Effect of molecular interactions on the O-H stretching force constants for associated water species

Abstract

Semi-empirical molecular-orbital calculations are reported for the O-H stretching force constants of different hydrogen-bonded species of water involving dimeric, trimeric, tetrameric and pentameric associations. These calculations show that- in general- a water molecule bonded to another water molecule through oxygen lone-pair electrons behaves as a stronger acid than an isolated water molecule, and a water molecule bonded to two water molecules through its two oxygen lone pairs is still stronger. This is reflected in the force constants of hydrogen-bonded O-H bonds. These results explain why O-H stretching bands in IR and Raman spectra of liquid water appear at significantly lower frequencies than for solutions of water in organic bases, even though water is a weaker base.