Chemistry: the middle kingdom

Chemistry occupies a unique middle position between physics and mathematics on the one side and biology, ecology, sociology and economics on the other. It is said that chemistry is reducible into physics and finally mathematics. However, in moving from the covalent to the non-covalent world we obtain a new chemistry, one that is a starting point for the emergence of the soft sciences. This article argues that this new chemistry represents a paradigm shift in the way in which chemists think about their subject today. Biology may be considered as emerging out of this new chemistry, which in itself cannot be reduced into physics and mathematics as was the case for chemistry thus far practiced. This dualistic nature of chemistry, reducible and irreducible, is a new development but one that ensures that the subject will remain alive and well in the foreseeable future

Hydrogen bonds and other intermolecular interactions in organometallic crystals

Organometallic compounds have been studied with X-ray crystallography from their very discovery. Yet structural organometallic chemists were almost exclusively concerned with the molecular structure and stereochemistry of organometallic compounds and clusters rather than with their crystal structures and packing characteristics. The growing importance of crystal engineering and supramolecular chemistry, however, led to interest in the nature of the interactions that bind organometallic molecules into crystals. In part, these interactions are similar to those found in purely organic crystals because the peripheries of these molecules often contain organic residues. Yet molecular features peculiar to organometallic compounds also do lead to distinctive supramolecular characteristics. Most notable among these intermolecular interactions are hydrogen bonds. Organometallic compounds contain a wealth and diversity of hydrogen bonds that are without counterpart in the organic world. These include C-H···O bonds to M-C≡O acceptors, and hydrogen bonds wherein the metal atom itself acts as a donor or as an acceptor. Even more exotic is the dihydrogen bond M1-H ··· H-M2. Despite this variety, all these weak interactions have properties that resemble those of the more familiar hydrogen bonds such as O-H ··· O, N-H ··· O, O-H ··· N and N-H ··· N. Other interactions that are distinctive to organometallic compounds are the agostic interaction to electron deficient metals (C-H) ··· M and the aurophilic interaction Au···Au. The Cambridge Structural Database (CSD) is an essential tool in the analysis of weak intermolecular interactions. Since the number of organometallic crystal structures in the CSD is very large, the weakest of intermolecular interactions may be studied with ever-increasing degrees of reliability. Through such analysis one is able to obtain a more complete idea of organometallic crystal architecture. Crystal engineering must pass through the stage of analysis before crystal synthesis can be attempted and organometallic crystal engineering is still in its infancy. However, the progress made so far in understanding the nature of intermolecular interactions in these crystals indicates that one may expect rapid progress in the engineering of organometallic crystals with desired structures and properties

Designer crystals: intermolecular interactions, network structures and supramolecular synthons

Questions relating to the prediction of the crystal structure or structures of a given organic molecule may be more gainfully reversed so that retrosynthetic analysis of a target crystal network leads to the identification of molecular precursors. Crystal engineering is solid state supramolecular synthesis and supramolecular synthons, units formed by synthetic operations involving intermolecular interactions, may be used to focus efforts in such logic-driven retrosynthesis

C-H … O hydrogen bonding and the deliberate design of organic crystal structures

The C-H…O interaction, though weak, is not van deer Wails in nature but electrostatic and resembles O-H…O and N-H…O hydrogen bonds in its geometrical properties. Spectroscopic evidence hints at some degree of covalent bonding between H and 0 atoms. The long-range, electrostatic character of the C-H…O interaction determines its important role in crystal engineering. Planar aromatic hydrocarbons adopt herringbone structures but if a critical number of O atoms is present, a planar molecule will cross the structural threshold from a C…H stabilised herringbone structure to a C-H…O stabilised layer structure wherein adjacent molecules are parallel and highly overlapped. These ideas have been used to design a family of crystal structures of alkoxyphenylpro-piolic acids that participate in an intermolecular, solid state Diels-Alder reaction. When the number of C-H…O interactions in a structure is unusually large or small, there is a possibility of distortions in the O-H…O network

Crystal and co-crystal

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Dimorphs of 4'-amino-4-hydroxy-2'-methylbiphenyl: assessment of likelihood of polymorphism in flexible molecules

The less stable polymorph of the title compound has the better synthon while the more stable polymorph the better packing. The possibility of polymorphism in a closely related isomer is examined computationally

Supramolecular equivalence of ethynyl, chloro, bromo and iodo groups. A comparison of the crystal structures of some 4-phenoxyanilines

4-(4'-Iodo)phenoxyaniline is isostructural to the corresponding bromo, chloro and ethynyl derivatives in contrast to 4-iodoaniline which is not isostructural to its bromo, chloro and ethynyl counterparts. This difference is rationalised in terms of conditional isomorphism

Correlation between molecular dipole moment and centrosymmetry in some crystalline diphenyl ethers

The presence of a large molecular dipole moment in diphenyl ethers leads unequivocally to a centrosymmetric crystal structure

Pseudopolymorphism: occurrences of hydrogen bonding organic solvents in molecular crystals

Multi-point recognition with strong and weak hydrogen bonds between solvent and solute molecules facilitates the retention of organic solvents in crystals

Van der waals and polar intermolecular contact distances: quantifying supramolecular synthons

Crystal structures are viewed as being determined by ranges and constraints on interatomic contact distances between neighboring molecules. These distances are considered to arise from environment-dependent atomic sizes, that is, larger sizes for isotropic, van der Waals type contacts and smaller sizes for more-polar, possibly ionic contacts. Although the idea of different, or anisotropic, radii for atoms is not new, we developed a method of obtaining atomic sizes that is based on a theoretical framework. Using different atomic sizes for the same atom in different environments, we were able to rationalize some structural observations and anomalies. For example, benzene with the Pbca structure may be described in terms of two types of C···H interactions: a longer contact largely of the van der Waals type, and a shorter, structure-determining type (Cδ -···Hδ +), which we term "n-polar". Our approach is illustrated with three examples: 1) the equivalence in crystal packing of fluorobenzene, benzonitrile, pyridine N-oxide, and pyridine/HF 1:1 molecular complex, all of which take the not-so-common tetragonal P41212 space group and are practically isomorphous; 2) the similarity of the Pa3 acetylene and Pbca benzene crystal structures; and 3) the equivalence between an increase in pressure and an increase in the "n-polar"contacts in Pbca benzene; in other words, the equivalence between hydrostatic pressure and chemical pressure. In the context of crystal engineering, we describe a method whereby the topological information conveyed in a supramolecular synthon is recast in a more quantitative manner. A particular synthon, and in turn the crystal structure to which it leads, is viable within small ranges of distances of its constituent atoms, and these distances are determined by chemical factors