An Analysis of the Through-bond Interaction Using the Localized Molecular Orbitals. I. Interactions between Lone-pair Orbitals in Azines: Pyridazine, Pyrimidine, and Pyrazine

Abstract

The CNDO/2 and INDO calculations were performed on three azines: pyridazine, pyrimidine, and pyrazine. In order to check the validity of the lone-pair orbital energies, the calculated orbital energies were compared with those obtained by ab initio calculations. These molecules have two lone-pairs, which can be transformed into symmetric and antisymmetric combinations. In pyridazine and pyrimidine, the symmetric one is lower than the antisymmetric one in energy, while in pyrazine the reverse is true. These results were subjected to an analysis from the standpoint of the through-bond interaction. That is, we transformed the canonical molecular orbitals obtained by the CNDO/2 method into localized molecular orbitals. By the use of the localized molecular orbitals thus obtained, the variation in the lone-pair orbital energies of these molecules was estimated in terms of the through-bond (and/or the through space) interaction according to a method which selectively considered the particular interaction between the specified localized molecular orbitals in a molecule. As a result of this analysis, it was found that the relative order of the lone-pair orbital energies was determined by the different types of interactions from molecule to molecule, and the energy variation in the lone-pair orbitals was elucidated in terms of the specified through-bond and through-space interactions.