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.