ИСТИНА

Binary ethylenediamine—water system is interesting particularly due to its unique physical and physicochemical properties that nonmonotonically change with the change in the relative content of components and reach extreme values at certain compositions, the values dramatically differing from those typical of individual substances. To clarify the structural and energetic features of the ethylenediamine—water system, nonempirical analysis of model (NH2CH2CH2NH2)m(H2O)n clusters with m=1—4 and n=2—18, including those with n=2m, was undertaken. Quantum chemical calculations were carried out at two levels, namely the second order of the Moeller—Plesset perturbation theory (MP2) and density functional approximation with B3LYP hybrid exchange—correlation functional (DFT-B3LYP), with a sufficiently flexible split valence Gaussian basis set augmented with diffuse and polarization functions (6-31++G(d,p)), which are known to provide reliable description of hydrogen-bonded systems. Analysis of the results obtained enabled us to identify the predominant local structure motive that originates upon the solvation of ethylenediamine amino groups with water molecules and is typical of individual amino groups in binary systems with a relatively low ethylenediamine content (10 mol %), as well as of «compact hydration bridges», which bind amino groups of the molecules in the systems with a high ethylenediamine content (about 30 mol %). In the systems of the latter kind, structural building blocks of (NH2CH2CH2NH2)2(H2O)4 composition were identified and shown to provide the possibility of constructing a continuous network of the corresponding supramolecular structure stabilized by the maximum possible number of hydrogen bonds. Hydrogen bonds in the suprastructure form predominantly square-rhombic rings with alternating covalent and hydrogen bonds in NH OH NH OH sequences, the neighboring rings being either coplanar or nearly orthogonal to each other. This kind of the mutual arrangement of molecules predetermines not only the highest stabilization of each structure unit, but also the possibility of extending any preformed structure by adding subsequent similar units. The discovered character of the spatial hydrogen-bond network in mixed ethylenediamine—water systems of diverse compositions can explain the nature of many peculiar physicochemical properties of the solutions and the processes that take place in them.