Fitting classical microcanonical unimolecular rate constants to a modified RRK expression: Anharmonic and variational effects

Abstract

Classical RRKM (i.e., microcanonical) rate constants are often calculated for analytic potential energy surfaces (PESs) developed to represent unimolecular decomposition reactions. The values for these rate constants reflect the complete anharmonicities of the PESs. A common procedure is to fit these rate constants with the expression of RRK theory, which is the harmonic limit of classical RRKM theory. These RRK fits often give values for s significantly less than 3n−6(5), which are not meaningful, since anharmonic rate constants are represented with a harmonic model. In this work different schemes are proposed for introducing anharmonic and also variational transition state effects into the RRK expression. These modified RRK schemes allow one to fit classical anharmonic RRKM rate constants with s=3n−6(5). The fits give information about the importance of anharmonicity in the unimolecular reactant’s density of states and the transition state’s sum of states.