ИСТИНА

Properties of numerous systems composed of molecules, which involve proton-donating and accepting groups, are determined by relative susceptibility of the corresponding structural fragments to be involved in more than one bond with the neighbors. Most typical groups are hydroxyl, amino, and fluoride ones. Hydrogen bonds can be intra- and intermolecular. Former bonds stabilize particular configurations of molecules, which in the case of hydrocarbon derivatives are of primarily gauch or twisted kind. Latter ones determine mutual arrangement of particles in ensembles composed of the molecules of either the same or different nature. Here, intermolecular bonds compete with intramolecular ones, and prevailing conformations of constituents can differ under different conditions and abruptly change when an energy increase, which accompanies the conformational transformation of an individual molecule, is counterbalanced by the formation energy of extended systems of intermolecular bonds. The latter trend is especially pronounced when the resulting system of bonds, which can be referred to as conjugated due to the alternation of covalent and hydrogen bonds, sews together all molecules within very large spatial domains. Such structural motives are characterized by peculiar variants of the electron density distribution, which makes the energy barrier of the initial stage of bond breaking much higher compared to that of an individual spatially separated hydrogen bond. At the same time, the peculiar density distribution provides correlation in nuclear states of numerous molecules, primarily in their vibrational motion, which is reflected in typical patterns of ir absorption spectra and in molecular dynamics induced by vibrational excitation. The aforementioned trends were discovered, analyzed, and explained during the investigation of clusters composed of water, ammonia, hydrogen fluoride, as well as hydrocarbon derivatives, which involve hydroxyl and amino groups in various proportions, including ethane bifunctional derivatives. Theoretical modeling was carried out with the use of quantum chemical methods, chiefly the second order of the Moeller—Plesset perturbation theory (MP2) and density functional approach with B3LYP hybrid exchange–correlation functional (DFT-B3LYP) and a sufficiently flexible split valence Gaussian basis set augmented with diffuse and polarization functions (6-31++G(d,p)). Stationary simulations were supplemented with dynamic modeling and analysis of relevant one-dimensional quantum mechanical problems, which help in clarifying energetic aspects of the existence and possible excitation of hydrogen-bonded systems.