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

Polycatenation, polythreading and polyknotting in coordination network chemistry

Interpretation of X-ray Powder Diffraction Patterns . By H. Lipson and H. Steeple. Pp. viii + 335 + 3 plates. (Mac-millan: London; St Martins Press: New York, May 1970.) £4.

X-Ray Diffraction

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

The occurrence of interpenetration in metal–organic and inorganic networks has been investigated by a systematic analysis of the CSD and ICSD structural databases. For this purpose, a novel version of TOPOS (a program package for multipurpose crystallochemical analysis) has been employed, where the procedure of recognition of interpenetrating nets is based on the representation of a crystal structure as a finite reduced graph. In this paper we report a comprehensive list (301 Refcodes) of interpenetrating metal–organic 3D structures from CSD, that are analyzed on the basis of their topologies. Interesting trends and novel features have been observed and distinct classes of interpenetrating nets have been envisaged, depending on the relationships of the individual motifs.

Interpenetrating metal–organic and inorganic 3D networks: a computer-aided systematic investigation. Part I. Analysis of the Cambridge structural database

A study of intramolecular hydrogen bonding in benzoylacetone (1-phenyl-1,3-butadione) has been carried out with 8.4(4)K X-ray data and 20(1)K neutron data. Analysis of the neutron data shows that the hydrogen, between the two oxygens in the keto−enol part of the molecular structure, is asymmetrically placed in a large flat potential well. The charge density obtained from X-ray and neutron data has been analyzed by using multipolar functions and topological methods, which gave evidence of extensive π-delocalization in the keto−enol group. The multipole populations show that there are large formal charges on the oxygens and the enol hydrogen, which impart polar character to the hydrogen bond. This effect is also evident in the Laplacian and in the electrostatic potential calculated from the X-ray data and it is found that the hydrogen position is stabilized by both electrostatic and covalent bonding contributions at each side of the hydrogen atom. The resonance assisted hydrogen bonding model has been refin...

Topological Analysis of the Charge Density in Short Intramolecular O−H···O Hydrogen Bonds. Very Low Temperature X-ray and Neutron Diffraction Study of Benzoylacetone

Binuclear platinum diphosphite complexes. Crystal structures of K4[Pt2(pop)4Br]·3H2O, a new linear chain semiconductor, and K4[Pt2(pop)4Cl2]·2H2O

Of the several hundred structures in the Cambridge Structural Database [version 4.6 (1992), Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, England] having space groups Pca2_1 or Pna2_1 and more than one molecule in the asymmetric unit (Z > 4), approximately three-quarters contain local centers of symmetry. These local centers, which are not crystallographic centers, occur predominantly near x = 1/8, y = ¼ in Pca2_1 or near x = 1/8, y = 0 in Pna2_1; this also holds for the limited number of examples we have examined of pseudo-centrosymmetric molecules with Z = 4. Local centers at these points create unusual correlations between corresponding atoms in the two molecules.

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Arrays with Local Centers of Symmetry in Space Groups Pca21 and Pna21

The first (so-called) lambda transition in solids was found in the specific heat measurements for NH4Cl at 242 K by F. Simon in 1922 [Simon (1922). Ann. Phys. 68, 241–280]. Analogous phenomena found in many other solids gave rise to doubts (expressed most clearly by A. R. Ubbelohde some 50 years ago) about the applicability of classical thermodynamics to some phase transitions [Ubbelohde (1956). Brit. J. Appl. Phys. 7, 313–321]. However, Y. Mnyukh's studies of enantiotropic phase transitions in eight organic crystals showed that all proceed by a nucleation-and-growth mechanism [summarized in Mnyukh (2001), Fundamentals of Solid State Phase Transitions, Ferromagnetism and Ferroelectricity. 1st Books]. Nucleation is localized at defects in the parent phase; growth can be epitaxic and oriented if parent and daughter phases have closely similar structures, or random (not oriented) if there are substantial structural differences. This conclusion is supported by a critical review of Mnyukh's eight examples and other results published in the interim. It seems that Ubbelohde's invocation of `hybrid crystals' and `smeared transitions' can mostly be accounted for by lack of equilibrium in the phase-transition studies cited by him. However, the intermediate phase in 4,4′-dichlorobenzophenone appears to have structural resemblances to Ubbelohde's' `hybrid crystal'.

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On the mechanism of some first-order enantiotropic solid-state phase transitions: from Simon through Ubbelohde to Mnyukh

Some 60 examples of crystal structures are presented which can be better described in space groups of higher symmetry than used in the original publications. These are divided into three categories: (A) incorrect Laue group (33 examples), (B) omission of a center of symmetry (22 examples), (C) omission of a center of symmetry coupled with a failure to recognize systematic absences (nine examples). Category A errors do not lead to significant errors in molecular geometry, but these do accompany the two other types of error. There are 19 of the current set of examples which have publication dates of 1996 or later. Critical scrutiny on the part of authors, editors and referees is needed to eliminate such errors in order not to impair the role of crystal structure analysis as the chemical court of last resort.

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Frank H. Herbstein

Papers

Topological Analysis of the Charge Density in Short Intramolecular O−H···O Hydrogen Bonds. Very Low Temperature X-ray and Neutron Diffraction Study of Benzoylacetone

Binuclear platinum diphosphite complexes. Crystal structures of K4[Pt2(pop)4Br]·3H2O, a new linear chain semiconductor, and K4[Pt2(pop)4Cl2]·2H2O

Arrays with Local Centers of Symmetry in Space Groups Pca21 and Pna21

On the mechanism of some first-order enantiotropic solid-state phase transitions: from Simon through Ubbelohde to Mnyukh

Some 60 new space-group corrections