Zero- and High-Pressure Mechanisms in the Complex Forming Reactions of OH with Methanol and Formaldehyde at Low Temperatures

A recent ring polymer molecular dynamics study of the reactions of OH with methanol and formaldehyde at zero pressure and below 100 K has shown the formation of collision complexes with long lifetimes, longer than 100 ns for the lower temperatures studied, 20–100 K (del Mazo-Sevillano et al., 2019). These long lifetimes support the existence of multicollision events with the He buffer-gas atoms under experimental conditions, as suggested by several transition state theory studies of these reactions. In this work, we study these secondary collisions, as a dynamical approach to study pressure effects on these reactions. For this purpose, the potential energy surfaces of He with H2CO, OH, H2O, and HCO are calculated at highly accurate ab initio level. The stability of some of the complexes is studied using path integral molecular dynamics techniques, determining that OH–H2CO complexes can be formed up to 100 K or higher temperatures, whereas the weaker He–H2CO complexes dissociate at approximately 50 K. The predicted IR intensity spectra show new features which could help the identification of the OH–H2CO complex. Finally, the He–H2CO + OH and OH–H2CO + He collisions are studied using quasi-classical trajectories, finding that the cross section to produce HCO + H2O products increases with decreasing collision energy, and that it is ten times higher in the He–H2CO + OH case.