Effect of hydrogen bridge geometry on the vibrational spectra of water: Three-parameter potential of H bond

The ability of water molecules to form a three-dimensional network of hydrogen bonds basically determines both the intrinsic structure and unique properties of this liquid and also a character of interactions with other molecules. However, the dependence of the H bond energy on the geometry of its hydrogen bridge, namely on the RO…O length and non-linearity, is unknown, i.e. the deviations of O—H group directions and the lone pair forming this bond from optimum ones (angles (φ = H—O…O and χ = —O…O correspondingly). Even in computer simulation methods, the contribution of H bonds to the total interaction potential is not separated; that does not allow one to define unequivocally these bonds in a model and to analyze quantitatively the features of their networks. The purpose of this work is to fill in this gap by expressing the energy E through geometric parameters (R, φ, χ). The obtained solution quality is proved by an agreement between the distributions of OH vibrational frequencies (which are very sensitive to the H bond strength) calculated with its help and the shape of experimental spectra in a wide temperature range. Based on this, the distributions of H bond energies P(E, T) and of their bend angles P(φ, T) and P(χ, T) are also calculated. It is shown that the main contribution to spectra is made by the shortest bonds with their lengths close to a minimum of the potential E(R). Thus, the low-frequency slope of a band corresponds to slightly bent H bonds, while the central part relates to also short but sufficiently nonlinear H bonds. LongH bonds are responsible for only well known high-frequency Walrafens’s wing near 3620 cm-1. Moreover, these weak bonds are very strongly bent.