Vibronic interactions in conjugated systems

A. It is first shown how the ordinary Born-Oppenheimer approximation for separating nuclear and electron motions in a molecule can be adapted to degenerate electronic states. Semi-empirical molecular orbital theory is then used to examine Jahn-Teller distortions in the ground states of conjugated hydrocarbon radicals. Numerical predictions are made for cyclic polyenes radicals and the mononegative ions of coronene and triphenylene. It is concluded that, except in the cyclic polyenes C_(4x) H_(4x), x = 0, 1, 2..., the barriers between "stable" distorted molecular configurations are negligible and that a dynamical coupling of nuclear and electronic motions exists in these radicals. B. It has been suggested in the literature that certain anomalies in the electronic spectra of coronene and triphenylene mononegative ions are due to Jahn- Teller distortions. The methods of the thesis are adapted to the Pariser and Parr molecular orbital scheme and benzene negative ion is treated in detail as a model for these systems. It is concluded that the Jahn-Teller effect cannot be responsible for the observed anomalies. The intensity of the ^1A_(1g) → ^1B_(1u) transition in benzene is calculated mainly as a test of the theory which is found to be adequate. C. A theoretical treatment of the pseudo-Jahn-Teller effect is presented and shows that several types of behavior arise. The ^1B_(1u) state of benzene undergoes a pseudo-Jahn-Teller interaction and a detailed calculation shows the state suffers a significant decrease in its e_(2g) CC stretching force constant, but is not permanently distorted. The ^1E_(1u) level of benzene does not experience a simple Jahn-Teller effect, but in addition to the pseudo-Jahn-Teller effect has a Jahn-Teller effect due to two electron perturbations. These perturbations result in changes in both bond lengths and valence angles, the changes being small and leading to a dynamical coupling