ESCA studies of the uracil base. The effect of methylation, thionation, and ionization on charge distribution

ESCA core line spectra have been obtained for uracil, the thiouracils, and some 1-methyl and 3-methyl analogues. In addition, anionsof the mono methyl derivatives have been prepared and studied. Core line binding energy shifts for all solid compounds, ionic and neutral, are found to correlate reasonably with CNDO/2 charge distributions calculated for the free molecules. The correlation line for ionic molecules is offset from that for neutrals by a constant factor ascribed to an intermolecular potential term. Specific effects of methylation, thionation, and ionization on the charge distribution are described. Methylation at either nitrogen leads to electron release from the nitrogen into the ring. Thionation results in a decrease in the charge at the site of thionation, while the electron density at adjacent atoms increases. The alterations in charge density are somewhat different for neutral and ionic compounds. Upon ionization of either N(1) or N(3) sites, much of the residual charge is located on the keto oxygens. The presence and position of the thionated carbon during ionization mainly affects the change in electron density at the C(6) position.