Behaviour of intermolecular interactions at extreme pressures
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Date
28/11/2019Author
Giordano, Nico
Metadata
Abstract
In organic solids, pressures of only a few gigapascals modify and rearrange
intermolecular contacts such as H-bonds and van der Waals contacts leading to
extensive phase diversity. Applications in this rich area of research include
searches for new phases and solvates of pharmaceutical materials; modelling of
detonation mechanisms of energetic materials, and modelling of the driving
forces of phase transitions. The overarching theme of this PhD thesis is to obtain
new, often difficult to isolate, high-pressure polymorphs of small molecules and
elucidate the role of intermolecular interactions in their phase stabilities.
The need to obtain precise structural information at atomic resolution
demands the use of single crystal diffraction methods but scattering intensities
are typically low, and the pressure apparatus used in these studies (the diamond
anvil cell) results in incomplete data. This can make direct structure
determinations for some materials difficult or even impossible. Third generation
synchrotron X-ray sources are therefore used for their brightness, high energies,
and small focused beams to extract as much structural information from
samples as possible.
The amino acid L-threonine, characterised by its hydrogen bond
network, has been structurally characterised at 22 GPa which is an unusually
high-pressure for a complex organic molecule. L-threonine undergoes two
isosymmetric phase transitions at ca. 2 and ca. 9 GPa, and a phase transition at
ca. 18 GPa that results in a loss of crystal symmetry. Structures of L-threonine
were determined by single-crystal X-ray diffraction to 22 GPa; which is the
highest-pressure structure ever reported for an amino acid. High-pressure
polymorphism in pyridine was studied extensively by single-crystal X-ray
diffraction, Raman spectroscopy and neutron powder diffraction. Pyridine has
at least three polymorphs in the narrow pressure range of ca. 1 to ca. 2 GPa but
the sluggish nature of the phase transitions has made isolating and
characterising one of the phases difficult, until now. Here, we used in situ crystal
growth in the diamond anvil cell to obtain a stable, diffraction quality single
crystal of the elusive phase III and determined its crystal structure for the first
time. A mechanism for the transformation is also proposed. The halogen
bonded molecule, 4-iodobenzonitrile was studied experimentally by single-crystal
X-ray diffraction and Raman spectroscopy up to 10 GPa.
4-iodobenzonitrile undergoes a reconstructive phase change above 5 GPa that
results in crystals breaking apart, making it difficult to obtain meaningful
diffraction data. Nevertheless, the structure of the new high-pressure phase was
determined for the first time by rapidly pressurising a crystal grown in situ to
8 GPa. Crystal lattice and intermolecular PIXEL energy calculations have been
validated for use with small organics to 22 GPa, as well as for halogen containing
molecules at very high pressures; allowing the roles of stabilising, or
destabilising, molecular interactions to be probed in high-pressure polymorphs
for a range of organic molecules.
Finally, a neon co-crystal was obtained on compression of a Cu2 Pacman
complex. This single-crystal structure represents one of only a few published
neon containing organometallic structures. Neon resides within the interstitial
voids as a result of the Pacman complex reconfiguring to allow neon-uptake.
The study shows the interplay between the pressure transmitting medium and
crystal structure and we discuss the potential applications of pressure mediated
guest-uptake in the Pacman complexes.