Richard S. Moog

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.

Solvent effects on electronic transitions in several coumarins

Abstract Absorption and emission energies of four coumarins (coumarins 1, 102, 152, 153) in a variety of alcohols and aprotic solvents are analyzed in terms of the solvatochromic parameter and dielectric continuum models. Although the results for all four solutes are well described using the empirical solvatochromic parameters, some deviations from the predictions of the more theoretically based approach are observed in alcohol solvents, particularly for those coumarins expected to exhibit better hydrogen-bond-accepting ability.

Dielectric friction and chemical dynamics in solution

The role of dielectric friction in solvation dynamics and rotational motion is examined. Using the connection between solvatiOn dynamics and time dependent dielectric friction developed by van der Zwan and Hynes, it is demonstrated that solvation times determined from time dependent fluorescence studies can be used to accurately gauge dielectric friction contributions to the rotational diffusion of an oxazine dye in alcohol solutions. The generality of these conclusions requires that additional probe molecules, spanning a large range of permanent dipole moments, be studied. A series of amino substituted fluorenones are shown to be an excellent choice of molecules for such studies.