The unimolecular dissociation of the propionyl radical: A classical dynamics study

The unimolecular dissociation of the propionyl radical to form CO and CH2CH 3 was investigated by classical trajectory calculations. Various types of i nitial sampling conditions were employed: Microcanonical for energies rangi ng from 27.8 to 72.8 kcal/mol above the zero-point energy (ZPE), and select ive excitations at 67.8 kcal/mol. A quasiclassical barrier sampling techniq ue, which circumvents the problem of ZPE leakage, was also used for the cal culation of product energy distributions. For energies above 43 kcal/mol, t he computations showed that the intramolecular vibrational relaxation is no t rapid as compared with the rate of reaction. On the other hand, it is fou nd that vibrational modes associated to the CCO moiety are significantly co upled to the reaction coordinate, in agreement with the suggestion reported by Zewail and co-workers [J. Phys. Chem. 100, 9202 (1996)]. However, the c alculations cannot predict the significant decrease of the dissociation rat e observed upon deuterium substitution on the alpha -carbon. Product energy distributions and CO vibrational populations computed for the different ex citation schemes are compared with those determined experimentally. For man y ensembles, the fraction of the available internal energy resulting in CO vibration agrees with that estimated experimentally. (C) 2001 American Inst itute of Physics.