Spinning, Sliding, Flexing, and Jumping of Theophylline Sulfate between Concomitant Polymorphs and Hydrates

The solid form screening of theophylline (TP) and sulfuric acid led to a collection of new crystals including two anhydrous sulfate salts (TPH+HSO4– form I and TPH+HSO4– form II) and two hydrated salts (bis-theophyllinium sulfate monohydrate ([TPH+]2SO42–·H2O) and theophyllinium hydrogen sulfate dihydrate (TPH+HSO4–·2H2O)). The new structures were determined and their stability was investigated, suggesting multiple modes of movement for the ions in the solid state. Anhydrous TPH+HSO4– form II exhibits unusually large anisotropic linear thermal expansion, associated with the elongation of a unique sulfate−π contact. Additionally, thermal dehydration of TPH+HSO4–·2H2O occurs in a salient, rocketlike manner that may be caused by the sudden release of water vapor trapped inside the particles.