Interpretation of Open-Shell SCF Calculations on the T and V States of Ethylene

Abstract

Self-consistent field calculations are reported on (planar and perpendicular geometric conformers of) the lowest energy triplet and singlet (pi,pi*) orbital configurations of ethylene, which are usually identified with the spectroscopic T and V states, respectively. For the planar conformation the calculation predicts a V state with the characteristics of a Rydberg state, but a T state of expected valence-shell character. The pi* orbital is much too large and the internuclear distance too small for the calculated V state, but are as expected for the T state. It is concluded that the calculated result for the supposed V state in the planar conformation is spurious due to the inadequacy of the Hartree–Fock single-configuration theory. The supposed V state here calculated may be identified with a Rydberg state which has been observed spectroscopically at ~ 1.4 eV higher energy than the V state in the optical spectrum. For the perpendicular conformation, the calculations yield resonable results for both the T and V states. Calculations also have been carried out on a singlet (sigma,pi*) state for planar ethylene; here the size of the pi* orbital is reasonable.