Molecular structures of mono- and diamino derivatives of NAl-dimethyluracil and their complexes with water are investigated by ab initio quantum chemical methods at the MP2/6-31G(d,p) level of theory. The results of the calculations demonstrate that the formation of the N...H-O hydrogen bonds with participation of the nitrogen atom of the amino group in the N,N'-dimethyl-5,6-diaminouracil complex with water is caused by the intrinsic properties of this substituent. An analysis of the geometries and water-amine interaction energies for different monohydrates of mono- and diamino derivatives of N,N'-dimethyluracil indicates that the formation of this H bond requires the presence of the neighboring proton donor group. The energy of the N...H-O hydrogen bonds depends on a degree of conjugation between the lone pair of the nitrogen atom and the T system of the rest of the molecule. In the case of amino derivatives of uracil, the weakest conjugation is observed for the substituent at the C(5) atom. Therefore, the N...H-O hydrogen bond is formed by this amino group. A comparison of the amine-water interaction energies and the relative stability of isomeric monohydrates allows for the conclusion that the formation of such nonstandard hydrogen bonds is the most favorable way for interactions of 1,2-diamines with water and other proton donors.