Analysis of the absorption spectrum (1 1B u←1 1A g) and resonance Raman excitation profiles of trans-1,3,5-hexatriene based on ab initio molecular orbital calculations

Abstract

The electronic absorption spectrum, the relative Raman intensities upon 0-0 excitation, and the resonance Raman excitation profiles of trans-1,3,5-hexatriene in the region of the 1 1Bu←1 1Ag transition are analyzed on the basis of the structures and vibrational force fields obtained from ab initio molecular orbital (MO) calculations. The second-order Mo/ller–Plesset perturbation (MP2) and the configuration interaction singles (CIS) methods are employed to describe the 1 1Ag and 1 1Bu states, respectively. The vibrational force fields obtained from ab initio MO calculations are scaled in order to fit the calculated frequencies to the observed. The Duschinsky rotation among all the modes of ag symmetry is fully taken into account. Both the calculated absorption spectrum and resonance Raman intensities are in agreement with the observed. This shows the usefulness of the CIS method for estimating the structure and vibrational force field in the 1 1Bu state of trans-1,3,5-hexatriene. On this basis, some refinements are made on the structure and force field in the 1 1Bu state in order to obtain a better fit between the observed and calculated results for the absorption spectrum and resonance Raman intensities. Effects of the frequency changes and Duschinsky rotation upon the electronic excitation are significant in the resonance Raman excitation profiles of some 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 spectrum and resonance Raman intensities.