A 2:1 sulfamethazine–theophylline cocrystal exhibiting two tautomers of sulfamethazine

In the title cocrystal, 4‐amino‐N‐(4,6‐dimethylpyrimidin‐2‐yl)benzenesulfonamide–4‐amino‐N‐(4,6‐dimethyl‐1,2‐dihydropyrimidin‐2‐ylidene)benzenesulfonamide–1,3‐dimethyl‐7H‐purine‐2,6‐dione (1/1/1), C₇H₈N₄O₂·2C₁₂H₁₄N₄O₂S, two sulfamethazine molecules cocrystallize with a single molecule of theophylline. Each molecule of sulfamethazine forms a hydrogen‐bonded ribbon along the b axis crosslinked by further hydrogen bonding. The two sulfamethazine molecules exhibit a hydrogen‐shift isomerization so that the crystal structure contains both tautomeric forms. Calculation of their relative energies showed that the tautomer protonated at the chain N atom is considerably more stable than the one where an N atom in the aromatic ring is protonated. The latter, here observed for the first time, is stabilized through strong intermolecular interactions with the theophylline molecules.