ИСТИНА

Water and ethylenediamine are compounds with the individual properties determined by the formation of more or less protracted hydrogen-bond networks based on hydroxyl and amino-groups respectively. Being mixed they produce a binary system with the properties that far extend the original individual characteristics. The reason for that is the difference in proton-donation and acceptance abilities and relative spatial arrangement of NH2 and OH groups, as well as the 1:1 ratio of hydrophilic amino-to-hydrophobic methylene groups in the organic counterpart. As a result, at certain molar ratios, physical properties of the binary solutions abruptly change. The reason for that can only be the character of the electron density distribution that strongly favors peculiar mutual arrangement of molecules of both kinds in the binary ensemble and the formation of a conjugated H-bond network with alternating covalent and hydrogen bonds. To understand possible reasons of structuring we modeled mixed clusters of water and ethylenediamine molecules with various molecular ratios assuming diverse initial arrangements of molecules. We have carried out quantum chemical simulations of (NH2CH2CH2NH2)m(H2O)n clusters with m up to six and n up to twenty. A particular attention was paid to systems with an n/m ratio equal to two, which was experimentally found as an outstanding one. The methods used were the second order of the Moeller—Plesset perturbation theory (MP2) and density functional approximation (DFT) with B3LYP hybrid exchange–correlation functional and a sufficiently flexible split valence Gaussian basis set augmented with diffuse and polarization functions (6-31++G(d,p)) that proved to provide reliable description of hydrogen-bonded systems. It was found that at a large fraction of water molecules, configuration of the hydrogen-bond network is chiefly determined by interactions between water molecules that are joined in a conjugated frame that envelops individual ethylenediamine molecules with prevailing twisted hexagons in the vicinity of ethylenediamine. At a small water fraction, the system involves only broken segments of conjugated H-bond network. At intermediate compositions, especially those of twice as large molecular fraction of water molecules, extended completely H-bonded fragments are observed, being seemingly determined by two structural trends. One is the formation of tetragonal tings composed of alternating NH and OH groups that are close neighbors of aforementioned twisted hexagons composed of a four-vertex segment of ethylenediamine skeleton and two water molecules. The other is the trend of ethylenediamine molecules to form hydrophobic cavities composed of methylene group segments of up to four hydrocarbon skeletons. The latter are surrounded with nearly conjugated H-bond sequences.