An experimental study of the gas-phase kinetics of reactions with hydrated hydronium(1+) ions(n = 1-3) at 298 K

Abstract

Flowing afterglow experiments are reported which follow the influence of stepwise hydration on the kinetics of pro- ton transfer from the hydronium ion to H2S, CH20, HCOOH, CH3OH, CH3CH0, C~HSOH, CH3COOH, HCOOCH3, (CH3)2O, (CH3)2CO, and NH3 at 298 f 2 K. Rate constants have been measured for the reactions of these molecules with H3O+.H2O, H3Of.(H20)2, and H30+.(H20)3. The reactions were observed to proceed according to the general equation H30+-(H20), + B - BH+.(H20), + (n - m + 1)HzO with 0 C m C n, albeit product distributions could not be unraveled. Comparisons are made with the rate constants for the nude reactions involving H30+. The reversal in the relative basicity of H20 and H2S upon hydration which can be deduced from known energetics of solvation is manifested by a sharp drop (by 2 IO3) in the observed reaction rate constant for n = 1, 2, and 3. For B = CH20, equilibrium-constant measurements are re- ported which indicate a trend toward the equalization of the basicities of H20 and CH20 upon hydration with up to three water molecules. All of the hydrated H3O+ ions are observed to react rapidly and irreversibly with the remaining, somewhat stronger, oxygen bases with k 2 cm3 molecule-' s-' and decreasing slightly and monotonically with increasing hydration. Comparisons with calculated capture rate constants indicate that these reactions proceed with approximately unit probability. The intriguing possibility of generating polymeric water molecules from the reactions investigated is discussed briefly.