Anion-anion repulsion, coordination number, and the asymmetry of hydrogen bonds

By examining which coordination numbers are not known for a large variety of cations, it is found that the closest approach of two O atoms, R(min), is given by, R(min) = 220 - 34.ln(s') pm, where s' is the projection onto the O-O vector of the valence of the bonds formed to a common cation. This equation is then used to show that the asymmetry of O-H...O hydrogen bonds is a consequence of the repulsion between the donor and acceptor O atoms. The model leads to a natural division of hydrogen bonds into three classes: strong, normal, and weak, and it gives good predictions for the geometries of each.