Photochemical reactions of the low-lying excited states of formaldehyde: T1/S0 intersystem crossings, characteristics of the S1 and T1 potential energy surfaces, and a global T1 potential energy surface.

Accurate ab initio calculations using the multireference configuration interaction method have been performed to characterize the potential energy surfaces (PESs) of low-lying excited states (S(1) and T(1)) of formaldehyde (H(2)CO) and hydroxymethylene (HCOH) with emphasis on their isomerization, dissociation, and the possible role of the T(1) state in the nonadiabatic photodissociation of H(2)CO. Two regions on the T(1) PES are found to contribute to the nonadiabatic transition to the ground (S(0)) state. Three minima on the seam of crossing (MSXs), 80-85 kcal/mol (above the S(0) global minimum), are located in the HCOH region; they, however, are blocked by a high-energy isomerization transition state at approximately 107 kcal/mol. The other MSX discovered in the H(2)CO region is reachable with energy </=91 kcal/mol and strong spin-orbit interaction; this may be a more important pathway for the T(1) to S(0) transition. A full-dimensional PES is generated for the T(1) state, fitted by a weighted least-squares method employing a many-body expansion in which each term is a function of the internuclear distances and is invariant under permutations of like atoms. The single global function covers the formaldehyde and the HCOH regions as well as dissociation pathways. The high quality of the fitted PES is demonstrated by the small root-mean-square fitting error of 119 cm(-1) and the close agreement between the critical points from ab initio calculations and from the fitted PES.