Intramolecular Charge Transfer in Aromatic Free Radicals

TLDR

The study theoretically investigates the rate of intramolecular odd‑electron transfer between the two phenyl groups in mononegative α,ω‑diphenyalkanes φ—(CH2)n—φ. The transfer proceeds via self‑trapping of the odd electron on one phenyl ring caused by solvent polarization and bond distortions, followed by an electronic resonance in a short‑lived thermally activated state where the rings appear equivalent. Calculations reveal that the polymethylene chain can be modeled by a pseudopotential, self‑trapping markedly slows transfer, the activation energy is ≈1000 cm⁻¹, and the rate decreases exponentially with chain length, dropping by at least a factor of ten per added methylene group.

Abstract

A theoretical analysis is made of the rate of intramolecular transfer of the odd electron between the two phenyl groups in the mononegative ions of the α,ω-diphenyalkanes, φ—(CH2)n—φ. The essential features of the calculations are: (a) It is shown that the polymethylene chain can be replaced by a pseudopotential corresponding to an effective direct transfer between the rings. (b) There is a strong tendency for self-trapping of the odd electron on one phenyl ring, or the other, due to solvent polarization and bond distortions in the rings. This self-trapping greatly reduces the rate of intramolecular charge transfer. (c) The intramolecular charge transfer occurs as an electronic resonance effect when a short-lived thermally activated molecular state is formed in which the two rings appear to the odd electron to be equivalent to one another. The activation energy is estimated to be of the order of 1000 cm—1. (d) It is found that the rate of intramolecular charge transfer decreases exponentially with the length of the polymethylene chain, the decrease being as much or more than a factor of ten for each added methylene group.