Potential-Energy Surface for Ultrafast Photochemistry. Static and Dynamic Aspects

The first singlet excited states (S-1) which control the ultrafast (i.e. subpicosecond) photochemistry of 2-cis-penta-2,4-dieniminium cation (2-cis-C5H6NH2+), all-trans-hexa-1,3,5-triene (all-trans-HT) and cyclohexa-1,3-diene (CHD) have been investigated using ab initio MCSCF and multireference MP2 theories. The structure of the corresponding potential energy surfaces (PESs) has been characterized by computing novel unconstrained and symmetry-constrained minimum-energy paths (MEP) starting from Franck-Condon and S-2/S-1 conical intersection points on S-1. Furthermore, analytical frequency computations have been used to produce quantitative information on the surface curvature. We show that the S-1 energy surface is characterized by two domains, region I and region II. Region I controls the initial acceleration of the excited state molecule. In contrast, region II is a low-lying region of S-1 and controls the evolution towards fully efficient decay to the ground state. The energy surface structure indicates that the double-bond isomerization of 2-cis-C5H6NH2+ and all-trans-MT and the ring-opening of CMD are prototypes of three classes of barrierless reactions characterized by a different excited state dynamics. In 2-cis-C5H6NK2+ and, more loosely, in all-trans-HT the initial relaxation results in the production of a totally symmetric S-1 transient. The following triggering of the S-1 -->S-0 decay requires energy redistribution along a symmetry-breaking (torsional) mode leading to an S-2/S-1 conical intersection (CI). In contrast, the shape of region I of CHD indicates that an almost direct (i.e. impulsive) motion towards an asymmetric S-1/S-0 CI occurs upon initial relaxation. Previously reported and novel semi-classical trajectory computations and the available experimental evidence seem to support these conclusions.