Raman spectroscopic studies of water-acetonitrile mixtures

The Raman spectra of H2O-HDO-CD3CN mixtures have been investigated over the whole composition range. Changes in the water stretching bands are discussed in terms of the fluctuation model of hydrogen bonding. Contrary to several other nonelectrolytes, acetonitrile does not stabilize the water structure in the range 0 < x2 <approximately 0.2. However, there is a strong tendency for self-association of water molecules down to relatively low water concentrations. This is confirmed both by differences in the Isub(parallel) and Isub(perpendicular) peak maxima of the H2O bending vibration and the changes observed in the shape of the CN band of CD3CN. In solutions with very low water concentrations the water clusters become unstable and the stretching bands show the characteristic features of 1:2 complexes of H2O with acetonitrile, which are now the predominant water species. (author)