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Selected Publications

  1. 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


  2. 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


  3. 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


  4. 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


  5. 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


  6. 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, 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 dx2y2 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


  7. 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


  8. 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


  9. 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


  10. 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