Hydrogen bonding: How much anharmonicity?

In moderately strong hydrogen bonds, hydrogen bond formation increases the anharmonicity constant of the high frequency stretching vibration, significantly but not dramatically. This increase tends to increase with the strength of the hydrogen bond. The main cause of the fine structure and breadth of this band is, however, coupling with both the low frequency stretching and bending vibrations of the bridge, despite the smallness of the coupling constants. Second–order perturbation theory is sufficient to interpret the observed frequencies in the case of moderately strong hydrogen bonds. HCN⋯HF, O–H⋯:O, O–H⋯:N, and N–H⋯:N hydrogen bonds are considered.