The nature of electronic transitions in absorption spectra of phenyl-substituted 1,3,4-oxadiazoles and 1,3,4-sym-triazoles

The electronic absorption spectra of 1,3,4-oxadiazole, 1,3,4-sym-triazole and their phenyl derivatives were calculated within the -electron approximation, by the PPP-Cl procedure. The nature of the electronic transitions was analysed in terms of special quantum-mechanical excitation indices: excitation-localization indices and charge-transfer indices. An orbital analysis was performed within the framework of the structure-orbital approach. The transitions calculated for unsubstituted azoles should be treated as perturbed ones of the models-cys-azabutadiene molecule. In the case of phenyl derivatives one can classify the transitions as being either local or molecular in character. The dependence of their characteristics on the -system structure has been studied for the latter transitions. The results of the theoretical treatment are in good agreement with experimental data.