Valence-Bond Calculations of Triplet-State Energies and Spin Densities of Unsaturated Organic Molecules

A theoretical study of triplet-state organic molecules is given by means of the semiempirical valence-bond (VB) method. Triplet-state energies and spin densities are calculated by means of a VB bond-diagram method with integral parameters from studies of ground-state and spin-dependent properties. For the two- to eight-carbon polyenes reasonable agreement is obtained between the calculated ground to lowest triplet-state energy separations and the experimental absorption maxima. It is found that a reversal of the polyene single and double bond lengths significantly lowers the triplet-state energy because of effective correlation which tends to concentrate the positive spin density on the terminal carbons. A discussion is given of the various factors which influence dihedral twisting in the polyenes.