Fluorosulfuric Acid-Arsenic Pentafluoride

Perfluoroalkanesulfonic Acid-Based Systems. CnFn+1S03H- 

It has not been possible to measure the acidity of the CF3S03H-SbFs system by spectrophotometry because of its very strong absorption in the UV-visible spectrum. In comparison with the strength of the related perfluoroalkanesulfonic acids, its acidity should be very close or slightly higher (by 1 H0 unit) than the acidity of the perfluoroethane- and perfluorobutanesulfonic acid-SbF5 mixtures, which was 

Acidity measurements were again limited for the lack of suitable indicator base and even 1,3,5-trinitrobenzene, the weakest base used, was fully protonated (Ho —18.5) in the 22mol% solution of boron triflate. Extrapolation of this system to 40% B(CF3S03)3 in CF3SO3H would lead to an H0 value of —20 in the Hammett scale. Consequently, the acidity is comparable to that of the ternary system HSO3F-SbF5-S03. 

Nevertheless, the acidity measurements based on the use of the benzhydryl cation indicator family87 show that the upper acidity limit of the HF-SbF5 system is reached with 10 mol% of SbF5. The weakest indicator available is protonated 4,4 -dimethoxy-benzhydryl cation (pA BH+ —23) and could not be further protonated, even in the most concentrated HF-SbF5 solutions. 

The HF-SbF5 system has also been reinvestigated by high-field NMR spectroscopy87 as well as by the theoretical methods such as ab initio molecular dynamics96 and DFT theory.97 

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