Solid-state NMR studies of theophylline co-crystals with dicarboxylic acids

•Three co-crystals of theophylline were studied using solid-state NMR and FT-IR.•The NMR assignments were supported by GIPAW calculations.•The application of ssNMR to analysis of the solid drug forms was described. In this work, three polycrystalline materials containing co-crystals of theophylline with malonic, maleic, and glutaric acids were studied using 13C, 15N and 1H solid-state NMR and FT-IR spectroscopy. The NMR assignments were supported by gauge including projector augmented waves (GIPAW) calculations of chemical shielding, performed using X-ray determined geometry. The experimental 13C cross polarization/magic angle spinning (CP/MAS) NMR results and the calculated isotropic chemical shifts were in excellent agreement. A rapid and convenient method for theophylline co-crystals crystal structure analysis has been proposed for co-crystals, which are potentially new APIs.