Probing hydrogen bonding environments: solvatochromic effects on the CN vibration of benzonitrile.

The energy of the nitrile stretching mode in benzonitrile is known to be solvent-dependent. Kamlet-Taft solvatochromic parameters pi* and alpha are used to model this dependence in both protic and aprotic solvents using multivariable linear regressions. Aprotic solvents induce red shifts that are shown to be proportional to pi*. In protic solvents, the positions of the two bands attributed to distinct solvation states, F and H, are both fit to the Kamlet-Taft solvatochromic equation. The inclusion of an alpha term yields excellent correlations with the H band, indicative of the strong influence of hydrogen bonding interactions occurring at the nitrile nitrogen in this solvation state. The F band also is best fit by inclusion of the alpha term, albeit with a weaker overall dependence and a much poorer fit than that for the H band. DFT calculations on simple single-molecule complexes with benzonitrile are consistent with the presence of pi-H bonds to the nitrile group as a possible source of the F band dependence on alpha.