The effect of solute-solvent interactions on infrared carbon-halogen stretching frequencies

Solvent shifts of the infrared carbon-halogen stretching frequencies v(C-Hal) for a number of solutes in solution have been measured. Using chloroethane as a reference solute, a multiple regression analysis is found to correlate the relative frequency shift (Δv/v) with three independent solvent parameters: two functions of the dielectric constant and refractive index of the solvent which reflect its dipolar and non-dipolar bulk dielectric contributions, respectively, and a relative Lewis basicity parameter which allows for specific solute-solvent interactions. Solvent variation constants, which scale solvent shifts for one solute to the corresponding shifts of a reference solute according to the method of Bellamy, Hallam and Williams, have been determined and are found to be directly derivable from the regression coefficients for the solvent parameters. The possibility of further predicting solvent variation constants, in terms of linear free energy relationships for the various solutes, is discussed. Solutes examined in the course of the analysis are: chloroethane, chloromethane, chlorocyclohexane, 1-chloro-2-methylpropene, chlorobenzene, chlorobenzene[D₅], 1,4-dichlorobenzene, bromomethane and iodoethane.