Configuration interaction calculations on the planar 1(π, π*) state of ethylene

Abstract

The 1(π,π*) state of ethylene has been thoroughly investigated at the configuration interaction level utilizing an extensive basis set and a configuration selection scheme which emphasizes a correct description of all correlation effects involving the π or π* electrons. The result, contrary to the Hartree-Fock prediction, but in agreement with the general experimental conclusion, is a mostly valence state as indicated by a value for 〈π*|x2|π*〉 of 8.8 a.u. using the π* natural orbital. A calculated excitation energy of 7.96 eV is in reasonable agreement with the 7.65 eV experimental absorption maximum. It is found that σ→σ* excitations relative to the ground state are essential to describe the left-right correlation of σ electrons with the σ* electron. This in turn compensates for the ionic πA2-πB2 nature of the 1(π,π*) state, allowing the SCF state to become contracted at the CI level. The π* orbital in the second 1(π,π*) state appears to be well characterized at a CI level as a 3d orbital constrained to be orthogonal to the mostly valence π* orbital of the lower state. Calculations employing the same configuration selection scheme were done on the 3(π,π*), 1(π,3s), and positive ion states, the characterization of these states being consistent with other theoretical and experimental work. Copyright © 1977 American Institute of Physics.