Stability, polarity, intramolecular interactions and π-electron delocalization for all eighteen tautomers rotamers of uracil. DFT studies in the gas phase

Abstract

Complete tautomeric equilibria were investigated for uracil at the DFT(B3LYP)/6-311+G(d,p) level to establish the stability order of all possible 18 tautomers-rotamers in the gas phase and to characterize their internal effects, polarity and aromatic character. Although the di-NH form strongly predominates (100%) in the mixture, the NH–OH, di-OH and CH–NH forms can be also considered. The favored tautomer is moderately stabilized by intramolecular interactions (attractions of the NH and C=O groups); it is also moderately polar and moderately delocalized. Stability of the functional groups (both amide functions) seems to be more important than intramolecular interactions, polarity and aromaticity. This is probably the main factor that dictates the tautomeric preferences in the uracil molecule.