The electronic structure of the azabenzenes an ab initio MO-SCF-LCAO study

Ab initio MO-SCF-LCAO calculations have been carried out for benzene and the azabenzenes pyridine, pyridazine, pyrimidine, pyrazine, s-triazine and s-tetrazine, and the results are presented.The general structure of the molecular orbitals is discussed and a classification scheme referring to an ideal case of cylindrical symmetry is described. Orbital energies, empirically corrected for reorganization and correlation, have been used to make assignments for the photoelectron spectra. A number of molecular properties have been computed and are discussed in relation to available experimental information.The electrostatic potential around each molecule is presented in the form of contour maps. Each nitrogen lone pair gives rise to a region of negative potential. The depths of these minima give some information about the relation between electronic structure and basicity. The potential in regions of high π-electron density is negative only in benzene and pyridine. This fact can be correlated with the increasing resistance against direct electrophilic substitution for the azabenzenes as compared to benzene itself.The calculations have been performed using contracted Gaussian functions as a basis. Four s- and two p-type atomic orbitals were used for carbon and nitrogen, whereas for hydrogen two s- and one pσ-type functions were used.