P. M. Zorkii

Isostructural and Nonisostructural Compounds in Series of Halogenated Organic Crystal Substances. Structure of Hal-Aggregates

The local characteristics and the form of intermolecular Hal aggregates (assemblies of contacting halogen atoms of neighboring molecules), existing in haloorganic crystal substances differing only in the nature of Hal atoms, are compared. Twenty three series of halogenated hydrocarbons involving 57 crystal structures are considered. Pronounced specifics of Hal-aggregates has been established for compounds with low and medium halogen contents. It is found that k(Hal) (coordination number of the Hal atom with respect to the neighboring Hal atoms) generally increases in the series F–Cl–Br–I; for constant k(Hal), = nearly always decreases (ri is the distance from the Hal atom in question to one of the k nearest Hal atoms, and RHal is the van der Waals radius).

Ordinary organic crystal chemistry. Interpretation of the most probable homomolecular structures

For homomolecular organic crystals, most characteristic types of molecular arrangement corresponding to most widespread structural classes and subclasses are named and interpreted. These characteristic structural variants cover about 70% of crystal substances consisting of chemically identical molecules. Examples of crystal structures are cited to illustrate the structural laws of organic crystals; the structures are given as easily perceptible representations taken from the album “Structure of Organic Crystals.” An important structural feature of ordinary molecular crystals is coexistence of molecules with four or two different orientations (64 and 28%, respectively). Basic notions and approaches of modern organic crystal chemistry are briefly defined.

Crystal-Chemical Classes of “Cambridge” Crystal Structures: Statistical Analysis of Topology

Crystals structures with “organic” carbon, i.e., organic and organometal compounds and coordination compounds with organic ligands are systematized based on the results of a statistical treatment of CSD (Cambridge Structural Database) data. The overwhelming majority of CSD structures are molecular crystals, which may be homomolecular or heteromolecular; the latter, in turn, are classified into molecular complexes, salts, crystal solvates, and crystal hydrates. Polymeric (nonmolecular) crystals occur much more rarely. For substances belonging to different crystal-chemical classes, distributions according to space groups and structural classes have been studied; considerable differences between the distributions have been found. In particular, it was established that chiral structures (with P212121, P21, etc. symmetry) are met much more rarely among inorganic CSD structures with metals and metalloids than among organic structures consisting exclusively of organogen elements; the most striking examples are hydrates and salts containing Cl–, Br–, and I– ions.

A comparative analysis of the crystal structures of carbamide and thiocarbamide

A comparative crystal chemical analysis was performed for carbamide and three modifications of thiocarbamide (including an exotic modulated structure containing 36 molecules in the orthogonal cell) using the concepts and approaches applied by P. M. Zorkii et al. in previous works. A detailed description is given for the structures to outline the technique which permits interpretation of their characteristic features. The original drawings clearly display the molecular arrangement in the crystals.

Orthogonal contacts between benzene rings: A special type of specific intermolecular interactions

The crystal structures of benzene and some of its derivatives are analyzed in detail. The structures are characterized by orthogonal contacts between the benzene cycles (OBzC). Although the orthogonal contacts conflict with the close packig law, they are an important structure-forming factor, being thus a specific type of intermolecular interactions. The OBzC system, which is inherent in the orthorhombic crystals of benzene, remains invariable in 2-aminophenol and 2-amino-4-chlorophenol crystals, notwithstanding other specific interactions (H-bonding and halogen...halogen contacts). OBzC are also observed in para- and ortho-cresol crystals. A classification of OBzC is given, and an especially effective projection method (rectangular projection) is suggested.

Similarities and differences of crystal structure of internal complex compounds of copper and nickel

Summary1.It can be predicted theoretically that if crystals of corresponding copper and nickel internal complexes are not isostructural then in at least one of them the metal atoms are not at the centers of symmetry (the complex is nonplanar). This is confirmed by experimental data.2.The existing (somewhat limited) experimental data show that in the M−4O and M−4S groups the corresponding copper and nickel compounds are not isostructural, but in the first of these groups copper atoms and in the second nickel atoms are at the centers of symmetry.3.Diamagnetic nickel compounds of the M−N,20 group (trans) usually form crystals isostructural with the crystals of the corresponding copper compounds, and in most cases the metal atoms are at the centers of symmetry. However, several exceptions are known. In known cases in the M→N,20(cis) group corresponding copper and nickel compounds are not isostructural.4.Because of the paucity of structure data on crystals of compounds in the M−4N group, no definite conclusions can be drawn. Cases of similarity as well as sharp difference between the structures of corresponding internal copper and nickel complexes are known.

Structure of Organic Crystals with Molecules Lying on Twofold Crystallographic Axes. Structural Class C2, Z = 2(2)

A comparative analysis is performed for 44 structures from the rare structural class C2, Z = 2(2) with molecules lying on the twofold crystallographic axes. The overwhelming majority of these structures (42 of 44) have distinct chains (or rods) where molecules are “strung” on the 2 axis forming very effective (according to their contribution to crystal energy) intermolecular contacts. The chains form layers grouped into two structural subclasses: 1) layers with a primitive lattice (12 structures) and 2) layers with a centered lattice (30 structures). Detailed crystal-chemical analysis has been performed for six structures from each subclass. Specific intermolecular contacts leading to formation of the given structural class and dictating the choice of the subclass are discussed.