Accounting for environmental effects in ab initio calculations of proton transfer barriers

The proton transfer between imidazole and water was studied by quantum chemical calculations in the presence of further ligand water molecules. In particular, we investigated the effect of the position of secondary waters relative to the proton transfer system. It is shown that the energy surface of transfer can be well reproduced when these waters are replaced by point charges. We found that at close distances the charges need to be enhanced to account for induced polarization. As a further simplification, the environmental effects of these secondary waters on the proton transfer barriers can be described analytically by the electrostatic interaction of fitted point charges placed at the position of the ligand waters using the Mulliken charges of imidazole and the primary water.