Abstract
MCSCF and MRCI calculations on the first three singlet states of cis-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 the terminal CH 2 groups. Geometry relaxations in and excitation energies to the 1 1B 2 and 2 1A 1 states are discussed in detail. For planar structures the covalent 2 1A 1 state is lower in energy than the 1 1B 2 state. If non-planar geometry relaxations are allowed we find that among the structures considered the lowest-lying non-planar excited singlet state is ionic with one terminal CH 2 group rotated by 90°. A detailed comparison with the results on trans-butadiene is also given.
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