S-1-S-2 vibronic coupling in cis-1,3,5-hexatriene. I. Electronic structurecalculations

The estimates for the vertical excitation energy of the 2 (1)A(1) state of cis-1,3,5-hexatriene (CHT) vary considerably and provide a good example of the difficulties that can arise in determining transition energies. The gre at uncertainty is surprising if one considers that this state has already b een characterized by high resolution techniques such as resonance enhanced multiphoton ionization (REMPI) and fluorescence excitation spectroscopy in free jet expansions. A theoretical analysis of this problem is clearly need ed and the present work, along with the following paper, represents an effo rt to investigate the nature of the 2 (1)A(1) and 1 B-1(1) states of CHT. I t is shown that a combination of nb initio electronic structure and quantum -mechanical wave packet calculations is required to systematically approach a question as involved as locating the energetical position of the 2 (1)A( 1), level. We characterize the energy dependence of the 1 (1)A(1), 2 (1)A(1 ), and 1 B-1(1) states of CHT as a function of the in-plane normal coordina tes for small displacements from the ground-state equilibrium geometry empl oying the single-state multiconfigurational second-order perturbation theor y (CASPT2) method. This information constitutes the basis for the construct ion of diabatic harmonic model potential-energy functions associated with t he three electronic states in the Franck-Condon region that is essential fo r the treatment of nonadiabatic dynamics. Five totally symmetric modes with high Franck-Condon and/or tuning activity are identified. Vibronic interac tion between the S-1 and S-2 states is primarily mediated by four vibration s of b(1) symmetry, nu (26), nu (27), nu (30), and nu (31). nu (30) and nu (31) are found to be exceptionally powerful interstate coupling modes and t he strong nonadiabatic effects induced by these modes in CHT are mainly res ponsible for the spectroscopic differences observed for the S-1 and S-2 sta tes of CHT and trans-1,3,5-hexatriene. (C) 2001 American Institute of Physi cs.