Thiouracils: Acidity, Basicity, and Interaction with Water

Abstract

The optimized geometries, harmonic vibrational frequencies, and the energies of the cyclic structures of monohydrated 2-thiouracil, 4-thiouracil, and 2,4-dithiouracil are calculated using density functional theory (B3LYP) combined with the 6-31+G(d,p) basis set. In the three most stable cyclic structures, the water molecule accepts the NH proton and donates a proton to the carbonyl oxygen or thiocarbonyl sulfur atoms. The intermolecular distances between the water molecule and the acceptor atom of thiouracils are about 0.5 Å longer for hydrogen bonds involving a sulfur atom. Less stable cyclic complexes involving the O4 atom and the C5H bond are also formed. The frequency shifts of the ν(OH) stretching vibrations of water and the ν(NH) stretching vibrations of thiouracils are compared with recent data on the 1:1 adducts of uracil and water. The proton affinity of the oxygen and sulfur atoms and the deprotonation enthalpy of the NH bonds of thiouracils are calculated at the same level of theory. Although intrinsic acidities and basicities are larger in thiouracils than in uracils, the binding energies with one water molecule do not differ markedly for uracil and thiouracils. Comparison with previous data obtained for the 1:1 adducts of uracil and thymine with water suggests that the same binding energy is obtained for a much larger proton affinity of the sulfur atom as compared with the oxygen atom. The complexes of the thiouracils with three water molecules are also investigated and the hydrogen bonding cooperativity is discussed. Comparison with uracil indicates an alteration of the first hydration shell caused by the substitution of the oxygen atom by the sulfur one.