Isotopic branching in methane oxidation: fate of deuterium in CH3D + OH

Ascertain whether, in the grain-surface reaction involving deuterated methane (CH3D) and OH within H2O-rich ices, the deuterium atom preferentially transfers to the methyl radical (forming CH2D) or to water (forming HDO), and determine whether this methane-oxidation pathway can subsequently produce deuterated methanol via CH3 + OH → CH3OH under prestellar core conditions.

Background

Methanol can form in H2O-rich ices before CO freeze-out via CH4 + OH → CH3 + H2O followed by CH3 + OH → CH3OH. If CH3D or OD is present, deuteration could occur along this route.

The isotopic fate of D in the initial CH4 + OH step (to CH2D vs. HDO) critically determines whether this pathway contributes to deuterated methanol formation.