IR spectroscopy of an exceptional H-bonded liquid: water

Abstract

Water is so familiar that we often consider it an ordinary liquid. It is however an exceptional one which displays unique physical, chemical and biological properties which are all due to its exceptionally high density of H-bonds. These unique properties may be considered as satisfactorily understood. In a paradoxical way, the more ordinary properties of water are not satisfactorily understood. Up to now, for instance, there has been no clear answer to the question of how such a species made of small molecules held together by directional H-bonds can still be a liquid. As a second paradox, water is a species which has been studied by almost all available techniques. One technique is, however, surprisingly lacking: IR spectroscopy in the fundamental region, which has for a long time been hampered by the exceptionally high absorptivity of water which makes usual transmission techniques inappropriate. Implementation of an ATR (Attenuated Total Reflection) set-up has given precise IR spectra of water for wavenumbers higher than 700 cm −1. Quantitative analyses of the variations of these spectra with temperature and H/D isotopic concentration has led to a novel description of the structure and dynamics of water at the molecular level. The fluidity of water at room temperature appears then to be due to the presence in an appreciable concentration of water molecules performing rotations of a diffusive nature around one of their axes only. It is not, as is usually thought, due to the presence of broken H-bonds which remain at a low concentration at temperatures lower than 100°C. Beyond this analysis, this ATR technique seems promising in making IR spectroscopy an appropriate technique for the study of aqueous media.