Calculations of Dissociation Dynamics of CHOH on a Global Potential Energy Surface Reveal the Mechanism for the Formation of HCOH; Roaming Plays a Role.

The experimental observation of hydroxymethylene, HCOH, following excitation of methanol at 193 nm, was reported recently (Hockey, E. K.; McLane, N.; Martí, C.; Duckett, L.; Osborn, D. L.; Dodson, L. G. Direct Observation of Gas-Phase Hydroxymethylene: Photoionization and Kinetics Resulting from Methanol Photodissociation. 2024, 146, 14416-14421, 10.1021/jacs.4c03090). This stimulated us to investigate a dynamic mechanism for its formation using a global potential energy surface for the ground electronic state (S) of methanol. We show via quasi-classical trajectory calculations that hydroxymethylene is indeed formed under the reasonable assumption that the initially excited state undergoes rapid internal conversion to S. From the trajectories, fractional yields of the six major product channels are determined as a function of excitation energy, and the rate of production of each is determined from the fractions and rate of methanol disappearance. Roaming is observed in trajectories leading to the OH and CH products as well as in those leading to CHOH + H and non-reactive trajectories. A "frustrated" roaming accounts for roughly 20% of the HCOH production.