Gas-phase and solution-phase proton transfer to H2O analyzed by high-level ab initio quantum chemistry including complete basis set and Gaussian theory schemes

Free energies of the gas-phase proton transfer to H2O are calculated for H2O, formic acid, acetic acid, phenol and 3-chlorophenol using high-level quantum chemistry including DFT, CBS and Gaussian theory. The overall best agreement with experimental data within 8 kJ/mol is achieved by MP2//6-311+G(2d,2p) and DFT. For the aromatic compounds, G1 and G2 yield errors of 600–750 kJ/mol, which can be traced back to artefacts of MP4-level basis set corrections. Comparison with solution-phase pKa suggests that protonated water clusters are energetically significant for the dissociation in aqueous solution.