Competitive threshold collision-induced dissociation: Gas-phase acidities and bond dissociation energies for a series of alcohols

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 Delta(acid)H(298)(CH3OH) = 1599 +/- 3 kJ/mol, Delta(acid)H(298)(CH3CH2OH) = 1586 +/- 5 kJ/mol, Delta(acid)H(298)((CH3)(2)CHOH) = 1576 +/- 4 kJ/mol, and Delta(acid)H(298)((CH3)(3)COH) = 1573 +/- 3 kJ/mol.