Deuterium Enrichment of Interstellar Methanol Explained by Atom Tunneling

We calculate, down to low temperature and for different isotopes, the reaction rate constants for the hydrogen abstraction reaction H + H3COH → H2 + CH2OH/CH3O. These explain the known abundances of deuterated forms of methanol in interstellar clouds, where CH2DOH can be almost as abundant as CH3OH. For abstraction from both the C- and the O-end of methanol, the barrier-crossing motion involves the movement of light hydrogen atoms. Consequently, tunneling plays a dominant role already at relatively high temperature. Our implementation of harmonic quantum transition state theory with on the fly calculation of forces and energies accounts for these tunneling effects. The results are in good agreement with previous semiclassical and quantum dynamics calculations (down to 200 K) and experimental studies (down to 295 K). Here we extend the rate calculations down to lower temperature: 30 K for abstraction from the C-end of methanol and 80 K for abstraction from the OH-group. At all temperatures, abstraction from the C-end is preferred over abstraction from the O-end, more strongly so at lower temperature. Furthermore, the tunneling behavior strongly affects the kinetic isotope effects (KIEs). D + H3COH → HD + CH2OH has a lower vibrationally adiabatic barrier than H + H3COH → H2 + CH2OH, giving rise to an inverse KIE (k H/k D < 1) at high temperature, in accordance with previous experiments and calculations. However, since tunneling is more facile for the light H atom, abstraction by H is favored over abstraction by D below ∼135 K, with a KIE k H/k D of 11.2 at 30 K. The H + D3COD → HD + CD2OD reaction is calculated to be much slower than the D + H3COH → HD + CH2OH, in agreement with low-temperature solid-state experiments, which suggests the preference for H (as opposed to D) abstraction from the C-end of methanol to be the mechanism by which interstellar methanol is deuterium-enriched.