The optimized geometries and vibrational properties of the N3 anions and X8-protonated cations (X = O, S) of 2-thiouracil, 4-thiouracil and 2,4-dithiouracil are studied using density functional theory (B3LYP) combined with the 631+G(d,p) basis set. The complexes formed with one water molecule at the N1H, X7 side of the anions and cations of thiouracils are investigated at the same level of theory. They are compared with the complexes involving the neutral thiouracils where only cyclic structures are formed. In their anionic forms, the X7 . . .H2O hydrogen bonds are shorter and the N1H . . .O-w hydrogen bonds are longer than in the neutral forms. In the cationic complexes, the X7 . . .H2O hydrogen bond is broken. The binding energies with water are substantially larger in the anionic and cationic complexes than in the neutral ones. The present results are discussed in terms of the electronic distribution on the atoms of thiouracils involved in their interaction with water. The curve representing the X7 . . .H-w(') distances as a function of the Mulliken net charges on the X7 atom is much less steep for S7 . . .H-w(') than for O7 . . .H-w(') hydrogen bonds. (C) 2001 Elsevier Science B.V. All rights reserved.