Infrared spectra of water. II : Dynamics of H2O(D2O) molecules

Abstract

Spectra of $\varepsilon''$ (imaginary dielectric constant) of water in the IR region, obtained as described in a previous article by using attenuated total reflection (ATR) techniques, are analyzed. Criteria are presented which allow us to define, on the basis of spectroscopical arguments only, the low temperature spectrum $\varepsilon''$(TL) and the high temperature spectrum $\varepsilon''$(TH) on which any spectrum at temperature T, in the range $-5~^\circ {\rm C}$ to $80~^\circ {\rm C}$, may be decomposed. These spectra display novel features : their νs bands (intramolecular stretch) have shapes which have great similarities in both spectra ; theδ band (intramolecular bend) of the high temperature spectrum $\varepsilon''$(TH), has a simple shape with no apparent structure, which may be approximated, with a good precision, by a Lorentzian curve. A discussion of the origin of these features reveals a new picture of water at a molecular level : it is made of H2O molecules which either perform rotations of a vibrational character (librations) around their three axes and appear in $\varepsilon''$(TL) or perform rotations around their C2 symmetry axis z of a diffusional (relaxational) character. H2O molecules of the latter type appear in $\varepsilon''$(TH). Their concentration reaches $\approx 35~\%$ at 0 ○C (1 atm.) and $\approx 90~\%$ at $100~^\circ {\rm C}$ (1 atm.). They have rotational energies around z greater than the maximum of the potential energy governing this rotation. Around their other two axes they perform librations which qualitatively do not differ from those performed by molecules appearing in $\varepsilon''$(TL). The estimated correlation time, or average time during which phase coherence of δ vibrations in these molecules is kept, falls in the range 10-14-10-13 s, as deduced from the Lorentzian shape of the δ band in $\varepsilon''$(TH). The analysis of these spectra also indicates that the “δ+ νL” band borrows its intensity from νs via a cubic term in the vibrational potential.