Caffeine Co-Crystal Mechanics Evaluated with a Combined Structural and Spectroscopic Approach

Abstract

In this report caffeine (CAF) co-crystallization with a fluoro-nitrobenzoic acid (F-NBA) yields a new solid form with superior tableting performance. A primary N···H-O synthon connects the basic nitrogen of CAF with the carboxylic acid of F-NBA to result in a layered structure with supportive CH3···O interactions. Over the entire compaction pressure range of nominally 50-300 MPa, the co-crystal displayed improved tensile strength relative to the individual co-formers that demonstrated tensile strengths consistently below 2 MPa. Qualitatively we interpret this tableting improvement for the co-crystal a result of increased plasticity manifested from the layered co-crystal structure, an observation consistent with previous co-crystal studies on modified material mechanics that suggest facile slip plane activation supports improved tableting. Powder Brillouin light scattering (BLS) is further introduced as a novel tool to rapidly evaluate elastic anisotropy to complement our structural interpretation of tableting performance. Each powder BLS spectra revealed two acoustic frequency distributions that we assign as longitudinal and transverse and permits rank-order of the elastic anisotropy. The shear mode distribution revealed an increasing population of low-velocity modes that mirrored the rank-order tableting performance of CAF:F-NBA > CAF > F-NBA. With further experimental support, we anticipate powder BLS may be utilized as a complementary tool to quantify and discriminate the mechanical properties of co-crystals and polymorphs for relation to their processing performance.