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

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Abstract
ESCA [Electron Spectroscopy for chemical analysis] core line spectra were obtained for uracil, thiouracils and some 1-methyl and 3-methyl analogs. Anions of the mono methyl derivatives were prepared and studied. Core line binding energy shifts for all solid compounds, ionic and neutral, correlate resonably with CNDO/2 [Complete Neglect of Differential Overlap] charge distribution 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 N leads to electron release from the N 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 O. The presence and position of the thionated C during ionization mainly affects the change in electron density at the C(6) position.

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