Axhausen, J.; Lux, K.; Kornath, A.Angew. Chem. Int. Ed. 2014. 53, Early View.
The homogeneous mixture of anhydrous hydrogen fluoride aHF and antimony pentafluoride AsF5 is known as a superacidic system. The high acidity is derived from the formation of [H2F]+ [AsF6]−. No experimental evidence exists for the existence of the free acid molecule HAsF6. The reaction of trimethylsilyl N,N-dimethylcarbamate in the binary system aHF/AsF5 led to decomposition of trimethylsilyl N,N-dimethylcarbamate at −50 °C to dimethylammonium hexafluoridoarsenate and cocrystallization of HAsF6. The single-crystal X-ray structure displays an HAsF6 molecule involved in an asymmetric hydrogen bridge to the hexafluoridoarsenate anion. As a result of the incalculable situation in the crystal lattice, the molecular structure of HAsF6 is calculated by quantum chemical structure optimization of the extreme cases of [FHF-AsF5]− (strong hydrogen bond) and HAsF6 (no hydrogen bond) at the PBE1PBE/6-311G(3df,3pd) level of theory.
Superacids are defined as stronger acids than 100% sulfuric acid. As you might know the story on the discovery of FSO3H/SbF5, when a member of Olah group put a candle into the acid during a Christmas party, the candle dissolved immediately.After that, FSO3H/SbF5 has been called Magic acid, known as one of the superacids. In this report, Kornath group show the protonation of trimethylsilyl-N,N-dimethylcarbamate in the superacid media HF/AsF5. This reaction gave not only dimethyl ammoniumhexafluoridoarsenate [Me2NH2]+[AsF6]–, but the hexafluoroarsenic(V) acid HAsF6 which was structurally characterized by X-ray diffraction by chance in co-crystalline.
 “Superacids”,Olah, G. A. ; Prakash, G. K.; Sommer, J. 2nd ed., Wiley, Hoboken, 2009. ISBN: 9780470421543
The chemistry of superacids has developed in the last two decades into a field of growing interest and importance. Now available in a new expanded second edition, this definitive work on superacids offers a comprehensive review of superacids and discusses the development of new superacid systems and applications of superacids in the promotion of unusual reactions.