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 fo r the ground and low-lying valence excited singlet states at the compl ete 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 nu mber of low-lying Rydberg excited singlet and triplet states make poss ible confident assignments of the main features reported in the experi mental 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 ch aracteristic for the longer polyenes. The agreement is within 0.05 eV for all excitation energies where comparison with experimental data is possible.