| The potential energy surfaces of the S0, S1 and S2 states of both trans-1,3,5-hexatriene (E-HT) and cis-1,3,5-hexatriene (Z-HT) are investigated in the Franck-Condon region at the CASPT2 level of theory. The S0 geometry optimization and force field calculation are carried out with the restricted Hartree-Fock (RHF) plus Moller-Plesset second-order perturbation theory (MP2) method with the aug-cc-pVDZ and cc-pVTZ basis sets. Approximate normal coordinates are constructed from the ab initio force field in terms of linear combinations of internal symmetry coordinates.;The CASPT2 method predicts that the vertical excitation energy of the 21Ag state is below the 1 1Bu state for E-HT, which is consistent with time-resolved photoionization yield and photoelectron spectroscopy experiments conducted by Cyr and Hayden [J. Chem. Phys. 104, 771 (1991)]. Analogously, for Z-HT, the CASPT2 method suggests a vertical ordering of the 21A1 state below the 11B1 state for Z-HT, which is also consistent with experiment.;The vibronic interaction parameters for a description of the 1 1Bu ← 11A g (11B1 ← 1 1A1) transition are computed at the CASPT2/ aug-cc-pVDZ level of theory. The values of the vibronic coupling constants indicate that five totally symmetric vibrations have high Franck-Condon and/or tuning activity for both E-HT and Z-HT. For both E-HT and Z-HT, low frequency modes of bu and b1 symmetry, respectively, are shown to couple the S1 and S2 states. |
