All-valence-electron configuration mixing calculations for the characterization of the 1(π, π *) states of ethylene

Abstract

Configuration mixing (CM) calculations are reported for the 1(π,π *) and various other electronic states of ethylene. The configuration subspaces considered include all single and double excitations relative to a series of the most important terms in the CM expansion of each state. The lowest-lying 1(π,π *) transition is found to be essentially intra-valence in character with a calculated ∫ value in the 0.25–0.30 range, in good agreement with experiment. Diffuse 3dπ AOs are nevertheless found to be quite important in the representation of the 1(π,π *) state, even after extensive CM: failure to include them in the AO basis results in an energy increase of at least 0.7 eV and substantial overestimation of the corresponding ∫ value by as much as 50%. The vertical 1(π,π *) excitation energy is calculated to be 8.1 eV, while the corresponding 0-0 transition (90° twisted conformation) is predicted in the 6.0–6.2 eV range; very little change in CC bond length is found to occur upon torsion of the CH 2 groups in this state.