ИСТИНА

Hydrogen bonds play a key role in innumerous chemical and biochemical processes. H-bond networks predetermine properties of the unique and, at the same time, universal solvent, water, and all aqueous solutions where either hydrophilic or hydrophobic solvation effects locally strengthen or weaken the inherent H-bond system of water. Monoatomic alkaline, alkaline-earth, and halide ions that are present in aqueous solutions act as either the so-called structure breaking or structure-making agents. Almost all biologically important or active particles and macromolecules form hydrogen bonds of one or another strength, and typical functional groups that provide the bonding are hydroxyl and amino ones, although weak H-bonds can appear also when a π-electron system of some usually aromatic molecular fragment act as a proton acceptor. As a result, aqueous solutions are characterized by extended hydrogen bond networks that involve local defects of Bjerrum's D or L kind, which can be traps of electron-excessive or electron-deficient particles, and are more or less distorted due to thermal motion, as well as rotational and vibrational excitation. All the particles involved in the extended H-bonded system can be considered as constituting a supramolecular ensemble, the internal dynamics of which is determined by the aforementioned factors. Our nonempirical quantum chemical simulations of diverse hydrogen-bonded clusters composed of water, hydrogen fluoride, and ammonia molecules of either individual or mixed chemical nature, as well as hydrates of different small ions and biologically important relatively small molecules, which involve hydroxyl and/or amino groups, enabled us to prove the hypothesis1 about the conjugation effect in hydrogen-bond networks that takes place when covalent and hydrogen bonds alternate within extended either open or closed H-bond sequences. The effect is manifested in the higher stability of clusters and in the non-additivity of H-bond energies. Another important manifestation of the effect, which is related to the peculiarities in the electron density distribution, is the appearance of characteristic vibrations within the highfrequency range that can be treated as a fingerprint of the conjugated H-bond systems. The larger the spatial domains of conjugated H-bond networks, the higher the absorption intensity in this narrow characteristic range.