Mode specificity in the unimolecular dissociation of formaldehyde (H2CO→H2+CO), a two-mode model

The reaction path (the minimum energy path in mass-weighted Cartesian coordinates) and all the coupling functions which fully characterize the reaction path Hamiltonian of Miller, Handy, and Adams [J. Chem. Phys. 72, 99 (1980)], have been calculated for the unimolecular dissociation of formaldehyde (H2CO→H2+CO) in its ground electronic state. The reaction coordinate and the four other in-plane vibrational modes are strongly coupled to each other, but the out-of-plane vibration is coupled directly only to the reaction coordinate. Calculations of the type of Waite and Miller [J. Chem. Phys. 73, 3713 (1980); 74, 3910 (1981)] for the state-specific unimolecular rate constants are carried out for a two-mode model consisting of the reaction coordinate and the out-of-plane vibration, and one observes a significant degree of mode specificity; i.e., the unimolecular rate constant for a given metastable state is not a smooth function of the energy of the state. It is suggested that this mode specificity may persist in the complete six-mode system.