Relative stability of keto-enol tautomers in 5,6-substituted uracils: Ab initio, DFT and PCM study

The relative stability orders in the tautomers of uracil and its derivatives (5-fluorouracil, 5-chlorouracil, 5-aminouracil, 5-hydroxyuracil, 5-methyluracil, 6-methyluracil, 5-hydroxy-6-methyluracil and 5-amino-6-methyluracil) were established using composite (G3MP2B3) and DFT (TPSS) methods. The stability orders were determined both in the gas phase and water solutions, taking into account specific and non-specific hydration. The primary solvation shell of uracils was modeled as a complex of a tautomer with 5 water molecules. An analysis of the factors which determine the stability of the enol forms of uracils was performed. The most important factor was found to be changes in the intramolecular conjugation at tautomerization. As was shown by the NBO analysis, the stabilization energy due to the nN → π∗ (or σ∗) interaction in the diketo tautomer is lost in the enol forms, but is partially compensated by an increase in the conjugation length. The effect of the substituent in the fifth position of the pyrimidine ring on the energy of tautomers is less prominent. It was shown that the hydration energy considerably differs for tautomers, and leads to substantial redistribution in the stability series of uracil tautomers. Both specific and non-specific solvation are of vital importance for stabilization of tautomers.