  Observations of star forming environments revealed that the abundances of
some deuterated interstellar molecules are markedly larger than the cosmic D/H
ratio of 10-5. Possible reasons for this pointed to grain surface chemistry.
How- ever, organic molecules and water, which are both ice constituents, do not
enjoy the same deuteration. For example, deuterated formaldehyde is very
abundant in comets and star forming regions, while deuterated water rarely is.
In this article, we explain this selective deuteration by following the
formation of ices (using the rate equation method) in translucent clouds, as
well as their evolu- tion as the cloud collapses to form a star. Ices start
with the deposition of gas phase CO and O onto dust grains. While reaction of
oxygen with atoms (H or D) or molecules (H2) yields H2O (HDO), CO only reacts
with atoms (H and D) to form H2CO (HDCO, D2CO). As a result, the deuteration of
formaldehyde is sensitive to the gas D/H ratio as the cloud undergoes
gravitational collapse, while the deuteration of water strongly depends on the
dust temperature at the time of ice formation. These results reproduce well the
deuterium fractionation of formaldehyde observed in comets and star forming
regions and can explain the wide spread of deuterium fractionation of water
observed in these environments.
