The unimolecular dissociation of the propionyl radical to form CO and CH2CH3 was investigated by classical trajectory calculations. Various types of initial sampling conditions were employed: Microcanonical for energies ranging from 27.8 to 72.8 kcal/mol above the zero-point energy (ZPE), and selective excitations at 67.8 kcal/mol. A quasiclassical barrier sampling technique, which circumvents the problem of ZPE leakage, was also used for the calculation of product energy distributions. For energies above 43 kcal/mol, the computations showed that the intramolecular vibrational relaxation is not rapid as compared with the rate of reaction. On the other hand, it is found that vibrational modes associated to the CCO moiety are significantly coupled to the reaction coordinate, in agreement with the suggestion reported by Zewail and co-workers [J. Phys. Chem. 100, 9202 (1996)]. However, the calculations cannot predict the significant decrease of the dissociation rate observed upon deuterium substitution on the alpha -carbon. Product energy distributions and CO vibrational populations computed for the different excitation schemes are compared with those determined experimentally. For many ensembles, the fraction of the available internal energy resulting in CO vibration agrees with that estimated experimentally. (C) 2001 American Institute of Physics.