High-Pressure X-ray Diffraction Studies on Organic Crystals

The high-pressure X-ray diffraction technique, commonly applied for inorganic crystals, is discussed in a view of its application for laboratory studies on molecular crystals of organic compounds – particularly when the weak X-ray scattering of such specimens causes experimental difficulties. To minimize the background-to-signal ratio, relatively big crystal samples (ca. 0.005 A³) were used and the mechanical properties of the gasket material were applied to obtain the optimum experimental conditions for pressure generation and lowering the background of reflections. The mechanical properties of tungsten, Inconel, and hardened steel gaskets are different and these properties can be helpful in planning high-pressure measurements with large high-pressure chamber to contain a big crystal sample. Several characteristic features of organic crystals – like their strong compressibility (depending on the nature of intermolecular interactions) and pressure-dependence of thermal vibrations of atoms – can have a direct impact on the conditions of high-pressure experiments. The properties of molecular crystals of organic compounds have been discussed on the basis of several high-pressure structural studies, mainly on simple cyclic β-diketoalkanes (enol forms). It has been attempted to assess the feasibility of investigating the details of molecular structure at high pressures by X-ray laboratory studies. A systematic pressure-induced change in the length of double C  O bonds is observed in this group of compounds, which can be correlated with the compression of the H-bonds involving the carbonyl groups.