Nonempirical parameter estimate for the statistical adiabatic theory of unimolecular fragmentation

Abstract

For the C-H bond fission in CH₃, the reaction path, the potential profile and the frequencies of the normal modes perpendicular to the reaction path are calculated by means of ab initio quantum chemical approaches (UHF, second-order Møller-Plesset perturbation theory). On the whole, these data confirm the assumptions made in the semiempirical statistical adiabatic channel model of Quack and Troe qualitatively; in particular, the decrease of the frequencies of reactant bending modes which transform into free rotations of the product is well reproduced by an exponential switching function with a common parameter. However, this single-parameter switching function cannot be used for all vibrations changing their eigenvalues. Furthermore, within the quantum chemical approaches employed, the Morse function with a constant parameter β is a crude approximation to the potential energy profile. On the basis of these theoretically estimated data, adiabatic rovibronic energy terms are constructed and high-pressure thermal rate constants are calculated; the latter turn out to be predominantly determined by the bond dissociation energy. For comparison, high-pressure recombination rate constants are given.