Electronic excitation spectrum of thiophene studied by symmetry-adapted cluster configuration interaction method

Electronic excitation spectrum of thiophene was investigated by the symmetr y-adapted cluster (SAC)/SAC configuration interaction method. Seventy singl et and four lowest triplet electronic states of thiophene were computed to give a detailed satisfactory theoretical interpretation of the vacuum ultra violet (VUV) spectrum and the electron energy loss spectrum of thiophene. T he present calculations gave the 2 (1)A(1) valence state at 5.41 eV and the 1 B-1(2) valence state at 5.72 eV with oscillator strengths 0.0911 and 0.1 131, respectively, and the 5 (1)A(1) valence state at 7.32 eV and the 4 B-1 (2) valence state at 7.40 eV with oscillator strengths 0.3614 and 0.1204, r espectively. These valence-excited states were assigned to the two strong a bsorption bands of the VUV spectrum centered around 5.5 and 7.05 eV, respec tively. A number of Rydberg transitions were obtained and assigned to the 6 .0, 6.6, and 7.5-8.7 eV, etc. energy regions. The similarities and differen ces in the electronic excitations between thiophene and other five-membered ring compounds were discussed. The accuracy and assignment of the present results are compared with those of the recent theoretical studies by CASPT2 and multireference double configuration interaction methods. (C) 2001 Amer ican Institute of Physics.