The temperature dependence of the 11B NMR spectra of the monosubstituted and protonated c/oso-hexaborates RB6H6-, R=CH3, C2H5, C3H7, C4H9 C8H17, (SCN)B6H6- and XB6H6 X = Cl. Br. I, has been determined in the range 180-300 K. The extra proton migrates at higher temperatures intramolecularly, probably across the edges of the B6 core. The extra proton is fixed to a facet of the octahedron below the coalescence temperatures 240 K (RB6H6-) and 210 K (XB6H6-) as recognized by the splitting of the signal of the equatorial B atoms due to the lowering of the symmetry from C4v to Cs . From the spin-spin coupling it can be deduced that the proton prefers with the softer Brönsted acids RB6H6- facets adjacent to the substituent, while with the stronger Brönsted acids XB6H6- facets including the antipodal B atom are occupied.
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