Studies of the binary reactions of H3O+⋅(H2O)0,1,2 ions and their deuterated analogues with D2O, H2O, and NH3

The rate coefficients and product ion distributions for the binary reactions of H3O+⋅(H2O)0,1,2 and D3O+⋅(D2O)0,1,2 ions with D2O and H2O, respectively, and with NH3 have been studied at 300 K using a selected ion flow tube (SIFT) apparatus. The ions were created in a flowing afterglow ion source and after mass filtering were injected at low energy into the SIFT. All the reactions proceeded at or near the gas kinetic limit. In the D2O and H2O thermoneutral isotopic exchange reactions, the distribution of H and D amongst the product ions and neutrals was seen to be purely statistical. This implies that these reactions proceed via the formation of an intermediate long-lived association ion in which total randomization of the H and D atoms takes place prior to unimolecular decomposition. No appreciable isotopic exchange occurred in the exothermic NH3 reactions which apparently proceeded via the simpler mechanisms of D+ (or H+) or D3O+ (or H3O+) transfer. The differing mechanisms for the H2O and D2O reactions compared to the NH3 reactions are rationalized in terms of the thermicities of the reactions and the lifetimes of the respective intermediate ions.