Crystallization of low-molecular-weight organic compounds for X-ray crystallography

General strategies are described to obtain crystals of low-molecular-weight compounds suitable for X-ray structure determination. A survey is given of a variety of crystallization techniques together with their advantages and drawbacks, illustrated by experiences with notoriously crystallization-resisting compounds. The methods discussed range from preliminary investigations using evaporation, batch crystallization and liquid-liquid diffusion methods, via the most frequently used methods such as sitting-drop vapor-phase diffusion and change of temperature, to methods such as gel crystallization, sublimation and solidification. The most successful method appears to be the sitting-drop vapor-phase diffusion. Complete crystallization routes are described, taking into account the results of preliminary investigations. The hard-to-crystallize vecuronium bromide is presented as a case study.