Rate coefficients at 297 K for proton transfer reactions with H2O. Comparisons with classical theories and exothermicity

Rate coefficients for proton transfer reactions of the type XH⁺ + H₂O → H₃O⁺ + X where X = H₂, CH₄, CO, N₂, CO₂ and N₂O and the type H₂O + X^? → XH + OH^? where X = H, NH₂ and C₂H₅NH have been measured at 297 K using the flowing afterglow technique. The results compare favourably with the predictions of the average-dipole-orientation theory. A trend is observed with exothermicity on a plot of (k_exp/k_ADO)_{298 K} versus ?ΔH_{298 K}⁰. The question is raised whether the relatively low probability observed for slightly exothermic proton transfer reactions is a consequence of reaction mechanism or results from the presence of a small activation energy barrier.