An analysis of dielectric models of solvatochromism

The manner in which orientational and distortional polarization contributions to solvatochromic effects are isolated within the McRae-Bayliss model of solvatochromism is shown to be untenable. An alternate separation scheme, christened modified McRae-Bayliss, is developed and predicts that distortional contributions to solvatochromic effects diminish in absolute magnitude as the dielectric constant of the solvent increases, an effect which is suported by experimental observation. The correlating ability of McRae-Bayliss, modified McRae-Bayliss, and alternate formulations discussed in the text is tested with both nonpolar and general solvent groups. In the nonpolar solvent limit, the various models are equivalent and suffer a common fate in which systematic class deviations (e.g., perfluorinated or alkane solvent groups) are clearly evident. It is clear that no model which relies solely on bulk dielectric properties will be able to correlate solvatochromic behavior in even nonpolar solvents without systematic lack of fit by solvent class. General solvent correlations are a more difficult issue. Development of a detailed correlation model incorporating suppression of distortional contributions to solvatochromism as the static permittivity increases would lead to the introduction of additional, highly correlated, parameters; this course was deemed untenable. However, correlations based on the two major model parameters are examined. Despite major differences in physical make-up and initial presumptions, it is virtually impossible to distinguish the correlating ability of the McRae-Bayliss, modified McRae-Bayliss, or alternate models. As with the nonpolar solvents, lack of fit patterning with respect to solvent class is discernible as is global patterning which appears related to the distribution of the input data.