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The Solvation of Acetonitrile

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posted on 06.04.1999, 00:00 by Jeffrey R. Reimers, Lachlan E. Hall
Acetonitrile is an extremely important solvent and cosolvent. Despite this, we have no general picture of the nature of mixed liquids containing acetonitrile applicable across-solvent families. We consider the properties of acetonitrile dissolved in 33 solvents, focusing on interpretation of the environment-sensitive solvent shift, Δν, of its CN stretch frequency, ν2. The two major models (dispersive and specific solvation) which have been proposed to interpret Δν are based on diverse experiments with incompatible conclusions. We ascertain the robust features of these models and combine them into a new one in which solvent−solvent and solvent−solute forces compete to determine the structure of the solution and hence Δν. First, Δν is analyzed in terms of solvent repulsive and dielectric effects combined with specific solvation effects. To interpret this specific solvation, 95 MP2 or B3LYP calculations are performed to evaluate structures and CN frequency shifts for CH3CN complexed with one molecule of either water, methanol, ethanol, 2-propanol, tert-butyl alcohol, phenol, benzyl alcohol, acetic acid, trifluoroacetic acid, 2,2,2-trifluoroethanol, 1,1,1,3,3,3-hexafluoro-2-propanol, acetonitrile, chloroform, carbon tetrachloride, tetrahydrofuran, formamide, pyridine, or Cl-, as well as 45 parallel calculations for the solvent monomers or dimers. The results are then convolved using known structural properties of the various solutions and/or related neat liquids, leading to an interpretation of the observed solvent shifts. Also, we measure Δν for acetonitrile in aqueous solution using Fourier transform Raman spectroscopy and show that the results are consistent with, but require modification of, microheterogeneity theories for the structure of acetonitrile−water solutions. Although such theories are still in their infancy, we suggest that microheterogeneity could also account for most known properties of acetonitrile−alcohol solutions and, in fact, be a quite general phenomenon.