Effect of Sr2+ association on the tautomerization processes of uracil and its dithio- and diseleno-derivatives

Abstract

The structures and relative stabilities of the complexes formed by uracil and its thio- and seleno-derivatives with the Sr²⁺ cation, in the gas phase, have been analyzed by means of G96LYP density functional theory (DFT) calculations. The attachment of the Sr²⁺ cation to the heteroatom at position 4 is preferred systematically. Although the enolic forms of uracil and its derivatives should not be observed in the gas phase, the corresponding Sr²⁺ complexes are the most stable. The enhanced stability of these tautomers is two-fold, on the one hand Sr²⁺ interacts with two basic sites simultaneously, and on the other hand an aromatization of the six-membered ring takes place upon Sr²⁺ association. Sr²⁺ attachment also has a clear catalytic effect in the tautomerization processes involving uracil and its derivatives. This catalytic effect increases when oxygen is replaced by sulfur or selenium. The Sr²⁺ binding energy with uracil and its derivatives is bigger than the tautomerization barriers connecting the dioxo forms with the corresponding enolic tautomers. Consequently, when associated with Sr²⁺, all tautomers are energetically accessible and should all be observed in the gas phase.