Exchange reactions between deuterium and hydrogen halides. I. Hydrogen chloride

Abstract
In the majority of gaseous exchange reactions, deuterium atoms, produced photochemically or by an electric discharge, have been employed to initiate the reaction. Only the thermal method offers some prospect of detecting exchange reactions of the type D2 + HA→DA + HD. At high temperatures in addition atomic reactions will occur initiated by thermally dissociated atoms which, although present in very low concentration, react with very much smaller activation energies. Thus thermal exchange reactions can be brought about by two different mechanisms, their relative velocities being mainly dependent on the activation energies of the elementary reactions involved. Using activation energies derived from band spectroscopic data, Eyring and his collaborators attempted to predict the mechanism for several reactions where such alternative mechanisms were possible (Eyring 1931). Thus in the case of the exchange reaction of hydrogen and deuterium the method was successful in eliminating the molecular and predicting the atomic reaction, in agreement with experiment. The reaction of deuterium and hydrogen chloride offers another possibility of testing such calculations. Previous work indicated the existence of a bimolecular reaction, and it seemed, probable that at higher temperatures the atomic mechanism would intrude (Gross and Steiner 1936). It seemed desirable to investigate whether this reaction really proceeded in both ways and whether the quantitative aspects of this more complicated behaviour could be determined. The reactions have an additional interest, in that the transition state theory can be applied to calculate their rate, since they are sufficiently simple to permit us to derive the necessary data (Wheeler, Topley and Eyring 1936). Experiments were performed with deuterium and hydrogen chloride mixtures, as well as with hydrogen and deuterium chloride. In addition the ortho-para-hydrogen conversion catalysed by hydrogen chloride has been studied. The bimolecular reaction was observed, but an additional atomic chain mechanism was found to take part. The results obtained lead to a consistent mechanism from which the rates of the elementary reactions can be derived. These are then compared with the theoretical calculations.

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