Researchers in Sweden have developed a new method to account for the vast abundance of deuterated compounds in space.  

Terry Frankcombe and Gunnar Nyman at the University of Gothenburg in Sweden have focussed their interest on ammonia, a gas with a pungent odour.  'The fraction that is deuterated vastly exceeds what would be statistically expected,' says Frankcombe.  'We have developed a combined dynamics and statistically based model to investigate the formation of NH₂⁺, one of the ammonia precursors. Our model leads us to suggest a new explanation for how the deuterium enrichment happens during the formation of the ion. In all isotopically substituted NH⁺ + H₂ reactions in which both hydrogen and deuterium are present, the NHD⁺ product is preferred.'

More earthly applications have partly motivated this work: 'In collaboration with the Copenhagen Centre for Atmospheric Research, Denmark, isotope effects have been used to better understand the composition of Earth's atmosphere and pin down sources and sinks for various species in the atmosphere,' says Frankcombe. But that isn't all.  This is also an interesting system to study simply because developing a practical yet accurate treatment is challenging, he explains.

Chris Williams, a theoretical chemist at Ohio State University, Columbus, US, explains that theoretical calculations are necessary because 'it is hard to reproduce the low temperatures and pressures of the interstellar environment in the laboratory. Conventional reactive scattering calculations can simulate this type of reaction but the computational cost is considerable.'  This new route using the 'adiabatic capture centrifugal sudden approximation has proved to be invaluable finding overall rate constants on this class of reactions'.

Frankcombe notes that even in the vacuum of interstellar space, treating the molecules in isolation isn't the whole story.  'The focus may shift to reactions occurring on and in grains of dust to complement the gas phase work, as it is has become clear that interstellar chemistry cannot be understood without understanding the condensed-phase chemistry occurring in and on grains.'  They warn that in that case, 'performing accurate dynamics studies with presently available methods becomes essentially impossible'.

Colin Batchelor