Solvent effects on proton transfer in ammonia-hydrogen halide complexes have been investigated by using the cluster, continuum, and combined discrete-continuum models, respectively. Our results show that for the hydrogen-bonded complexes with less acidic HF subunit, the structure obtained by the continuum model is of neutral type whereas in clusters with at least three molecules of water the ion pair structures have been found. These results suggest that for such hydrogen bonded complex the simple continuum model is not adequate and explicit water molecules should be added in order to take into account specific hydrogen bonds. For the hydrogen-bonded complexes with strong acidic HCl and HBr subunits, a weak solvent effect is adequate to describe proton transfer processes in aqueous solution. The ion pair structures have been found by using the cluster model or the continuum model, and their results are in agreement with those in the combined discrete-continuum model. Because specific solute-solvent interactions were described at the quantum level and long-range interactions were introduced through a continuum model, the combined discrete-continuum model would lead to more accurate solvation properties and was suitable for hydrogen-bonded complexes with both strongly and weakly acidic HX subunit.

Key words: XH-NH₃(X=F,Cl,Br), proton transfer, solvent effect, cluster model, continuum model, combined discrete-continuum model