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

Eight stable tautomers of the molecular uracil were presented as the result s of density functional theory (DFT) studies with various exchange-correlat ion functionals such as the local density functional SL, the nonlocal densi ty functional BLYP, the hybrid functional B3LYP, and the half-and-half func tional BHandH, using the 6-31G*, 6-31 + G*, and 6-31G** basis sets. The geo metries of the eight tautomers were fully optimized at the B3LYP/6-31G*, B3 LYP/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 geome trical optimization of the ring structures except for the variance within 2 degrees 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, wh ere 2,4-dioxo-dilactam tautomer (U1) was the most stable. The relative ener gies including corrections of zero-point vibrational energies (ZPVEs) and t he dipole moments and polarizabilities of the tautomers were tabulated in d etail, with references to the experimental data and ab initio data in liter ature. (C) 1999 Elsevier Science B.V. All rights reserved.