Potential energy and dipole moment surfaces for simultaneous torsion and pyramidalization of ethylene in its lowest-lying singlet excited states: A CI study of the sudden polarization effect

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Abstract
A b initio CI calculations have been carried out in a double-zeta AO basis for the simultaneous torsion and pyramidalization energy and dipole moment surfaces of the two lowest (V and Z) singlet excited states of ethylene. In nonpyramidalized geometries these two electronic states are found to undergo a potential crossing for a twisting angle of ϑ=82 °, with the Z(1A1) species being the more stable for the perpendicular D2d conformation. The dipole moments of these states are found to increase quite rapidly with pyramidalization for the entire range of twisting angle from ϑ=75 ° to ϑ=90 °, but this effect is found to reach a definite maximum in the neighborhood of the 82 ° crossing region for the unpyramidalized species and it is argued that these two phenomena are in fact closely related to one another. The CI results are found to be strongly dependent on the choice of one-electron basis in the significant portion of key structural regions and it is concluded that the use of natural orbitals optimized for one of the two nearly degenerate singlet states leads to excessively ionic charge distributions. Finally, the minimum-to-minimum energy difference (Te) between the ground and lowest singlet excited state is calculated to be 5.83 eV, suggesting a corresponding T0 value for this transition of 5.6–5.7 eV which is in very good agreement with McDiarmid’s recent experimental determination of this quantity.