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-degrees-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 angstrom at -50-degrees-C; final R value with 515 independent observed MoK-alpha 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.