Photodissociation Dynamics of Formic Acid at 230 nm: A Computational Study of the CO and CO2 Forming Channels.

Abstract

Recent photolysis experiments with formic acid suggest that the roaming mechanism is a significant CO-forming pathway at a photolysis energy of 230 nm. While previous computational studies have identified multiple dissociation pathways for CO-forming channels, the dynamic features of these pathways remain poorly understood. This study investigates the dissociation dynamics of the CO + H₂O and CO₂ + H₂ channels in the ground state (S₀) of formic acid using direct dynamics simulation and the generalized multi-center impulsive model (GMCIM) at 230 nm. Computational results summarize the characteristics of the product states from six different dissociation pathways, including two roaming pathways. A comparison of the simulated speed distribution of CO products with experimental observations shows that high-rotational CO products predominantly originate from the three-center dissociation pathway. Furthermore, while experimental results reveal a bimodal speed distribution of CO at low rotational states, our findings suggest that the OH roaming pathway contributes to the fast component of this distribution, rather than the slow component. Furthermore, another isomerization-mediated four-center pathway contributes negligibly to the experimental results. The agreement between computational results and experimental observations at 230 nm supports the previously proposed dissociation mechanism of the CO + H₂O channel. For the CO₂ + H₂ channel, this study provides useful information for experimental identification of dissociation pathways in the future.