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Mechanochemical cocrystal formation studied by in situ Raman spectroscopy

Cocrystals have become a widely researched class of materials with a great potential in pharmaceutical industry (Qiao et al., 2011) and in the study of solid-state reactions (Friščić & Jones, 2009). Neat grinding and liquid- assisted grinding are fast screening techniques for cocrystal formation which can even result in cocrystals that cannot be prepared by classical solution synthesis. In this study the mechanochemical formation of nicotinamide cocrystals with benzoic, salicylic or anthranilic acid and the influence of small quantities of added primary alcohols was investigated by using in situ Raman spectroscopy (Gracin et al., 2014). Obtained cocrystals of nicotinamide with benzoic and anthranilic acid were previously not known and their crystal structures were elucidated using powder X-ray diffraction. For all cocrystals, their rates of formation were different when using different primary alcohols as additives in the reaction mixture. Particularly interesting were cocrystals of nicotinamide and benzoic acid where three new phases were detected while the crystal structures were elucidated for the two of them (polymorphs I and II). In situ spectroscopy revealed that neat grinding results in the formation of phase III that converts to polymorph I while liquid- assisted grinding results in the formation of the polymorph II that converts to phase III. Rates of conversion were dependent on the alcohol additive in the reaction mixture.

cocrystal, in situ Raman spectroscopy, neat grinding, nicotinamide, benzoic acid, anthranilic acid, salicylic acid

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