DFT calculations on the protonation of alkanes on HF/SbF5 superacids usingcluster models

Calculations at B3LYP/6-31++G** + RECP (Sb) level have been performed for t he protonation of C-H and C-C bonds of methane, ethane, propane, and isobut ane by models of the liquid superacid media HF/SbF5. The antimony atoms wer e dealt with by relativistic effective core potentials. The species H2F+. S b2F11- 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-carboni um ion interacting with the anion moiety. The enthalpies of activation for WH exchange of alkanes were calculated in the range of 19 to 21 kcal/mol. F or the protonation of the C-C bond, the enthalpy of activation strongly dep ends on the structure of the hydrocarbon being attacked, and was always hig her than the enthalpy of activation for H/H exchange. This suggests the exi stence of steric demand for the C-C protonation.