S 1 –S 2 vibronic coupling in trans-1,3,5-hexatriene. I. Electronic structure calculations

The potential-energy surfaces of the 1 1Ag, 2 1Ag, and 1 1Bu states of trans-1,3,5-hexatriene (THT) are explored in the vicinity of the ground state equilibrium structure. The S0 geometry optimization and force field calculation have been carried out with the restricted Hartree–Fock plus Mo/ller–Plesset second-order perturbation theory method. Vibronic coupling constants for the normal coordinates of ag and bu symmetry were computed with the complete-active-space self-consistent-field (CASSCF) and single state multiconfigurational second-order perturbation theory (CASPT2) electronic structure models. The CASSCF/CASPT2 method unequivocally places the vertical excitation energy of the dark 2 1Ag “phantom state” below the 1 1Bu level and predicts an energy difference of ca. 0.5 eV. The results are consistent with time-resolved photoionization yield and photoelectron spectroscopy experiments that indicate the existence of a low lying S1–S2 conical intersection which induces rapid 1 1Bu→2 1Ag internal conversion on a time scale of 40 fs to 50 fs [Cyr and Hayden, J. Chem. Phys. 104, 771 (1996)]. Based on the vibronic coupling constants five totally symmetric vibrations with high Franck–Condon and/or tuning activity have been identified. The S1 and S2 states interact primarily via the two bu normal modes ν24 and ν26. Other ag and bu normal vibrations do not appear to couple significantly to the lowest lying π→π* transition. The modeling of the ultrafast relaxation processes following optical excitation of the 1 1Bu state of THT and the calculation of absorption and resonance Raman spectra are discussed in the following paper.