Single Crystal Diffraction


Single crystal diffraction pattern

Figure 1 Example of a single crystal diffraction pattern


As hydrates represent crystalline phases, they can be unambiguously described and identified by crystallographic techniques. Crystals are made of small units, unit cells, which show periodic repetition in three dimensions. Thus, crystalline material diffracts short wavelength electromagnetic radiation giving rise to diffraction methods. Using a single crystal, a well-defined three-dimensional interference pattern (Figure 1) can be obtained, from which the unit cell dimensions can be extracted from the spot positions and the unit cell contents from the spot intensities. After integration of the spot intensities, a structural model can be solved from the diffraction pattern and refined. This structural model is the most accurate description of a crystalline phase at the moment, as it gives the exact coordinates of every atom as well as their connectivity. The single crystal structure gives information about non-covalent interactions such as hydrogen and halogen bonds, π-stacking and van-der-Waals interactions. In some cases, the structure solution benefits from the information gained from other methods (e.g. thermal gravimetry), as atom positions can deviate from full occupancy. Especially non-stoichiometric solvates/hydrates show lower occupancy of the solvent/water positions depending on the conditions the crystal was prepared in.