Effects of the H-bond bridge geometry on the vibrational spectra of water: The simplest models of the H-bond potential

The approach suggested in this work within the fluctuation theory of the hydrogen bond allows one to correlate the vibrational band shape with the statistical distribution of the geometrical parameters of the O-H...O hydrogen bridge, generated by the local fluctuations of the molecular surroundings in liquids. The correlation may be established either exclusively from spectral data or by also using the known empirical correlations with structural data. The existing explanations to the anomalous broadening of the spectral bands of associated liquids, namely, the dominant effect of the RO...O bond length or the φ(H-O...O ) bending angle on the OH vibration frequency were tested. Both hypotheses provided an adequate description of the experimental spectra and their temperature dependence. However, the required relationship between the frequency and geometrical parameters conflicts with several empirical facts, indicating that the one-parameter potentials are insufficient for a quantitative description of H-bonds in liquids. A method for the design and verification of more complex potentials containing all geometrical parameters of the hydrogen bridge affecting the H-bond energy is suggested.