Out-of-Plane Modes ofcis-1,3,5-Hexatriene:  Frequency Shifts in the 2¹A₁and 1¹B₁Excited 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-active-space self-consistent-field (CASSCF) electronic structure model is employed. Je and a orbitals are included in the active space to account for the mixing of both types of molecular orbitals at nonplanar conformations of CHT. On the basis of the Hessian matrices and the resulting approximate normal frequencies, an interpretation of recent spectroscopic evidence for the valence excited states of CHT is attempted. In particular, we make assignments 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 coordinate rotations induced by electronic excitation are shown to play an important role for the intensity distributions observed by REMPI and resonance Raman spectroscopy.