Formation and decomposition of chemically activated ethyl radicals is simulated by the Monte Carlo quasiclassical trajectory method. The results are compared with a previous study of a microcanonical ensemble of ethyl radicals. Chemically activated ethyl radicals have an apparent non-RRKM lifetime distribution as a result of nonrandom excitation. However, rapid intramolecular relaxation processes give a lifetime distribution after approximately 5 X 10~12 s which agrees with that for a microcanonical ensemble. A statistical model for partitioning of energy and angular momenta at the dissociation barrier is given, and the trajectory results agree with the predictions of the model.(60) G. C. Schatz, J. Phys. Chem., 83, 989 (1979).

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Classical trajectories are used to study the unimolecular decomposition of H–C–C* radicals formed by the H+C=C chemical activation step. The lifetime distribution of the H–C–C* radicals is strongly nonexponential. A long time exponential tail to the lifetime distribution has a unimolecular rate constant ten times smaller than that for a microcanonical ensemble. The relationship between the lifetime distribution and the collision averaged unimolecular rate constant is considered. A discussion is presented on the effect of quasiperiodic trajectories in the molecular phase space on the quantum mechanical unimolecular rate constant.

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