TUNNELING CORRECTIONS TO UNIMOLECULAR RATE CONSTANTS, WITH APPLICATION TO FORMALDEHYDE

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 microcanonical 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 (H2CO → H2 + CO2 trans-HCOH → H2CO) of interest in the collisionless photochemistry of formaldehyde indicates that tunneling effects are quite significant for rates of 109 s-1 or slower. Isotope effects are also considered and seen to be quite interesting. © 1979, American Chemical Society. All rights reserved.