Electronic structure of dicarbonyls. Ground state of glyoxal

Abstract

The ground electronic state of the simplest ..cap alpha..-dicarbonyl, glyoxal, (CHO)/sub 2/, has been studied by ab initio self-consistent field (SCF) methods using a double-zeta basis set of contracted Gaussian functions. The internal rotation potential curve was determined by optimizing the five structural parameters in the cis and trans forms and three intermediate geometries. The cis-trans energy separation was found to be 5.9 kcal/mol with the trans being the lower form. The internal rotation barrier was 7.5 kcal/mol with respect to the trans minimum, and the potential maximum occurs at an angle of about 102/sup 0/ away from the trans planar form. The energy ordering of the highest occupied molecular orbitals was determined to be n, ..pi.., n, and ..pi.., in order of increasing Koopmans theorem ionization potential. The close correspondence of these orbitals with those of two interacting CO molecules made possible generalizations of ground electronic state structure of ..cap alpha..-dicarbonyls and some predictions concerning excited electronic states. (auth)