Semiempirical Studies of Core Electron Binding Energies Part 10. The SCC-MO Calculations on some Purines

The solid state binding energies of 8-azaguanine, 8-azaxanthine, hypoxanthine and xanthine were calculated by using semiempirical self-consistent charge intramolecular electrostatic potentials expressed in the point-charge approximation. The results are essentially in good agree-ment with the experimental data giving in the same time a very simple and transparent inter-pretation of ESCA spectra which is close to the chemical intuition. Splitting of some unresolved N(1s) peaks is proposed and separate binding energies are attributed to particular nitrogen atoms. Since the point-charge model describes a number of molecular properties, it is concluded that formal atomic charges are meaningful within the adopted theoretical framework despite the fact that they can not be defined in a unique fashion. The relaxation energy and work functions of the studied molecular crystals are briefly discussed.