Electron-vibrational spectra of pyridine, pyrazine and pyrimidine. Molecular structure in excited electron states

Abstract

Direct computations are given of the electron-vibrational spectra for pyridine, pyrazine, pyrimidine and their deuterosubstituted analogues. A detailed analysis is given of the nature of the first allowed nπ*-transition of these molecules, by using charts of redistribution of electronic densities. Selection of molecular models of excited electron states was made using the previously suggested methods, i.e., variations in the bond lengths and force constants of the bonds were established from semi-empirical correlations between the length, force constant and π-order (index) of the bond; whilst variations in the valence angles were defined by a semi-empirical method based upon the use of an hybridized-AO basis for the approximation of electronic density distribution in the combination electronic states (the HAO method). The possibility of determining the direction of HAOs corresponding to the unshared electron pairs (which must be predetermined when using the HAO method), effected with the aid of the electronic-density charts, is illustrated. Electron—vibrational spectra have been interpreted. The computed spectra are quantitatively in agreement with experiment, thus offering further proof that the chosen molecular models are adequate for describing the real molecular structure in the excited state, and to the viability and efficiency of the method as a whole.