Ab initio calculation on the photochemistry of formaldehyde : the search for a hydroxycarbene intermediate

Ab initio calculations on the H2CO-HCOH rearrangement have been performed. The electronic coupling between the SI and SO surfaces, which can induce internal conversion, is calculated for this rearrangement and for the reaction coordinates leading directly to radical and molecular products. The coupling is calculated with true adiabatic Born-Oppenheimer functions, i.e., the wave functions and coupling integrals are explicitly calculated as functions of the nuclear geometry. The coupling for the hydroxycarbene rearrangement turns out to be the largest one. This indicates that the hydroxycarbene can serve as an intermediate state in the formaldehyde photochemistry. We also report calculations on the bimolecular H2COHCOH rearrangement; this interaction gives rise to a decrease of the energy barrier involved.