How many uracil tautomers there are? Density functional studies of stability ordering of tautomers Original Research Article

Eight stable tautomers of the molecular uracil were presented as the results of density functional theory (DFT) studies with various exchange-correlation functionals such as the local density functional SL, the nonlocal density functional BLYP, the hybrid functional B3LYP, and the half-and-half functional BHandH, using the 6-31G∗, 6-31+G∗, and 6-31G∗∗ basis sets. The geometries of the eight tautomers were fully optimized at the B3LYP/6-31G∗, B3LYP/6-31+G∗, and BLYP/6-31G∗∗ level, and it was noted that the polarized p-orbital function added to hydrogen atom and the diffuse functions added to the heavy atoms (nitrogen and oxygen atoms) did not contribute to the geometrical optimization of the ring structures except for the variance within 2° for HOC bond angles. All of the present DFT results suggested that the stability order should be: U1 > U2 > U4 > U5′ > U5 > U3 > U6′ > U6, where 2,4-dioxo-dilactam tautomer (U1) was the most stable. The relative energies including corrections of zero-point vibrational energies (ZPVEs) and the dipole moments and polarizabilities of the tautomers were tabulated in detail, with references to the experimental data and ab initio data in literature.