On the solvation of (quasi-)degenerate solutes: a continuum analysis based on perturbation-response theory

The solvation of a solute possessing two degenerate or quasi-degenerate non-polarizable states is considered from a theoretical point of view. The analysis is based on the perturbation theory of solute/solvent interactions in the form of linear response theory for condensed systems, and the results are interpreted in terms of macroscopic dielectric functions of the solvent. It is found that, with respect to the usual expression, the solvation energy is enriched with two terms, one of which is a non-standard one. The non-standard term is further divided into diagonal and off-diagonal, acoustic/orientational and electronic contributions. The various additional contributions are discussed, and they produce two main effects: (a) an enhancement of the solvation energy (due to the coupling between the orientational motions of the solvent molecules and the electronic degrees of freedom of the solute); (b) a resonance of the solute wavefunction between the two quasi-degenerate states.