| Abstract
| - Ab initio multistate second-order perturbation theory (MS−CASPT2) calculations are used to map the reactionpath for the ultrafast photochemical electrocyclic ring-opening of cyclohexa-1,3-diene (CHD). This path ischaracterized by evolution along a complex reaction coordinate extending over two barrierless excited statepotential energy surfaces and ultimately leading to deactivation through a S1/S0 conical intersection. Theobserved excited-state dynamics involve three sequential phases with lifetimes (traveling times) of 10, 43,and 77 fs, respectively. In this work we associate each phase to the evolution of the CHD molecular structurealong a different mode. In particular, we show that (a) the decay of CHD from its spectroscopic (1B2) stateto a lower lying dark (2A1) excited state involves motion along a highly curved coordinate corresponding toa mixture of σ bond expansion and symmetry breaking skeletal bending, (b) the evolution on the 2A1 (S1)state and the final 2A1→1A1 (i.e., S1→S0) decay involve a large amplitude displacement along the sameasymmetric bending mode which ultimately leads to a S1/S0 conical intersection, and (c) the application of anovel strategy for mapping the multidimensional S1/S0 intersection space indicates that the ultrashort 77 fslifetime of the 2A1 excited state is due to the existence of an extensive set of S1/S0 conical intersection pointsspanning the low-lying part of the 2A1 energy surface. Points (a) and (b) are validated by discussing theresults of previously reported and new femtosecond time-resolved spectroscopic data on CHD and on thetwo dialkyl derivatives α-terpinene and α-phellandrene. An interpretation in terms of driving forces is alsogiven.
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