Theoretical and Computational Studies of Non-RRKM Unimolecular Dynamics

Abstract

A survey is presented of theoretical models and computational studies for unimolecular reaction dynamics. Intrinsic RRKM and non-RRKM dynamics are described, and properties of the unimolecular reactant's classical phase space giving rise to these dynamics are discussed. Quantum dynamical calculations of isolated resonances and state-specific decomposition are reviewed, and the resulting possible mode-specific or statistical state-specific decomposition is delineated. The relationship between the latter and RRKM theory is described. Computational studies give the probability that a molecule dissociates in a time interval of t --> t + dt, that is, the lifetime distribution P(t), and determining unimolecular rate constants versus pressure, energy, and temperature from P(t) is outlined. Non-RRKM behavior evident in P(t) is not always present in the rate constants. The need to include anharmonicity and the proper treatment of the K quantum number, in calculating the RRKM unimolecular rate constant, is explained. The possibility of observing "steps" in unimolecular rate constants is considered. The extensive experimental non-RRKM dynamics found for several classes of chemical reactions are surveyed. The direct coupling of chemical dynamics with electronic structure theory, that is, direct dynamics, has allowed one to study the atomic-level dynamics for numerous unimolecular reactions, and extensive non-RRKM and nonintrinsic reaction coordinate (IRC) dynamics have been discovered. These dynamics for OH(-) + CH(3)F and F(-) + CH(3)OOH are reviewed.