J. Appl. Cryst.の発刊に際して

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

The Halogen Bond

Publisher Summary This review is concerned with the neglected class of inorganic compounds, which contain ions of the same element in two different formal states of oxidation. Although the number of references cited in our review show that many individual examples of this class have been studied, yet they have very rarely been treated as a class, and there has never before, to our knowledge, been a systematic attempt to classify their properties in terms of their electronic and molecular structures. In the past, systems containing an element in two different states of oxidation have gone by various names, the terms “mixed valence,” nonintegral valence,” “mixed oxidation,” “oscillating valency,” and “controlled valency” being used interchangeably. Actually, none of these is completely accurate or all-embracing, but in our hope to avoid the introduction of yet another definition, we have somewhat arbitrarily adopted the phrase “mixed valence” for the description of these systems. The concept of resonance among various valence bond structures is one of the cornerstones of modern organic chemistry.

Mixed Valence Chemistry-A Survey and Classification

Crystal polymorphism is encountered in all areas of research involving solid substances. Its occurrence introduces complications during manufacturing processes and adds another dimension to the complexity of designing materials with specific properties. Research on polymorphism is fraught with unique difficulties due to the subtlety of polymorphic transformations and the inadvertent formation of pseudopolymorphs. In this report, a summary of thermodynamic, kinetic and structural considerations of polymorphism is presented. A wide variety of techniques appropriate to the study of organic crystalline polymorphism and pseu-dopolymorphism is then surveyed, ranging from simple crystal density measurement to observation of polymorphic transformations using variable-temperature synchrotron X-ray diffraction methods. Application of newer methodology described in this report is yielding fresh insights into the nature of the crystallization process, holding promise for a deeper understanding of the phenomenon of polymorphism and its practical control.

Crystalline Polymorphism of Organic Compounds

Dendrimers in Supramolecular Chemistry: From Molecular Recognition to Self-Assembly.

Equimolar mixtures of ortho- and meta-, or ortho- and para-, or meta- and para-tetrachlorodicyanobenzene, C6Cl4(CN)2, form structures with disordered layers containing both kinds of molecules in rhombohedral or pseudo-rhombohedral cells. These structures are similar to the structure of the rhombohedral form of the meta isomer. The guiding force appears to be an intermolecular synthon, in which a nitrile group in one molecule interacts with Cl atoms in two adjacent molecules with N⋯Cl distances of approximately 3.1 A.

Planar packing of tetrachlorodicyanobenzenes. II

4,5-dibromo-1-methyl-1H-imidazole

Potassium Tetranitrodiammine cobaltate(III)

The title compound, C13H9NO, crystallizes with four molecules in the asymmetric unit. Each of the four crystallographically independent molecules forms a chain parallel to the a axis with symmetry-equivalent molecules. These chains are held together by similar O-H...NC hydrogen bonds, with approximately linear O-H...N angles and significantly bent H...N-C angles. The four different molecules are related by strong elements of pseudosymmetry. To better describe the pseudosymmetry, the structure has been reported in the non-standard space group C-1.

4'-Hydroxybiphenyl-4-carbonitrile.

5,6-Dimethylbenzofurazan 1-oxide (Me2BF), C8H8N2O2, occurs in four polymorphic forms that are polytypes of each other. Each polymorph of Me2BF contains molecules disordered about pseudo-twofold axes and arranged head-to-tail in ribbons, with the ribbons forming approximately planar layers held together by weak C—H⋯N and C—H⋯O interactions. Adjacent layers interact in different ways in the different polymorphs. In addition to twinning in the individual polymorphs, four examples of allotwining, that is, oriented overgrowths between different polymorphs, were found.

Doyle Britton

Papers

Planar packing of tetrachlorodicyanobenzenes. II

4,5-dibromo-1-methyl-1H-imidazole

Potassium Tetranitrodiammine cobaltate(III)

4'-Hydroxybiphenyl-4-carbonitrile.

Four polymorphs (polytypes) of 5,6-dimethylbenzofurazan 1-oxide