Inclusion of hydrogen atoms in the least squares refinement can have a significant effect on the overall precision of the structural result, especially for organic structures. Hydrogen atom refinement is normally a prerequisite for the determination of a molecule's absolute configuration. Hydrogen atom location and refinement can be performed for most structures, but it does require high quality from high quality crystals. This increases the quality and resolution of the X-ray data and, consequently, the accuracy of the atomic positions. Compounds that are unstable at room temperature, due to thermal instability, solvent loss or sensitivity to oxygen, moisture or light, are usually stable indefinitely at low temperature.
Some compounds exhibit different physical properties below ambient temperature.
It is possible to determine crystal structures at specific temperatures where properties of interest are exhibited. MSC utilizes stable, liquid-nitrogen based low temperature systems capable of maintaining single crystals at temperatures between ambient and 100 K with a stability of one degree. The low temperature surcharge is based on the number of unique non-hydrogen atoms. Analytical absorption correction For well defined crystals, the most accurate method of correcting for X-ray absorption is to use the analytical technique. This method requires that the crystal shape be accurately measured and that the crystallographic Miller indices be assigned to the crystal faces.