Competitive Threshold Collision-Induced Dissociation: Gas-Phase Acidity and O−H Bond Dissociation Enthalpy of Phenol

Energy-resolved competitive collision-induced dissociation methods are used to measure the gas-phase acidity of phenol relative to hydrogen cyanide. The competitive dissociation of the [C 6 H 5 O·H·CN] - complex into C 6 H 5 OH + CN - and C 6 H 5 O - + HCN is studied using a guided ion beam tandem mass spectrometer. The reaction cross sections and product branching fractions are measured as a function of collision energy. The enthalpy difference between the two reaction 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 phenol gas-phase acidity, Δ acid H 0 (C 6 H 5 OH) = 1448 ± 8 kJ/mol, is determined relative to the established literature value of hydrogen cyanide, Δ acid H 0 (HCN) = 1462.3 ± 0.9 kJ/mol. We then derive Δ acid H 298 (C 6 H 5 OH) = 1454 ± 8 kJ/mol and the bond dissociation energy of D 298 (C 6 H 5 O−H) = 359 ± 8 kJ/mol.