Electronic structures

Abstract

The studies pertain mainly to the correlation of electronic structure and physicochemical as well as biological properties of five different sets of biologically important heteroaromatic N?oxides, employing molecular orbital techniques. The N?oxide sets studied are the 4?substituted pyridine N?oxides, diazine N?oxides, purine N?oxides, quinoline N?oxides, and phenazine N?oxides. Each set consists of biologically active compounds as well as model compounds that are structurally related to the active compounds, so that a comparative study is possible. The structure of the N?oxide moiety and its interaction with various N?heterocyclic ring systems is of special interest. Another objective of the present study is to compare the relative merits and effectiveness of different ??electron as well as all?valence?electron MO procedures. The simple PPP ??electron method and its modification developed in the present study give satisfactory descriptions of the electronic spectra of the N?oxides. Three basically different all?valence?electron MO procedures—IEH, CNDO/2, and MINDO/2—give very good agreement for dipole moments, ionization potentials, and charge distribution. IEH provides comparable values with first ionization potentials, whereas CNDO/2 and MINDO/2 give better correlation with electron?acceptor and electron?donor properties. The frontier electron densities and superdelocalizabilities enable a fairly satisfactory correlation with antifungal, antibiotic, antibacterial, and carcinogenic activities of the N?oxides. The overall conclusion is that molecular orbital techniques are very helpful in studying molecular electronic structures of chemical as well as biological systems.