The alkane σ-bond basicity scale revisited

The energy of the n-butonium and isobutonium cations was calculated. At the MP4/6-311++G**//MP2(fu)/6-31G** level, the C-carbonium ions were more stable than the H-carbonium ions. The results are in agreement with gas-phase data of n-butane and isobutane protonation but disagree with results in liquid superacid, where protonation of the tertiary C-H of isobutane is preferred over C-C protonation. Additional calculations, including the superacid moiety, revealed that the activation energy for C-C protonation is higher than the energy for attack at the tertiary C-H. This suggests that the sigma bond reactivity in the liquid superacid system is controlled by the activation energy for proton transfer, rather than by the intrinsic basisity of the bond. The higher stability of the C-carbonium relative to the H-carbonium ions was ascribed to a better charge distribution among the atoms and groups of the three center bond.