THEORETICAL-STUDY OF THE ELECTRONIC-SPECTRUM OF ALL-TRANS-1,3,5,7-OCTATETRAENE

Ab initio calculations have been performed for the electronic spectrum of all-trans-1,3,5,7-octatetraene including geometry optimizations for the ground and low-lying valence excited singlet states at the complete active space self-consistent-field level. Excitation energies have been calculated using multiconfigurational second-order perturbation theory (CASPT2). Computed excitation energies for the valence and a number of low-lying Rydberg excited singlet and triplet states make possible confident assignments of the main features reported in the experimental spectra. The computed vertical and nonvertical excitations and the fluorescence maxima confirm the suggested presence of the 2(1)A(g) valence state below the 1 1B(u) optically allowed state, as is the characteristic for the longer polyenes. The agreement is within 0.05 eV for all excitation energies where comparison with experimental data is possible.