of SbF5. The resolution of the signals decreases as well, because of the increasing viscosity of the liquid system. Strength All proton-producing acids stronger than 100% sulfuric acid are considered superacids, and are characterized by low values of the Hammett acidity function. For instance, sulfuric acid, H2SO4, has a Hammett acidity function, H0, of −12, perchloric acid, HClO4, has a Hammett acidity function, of −13, and that of the 1:1 magic acid system, HSO3F·SbF5, is −23. Fluoroantimonic acid, the strongest known superacid, is believed to reach extrapolated H0 values down to −28. Uses Observations of stable carbocations Magic acid has low nucleophilicity, allowing for increased stability of carbocations in solution. The "classical" trivalent carbocation can be observed in the acid medium, and has been found to be planar and sp2-hybridized. Because the carbon atom is surrounded by only six valence electrons, it is highly electron deficient and electrophilic. It is easily described by Lewis dot structures because it contains only two-electron, single bonds to adjacent carbon atoms. Many tertiary cycloalkyl cations can also be formed in superacidic solutions. One such example is the 1-methyl-1-cyclopentyl cation, which is formed from both the cyclopentane and cyclohexane precursor. In the case of the cyclohexane, the cyclopentyl cation is formed from isomerization of the secondary carbocation to the tertiary, more stable carbocation. Cyclopropylcarbenium ions, alkenyl cations, and arenium cations have also been observed. As use of the Magic acid system became more widespread, however, higher-coordinate carbocations were observed. Penta-coordinate carbocations, also described as nonclassical ions, cannot be depicted using only two-electron, two-center bonds, and require, instead, two-electron, three (or more) center bonding. In these ions, two electrons are delocalized over more than two atoms, rendering these bond centers so electron deficient that they enable saturated alkanes to participate in electrophilic reactions. The discovery of hypercoordinated carbocations fueled the nonclassical ion controversy of the 1950s and 60s. Due to the slow timescale of 1H-NMR, the rapidly equilibrating positive charges on hydrogen atoms would likely go undetected. However, IR spectroscopy, Raman spectroscopy, and 13C NMR have been used to investigate bridged carbocation systems. One controversial cation, the norbornyl cation, has been observed in several media, Magic acid among them. The bridging methylene carbon atom is pentacoordinated, with three two-electron, two-center bonds, and one two-electron, three-center bond with its remaining sp3 orbital. Quantum mechanical calculations have also shown that the classical model is not an energy minimum.