The fluctuation hypothesis of hydrogen bonding

Abstract

On the basis of the fluctuation hypothesis of hydrogen bonding analytical formulae are obtained for the band shape in the infra-red and Raman spectra of stretching vibrations of H₂O molecules with only one OH group involved in H-bonding (so-called complexes of the type 1-1 [1]). The statistical distribution of stretching frequencies ω_OH affected by H-bonds of different strengths is derived from the spectrum of HOD molecules in a similar complex using empirical correlation between the half-width of the band and the shift of its maximum relative to monomeric frequency. The coupling of intramolecular vibrations has been taken into account by means of the method of partial oscillators developed in [3], and the frequencies and intensities have been calculated in vibrational spectra of a continuum of water molecules differently perturbed by H-bonding. The series of computations are performed with varying values of basic parameters of the model in order to establish their influence on the spectra. The results of calculations demonstrate the broadening of the line of ‘free’ OH oscillators, the additional divergence of maxima positions, and redistribution of intensities between ‘free’ and H-bonded oscillators produced by their coupling. A specific behaviour of the depolarization ratio across the Raman bands follows also from calculations. Some of the predictions are in good agreement with experimental data while others are still waiting for experimental verification.