Can a photochemical reaction be concerted? A theoretical study of the photochemical sigmatropic rearrangement of but-1-ene

MC-SCF computations at the 4-31G level using a complete active space (CAS) of four orbitals demonstrate the existence of a concerted photochemical pathway for [1,2] and [1,3] alkyl sigmatropic shifts. The central feature of this concerted path is a conical intersection (e.g., a genuine crossing) between ground and excited state from which a fully efficient return to the ground state is possible. Thus the excited-state surface has no minimum with zero gradient (i.e., a critical point) but only a singularity which corresponds to the lowest energy point of a conical intersection between ground and excited states. Thus there is no bottleneck corresponding to a short-lived intermediate that would correspond to the minimum on the excited-state surface at an avoided crossing. Intrinsic reaction coordinate computations have been performed on the excited-state surface that demonstrate the existence of two "channels" on the excited-state surface that simply continue on the ground-state surface. One of these channels leads to a [1,2] sigmatropic shift, the other to a [ 1,3] sigmatropic shift. The proposed mechanism is consistent with experimental observations where both [1,2]- and [1,3]-shift products are observed, and where the migrating group moves according to a supra process with retention of configuration of the migrating group.