787 Table I. Rate Constants from Esr Studies on CH3CN
Evidence for Hydrogen Atom Abstraction by Methyl Radicals in the Solid State at 77°K'
Sir: Since thermal reactions of free radicals by hydrogen atom abstraction in the gas and liquid states are generally characterized by activation energies in excess of 5 kcal mol-', it has been customary to exclude consideration of such processes in the solid state at 77°K.2 From detailed studies on the reactions of methyl radicals in y-irradiated acetonitrile at low temperatures, we have now obtained definitive evidence for a process of hydrogen atom abstraction with an activation energy of only 1.4 kcal mol-'. This result suggests either a large contribution from tunnelling at low temperature or a drastic modification of the potential energy surface for the reaction upon going to the solid state. When y-irradiated acetonitrile is bleached with visible light at low temperatures (77-12OoK), methyl radicals are p r o d u ~ e d . ~Recent ,~ esr studies in this laboratory on single crystals of 13CD3CNand CD3I3CN have unambiguously confirmed this assignment. The subsequent thermal decay of these radicals in both acetonitrile-ha and acetonitrile-d3 is now understood in terms of a simple kinetic competition between two parallel first-order p r o c e ~ s e s . ~Reaction 1 leads to the
+ CN- + CHKN -+ e-(CH3CN)z CHI* + CHaCN +CHI + ~CHICN
CH,.
(1)
(2)
recovery of the photobleachable electron-excess center,4 while reaction 2 results in hydrogen atom abstraction from a neighboring acetonitrile molecule. 3,5-7 The results presented below refer to the high-temperature crystalline phase of acetonitrile which can be studied at low temperatures by quenching the sample to 77°K; in this phase the photobleachable center produced by yirradiation has been identified by esr as an excess electron bound to two acetonitrile molecules. 4,8 In kinetic studies on the above reactions, the decay of CH3. and the growth of sCHZCNwere followed simultaneously by esr, while the recovery of the electronexcess center was monitored in separate experiments by its optical absorption s p e ~ t r u m . ~Table I summarizes the rate constants obtained from esr measurements on acetonitrile-h3 at 77 and 87°K. The sum of the rate k2, was obtained by following the decay constants, kl of CH3. in the dark after photobleaching. Independently, we evaluated k2 both from the decay of CH3. and by the growth of . CH2CN during steady illumination. Under the latter conditions the methyl radicals decay only by means of reaction 2, since reaction 1 is continuously reversed by the photobleaching process. The kinetic coincidence as demonstrated by the essen-
+
(1) This work was supported by the U. S. Atomic Energy Commission under Contract No. AT-(40-1)-2968. This is AEC Document No. 010-2968-54. (2) For a review, see J. E. Willard in "Fundamental Processes in Radiation Chemistry," P. Ausloos, Ed., Wiley-Interscience, New York, N. Y., 1968, p 599. (3) P. B. Ayscough, R. G. Collins, and T. J. Kemp, J. Phys. Chem., 70, 2220 (1966). (4) M. A. Bonin, K. Tsuji, and F. Williams, Nature (London), 218, 946 (1968). (5) I
