Direct ab initio molecular dynamics study of the two photodissociation channels of formic acid

A total of ∼1200 trajectories have been integrated for the two photodissociation channels of formic acid, HCOOH → H2O + CO (1) and HCOOH → CO2 + H2 (2), which occur with 248 and 193 nm photons, using the direct ab initio molecular dynamics method at the RMP2(full)/cc-pVDZ level of theory. It was found that the percentage of the energy distributed to a relative translational mode in reaction (2) is much larger than that in reaction (1). This is mainly due to the difference in the geometry of transition state (TS); the H2O geometry in the TS of reaction (1) was predicted to significantly deviate from the equilibrium one, whereas the CO2 and H2 geometries in the TS of reaction (2) were found to be more similar to their equilibrium ones. It was also found that the product diatomic molecules, CO and H2, are both vibrationally and rotationally excited. The calculated relative population of the vibrationally excited CO for the 248 nm photodissociation was consistent with experiment.