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 – 34.1n(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.