The Variable Electronegativity Method. IV. Glyoxaline, Its Cation and Anion

Results of VESCF calculations of the π-electron distribution and ionization potentials in glyoxaline and its cation and anion are reported. The electron distributions in these systems are in accordance with the electrophilic substitution reactions observed for glyoxaline and bear out the concept of electronegativity reversal in the anion previously suggested to account for chemical properties of glyoxaline. The dipole moment of glyoxaline, evaluated from the calculated π-electron distribution, agrees with the observed moment, providing further vindication for the VESCF method and support for the suggestion that σ-bond polarizations in heterocycles are slight. Values of the Huckel coulomb and resonance parameters required to make the H�ckel method reproduce the VESCF results for these heterocycles are considered in some detail and it seems feasible to set up empirical rules for selecting appropriate values for the coulomb parameters.