A selection of key publications by the group. The Effect of High Pressure on Polymorphs of a Derivative of Blatter’s Radical: Identification of the Structural Signatures of Subtle Phase Transitions., Crystal Growth & Design, 2023, 23(3), pp.1915-1924. Edward T. Broadhurst, Cameron J. G. Wilson, Georgia A. Zissimou, Mayra A. Padrón Gómez, Daniel M. L. Vasconcelos, Christos P. Constantinides, Panayiotis A. Koutentis, Alejandro P. Ayala, and Simon Parsons. The effect of pressure on the α and β polymorphs of a derivative of Blatter’s radical, 3-phenyl-1-(pyrid-2-yl)-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl, has been investigated using singlecrystal X-ray diffraction to maximum pressures of 5.76 and 7.42 GPa, respectively. The most compressible crystallographic direction in both structures lies parallel to π-stacking interactions, which semiempirical Pixel calculations indicate are also the strongest interactions present. The mechanism of compression in perpendicular directions is determined by void distributions. Discontinuities in the vibrational frequencies observed in Raman spectra measured between ambient pressure and ∼5.5 GPa show that both polymorphs undergo phase transitions, the α phase at 0.8 GPa and the β phase at 2.1 GPa. The structural signatures of the transitions, which signal the onset of compression of initially more rigid intermolecular contacts, were identified from the trends in the occupied and unoccupied volumes of the unit cell with pressure and in the case of the β phase by deviations from an ideal model of compression defined by Birch−Murnaghan equations of state. DOI: 10.1021/acs.cgd.2c01422 Discerning subtle high-pressure phase transitions in glyphosate. CrystEngComm, 2023, 25(6), pp.988-997. Cameron J. G. Wilson, Peter A. Wood and Simon Parsons The common garden herbicide glyphosate, N-(phosphonomethyl)glycine, has been studied between ambient pressure and 5.17 GPa using single crystal X-ray diffraction. Glyphosate forms a structure composed of layers parallel to the (1 0 −2) planes. Hydrogen bonds form along the stacking direction, which are very incompressible so that the effects of pressure are accommodated mostly within the layers. This study has confirmed two high pressure phase transitions previously observed by Raman spectroscopy, enabling the structural signatures of the transitions to be identified. Both transitions are very subtle and second order, involving changes to the way the structure responds to pressure rather than changes to the structure. The first transition occurs between 0.93–1.21 GPa and corresponds to the onset of greater compressibility within the layers. The second transition between 3.78–4.23 GPa is an intramolecular feature signalling a deformation of the molecular backbone. In the absence of a first order phase transition, the packing remains in a compressed form of its ambient pressure form up until the highest pressure measured. A reconstructive phase transition occurs at 5.98 GPa forming a polycrystalline high-pressure phase. DOI: 10.1039/d2ce01616h Behavior of Occupied and Void Space in Molecular Crystal Structures at High Pressure Crystal growth & design, 2022, 22(4), pp.2328-2341. Cameron J. G. Wilson, Tomas Cervenka, Peter A. Wood, and Simon Parsons We report a Monte Carlo algorithm for calculation of occupied (“network”) and unoccupied (“void”) space in crystal structures. The variation of the volumes of the voids and the network of intermolecular contacts with pressure sensitively reveals discontinuities associated with first- and second-order phase transitions, providing insights into the effect of compression (and, in principle, other external stimuli) at a level between those observed in individual contact distances and the overall unit cell dimensions. The method is shown to be especially useful for the correlation of high-pressure crystallographic and spectroscopic data, illustrated for naphthalene, where a phase transition previously detected by vibrational spectroscopy, and debated in the literature for over 80 years, has been revealed unambiguously in crystallographic data for the first time. Premonitory behavior before a phase transition and crystal collapse at the end of a compression series has also been detected. The network and void volumes for 129 high-pressure studies taken from the Cambridge Structural Database (CSD) were fitted to equation of state to show that networks typically have bulk moduli between 40 and 150 GPa, while those of voids fall into a much smaller range, 2–5 GPa. These figures are shown to reproduce the narrow range of overall bulk moduli of molecular solids (ca. 5–20 GPa). The program, called CellVol, has been written in Python using the CSD Python API and can be run through the command line or through the Cambridge Crystallographic Data Centre’s Mercury interface. DOI: 10.1021/acs.cgd.1c01427 A first-order phase transition in Blatter's radical at high pressure Acta Crystallographica Section B: Structural Science,2022, Crystal Engineering and Materials, 78(2) Edward T. Broadhurst, Cameron J. G. Wilson, Georgia A. Zissimou, Fabio Nudelman, Christos P. Constantinides, Panayiotis A. Koutentis and Simon Parsons The crystal structure of Blatter's radical (1,3-diphenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl) has been investigated between ambient pressure and 6.07 GPa. The sample remains in a compressed form of the ambient-pressure phase up to 5.34 GPa, the largest direction of strain being parallel to the direction of π-stacking interactions. The bulk modulus is 7.4 (6) GPa, with a pressure derivative equal to 9.33 (11). As pressure increases, the phenyl groups attached to the N1 and C3 positions of the triazinyl moieties of neighbouring pairs of molecules approach each other, causing the former to begin to rotate between 3.42 to 5.34 GPa. The onset of this phenyl rotation may be interpreted as a second-order phase transition which introduces a new mode for accommodating pressure. It is premonitory to a first-order isosymmetric phase transition which occurs on increasing pressure from 5.34 to 5.54 GPa. Although the phase transition is driven by volume minimization, rather than relief of unfavourable contacts, it is accompanied by a sharp jump in the orientation of the rotation angle of the phenyl group. DFT calculations suggest that the adoption of a more planar conformation by the triazinyl moiety at the phase transition can be attributed to relief of intramolecular HH contacts at the transition. Although no dimerization of the radicals occurs, the π-stacking interactions are compressed by 0.341 (3) Å between ambient pressure and 6.07 GPa. DOI: 10.1107/S2052520622000191 Use of a miniature diamond-anvil cell in a joint X-ray and neutron high-pressure study on copper sulfate pentahydrate. IUCrJ. 2021;9(1). Giulia Novelli, Konstantin V. Kamenev, Helen E. Maynard-Casely, Simon Parsons, Garry J. McIntyre. Single-crystal X-ray and neutron diffraction data are usually collected using separate samples. This is a disadvantage when the sample is studied at high pressure because it is very difficult to achieve exactly the same pressure in two separate experiments, especially if the neutron data are collected using Laue methods where precise absolute values of the unit-cell dimensions cannot be measured to check how close the pressures are. In this study, diffraction data have been collected under the same conditions on the same sample of copper(II) sulfate pentahydrate, using a conventional laboratory diffractometer and source for the X-ray measurements and the Koala single-crystal Laue diffractometer at the ANSTO facility for the neutron measurements. The sample, of dimensions 0.40 × 0.22 × 0.20 mm3 and held at a pressure of 0.71 GPa, was contained in a miniature Merrill–Bassett diamond-anvil cell. The highly penetrating diffracted neutron beams passing through the metal body of the miniature cell as well as through the diamonds yielded data suitable for structure refinement, and compensated for the low completeness of the X-ray measurements, which was only 24% on account of the triclinic symmetry of the sample and the shading of reciprocal space by the cell. The two data-sets were combined in a single `XN' structure refinement in which all atoms, including H atoms, were refined with anisotropic displacement parameters. The precision of the structural parameters was improved by a factor of up to 50% in the XN refinement compared with refinements using the X-ray or neutron data separately. DOI: 10.1107/S2052252521010708 Revealing the early stages of carbamazepine crystallization by cryoTEM and 3D electron diffraction IUCrJ. 2021;8(6) Edward T Broadhurst, Hongyi Xu, Simon Parsons, Fabio Nudelman Time-resolved carbamazepine crystallization from wet ethanol has been monitored using a combination of cryoTEM and 3D electron diffraction. Carbamazepine is shown to crystallize exclusively as a dihydrate after 180 s. When the timescale was reduced to 30 s, three further polymorphs could be identified. At 20 s, the development of early stage carbamazepine dihydrate was observed through phase separation. This work reveals two possible crystallization pathways present in this active pharmaceutical ingredient. DOI: 10.1107/S2052252521010101 Accurate H‐atom parameters for the two polymorphs of l‐histidine at 5, 105 and 295 K. Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials. 2021;77(5):785-800. Giulia Novelli, Charles J McMonagle, Florian Kleemiss, Michael Probert, Horst Puschmann, Simon Grabowsky, Helen E Maynard-Casely, Garry J McIntyre, Simon Parsons The crystal structure of the monoclinic polymorph of the primary amino acid L-histidine has been determined for the first time by single-crystal neutron diffraction, while that of the orthorhombic polymorph has been reinvestigated with an untwinned crystal, improving the experimental precision and accuracy. For each polymorph, neutron diffraction data were collected at 5, 105 and 295 K. Single-crystal X-ray diffraction experiments were also performed at the same temperatures. The two polymorphs, whose crystal packing is interpreted by intermolecular interaction energies calculated using the Pixel method, show differences in the energy and geometry of the hydrogen bond formed along the c direction. Taking advantage of the X-ray diffraction data collected at 5 K, the precision and accuracy of the new Hirshfeld atom refinement method implemented in NoSpherA2 were probed choosing various settings of the functionals and basis sets, together with the use of explicit clusters of molecules and enhanced rigid-body restraints for H atoms. Equivalent atomic coordinates and anisotropic displacement parameters were compared and found to agree well with those obtained from the corresponding neutron structural models. DOI: 10.1107/S205252062100740X Mapping the cooperativity pathways in spin crossover complexes. Chem. Sci., 2020, 12, 1007-1015 Matthew G Reeves, Elodie Tailleur, Peter A Wood, Mathieu Marchivie, Guillaume Chastanet, Philippe Guionneau, Simon Parsons Crystal packing energy calculations are applied to the [Fe(PM-L)2(NCS)2] family of spin crossover (SCO) complexes (PM-L = 4-substituted derivatives of the N-(2-pyridylmethylene)-4-aminobiphenyl ligand) with the aim of relating quantitatively the cooperativity of observed SCO transitions to intermolecular interactions in the crystal structures. This approach reveals a linear variation of the transition abruptness with the sum of the magnitudes of the interaction energy changes within the first molecular coordination sphere in the crystal structure. Abrupt transitions are associated with the presence of significant stabilising and destabilising changes in intermolecular interaction energies. While the numerical trend established for the PM-L family does not directly extend to other classes of SCO complex in which the intermolecular interactions may be very different, a plot of transition abruptness against the range of interaction energy changes normalised by the largest change shows a clustering of complexes with similar transition abruptness. The changes in intermolecular interactions are conveniently visualised using energy difference frameworks, which illustrate the cooperativity pathways of an SCO transition. DIO: 10.1039/D0SC05819J Effect of High Pressure on the Crystal Structures of Polymorphs of l-Histidine. Cryst. Growth Des. 2020, 20, 12, 7788–7804 Giulia Novelli, Helen E Maynard-Casely, Garry J McIntyre, Mark R Warren, Simon Parsons The effect of pressure on the crystal structures of the two ambient-pressure polymorphs of the amino acid l-histidine has been investigated. Single-crystal diffraction measurements, up to 6.60 GPa for the orthorhombic form I (P212121) and 6.85 GPa for the monoclinic form II (P21), show their crystal structures undergo isosymmetric single-crystal-to-single-crystal first-order phase transitions at 4.5 and 3.1 GPa to forms I′ and II′, respectively. Although the similarity in crystal packing and intermolecular interaction energies between the polymorphs is remarkable at ambient conditions, the manner in which each polymorph responds to pressure is different. Form II is found to be more compressible than form I, with bulk moduli of 11.6(6) GPa and 14.0(5) GPa, respectively. The order of compressibility follows the densities of the polymorphs at ambient conditions (1.450 and 1.439 g cm–3 for phases I and II, respectively). The difference is also related to the space-group symmetry, the softer monoclinic form having more degrees of freedom available to accommodate the change in pressure. In the orthorhombic form, the imidazole-based hydrogen atom involved in the H-bond along the c-direction swaps the acceptor oxygen atom at the transition to phase I′; the same swap occurs just after the phase transition in the monoclinic form and is also preceded by a bifurcation. Concurrently, the H-bond and the long-range electrostatic interaction along the b-direction form a three-centered H-bond at the I to I′ transition, while they swap their character during the II to II′ transition. The structural data were interpreted using periodic-density-functional theory, symmetry-adapted perturbation theory, and semiempirical Pixel calculations, which indicate that the transition is driven by minimization of volume, the intermolecular interactions generally being destabilized by the phase transitions. Nevertheless, volume calculations are used to show that networks of intermolecular contacts in both phases are very much less compressible than the interstitial void spaces, having bulk moduli similar to moderately hard metals. The volumes of the networks actually expand over the course of both phase transitions, with the overall unit-cell-volume decrease occurring through larger compression of interstitial void space. DOI: 10.1021/acs.cgd.0c01085 MrPIXEL: automated execution of Pixel calculations via the Mercury interface. J. Appl. Cryst. 2020. 53, 1154-1162 Matthew G Reeves, Peter A Wood, Simon Parsons The interpretation of crystal structures in terms of intermolecular interaction energies enables phase stability and polymorphism to be rationalized in terms of quantitative thermodynamic models, while also providing insight into the origin of physical and chemical properties including solubility, compressibility and host–guest formation. The Pixel method is a semi-empirical procedure for the calculation of intermolecular interactions and lattice energies based only on crystal structure information. Molecules are represented as blocks of undistorted ab initio molecular electron and nuclear densities subdivided into small volume elements called pixels. Electrostatic, polarization, dispersion and Pauli repulsion terms are calculated between pairs of pixels and nuclei in different molecules, with the accumulated sum equating to the intermolecular interaction energy, which is broken down into physically meaningful component terms. The MrPIXEL procedure enables Pixel calculations to be carried out with minimal user intervention from the graphical interface of Mercury, which is part of the software distributed with the Cambridge Structural Database (CSD). Following initial setup of a crystallographic model, one module assigns atom types and writes necessary input files. A second module then submits the required electron-density calculation either locally or to a remote server, downloads the results, and submits the Pixel calculation itself. Full lattice energy calculations can be performed for structures with up to two molecules in the crystallographic asymmetric unit. For more complex cases, only molecule–molecule energies are calculated. The program makes use of the CSD Python API, which is also distributed with the CSD. DOI: 10.1107/S1600576720008444 Pressure-induced inclusion of neon in the crystal structure of a molecular Cu 2 (pacman) complex at 4.67 GPa. Chem. Commun., 2020, 56, 3449 Nico Giordano, Christine M Beavers, Konstantin V Kamenev, Jason B Love, James R Pankhurst, Simon J Teat, Simon Parsons Crystals of a Cu complex of the macrocyclic Schiff-base calixpyrrole or ‘Pacman’ ligand, Cu2(L), do not contain any solvent-accessible void space at ambient pressure, but adsorb neon at 4.67 GPa, forming Cu2(L)3.5Ne DOI: 10.1039/c9cc09884d Polymorph evolution during crystal growth studied by 3D electron diffraction. IUCrJ. 2020;7(1):5-9. Edward T Broadhurst, Hongyi Xu, Max TB Clabbers, Molly Lightowler, Fabio Nudelman, Xiaodong Zou, Simon Parsons 3D electron diffraction (3DED) has been used to follow polymorph evolution in the crystallization of glycine from aqueous solution. The three polymorphs of glycine which exist under ambient conditions follow the stability order β < α < γ. The least stable β polymorph forms within the first 3 min, but this begins to yield the α-form after only 1 min more. Both structures could be determined from continuous rotation electron diffraction data collected in less than 20 s on crystals of thickness ∼100 nm. Even though the γ-form is thermodynamically the most stable polymorph, kinetics favour the α-form, which dominates after prolonged standing. In the same sample, some β and one crystallite of the γ polymorph were also observed. DOI: 10.1107/S2052252519016105 High-pressure polymorphism in L-threonine between ambient pressure and 22 GPa. CrystEngComm 2019, 21 (30), 4444. Giordano, N.; Beavers, C. M.; Kamenev, K. V.; Marshall, W. G.; Moggach, S. A.; Patterson, S. D.; Teat, S. J.; Warren, J. E.; Wood, P. A.; Parsons, S. The crystal structure of L-threonine has been studied to a maximum pressure of 22.3 GPa using single-crystal X-ray and neutron powder diffraction. The data have been interpreted in the light of previous Raman spectroscopic data by Holanda et al. (J. Mol. Struct. (2015), 1092, 160–165) in which it is suggested that three phase transitions occur at ca. 2 GPa, between 8.2 and 9.2 GPa and between 14.0 and 15.5 GPa. In the first two of these transitions the crystal retains its P212121 symmetry, in the third, although the unit cell dimensions are similar either side of the transition, the space group symmetry drops to P21. The ambient pressure form is labelled phase I, with the successive high-pressure forms designated I′, II and III, respectively. Phases I and I′ are very similar, the transition being manifested by a slight rotation of the carboxylate group. Phase II, which was found to form between 8.5 and 9.2 GPa, follows the gradual transformation of a long-range electrostatic contact becoming a hydrogen bond between 2.0 and 8.5 GPa, so that the transformation reflects a change in the way the structure accommodates compression rather than a gross change of structure. Phase III, which was found to form above 18.2 GPa in this work, is characterised by the bifurcation of a hydroxyl group in half of the molecules in the unit cell. Density functional theory (DFT) geometry optimisations were used to validate high-pressure structural models and PIXEL crystal lattice and intermolecular interaction energies are used to explain phase stabilities in terms of the intermolecular interactions. DOI: 10.1039/c9ce00388f The Effect of Pressure on Halogen Bonding in 4-Iodobenzonitrile. Molecules 2019, 24 (10), 2018. Giordano, N.; Afanasjevs, S.; Beavers, C. M.; Hobday, C. L.; Kamenev, K. V.; O'Bannon, E. F.; Ruiz-Fuertes, J.; Teat, S. J.; Valiente, R.; Parsons, S. The crystal structure of 4-iodobenzonitrile, which is monoclinic (space group I2/a) under ambient conditions, contains chains of molecules linked through C≡N···I halogen-bonds. The chains interact through CH···I, CH···N and π-stacking contacts. The crystal structure remains in the same phase up to 5.0 GPa, the b axis compressing by 3.3%, and the a and c axes by 12.3 and 10.9 %. Since the chains are exactly aligned with the crystallographic b axis these data characterise the compressibility of the I···N interaction relative to the inter-chain interactions, and indicate that the halogen bond is the most robust intermolecular interaction in the structure, shortening from 3.168(4) at ambient pressure to 2.840(1) Å at 5.0 GPa. The π∙∙∙π contacts are most sensitive to pressure, and in one case the perpendicular stacking distance shortens from 3.6420(8) to 3.139(4) Å. Packing energy calculations (PIXEL) indicate that the π∙∙∙π interactions have been distorted into a destabilising region of their potentials at 5.0 GPa. The structure undergoes a transition to a triclinic ( P1¯ ) phase at 5.5 GPa. Over the course of the transition, the initially colourless and transparent crystal darkens on account of formation of microscopic cracks. The resistance drops by 10% and the optical transmittance drops by almost two orders of magnitude. The I···N bond increases in length to 2.928(10) Å and become less linear [<C−I∙∙∙N = 166.2(5)°]; the energy stabilises by 2.5 kJ mol−1 and the mixed C-I/I..N stretching frequency observed by Raman spectroscopy increases from 249 to 252 cm−1. The driving force of the transition is shown to be relief of strain built-up in the π∙∙∙π interactions rather than minimisation of the molar volume. The triclinic phase persists up to 8.1 GPa. DOI: 10.3390/molecules24102018 Computational analysis of M–O covalency in M(OC6H5)4(M = Ti, Zr, Hf, Ce, Th, U). Dalton Transactions 2019, 48 (9), 2939. Berryman, V. E. J.; Whalley, Z. J.; Shephard, J. J.; Ochiai, T.; Price, A. N.; Arnold, P. L.; Parsons, S.; Kaltsoyannis, N. A series of compounds M(OC6H5)4 (M = Ti, Zr, Hf, Ce, Th, U) is studied with hybrid density functional theory, to assess M–O bond covalency. The series allows for the comparison of d and f element compounds that are structurally similar. Two well-established analysis methods are employed: Natural Bond Orbital and the Quantum Theory of Atoms in Molecules. A consistent pattern emerges; the U–O bond is the most covalent, followed by Ce–O and Th–O, with those involving the heavier transition metals the least so. The covalency of the Ti–O bond differs relative to Ce–O and Th–O, with the orbital-based method showing greater relative covalency for Ti than the electron density-based methods. The deformation energy of r(M–O) correlates with the d orbital contribution from the metal to the M–O bond, while no such correlation is found for the f orbital component. f orbital involvement in M–O bonding is an important component of covalency, facilitating orbital overlap and allowing for greater expansion of the electrons, thus lowering their kinetic energy. DOI: 10.1039/c8dt05094e A jumping crystal predicted with molecular dynamics and analysed with TLS refinement against powder diffraction data. IUCrJ 2019, 6 (1), 136. van de Streek, J.; Alig, E.; Parsons, S.; Vella-Zarb, L. A By running a temperature series of molecular dynamics (MD) simulations starting from the known low-temperature phase, the experimentally observed phase transition in a `jumping crystal' was captured, thereby providing a prediction of the unknown crystal structureof the high-temperature phase and clarifying the phase-transition mechanism. The phase transition is accompanied by a discontinuity in two of the unit-cell parameters. The structure of the high-temperature phase is very similar to that of the low-temperature phase. The anisotropic displacement parameters calculated from the MD simulations readily identified libration as the driving force behind the phase transition. Both the predicted crystal structure and the phase-transition mechanism were verified experimentally using TLS (translation, libration, screw) refinement against X-ray powder diffraction data. DOI: 10.1107/s205225251801686x Elastically Flexible Crystals have Disparate Mechanisms of Molecular Movement Induced by Strain and Heat. Angewandte Chemie, International Edition 2018, 57 (35), 11325. Brock, A. J.; Whittaker, J. J.; Powell, J. A.; Pfrunder, M. C.; Grosjean, A.; Parsons, S.; McMurtrie, J. C.; Clegg, J. K. Elastically flexible crystals form an emerging class of materials that exhibit a range of notable properties. The mechanism of thermal expansion in flexible crystals of bis(acetylacetonato)copper(II) is compared with the mechanism of molecular motion induced by bending and it is demonstrated that the two mechanisms are distinct. Upon bending, individual molecules within the crystal structure reversibly rotate, while thermal expansion results predominantly in an increase in intermolecular separations with only minor changes to molecular orientation through rotation. DOI: 10.1002/anie.201806431 Probing the origin of the giant magnetic anisotropy in trigonal bipyramidal Ni(II) under high pressure. Chemical Science 2018, 9 (6), 1551. Craig, G. A.; Sarkar, A.; Woodall, C. H.; Hay, M. A.; Marriott, K. E. R.; Kamenev, K. V.; Moggach, S. A.; Brechin, E. K.; Parsons, S.; Rajaraman, G.et al. Understanding and controlling magnetic anisotropy at the level of a single metal ion is vital if the miniaturisation of data storage is to continue to evolve into transformative technologies. Magnetic anisotropy is essential for a molecule-based magnetic memory as it pins the magnetic moment of a metal ion along the easy axis. Devices will require deposition of magnetic molecules on surfaces, where changes in molecular structure can significantly alter magnetic properties. Furthermore, if we are to use coordination complexes with high magnetic anisotropy as building blocks for larger systems we need to know how magnetic anisotropy is affected by structural distortions. Here we study a trigonal bipyramidal nickel(II) complex where a giant magnetic anisotropy of several hundred wavenumbers can be engineered. By using high pressure, we show how the magnetic anisotropy is strongly influenced by small structural distortions. Using a combination of high pressure X-ray diffraction, ab initio methods and high pressure magnetic measurements, we find that hydrostatic pressure lowers both the trigonal symmetry and axial anisotropy, while increasing the rhombic anisotropy. The ligand–metal–ligand angles in the equatorial plane are found to play a crucial role in tuning the energy separation between the dx2−y2 and dxy orbitals, which is the determining factor that controls the magnitude of the axial anisotropy. These results demonstrate that the combination of high pressure techniques with ab initio studies is a powerful tool that gives a unique insight into the design of systems that show giant magnetic anisotropy. DOI: 10.1039/c7sc04460g Determination of absolute configuration using X-ray diffraction. Tetrahedron: Asymmetry 2017, 28 (10), 1304. Parsons, S. Methods for determination of absolute structure using X-ray crystallography are described, with an emphasis on applications for absolute configuration assignment of enantiopure light-atom organic compounds. The ability to distinguish between alternative absolute structures by X-ray crystallography is the result of a physical phenomenon called resonant scattering, which introduces small deviations from the inherent inversion symmetry of single-crystal X-ray diffraction patterns. The magnitude of the effect depends on the elements present in the crystal and the wavelength of the X-rays used to collect the diffraction data, but it is always very weak for crystals of compounds containing no element heavier than oxygen. The precision of absolute structure determination by conventional least squares refinement appears to be unduly pessimistic for light-atom materials. Recent developments based on Bijvoet differences, quotients and Bayesian statistics enable better and more realistic precision to be obtained. The new methods are sensitive to statistical outliers, and techniques for identifying these are summarised. DOI: 10.1016/j.tetasy.2017.08.018 Phase transition sequences in tetramethylammonium tetrachlorometallates by X-ray diffraction and spectroscopic measurements. Acta Crystallographica, Section B: Structural Science, Crystal Engineering and Materials 2017, 73 (5), 844. Binns, J.; McIntyre, G. J.; Barreda-Argueso, J. A.; Gonzalez, J.; Aguado, F.; Rodriguez, F.; Valiente, R.; Parsons, S. The phase transition sequences of two members of the tetramethylammonium tetrachlorometallate(III) family of hybrid organic–inorganic salts have been determined and structurally characterized as a function of temperature for the first time. Unusually, a reduction in point-group symmetry with increasing temperature until reaching a cubic prototype phase is observed. Two additional intermediate phases are observed for Fe3+. First-principles calculations and the presence of short Cl⋯Cl contacts for Ga3+ suggest the [GaCl4]− anion to be conformationally hindered due to stronger lone-pair–σ-hole interactions. The conformationally more flexible Fe3+ structures show sublattice melting with the onset of rotational disorder in the [NMe4]+ cations occurring 40 K below the corresponding onset of rotational disorder in the [FeCl4]− sublattice. DOI: 10.1107/s2052520617006412 ζ-Glycine: insight into the mechanism of a polymorphic phase transition. IUCrJ 2017, 4 (5), 569. Bull, C. L.; Flowitt-Hill, G.; de Gironcoli, S.; Kucukbenli, E.; Parsons, S.; Pham, C. H.; Playford, H. Y.; Tucker, M. G. Glycine is the simplest and most polymorphic amino acid, with five phases having been structurally characterized at atmospheric or high pressure. A sixth form, the elusive ζ phase, was discovered over a decade ago as a short-lived intermediate which formed as the high-pressure ∊ phase transformed to the γ form on decompression. However, its structure has remained unsolved. We now report the structure of the ζ phase, which was trapped at 100 K enabling neutron powder diffraction data to be obtained. The structure was solved using the results of a crystal structure prediction procedure based on fully ab initio energy calculations combined with a genetic algorithm for searching phase space. We show that the fate of ζ-glycine depends on its thermal history: although at room temperature it transforms back to the γ phase, warming the sample from 100 K to room temperature yielded β-glycine, the least stable of the known ambient-pressure polymorphs. DOI: 10.1107/s205225251701096x Reversible Pressure-Controlled Depolymerization of a Copper(II)-Containing Coordination Polymer. Chemistry - A European Journal 2017, 23 (51), 12480. Clegg, J. K.; Brock, A. J.; Jolliffe, K. A.; Lindoy, L. F.; Parsons, S.; Tasker, P. A.; White, F. J. A unique pressure‐induced Cu−N bond breaking/bond forming reaction is reported. The variation of pressure on a single crystal of a one‐dimensional copper‐ (II)‐containing coordination polymer (Cu2L2(1‐methylpiperazine)2]n, where H2L is 1,1′‐(1,3‐phenylene)‐bis(4,4‐dimethylpentane‐1,3‐dione)), was monitored using single crystal X‐ray diffraction with the aid of a diamond anvil cell. At a very low elevated pressure (≈0.05 GPa) a remarkable reversible phase change was observed. The phase change results in the depolymerization of the material through the cleavage and formation of axial Cu−N bonds as well as “ring flips” of individual axially coordinated 1‐methylpiperazine ligands. Overall, the pressure‐induced phase change is associated with a surprising (and non‐intuitive) shift in structure‐from a 1‐dimensional coordination polymer to a discrete dinuclear complex. DOI: 10.1002/chem.201703115 Pressure induced enhancement of the magnetic ordering temperature in rhenium(IV) monomers. Nature Communications 2016, 7, 13870. Woodall, C. H.; Craig, G. A.; Prescimone, A.; Misek, M.; Cano, J.; Faus, J.; Probert, M. R.; Parsons, S.; Moggach, S.; Martinez-Lillo, J.et al. Materials that demonstrate long-range magnetic order are synonymous with information storage and the electronics industry, with the phenomenon commonly associated with metals, metal alloys or metal oxides and sulfides. A lesser known family of magnetically ordered complexes are the monometallic compounds of highly anisotropic d-block transition metals; the ‘transformation’ from isolated zero-dimensional molecule to ordered, spin-canted, three-dimensional lattice being the result of through-space interactions arising from the combination of large magnetic anisotropy and spin-delocalization from metal to ligand which induces important intermolecular contacts. Here we report the effect of pressure on two such mononuclear rhenium(IV) compounds that exhibit long-range magnetic order under ambient conditions via a spin canting mechanism, with Tc controlled by the strength of the intermolecular interactions. As these are determined by intermolecular distance, ‘squeezing’ the molecules closer together generates remarkable enhancements in ordering temperatures, with a linear dependence of Tc with pressure. DOI: 10.1038/ncomms13870 This article was published on 2024-08-30
