The first singlet excited states (S1) which control the ultrafast (i.e. sub-picosecond) 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 abinitio 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 S2/S1 conical intersection points on S1. Furthermore, analytical frequency computations have been used to produce quantitative information on the surface curvature.
We show that the S1 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 S1 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-HT and the ring-opening of CHD are prototypes of three classes of barrierless reactions characterized by a different excited state dynamics. In 2-cis-C5H6NH2+ and, more loosely, in all-trans-HT the initial relaxation results in the production of a totally symmetric S1 transient. The following triggering of the S1→S0 decay requires energy redistribution along a symmetry-breaking (torsional) mode leading to an S1/S0 conical intersection (CI). In contrast, the shape of region I of CHD indicates that an almost direct (i.e. impulsive) motion towards an asymmetric S1/S0 CI occurs upon initial relaxation. Previously reported and novel semi-classical trajectory computations and the available experimental evidence seem to support these conclusions.
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