![]() Somewhat aside stands a substantial family of bonds (intra- as well as intermolecular) that can be regarded as weakened covalent bonds, or (as sometimes claimed) as ‘stills’ from the process of making/breaking a chemical bond, tracing a reaction pathway, e.g. Each of these ‘bonds’ corresponds to a peculiar combination of the forces mentioned above, and should not be regarded as something physically unique see the illuminating discussion by Dunitz & Gavezzotti (2005 ▸, 2012 ▸). the causes and effects of parallel stacking of aromatic molecules, remained a disputed issue for a long time, partly due to a mistaken analogy with charge-transfer complexes (see the discussion in Hunter & Sanders, 1990 ▸) and equally fruitless explanations in terms of quadrupole moments (Williams, 1993 ▸). Meanwhile, so-called ‘π–π interactions’ i.e. In a similar vein, Scheiner (2013 ▸) introduced the pnicogen bond. Mono-coordinate Cl, Br or I atoms ( X) have a depletion of electron density (σ-hole) opposite to the covalent bond (Politzer et al., 2007 ▸), therefore Y- X⋯ D contacts with electron-donor atoms D can be stabilizing with the energy varying widely, from 10 to 200 kJ mol −1. ![]() More recently, Metrangolo and co-workers (Metrangolo & Resnati, 2001 ▸ Metrangolo et al., 2005 ▸, 2006 ▸) introduced the concept of the ‘halogen bond’. C-H⋯O ( ca 5 kJ mol −1) and C-H⋯π, which can have energies as low as 0.2 kJ mol −1, imperceptibly merging into ‘unspecified’ van der Waals interactions. Later the concept of the ‘hydrogen bond’ was expanded, with substantial controversy (Bernstein, 2013 ▸), to include ‘weak’ hydrogen bonds, e.g. These bonds have energies of ca 20–40 kJ mol −1, while the strongest (charge-assisted or resonance-assisted) hydrogen bonds of ca 150 kJ mol −1 are comparable in energy with covalent bonds proper. The crucial role of such bonds in the structure of water (ice), proteins and DNA is well known. Originally, this term was applied only to D-H⋯ A interactions where both donor ( D) and acceptor ( A) were very electronegative atoms (O and N, but also F and Cl). The hydrogen bond is by far the most important and the most studied – in fact, this concept even predates (Moore & Winmill, 1912 ▸) the discovery of X-ray diffraction. Several chemically specific types of weak interactions are often singled out. All these forces are ubiquitous in all molecular crystals – and beyond, in amorphous solids, liquids and even gases. The latter, of course, are ‘weak’ only at or near the equilibrium intermolecular distances and increase exponentially when molecules are forced closer together under pressure, quickly becoming anything but weak. Physically, these weak interactions can be classified into Coulombic forces between (usually) not very polar species, the effects of mutual polarization between molecules (polarization forces), the dispersion (van der Waals) forces, and the forces of mutual repulsion between closed electron shells due to the Pauli exclusion principle. All along, the crystal needs to be seen in a dynamic, rather than static, way – from thermal vibrations to phase transformations to solid-state reactions. ![]() Both, in turn, must be stepping stones to understanding, predicting and (hopefully) engineering the properties of crystals. However, what makes crystal (and molecular) structures stable and drives chemical reactions and phase transitions, is not the geometry per se but free energy, which is not so easy to visualize. The all-too-common pitfall in a crystallographic paper is to make the discussion of geometrical details an end in itself. The theme of this issue is an integrated approach to weak interactions in crystals. The purpose of the present paper is much more modest: to draw attention to the recent fascinating developments in this field (while also briefly tracing their historical roots) and some unfinished business of the past which now can, and should, be reassessed – and, of course, to provide an introduction to the following research papers. Because it is these forces that hold together a molecular crystal, their study is almost synonymous with the science of organic crystal chemistry – and would require volumes to review. In structural chemistry and crystallography, the term ‘weak interactions’ usually brackets together everything weaker than a single covalent bond or an electrostatic interaction between directly contacting fully charged ions of opposite sign ( i.e. ![]()
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