|dc.description.abstract||Hydrogen bonds are of great interest in the solid state due to their importance
in structural, functional and dynamical properties of chemical systems. Moderate
hydrogen bonds have been linked with proton transfer, whereas the short, strong
hydrogen bonds enable proton migration.
Previous work in our group on relatively simple hydrogen bonded adducts
relied on the combination of ab initio computational modelling (molecular dynamics)
with variable temperature diffraction results (X-ray and neutron). These
demonstrated that the interplay of these techniques was successful in studying the
phenomena of proton transfer and migration.
The present work follows on from that, and focuses on the effects of
temperature and pressure on proton transfer and migration using both experimental
and computational methods. The systems studied continue to encapsulate adducts
with N…O and O…O hydrogen bonds.
The study of the adduct formed between squaric acid and 4,4’-bipyridine was
found to exhibit proton transfer associated with a single-crystal to single-crystal
phase transition at 450 K that is coupled to a colour change (yellow to red). X-ray
and neutron diffraction initially revealed the heavy atom structure and secondly the
location of the hydrogen atoms along the moderate N…O hydrogen bond (ca. 2.6 Å).
Computational modelling supported this and deduced the reason for the striking
colour change. Pressure studies also determined that the adduct underwent two
phase-transitions with a similar colour change, indicating that proton transfer is also a factor here, but with powder patterns different from the high temperature form,
indicating that further polymorphs for this interesting system must exist.
In an attempt to lower the temperature at which proton transfer would occur
the base was changed to one of a more basic nature, i.e. co-crystallisation of squaric
acid and 2,2’-dimethyl–4,4’-bipyridine was pursued. This lead to the formation of
two red crystals that were found to posses the base doubly protonated at all
temperatures studied (from 300 K to 100 K).
The adduct of N,N-dimethylurea with phosphoric acid was obtained from a
systematic study designed to follow the success of a previously reported system that
showed proton migration (the adduct of urea and phosphoric acid). The new material
was found to crystallise as the 2:1 adduct and maintained the short, strong hydrogen
bonds characteristic of the parent structure.
As part of the systematic approach undertaken throughout the research
presented here, co-crystallisation of a combination of acids and bases were attempted
in order to synthesise new materials containing short, strong hydrogen bonds. These
yielded the adducts between oxalic acid and 2,2’-dimethyl-4,4’-bipyridine, and
oxamic acid and 4,4’-bipyridine. In addition to these adducts some compounds ended
up reacting to create new ones, e.g. the fusing of dimethyl urea and squaric acid to
give N-(2-hydroxycyclobutene-3,4-dione)-N’,N’-dimethylurea and N-(2-
hydroxycyclobutene-3,4-dione)-N,N'-dimethylurea, while a new polymorphs of one
of the precursors on its own was also obtained (N,N’-dimethylurea). The resulting
co-crystallisations did not all follow the design quite as intended. They did, however,
yield interesting new structures, some of which have the potential to be proton
migration and transfer systems.||en