Explorations in the dynamics of crystalline solids and the evolution of crystal formation processes

This article describes a few selected areas of research within the field of structural chemistry, with emphasis on aspects that have been influenced and inspired by the seminal work of Jack Dunitz. The topics covered include the study of dynamic properties of crystalline materials, focusing on the use of solid-state 2H NMR spectroscopy to unravel details of dynamic hydrogen-bonding arrangements in crystalline alcohols and amino acids, as well as the use of in situ Raman microspectrometry to explore molecular transport processes through porous crystals. A case study involving the determination of both structural properties and dynamic properties of a material (ammonium cyanate) that is not amenable to structural characterization by single-crystal X-ray diffraction is also presented. On the theme of exploring the time evolution of crystallization pathways, the recent development and application of in situ solid-state NMR techniques for mapping time-dependent changes that occur in the solid phase during crystallization processes are discussed. Finally, the article contemplates the prospects for deriving a fundamental physicochemical understanding of crystal nucleation processes, which is identified as perhaps the most significant challenge in structural chemistry in the next few decades

New in situ solid-state NMR strategies for exploring materials formation and adsorption processes: prospects in heterogenous catalysis

Solid-state NMR spectroscopy is a powerful technique for studying structural and dynamic properties of solids and has considerable potential to be exploited for in situ studies of chemical processes. However, adapting solid-state NMR techniques and instrumentation for in situ applications are often associated with technical challenges, and for this reason, the opportunities remain underexploited. This paper highlights two experimental strategies that we have developed in recent years for in situ solid-state NMR investigations of solid-state processes. One technique is focused on probing details of the time evolution of materials formation processes, and the other technique is focused on understanding the time evolution of adsorption processes in microporous and mesoporous solid host materials. Each of these in situ solid-state NMR techniques has significant prospects for applications in areas relating to heterogeneous catalysis

Some of tomorrow's catalysts for processing renewable and non-renewable feedstocks, diminishing anthropogenic carbon dioxide and increasing the production of energy

This review provides a wide-ranging summary of several aspects of heterogeneous catalysis and its impact on the increasing need to generate more energy, less CO2 and the production of more commodities required by an expanding world population. Particular attention is paid to the options (some of which are already a practical reality) now available for the use of anthropogenic CO2 as a source for the production of platform chemicals required to sustain civilized life. In this connection, Rubisco-inspired methods of utilizing CO2 are discussed, as is the utilization of algae to yield ethanol and O2 from water, CO2 and sunlight. In addition, the increasing use of methanol (derived from CO2) as an energy vector, as well as a source of ethene and propene (which are in growing worldwide demand), is adumbrated. As far as strategies for the design of new solid catalysts are concerned, summarizing accounts are given of the emerging popularity and recent successes of supported “single-atom”, chemo-selective catalysts (of Pt, Pd, Ir and Au), of so-called “single-atom alloy” catalysts for selective hydrogenations, and of monophasic single-site heterogeneous catalysts (SSHCs) for a range of chemical processes, some of which have already been commercialized. SSHCs can, in general, be assembled from earth-abundant elements (C, N, O, Mg, Al, P, Fe), and are effective for shape-selective, regio-selective and enantio-selective catalytic conversions. We also briefly discuss the prospect of converting anthropogenic CO2 into CH4, and touch upon the action needed to reduce atmospheric CO2 so as to fulfil the aims of the recent (December 2015) UN Climate Change Conference in Paris (COP-21)

Calculation of solid-state NMR Lineshapes using contour analysis

Two new methods for calculating lineshapes in solid-state NMR spectra are described. The first method, which we refer to as semi-analytical, allows the rapid calculation of quadrupolar central-transition lineshapes in both static and magic-angle spinning cases. The second method, which is fully numerical, allows the calculation of lineshapes resulting from any combination of interactions, including quadrupolar, dipolar and chemical shift anisotropy, and is not restricted to cases in which the principal axis systems for the different interactions are aligned. Both methods are derived from consideration of the contour lines on a plot of the resonance frequency against the Euler angles, allowing the intensity of the lineshape to be calculated at each frequency. Consequently, highly accurate lineshapes can be calculated more rapidly than previously possible, since only orientations contributing to each specific frequency are considered. For our semi-analytical method, the intensity of each point in the lineshape can be directly calculated in tens of milliseconds on a standard PC. In contrast, established methods can take several hours to calculate the same lineshape

Complexes of Thiourea with alkali metal bromides and iodides: Structural properties, mixed-halide and mixed-metal materials, and halide exchange processes

We report the preparation and structural properties of complexes of metal halides and thiourea with composition MX[thiourea]4 (MX = KBr, KI, RbI, CsI), together with the mixed-halide materials KBrnI1–n[thiourea]4 (0 < n < 1) and the mixed-metal materials KnCs1–nI[thiourea]4 (0 < n < 1). These materials are isostructural, with a tetragonal structure (space group P4/mnc) characterized by M+[thiourea]4 coordination columns along the 4-fold axis and halide anions located in channels in the region of space between adjacent columns and running parallel to the columns. For the mixed-halide materials, the stoichiometry KBrnI1–n[thiourea]4 depends on the bromide/iodide ratio in the crystallization solution; the crystalline complexes have a higher bromide/iodide ratio than the crystallization solution, indicating preferential incorporation of bromide within the complex. Soaking crystals of KBr0.61I0.39[thiourea]4 in iodide containing solutions leads to halide exchange, with the iodide to bromide ratio increasing relative to the parent crystal. Further experiments produced no evidence that these thiourea complexes can accommodate extended polyiodide networks

Exploiting powder X-ray diffraction for direct structure determination in structural biology: the P2X4 receptor trafficking motif YEQGL

We report the crystal structure of the 5-residue peptide acetyl-YEQGL-amide, determined directly from powder X-ray diffraction data recorded on a conventional laboratory X-ray powder diffractometer. The YEQGL motif has a known biological role, as a trafficking motif in the C-terminus of mammalian P2X4 receptors. Comparison of the crystal structure of acetyl-YEQGL-amide determined here and that of a complex formed with the μ2 subunit of the clathrin adaptor protein complex AP2 reported previously, reveals differences in conformational properties, although there are nevertheless similarities concerning aspects of the hydrogen-bonding arrangement and the hydrophobic environment of the leucine sidechain. Our results demonstrate the potential for exploiting modern powder X-ray diffraction methodology to achieve complete structure determination of materials of biological interest that do not crystallize as single crystals of suitable size and quality for single-crystal X-ray diffraction

Structural diversity of solid solutions formed between 3-chloro-trans-cinnamic acid and 3-bromo-trans-cinnamic acid

The formation and structural properties of solid solutions containing 3-chloro-trans-cinnamic acid (3-ClCA) and 3-bromo-trans-cinnamic acid (3-BrCA) are explored across a range of compositions. Two distinct γ-type structures of 3-ClCA/3-BrCA solid solutions and two distinct β-type structures of 3-ClCA/3-BrCA solid solutions are reported and structurally characterized. One of the γ-type structures is isostructural with the known γ polymorphs of pure 3-ClCA and pure 3-BrCA, whereas the other γ-type structure has not been observed previously for either pure 3-ClCA or pure 3-BrCA (representing a rare case in which the structure of the solid solution is not known for the pure phases of either of the constituent molecules). One of the β-type structures of the 3-ClCA/3-BrCA solid solutions is similar to the β polymorph of pure 3-ClCA, whereas the other β-type structure is similar to the β polymorph of pure 3-BrCA. The specific β-type structure formed is found to depend on the relative amounts of 3-BrCA and 3-ClCA in the solid solution. UV irradiation of the β-type 3-ClCA/3-BrCA solid solution with 1:1 composition yields three different photodimers, with substituents {Cl,Cl}, {Cl,Br}, or {Br,Br} in the approximate ratio 1:2:1 respectively, consistent with the occurrence of a topochemical reaction in a solid solution with a random distribution of 3-ClCA and 3-BrCA molecules

Establishing the transitory existence of amorphous phases in crystallization pathways by the CLASSIC NMR technique

With the growing realization that crystallization processes may evolve through a sequence of different solid forms, including amorphous precursor phases, the development of suitable in situ experimental probes is essential for comprehensively mapping the time‐evolution of such processes. Here we demonstrate that the CLASSIC NMR (Combined Liquid‐ And Solid‐State In situ Crystallization NMR) strategy is a powerful technique for revealing the transitory existence of amorphous phases during crystallization processes, applying this technique to study crystallization of DL menthol and L menthol from their molten liquid phases. The CLASSIC NMR results provide direct insights into the conditions (including the specific time period) under which the molten liquid phase, transitory amorphous phases and final crystalline phases exist during these crystallization processes

Controlling spatial distributions of molecules in multicomponent organic crystals, with quantitative mapping by confocal Raman microspectrometry

We report four experimental strategies for controlling the three-dimensional arrangement of molecules in multicomponent organic crystals, exploiting confocal Raman microspectrometry to quantify the three-dimensional spatial distributions. Specifically, we focus on controlling the distribution of two types of guest molecule in solid organic inclusion compounds to produce composite core–shell crystals, crystals with a homogeneous distribution of the components, crystals with continuous compositional variation from the core to the surface, and crystals with alternating shells of the components. In this context, confocal Raman microspectrometry is particularly advantageous over optical microscopy as it is nondestructive, offers micrometric spatial resolution, and relies only on the component molecules having different vibrational properties