DFT Calculations on the Protonation of Alkanes on HF/SbF5 Superacids Using Cluster Models

Abstract

Calculations at B3LYP/6-31++G** + RECP (Sb) level have been performed for the protonation of C−H and C−C bonds of methane, ethane, propane, and isobutane by models of the liquid superacid media HF/SbF5. The antimony atoms were dealt with by relativistic effective core potentials. The species H2F+·Sb2F11- was considered as the model electrophile. The transition states for the protonation of the C−H bonds (H/H exchange) are similar to an H-carbonium ion interacting with the anion moiety. The enthalpies of activation for H/H exchange of alkanes were calculated in the range of 19 to 21 kcal/mol. For the protonation of the C−C bond, the enthalpy of activation strongly depends on the structure of the hydrocarbon being attacked, and was always higher than the enthalpy of activation for H/H exchange. This suggests the existence of steric demand for the C−C protonation.