At present, the theoretical concepts of the hydrogen bond (H-bond) in condensed media, for example, in living systems, biomolecules, are not fully solved. Quantum chemical modeling is used as one of the methods for studying the nature and determining the strength of the H-bond. In this paper, we continue to study the system of hydrogen bonds in molecular crystals of alanine and tyrosine. The dimers of these amino acids were modeled using the DFT method using the B97D functional with bases 6-31++G**. In the framework of NBO analysis, the stabilization energies of the formed hydrogen bond and the value of the transferred charge are calculated. It is shown that in alanine dimers, the main factor affecting the h-bond stabilization energy is the geometric parameters and, first of all, (N-H...O). The binding σ-orbital of the hydrogen bond is the result of the interaction of a hybrid NBO of the lone electron pairs of an oxygen atom and a loosening σ*-NBO N−H bond. The nature of bond formation in all three cases is the same, and the charge transfer value is greater than the value of the bond criterion, which indicates the presence of hydrogen bonds in all analyzed alanine systems. In tyrosine dimers, two H-bonds are formed that are similar in nature, as well as in geometric and energy parameters. The third H-bond is very weak, and the amount of charge transfer indicates its absence. The main interaction between the molecules in the third tyrosine dimer is the H-bond between the –СОО− and –OH groups. It was found that the scheme of formation of hydrogen bonds in molecular crystals of tyrosine is somewhat different from that of alanine.
Automated generation of radical species in crystalline carbohydrate using ab initio MD simulations
