Laboratory studies of H/D exchange in the reactions of H3+ with H2

The overall forward (exoergic) rate coefficients ksub(f), and the overall reverse (endoergic) rate coefficients, ksub(r), the product ion distributions are obtained at one or more temperatures. The equilibrium constants, K=ksub(f)/ksub(r) and hence ΔH0 and ΔS0 for the individual reaction channel can be determined from these data. Of the ten reactions in the H3+ + H3 series, four representative reactions are discussed. The studies demonstrate the controlling influence of the enthalpy and entropy differences on isotopic exchange reactions. They also demonstrate that isotopic scrambling occurs in relatively long lived intermediate complexes at low temperatures, whereas at higher temperatures the simpler mechanism of proton (or deuteron) transfer is favoured. The data on these reactions also add to the body of data on ion-molecule isotope exchange reactions which invariably show that the rate coefficients for the exoergic reactions, ksub(f), approach the collisional limiting values, ksub(c) at low temperatures (i.e. when T < |ΔH0/R| )