Out-of-plane modes of cis-1,3,5-hexatriene: Frequency shifts in the 2(1)A(1) and 1(1)B(1) excited states

The calculation of Hessian matrices for the out-of-plane modes of cis-1,3,5 -hexatriene (CHT)in the 1(1)A(1), 2(1)A(1), and 1(1)B(1) states is reported at the equilibrium geometry of the electronic ground state. The complete-a ctive-space self-consistent-field (CASSCF) electronic structure model is em ployed. Je and a orbitals are included in the active space to account for t he mixing of both types of molecular orbitals at nonplanar conformations of CHT. On the basis of the Hessian matrices and the resulting approximate no rmal frequencies, an interpretation of recent spectroscopic evidence for th e valence excited states of CHT is attempted. In particular, we make assign ments of transitions in the low-frequency region of the hv + hv, resonance enhanced multiphoton ionization (REMPI) spectrum of the 2(1)A(1) state. The predicted excited-state frequencies appear to be accurate acid normal coor dinate rotations induced by electronic excitation are shown to play an impo rtant role for the intensity distributions observed by REMPI and resonance Raman spectroscopy.