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 C N 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 C N 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 C N 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 C N 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 C N 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.

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Experimental studies of the Raman scattering of the band of C = O vibrations of acetone (1710 cm–1) showed that the parallel and perpendicular polarized components have a large half-width (respectively, 11.6 and 18 cm–1) and also the bands' maxima of these components are shifted by ~5 cm–1. In the neutral solvent (heptane), the difference of the maxima position of the bands decreases. Calculations showed that the molecules of acetone can aggregate to form a dimer with the energy gain of 10.1 kJ/mole. In the dimer several hydrogen bonds are formed between the oxygen atom of one molecule and the hydrogen atoms of CH3 -group of another molecule. In an aqueous mixture of acetone, according to calculations, there is a possibility for formation of dimers and closed trimer aggregates with the energy gain, respectively, 19.1 and 45.8 kJ/mole. Calculation showed that symmetric and antisymmetric O–H vibrations of water are displaced in the interaction with acetone to lower frequencies, respectively, to 3808.4 and to 3931.8 –1.