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⁺]₂SO₄^2–·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.

The solid form landscape investigation of theophylline and sulfuric acid reveals a rich collection of salts, polymorphs, and hydrates. Such a variety of crystal forms arises, thanks to a remarkable mobility of the chemical constituents in the solid state, while their crystals show dynamism and thermosalience.