Nonstatistical effects in the unimolecular dissociation of the acetyl radical

Classical trajectory and statistical variational efficient microcanonical sampling transition state theory calculations were carried out to investigate the dissociation dynamics of the acetyl radical. For this purpose, an analytical potential function was developed based on ab initio and experimental data reported in the literature. This potential function reproduces reasonably well the geometries, frequencies, and energies of the stationary points of the ground state potential energy surface. The dynamics of the reaction was shown to be intrinsically non-Rice-Ramsperger-Kassel-Marcus (RRKM) at high energies and particularly at 65.9 kcal/mol, at which experimental work showed evidence for nonstatistical behavior. On the other hand, initial excitations of normal modes 507 (CCO bend), 1079 (CC stretch), 1504 (CH3 umbrella vibration), and 1939 (CO stretch) enhance significantly the rate of reaction; specifically, excitation of the CO stretch gives a rate coefficient an order of magnitude higher than the rate obtained under random initial conditions. These mode specific effects are explained in terms of a restricted intramolecular vibrational redistribution (IVR). Under statistical initial conditions, the classical trajectory calculations showed a normal isotope effect at the two lowest energies studied, and a slight inverse isotope effect at 65.9 kcal/mol, a result that can be explained with the presence of a methyl free-rotor at the transition state. In contrast, upon initial excitation of the CC and CO stretches and CCO bending at 65.9 kcal/mol, the calculations predicted a normal isotope effect, which agrees with the experimental findings. (C) 1999 American Institute of Physics. [S0021-9606(99)01223-4].