Nonadiabatic coupling between low lying singlet states of geometrically relaxed olefines: Ethylene and propylene

Abstract

Geometrical relaxation in low lying singlet excited states of biradicaloids with equal and different radical centers has been investigated employing ab initio large-scale CI treatments. The nonadiabatic coupling matrix elements between three singlet electronic states through twisting of the olefinic bond in propylene, unpyramidalized ethylene, and ethylene pyramidalized at one or both ends have been determined. The results are analyzed in terms of ionic and covalent contributions to the electronic wave functions. For large twisting angles a crossing between the two lowest excited states occurs for nonpyramidalized and dipyramidalized ethylene. The crossing becomes avoided for systems with two unequal radical centers. In the latter case both excited states exhibit large charge separation of opposite direction. In the region of avoided crossing the nonadiabatic coupling function between two excited states exhibits a peak which goes over into a broad shape curve as the difference in the electronegativity of the two radical centers increases. If the large polarity of the excited states is not localized within a few degrees of the twisting angle, the correction to the Born–Oppenheimer states through nonadiabatic coupling does not substantially change the main features of these states. The nonadiabatic coupling functions between the ground state and each of the two excited states are broad curves symmetrical with respect to the 90° twist. It is concluded that internal conversion from the excited into the ground state is not likely to be extremely fast.