Competitive Threshold Collision-Induced Dissociation: Gas-Phase Acidities and Bond Dissociation Energies for a Series of Alcohols

Abstract

Energy-resolved competitive collision-induced dissociation methods are used to measure the gas-phase acidities of a series of alcohols (methanol, ethanol, 2-propanol, and 2-methyl-2-propanol). The competitive dissociation reactions of fluoride−alcohol, [F-·HOR], alkoxide−water, [RO-·HOH], and alkoxide−methanol [RO-·HOCH3] proton-bound complexes are studied using a guided ion beam tandem mass spectrometer. The reaction cross sections and product branching fractions to the two proton transfer channels are measured as a function of collision energy. The enthalpy difference between the two product channels is found by modeling the reaction cross sections near threshold using RRKM theory to account for the energy-dependent product branching ratio and kinetic shift. From the enthalpy difference, the alcohol gas-phase acidities are determined relative to the well-known values of HF and H2O. The measured gas-phase acidities are ΔacidH298(CH3OH) = 1599 ± 3 kJ/mol, ΔacidH298(CH3CH2OH) = 1586 ± 5 kJ/mol, ΔacidH298((...