11 - Observing the Water Molecule

Abstract

This chapter examines experimental and theoretical methods to observe or describe H2O molecules. It mainly concerns infrared (IR) spectrometry, which is the most precise and powerful method to study H-bonds but has up to now scarcely been used to observe H2O molecules for paradoxical reasons. Most classical methods of physical chemistry can give only partial views on the structure and dynamics of the extended H-bond network this small molecule always develops around itself because of its exceptional structure that allows it to donate two H-bonds and accept other two H-bonds at the same time. The method that is by far the most precise and the most powerful to observe H-bonds—namely, infrared (IR) spectroscopy—is consequently the one that allows one to observe H2O molecules with the greatest accuracy. The paradox is that it has for long been hindered as a method of observation of H2O molecules because of its hypersensitivity to H-bonds. This paradox was so developed that H2O molecules were considered a poison for IR spectroscopy. This hindrance has only recently been recognized. After its origin had been identified, methods could then be devised that make IR spectrometry, or quantitative spectroscopy, a very precise tool to observe this H-bond network. It is however too recent to be a routine method. There is no doubt that its great efficiency may make it so in the near future. Relative intensities of bands can thus be accurately measured because of its very good signal-to-noise ratio. The exploitation of these measurements allows one to translate hydration spectra.