Mechanism of superacid catalyzed alkane activation: theoretical ab initio studies of pentacoordinated carbonium ion rearrangement

A theoretical ab initio study of the interconversion of pentacoordinated carbonium ions was carried out. For the isobutonium cations it was found that the respective C-carbonium ions were lower in energy than the H-isobutonium ions. Nevertheless, the interconversion of the 1-H-isobutonium cation in the C-isobutonium ion is a barrierless process. This suggests that product arisen from C-C protonation in liquid superacid and zeolite catalyzed alkane activation may be formed by protonation in the outer and more accessible primary C-H bonds of isobutane, rather than by direct protonation of the inner and more steric demanding C-C bonds.