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P. B. AYSCOUGH, R. G. COLLINS, AND T. J. KEMP
Electron Spin Resonance Studies of Fundamental Processes in Radiation and Photochemistry.
11. Photochemical Reactions in 7-Irradiated
Nitriles at 77°K
by P. B. Ayscough, R. G . Collins, and T. J. Kemp School of Chemistry and Cookridge R a d i a t w n Laboratories, T h e University, Leeds 3, England (Received April 14, 1966)
The y radiolysis of acetonitrile at 77°K in the absence of light results in the formation of trapped CH2CN radicals together with a highly photosensitive species which yields methyl radicals when irradiated with infrared or red light. Irradiation with white light results in the disappearance of methyl radicals and the formation of further CH2CNradicals. These reactions are interpreted in terms of a mechanism involving both trapped electrons and positive “holes.” The comparative behavior of propionitrile and malononitrile is also discussed briefly.
In part I of this series‘ we presented a general survey of reactions observed by means of electron spin resonance in y-irradiated aqueous, alcoholic, ethereal, and olefinic glasses a t low temperatures and attributed to mobile thermal electrons. Many of these reactions could be simulated by the photochemical generation of electrons or by remobilization of trapped electrons by thermal or photochemical methods. When acetonitrile was incorporated into a glassy matrix of 8 N sodium hydroxide a t 77”K, methyl radicals were formed by the reaction of mobile electrons according to the dissociative capture process (l), but in a report by Dunbar, Hale, Harrah, Rondeau, and Zakancyz2 there is no mention of the appearance of methyl radiCH3CN
+ e-
-3
(CH3CN-) +CHS
+ CN-
(1)
cals in y-irradiated pure acetonitrile between 77°K and room temperature. The present investigation was undertaken to attempt to resolve this apparent contradiction and to examine the possibility of observing the reactions of electrons trapped in this polycrystalline matrix.
Experimental Section Electron spin resonance spectra were recorded by means of a Varian V-4500 epr spectrometer with 100 kc/sec modulation and a low-power microwave bridge. T h e Journal of Physical Chemistry
Acetonitrile was of spectroscopic grade and was not further distilled. Propionitrile and malononitrile were obtained from B.D.H. Ltd. and Eastman Kodak Ltd., respectively, and were freshly distilled before use. All samples were degassed by the freeze-pump-thaw procedure before being distilled into Spectrosil grade quartz sample tubes for irradiation and examination by esr. y-Irradiations were carried out using a (nominal) 1700-curie 6oCosource and doses of 2 to 12 X lo5 rads were given in complete darkness. Samples were kept in darkness by means of a light-tight brass housing while esr spectra were recorded. A shutter was provided so that light could be admitted from a tungsten lamp through a 1-cm2 aperture in front of which was a close-fitting attachment for holding spectral filters. Note : irradiated samples of propionitrile invariably shattered violently when melted.
Results Figure l a depicts the spectrum obtained from acetonitrile after a dose of 2 X lo5rads at 77°K in darkness. The triplet composed of a narrow central line and asymmetric outer peaks is characteristic of a rotating methyl(1) P. B. Ayscough, R. G. Collins, and F. s. Dainton, Nature, 205, 965 (1965). (2) D. Dunbar, D. Hale, L. Harrah, R. Rondeau, and S. Zakanycz, Develop. A p p l . Spectry., 3, 361 (1964).
PHOTOCHEMICAL REACTIONS IN y-IRRADIATED NITRILES
1 0 0 paus
Figure 1. Electron spin resonance spectra of acetonitrile r-irradiated a t 77°K in darkness. Dose 2 x 105 rads: ( a ) observed a t 77°K in darkness; (b) observed a t 77°K after 10-sec illumination with red light in cavity; (c) observed a t 77°K after l-min illumination with visible light in cavity.
ene group and may be attributed without doubt to e H z C N radicah2 The additional minor peaks indicate the presence of a small number of methyl radicals. Figure l b shows the spectrum observed after a 10-sec exposure to red light (using a Chance OR-1 filter). The methyl radical quartet has increased enormously in size, but there is no change in the size of the CHzCN signal. Further exposure to unfiltered light for about 1 min or to red light for about 40 min causes the disappearance of the methyl quartet and a 60% increase in the size of the CHzCN peak (see Figure IC). I n other experiments it was shown that the most careful screening of light from the sample during y irradiation did not entirely eliminate the trace of methyl radicals in the original spectrum. It was also shown that exposure of y-irradiated samples to infrared light (>8000 A) also brought about the increase in concentration of methyl radicals shown in Figure l b though much more slowly. Incorporation of electron scavengers in the samples before y irradiation caused little or no change in the observed spectra. For example, 0.2 M nitrous oxide and 0.04, 0.1, 0.4, and 1 M sulfuric acid have no effect; in a 4 M solution of sulfuric acid (glassy rather than polycrystalline) the yield of CHzCN radicals was more than doubled, and a longer period of photolysis was needed to give a rather less intense methyl radical signal subsequently. The behavior of CD3CN was identical apart from the replacement of the CHzCN triplet by the pentet of ~ D z C and N the CH3quartet by the c1/D3septet. When propionitrile was subjected to the same treat-
2221
ment, the initial spectrum consisted of approximately equal proportions of the normal twelve-line spectrum of CzHSand a broader five-line spectrum (average peak separation 20 gauss) attributed to CHSCHCN. Illumination with red light increased the concentration of ethyl radicals, but further illumination with unfiltered light did not bring about any further change. It was found that malononitrile required approximately six times the radiation dose to achieve the same radical concentration as acetonitrile, and an illumination time two orders of magnitude greater was needed to bring about the change shown in Figure 2b. We interpret Figure 2a as being composed mainly of eH2CN with smaller amounts of CH8 and perhaps CH(CN)CHzCN, the outer peaks of which appear outside the CH3 quartet. The final spectrum obtained after illumination with unfiltered light is a predominant doublet, probably attributable to CH(CN)2. When irradiated in the presence of light, the spectrum is mainly C H ( C N ) ~(uH = 22.0 gauss, U N = 9.5 gauss) and no change occurs on further illumination. Hydrogen atoms were present in all irradiated samples but were unaffected by illumination.
Discussion Those observations relating to the final spectra observed in acetonitrile are in agreement with those of earlier reports,*s3 but the additional information derived from the other nitriles and from the photolysis experiments suggests some modification of the reaction mechanism. We must distinguish between processes
;:1
r-
I O 0 qwss
Figure 2. Electron spin resonance spectra of malononitrile r-irradiated a t 77°K. Dose 1.2 X lo6 rads: (a) observed a t 77°K in darkness; (b) observed a t 77°K after 10-min illumination with visible light outside cavity. ~
~~~~~
(3) N. V. Eliseeva, B. V. Kotov, V. A. Sharpatzi, and A. N. Pravednikov, Opt. Spectry. (USSR),18,470 (1965).
Volume 70, Number 7
July 1966
2222
P. B. AYSCOUGH, R. G. COLLINS,AND T. J. KEMP
occurring in the dark at 77°K and those occurring under the influence of infrared and/or visible radiation. In our work and in that of Dunbar, et al., the main species trapped initially a t 77°K is CH&N suggesting a primary ionization (2) followed by the ion-molecule reaction (3a) or the dissociative electron-captive reaction (3b).
+ e-
(2)
+ CH2CN
(3a)
CH3CX --+ CH3CN+ CHBCNH+ CHsCN
/
7
CHSCN + \,e-
\
L CH3CN* +CH&N
+H
(3b)
Dunbar, et al., suggest that a poorly resolved triplet coincident with the central peak of CHzCN indicates the presence of CN, but our own observations that these features follow exactly the changes in the 6H2CN concentration during photolysis experiments lead us to identify them as partially resolved hyperfine interaction with 14N in CH2CN itself. This conclusion is supported by (a) the absence of CZK2or HCN in the products and (b) the exact equivalence of g factors for CHZCN and the “CN” radical. I n propionitrile the radicals CH3CHCN and C2H5 are observed (products of reactions analogous to (2) and (3)) but again no CN is found. The observation by Dunbar, et al., of a purple color in “quick-frozen” samples of acetonitrile which disappeared on warming to 147°K accompanied by a 40% increase in 6HZCN concentration closely parallels our own observations of the effect of photolysis a t 77”K, apart from the transient appearance of large numbers of methyl radicals during the photolysis. Dunbar, et al., suggest that the increase in CHzCN is caused by the thermal dissociation of an excited species CH3CN*, possibly a triplet-state molecule, which is trapped at 77°K and is responsible for the purple color (reaction 4). Our own experiments suggest that CH3 is a preCH3CN* (or CH&&) +6H2CN
+H
(4)
cursor of CHzCN certainly during the illumination and probably during thermal annealing of irradiated samples also. This might suggest the sequence of reactions 5 and 6. However, the failure to detect CN CH3CN* (or CH3Cfi) +CH3 CH3
+ CN
+ CH&N +CH4 + c H & N
The Journal of Physical Chemistry
CN
+ CH3CN +HCN + CHzCN
(7) even when formed by the extremely low-energy process (5) induced by infrared radiation is powerful evidence against this interpretation. Furthermore, no HCN was found by Dunbar, et al. It is important to stress that during the photolysis of y-irradiated acetonitrile a t 77°K there is a large increase in the absolute concentration of trapped radicals detected by their esr spectra. This can arise only by dissociation of a diamagnetic species or from reaction of a paramagnetic species which fails to give an observabIe esr spectrum. When combined with the requirement that this species should adsorb in the infrared and yield methyl radicals, directly or indirectly, with great efficiency and a very low quantum of energy, the suggestion that the species is a trapped electron is more acceptable than a triplet state of CH3CN. The failure to observe the characteristic spectrum of trapped electrons as observed in polar matrices such as aqueous or alcoholic glasses might be attributed to line broadening caused by interaction with I4N nuclei in cyanide ions close to the trap. Also, the failure of electron scavengers such as nitrous oxide to interfere with the photolysis may well be caused by the strong electrophilic character of acetonitrile itself and to the fact that the migration distance of the electron is less than the dimensions of the microcrystal of acetonitrile. Since this is polycrystalline, the scavenger molecules are likely to be in the interstices. Dissociative electroncapture processes such as (1) would then account for the photolytic generation of methyl and ethyl radicals without the simultaneous formation of CN radicals. The subsequent disappearance of methyl radicals on prolonged irradiation with visible light is probably a consequence of photoionization of CN-, a much less efficient process than that by which the radicals are formed. Van Dusen and Truby4 have proposed a similar mechanism to explain the effect of photolysis of y-irradiated methyl bromide in cyclohexane a t 77°K. They suggest that electrons formed by photoionization of bromide ions combine with a positive “hole” while the bromine atom recombines with a methyl radical initially formed in close proximity. I n acetonitrile the positive “hole” is probably CH3CNH+ formed by reaction 3a, so reaction of the electron photoejected from CN- may take the course represented by (8) and (9) where H* is the “hot” hydrogen atom. The disappearance of methyl and the simultaneous formation of CH&N would then be explained by a mechanism
(5) (6)
(4) W. Van Dusen, Jr., and F. K. Truby, J . Am. Chem. Soc., 87,
188 (1965).
PHOTOCHEMICAL REACTIONS IN 7-IRRADIATED NITRILES
+ e- -+ CHsCN + H* H* - CH&N CH&N + Hz
CH&NH+
(8)
(9)
very similar to that proposed for the photolysis of 7irradiated methanol6a t low temperatures. There is no evidence for the participation of any highly photoserisitive intermediate in the radiolysis of malononitrile, and the presence of methyl radicals can be explained only by suggesting that they are derived from acetonitrile formed during the prolonged radiolysis. The change brought about by continued photolysis with visible light is similar to that observed in the final stages of the photolysis of acetonitrile, though very much slower. It seems likely that the same mechanism applies though the reasons for the differences in yields from the radiolysis and the subsequent photolysis are not yet clear. It appears, therefore, that our observations can be interpreted more readily in terms of a mechanism
2223
involving trapped electrons which can be remobilized by photolysis than one involving trapped excited singlet or triplet acetonitrile molecules, though the nature of the electron trap is not well defined. In general, the prerequisite for the formation of a “good” electron trap is a glassy matrix of high dielectric constant in which the electron can cause local polarization by some reorientation of the solvent dipoles. Acetonitrile has a high dielectric constant, but reorientation of its dipole around a thermalized electron is difficult because of its polycrystalline nature so that stabilization energy of the electron is likely to be low.
Acknowledgment. We are grateful to the Science Research Council for financial assistance and to Professor F. S. Dainton, F. R. S., for helpful discussions. ( 5 ) F. S. Dainton, G. A. Salmon, and J. Teply, Proc. Roy. SOC.
(London), A286, 27 (1965).
Volume YO, Number Y
J u l y 1066
