Intensity Enhancement of Forbidden Electronic Transitions by Weak Intermolecular Interactions

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Abstract
This paper discusses the phenomenon where an originally ``forbidden'' electronic transition in a molecule (M) becomes strikingly enhanced through weak interactions with a neighboring perturber (P). The purpose of the paper is to present a general mechanism by which both symmetry‐forbidden and multiplicity‐forbidden transitions of a molecule in a perturbing environment can gain intensity. The mechanism for intensity enhancement is based on perturbation theory, diagrams being introduced in order to simplify the discussion. The paper emphasizes the fact that the enhanced transition moment may be gained not only from strong transitions in the molecule itself, but also from transitions in the perturbing environment. A number of examples of intensity enhancement are presented and analyzed. The so‐called Ham bands of benzene in carbon tetrachloride, for example, are shown to arise from intensity borrowed from the solvent. In spin‐forbidden transitions a requirement for enhancement is that either P is paramagnetic (O2 effect) or that strong spin—orbit coupling in the excited states of P exists (external heavy‐atom effect). Besides spin‐forbidden and symmetry‐forbidden transitions, double and multiple transitions made allowed through weak intermolecular interactions are discussed. Double transitions in molecular oxygen are analyzed, and it is concluded that intensity is borrowed from the Schuman—Runge band system. In this case an intermolecular interaction energy of only 5 cm−1 is required to give the observed intensity, so the transition can hardly be thought of as being due to the ``O4 molecule.'' A study of these weak interactions allows knowledge to be gained about molecular eigenfunctions in the important but often neglected regions of intermolecular space.