  The formation of singly and doubly deuterated isotopomers of formaldehyde and
of singly, doubly, and multiply deuterated isotopomers of methanol on
interstellar grain surfaces has been studied with a semi-empirical modified
rate approach and a Monte Carlo method in the temperature range 10-20 K.
Agreement between the results of the two methods is satisfactory for all major
and many minor species throughout this range. If gas-phase fractionation can
produce a high abundance of atomic deuterium, which then accretes onto grain
surfaces, diffusive surface chemistry can produce large abundances of
deuterated species, especially at low temperatures and high gas densities.
Warming temperatures will then permit these surface species to evaporate into
the gas, where they will remain abundant for a considerable period. We
calculate that the doubly deuterated molecules CHD2OH and CH2DOD are
particularly abundant and should be searched for in the gas phase of
protostellar sources. For example, at 10 K and high density, these species can
achieve up to 10-20% of the abundance of methanol.

Astrophysics