We have applied a combination of spectroscopic and diffraction methods to study the adduct formed between squaric acid and bypridine, which has been postulated to exhibit proton transfer associated with a single-crystal to single-crystal phase transition at ca. 450 K. A combination of X-ray single-crystal and very-high flux powder neutron diffraction data confirmed that a proton does transfer from the acid to the base in the high-temperature form. Powder X-ray diffraction measurements demonstrated that the transition was reversible but that a significant kinetic energy barrier must be overcome to revert to the original structure. Computational modeling is consistent with these results. Modeling also revealed that, while the proton transfer event would be strongly discouraged in the gas phase, it occurs in the solid state due to the increase in charge state of the molecular ions and their arrangement inside the lattice. The color change is attributed to a narrowing of the squaric acid to bipyridine charge-transfer energy gap. Finally, evidence for the possible existence of two further phases at high pressure is also presented.

The phenomenon of solid-state proton migration within molecular complexes containing short hydrogen bonds is investigated in two dimethylurea-oxalic acid complexes. Extensive characterisation by both X-ray and neutron diffraction shows that proton migration along the hydrogen bond can be induced in these complexes as a function of temperature. This emphasises the subtle features of the hydrogen bond potential well in such short hydrogen bonded complexes, both intrinsically and in the effect of the local crystalline environment. Based on these findings, the synthesis and analysis of a series of solid-state molecular complexes is shown to be a potential route to designing materials with tuneable proton migration effects.

The basal surfaces of phyllosilicate minerals have been widely studied, whereas the edge surfaces have received little attention. However, in order to simulate complete clay particles at the atomic level, the modeling of edge surfaces becomes crucially important, and such surfaces are likely to be far more active. We used a combination of quantum and potential based techniques to evaluate the structure of the edge surfaces of pyrophyllite and their interaction in an aqueous environment. These include {110}, {100}, {010}, {1̅ 10}, {130}, and {1̅ 30}. We found that the CLAYFF force field is an effective model for reproducing the DFT results. Furthermore, the results show that, for this notorious natural hydrophobic clay, all edge surfaces show hydrophilic behavior and that the precise structure of water above these surfaces is influenced by both the presence of hydroxyl groups and under-coordinated surface Al atoms; this will impact both geological processes where natural clays are involved and processes where such clays act as primary retention barriers to the dispersion of contaminants.