A theoretical study of alkane protonation in HF/SbF5 superacid system

Ab initio calculations for the protonation of the C-H and C-C bonds of meth ane, ethane, propane and isobutane by a superacid moiety was carried out. F or the C-H protonation (H/H exchange) the transition state resembles an H-c arbonium ion coordinated with the superacid. The activation energy for the H/H exchange was about 16 kcal.mol(-1), at B3LYP/6-31++G"* + RECP (Sb) leve l, regardless the type of C-H bond being protonated. For the C-C protonatio n the activation energy depends on the structure of the hydrocarbon and was always higher than the activation energy for C-H protonation, indicating a higher steric demand.