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

The potential energy surfaces of the 1 1Ag, 2 1Ag, and 1 1Bu 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 1Bu and 2 1Ag 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 bu symmetry is found to be active as 1 1Bu–2 1Ag coupling mode. On the basis of these ab initio results, a simple five-mode vibronic coupling model of the 1 1Bu–2 1Ag conical intersection is proposed.