Nonstatistical effects in the unimolecular dissociation of the acetyl radical

Classical trajectory and statistical variational efficient microcanonical s ampling transition state theory calculations were carried out to investigat e the dissociation dynamics of the acetyl radical. For this purpose, an ana lytical potential function was developed based on ab initio and experimenta l data reported in the literature. This potential function reproduces reaso nably well the geometries, frequencies, and energies of the stationary poin ts of the ground state potential energy surface. The dynamics of the reacti on was shown to be intrinsically non-Rice-Ramsperger-Kassel-Marcus (RRKM) a t high energies and particularly at 65.9 kcal/mol, at which experimental wo rk showed evidence for nonstatistical behavior. On the other hand, initial excitations of normal modes 507 (CCO bend), 1079 (CC stretch), 1504 (CH3 um brella vibration), and 1939 (CO stretch) enhance significantly the rate of reaction; specifically, excitation of the CO stretch gives a rate coefficie nt an order of magnitude higher than the rate obtained under random initial conditions. These mode specific effects are explained in terms of a restri cted intramolecular vibrational redistribution (IVR). Under statistical ini tial conditions, the classical trajectory calculations showed a normal isot ope 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 e xcitation 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 exper imental findings. (C) 1999 American Institute of Physics. [S0021-9606(99)01 223-4].