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 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.

Recent Advances in linear and nonlinear Raman spectroscopy I

“…Studying the CN triple bond vibration of acetonitrile in aqueous and other solutions, Tukhvatullin et al 255 were able to show by quantum chemical calculations that there is a possibility of dimer formation and unlike molecule aggregates in these solutions. Studying the CN triple bond vibration of acetonitrile in aqueous and other solutions, Tukhvatullin et al 255 were able to show by quantum chemical calculations that there is a possibility of dimer formation and unlike molecule aggregates in these solutions.…”

Intermolecular H‐bond in propan‐2‐ol and its solutions with acetonitrile

“…1(d)). 13,14 The dipole moment of such an aggregate is 5.05 D, which is higher than the additive sum of the dipole moments of interacting molecules. Accordingly, aggregates propan-2-ol-acetonitrile are formed and should exist for a long time.…”

Raman stpectroscopy of vibrational and rotational relaxation of acetonitrile molecules dissolved in ionic liquids

“…First of all the rotational and translational dynamics of acetonitrile molecules in the neat AN as well as in various solvents have been studied using a wide kit of experimental techniques (FTIR, Raman, NMR relaxations, etc, see [8,[10][11][12] and the references therein). These data are very useful for comparative and metrological purposes.…”

“…Secondary band structures, noted and considered in some earlier studies [9,11,13], are observed at the present instrumental setting as well. Asymmetric shoulders are attributed in some works to the "hot bands" [13], whereas in others [8,10] those extra bands are believed to originate from intermolecular effects (aggregation of AN or H-bonding with solvent molecules). Without getting deep into polemic at the present stage of work, i. e. before starting on AN dynamics in IL environment, it is only important to note that the half-widths and the values of τ vib and τ rot (2) obtained by digital processing of ν2 and ν4 band profiles pertain well to the data available in literature.…”

“…3) disappear in certain solutions, e. g. in water, formic acid, water / polymer gels [10,13]. With increasing dissolution the overlap of bands increases, the sum-contour loses the asymmetry, and in the sequel the overlapping band components can not be resolved by statistical processing using least-squares fitting procedures [13].…”