Geometry relaxation effects in the 1 Bu and 2 Ag states of trans-1,3-butadiene

MCSCF and MRCI calculations on the first three singlet states of trans-1,3-butadiene are presented. Flexible basis sets were applied and full geometry optimization was carried out at the MCSCF level for planar and selected non-planar structures including twisting and pyramidalization of terminal CH₂-groups. Geometry relaxations in and excitation energies to 1 ¹B_u and 2 ¹A_g states are discussed in detail. For planar structures the covalent 2 ¹A_g state is lower in energy than the 1 ¹B_u state. If non-planar geometry relaxations are allowed, the lowest lying non-planar excited singlet state turns out to be ionic with one terminal CH₂ group rotated by 90°. Limitations of the current investigations due to restrictions in the MRCI treatment and because of incomplete scanning of excited state surfaces are pointed out.