Hydrate der Fluorsulfonsäure: Das Schmelzdiagramm des Systems FSO3H?H2O und die Kristallstruktur des Monohydrats, (H3O)FSO3

Hydrates of Fluorosulfuric Acid: The Melting Diagram of the System FSO₃H? H₂O and the Crystal Structure of the Monohydrate, (H₃O)FSO₃ The system FSO₃H? H₂O has been investigated by difference thermal analysis. By adhering to exclusively lower temperatures in the course of preparation and manipulation of the measuring samples, it was possible to avoid hydrolysis of the S? F bond and to obtain a quasi-binary melting diagram. This reveals the existence of four crystalline hydrates FSO₃H · nH₂O with n = 1, 2, 3, and 4, melting at –12, –53 (dec.), –46, and ?63°C (dec.), respectively. The structure of the monohydrate has been determined (crystal system orthorhombic, space group Pnma, Z = 4 formula units per unit cell; lattice constants a = 8.055, b = 6.465, c = 7.459 Å at ?50°C; final R value with 515 independent observed MoKα diffractometer data at 0.043). The result is a typical oxonium salt, (H₃O)FSO₃, characterized by strong hydrogen bonds only of the kind O? H…?O and with interesting relations to the isosteric, dimorphic compound (H₃O)ClO₄.