Proton transfer as a function of hydration number and temperature: rate constants and product distributions for OH-(H2O)0,1,2,3 + HF at 200-500 K

Abstract

Proton-transfer reactions which are exothermic in the gas phase are exceedingly efficient - most reacting at essentially every collision. Several studies have shown how this extreme reactivity is influenced by solvation in the gas phase at 300 K. Here we report how hydration influences the rate constants and the product distributions of the reaction OH/sup -/ + HF = H/sub 2/O + F/sup -/ ..delta..H/sup 0/ = -19.5 kcal/mol/sup 17/ (1) throughout the temperature range 200-500 K. Such studies in the gas phase - where the solvation number is systematically changed - examine the role of solvate in the transfer of protons. Reaction 1 - a proton-transfer reaction involving only four atoms - has been studied in a selected ion flow tube (SIFT) in the temperature range 200-500 K. We found that the experimental rate constants (each a mean of at least three determinations) decrease monotonically with increasing temperature. The data agree, within the limits of experimental error, with theoretical predictions for collision rate constants shown as solid lines. These are derived by using the adiabatic capture model (ACCSA) due to Clary: this model has already shown impressive predictive success for proton-transfer reactions in this temperature range. We therefore concludemore » that the temperature dependence is that of the physical collision efficiency and not that of the chemical reaction efficiency. Proton transfer, efficient at 300 K, is efficient throughout 200-500 K.« less