Andrew G. P. Maloney

Competition between hydrogen bonding and dispersion interactions in the crystal structures of the primary amines

The crystal structures of the primary amines from ethylamine to decylamine have been determined by X-ray diffraction following in situ crystallisation from the liquids. In the series from propylamine to decylamine structures remain in the same phase on cooling from the melting point to 150 K, and the structures of these compounds were determined by single-crystal methods. By contrast, ethylamine undergoes a slow reconstructive phase transition on cooling to 150 K. The structure of the high-temperature form was determined by single-crystal methods at 180 K, while that of the low-temperature form was determined by powder diffraction at 150 K. The stability of the low-temperature form can be ascribed in part to more energetic hydrogen bond formation. PIXEL calculations indicate that hydrogen bonding and methyl–methyl interactions at the chain ends are optimised in the early members of the series, with particularly inefficient inter-chain interactions observed for propylamine and pentylamine. In the later members of the series dispersion interactions become the principal structure-directing interaction and the energies of the hydrogen bonds and methyl–methyl interactions become weaker to accommodate more efficient inter-chain packing. The weakest methyl–methyl interactions occur in heptyl- and nonyl-amines. Overall, intermolecular interactions in the even membered amines are stronger and the packing more efficient than in the odd members, leading to an alternation in melting points along the series, an effect reminiscent of results obtained for the alkanes, carboxylic acids and several α–ω alkyl derivatives.

Intermolecular interaction energies in transition metal coordination compounds

Parameters required to perform PIXEL energy calculations, a semi-empirical method for evaluating intermolecular interactions, have been defined for the transition metals. Using these parameters, lattice energies of thirty-two 1st row, five 2nd row and six 3rd row transition metal complexes have been calculated and compared to experimental values giving correlations of calculated sublimation enthalpies comparable to those obtained for organic crystal structures. Applications of the method are illustrated by analysis of the intermolecular interactions in chromium hexacarbonyl, stacking interactions in bis(acetylacetonato)-oxo-vanadium(IV) and dihydrogen bonding. The results extend the applicability of the PIXEL method from organic materials (ca. 40% of the Cambridge Structural Database (CSD)) to a much wider range of organic and organometallic systems (ca. 85% of the CSD).

Predicting mechanical properties of crystalline materials through topological analysis

With the aim to develop simple, programmatically generated, topology-based descriptors of crystal structures for application to mechanical properties prediction methods, we have developed a new geometric analysis protocol using the CSD Python API. By scanning a crystal structure for Miller planes with the least physical impediment to translational slipping, we are able to predict plausible slip planes for a crystal structure – a feature shown to correlate strongly with tabletability. A simple, automatic hydrogen-bond network dimensionality analysis method has also been developed which, when used in conjunction with the slip plane analysis, can detect whether the proposed slip plane is bridged by hydrogen bonding interactions. These methods are combined with other calculated topological features into a set of crystal structure descriptors, which provide a fast, qualitative way to describe the aspect of crystal plasticity that results from packing geometry. While intended for use in conjunction with other methods, these descriptors alone are shown to correctly predict the relative tabletability measured for multiple drug systems.

Harnessing the knowledge of metal–organic frameworks

Research into the rational design and synthesis of extended materials has grown considerably over the last 20 years, with these materials finding applications in areas such as gas storage, catalysis and drug delivery. The Cambridge Structural Database (CSD) contains over 875,000 small molecule crystal structures, including tens of thousands of metal-organic frameworks (MOFs) and other extended materials. The CSD is therefore an excellent resource for both identifying existing structures with promising characteristics and for guiding future research developments. A major issue in this area though is the huge diversity in the composition of extended materials, and the definition of such materials, which makes collating crystal data for all the extended materials in the CSD a challenging prospect.

Can predicted solid form landscapes provide insight into structure–property correlations?

Crystal structure prediction (CSP) has emerged in recent years as a powerful technique to complement and augment experimental studies of organic solid forms. While the ultimate goal of CSP is often to predict the thermodynamically stable form of a given system from only the chemical diagram, hundreds or even thousands of plausible crystal structures are generated along the way. This landscape of potential solid forms contains a wealth of information on possible packing arrangements and intermolecular interactions, all of which can be used to infer the characteristic properties of the material in question.