Polar structures in the theory of conjugated molecules I. Identification of the ethylene π-electron states

Abstract

This and two of three parts to be published subsequently are concerned mainly with the so-called valence-bond theory of conjugated and aromatic molecules. An improvement to the method is described, which consists in adding to the usual set of structures some extra ones which are ‘polar’ in the sense that they show two of the π-electrons on one centre, and none on another centre, making these two centres carry respectively negative and positive charges. This adds a certain flexibility to the description of molecular states which is lacking when the electrons are supposed to be distributed one to each centre throughout. In this part a preliminary question is treated which bears on getting the new empirical parameters needed for including polar structures in the theory. This question is the assignment of the long wave-length bands in the spectrum of ethylene. The assignment is made with the help of a theoretical study of the ethylene energy levels in an approximation using antisymmetric molecular orbitals. Using this calculation as a guide, two transitions are assigned. A weak band, appearing at about 2000 Å, is taken to be 1 A g - 1 A g9 and a strong one, having its maximum at about 1630 Å, is taken to be 1 A g – 1 B lu .