Features of pnictogen O…N bonds were considered from the point of view of analyzing various geometric orientations of the neighboring nitro groups NO 2 …NO 2 observed in molecular crystals. The "idealized" orientation for a pnictogen bond, where the nucleophilic fragment of the atom O of one nitro group was directed to the electrophilic site of the atom N of the other one, and the "stacking" orientation, which was characterized by the parallel stacking of nitro groups, differ in the features of the electron density and electrostatic potential distribution between O and N atoms. The applied electronic criterion, obtained with the use of quantum chemical calculations with periodic boundary conditions, confirmed that for the considered O…N interactions the necessary condition for their classification as pnictogen bonds was met.
The inner-crystal quantum electronic pressure was estimated for unstrained C6Cl6, C6Br6, and C6I6 crystals and for those under external compression simulated from 1 to 20 GPa. The changes in its distribution were analyzed for the main structural elements in considered crystals: for triangles of the typical halogen bonds assembled in Hal3-synthons, where Hal = Cl, Br, I; for Hal...Hal stacking interactions, as well as for covalent bonds. Under simulated external compression, the quantum electronic pressure in the intermolecular space reduces as the electron density increases, indicating spatial areas of relatively less crystal resistance to external compression. The most compliant C6Cl6 crystal shows the largest changes of quantum electronic pressure in the centre of Cl3-synthon while the deformation of rigid I3-synthon under external compression depends only on the features of I...I halogen bonds.
In order to understand the phenomenon of negative linear compressibility (NLC) in organic crystals, it is necessary to investigate not only the structural features but also the electronic changes taking place under external hydrostatic pressure. It is also necessary to clarify which electronic properties allow the quantification and comparison of the compressibility of crystals. In our study, the crystal structures of sodium and cadmium formates under hydrostatic compression were modeled, as well as the α and β-phases of calcium formate. The changes in cell parameters and spatial dependences of the linear compressibility were analyzed, and the ranges of external pressure, which must be applied for NLC onset, were predicted for the sodium and α-calcium formates. Although the behavior of chemical bonds is not predicted by the sign or absolute value of the quantum electronic pressure, its relative change under external pressure clearly distinguishes the soft and rigid regions in a crystal. The relationship between the NLC values and the changes in quantum electronic pressure in the cavities of formate crystals was established.
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