The alkane sigma-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 stab le than the H-carbonium ions. The results are in agreement with gas-phase d ata of n-butane and isobutane protonation but disagree with results in liqu id superacid, where protonation of the tertiary C-H of isobutane is preferr ed over C-C protonation. Additional calculations, including the superacid m oiety, revealed that the activation energy for C-C protonation is higher th an 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 activat ion energy for proton transfer, rather than by the intrinsic basisity of th e 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 grou ps of the three center bond.