Effect of high pressure on structural oddities
View/ Open
JohnstoneSupplemental2009.zip (9.304Mb)
Date
2010Author
Johnstone, Russell D. L.
Metadata
Abstract
This thesis describes the effect of pressure on crystal structures that are in some way
unusual. The aim was to investigate whether pressure could be used to force these
‘structural oddities’ to conform to more conventional behaviour. In many cases
pressure-induced phase transitions were observed, and the driving forces of these are
considered.
L-serine monohydrate crystallises with layers of hydrogen bonded serine molecules.
Layers are linked together by H-bonds from the donor atoms of water molecules. The
orientation of the water molecules between the layers is uncommon for other layered
hydrates in the CSD. A single crystal of serine hydrate undergoes a pressure-induced
phase transition at 5 GPa, which is characterised by a rotation of the water molecules
to an orientation which is more frequently observed. PIXEL calculations show that
the transition is driven by the PV term in the equation G = U - TS + PV. An attempt
to reproduce the transition in another layered hydrate with a similar topology was
partially successful in the compression of S-4-sulfo-L-phenylalanine monohydrate,
which undergoes a similar phase transition at 1 GPa.
Methyl 2-(9H-carbazol-9-yl)benzoate crystallises unusually with eight molecules in
the asymmetric unit (Z’ = 8). Compression of a single crystal results in a phase
transition at ca. 5 GPa to give a Z’ = 2 polymorph. The PV term is an important
contributor to the driving force of the transition. The geometries of the molecules in
phase-II are significantly less stable than in phase-I, and as pressure is released on
phase-II the need to adopt a more stable molecular conformation eventually
outweighs the PV advantage. The Z’ = 8 structure is eventually re-established at 4.6
GPa. This work illustrates how low Z’ polymorphs of the same structure are not
always the thermodynamically more stable forms.
When recrystallised in situ from a 4:1 by volume solution of methanol and ethanol, a
new polymorph of salicylamide is obtained at 0.2 GPa. The ambient pressure phase
appears in the CSD to contain a number of abnormally short H…H contacts. We find
this model to be incorrect, and have re-determined the structure to find no short
H…H contacts. PIXEL and DFT calculations indicate that the high-pressure
polymorph is favoured over the ambient phase by the PV term, the zero point energy
and entropy. Low completeness that often occurs as a result of shading from the
high-pressure cell was improved by the inclusion of multiple crystals within the
sample chamber.
Bianthrone changes colour from yellow to green on grinding, though this does not
occur when subjected to hydrostatic pressure to 6.5 GPa. There is, however, a subtle
colour change from bright yellow to dark orange as pressure is applied, and it is
likely that this is caused by changes in the - stacking distances. This work
highlights how a system can react differently to hydrostatic and non-hydrostatic
conditions.