T J Millar (UMIST), H Roberts (UMIST), A J Markwick (UMIST) and S B Charnley (NASA Ames)
The H2D+ ion is the most important ion in the deuteration of interstellar molecules at low temperatures. We shall briefly discuss the means by which H2D+ is fractionated and the identify the critical parameters which influence the degree of fractionation. We shall describe how deuteron transfer spreads fractionation among interstellar molecules and how observations of these can lead to the derivation of important physical and chemical parameters in interstellar clouds. We shall discuss how the interaction with dust grains and non-thermal destruction of H2D+ can affect the fractionation. Finally, we shall briefly mention some cosmological implications of the deuterium chemistry.