Compressibility Studies of Zirconium Based Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are a well-studied class of porous materials with the potential to be used in many applications such as gas storage and catalysis.[1] UiO-67 (UiO = University of Oslo), a MOF built from zirconium oxide units connected with 4,4-biphenyldicarboxylate (BDC) linkers, forms a face centred cubic structure. Zirconium has a high affinity towards oxygen ligands making these bridges very strong, resulting in UiO-based MOFs having high chemical and thermal stability compared to other MOF structures. Moreover, UiO-67 has become popular in engineering studies due to its high mechanical stability.[2] Using high pressure x-ray crystallography we can exert MOFs to GPa pressures, experimentally exploring the mechanical stability of MOFs to external pressure. By immersing the crystal in a hydrostatic medium, pressure is applied evenly to the crystal. On surrounding a porous MOF with a hydrostatic medium composed of small molecules (e.g. methanol), the medium can penetrate the MOF, resulting in medium-dependant compression. On compressing MOF-5 (Zn4O(BDC)3) using diethylformamide as a penetrating medium, the framework was shown to have an increased resistance to compression, becoming amorphous several orders of magnitude higher in pressure than observed on grinding the sample.[3] Here we present a high-pressure x-ray diffraction study on the UiO-based MOF UiO-67, and several new synthesised derivatives built from same metal node but with altered organic linkers, allowing us to study in a systematic way, the mechanical stability of the MOF, and its pressure dependence on both the linker, and pressure medium.

Pressure-induced phase transitions in energetic materials

Explosives and propellants, known generically as energetic materials, are widely used in applications that include mining, munitions, and automotive safety. Key properties of these materials include: reliable performance under a range of environmental conditions; long-term stability; environmental impact; processability; sensitivity to accidental initiation through stimuli such as impact, shock, friction, and electrostatic discharge. Many of these properties are affected by the crystal structure of the energetic material. Explosives experience elevated pressures and temperatures under detonation conditions - such conditions often induce phase transitions in the energetic material. Hence detailed studies of pressure-induced structural changes in these materials are essential in order to understand and model fully their behaviour. This presentation will describe some recent high-pressure studies (using a combination of X-ray and neutron diffraction techniques) on 2,4-dinitroanisole (DNAN), an insensitive explosive that is replacing TNT in some applications [1,2]. DNAN shows rich pressure-induced polymorphism, with at least four high-pressure forms having been identified to date. One of the structures provides insight into as to why DNAN is particularly insensitive to initiation by shock. The presentation will also describe the interplay between experiment and theory, which will be illustrated by experimental and computational high-pressure studies of 1,1-diamino-2,2-dinitroethene (DADNE or FOX-7). A very subtle phase transition has been identified at a pressure of ~2.0 GPa and the implications of this will be discussed in relation to the observed structural changes and properties of this material.

Serotonin in different compounds: crystallographic and computational insight

Biologically active substances are in the focus of pharmaceutical and chemical research. Serotonin, one of the most common neurotransmitters, is widely studied in relation to its effect on humans from cellular to neurological levels. Although serotonin plays a key role in some biological processes, its chemistry and crystallography are not sufficiently understood. The aim of the present study was to crystallize serotonin adipate and creatinine sulfate monohydrate, determine their crystal structures, and analyze them in a comparison with other previously known serotonin crystal structures. Special attention was paid to the interrelation between the molecular conformation and crystalline environment. This issue was addressed using crystallographic and computational chemistry (DFT-D, MD) approaches. In our research was shown that the crystal structure of the creatinine sulfate complex significantly differs from what was previously determined. The conformation of serotonin in the new structure differs from serotonin conformations in all other known complexes, as well as from the most stable conformation, predicted by the adiabatic conformational analysis using quantum chemical calculations (DFT, MP) in different phases. This work has explicitly shown the influence of different interactions on serotonin molecular conformation in the crystalline state, described from a crystallographic and theoretical point of view. It has been previously demonstrated that salt formation in the presence of different anions produces variation in pharmacological, therapeutic and physic-chemical properties. This study has shown that alterations of the anion affects the molecular geometry of the bioactive substance and invite further investigation to rationalize the geometry changes. The work was supported by the RFBR Grants No.14-03-31866, 13-03-92704, Russian Ministry of Science and Education and RAS, Siberian Supercomputer Center SB RAS Integration Grant No.130, Edinburgh Compute and Data Facility