EFFECT OF INTERMOLECULAR INTERACTION ON INTENSITY OF WEAK OF FORBIDDEN ELECTRONIC TRANSITIONS

The effect of a solvent on the intensity of a forbidden electronic transition of a solute is studied by applying first order perturbation theory to the system consisting of a solute molecule A and its perturber B. The interaction energy between A and B is expressed in terms of the Longuet-Higgins and Salem formulation of intermolecular forces, with the result that there are two mechanisms by which the forbidden transition gains intensity-(a) borrowing from allowed transitions of A by virtue of the static electric field of the solvent B, and (b) borrowing from intense transitions of the solvent B by virtue of terms related to the dispersion interaction between A and B. Second order perturbation terms are shown to add nothing new, and a cross interaction is important if the forbidden transition of A is already vibronically perturbed. The solvent enhancement of the 0-0 band of the 2600 Å transition of benzene is discussed as an example. The static electric fields of polar solvents such as water and alcohols induce intensity borrowing from allowed benzene transitions, mainly 1E1u-1A1g. In carbon tetrachloride and other chlorinated solvents the theory supports the Robinson mechanism by which intensity is borrowed from a transition of the solvent. © 1969.