Structures and binding energies of the cyclic forms of the uracil-water and thymine-water complexes, in their lowest-lying triplet state, were determined using the UB3LYP/6-311++G(d,p) method. The calculated binding energies, ranging from -20 to -30 kJ/ mol, are about 1-3 kJ/mol smaller than those of the corresponding complexes in the singlet state. Proton affinities (PAs) at oxygen atoms and deprotonation enthalpies (DPEs) of the NH and CH groups of triplet uracil and thymine were evaluated at the same level. Compared to the singlet counterparts, DPEs of the NH bonds of triplet pyrimidine nucleobases are found to be slightly smaller, whereas DPEs of the C5H5 and C6H6 bonds are significantly decreased by 100 and 47 kJ/mol, respectively. PAs at oxygen of the triplet bases are slightly increased by 2-7 kJ/mol, relative to the singlet values. Again, it has been found that the most stable cyclic hydrogen bond is formed at the O-site characterized by the smallest PA and at the NH site having the smallest DPE. A certain correlation has been found between the hydrogen bonding energies in the studied complexes and the PAs and DPEs of the sites involved in the interaction. The calculated frequency shifts of hydrogen stretching modes confirm the established bond characteristics. A NBO analysis was also carried to probe further the interaction of the triplet uracil and water. Protonation at O-sites and NH-deprotonation significantly influence the vertical singlet-triplet gap of uracil. (C) 2005 Elsevier B.V. All rights reserved.