Abstract
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 long lived 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 multi collision 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 H(2)CO, OH, H(2)O 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-H(2)CO complexes can be formed up to 100 K or higher temperatures, while the weaker He-H(2)CO complexes dissociate at approximately 50 K. The predicted IR intensity spectra shows new features which could help the identification of the OH-H(2)CO complex. Finally, the He-H(2)CO + OH and OH-H(2)CO + He collisions are studied using quassi-classical trajectories, finding that the cross section to produce HCO + H(2)O products increases with decreasing collision energy, and that it is ten times higher in the He-H(2)CO + OH case.