Theoretical Treatment of Environmental Effects on Spectra and Chemical Reactivity

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This research is supported by the NSF theoretical and computational chemistry program. Reaction field models will be developed for the prediction of solvent effects on structure, reactivity and spectroscopic properties of large molecular systems. Initial tests of the theory will employ semi-empirical molecular electronic structure methods. Subsequent tests will be carried out using ab initio calculations for small molecular systems. Specific aspects of the reaction field model to be studied include the shape of the solute cavity and the nature of the electronic distribution that generates the reaction field. The theoretical model will be applied to the calculation of: (a) solvatochromic effects in dyestuffs, (b) solvation effects on reaction rates and equilibria, and (c) absolute solvation energies for various solutes ranging from nonpolar to zwitterionic to ionic. Remarkable success has been achieved in recent years in understanding and predicting the properties of gas phase molecules from first principles (by approximately solving the electronic Schroedinger equation), but far less success had been achieved for molecules dissolved in liquid solution. It is well known that dissolving a molecule (the solute) in a solvent can dramatically change its color, its other spectroscopic properties, and its chemical reactivity. Reaction field models attempt to describe the solute-solvent forces responsible these observed effects. A number of such reaction field models have been proposed, but considerable testing is still required to determine the validity of the many mathematical approximations which must be introduced if the model is to be of practical use for large molecules such as dyes or various light-sensitive pharmacological agents.