Further dynamical studies of the dissociation and elimination reactions of methyl nitrite

摘要

Classical trajectory calculations were performed to investigate the effects of molecular rotation, deuterium substitution, and the possibility of mode-specific effects in the two unimolecular channels that initiate the thermal decomposition of methyl nitrite (MeONO): O-N bond dissociation giving CH_3O and NO and concerted elimination to produce CH_2O and HNO. The calculations were carried out at a total energy of 140 kcal/mol, at which a microcanonical ensemble of excited molecules is maintained throughout the decomposition. Total and individual rate coefficients were evaluated under several types of initial sampling conditions: microcanonical (i. e., random) distribution of vibrational energy, selective excitation of normal modes, and various angular momentum orientations. Comparisons of the results obtained from random initial conditions and normal mode excitations show that there is significant enhancement of the decomposition rates for excitations of several vibrational modes (apparent non-RRKM behavior). The calculations predict rapid energy exchange among modes 465 (ONO bend), 715 (CO stretch), and 931 (O-N stretch) as well as strong coupling between modes 246 (CONO torsion) and 1670 (N = O stretch). The vibrational state distributions for the nascent NO species computed under excitations of modes 246 and 1670 are much broader than that obtained under random initial conditions. This gives further evidence for incomplete relaxation of vibrational energy on the time scale of reaction. Molecular rotation enhances the decomposition rates significantly. More specifically, exciting the symmetric top axis promotes elimination, while exciting either of the remaining two axes promotes dissociation. The presence of two-dimensional rotors at the dissociation transition state may explain the inverse isotope effect found in our previous classical trajectory calculations [J. Chem. Phys. 109, 8907 (1998)]. Finally, the importance of anharmonicity in the unimolecular density of states was estimated by fits of modified RRK schemes to our previously reported microcanonical rate coefficients.