Rate and equilibrium constant measurements for gas-phase proton-transfer reactions involving H2O, H2S, HCN, and H2CO

Abstract

The flowing afterglow technique has been employed in the measurement of rate and equilibrium constants at 296 ± 2 K for unsolvated proton transfer reactions of the type [Formula: see text] and several solvated proton transfer reactions of the type [Formula: see text] where X and Y may be H2O, H2S, HCN, or H2CO. Where possible, direct comparisons are made with similar measurements performed with other techniques. The equilibrium constant measurements provide a measure of the relative proton affinities of H2O, H2S, HCN, and H2CO and absolute values for PA(H2O) = 166.4 ± 2.4 kcal mol−1, PA(H2S) = 170.2 ± 1.8 kcal mol−1, and PA(HCN) = 171.0 ± 1.7 kcal mol−1 when reference is made to PA(H2CO) = 170.9 ± 1.2 kcal mol−1 which can be derived from available thermochemical information. The rate constant measurements reinforce the generalization that unsolvated proton transfer involving simple molecules proceeds with high efficiency and provide information about the influence of solvation on this efficiency.