Raman spectra of CN vibrations of acetonitrile in aqueous and other solutions. Experimental results andab initio calculations

Abstract

Quantum-chemical calculations showed a possibility of formation of dimers in pure acetonitrile, as well as molecular aggregates in its solutions with water and formic acid. Accordingly, a low-wavenumber asymmetry of the CN band in Raman spectra of liquid acetonitrile can be related to presence of dimers. In neutral solvents within the range of concentrations 1–0.1 mole fraction, the asymmetry of the CN band is preserved. However, in aqueous solutions and solutions with formic acid, an asymmetry of another type is observed. First of all, with dilution of acetonitrile, a high-wavenumber asymmetry is observed, and the band broadens. At concentrations that are lower than 0.5 mole fraction, the low-wavenumber asymmetry is observed again. In the present work such peculiarities of the CN band's behavior are related to the presence of aggregates, which consist of molecules of acetonitrile and the proton-donor solvent. A difference of the wavenumber of the CN band's maxima in parallel and perpendicular polarizations of the scattered light also indicates the presence of aggregates in acetonitrile–water solution. This difference changes with changing concentration of acetonitrile. The wavenumber difference is connected with the complexity of the band, since within the band there is an overlap of the CN bands of acetonitrile molecules, which either form H-bonds with molecules of water or are free from H-bonds. The overlapping bands are different with respect to wavenumber and depolarization ratio. Copyright © 2005 John Wiley & Sons, Ltd.