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→H ₂ O+CO (1) and HCOOH→CO ₂ +H ₂ (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 H ₂ O geometry in the TS of reaction (1) was predicted to significantly deviate from the equilibrium one, whereas the CO ₂ and H ₂ 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 H ₂ , are both vibrationally and rotationally excited. The calculated relative population of the vibrationally excited CO for the 248 nm photodissociation was consistent with experiment.