Ab initio investigation of the potential energy surfaces involved in the photophysics of s-trans-1,3-butadiene

The potential energy surfaces of the 1 ¹A_g, 2 ¹A_g, and 1 ¹B_u electronic states of trans-butadiene have been investigated with the complete-active-space self-consistent-field (CASSCF) method as well as second-order perturbation theory based on the CASSCF reference (CASPT2). Symmetry-adapted valence internal coordinates are employed to describe large-amplitude deformations from the ground-state equilibrium geometry. Both in-plane and out-of-plane deformations have systematically been explored. Four coordinates are identified as particularly effective in inducing a crossing of the 1 ¹B_u and 2 ¹A_g surfaces: two totally symmetric coordinates (symmetric C-C double-bond stretching and symmetric C-C-C bending) as well as the conrotatory and disrotatory torsions of the terminal CH₂ groups. A single coordinate of b_u symmetry is found to be active as 1 ¹B_u-2 ¹A_g coupling mode. On the basis of these ab initio results, a simple five-mode vibronic coupling model of the 1 ¹B_u-2 ¹A_g conical intersection is proposed.