Ab initio multiple spawning dynamics of excited butadiene: Role of charge transfer

Ab initio multiple spawning simulations of the photochemical reaction dynamics of s-frans-l,3-butadiene were performed. It is found that nonadiabatic events involving two low-lying excited states begin as early as 10 fs after excitation, resulting in the population being split between the bright 1 ¹B_u state and the dark 2¹A_g state. The molecule subsequently twists about a terminal carbon-carbon bond regardless of whether it is on the 1¹B_u or 2¹A_g electronic state. This twisting motion leads to conical intersections between S₁ and S₀. Several regions of the intersection seam involving states of differing character are accessed. The regions of the seam involving intersection between a state of charge-transfer character and a state of covalent character dominate the quenching dynamics, but intersections between two covalent states are also accessed a small percentage of the time. The existence and relative energies of these intersections are validated by optimization at the multistate complete active space second-order perturbation level of theory (MS-CASPT2). Our results point to a new mechanism for photoisomerization of butadiene that emphasizes the role of charge-transfer states.