Modelling deuterium fractionation in interstellar clouds

Over 20 molecules containing deuterium have been detected in interstellar clouds. Two molecules, D2CO and NHD2, contain two deuterium atoms. The abundances of these deuterated molecules, relative to their hydrogenated analogues, have values which can be up to 10,000 times greater than the cosmic D/H ratio. I review the primary processes by which deuterium is fractionated in interstellar molecules, describe the routes by which secondary fractionation occurs and compare the results of model calculations of fractionation with observation. The roles of accretion and surface chemistry in enhancing the level of fractionation are discussed, particularly in the context of synthesising doubly deuterated species. Non-thermal effects, which can alter fractionation on small scales, are also discussed briefly.