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.

The wavenumbers of peaks of parallel and perpendicular polarized components of the C O vibration band for dimethylformamide in the Raman spectra differ by 14.5 cm −1 . The difference in wavenumber values decreases and falls to zero on strong dilution with CCl 4 and dimethyl sulfoxide. A doublet structure of the band is observed in solutions with CCl 4 in the 0.2 mole fraction concentration region. The abovementioned non-coincidence of C O vibration band wavenumbers in pure liquid is explained by the complexity of the band, i.e. the presence within the band of two close and overlapped lines with different magnitudes of depolarization ratio belonging to monomeric molecules (high wavenumber) and aggregate formation. In a mixture at strong dilution there remains only one type of formation, and the band acquires a simple shape of a separate line, and the above-mentioned wavenumber difference falls to zero. The experimental results are supplemented by ab initio calculations of normal vibrational wavenumbers and of some parameters for monomeric and dimeric dimethylformamide molecules.

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