ANALYSIS OF THE ABSORPTION-SPECTRUM (1-B-1(U)[-1 (1)A(G)) AND RESONANCE RAMAN EXCITATION PROFILES OF TRANS-1,3,5-HEXATRIENE BASED ON AB-INITIO MOLECULAR-ORBITAL CALCULATIONS

The electronic absorption spectrum, the relative Raman intensities upo n 0-0 excitation, and the resonance Raman excitation profiles of trans -1,3,5-hexatriene in the region of the 1 B-1(u) <-- 1 (1)A(g) transiti on are analyzed on the basis of the structures and vibrational force f ields obtained from ab initio molecular orbital (MO) calculations. The second-order Moller-Plesset perturbation (MP2) and the configuration interaction singles (CIS) methods are employed to describe the 1 (1)A( g) and 1 B-1(u) states, respectively. The vibrational force fields obt ained from ab initio MO calculations are scaled in order to fit the ca lculated frequencies to the observed. The Duschinsky rotation among al l the modes of a(g) symmetry is fully taken into account. Both the cal culated absorption spectrum and resonance Raman intensities are in agr eement with the observed. This shows the usefulness of the CIS method for estimating the structure and vibrational force field in the 1 B-1( u) state of trans-1,3,5-hexatriene. On this basis, some refinements ar e made on the structure and force field in the 1 B-1(u) state in order to obtain a better fit between the observed and calculated results fo r the absorption spectrum and resonance Raman intensities. Effects of the frequency changes and Duschinsky rotation upon the electronic exci tation are significant in the resonance Raman excitation profiles of s ome bands. The structure and vibrational force field obtained for the ground electronic state by the simple Hartree-Fock method do not give an appropriate set of parameters for calculating the absorption spectr um and resonance Raman intensities.