The molecular structure of the a form of o-acetamidobenzamide at 30K, with thermal ellipsoids drawn at the 50% level.
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Using the ISIS instrument SXD to obtain accurate crystal structures.
Computational methods of crystal structure prediction (CSP) can easily generate a great many trial structures for any given molecule; predicting which structures actually occur in the solid state is notably harder. Fortunately, a great deal of uncertainty can be removed from CSP by employing accurate molecular starting models, particularly in respect of hydrogen atom positions. By employing single-crystal neutron diffraction on SXD, we have obtained accurate crystal structures for the a and b forms of o-acetamidobenzamide that represent a significant improvement upon existing X-ray derived structures. Hydrogen atoms are located with the same accuracy as non-H atoms and the thermal displacement parameters are refined anisotropically for all atoms. As a consequence, intra- and intermolecular bonds, including weak H-bonds, are now much better defined than was previously the case. These crystal structures are now being used in a CSP study into the energy difference between the two forms which is larger than that normally observed for two polymorphs of the same compound.
CK Broder, M Gutmann, K Shankland (ISIS), SA Barnett (UCL), CC Wilson (University of Glasgow)
Research date: December 2006
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