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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

  • Author(s): Meyer, MM
  • Wang, XB
  • Reed, CA
  • Wang, LS
  • Kass, SR
  • et al.

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(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. © 2009 American Chemical Society.

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