Thiouracils: Acidity, basicity, and interaction with water

The optimized geometries, harmonic vibrational frequencies, and the energie s of the cyclic structures of monohydrated 2-thiouracil, 4-thiouracil, and 2,4-dithiouracil are calculated using density functional theory (B3LYP) com bined with the 6-31+G(d,p) basis set. In the three most stable cyclic struc tures, 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 Angstrom longer for hydrogen bonds involving a sulfur atom. Less stable cyclic complexes involving the O4 atom and the C5H bond are also formed. T he frequency shifts of the v(OH) stretching vibrations of water and the V(N H) stretching vibrations of thiouracils are compared with recent data on th e 1:1 adducts of uracil and water. The proton affinity of the oxygen and su lfur atoms and the deprotonation enthalpy of the NH bonds of thiouracils ar e 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 th ymine with water suggests that the same binding energy is obtained for a mu ch larger proton affinity of the sulfur atom as compared with the oxygen at om. The complexes of the thiouracils with three water molecules are also in vestigated 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.