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Solution structures and precrystallization molecular association


Nucleation—the initial stage of crystal growth—is a challenging area to study experimentally, but an understanding of the process is essential as increasing numbers of important materials are found to exhibit polymorphism, even when grown under seemingly identical conditions. To examine the assembly of potential supramolecular building blocks in crystallization processes, this study focuses on initial molecular assembly in the solution state; knowledge of this early stage assembly can help understand the dominance of certain polymorphs in the crystalline state.


The RAPID II was used to conduct wide angle liquid scattering on benzoic acid derivatives, which consistently form dimers in the solid state, in solution. Detecting the existence of such dimers in solution could indicate the initial building blocks prior to crystallization. It is important in liquid scattering to be able to correct for the background scattering arising from the capillary and the solution as this dominates the images. Making sure that the resolution of the resulting solute-only scattering is good enough does not depend on the quality of the raw images as the scattering arising from the solution and the capillary is generally much greater than that of the solute.

Conventionally, liquid measurements need to be taken on synchrotron sources. However, the flexibility and wide dynamic range of the RAPID II allow these experiments to be made in the home lab. Since overall signal was the most important factor in the experiment, a 0.8mm collimator and 0.7 mm capillary yielded the best measurements in the fastest time. The most straightforward way of collecting data, collecting a single long exposure, overloaded the detector with signal from the capillary. To avoid this situation, multiple 2-hour, 1° images were collected and merged with the AreaMax software. For each solution measurement, a corresponding background measurement (corresponding to the capillary and solution) was taken and the images were corrected for background before merging.

Solution structure
The liquid scattering arising from benzoic acid in a methanol solution. The raw data is corrected for the methanol and capillary before being integrated and Fourier transformed to give a pair distribution function.

The raw data image of benzoic acid in methanol is dominated by the methanol and capillary so looks very similar to the pure methanol. However, on subtraction of a background image, structure specific to the benzoic acid is apparent. When this is corrected and Fourier transformed, peaks in the pair distribution function are observed at approximately 2.5 è, and 4.8 è. Results courtesy of Andy Parkin, Lynne H Thomas and Chick C Wilson, Structural Chemistry Group, University of Glasgow.