Change of Prototropic Equilibria for Uracil when Going from Neutral Molecule to Charged Radicals. Quantum-Chemical Studies in the Gas Phase

To understand various substitution, oxidation or degradation reactions for uracil, quantum-chemical calculations were performed for two classes of charged radicals (cations and anions) of uracil (U+center dot and U-center dot) and model compounds: phenol (P+center dot and P-center dot) and hydroxyazines (HP1-4(+center dot) and HP1-4(-center dot)). In calculations, all possible eighteen prototropic tautomers-rotamers of U and all possible five prototropic tautomers-rotamers of P and HP1-4 were considered. Stabilities, internal effects and aromatic character, estimated for charged radicals, were compared with those observed previously for neutral molecules. The greatest changes of the tautomeric preferences take place for radical anions. One-electron reduction stabilizes the keto and amide functions in studied compounds, whereas one-electron oxidation favors the enol function for phenol and the amide function for uracil and hydroxyazines.