Spectroscopic studies of vibrational relaxation and chemical exchange broadening in hydrogen bonded systems. IV. Analysis of the isotropic Raman bands of pyridine/water system

Abstract

Raman (isotropic) band profiles for the v 1 band of pyridine in binary mixtures with water have been interpreted in terms of a dynamic exchange model which includes multiple ‘sites’, according to a reasonable equilibrium distribution of ‘complex’ species. At the simplest level, application of the Bratos-Strauss two-site model results in a k 21 value near 3 × 1010 s-1. This exchange rate leads to a rather small additional broadening and, indeed, has proved difficult to detect in the overall band profile. Extension to a three-site model, while useful to achieve a better ‘fit’ to the observed spectra, does not, for this system at least, cast additional light on the relaxation processes involved. In fact, the site-site exchange models may not be adequate for binary mixtures of water with organic bases. This is due to the existence of a very wide distribution, on the time scale of the experiment, of hydrogen bonded environments.