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Open Access Publications from the University of California

High-pressure polymorphism in l-threonine between ambient pressure and 22 GPa

  • Author(s): Giordano, N
  • Beavers, CM
  • Kamenev, KV
  • Marshall, WG
  • Moggach, SA
  • Patterson, SD
  • Teat, SJ
  • Warren, JE
  • Wood, PA
  • Parsons, S
  • et al.

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 P2 2 2 symmetry, in the third, although the unit cell dimensions are similar either side of the transition, the space group symmetry drops to P2 . 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. 1 1 1 1

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