Energetic Requirements for Geometric Deformation

The H-bond stretching frequency provides a measure of the curvature of the bottom of the well in the intermolecular potential. It is therefore instructive to point out that correlates fairly well with the interaction energy. These frequencies are rather small, on the order of 100 cm , and occur in the far-IR region of the spectrum. As a point of comparison, the stretching frequency of the C—F covalent bond typically appears at approximately 1000 cm . Since the force constant is proportional to the square of the frequency, one can say that the curvature of the H-bond potential is only some 1/100 that of the C —F bond. 

V( make it apparent that it is more difficult to bend the proton donor molecule within the plane of the H-bond than out of this plane. 

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