J. Appl. Cryst.の発刊に際して

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

We have recently described a remarkable new way of exploring packing modes and intermolecular interactions in molecular crystals using a novel partitioning of crystal space. These molecular Hirshfeld surfaces reflect intermolecular interactions in a novel visual manner, offering a hitherto unseen picture of molecular shape in a crystalline environment. The surfaces encode information about all intermolecular interactions simultaneously, but sophisticated interactive graphics are required in order to extract the information most efficiently. To overcome this we have devised a two-dimensional mapping which summarizes quantitatively the nature and type of intermolecular interaction experienced by a molecule in the bulk, and presents it in a convenient graphical format. The mapping takes advantage of the triangulation of the Hirshfeld surfaces, and plots the fraction of points on the surface as a function of the closest distances from the point to nuclei inside and outside the surface.
In this manner all interaction types (for example, hydrogen bonding, close and distant van der Waals contacts, C–H⋯π interactions, π–π stacking) are readily identifiable, and it becomes a straightforward matter to classify molecular crystals by the nature of interactions, and to rapidly identify similarities and differences which can become obscured when examining crystal packing diagrams. These plots are a novel visual representation of all the intermolecular interactions simultaneously, and are unique for a given crystal structure and polymorph. Applications to a wide variety of molecular crystals and intermolecular interactions are presented, including polymorphic systems, as well as crystals where Z′ > 1.

Fingerprinting intermolecular interactions in molecular crystals

The defect structure of anion-excess (Ca1−xYx)F2+x with x = 0.06

Co-crystallisation of pentachlorophenol (PCP) with the series of dimethylpyridines (lutidines) results in the formation of molecular complexes that show strong hydrogen bonding in the solid state, as anticipated. Competition for the hydrogen atom in these strong hydrogen bonds (HBs) by the pKa-matched molecules leads to a variable degree of hydrogen transfer from PCP towards the lutidine, dependent on the ΔpKa value in a particular complex. The influence of weak C–H⋯O HBs in the vicinity of the strong O⋯H⋯N HB on this hydrogen transfer is discussed.

Hydrogen transfer in pentachlorophenol – dimethylpyridine complexes

Solid-state lattice-dynamics calculations within the hybrid density-functional approach are applied to the study of the thermally induced ${\text{Fe}}^{2+}$ $\text{low}\text{ }\text{spin}(\text{LS};S=0)\ensuremath{\leftrightarrow}\text{high}\text{ }\text{spin}(\text{HS};S=2)$ crossover (SCO) in the extended network of the $\text{CsFe}[\text{Cr}{(\text{CN})}_{6}]$ Prussian blue analog. The variations in the thermodynamic parameters defining the SCO transition with the Fock exchange content $({F}_{0})$ of the functional are obtained and discussed, where, in keeping with the findings of previous studies of isolated complexes, it is found that an admixture ${F}_{0}\ensuremath{\approx}14\mathrm{%}$ provides reliable values. The transition is shown to be dominated by the entropy difference, $\ensuremath{\Delta}S$, associated with the softening of low-frequency vibrational (vib) modes in the HS state, as has been suggested previously for a wide range of SCO materials, more than half of $\ensuremath{\Delta}{S}_{\text{vib}}$ deriving from modes with wave numbers of $250\text{ }{\text{cm}}^{\ensuremath{-}1}$ or less. Analysis of the influence of the spectroscopic selection rules upon the apparent SCO thermodynamics reveals that determinations based solely upon infrared or Raman frequencies, or upon their combination, lead to significant errors. The effect upon the SCO transition of the electronic entropy associated with the degenerate ${\text{Fe}}^{2+}$ HS $({e}_{\text{g}}^{2}{t}_{2\text{g}}^{4})$ configurations is also detailed, evidence for the existence of an associated dynamic Jahn-Teller distortion being presented. Optimized structures, bulk moduli, $\ensuremath{\Gamma}$-point vibrational frequencies, and crystal-field energy models are discussed for all relevant spin states.

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Spin crossover in the CsFe II [Cr III (CN) 6 ] Prussian blue analog: Phonons and thermodynamics from hybrid functionals

A family of multicomponent haloaniline/3,5-dinitrobenzoic acid molecular crystals with striking red-to-colourless temperature-induced thermochromism is identified and characterised. Four thermochromic pairs of 1 : 1 neutral cocrystals and ionic salts are identified which, unusually, grow concomitantly under the same conditions. The coloured cocrystals are found to be metastable, kinetically trapped during crystallisation, and convert via proton transfer to the more stable salt forms on heating. The colour of the neutral form and the temperature of the transition can be tuned through the halogen and by chemical substitution on the aniline component. From structural characterisation and first-principles modelling, we elucidate the origin of the metastability of the cocrystals and link structural changes through the phase transition to the striking visible colour change. By deliberately exploiting the uncertainty of the salt-cocrystal continuum, where the small pKa difference between components enables significant solid-state structural rearrangements induced by proton transfer, this work highlights a novel design paradigm for engineering new organic thermochromics with tailored physical properties.

/pdf/living-in-the-salt-cocrystal-continuum-indecisive-organic-4eayvpvfwy.pdf

Living in the salt-cocrystal continuum: indecisive organic complexes with thermochromic behaviour

We have investigated the high-temperature superionic transition of the anion-excess fluorite with x = 0.06 using both monochromatic and time-of-flight Laue single-crystal neutron diffraction. The measured Bragg intensities indicate that the cuboctahedral defect clusters found at ambient temperature start to break up into smaller fragments even below the superionic transition temperature, . Information concerning the local defect configuration at T = 1173 K has been provided by modelling the measured distribution of the coherent elastic diffuse scattering within the plane of reciprocal space. The high-temperature defects are of the `Willis' type and strongly resemble the short-lived Frenkel clusters found in the pure fluorites such as above .

http://iopscience.iop.org/article/10.1088/0953-8984/9/4/005/pdf

Chick C. Wilson

Papers

The defect structure of anion-excess (Ca1−xYx)F2+x with x = 0.06

Hydrogen transfer in pentachlorophenol – dimethylpyridine complexes

Spin crossover in the CsFe II [Cr III (CN) 6 ] Prussian blue analog: Phonons and thermodynamics from hybrid functionals

Living in the salt-cocrystal continuum: indecisive organic complexes with thermochromic behaviour

A neutron diffraction study of the superionic transition in ? with x = 0.06