The solvation of acetonitrile

Acetonitrile is an extremely important solvent and cosolvent. Despite this, we have no general picture of the nature of mixed liquids containing aceto nitrile applicable across-solvent families. We consider the properties of a cetonitrile dissolved in 33 solvents, focusing on interpretation of the env ironment-sensitive solvent shift, Delta upsilon, of its CN stretch frequenc y, upsilon(2). The two major models (dispersive and specific solvation) whi ch have been proposed to interpret Delta upsilon are based on diverse exper iments with incompatible conclusions. We ascertain the robust features of t hese models and combine them into a new one in which solvent-solvent and so lvent-solute forces compete to determine the structure of the solution and hence Delta upsilon. First, Delta upsilon is analyzed in terms of solvent r epulsive and dielectric effects combined with specific solvation effects. T o interpret this specific solvation, 95 MP2 or B3LYP calculations are perfo rmed to evaluate structures and CN frequency shifts for CH3CN complexed wit h one molecule of either water, methanol, ethanol, 2-propanol, tert-butyl a lcohol, phenol, benzyl alcohol, acetic acid, trifluoroacetic acid, 2,2,2-tr ifluoroethanol, 1,1,1,3,3,3-hexafluoro-2-propanol, acetonitrile, chloroform , carbon tetrachloride, tetrahydrofuran, formamide, pyridine, or Cl-, as we ll as 45 parallel calculations for the solvent monomers or dimers. The resu lts are then convolved using known structural properties of the various sol utions and/or related neat liquids, leading to an interpretation of the obs erved solvent shifts. Also, we measure Delta upsilon for acetonitrile in aq ueous solution using Fourier transform Raman spectroscopy and show that the results are consistent with, but require modification of, microheterogenei ty theories for the structure of acetonitrile-water solutions. Although suc h theories are still in their infancy, we suggest that microheterogeneity c ould also account for most known properties of acetonitrile-alcohol solutio ns and, in fact, be a quite general phenomenon.