Effect of High Pressure on the Crystal Structures of Polymorphs of l-Histidine

Abstract

adapted perturbation theory and semi-empirical Pixel calculations, which indicate that the transition is driven by minimisation of volume, the intermolecular interactions generally being destabilised 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 volume of the network actually expand over the course of both phase transitions, with the overall unit-cell-volume decrease occurring through larger compression of interstitial void space.

Accurate H-atom parameters for the two polymorphs of L-histidine at 5, 105 and 295 K

“…The total lattice energies of the two polymorphs become more positive with temperature, being À303.3 and À298.1 kJ mol À1 for the orthorhombic and monoclinic forms, respectively. The lattice energy difference of 5.2 kJ mol À1 is in good agreement with that calculated by Novelli et al (2020) for the two polymorphs at a similar temperature (3.6 kJ mol À1 at 298 K).…”

“…The intermolecular interaction energies within the first coordination sphere of the two polymorphs, which contains 12 molecules in six symmetry-related pairs, were calculated with the Pixel method (Table 5). The molecule-molecule energies are grouped into six symmetry-equivalent pairs labelled A/A 0 , B/B 0 , etc, and they are broken down into their Coulombic, The comparison between the hydrogen bonding and packing in the two phases, presented below, is informed by comments made by a referee with respect to our earlier paper (Novelli et al, 2020). The crystal packing in the ab layers by the N1-H1BÁ Á ÁO2 ii and N1-H1CÁ Á ÁO2 iii hydrogen bonds (contacts B and C, respectively) is identical in the orthorhombic [Fig.…”

“…We thank a referee to our earlier paper (Novelli et al, 2020) for insight into the crystal packing of the two polymorphs of l-histidine. We thank the Australian Centre for Neutron Scattering (ANSTO) for the allocation of neutron beam time and for studentship funding to GN, who was also supported by The University of Edinburgh and the Engineering and Physical Sciences Research Council (UK).…”

“…The location of the H atoms was also less definitive than is usual in neutron diffraction analysis. More recently, the effect of high pressure on the two polymorphs of l-histidine has been investigated using single-crystal X-ray diffraction (Novelli et al, 2020). Despite their similarity with respect to crystal packing and intermolecular interaction energies under ambient conditions, the way in which the polymorphs respond to pressure is different.…”

Suppression of isotopic polymorphism

“…We calculate the volume of both the voids and the network of intermolecular contactsthe space enclosed within the VdW radii of all atoms-using a Monte-Carlo procedure, as described in Ref. 38. These are plotted in Figure 2.…”