Conformational polymorphism in N-(4′-methoxyphenyl)-3-bromothiobenzamide

Three conformational polymorphs of N-(4'-methoxyphenyl)-3-bromothiobenzamide, yellow a, orange beta, and yellow gamma, have been identified by single-crystal X-ray diffraction. The properties and structure of the polymorphs were examined with FT Raman, FTIR (ATR), and UV/Vis spectroscopy, as well as differential scanning calorimetry. Computational data on rotational barriers in the isolated gas-phase molecule indicate that the molecular conformation found in the a form is energetically preferred, but only by around 2 kJ mol(-1) over the gamma conformation. The planar molecular structure found in the beta form is destabilized by 10-14 kJ mol(-1), depending on the calculation method. However, experimental evidence suggests that the beta polymorph is the most stable crystalline phase at room temperature. This is attributed to the relative planarity of this structure, which allows more and stronger intermolecular interactions, that is, more energetically effective packing. Calculated electronic-absorption maxima were in agreement with experimental spectra.