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

A b initio MCSCF gradient method is applied to explore the potential surfaces of the low-lying excited states of 1, 3-trans-butadiene. The determination of the equilibrium geometries and the force constants shows that there are several local minima for the 1 3Bu (T1), 2 1Ag (S1), and 1 1Bu (S2) 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 1Ag state is found to have four minima with low symmetry (Ci, C2, and C1). A planar local minimum with C2h symmetry is found on the 1 1Bu surface. The comparable planar minimum was found for the 1 1Bu state of trans-hexatriene. This is the first ab initio 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.