Hydrogen bonds and their fluctuations are one of the factors that determine the unique properties of water . Building models of formation and rupture of hydrogen bonds due to non-eigen vibrations of a molecule of water is to a large extent determined by the availability of accurate information on the geometric structure of the water molecule. Geometric parameters of the water molecule have been well studied for the gaseous state. This was aided by the possibility of an experimental study of the regularities in the rotational spectra of molecules. However, some questions about the geometry of the water molecule in the liquid state remain unanswered. For example, many sources state that the valence angle of the water molecule decreases during the transition into the liquid state . Based on the experimental data of molecular vibration spectra in D<sub>2</sub>O and H<sub>2</sub>O molecules , the authors have estimated valence angle of water in the liquid state. Consequently, the value of the valence angle of water in liquid state was determined to be (89 ±2)°. A question of determination of libration vibrations of water molecule, as well as the analysis of its consequent inversion doubling, based on the new information on the equilibrium angle of the water molecules in the liquid state, constitutes an interest and is discussed in the present paper.
This research is devoted to the vibrational spectroscopy inverse problem solution that gives a possibility to design a
molecule and make conclusions about its geometry. The valence angle finding based on the usage of inverse spectral
vibrational spectroscopy problem is a well-known task. 3N-matrix method was chosen to solve the proposed task. The
usage of this method permits to make no assumptions about the molecule force field, besides it can be applied to
molecules of matter in liquid state. Anharmonicity constants assessment is an important part of the valence angle finding.
The reduction to zero vibrations is necessary because used matrix analytical expression were found in the harmonic
approach. In order to find the single-valued inverse spectral problem of vibrational spectroscopy solution a shape
parameter characterizing “mixing” of ω<sub>1</sub> and ω<sub>2</sub> vibrations forms must be found. The minimum of such a function Υ called a divergence parameter was found. This function characterizes method’s accuracy. The valence angle assessment was reduced to the divergence parameter minimization. The β value concerning divergence parameter minimum was interpreted as the desired valence angle. The proposed method was applied for water molecule in liquid state: β = (88,8 ±1,7)° . The found angle fits the water molecule nearest surrounding tetrahedral model including hydrogen bond curvature in the first approximation.