e x . ~ - ~ Contrary to the initial reports,2 the uncomplexed methyl formate cation is not observed following diss~ciation.~-~ It appears, therefore, that the nature of the bonding between the radical cation and the CFC13 solvent 'protects' these very reactive u radicals from undergoing the irreversible internal hydrogen-atom transfer reaction until dissoci- ation takes place, as depicted in scheme 1.1+' Scheme 1. Turning to the smaller chlorine hyperfine interactions between the aldehyde radical cations and CFC13 which are reported in our paper, we have presented reasons why the reversible loss of the 35Cl coupling with temperature in this case is more likely to be due to a motional averaging of the hyperfine anisotropy associatedI06 GENERAL DISCUSSION with the cation-solvent complex than to a dissociation of the complex. In the light of the results for the methyl formate5-' and trimethyl phosphate' complexes, an additional argument against dissociation is that internal hydrogen-atom transfer to oxygen is not observed for the propionaldehyde radical cation between 120 and 140 K despite the fact that the chlorine interaction is already lost and methyl group rotation sets in between these temperatures [see fig. 2 ( a ) and 3 ( c ) of our paper]. However, it must be admitted that this argument loses its force if the propionaldehyde cation adopts conformation 2, which is unfavourable for this hydrogen-atom transfer reaction. The possibility of dissociation to give an unreactive aldehyde radical cation can therefore not be ruled out at this time. Despite the fact that the substructure in the e.s.r. spectrum of the acetaldehyde cation in CFCI3 (fig. 1 of our paper) cannot be analysed in detail, we have confidently attributed this substructure to a matrix interaction resulting mainly from one 35Cl nucleus because deuteration of the methyl group produces only linewidth changes in the e.s.r. spectra of MeCHO'+ and MeCD0'+.8 This conclusion has a corollary, namely that the aforementioned substructure should be eliminated in the e.s.r. spectrum of the acetaldehyde cation in a neon matrix. Prof. L. B. Knight Jr has recently shown this to be the case.' The species was generated using an open-tube neon discharge lamp with 17 eV photons and then trapped in a neon matrix at 4 K, the technique being similar to that used in the previous investigation of the formal- dehyde radical cation." Small hyperfine couplings to the hydrogens of the methyl group, which were masked by the solvent interaction in the Freon matrix, are now clearly revealed together with I3C satellite lines in natural abundance for partially oriented CH3CHO'+. A detailed study is underway, this species being of particular interest because it is the first cation to be investigated by e.s.r. using both the Freon method emphasised in this Discussion and Knight's technique." ' X-Z. Qin, B. W. Walther and F. Williams, J. Chem. Soc., Ghem. Commun., in press. * ( a ) D. Becker, K. Plante and M. D. Sevilla, J. Phys. Chem., 1983, 87, 1648; ( b ) G. W. Eastland, D. N. R. Rao, J. Rideout, M. C. R. Symons and A. Hasegawa, J. Chem. Res. ( S ) , 1983, 258. V. I. Vovna, S. N. Lopatin, R. Pettsol'd and F. I. Vilesov, Khim. Vys. Energii, 1975, 9, 9. K. Watanabe, T. Nakayama and J. Mottl, J. Quant. Spectrosc. Radiar. Transfer, 1962, 2, 369. M. Iwasaki, H. Muto, K. Toriyama and K. Nunome, Chern. Phys. Lett., 1984, 105, 586. M. D. Sevilla, D. Becker, C. L. Sevilla, K. Plante and S. Swarts, Faraday Discuss. Chem. Soc., 1985, 78, 71 M. D. Sevilla, D. Becker, C. L. Sevilla and S. Swarts, J. Phys. Chem., in press. L. B. Knight Jr, personal communication. * L. D. Snow and F. Williams, Chem. Phys. Lett., 1983, 100, 198. 10 L. B. Knight Jr and J. Steadman, J. Chem. Phys., 1984, 80, 1018.

ISSN:0301-7249    

DOI:10.1039/DC9847800083

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

Faraday Discuss. Chem. Soc., 1984, 78,107-1 19 Charge-transfer Processes in the Quenching of Excited Uranyl Ion by Organosulphur, Organohalogen and Organometal Species BY HANNA B. AMBROZ,~ KEVIN R. BUTTER AND TERENCE J. KEMP" Department of Chemistry, University of Warwick, Coventry CV4 7AL Received 2nd May, 1984 Strong quenching of the emission of excited uranyl ion, [U022+]*, is found on addition of the molecules RHal (Hal = Br, I), R2S and R4M (M = Si, Ge, Sn, Pb), both in terms of the intensity and lifetime of the emitting species as determined by 347 nm laser flash photolysis in acetone solution. Kinetic studies have been supplemented by quantum-yield determinations (for UlV), product analysis and, in certain cases, by e.s.r. investigation of the irradiated system at 77 K.Second-order rate constants k2 for the quenching process fall in the region 107-6x lo9 dm3 mol-' s-' and, in general, correlations can be found between log k2 and the ionisation potential of the donor for a particular series. Plots for the series R4Sn and RHal yield gradients of - 1.12 f 0.10 and - 1.6 I f 0.07 eV-', respectively, while that for the series cyclo-R2S is markedly different (-0.69 k0.07 eV-I). (None of these gradients conforms to the figure of - 16.9 1 eV-' expected from the linear sloping section of a so-called Weller plot.) +(U'") is generally rather low (< 0.07) and in the case of RHal extremely so, although U'" yields could not always be determined because of precipitation of photochemical products. Organic products were identified by gas-chromatography-mass-spectrometry. E.s.r.examin- ation indicated that one-electron transfer from R2S to [U02"]* takes place to yield the species (R2S)2'+ as the identifiable radical product, although a monomeric radical is found under certain conditions. (R2S)2'+ was also observed as a transient species during laser flash photolysis of tetrahydrothiophene in the presence of U02'+ ion, while Me2S, yielded Me2S2'+. Mechanisms of the quenching processes are discussed in terms of fast, largely reversible electron transfer and the intermediacy of exciplexes. Excited-state processes of the dioxouranium(vr) (or uranyl) ion, [UO,]", have been the subject of intensive study over many years for a variety of reasons, both academic and applied, covering the analysis, separation and purification of uranium, including isotopic separation, and they have been reviewed from time to These processes are marked by their extreme diversity, including lumines- cence from the monomeric dictation,'-3 from an excimer4 and from ex~iplexes,~ non-radiative processes uia both thermally activated and non-activated pathways,6 and both reversible and irreversible electron- and hydrogen-atom-transfer processes from suitable which include inorganic and covalent molecules" and saturated, unsaturated and aromatic molecules.273 Interaction with organometallic molecules such as metallocenes' I and metal carbonyls12 appears to involve principally simple electron transfer to give reasonable yields of redox products, Cp,M'+ and M(CO),'+.However, interaction with iodo- and bromo-alkanes, while being kinetically extremely efficient, involves no detectable net charge transfer, and fast, reversible exciplex formation offers the best expla- nation.I3 t On leave from the Institute of Atomic Energy, ORiPI, Swierk, Poland.107108 QUENCHING OF EXCITED URANYL ION Table 1. Quenching of [U022+]* by alkylmetals {[U022+] = 0.08 mol drn-,, medium: acetone (no added acid)) compound (number) I/eV AG&"/kJ mol-l k,/ lo9 dm3 mol-' s-' SnMe, SnEt, SnPr", SnBU", SnPr', SnBu", Sn Et Me, SnBu"Me3 PbMe, PbEtMe, Pb Et, Me, PbEt3Me PbEt, SiMe, SiEt, SiEtMe, Si Bu'Me, GeMe, GeEt, 1 9.69' 2 8.93' 3 8.82' 4 8.76' 5 8.46' 6 8.45' 7 9. I 0' 8 9.00' 1 8.90' 2 8.65' 3 8.45 4 8.26' 5 8.13' 1 9.98' 2 9.78' 3 9.70' 4 9.34' 1 10.02' 2 9.41' ( a ) M=Sn -15.01 -70.75 -78.8 1 -83.2 1 -105.21 -89.08 -58.28 -65.6 1 ( b ) M=Pb -72.75 -9 1.28 - 105.95 -1 19.88 -129.41 ( c ) M=Si +6.25 -8.41 - 14.28 -49.68 ( d ) M=Ge +9.I9 -35.55 0.16 * 0.007 1.7 1 * 0.05 1.40 * 0.05 1.70 f 0.08 4.60 f 0.12 2.96 * 0.06 0.56 f 0.01 0.80*0.015 2.77 f 0.14 3.71 f0.17 5.54 f 0.28 5.48 * 0.17 5.66 * 0.26 < 0.002 0.0243 * 0.0077 0.0096 f 0.0005 0.059 f 0.0029 0.0 178 * 0.0009 0.0429 f 0.0022 a Calculated from AG;, = E"(D/D+) - E(A-/A) - 'AE,,,(A*) [ref. (16)], but Data neglecting the term e;/&r; 1 converted to E"(D/D+) by treatment of ref. (17). from ref. (17). Data from ref. (18). We describe here the results of quenching [UOz2+]* with a range of organometals (Group IVB alkyls), organosulphur compounds and further halogenoalkanes, aug- menting the kinetic work, which is based on 347 nm laser flash photolysis, by product and quantum-yield studies and low-temperature (77 K) e.s.r.spectra in the case of dialkyl sulphides. The results are discussed in terms of established free-energy relationships for excited-state electron transfer. EXPERIMENTAL Laser flash photolysis was carried out at 347 nm (50 ns pulse of 100 mJ energy) as described previously," using either AnalaR grade or purified acetone (which gave similar results) or 50% (v/v) acetone +water as solvents. Quantum-yield measurements were also performed as before." E.s.r. spectra were recorded at 77 K with a Bruker model ER 200 tt spectrometer. Samples were prepared by various methods, all of which gave similar results; typically the liquid thioether was agitated with (moist) crystals of uranyl perchlorate (sometimes with added HC10,) and the organic layer taken and frozen to 77 K prior to photolysis for 1-4 h with a 900 W Xe/Hg point source, the output of which was filtered through both Schott UG5 and Pyrex filters, i.e.hirr = 330-410 nm. Mass spectra were recorded on a Kratos model MS80 spectrometer coupled to a Carlo Erba gas chromatograph equipped with an S.E. 30 column operated at 80 "C.H. 9. AMBROZ, K. R. BUTTER AND T. J . KEMP 109 Table 2. Quenching of [U022+]* by organosulphur compounds ([UO,"] = 0.08 mol dmd3, [HClO,] = 0.30 mol dm-3, medium: acetone) compound (number) I">'/eV AG;,'/kJ mol-,' k2/ lo9 dm3 mol-' s-' ethylene sulphide trimethylene sulphide tetrahydrothiophene pentamethylene sulphide thiophene 2-ethylthiophene 2-propylthiophene thiophene-2-carbonitrile 1,4-dithiane 1,3-dithiane 2-acetylthiophene 3-acetylthiophene thiolacetic acid methylthiocyanate methylthioacetonitrile pro panethiol butanethiol dimethyldisulphide di-n-butylsulphide ( a ) cyclic compounds 1 8.42 - 1 10.60 2 8.65 -90.85 3 8.62 -93.42 4 8.62 -93.42 5 8.87 -7 1.95 6 8.8 -77.96 7 8.6 -95.14 8 9.83 + 10.49 9 8.75 -82.26 10 8.33 - 1 18.33 11 9.20 -43.6 1 12 9.30 -35.03 ( b ) linear compounds I 10.06 +30.24 2 9.96 +2 1.65 3 9.77 i-5:34 4 9.19 -44.47 5 9.15 -47.9 1 6 8.7 1 -85.69 7 8.40 -112.31 2.14* 0.10 2.76k0.14 1.77 f 0.16 1.69 f 0.07 1.50*0.11 1.53 f 0.05 1.91 k0.05 0.24 f 0.04 1.60 f 0.07 2.06f0.10 0.79 * 0.02 0.53 f 0.03 0.48 f 0.04 0.39 f 0.02 1.82 f 0.07 1.44 f 0.1 0 1.81 f0.10 2.83*0.10 I .41 f 0.06 a*b Data from published ref.(19) and (20). ' See footnote ( a ) to table 1; E(D/D+) calculated from I values via correlation E(D+/D+) = 0.89 1-6.04 [ref. (21)]. Simple alkylmetals were obtained commercially, otherwise symmetrical tetra-alkyltin compounds were prepared by the standard reaction of anhydrous SnCl, with the correspond- ing alkyl Grignard reagent in Et20.14 Unsymmetrical compounds were prepared using the appropriate alkylchlorotin compound and the alkyl Grignard reagent.14 Bu'SiMe, was pre- pared from the appropriate alkylithium and SiCI,, followed by further reaction with the appropriate Grignard reagent." The lead alkyls were gifts from The Associated Octel Co. Ltd (Ellesmere Port), to whom we express our thanks.RESULTS LASER FLASH PHOTOLYSIS The lifetime of [U02"]* determined at 510nm in solution was systematically reduced on addition of the various quencher species. Pseudo-first-order rate con- stants were determined at ten concentrations of each quencher to give the statistically averaged values of the second-order quenching rate constant, k2, presented in tables 1-3. Radical-cation spectra were clearly apparent when relatively high concentrations of certain organosulphur compounds were flashed in the presence of aqueous U022+ ion (0.16 mol dmP3); thus tetrahydrothiophene (0.025 mol dmW3) yielded a broad spectrum with A,,, = 465 nm (7 =: 35 ps) essentially identical to that published for the dimer cation while dimethyl disulphide (0.025 rnol dm-3) yielded a similar broad spectrum, A,,, = 460 f 5 nm, attributable by comparison with Me2S2'+, a species known from pulse radiolysis (A,,, = 440 nm in water)23 and e .~ . r . ~ ~ studies.110 QUENCHING OF EXCITED URANYL ION Table 3. Quenching of [UO,,+]* by halogenoalkanes {[UO,”] = 0.08 mol drnp3, [HCIO,] = 0.30 mol dm-3, medium: acetone + H 2 0 (50: 50)) compound (number) IaYb/eV AG”,‘‘/kJ mol-l k,/ lo8 dm3 mol-l s-’ - iodomethane 1 9.54 - 14.40 5.1 1 zk0.31 1 -iodopropane 3 9.26 -38.46 11.30*0.01 2-iodopropane 4 9.18 -45.33 13.4* 0.035 1 -iodobutane 5 9.23 -4 1.04 10.60 f 0.02 1 bromoform 6 10.47 65.45 0.155 * 0.066 bromoethane 7 10.28 49.13 0.326f0.014 I -bromopropane 8 10.18 40.55 0.265 f 0.01 3 2-bromopropane 9 10.07 31.10 0.388 f 0.063 1 -bromobutane 10 10.15 37.97 0.31 *0.01 2-bromobutane I I 10.10 33.68 0.448 f 0.045 1 -iodoethane 2 9.33 -32.40 8.50 f 0.34 Data from ref.(19) and (20). See footnote (c) to table 2. Table 4. Quantum yields for UIv appearance ([UO,”] = 0.08 rnol drnp3, [HCIO,] = 0.30 mol dmP3, irradiation wavelength 401 nm) compound medium #(U‘”> ( a ) organosulphur compounds propanethiol acetone + H20 a butanethiol acetone + H20 di-n-butylsulphide acetone + H 2 0 a 1,3-dithiane acetone + H 2 0 a pentamethylene sulphide acetone + H20 a tetrahydrothiophene acetone + H 2 0 a SiEt, acetone GeMe, acetone iodomethane acetone + H20 1 -bromopropane ( b ) organometallic compounds (c) halogenoalkanes acetone + H 2 0 a 0.0 13 0.0 13 0.023 0.01 1 0.067 0.037 (0.0 1 co.01 <0.005 <0.005 a Acetone +water mixtures were 50 : 50 (v/v).QUANTUM-YIELD DETERMINATIONS These were made for selected compounds, regarded as typical, and are sum- marked in table 4. E.S.R. SPECTRA The photo-oxidation by uranyl ion at 77 K of a number of molecules R2S [R2 = Me,, Et,, Bun2, (CH,),, (CH,), and (CH),] yielded two main paramagnetic species in every case, typified particularly well by Me,S and tetrahydrothiophene. In fig. 1 are shown the radicals from Me2S ( a ) when saturated with uranyl perchlorate (either as wet crystals or dissolved in 1.0 rnol dmP3 HClO,) and ( b ) saturated withH. B. AMBROZ, K. R. BUTTER A N D T. J. KEMP 1 1 1 Fig. 1. Second-derivative e.s.r. spectra obtained on photo-oxidation of Me2S by uranyl perchlorate at 77 K: ( a ) Me$ presaturated with uranyl perchlorate either as crystals (moist) or dissolved in HClO, (1 .O mol dm-3) ; ( b ) Me2S presaturated with uranyl perchlorate dissolved in 70% HC104.Arrow signifies DPPH standard. uranyl perchlorate dissolved in 70% HClO,. Spectrum ( b ) , with a( 12H) = 0.62 mT and g,, = 2.0101 , can be readily assigned to (Me2S)2'+ by comparison with spectral data both in solution and in the solid Spectrum (a), with a(6H) = 2.24 mT and g,, = 2.0041, is regarded as being from a monomeric species. Fig. 2 illustrates the radicals obtained from tetrahydrothiophene ( a ) following saturation with moist uranyl perchlorate prior to photolysis, featuring a monomeric species with seven (?) lines and g,, = 2.0042 [one analysis indicates a(2H) = 3.72 mT and a(2H) = I .86 mT], (c) following saturation with uranyl perchlorate dissolved in 70% HClO, featuring the e s t a b l i ~ h e d ~ ~ , ~ ~ dimeric cation radical with a( 16H) = 0.605 mT and g,, = 2.0103 and ( b ) following saturation with uranyl perchlorate in HClO, (1.0 mol dm-3), when a mixture of the monomer and dimer radicals is obtained.The slightly anisotropic appearance of all spectra is due to the contribution of a minor neutral, sulphur-centred species with high g-tensor anisotropy with g, == 2.06, g,, == 2.00 and g, = 1.97. PRODUCT IDENTIFICATION After prolonged irradiation (36 h) at 364-414 nm of acetone solutions of organotin compounds (0.2 mol dm-3) in the presence of uranyl ion (0.08 mol dm-3), a fine dark green precipitate was formed. The solution was centrifuged to separate the filtrate.The precipitate was washed with acetone and dried at I10 "C. A sample was dissolved in HClO, (6.0 mol dm-3) and its u.v.-visible spectrum taken; a further portion was submitted to analysis by atomic absorption spectroscopy. The sample112 QUENCHING OF EXCITED URANYL ION Fig. 2. Second-derivative e.s.r. spectra obtained on photo-oxidation of tetrahydrothiophene by uranyl perchlorate at 77 K; tetrahydrothiophene presaturated with ( a ) (moist) uranyl perchlorate crystals, (15) uranyl perchlorate dissolved in HC104 ( 1 .O mol dm-3) and (c) uranyl perchlorate dissolved in 70% HC104. Arrow signifies DPPH standard. was found to contain 10% UIv and 45% Uvl (based on the weight of the precipitate); the atomic absorption analysis gave a zero result for Sn.The u.v.-visible spectrum of the filtrate gave only the bands of the UO;+ ion, while the g.c.-m.s. analysis of the photolysate from tetra-n-butyltin indicated the presence of three products, uiz. (i) those attributable to the direct photolysis of acetone solvent, (ii) octane and (iii) heptan-2-one. We could not detect butane but cannot be confident of its absence. DISCUSSION Tables 1-3 indicate highly effective quenching of [U022f]* by a variety of alkylmetals, organosulphur compounds and halogenoalkanes. In fig. 3-5 are presen- ted plots of log k, against I from which the following features are apparent. (i) All plots are linear with the partial exception of data for the organolead compounds, which do tend to a plateau value near the diffusion-controlled rate.The slopes of these plots are as follows: cyclic organosulphur compounds, -0.69 * 0.07 eV-' : organotin compounds, - 1.12 f 0.10 eV-' ; halogenoalkanes, - 1.6 1 f 0.07 eV--'. None of these figures corresponds to that of -16.9 eV-' expected for full electron transfer in the endoergonic region referring to the model devised by Rehm and Weller,16 even though some of the data relate to this region. (The proportionality between the ionisation-energy and electrode-potential scales idnot unity, and various correla- tions have been suggested and reviewed;28 the proportionality constant is in the range 0.7-0.9 depending on the class of compound.) (ii) In the highly exoergonicH. B. AMBROZ, K. R. BUTTER AND T. J. KEMP 113 9.8 9 . 5 9 . 0 8 - 5 Y 00 - 8 - 0 8 .5 7.0 2 R4 Ge 0 1 R4Si and 03 I I 1 1 8 .1 8 . 5 9.0 9 . 5 10.0 I l e V Fig. 3. Dependence of log k2 upon ionization energy ( I ) for quenching of [UOZ2+]* by alkylmetals in acetone solution: 0, lead alkyls; A, tin alkyls; +, silicon alkyls: V, germanium alkyls. Numbering as in table 1 . region the rates generally fail to achieve those expected for electron transfer. (iii) In the endoergonic region the rates generally surpass those expected for full electron transfer. The models for electron-transfer processes developed by Marcus,29 Rehm and WellerI6 and Scandola and Balzani3' have been well reviewed by Eberson2* and will not be reiterated in full here. The kinetic scheme is as follows: k2 I k32 which yields for the observed quenching rate constant, k2, kl2 k2 = k231 I4 QUENCHING OF EXCITED URANYL ION - \*OX6 3 X I I 1 1 I 1 1 I \ I 1 8-6 9 - 0 9.4 9 -8 IleV Fig.4. Dependence of log k2 upon ionization energy (I) for quenching of [UOz2’]* by organosulphur compounds in acetone solution: 0, cyclic compounds; x , linear compounds. Numbering as in table 2. For the electron-transfer step, k 2 3 , we have k23 = K O exp ( - A Gg3/ RT) k 2 3 / k32 = K23 = exp ( - AG023/ RT). (3 1 (4) Simplifying assumptions lead to eqn ( 5 ) , with k 2 1 / ~ 0 often being taken as 0.25: ( 5 ) kl2 1 +KO [exp (AG:,/RT) +exp (AG&/RT)] k, = k2 I while AG;3 is related to the calculable AGz3 by an empirical relation: where AGi3(0) is the ‘intrinsic barrier’ to electron transfer, ie. the activation free energy when AG& equals zero. AGi3(0) was taken originally’6 as 10.05 kJ mol-’, and such a value operates satisfactorily for a large number of electron-transfer systems,28 although several have emerged requiring much higher values of AGl3(0),H. B.AMBROZ, K. R. BUTTER AND T. J. KEMP 115 9 . 2 9 . 6 10-0 10.4 IleV Fig. 5. Dependence of log k2 upon ionization energy ( I ) for quenching of [U0,2']* by halogenoalkanes in acetone +water solution. Numbering as in table 3. e.g. the oxidation of aliphatic amines in MeCN by excited tris(2,2'-bipyridine) complexes of CrIII, when AGi3(0) = 20 kJ m ~ l - ' , ~ ~ , ~ ' and the fluorescence quenching of various aromatic ion radicals, when values > 80 kJ mol-' appear necessary.32 Another system showing strongly deviant behaviour is the quenching of aromatic triplet states by Eu3+, when the maximum or 'plateau' rate in the exoergonic region is ca.lo6 dm3 mol-' s-I, a shortfall attributed to an anomalously small transmission ~oefficient.~~ While the Rehm-Weller treatment predicts a linear region in the plot of log k2 against AG;3 of slope -1/2.303RT when AG023 is highly endoergonic, there are reports of linearity over a much wider range of AG;3, e.g. from -90 to 90 kJ mol-' in the fluorescence quenching of various aromatic molecules by inorganic anions34 and from -25 to 90 kJ mol-' in the quenching of triplet ketones by inorganic anions.35 Linear plots covering a range in AG& of 90 kJ mol-' are also reported by Kuzmin et al. for quenching of 9,lO-dicyanoanthracene in heptane and of exciplexes in benzene.36 These various groups employ the Polanyi equation:" AGi3 = aAGZ3 +p.(7) p is equivalent to AGi3(0), and one may simplify eqn (1) by assuming that k30 >> k 3 2 and kZ1 >> k23, yielding eqn (8): k2 = k 1 2 k 2 3 / k 2 1 - (8)116 QUENCHING OF EXCITED URANYL ION Table 5. Values of a and p from eqn (7) for quenching of [UO,'+]* and other systems system CY P/kJ mol-' [ U 0 22 '1 *-S n R4 1 1.66 f 0.72 [U02*+]*-cyclic R2S 0.046 f 0.004 8.46 f 0.40 [U022+]*-halogenoalkanes 0.1 10 f 0.004 9.69 f 0.20 transition metal organometallic molecules 0.120 f 0.003 26.0 1 f 0.40 ['ArH]*-inorganic anions a 0.1 14 11.6 [3Ar2CO]-inorganic anions 0.207 10.3 0.088 f 0.008 Data from ref. (38); ' data from ref. (35). This may be rewritten as and again as k2=(1 ~ 1 0 ~ ~ ) e x p ( - A G ~ , / R T ) .(10) Plots of our various data in the form log k2 against AG;, (fig. 6) indicate a strong discrepancy with the Weller equation using AG:,(O) = 10.05 kJ mol-'. The pronoun- ced linearities suggested an attempted fit to the Polanyi equation in the form of plots of AG:,, calculated from eqn (lo), against AG;,, which should be linear according to eqn (7). These are shown in fig. 7, together with data for the quenching by metal carbonyls]' and metallocenes" of [U022+]*, and yield values for a and p listed in table 5. The data utilised in fig. 7 cover a wide variation in AG;, and confirm the applicability of the Polanyi equation to luminescence quenching of [Uo2"]*. The values of &,dox in these systems are always low (exceedingly so in the case of the halogenoalkanes), implying either highly efficient back electron transfer between the ion radicals, RHal'+ and Uv or a mechanism based on an exciplex involving weak charge transfer from a lone pair on the halogen atom to U022'. While &.dox as measured by 4(U'") is very low for tetra-alkylmetals, in the case of tetra-n-butyltin both octane and heptan-2-one were found, indicating path- ways (1 1)-( 14): Bun4Sn + [UO,"]* + Bu",Sn" + Uv (1 1) Bu",Sn'+ --+ Bu",Sn' +Bun' (12) 2Bu"' --* n-CgHig (13) (14) Bun' +CH,COCH, (ex solvent) --+ n-CSHI ]COCH3.In the case of the sulphur compounds, @(U'") is appreciably greater, and in a couple of instances the sulphur-centred cation radical was observed directly following the laser pulse. This process was confirmed in other cases by e.s.r.identification of radical products on photolysis of a glassy matrix of the organosulphur compound containing U022+ perchlorate at 77 K. At relatively high acidities the product was unmistakably the dimer cation identified by its g-tensor and the proton- coupling pattern (fig. 1 and 2); at low acidity or in neutral solution another species is produced with intriguing features, namely the proton-coupling pattern and hyper- fine coupling constant expected of the monomeric species R2S'+ but with an isotropicH. B. AMBROZ, K. R. BUTTER AND T. J. KEMP 117 \ - - - - _ _ - - - \ 10.0 " . O i \ \ 1 -._ R,Pb \ R W I D-m--D._ \ \ 2 nn 9 * 0 t 8 . 0 . A - + :: \ I \ I I I I I 1 I 1 I 1 -120 -80 - 40 0 40 80 AG&/kJ mol-' Fig. 6. Plots of log k, against AG& for quenching of [U02'+]* by lead alkyls (O), tin alkyls (A), cyclic organosulphur compounds ( X ) and halogenoalkanes (0).The broken curve denotes rates calculated on the basis of the Rehm-Weller treatment, using AG&(O)= 10.05 kJ mol-'. g-value of only 2.0041 *0.0001 (for both Me,S and tetrahydrothiophene), which is far from that associated with this type of species, e.g. 2.0113 for Me2S.+.39 A carbon-centred radical seems to be excluded by the seven-line and binomial spectrum exhibited from Me,S and a five-line binomial spectrum given by Et,S ( a , = 2.04 mT, g = 2.0029), and we favour a monomeric cation-radical species experiencing a g-shift by virtue of a local environmental effect. The pronounced effect of acidity upon this type of radical is well established for sulphur compounds in solution although in the solid state matrix effects must also be significant in controlling ion-molecule reactions.The minor species found in all spectra is attributed to RS- SR2, which features similar values for the components of the g-tensor.,' Finally, the interaction of [UOz"]* with the rnetallocenes gives a high yield of charge-separated species, with +(Cp2M+) = 0.62 for M = Fe," and the anomalous behaviour of the organometallics must refer to photoelectron transfer rather than an alternative process. In summary, quenching of [UOZ2+]* by sulphur-, metal- and halogen-centred compounds proceeds, with the possible exception of the last, by electron transfer to give U'", cation radicals and, ultimately, organic products derived from these. The relations between k2 and AG;3 appears to fit the Polanyi equation rather well.118 QUENCHING OF EXCITED URANYL ION 4 I t I I I I I -200 -1 60 -120 -80 -40 0 40 80 AG';,/kJ mol-' Fig.7. Plots of AGi3 [where k2 = 1 X 10" exp (-AG:,/RT)] against AG;3 for the quenching of [UO,"]* by tin alkyls (A), cyclic organosulphur compounds ( x ), halogenoalkanes (0) and organo-transition metals (OTM, +); the latter refer to quenching by metal carbonyls" and metallocenes.' We thank British Nuclear Fuels Ltd. and the S.E.R.C. for support of K.R.B. through a CASE award and the S.E.R.C. for a Visiting Fellowship to H.B.A. Mr M. A. Shand began exploratory work on the Uv'-organosulphur systems. We thank Mr H. G. Beaton for the synthesis of a number of organometallic compounds.I E. Rabinowitch and R. L. Belford, Spectroscopy and Photochemistry of Uranyl Compounds (Per- gamon Press, London, 1964). H. D. Burrows and T. J. Kemp, Chem. SOC. Rev., 1974, 3, 139. H. Giisten, in Gmelin, Handbook of Inorganic Chemistry- Uranium (Springer-Verlag, Berlin, 8th edn, 1983), suppl. vol. A6, chap. 3. M. D. Marcantonatos, Inorg. Chim. Acta, 1977, 24, L37. M. D. Marcantonatos and M. Deschaux, Chem. Phys. Lett., 1981,80, 327 and references therein. A. Cox, T. J. Kemp, W. J. Reed and 0. Traverso, J. Chem. Soc., Faraday Trans. 1, 1980, 76, 804. H. D. Burrows, S. J. Formosinho, M. da Graca Miguel and F. Pinto Coelho, J. Chem. Soc., Faraday Trans. 1, 1976, 72, 163. M. D. Marcantonatos, J. Chem. SOC., Faraday Trans. 1, 1979, 75, 2252.K. R. Butter and T. J. Kemp, J. Chem. SOC., Dalton Trans., 1984,923. 0. Traverso, R. Rossi, L. Magon, A. Cinquantini and T. J. Kemp, J. Chem. SOC., Dalton Trans., 1978, 569. S. Sostero, 0. Traverso, P. Di Bernard0 and T. J. Kemp, J. Chem. Soc., Dalton Trans., 1979, 658. R. K. Ingham, S. D. Rosenberg and H. Gilman, Chem. Rev., 1960, 60, 459. ' R. Matsushima, H. Fujimori and S. Sakuraba, J. Chem. SOC., Faraday Trans. 1, 1974, 70, 1702. 10 12 l 3 M. Ahmad, A. Cox, T. J. Kemp and Q. Sultana, J. Chem. Soc., Perkin Trans. 2, 1975, 1867. l 5 L. J. Tyler, L. H. Sommer and F. C. Whitmore, J. Am. Chem. Soc., 1947, 69, 981. l 6 D. Rehm and A. Weller, Isr. J. Chem., 1970, 8, 259. l7 ( a ) R. J. Klinger and J. K. Kochi, J. Am. Chem. Soc., 1980, 102, 4790; (b) C.L. Wong and J. K. 14 Kochi, J. Am. Chem. Soc., 1979, 101, 5593. G. G. Hess, F. W. Lampe and L. H. Sommer, J. Am. Chem. Soc., 1965, 87, 5327. 18H. B. AMBROZ, K. R. BUTTER A N D T. J . KEMP 119 R. M. Rosenstock, K. Drasch, B. W. Steiner and J. T. Herron, Energetics ofGaseous Ions (National Bureau of Standards, Washington D.C., 1977). 2o R. D. Levin and S. G. Lias, Zonisation Potential and Appearance Potential Measurements, 1971-1981, (National Bureau of Standards, Washington D.C., 1982). L. L. Miller, G. D. Nordblom and E. A. Mayeda, J. Org. Chem., 1972, 37, 916. 22 M. Bonifacic, H. Mockel, D. Bahermann and K-D. Asmus, J. Chem. SOC., Perkin Trans. 2, 1975, 675. ( a ) H. Mockel, M. Bonifacic and K-D. Asmus, J. Phys. Chem., 1974, 78, 282; (b) M. Bonifacic, K. Schafer, H. Mockel and K-D. Asmus, J. Phys. Chem., 1975, 79, 1496. R. L. Petersen, D. J. Nelson and M. C. R. Symons, J. Chem. SOC., Perkin Trans. 2, 1978, 225. 25 M. C. R. Symons, J. Chem. SOC., Perkin Trans. 2, 1974, 1618. 26 B. C. Gilbert, D. K. C. Hodgeman and R. 0. C. Norman, J. Chem. SOC., Perkin Trans. 2,1973, 1748. 27 W. B. Gara, J. R. M. Giles and B. P. Roberts, J. Chem. SOC., Perkin Trans. 2, 1979, 1444. 28 L. Eberson, Adv. Phys. Org. Chem., 1982, 18, 79. 29 R. A. Marcus, Annu. Rev. Phys. Chem., 1964. 15, 155. 30 F. Scandola and V. Balzani, J. Am. Chem. SOC., 1979, 101, 6140. 3 1 R. Ballardini, G. Varani, M. T. Indelli, F. Scandola and V. Balzani, J. Am. Chem. SOC., 1978, 100, I9 21 23 24 72 19. J. Eriksen, H. Lund and A. I. Nyvad, Acta Chem. Scand., 1983, B37, 459. N. Sabbatini, M. T. Indelli, M. T. Gandolfi and V. Balzani, J. Phys. Chem., 1982, 86, 3585. 3 2 33 34 H. Shizuka, M. Nakamura and T. Morita, J. Phys. Chem., 1980, 84, 989. 35 H. Shizuka and H. Obuchi, J. Phys. Chem., 1982, 86, 1297. M. G. Kuzmin, N. A. Sadovskii and I. V. Soboleva, Chem. Phys. Lett., 1978, 56, 519. J. Horiuchi and M. Polanyi, Acta Physiochim. URSS, 1935, 2, 505. 38 H. Shizuka, T. Saito and T. Morita, Chem. Phys. Lett., 1978, 56, 519. 39 J. T. Wang and F. Williams, J. Chem. Soc., Chem. Commun., 1981, 1184. 36 37

ISSN:0301-7249    

DOI:10.1039/DC9847800107

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1984, 78,12 1 - 133 A Theoretical Interpretation of the Electron Spin Resonance Spectrum for BH, Radicals and Comparison with Results for CH,' Radical Cations BY THOMAS A. CLAXTON, TSING CHEN AND MARTYN C. R. SYMONS" Department of Chemistry, University of Leicester, Leicester LE 1 7RH AND CHRISTOPHER GLIDEWELL Department of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9s Received 26th June, 1984 Exposure of sodium borohydride to 6oCo y-rays at 77 K gave a novel radical identified as BH, from its e.s.r. spectrum. This is dominated by a large hyperfine coupling (ca. 105 G) to two protons, but smaller couplings to two other protons and a boron atom were also detected. The proton hyperfine coupling constants are surprisingly similar to those for the recently identified CH; radical, which also has C,, symmetry. These results are strongly supported by ab initio calculations which confirm that such a distortion is most favourable.The prototype alkane radical cation CHB is a well established gas-phase species. Optical and photoelectron spectroscopic studies concur in assigning a structure of D2d symmetry when the ion is in its ground electronic state. This is maintained to be a consequence of a large first-order Jahn-Teller distortion6y7 of the ,T2 orbitally degenerate electronic ground state expected for a CH,' anion of tetrahedral symmetry initially formed by vertical ionisation of the neutral precursor, CH,. Only recently' has an observation, using e.s.r. techniques, of a species thought to be CH,' been reported despite the established observations by e.s.r.of a wide range of alkane cations9-I6 including C3H8+ and C2H6+. The e.s.r. spectrum was more consistent with a species of C2, symmetry because of the spectrum assigned to the species C2D2Hl, although the four protons in the spectrum of CH,' were equivalent. Prior to this, we reported an e.s.r. study of a radical thought to be the isoelectronic BH,." This species has only two strongly coupled protons and therefore has the same C2, symmetry as that of matrix-isolated CH,' cations. The unexpected behaviour of CH,', and in particular the observation of only one of the possible C2D2Hl isomers, has stimulated a further effort to characterise the isoelectronic BH, radical. The importance of the original observation of the BH, radical anion, which is isoelec- tronic with CH3,'8919 should be noted since it is possible that BH, is an intermediate in the production of BH,: BH, + hv -+ BH, +e&.(1) Sprague and Williams,20 using a tetramethylammonium salt, confirmed the solid-state result for BH,, but the more recent liquid-phase study of Roberts and coworker^^^-^^ is important since it seems clear that BH, is an intermediate in reaction (1). However, it was not detected, probably because the reaction was too rapid.23 Earlier, Baxendale and BH, +NH, -+ BH, +NH,' (2) in a pulse-radiolysis study of 121122 E.S.R. SPECTRUM OF -BH4 aqueous borohydride ions, observed transient optical bands at 260 and 400 nm, which were tentatively assigned to the BH, radical.An attempt to verify this assignment2, by low-temperature radiolysis and e.s.r. detection was complicated by the presence of other paramagnetic species, in particular the BH, radical, so that only the outermost features of the assumed BH, radical species were d e t e ~ t e d . ' ~ Concurrently with this e.s.r. study, both semi-empirical and ab initio calculations have been carried out on the isoelectronic series CH;, BH, and BeH, and the related A1H4 to assist in the identification. GEOMETRY OF CH; AND BH, The parent molecules, CH, and BH,, have Td symmetry and electronic ground state (1 a1)2(2a,)2( 1 t2)6;1A,, both theoretically and experimentally. The tetrahedral structure gives rise to an orbitally degenerate electronic state (1 ~ , ) ~ ( 2 a , ) ~ ( 1 t2)5;2T2, on removal of an electron. This must distort in accordance with the Jahn-Teller theorem.677 Apart from a totally symmetric vibration, vl, there is one doubly degenerate vibration, v2, and two triply degenerate vibrations, v3 and v,.Distortion along the normal coordinate of u2 effectively 'flattens' the molecule, lowering its symmetry to DZd (until it is eventually planar with symmetry D4h). The distortions induced by vibrations v3 and v4 lower the symmetry to C,," or C2".*' It is questionable whether the influence of vibrations in the parent molecules should be used in deciding which is the final distorted structure, particularly in 'trapped' radical situations frequently found in e.s.r. experiments. However, it is not diffcult to show qualitatively that all these distortions lead to structures which have non-degenerate electronic states.QUALITATIVE MOLECULAR-ORBITAL THEORY If a molecule has sufficiently high symmetry it is possible to predict its molecular- orbital energy diagram using qualitative arguments with a degree of confidence. The prime purpose of LCAO-MO theory is to form linear combinations of atomic (basis) orbitals such that all resultant combinations are orthonormal and all reson- ance integrals are zero. If the problem is formulated in matrix terminology this is equivalent to a matrix diagonalisation. One procedure, due to Jacobi, which was in common usage, was to take successively pairs of rows (and columns) and diagonalise the 2 x 2 matrix until all off -diagonal elements were zero. The procedure used here is an adaption of that matrix-diagonalisation approach.Qualitative MO models are still used extensively in chemistry since these models enable the more important aspects to be extracted and discussed. More complete theoretical calcula- tions use extended basis sets which are not always readily interpreted into chemical terminology. For our qualitative description it is also proposed that for molecules of the type ML, (M is a central atom bonded to all the ligands L), if all bond lengths are taken to be equal, it should be possible to neglect the central valence orbitals entirely. These orbitals (the 2s and the three 2p orbitals) of both boron and carbon provide a spherically symmetric core, and as long as all the bond lengths are equal their inclusion in the MO energy diagram should not alter the order of the orbitals whose symmetry is defined solely by the hydrogen atomic orbitals.The labelling scheme used is defined in fig. 1. Distorting a tetrahedral molecule, symmetry Td, to C3v symmetry can be achieved, without altering bond lengths, by increasing or decreasing the angles 012, 013 and 614 in their respective planes equallyT. A. CLAXTON, T. CHEN, M. C. R. SYMONS AND C. GLIDEWELL 123 Fig. 1. Numbering used to label atoms and angles in BH4 and CH;. * I - 9 = 109" 28' I:::- -...- t I- , , \ , ,-I---\ Fig. 2. Qualitative molecular-orbital energy diagrams for a tetrahedral arrangement of orbitals undergoing the distortions indicated. (a) C,, symmetry: 4, = 8,, = OI4 = 8 ; (b) DZd symmetry: 8,, = 834 = 8; ( c ) C,, symmetry.I24 E.S.R.SPECTRUM OF *BH4 (all equal to 8). This divides the ligands into two sets, an equilateral triangular arrangement (orbitals 2, 3 and 4) and orbital 1 placed perpendicularly above the centre. The equilateral triangular arrangement is well known in qualitative MO theory and gives rise to a low-energy nodeless orbital [labelled 0 in fig, 2 ( a ) ] and two degenerate orbitals with nodes that pass through orbital 1 (implying that the corresponding resonance integrals are zero and are therefore suitable molecular orbitals). The remaining molecular orbitals are obtained from the linear combina- tions of orbital 1 with the nodeless orbital 0. The MO energy diagrams are drawn in fig. 2 ( a ) . Note that the energy gap between the nodeless orbital 0 and the degenerate orbitals arising from orbitals 2 , 3 and 4 increases as 8 increases, whereas the energy gap between the linear combinations of nodeless orbital 0 with orbital I decreases as 8 increases (since the distance between orbital 1 and the triangular plane increases). The effect of these changes is demonstrated in the 3 diagrams in fig.2 ( a ) , showing that 8 should be less than the tetrahedral angle if the ground electronic state of CH: and BH4 is to be orbitally non-degenerate. The final MO have the same symmetries as the s and p atomic orbitals on the central atom, and the pairwise linear combinations are expected to give bonding orbitals of the same symmetry in the same order as shown. The D2d structure [fig. 2 ( b ) ] is obtained by altering, by the same amounts, the angles 8,, and 834 in their respective mutually orthogonal planes.It is convenient here to divide the ligands into two sets: I , 2 and 3 , 4. Once it is noticed, from symmetry, that the antibonding combinations of both these orbital pairs have zero resonance integrals with all other orbital combinations, the MO energy diagrams are almost trivial to construct. The ground state is only orbitally non-degenerate if the bond angles are increased above their tetrahedral values. Eventually on increas- ing these angles the molecule becomes planar ( D 4 h symmetry) still with a non- degenerate ground state. and 834 vary differently. This structure always appears to predict a non-degenerate ground state. Calculations have been performed on many of these structures.Dixon4 has found the minimum-energy configuration of CH: to have C,, symmetry, when all bond lengths were constrained to be the same, and that the energy minimised at 8 = 8,, = 813 = OI4 = 96.9" with a non-degenerate electronic state. This is in accord- ance with the diagram for 8 < 109" 28' in fig. 2 ( a ) . Similar calculations on D2d structures gave an optimised non-degenerate ground-state structure with 8 = = 834= 141.2" as predicted by the 8> 109'28' structure in fig. 2(b). More extensive calculations have been published by Meyer,26 but comparison can be made only with the D2d symmetry structure as this requires equal bond lengths. Here 8 = 140.8", in good agreement with Dixon's results. Arents and Allen,*' using basis sets with and without polarisation functions, also found that the total energy increased in order DZd < C3, < D 4 h < T d .Without any geometrical constraints the C2, symmetry is lowest in energy followed in order by DZd and C,,, in contrast to the prediction that the D2d state is lowest in energy.28 The qualitative MO energy diagrams predict that for the C,, symmetry the unpaired electron is to be found in the antibonding combination of the two closest hydrogen atoms. This will form, in turn, a bonding combination with a p orbital on the central atom, but nevertheless the unpaired electron is in an orbital which has a nodal plane containing the heavy atom and hydrogen atoms I and 2 (if 8,*> 109" 28'). In the rest of this paper it is assumed that hydrogen atoms 1 and 2 lie in the nodal plane of the unpaired electron.The similarities of the environment The C,, structure is related to the DZd structure in that the anglesT. A. CLAXTON, T. CHEN, M. C . R. SYMONS AND C. GLIDEWELL 125 of these hydrogen atoms and those in CH,' should be noted. For example, hydrogen atoms 1 and 2 now 'see' a distorted 'p-type' orbital inclined towards atoms 3 and 4 owing to orbital overlap. The anisotropic tensors should reflect this distortion. Although this is a simplified picture is it is supported by our quantitative calculations. THEORETICAL CALCULATIONS The unrestricted Hartree-Fock (UHF) method29 was used. A UHF wavefunction T is an eigenfunction of the S, operator with eigenvalue s = O S ( p - q ) ; p and q are the number of electrons with spins a and p, respectively.It is not an eigenfunction of the S2 operator with eigenvalue s( s + 1) but a projection operator P can be defined such that S2(PT) = s(s+ l)(P?P). Although not a trivial extension of the UHF procedure, it is possible to optimise the function PT = Q, with respect to the energy, in which case Q, is known as the extended Hartree-Fock (EHF) wavefunction. Unfortunately the EHF method consistently predicts hyperfine coupling con- s t a n t ~ ~ ' - ~ ~ which are too large, particularly where spin polarisation is expected to be dominant. The reasons for this have been discussed before32 but it was concluded that satisfacory results could be obtained using PfP. However, it is much simpler partially3' or completely34 to annihilate only the major unwanted spin component in fP (with spin s + 1) without changing the results significantly. The large differences r e p ~ r t e d ' ~ for some radicals using annihilation and complete projection procedures were not confirmed.32 Here the results are quoted from the UHF wavefunction after partial annihilation of the quartet spin state (and labelled UHFAA).It is possible to express the UHF wavefunction as a very limited configuration interaction (CI) expansion using the UHF natural orbitals (NO) as basis functions to construct the configuration^.'^ The coefficients in the CI expansion are calculated from the occupation numbers of the NO. By including all possible single- and double-replacement configurations it is easy to extract the doublet-state wavefunc- tion which should be an approximation to the completely projected PT wavefunc- tion.The hyperfine coupling constants calculated from this CI expansion are also given (labelled the ABC method after Amos, Beck and Cooper"). Previous work has shown that the configurations used (only 54 in these calculations) by the ABC method generally correspond to those with large coefficients in the conventional singles and doubles CI method (which would require over 10 000 configurations here). The conventional CI method would not be appropriate to use initally in this investigation. Previous calculations on BeH3' showed, however, that Moller-Plesset theory3' will give energy corrections that approximate to the conventional (S + D)CI energies. Therefore Moller-Plesset theory, up to third order, was used here.Various levels of approximation were also used. Semi-empirical calculations were made using the MNDO with, where appropriate, the parametrization for boron.40 All the rest of the calculations were at the a6 initio level. Calculations on CH: and BH4 were carried out (on a CDC Cyber 73 computer) using the HONDO~' integral package supplemented by routines to calculate the integrals necessary for the anisotropic coupling constants.42 The basis sets were of double-zeta quality and generally incorporated polarisation functions: ( 9 , 5 , 1)43 contracted to [4,2, 1]44 on carbon or boron and (4, 1) contracted to [2, 13 on hydrogen. The d-orbital gaussian exponent was chosen to be 0.98 for both carbon and boron and the p-orbital gaussian exponent on hydrogen was 1.0. The main purpose of the HONDO calculations was to provide an independent check (with a different basis126 E.S.R.SPECTRUM OF *BH4 Table 1. Calculated molecular geometries (see fig. 1, all bond lengths in A) description of calculation molecular radical symmetry parameter MNDO 4-31G* 6-3 1 lG** [4,2, 11 BH4 AlH4 CH: c 2 v D2 d c3 u c2, O2 d c3 v BeH, c 2 v D2 d c3 v c2 v D2 d 1.094 1.205 116.9" 62.2' 143.3' 1.135 1.093 1.164 1 17.5' 1.157 1.267 122.6" 58.3' 142.0' 1.192 1.155 1.223 1.493 1.343 46.9" 1.380 1.342 1.417 119.5' 123.1' 149.7' 121.7' 1.401 1.540 124.5" 45.5" 147.2' 1.446 1.074 1.164 122.9' 59.4" 139.9" 1.1 11 1.075 1.147 117.7' 1.182 1.275 125.8' 55.0" 138.7" 1.219 1.183 1.261 119.4" 1.534 1.417 126.3' 49.8" 144.6" I .459 1.417 1.511 1 S95' 1.752' 121.2" I 24.4'' 56.8"' I 52.2"' 1 .662' 1.075 1.074 1.176 1.169 124.7' 124' 55.2' 56' 1.112 140.4' 1.182 1.183 1.288 1.2289 128.4' 129.5" 48.4" 47.5" 1.22 1 1.184 1.266 139.3" 119.9' 1.528" 1.41 8" 128.0"" 46.4"" 1.460 144.5' Using 6-3 lG* basis set.The 6-31 IG** basis set calculates BeH, to dissociate to BeH; and H2. ' Using 6-21G basis set. set of orbitals) on the following calculations at the a6 initio level and to calculate anisotropic hyperfine coupling constants at the optimum geometries. Only C,, symmetries were studied. Most calculations were carried out using the GAUSSIAN-80 system45 employing the basis sets internal to this system on a VAX 11/780 computer. For each molecular system (table 1) the energy was minimised both without geometric constraints and also subject to the constraints of particular point groups.For CH,', BH, and BeH, the geometry optimisations were made using both the 4-3 l G*46 and 6-3 1 1 G**47 basis sets although the Moller-Plesset corrections3'were calculated for 6-3 1 1 G** set only. A study of the residual forces on the nuclei in these optimisations indicated that the C,, and D2d energy minima are genuine for CH,', BH4 and BeH, but theT. A. CLAXTON, T. CHEN, M. C. R. SYMONS AND C. GLIDEWELL 127 Table 2. Calculated total energies at optimised geometriesa UHF calculations Moller-Plesset MNDO calcn (4-31G*) (6-3 1 1 G**) (6-31 1G**) A H ; radical symmetry /kJ mol-' hartree O2 d GUb O2 d C3vb 1148.7 1140.8 1234.7 222.4 247.1 33 1.2 4.1 69.9 136.6 376.5 347.2 397.0 -39.7 15 8005 -39.71 1 8322 -39.677 3559 -26.856 92 19 -26.838 8689 -26.8 17 0772 -16.819 3901 - 16.789 866 1 -16.7799318 -244.066 6702g -244.039 6729 d -39.778 9 138 -39.769 22 13 -26.900 1894 -26.876 7939 -26.855 2967 d - 16.8 16 4966 C C -39.935 7199 -39.930 2536 C -27.039 2238 -27.020 9555 -26.992 2704 C - 16.939 9852 C a Geometries fully optimised in MNDO calculation, and in SCF calculation at 4-31G* Imposed At 6-31 1G** level, At 6-21 G level, optimisation indi- and 6-3 1 1G** geometries: Moller-Plesset energies calculate at SCF geometries.C3, symmetry does not yield a geniune minumum. optimisation indicated dissociation: BeH, + BeH, + H2. cated dissociation: AlH4 + AlH +H,. Not studied. C,, minima are not. In the case of BeH, the C,, isomer was calculated to be found using the 4-3 1 G* and 6-3 1G* basis sets but it dissociated into BeH, and H2 using the 6-31 lG** set.This may be contrasted with the behaviour of calculations on AlH4 (using a 6-21G48 basis set), the heavy congener of BH4, which indicated genuine minima for the C2, and D 2 d symmetries but a tendency to dissociate to AlH3 and H for the C,, symmetry. THEORETICAL RESULTS Table 2 lists the energies at the optimised geometries given in table 1. The calculations which were used to obtain the anisotropic coupling constants (fig. 3) are collected in table 3 (for CH,') and table 4 (for BH4). All coupling constants are in mT (1 G = T). DISCUSSION OF THEORETICAL RESULTS Tables 1 and 2 clearly show that, at all levels of theory used here, the radical BH4 has its lowest energy when the symmetry is C,,.The alternative D 2 d structure is always calculated to be significantly higher in energy ranging from ca. 25 kJ mol-' calculated in the MNDO approximation through ca. 50 kJ mol-' using Moller- Plesset third-order perturbation theory to ca. 60 kJ mol-' for the largest calculations which use a basis set of double zeta quality plus ploarisation functions. In contrast the analogous results for CH,' show that the difference in energies between the CZu and D2d symmetries are much closer to the extent that the MNDO theory calculates DZd to be lower in energy. All studies of the photoelectron spectrum of CH,' have been i n t e r ~ r e t e d ~ - ~ in terms of a Jahn-Teller distortion of the ground state from Td128 E.S.R.SPECTRUM OF -BH4 Table 3. Results for CH: (1 a.u. = 0.0529 nm; 1 G = lop4 mT): R , = 2.03, R2 = 2.21 a.u., 9, = 124", O2 = 56" isotropic hyerfine coupling constants/mT basis set total energy total spin calcn on carbon /hartree aC a H I ( s2> ~ ~~ UHF UHFAA ABC RHF" UHF UHFAA ABC R H F ~ ~~ -39.775 532 2.548 -39.777 5 18 0.89 1 0.889 -39.745 93 -39.775 593 -39.766 860 3.064 -39.768 353 1.062 1.059 -3.369 12.350 0.755 212 -1.107 11.615 0.750015 -1.105 11.615 0.75 0.75 0.75 -3.714 12.416 0.754616 -1.219 11.710 0.750013 -1.217 11.710 0.75 Ref. (27). D2d symmetry (no polarisation functions on H). Ref. (26). R , = 1.175 A, R2 = 1.077 A, 8, = 121.9", 8, = 56.3": [3, 13 on H. 12, I] on hydrogens. Table 4. Results for BH,: (1 a.u. = 0.0529 nm; 1 G = mT): R , = 2.235 a.u., R2 = 2.44 a.u., 01 = 129.5", 02 = 47.5" isotropic hyperfine coupling constants/mT basis set total energy total spin calcn on boron / hartree a B 1 OHz ( S 2 > UHF [4,2, 11 -26.891 799 1.387 -2.594 12.442 0.754 608 UHFAA [4,2,1] -26.893 244 0.485 -0.854 11.622 0.750 012 ABC [4,2, I ] 0.484 -0.853 11.622 0.75 UHF [4,2,0] -26.885 053 1.809 -2.730 12.410 0.754 443 UHFAA [4,2,0] -26.886 345 0.627 -0.898 1 1.585 0.750 0 1 1 ABC [4,2,0] 0.626 -0.897 11.585 0.75 to DZd symmetry.A set of calculations designed to find an acceptable transition-state structure between the two isomers were carried out. These proved unsatisfactory. It seems that an extensive exploration of the potential-energy surface in the C2u/ D2d region is necessary before the dynamic structure of CH; will be understood.All the radicals studied did not give a genuine minimum for the C3, structure but, as with BH4, calculations for both BeH, and AlH4 indicated that the structure with C2, symmetry was much more stable than D2d. This pattern of behaviour may be contrasted with (in a chemical sense) the closely related and formally isoelectronic permethyl cations Me4Mf (M =C," Si,49 Ge,49 or Sn"). None of these cations adopt a DZd structure in frozen solutions, but Me4C+ and Me4Sn+ both distort to give C,, products while Me4Si' and Me4Ge+ give C2, products. Although the heavy-atom basis set [4,2, 11 was used to obtain the optimum geometries the basis set [4,2,0] ( i e . the d function was neglected) gave almost identical geometries and this latter set was used to calculate the anisotropic coupling constants.The components are described later in fig. 4. As was indicated above the deviation of the tensor components from the bond axes on hydrogen atoms 1 and 2 is eniphasised by the dotted lines. The high symmetry of the radicals CH3T. A. CLAXTON, T. CHEN, M. C. R. SYMONS AND C. GLIDEWELL 129 and BH, requires that one of the diagonalised components of the anisotropic tensor on each hydrogen atom should lie along its chemical bond to the heavy atom. The corresponding components of the tensors on hydrogen atoms 1 and 2 in both CH,' and BH, intersect behind the heavy atom, which qualitatively suggests that the 'centre of gravity' of the p orbital has moved towards atoms 3 and 4. For CH; this is only 1 nm removed from the carbon atom, but the shift is 0.025 nm from boron in BH,.Although changes in bond lengths between these two species must account for some of this difference, the bulk of it must be associated with the electronegativity difference of the two heavy atoms. The ABC results are included (no energies are calculated) to show that the UHFAA method gives hperfine coupling constants very close to the fully projected values. EXPERIMENTAL The samples were prepared, and immediately used for e.s.r. study, from the best commer- cially available grades of NaBH,, NaBD, and KBH,, which were opened and handled in a dry atmosphere. After cooling fine powders of these materials to 77 K they were irradiated at the same temperature in a Vicrad 6oC0 y-ray source for up to 2 Mrad at a dose rate of ca.1 Mrad h-'. Maintaining the same temperature the spectra were measured on a Varian El09 spectrometer. Spectra were recorded at ca. 4 K, using an Oxford Instruments liqud- helium cryostat. Samples were annealed while the e.s.r. spectrum was monitored constantly and were subsequently recooled to 77 K for detailed study. E.S.R. RESULTS The second derivative of the e.s.r. absorption spectrum obtained from a y- irradiated sample of NaBH, is reproduced [fig. 3(a)] together with a spectral simulation5' [fig. 3 ( b ) ] using the following parameters: AII('H)= 10.52mT, All('H)=2.03 mT, AII("B)=4.34mT, linewidth = 0.9 mT A,('H) = 0 mT, linewidth = 0.6 mT. The spectrum from a sample of KBH, similarly treated was so similar that there is no significant difference in these parameters. The spectrum of NaBD, is unfxtu- nately complicated by the existence of extra, unassigned weak lines, some from incompletely deuterated material and others from the "BD, radical.A,('H) = 10.80 mT, A,("B) = O mT, DISCUSSION The hyperfine coupling constants from the spectral simulation should be regarded as only accurate to 2-3%, as variations of this magnitude do not alter the acceptability of the 'fit' significantly. This is partly due to the fact that the centre of the observed spectrum is contaminated with another species since on annealing above 77 K these features grew at the expense of those regions which could be simulated and eventually gave the spectrum assigned to the BH, radical. Cooling the sample, before annealing to ca.4 K , gave a general broadening of the features. Since the e.s.r. spectra assigned to the radicals CHI: and CD2H2f have some unex- plained characteristics it is not surprising that BH, also gives a spectrum which has130 E.S.R. SPECTRUM OF .BH4 + 321713 (9.139GHz) V Fig. 3. ( a ) Second-derivative e.s.r. spectrum of finely powdered NaBH, at 77 K. ( b ) Computer simulation using the parameters given in the text. certain peculiarities. For example the perpendicular features require a much nar- rower linewidth in the spectral simulation than the parallel features. This is not uncommon in such powder spectra. This may be due to some specific motional narrowing, perhaps arising from rapid selective isomerisation. The zero (or very small value) of A,("B) is not unreasonable, being a characteristic of 7~ radicals.However, it is more difficult to rationaiise a zero value for the perpendicular couplings of the two weakly coupled protons. This latter value may be averaged since the two perpendicular components (x, y ) could conceivably have different signs. Alternatively, outer components of the expected triplet may be greatly broadened. However, these small coupling constants do allow the experimental isotropic coupling constants to be calculated from these values with more certainty than usual from powder spectra since ambiguity with the relative signs of the observed values does not arise. The isotropic coupling constants are la('H)I =* 10.7 mT, la('H)I =0.7 mT and la("B)I = 1.4 mT, the smaller proton coupling being very uncertain.At this stage we would like to give prominence to the pioneering work of Knight et aLS on the e.s.r. spectrum of the CH; radical, which comprised an isotropic quintet with a ( ' H ) = 5.48 mT. In marked contrast, the deuterated species CD2HZ did not show such averaging, the deuterons being exclusively in the nodal plane positions and the two protons in the strongly coupled sites. The hyperfine coupling constants,T. A. CLAXTON, T. CHEN, M. C. R. SYMONS AND C. GLIDEWELL 13 1 including the derived proton coupling constant, for this deuterated species are la('H)) = 12.17 mT, la('H)I =0.222 mT and la('H)I = 1.46 mT. Since the smaller proton coupling certainly arises from the nodal-plane positions these are probably negative. This leads to an average proton hyperfine coupling constant for CH,' of 5.36 mT which is very close to that actually observed.The averaged value for BH4 is very similar, 5 mT, which draws attention to the marked similarity between the calculated proton coupling constants for CH; (table 3) and BH4 (table 4). It is well known that molecular vibrations can make significant contributions to the observed hyperfine coupling constants, particularly if the nuclei concerned lie in the nodal plane of the orbital containing the unpaired electron. F e ~ s e n d e n ~ ~ drew attention to this possibility for the I3C coupling of the radical CH3, where the observed hyperfine coupling constant at 77 K [a(C) = 3.85 mT] included as much as 1 mT which would be associated with the effect of a zero-point out-of-plane vibra- tional motion.A similar explanation was used to interpet the discrepancy between the calculated and observed coupling constant, aN, for the isoelectronic radical NH,' by theoretically estimating the vibrational c~ntribution.~~ CH: and BH, could be similarly affected since two of the hydrogen atoms (1 and 2 in fig. 1 if 012 > 109'28') as well as the heavy atom in each radical lie in the nodal plane of the unpaired electron. The effect on the hydrogen nuclei of any 'out-of-plane' vibration is expected to be small (using past experience) but the heavy-atom coupling constants would be significantly increased. In the procedure used to calculate the optimum geometries all changes in the structure reflected the CZv symmetry and therefore it is difficult to estimate the effect of such a vibration.However, plotting hyperfine coupling constants as a function of 8, 4, rl and r2 shows, in general, a linear relationship indicating only a negligible contribution to the coupling constants for those molecular vibrations which use these distortions. There is one exception to this and that is the effect of the variation of 8 on the coupling constants to protons 1 and 2, which is U-shaped about the value of 8 corresponding to the minimum-energy value. This implies that these coupling constants would be more negative than observed (in opposition to the effect of an out-of-plane vibration described above). It would be reasonable to suggest that the calculated hydrogen isotropic hyperfine coupling constants for the rigid molecule can be compared directly with experiment, but the heavy-atom coupling constants may be significantly larger than calculated.From table 4 the calculated proton hyperfine coupling constants (UHFAA using a [4,2, I] basis set) agree well with those derived from experiment (above). The difference between theory and experiment for the boron isotropic hyperfine coupling constants is larger: 1.4 mT from experiment and 0.48 mT from theory, which may be due to one of the molecular vibrations. Such a comparison is unfortunately not yet available for CH,'. However, the agreement between theory (see table 3, UHFAA using a [4, 2, 11 basis set) and experiment for the proton coupling constants is very good. a(I3C) is predicted to be significantly larger than 0.9 mT.Although spectral interpretation does present some difficulties, the hyperfine coupling constants derived from it are close to the theoretical values. The hyperfine coupling constants for boron are [if we use the 'experimental' isotropic coupling constant of 1.4 mT and the anisotropic coupling constants from fig. 4(b)] A,, = 3.2 mT, A, = 0.4 mT and Ayy = 0.6 mT, which confirms that our need to set A , = 0 mT is reasonable. Also from fig. 4(b) the same argument can be applied to give the expected hyperfine couplings for protons 1 and 2, but A, == 0 mT for the weskly coupled protons does not seem reasonable. There is no experimental evidence to suggest a mechanism by which this result may be explained. The theoretical calculation of anisotropic coupling constants is normally considered to be much132 E.S.R.SPECTRUM OF *BH4 1.274 \ -2,110 /--0.22; -2.232 -\%4 31 \ t / ' t 2 1-99 P -1.149 +-- -0.841 z -0.555 '\r 0's87 Fig. 4. Anisotropic tensors ( B ) for ( a ) CH,' and ( b ) BH4. Numbers not pointed to by an arrow (this is an axis rather than a direction) refer to a tensor component whose axis is perpendicular to the paper. The dotted lines are a continuation of the axis of these selected components discussed in the text. more reliable than isotropic coupling constants, at least at this level of approxima- tion. CONCLUSIONS It is concluded that the observed species is consistent with a radical containing two strongly coupled and two weakly coupled protons and one boron atom. The qualitative and quantitative descriptions of the structure of a radical with a formula BH4 predict a species which should show two strongly coupled and two weakly coupled protons because the unpaired electron occupies an orbital which involves the boron 2 p atomic orbital and two of the protons.The remaining protons are in the nodal plane. The atoms in the nodal plane would be expected to have coupling constants similar to (but probably less in magnitude) than those for BH,. It has already been discussed that the unpaired electron may be considered to be a p orbital distorted towards hydrogen atoms 3 and 4. In view of the discussion and in particular the large measure of agreement with the quantitative calculations there seems little doubt about the correctness of the identification.Nevertheless the possibility of alternative radicals must not be over- looked. These must include BH2, HBO- and H2B0 radicals. However, it would be necessary to include an extra species to account for the splittings assigned to two weakly coupled protons. The possibility that this could arise from a coupling to 23Na nuclei, as found, for example, with COY can be discounted since the same spectrum is obtained if KBH4 is using as the starting material. Also BH, is a sigma and should exhibit a large isotropic hyperfine coupling to both the boron nucleus and the protons. The radicals HBO- and H2B0 may be formed from impurities; indeed if the starting material is handled in a damp atmosphere over longer periods of time before irradiation increasing yields of HBO- radicals are observed.In any case the study of HBO- radicals in KBH4'9 eliminates this possibility. ' B. P. Pullen, T. A. Carlson, W. E. Moddeman, G. K. Schwietzer, W. E. Bull and F. A. Grimm, ' C. R. Brindle, M. B. Robin and H. Basch, J. Chern. Phvs., 1970, 53, 2196. J. Chem. Phys., 1970, 53, 768. J. W. Rabelais, T. Bergmark, L. 0. Werme, L. Karlsson and K. Siegbahn, Phys. Scr., 1971, 3, 13.T. A. CLAXTON, T. CHEN, M. C. R. SYMONS AND C. GLIDEWELL 133 R. N. Dixon, Mol. Phys., 1971, 20, 113. A. W. Potts and W. C. Price, Proc. R. SOC. London, Ser. A, 1972, 326, 165. H. A. Jahn and E. Teller, Proc. R. SOC. London, Ser. A, 1937, 161, 220. H. A. Jahn, Proc. R. SOC. London, Ser. A , 1938, 164, 117. L. B. Knight, J. Steadman, D. Feller and E. R. Davidson, J.Am. Chem. SOC., 1984, in press. M. C. R. Symons and I. G. Smith, J. Chem. Res. (S), 1979, 382. I ” J. T. Wang and F. Williams, J. Phys. Chem., 1980, 80, 3156. I ’ M. Iwasaki, K. Toriyama and K. Nunome, J. Am. Chem. SOC., 1981, 85, 2149. I’ K. Toriyama, K. Nunome and M. Iwasaki, J. Phys. Chem., 1981, 85, 2149. l 3 R. W. Alder, R. B. Sessions and M. C. R. Symons, J. Chem. Res. (S), 1981, 82. l 4 J. T. Wang and F. Williams, Chem. Phys. Lett., 1981, 82, 177. l 6 K. Toriyama, K. Nunome and M. Iwasaki, J. Chem. Phys., 1982, 77, 5891. K. Toriyama, M. Iwasaki and M. Fukaya, J. Chem. SOC., Chem. Commun., 1982, 1293. M. C. R. Symons, Radiat. Phys. Chem., 1976, 8, 381. M. C. R. Symons and H. W. Wardale, Chem. Commun. 1967, 758. 15 17 I 9 R. C. Catton, M. C. R. Symons and H.W. Wardale, J. Chem. SOC. A. 1969, 2622. ’O E. D. Sprague and F. Williams, Mol. Phys., 1971, 20, 375. R. M. Giles and B. P. Roberts, J. Chem. SOC., Chem. Commun., 1981, 360. J. Chem. SOC., Perkin Trans. 2, 1982, 1699. 21 22 J. A. Baban and B. P. Roberts, J. Chem. SOC., Perkin Trans. 2, 1981, 161. 23 J. A. Baban, J. C. Brand and B. P. Roberts, J. Chem. SOC., Chem. Commun., 1983, 315. 24 J. H. Baxendale, A. Breceia and M. D. Ward, Radiat. Phys. Chem., 1970, 2, 167. 25 J. R. Bews and C. Glidewell, J. Mol. Strucr., 1980, 64, 87. 26 W. Meyer, J. Chem. Phys., 1973, 58, 1017. 27 J. Arents and L. C. Allen, J. Chem. Phys., 1970, 53, 73. W. A. Lathan, W. J. Hebre, L. A. Curtiss and J. A. Pople, J. Am. Chem. SOC., 1971, 93, 6377. 29 J. A. Pople and R. K. Nesbet, J. Chem. Phys., 1954, 22, 571.30 B. Burton, T. A. Claxton, J. Tino and V. Klimo, J. Chem. SOC. Faraday Trans. 2, 1979, 75, 1307. 3 1 B. Burton, T. A. Claxton, J. Tino and V. Klimo, J. Chem. SOC., Faruday Trans. 2, 1980, 76, 1655. 3 2 T. A. Claxton, J. Chem. Phys., 1981, 75, 1888. 33 A. T. Amos and G. G. Hall, Proc. R. SOC. London, Ser. A, 1961, 263, 483. 34 A. T. Amos ad L. C. Snyder, J. Chem. Phys., 1964, 41, 1773. 35 D. M. Chipman, J. Chem. Phys., 1979, 71, 761. 37 C. Moller and M. S. Plesset, Phys. Reu., 1934, 46, 618. 38 M. J. S. Dewar and W. Thiel, J. Am. Chem. SOC., 1977, 99, 4899. 39 W. Thiel, P. Weiner, J. Stewart and M. J. S. Dewar, Quantum Chemistry Program Exchange, no. 428. 40 M. J. S. Dewar and M. L. McKee, J. Am. Chem. SOC., 1977, 99, 5231. 41 M. Dupuis, J. Rys and H. F. King, J. Chern. Phys., 1976, 65, 111. 43 S. Huzinaga, J. Chem. Phys., 1965, 42, 1293. 44 T. H. Dunning Jr, J. Chem. Phys., 1970, 53, 2823. 45 J. S. Binkley, R. A. Whiteside, R. Krishnan, R. Seeger, D. J. De Frees, H. B. Schlegel, S. Topiol, 46 P. C. Hariharan and J. A. Pople, Theor. Chim. Acra, 1973, 28, 213. 47 R. Krishnan, J. S. Binkley, R. Seeger and J. A. Pople, J. Chem. Phys., 1980, 72, 650. A. T. Amos, D. R. Beck and I. L. Cooper, Theor. Chim. Acta, 1979, 52. 329. 36 T. A. Claxton and N. A. Smith, Trans. Faraday Soc., 1971, 67, 1859. 42 L. R. Khan and J. A. Pople, personal communication, 1980. M. S. Gordon, J. S. Binkley, R. Seeger, J. A. Pople and W. J. Pietro, J. Am. Chem. SOC., 1982, 104, 2797. M. C. R. Symons, J. Chem. SOC., Chem. Commun., 1982, 869. 48 49 B. W. Walther and F. Williams, J. Chem. SOC., Chem. Commun., 1982, 270. ” P. Trousson, J. Magn. Reson., to be published. 52 R. W. Fessenden, J. Phys. Chem., 1967, 71, 74. 53 T. A. Claxton and N. A. Smith, Trans. Faraday SOC., 1970, 66, 1825. s4 D. W. Ovenall and D. H. Whiffen, Mol. Phys., 1961,4, 135. ” J. A. Brivati, N. Keen, M. C. R. Symons and P. A. Trevelion, Proc. Chem. SOC., 1961, 66. 56 J. H. Sharp and M. C. R. Symons, J. Chem. SOC. A, 1970, 3075. 57 G. Herzberg and J. W. C. Johns, in Electronic Spectra of Polyatomic Molecules, ed. G. Herzberg (Van Nostrand, Princeton N.J., 1960), p. 490.

ISSN:0301-7249    

DOI:10.1039/DC9847800121

出版商:RSC    

年代:1984

数据来源: RSC