Investigating the weak to evaluate the strong: An experimental determination of the electron binding energy of carborane anions and the gas phase acidity of carborane acids

(Chemical Equation Presented) Five CHB₁₁X₆Y ₅^- carborane anions from the series X = Br, Cl, I and Y = H, Cl, CH₃ were generated by electrospray ionization, and their reactivity with a series of Brønsted acids and electron transfer reagents were examined in the gas phase. The undecachlorocarborane acid, H(CHB ₁₁Cl₁₁), was found to be far more acidic than the former record holder, (1-C₄F₉SO₂)₂NH (i.e., ΔH°_acid = 241 ± 29 vs 291.1 ± 2.2 kcal mol^-1) and bridges the gas-phase acidity and basicity scales for the first time. Its conjugate base, CHB₁₁Cl₁₁^-, was found by photoelectron spectroscopy to have a remarkably large electron binding energy (6.35 ± 0.02 eV) but the value for the (1-C₄F ₉SO₂)₂N^- anion is even larger (6.5 ± 0.1 eV). Consequently, it is the weak H-(CHB₁₁Cl ₁₁) BDE (70.0 kcal mol^-1, G3(MP2)) compared to the strong BDE of (1-C₄F₉SO₂)₂N-H (127.4 ± 3.2 kcal mol^-1) that accounts for the greater acidity of carborane acids.