The gas phase acidity of HBF4 (HF-BF3)

Abstract

The gas phase acidity of the complex superacid HBF₄ and the interaction of BF₃ with HF as well as the F^- detachment from BF₄^- have been calculated ab initio using high accuracy energy models [CBS-4, CBS-Q, CBS-QB3, G1, G2(MP2) and G2] as well as the density functional B3LYP formalism. All methods but CBS-4 and G1 provide practically equal acidity data for the gas phase and one may recommend 288-289 kcal mol^-1 for the Gibbs free energy and 291-292 kcal mol^-1 for the corresponding enthalpy at 298 K. The complex HF-BF₃ is unstable in vacuo as the Gibbs free energy of decomposition into HF and BF₃ is predicted to be between -3.6 and -4.6 kcal mol^-1 depending on the method used. The conjugated base BF₄^- is stable with respect to fluoride ion detachment. We recommend 81-82 kcal mol^-1 and 72-73 kcal mol^-1 for the enthalpy and the Gibbs free energy, respectively.