Single Crystal Diffraction Studies at Low Temperatures

In the previous sections single crystal diffraction has been described and an overview of the typical information one may wish to obtain from such an experiment has been presented. This includes chemical composition, atomic positions and molecular geometries, atomic displacements, and, in the context of crystal engineering, the identification of supramolecular synthons, network topologies, cavity sizes, etc. The study of non-covalent interactions is one of the main research 

Reduction of the temperature of the crystal decreases the thermal motion of the atoms, which can be expressed in terms of the temperature factor (B). A reduction in B enhances the atomic scattering factors (Fig. 3.1.2) and thus the corresponding structure factors and measured intensities (Table 3.1.1), particularly at higher Bragg angles. 

An illustration of the influence of temperature on diffraction intensities is provided in Fig. 3.1.3, which shows two frames collected with a CCD area detector using a 0.3° scan over a 10 s time interval. Each has been collected for the compound 

The remainder of this section includes some illustrative examples of the use of low temperature crystallography of relevance to crystal engineering. 

Many hydrogen bonded systems exhibit temperature-dependent properties, such as proton migration or hydrogen atom disorder [15], and variable temperature neutron diffraction is the diffraction technique of choice to address these issues [16]. Nevertheless, some recent experiments using variable temperature X-ray diffraction have successfully enabled distinction between static and dynamic disorder of the H-bond proton (Fig. 3.1.4) [17]. Very recently a combination of both neutron and X-ray diffraction with variable temperature has been applied in the study of temperature-dependence of the hydrogen bond present in potassium hydrogen phthalate, and shows no evidence of disorder or proton migration dependence with temperature (Fig. 3.1.5) [18]. 

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