Vibronic activity in trans,trans-1,3,5,7 octatetraene: The S-0->S-1, spectrum

Various levels of configuration interactin are used to investigate the vibronic intensity pattern of the false origins of the 1 (1)A(g) --> 2 (1)A(g) transition of all-trans ocetatetraene. The vibronically induced mixing of the A(g) with the B-u states is best simulated when polarization, i.e., d, functions are added to the basis set of atomic orbitals. Normal mode rotation upon electronic excitation plays an important role in the intensity distribution of the b(u) false origins. The progressions of the totally symmetric modes built on the b, bands is satisfactorily simulated at the CASSCF/6-31G* level. The calculations also show that the overtones of at least four out-of-plane modes should be present in the spectra whose assignment is tentatively proposed. Through comparison of the pure electronic intensities calculated for cis-trans octatetraene with the vibronically induced intensities of trans-trans octatetraene, it is found that the presence of a cis linkage induces a spectral perturbation similar to that of the most active b(u) mode. (C) 1995 American Institute of Physics.