Isotope exchange in the reactions H 3O + + D 2O, NH 4+ + ND 3, CH 5+ CD 4 and their mirror reactions at thermal energies

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Results are presented for the rate coefficients and production distributions of the near-thermoneutral reactions: H 3O + + D 2O and D 3O + + H 2O at 295 K; NH 4 + + ND 3 and ND +4 + NH 3 at 204, 295 and 475 K; and CH 5 + + CD 4 and CD 5 + + CH 4 at 80, 204, 295 and 475 K. The data obtained are interpreted in terms of the lifetimes τ d with respect to unimolecular decomposition of the intermediate complexes formed in the reactions, and the shuttling times τ s of the protons and deuterons within the complexes. For the strongly bonded complexes formed in the water reactions, τ d ⪢ τ s, and consequently the rate coefficients are near to the collisional-limiting values and the product distributions are statistical. For the ammonia reactions, the complexes are somewhat more weakly bonded, and consequently τ d is not very much larger than τ s, the rate coefficients are nearly collisional, and the product distributions are not quite statistical. For the methane reactions, the complexes are short-lived and consequently τ d, is smaller than τ s; this is manifested by marked changes of the rate coefficients with temperature and by product distributions which are far from statistical. In an accompanying paper (M.J. Henchman, D. Smith and N.G. Adams, Int. J. Mass Spectrom. Ion Phys., 42 (1982) 25–32) a procedure is described where the zero-point energies of the reactants and products, and in the case of the methane reactions by utilising the production distributions presented in this paper.