Coupling of three intramolecular vibration modes of liquid H2O molecules in the framework of the fluctuation theory of hydrogen bonding

• Published in: Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy Molecular and biomolecular spectroscopy, 2021-01, Vol.244, p.118772, Article 118772
• Main Authors: Efimov, Yu.Ya, Naberukhin, Yu.I.
• Format: Article
• Language: English
• Subjects: Fermi resonance; Fluctuation theory, vibration spectra; H-bonding; Water
• ISSN: 1386-1425
• DOI: 10.1016/j.saa.2020.118772

Vibrational spectra of liquid water contain a wealth of information about its structure and dynamics but there are no generally acknowledged interpretation of their band profiles, unfortunately. We have tried to calculate them taking into account only the coupling of three intramolecular vibrations in the set of H2O molecules with different initial oscillator frequencies and intensities. The matter is that each water molecule forms hydrogen bonds of different strengths; thus the OH stretching band spans several hundreds of wave numbers (the fluctuation theory of hydrogen bonding). This distribution overlaps with the similar band of the first overtone of the HOH bending frequencies thus triggering a Fermi resonance between three vibrations. There were some problems causing some simplifications in previous theoretical modeling of vibrational transitions in condensed water. To solve them we extract the statistical distribution of OH frequencies of H2O molecules directly from the experimental spectra of HOD molecules at the same conditions instead of defining it theoretically. Also the bending overtone is allowed to have non-zero intrinsic intensity when calculating the Fermi resonance. The test calculation of the isotropic component of the Raman spectrum which is most critical for interpretation shows that our algorithm can reproduces the characteristic peculiarities of the experiment. The spectrum consists of three non-Gaussian contours. The overtone of bending vibration, being strengthened by Fermi resonance, makes the greatest contribution to the dominating low-frequency spectrum component that was previously attributed to intermolecular coupling of adjacent OH oscillators by some authors. Further we plan to calculate similar band profiles in the IR and also Raman isotropic and anisotropic spectra of Н2О and D2О molecules within a wide temperature range for their quantitative comparison with experiment. 3 variants of isotropic Raman band of liquid H2O calculated from the HOD spectrum: 1,2: Fermi doublet; 3: out-of-phase stretching vibrations. Solids: their sums. Red: Iov = 0. Blue and black: Iov = 0.1; W = +55 cm−1(blue) and W = −55 cm−1 (black). [Display omitted] •The peak at 3250 cm-1 of H2O water can be explained by a Fermi resonance