From molecules to crystal engineering: supramolecular isomerism and polymorphism in network solids.

The hydrogen bond is the most important of all directional intermolecular interactions. It is operative in determining molecular conformation, molecular aggregation, and the function of a vast number of chemical systems ranging from inorganic to biological. Research into hydrogen bonds experienced a stagnant period in the 1980s, but re-opened around 1990, and has been in rapid development since then. In terms of modern concepts, the hydrogen bond is understood as a very broad phenomenon, and it is accepted that there are open borders to other effects. There are dozens of different types of X-H.A hydrogen bonds that occur commonly in the condensed phases, and in addition there are innumerable less common ones. Dissociation energies span more than two orders of magnitude (about 0.2-40 kcal mol(-1)). Within this range, the nature of the interaction is not constant, but its electrostatic, covalent, and dispersion contributions vary in their relative weights. The hydrogen bond has broad transition regions that merge continuously with the covalent bond, the van der Waals interaction, the ionic interaction, and also the cation-pi interaction. All hydrogen bonds can be considered as incipient proton transfer reactions, and for strong hydrogen bonds, this reaction can be in a very advanced state. In this review, a coherent survey is given on all these matters.

The hydrogen bond in the solid state.

In the last few years the analysis of molecular crystal structures using tools based on Hirshfeld surfaces has rapidly gained in popularity. This approach represents an attempt to venture beyond the current paradigm—internuclear distances and angles, crystal packing diagrams with molecules represented via various models, and the identification of close contacts deemed to be important—and to view molecules as “organic wholes”, thereby fundamentally altering the discussion of intermolecular interactions through an unbiased identification of all close contacts.

Hirshfeld surface analysis

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

A new way of exploring packing modes and intermolecular interactions in molecular crystals is described, using Hirshfeld surfaces to partition crystal space. These molecular Hirshfeld surfaces, so named because they derive from Hirshfeld's stockholder partitioning, divide the crystal into regions where the electron distribution of a sum of spherical atoms for the molecule (the promolecule) dominates the corresponding sum over the crystal (the procrystal). These surfaces reflect intermolecular interactions in a novel visual manner, offering a previously unseen picture of molecular shape in a crystalline environment. Surface features characteristic of different types of intermolecular interactions can be identified, and such features can be revealed by colour coding distances from the surface to the nearest atom exterior or interior to the surface, or by functions of the principal surface curvatures. These simple devices provide a striking and immediate picture of the types of interactions present, and even reflect their relative strengths from molecule to molecule. A complementary two-dimensional mapping is also presented, which summarizes quantitatively the types of intermolecular contacts experienced by molecules in the bulk and presents this information in a convenient colour plot. This paper describes the use of these tools in the compilation of a pictorial glossary of intermolecular interactions, using identifiable patterns of interaction between small molecules to rationalize the often complex mix of interactions displayed by large molecules.

Novel tools for visualizing and exploring intermolecular interactions in molecular crystals.

Following a brief historical review of solid-state photochemistry current research is discussed under four parallel headings: first, analysis of the topochemical postulate according to which the course of the solid-state reaction and the stereochemistry of the photodimer (if any) can be predicted from the configuration and nearest-neighbour geometry of closest monomer molecules in the crystal lattice; secondly, the study of the lOCUS of the reaction, that is the dependence of the course of the reaction or (dimerization, cis — trans isomerization) on crystal texture (dislocation, grain and phase boundaries); thirdly, a better understanding of the packing principles of organic molecules in their crystal structure to enable us to construct photoreactive (or lightstable) crystal structures of any given monomer as well as mixed crystals of potentially codimerizable monomers for synthetic purposes or energy-transfer studies. Several approaches to this problem of 'crystal engineering' based on generalizations of packing modes of primary amides and of dichiorophenyl derivatives are outlined, and their application to a systematic photochemistry of the solid state discussed.

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Photodimerization in the solid state

Reactions in chiral crystals. Optically active heterophotodimer formation from chiral single crystals

Reactions in Chiral Crystals: An Absolute Asymmetric Synthesis

The solid-state photochemistry of several substituted anthracenes has been investigated with particular reference to the behaviour of polymorphic forms of the monomers. A class of 9-substituted anthracenes (methyl, methoxycarbonyl, chloro, bromo) crystallising in anti-parallel, sandwich packing arrangements give centrosymmetric dianthracenes in accordance with the topochemical principle, which also operates on light-stable monomers packing in non-centrosymmetric space groups having axes > 8 A (e. g. 9-methoxy, 9-ethoxy-, 1-cyano-) and showing no short parallel interplanar contacts. Some but not all monomers, crystallising in the parallel-stack type with a 4 A axis, dimerise to the non-topochemical head-to-tail dimer. Two monomers substituted elsewhere than in the 9-position give the theoretical number of solution dimers but yield only one dimer in the solid state.

Topochemistry. Part XXXIII. The Solid‐State Photochemistry of Some Anthracene Derivatives

effects are negligible and these two models differ only in this respect. The differences are essentially zero. (The small deviation from zero of the plotted differences at low atomic number arises for the most part from small inaccuracies in the analytic fits used to compute the differences). At the higher atomic numbers the difference is always negative because the relativistic DS atomic model is more compact than the nonrelativistic HFS model. As a general rule, the inclusion of exchange has a greater effect on the scattering factors than does relativity. At the heaviest elements the two effects appear to be comparable.

Gerhard M. J. Schmidt

Papers

Photodimerization in the solid state

Reactions in chiral crystals. Optically active heterophotodimer formation from chiral single crystals

Reactions in Chiral Crystals: An Absolute Asymmetric Synthesis

Topochemistry. Part XXXIII. The Solid‐State Photochemistry of Some Anthracene Derivatives

The crystal structure of [18]annulene, I. X‐ray study