A theory of solvent effects in infra-red spectra

Abstract

The vibration frequencies of a molecule change on immersion in a solvent as a result of the forces between the molecule and the surrounding solvent molecules. For non-polar solvents, in the absence of strongly orientation-dependent forces such as hydrogen bonding, dipole-induced dipole and dispersion forces are probably most important. The former can be approximated by considering the solute molecule to be in a spherical cavity of radius a in a uniform dielectric. This over-simple model of the solvent facilitates consideration of the effect of varying solute parameters upon ∆ ν/ν , the fractional decrease of frequency on solution in a given solvent from the vapour phase, and indicates, at least qualitatively, the character of the solute-solvent interaction in most cases. It can be shown (Pullin 1958) that if the dilute vapour phase potential and dipole moment functions of a diatomic molecule are given by V ( x ) = ½ kx 2 - bx 3 + cx 4 , (1) μ ( x ) = μ 0 + M ' x + M " x 2 , (2) then the potential function V s for the system, molecule and surrounding dielectric, is V s ( x ) = V ( x ) - ½ fμ 0 2 - fμ 0 M ' x - ½ fM ' 2 x 2 - fμ 0 M " x 2 - fM ' M " x 3 - ½ fM "2 x 4 , (3)