Tunneling corrections to unimolecular rate constants, with application to formaldehyde

Abstract

Tunneling corrections to the rate constant for unimolecular reactions in an isolated molecule are; treated within the standard transition state (i.e., RRKM) theory of such processes.The micro canonical distribution relevant to the unimolecular case causes tunneling effects to enter in a somewhat more complicated fashion than in the analogous transition state theory for thermally averaged bimolecular rate constants; e.g., even within the separable approximation they do not enter as simply a multiplicative correction factor.Application of the theoretical expressions to some unimolecular processes (HZCO + HZ + CO, trans HCOH + HZCO) of interest in the collisionless photochemistry of formaldehyde indicate that tunneling 9-1 effects are quite significant for rates of 10 secor slower.Isotope effects are also considered and seen to be quite interesting.