Knowledge Bank

A method has been developed for the determination of the potential surfaces of excited electronic states of conjugated molecules. By expressing the second-order SCF-MO-LCAO energy in analytic form, the first and second derivatives of the energy are obtained. These serve as the basis for calculation of the minimum energy conformation and for evaluation of the normal modes of the ground and excited electronic states. The calculated normal modes in each electronic potential surface are used for determining the vibronic transition intensities, thereby avoiding the need, inherent in most previous models, for restricting the treatment to only a single mode (e.g., pure $C=C$ bond stretching). The method is of particular importance for treating transitions which involve drastic conformational changes, such as torsions around $C-C$ bonds. As examples of the method, the N $\to$ V transition of ethylene and ethylene-d, has been calculated and the experimental vibrational structure of the V band is reproduced. New weak transitions are predicted in butadiene and hexatriene, and the vibronic structure of the first singlet of 1,3-cyclo-hexadiene is calculated.