FLUORIDES AND FLUORO ACIDS .30. CRYSTAL-STRUCTURES OF ACID HYDRATES AND OXONIUM SALTS .34. TRIFLUOROACETIC-ACID TETRAHYDRATE - A UNIQUE CHANGE FROM AN IONIC TO A MOLECULAR-CRYSTAL STRUCTURE ON DEUTERATION

Trifluoroacetic acid has been found from DTA, DSC, and temperature-dependent X-ray powder diffraction to form, besides other stable and metastable hydrates, both an undeuterated (reinvestigation) and perdeuterated stable tetrahydrate. For the undeuterated phase (mp-12-degrees-C) an ionic structure, (H5O2)[(CF3COO)2H].6H2O, has been confirmed by X-ray analysis. In contrast, the perdeuterated tetrahydrate (mp-15-degrees-C) has been found to be molecular, i.e. with CF3COOD.4D2O not only as the empirical but also as the pertinent structural formula. This unprecedented effect of a deuteration is distinct and unmistakable, since there are specific and consistent differences of position not only for certain H and D atoms but also for crucial heavier atoms. In other ways the two structures are strikingly similar, with three of the four water molecules hydrogen-bonded in characteristic layers of identical topologies.