An MCSCF study of the low-lying states of t r a n s-butadiene

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Abstract
A b i n i t i o MCSCF gradient method is applied to explore the potential surfaces of the low‐lying excited states of 1, 3‐t r a n s‐butadiene. The determination of the equilibrium geometries and the force constants shows that there are several local minima for the 1 3 B u (T 1), 2 1 A g (S 1), and 1 1 B u (S 2) states. Since each excited state has a different electronic character, its potential energy surface is complicated with respect to the rotation of C–C double bond and C–C stretching motions. The second 1 A g state is found to have four minima with low symmetry (C i , C 2, and C 1). A planar local minimum with C 2h symmetry is found on the 1 1 B u surface. The comparable planar minimum was found for the 1 1 B u state of t r a n s‐hexatriene. This is the first a b i n i t i o confirmation on the experimental analysis for the planar structure of the state. The present full π space MCSCF calculation gives not only reasonable transition energies but also detailed structures for low‐lying states.