On the possible interconversion between pairs of deuterated isotopomers of methanol, its ion, and its protonated ion in star-forming regions

Deuterated isotopomers of methanol have been detected both in hot cores and in the protostellar source IRAS 16293-2422. It is currently thought that these species are formed on cold grain surfaces in a previous era by the hydrogenation and deuteration of CO via reactions with atomic H and D. Methanol and its singly, doubly, and multi-deuterated isotopomers then evaporate or are sputtered into the gas as star formation occurs. In studying the post-evaporative gas-phase chemistry of these isotopomers, it is important to know if pairs of isotopomers with D atoms in different places (e.g. CH3OD and CH2DOH) can be interconverted or whether they can be viewed as separate entities with depletion mechanisms that are independent of each other. Here we show that it is difficult to exchange protons and deuterons on the two different parts of the methanol backbone. It is then possible that CH3OD can be destroyed on a Much faster time scale than CH2DOH, an explanation that may partially explain the relatively low abundance of the former in IRAS 16293-2422.