Temperature dependent solid-state proton migration in dimethylurea-oxalic acid complexes.

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.

A new polymorph of N,N'-dimethylurea characterized by X-ray diffraction and first-principles lattice dynamics calculations.

In this study we present a combined crystallographic and computational study of a new polymorph of N,N'-dimethylurea (DMU) with P2(1)2(1)2 space group symmetry, along with a revised theoretical study of the previously known phase in its corrected space group (Fdd2). X-ray diffraction studies show crystal structures that are very similar, differing only in the relative orientation of the hydrogen-bonded molecular chains that are common to both phases. The vibrational spectra were obtained from B3LYP hybrid functional lattice dynamics calculations and compared with the experimental data for the known phase. The free-energy difference between the forms is derived from the Gamma-point optical mode frequencies, and amounts to less than 1 kJ mol(-1) across the temperature range of interest. The electronic densities-of-states of both phases are also computed, yielding only marginal differences in valence and conduction band compositions and band gap widths. Taken together, the results highlight the small but important differences separating the two crystal lattices.

Temperature- and pressure-induced proton transfer in the 1:1 adduct formed between squaric acid and 4,4'-bipyridine.

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.