recent progress in free radical spectroscopy

RECENT PROGRESS IN FREE RADICAL SPECTROSCOPY

bY GEORGE PORTER

Department of Physical Chemistry, Sheffield University

SUMMARY

New techniques have revolutionized the study of the spectra of free radicals and other unstable molecules. Some of the more important are flash photolysis, matrix isolation methods and paramagnetic resonance. Recent progress is reviewed, particularly in the field of the electronic spectra of complex molecules.

LIKE THE SPECTROSCOPY of more stable molecules, free radical spectroscopy serves two purposes. First, analysis of the spectrum leads to structural and energetic information about the radical. Second, once the spectrum has been assigned it provides a means of identification and-if the extinction coefficient can be determined-of quantitative estimation. But the place of spectroscopy in free radical chemistry is a particularly important one because it is frequently the only method by which this type of information can be obtained.

Recent progress in the field has been very rapid. Six years ago the only spectra which were definitely assigned to free radicals were those of diatomic molecules (I), if we exclude the stable free radicals which can be observed under equilibrium conditions such as NO,, CIOa and triphenyl methyl. The transient radical spectra which were known had been studied exclusively in the gas phase and mainly in emission, but the organic chemist is particularly interested in the liquid phase and absorption spectra are more useful for the determination of radical concentrations and the study of kinetic behaviour. The absorption spectra of the closely related species-the triplet states of molecules-were unknown in solution and in the gas phase, and indeed it was not at all clear whether their detection would be possible in fluid media. Finally the spectroscopy of transient molecules was confined to electronic spectra in the visible and quartz ultra- violet regions. All these restrictions have now been to a large extent removed, with the important exception that satisfactory general methods for the detection and study of radical spectra in the infra- red and microwave regions have still to be developed.

Experimental Techniques It is often the case that a rapid advance in a particular branch of

261

262 GEORGE PORTER

science is a result of the introduction of a few special techniques and this is certainly true of the spectroscopy of unstable molecules. Experimentally there are two distinct problems in this field (1) the production of the unstable molecules in sufficient concentration and (2) the recording of their spectrum during the short period of their existence. Most earlier work on the spectroscopy of transient species solved both these problems at once by studying emission spectra in flames and electrical discharges. Here the dissociation and the excitation occur together and the low stationary concentration of the intermediates is of no consequence since the integrated in- tensity of emission over a relatively long period of time is the important factor. The conditions necessary for excitation imply a high energy and consequently a rather indiscrimate series of chemical reactions with the result that most emission spectra consist mainly of diatomic and other simple radicals which are relatively stable physically. Nevertheless, by using carefully controlled conditions, the emission spectra of quite complex intermediates can be recorded and the electrical discharge has been used effectively by SCH~LER et al., particularly in the study of aromatic free radicals (2).

Other methods involve the separation of the radical production and detection processes and have the great advantage that they make possible the study of the time dependence and hence the chemical behaviour of the intermediates. Unless the radicals are stabilized in some way it is necessary to effect their preparation in a time which is short compared with their lifetime under the prevailing conditions, and only two methods are at present available for this purpose. The first utilizes the rapid heating produced in a shock wave. This is limited to the gas phase and is discussed by Gaydon in another paper at this symposium. The second uses a pulse of radiation and although, in principle, pulses of electrons, X-rays and y-rays might be used, the only application of this method which has so far been generally successful is that using visible and ultra-violet radiation- i.e. flash photolysis. Although there have been some modifications of detail the method is still essentially the same as that originally described (3). Attempts to produce flashes of shorter duration than a few microseconds have not been successful since, although flashes of a few millimicroseconds duration are possible, this is only achieved by a prohibitive reduction in flash energy. A major improvement in this direction would be of very great importance since it would make possible the study of the absorption spectra of all the excited electronic states of molecules. Only metastable states of lifetime 1O-s set

or greater are at present detectable in absorption. For the detection of the intermediates, once they have been formed

at a suitable concentration, electronic absorption spectroscopy in

RECENT PROGRESS IN FREE RADICAL SPECTROSCOPY 263

the visible and ultra-violet regions is still the most generally powerful method. The spectrum is recorded by flash spectroscopy and the photographic plate or by photoelectric detectors. The latter usually have to be employed at a single wavelength for each recording, which makes the mapping of a complete spectrum a tedious and rather inaccurate procedure, but photoelectric recording is very suitable for kinetic studies of the spectrum at a single wavelength. Scanning spectrometers have not yet a sufficient speed and sensitivity for the study of spectra whose lifetimes are of the order of milliseconds or microseconds. One of the most outstanding requirements of free-radical spectroscopy is the extension of investigations to the infra-red and microwave regions since practically no vibrational or rotational spectra of these species are known. Although the last few years has seen considerable advances in rapid recording infra- red detectors, response times of the order of 1 psec now being available in the region out to 6 p, they have yet to be applied to the spectroscopy of free radicals. The use of microwaves presents many practical difficulties and attempts to record the microwave spectra of short-lived species have not yet been very successful.

Flash spectroscopy has recently been applied to the vacuum ultra- violet region by HERZBERG and SHDOSMITH (4), whose recording of the Rydberg spectrum of the methyl radical is a notable achievement. The conventional electronic flash has a good continuous spectrum at least as far as the fluorite limit and the extension of flash spectroscopy into the region presents no serious difficulties. On the other hand the use of vacuum ultra-violet flash photolysis techniques involving the construction of flash tubes from sapphire (5) or other transparent material is more difficult and the problem of containing a high-energy flash without rapid destruction of the tube has not yet been solved.

The most important spectroscopic development in this field is undoubtedly the recent application of electron magnetic resonance techniques to free radical problems. As yet the method has only been applied to stable radicals, or to systems in which unstable radicals have been trapped, and techniques have not yet been developed for the recording of transient spectra in low concentration although evidence about the rates of very fast radical reactions can be obtained from a study of broadening of the spectra.

FREE RADICAL STABILIZATION

The difficulty of recording spectra in times of the order of microseconds and with high sensitivity emphasizes the importance of methods for the stabilization of radicals for longer periods. Such methods have recently been developed and they provide a completely

264 GEDRGE PORTER

new approach to the study of transient intermediates. Although stabilization automatically preludes the study of the chemical properties of the intermediate under normal conditions it enor- mously simplifies the problem of recording such physical properties as absorption and magnetic resonance spectra and in the many cases where rapid recording techniques are not possible it provides the only means of study.

There are two major causes of instability in a molecule or a free radical. First it may dimerize rapidly, as most free radicals do, to form a stable molecule. This process may have a zero or even a negative temperature coefficient and occur with nearly unit collision efficiency. It can therefore only be prevented by keeping the molecules apart-by trapping them in a rigid matrix for example. The second class of reaction is dissociation of the radical or reaction between the radical and the matrix. Such reactions are almost invariably ac- companied by a considerable activation energy and can therefore be prevented by working at a sufficiently low temperature. If the radicals are trapped in an inert rigid matrix at low temperatures both conditions are satisfied and the radicals are stabilized indefinitely.

During the last few years several workers have independently set about the application of these ideas to the stabilization of radicals, although different methods were used by each of them. The methods are of two main types :

(1) The radicals are formed in a gas phase flow system by thermal or electrical dissociation and the mixture of products is then rapidly condensed on a cold finger (6, 7, 8, 9).

(2) A rigid solution is formed at a very low temperature and the free radicals or other unstable species are formed in situ by irradiation (10, 11, 12, 13).

The species trapped in this way have been examined by ultra- violet, visible and infra-red spectroscopy, and by paramagnetic resonance spectroscopy. The observations can be made at leisure over periods of many hours if the solution is kept frozen. In addition, on raising the temperature of the glass, emission spectra which presum- ably arise as a result of recombination ind other reactions which occur when the trapped radicals are freed, are often observed. For many purposes liquid nitrogen is a suitable refrigerant but liquid hydrogen and helium have also been used. The rigid matrix may be simply the parent molecule itself-as for example when ice is irradiated at low temperatures. In general a separate material must be used and, for this purpose, argon, nitrogen, paraffins and alcohols have been applied. If absorption spectra are to be recorded the matrix must be transparent and form a clear glass with a minimum of scattering, as well as ‘satisfying the requirements of inertness and of being a suitable


solvent. For visible and ultra-violet spectra these are stringent requirements but are reasonably well met by paraffin and alcohol mix- tures. These are, of course, rather unsuitable in the infra-red region where there are few really satisfactory matrix materials but fortunate- ly the lower scattering in this region makes the need for a clear glassy solution less essential.

One other requirement of methods of the second type, involving irradiation in situ, is that quantum yields of photochemical processes and G-values of radiation chemical processes shall not be greatly reduced in rigid media. Experience so far indicates that this is not generally a serious restriction and dissociation in rigid solvents usually occurs quite readily.

Apart from its usefulness in the study of unstable molecules the spectroscopy of rigid solutions at low temperatures provides a wealth of new phenomena and is likely to become a major field of spectroscopic investigation.

Spectra of Simple Polyatomic Radicals One of the main results of the application of the new techniques

has been the detection, assignment and partial analysis of a number of the spectra of unstable polyatomic radicals. Those which have been definitely assigned are of two kinds. (a) Triatomic or very simple polyatomic radicals containing light atoms, whose spectrum can be resolved and interpreted su~ciently well to make an assignment possible by spectroscopic methods alone. (b) More complex radicals containing a conjugated system and having spectra with sufficient structure for identification purposes. In many cases of this kind unequivocal assignments are also possible on the basis of the study of a series of related molecules and investigations of the dissociation processes.

With radicals of the first type definite assignments are few and nearly all are triatomic. CHO and NH, have been quite definitely assigned by RAMSAY as a result of flash photolysis studies of alde- hydes and of ammonia respectively and vibrational-rotational analyses have shown that both radicals have bent ground states and a linear excited state (14). PH, has been detected in a similar way during the flash photolysis of phosphine. The 4050 A system, first observed in comets and attributed to CH2 is now known to be due to C, and has been carefully studied by DOUGLAS (15). A similar system, probably due to Sic, is also known (16). Spectra assigned to HS2 (17) and N, (18) have been observed during flash photolysis of H,S and HN3 and the Iatter also from the photolysis of HN3 in rigid media (19). The alkaline earth hydroxides CaOH, SrOH and BaOH have been observed in the emission spectra of flames. The

266 GEORGE PORTER

spectra of the important radicals CH, and HO2 still elude all attempts at detection.

The only spectrum of intermediate complexity which has been definitely assigned is that of the tetratomic radical CH,. HERZBERG and SHGGSMITH (5) detected its spectrum during flash photolysis of mercury dimethyl and of several other methyl derivatives and showed that it must have a nearly planar structure in its ground state. Its ionization potential, determined from the Rydberg series limit was 9.84 eV. Other spectra of relatively simple radicals are observed in the fluorescence of vapours excited by radiation in the vacuum ultra-violet region (22) but their identification is still rather un- certain.

Electronic Spectra of Aromatic Radicals The first free radical spectrum to be reported was that of triphenyl

methyl, observed by Gomberg in 1900, and many other resonance stabilized radicals of this type have since been detected and studied in solution. These investigations seem far removed from those which have been discussed in this paper, which have so far been confined to gas phase studies of radicals containing only a few atoms. The organic chemist is often greatly interested in radicals of intermediate complexity and in their reactions in solution as well as the gas phase. For example we know quite a lot about the spectra and chemical behaviour of the very unstable methyl radical in the gas phase and the stable radical triphenyl methyl in solution. What of the radicals diphenyl methyl and phenyl methyl (benzyl) in both phases?

The spectra of many radicals of this kind have recently been reported and a number of them are now definitely assigned. They have been observed in the gas phase, both in emission and absorption as well as in ordinary solutions and in rigid glasses. Most of them have very sharp characteristic band spectra which, although rather too complex for vibrational analysis, are ideal for identification purposes. As in the spectra of stable aromatic molecules the main spectroscopic interest lies in the positions of the electronic energy levels rather than in structure determination. In this respect the benzyl radical is of considerable interest since information about its lower electronic states has been obtained in three separate ways.

Emission spectrum. SCH~~LER and his colleagues have obtained a large number of emission spectra by excitation of aromatic vapours in the electrical discharge. The method is rather catholic in its production of radical spectra but by studying a number of related molecules it appears probable that some of the spectra belong to aromatic radicals and of these the so called “V” spectrum. with maximum at


4477 A, is the best studied. It was also detected by WALKER and BARROW (23) and has been assigned to CGH,CH,, C,HJH, C,H,C and also to smaller species such as C,H. Recently SCH~~LER has supported the assignment to benzyl(2).

Absorption spectrum. PORTER and. WRIGHT (24) studying the flash photolysis of aromatic vapours detected a sharp band spectrum, with absorption maximum at 3053 A, from toluene, benzyl chloride and a number of benzyl derivatives. NORMAN and PORTER (11) obtained the spectrum from the same compounds by photolysis in rigid hydro- carbon glasses at the temperature of liquid nitrogen and PORTER and WINDSOR (25) obtained the same spectrum by photolysis of ordinary solutions at room temperature-the first case of the detection of a short-lived free radical spectrum in solution. STRACHAN (26) has recently extended the work in rigid glasses and has confirmed that this spectrum can be assigned unequivocally to the benzyl radical.

The results of the emission and absorption experiments seem to conflict, but SCH~~LER has pointed out that emission spectra of polyatomic molecules almost invariably arise only from the first excited state and that if this transition had a low probability it might not be observable in absorption so that the emission and absorption spectra might correspond to different electronic transitions. Dr. Strachan has recently confirmed this interpretation and the assignment of the V spectrum to benzyl by detecting this much weaker transition in absorption from rigid glasses containing the benzyl radical.

2Xl - 2x7 m

42

2x1 + 2x2

f 42

22.100 cm-l (PORTER and

STRACHAN)

21,500 cm-’ (BINGEL)

2XO

-

-

22,330 cm -I @CHOLER and MICHEL)

J-

f

-1

27,900 (LONGUET-HIGGINS

and POPLE)

Fig. 1. Experimental and theoretical values of towest transitions in benzyl (states are labelled according to Longuet-Higgins and Pople).

-

32,760 cm-l ‘P;;;;Hzd

H

33,700 cm-l (LONGUET-HIGGINS

and POPLE)

268 GEORGE PORTER

Molecular orbital calculations. These have been carried out for benzyl by BINGEL (27), DEWAR and LONGUET-HIGGINS (28), and LONGUET-HIGGINS and POPLE (29). It is found that benzyl should have a weak transition at long wavelengths and a stronger transition at shorter wavelengths. These predictions and the experimental observations are summarized in Fig. 1 from which it will be seen that the agreement between these several approaches to the spectrum of benzyl radical is very satisfactory.

Benzyl is the simplest of a large number of aromatic radicals in which the electron is delocalized owing to the possibility of benzenoid and quinonoid forms. Some of the absorption spectra of typical radicals of this kind which have recently been measured are as follows:

Radical Phase A (max.) ReJ

C,H&H, Gas. Liq. Glass 3053 (gas.) 24,25, 11,26 C,H5CHC,HS Liq. Glass 3345 (liq.) 25, 11,26

PC~-J&J-KH, Gas. Glass 3100 (gas.) 24,11 C,HSN H Gas. Glass 3008 (gas.) 24,ll C,H,G Gas. Glass 2920 (gas.) 24,l I

pHOC,H,O Liq. Glass 3940 (liq.) 25,30,31,26

The last of these radicals, benzosemiquinone, is to be distinguished from the relatively stable ionized form which is typical of a large class of stable semiquinone radicals. The spectra of the two forms are easily distinguished, have been recorded by flash photolysis of a number of quinones and used to carry out detailed kinetic studies of their reactions (30). One of the interesting observations in this work was the direct spectroscopic measurement of the rate of proton separation from the durosemiquinone radical:

OH O-

The trip/et state. Detection of the absorption spectra of the triplet states of a number of aromatic molecules was reported at the last discussion of this group (32). Similar absorption spectra have now been observed in the gas phase following flash photolysis of aromatic vapours (33). Recently the absolute extinction coefficients have been measured for triplet-triplet transitions in a number of aromatic molecules and f-values have been determined (34). The electronic

RECENT PROGRESS IN FR& RADICAL SPECTROSCOPY 269

energy levels and f-values of triplet transitions in the linear poly- scenes up to pentacene have been compared with theoretical calculations of PARISER (35) and agreement found which is probably within the combined errors of the experimental and theoretical values. Much remains to be done in this field-particularly in the study of radiationless transitions between states of different multiplicity.

Puramugnefic resonmce specrru. Paramagnetic resonance is, of course, limited in its application to molecules of the free radical class and is likely to prove one of the most powerful of all methods for their spectroscopic investigation. Electron resonance spectra have been reported in almost every conceivable type of material but few of these spectra are understood or assigned. Most progress in interpreting the spectra has been made in the study of stable radicals and ions whose identity is known. One of the most interesting possibilities of the method is the investigation of unstable radicals trapped in rigid matrices and considerable advances have already been made in this field. The simplest and most certain is the detection of atomic hydrogen in irradiated solids (13, 36) and the OH radical has now been fairly definitely detected in irradiated calcium hy- droxide crystals (13) and in ice (36). MATHESON, SMALLER and AVERY have obtained very good evidence for the presence of CH, in methane irradiated at 4°K and of CBH, from ethane, ethyl chloride and ethy- lene under the same conditions and spectra have also been attributed to these radicals by other workers (37). Over a hundred organic substances have been studied in this way and most of them show resonance spectra, but few of these spectra can yet be assigned and many of the reports are conflicting. Most of this work has appeared during the last two years and there is no fundamental reason why most of these spectra should not be assigned and interpreted in the near future.

So far paramagnetic resonance spectra have not been detected from the excited triplet states of molecules although conditions under which resonances were to be expected have been attained in several laboratories. The reason for the absence of resonance from these molecules is not entirely clear, and is of considerable theoretical importance.

None of the techniques which are now making major contribu- tions to free radical spectroscopy was known 10 years ago. When it is remembered that, although relatively few radical spectra are yet known, the number of unstable molecules exceeds that of all stable molecules the field is almost limitless and very rapid progress is to

270 GEORGE PORTER

be expected during the next decade. Particularly interesting develop- ments should result from the further study of electron magnetic resonance, of stabilized radicals in rigid matrices and of the kinetic spectroscopy of unstable molecules including the triplet state.

REFERENCES

(I) PORTER, G., J. Phys. Radium, 1954, 15, 497. (2) SCHULER, H., REINEBECK, L. and K~BERLE, R., Z. Now-, 1952, 7a, 421.

SCH~~LER, H. and MICHEL, A., ibid., 1955, lOa, 459. (3) PORTER, G., Proc. roy. Sot. A, 1950, 200, 284. r4) HERZBERG, G. and SHOOSMITH, J., Canad. J. Phys., 1956,34, 523. (5) NELSON. L. S. and RAMSAY, D. A., J. them. Phys., 1956, 25, 372. (6) RICE, F. 0. and FRAEMO, M., J. Amer. them. Sot., 1951, 73.5529; J. Amer.

them. Sot., 1953, 75, 548. (7) GIGUERE, P. A., J. them. Phys., 1954,22, 2085. (8) WHITTLE, E., Dows, P. A. and PIMENTEL, G. C., J. them. Phys., 1954, 22,

1943. (9) BROIDA, H. P. and PELLAM, J. R., Phys. Rev., 1954,95, 845.

(10) LEWIS, G. N. and LIPKIN, D., J. Amer. them. Sot.. 1942, 64, 2801. (1 I) NORMAN, I. and PORTER, G., Nature, Lond. 1954, 174, 508 ; Proc. roy. Sot.

A, 1955,230, 399. f 12) MADOR, I. L. and WILLIAMS, M. C., J. them. Phys., 1954,22, 1627. (13) LIVINGSTON, R., ZELDES, H. and TAYLOR, E. H., Phys. Reo., 1954, 94, 725. (14) RAMSAY, D. A., Ann. N. Y. Acad. Sci., 1957, 67, 485. (I 5) DOUGLAS, A. E., Astrophys. J., I95 I, 114, 466.

CLUSIUS, K. and DOUGLAS, A. E., Canad. J. Phys., 1954,32, 319. (I 6) KLEMAN, B., Astrophys. J., 1956, 123, 162. (17) PORTER, G., Disc. Faraday Sot., 1950, 9, 60. (I 8) THRUSH, B. A., Proc. roy. Sot. A, 1956, 235, 143. (19) BECKER, E. D. and PIMENTEL, G. C., J. them. Phyq., 1956,25,224. (20) JAMES, C. G. and SUGDEN, T. M., Nafure, Land. 1955, 175, 333. (21) GAYWN, A. G., Proc. ray. Sot. A, 1955,231,437. (22) STYLE, D. W. G. and WARD, J. C., Trans. Faraday Sot., 1953.49, 999. (23) WALKER, S. and BARROW, R. F., Trans. Faraday Sot., 1954, 50, 541. (24) PORTER, G. and WRIGHT, F. J., Trans. Faraday Sot., 1955, 51, 395. (25) PORTER, G. and WINDSOR, M. W., Nature, Land. 1957, 180, 187. (26) STRACHAN, E., Unpublished work. (27) BINGEL, W., Z. Natur-, 1955, 1Oa.

(28) DEWAR, M. J. S. and LONGUET-HIGGINS, H. C., Proc. phys. Sot. Lond., 1954, 67, 795.

(29) LONGUET-HIGGINS, H. C. and POPLE, J., Proc. phys. Sot. Lond., 1955,68,591. (30) BRIDGE, K. and PORTER, G., Proc. roy. Sot. A, 1958. In press. (3 I) LINSCHITZ, H., RENNERT, J. and KORN, T. M., J. Amer. them. Sot., 1954,76,

5839. (32) PORTER, G. and WINDSOR, M. W., Molecular Spectroscopy, p. 6, Institute of

Petroleum, 1955. (33) PORTER, G. and WRIGHT, F. J., Trans. Faraday Sot., 1955,51,393. (34) PORTER, G. and WINDSOR, M. W., Proc. roy. Sot. A. In press. (35) PARISER, R., J. them. Phys., 1956, 24, 250. (36) MATHESON, M. S. and SMALLER, B., J. them. Phys., 1955,23, 521. (37) REXROAD, H. V. and GORDY, W., A.P.S. Bulletin, 1957,4, 227.

recent progress in free radical spectroscopy

Documents