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.
Five CHB(11)X(6)Y(5)(-) carborane anions from the series X = Br, Cl, I and Y = H, Cl, CH(3) 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(11)Cl(11)), was found to be far more acidic than the former record holder, (1-C(4)F(9)SO(2))(2)NH (i.e., DeltaH degrees (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, CHB(11)Cl(11)(-), was found by photoelectron spectroscopy to have a remarkably large electron binding energy (6.35 +/- 0.02 eV) but the value for the (1-C(4)F(9)SO(2))(2)N(-) anion is even larger (6.5 +/- 0.1 eV). Consequently, it is the weak H-(CHB(11)Cl(11)) BDE (70.0 kcal mol(-1), G3(MP2)) compared to the strong BDE of (1-C(4)F(9)SO(2))(2)N-H (127.4 +/- 3.2 kcal mol(-1)) that accounts for the greater acidity of carborane acids.
Meyer
Matthew M
MM
Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, USA.
Wang
Xue-Bin
XB
Reed
Christopher A
CA
Wang
Lai-Sheng
LS
Kass
Steven R
SR