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. © 2024 The Authors. Published by American Chemical Society.