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

The proton transfer between imidazole and water was studied by quantum chem ical calculations in the presence of further ligand water molecules. In par ticular, we investigated the effect of the position of secondary waters rel ative 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 ch arges. We found that at close distances the charges need to be enhanced to account for induced polarization. As a further simplification, the environm ental 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 cha rges of imidazole and the primary water.