Protonated isobutane. A theoretical ab initio study of the isobutonium cations

The structure and energy of the isobutonium cations, protonated isobutane, were studied by ab initio methods. At MP2(full)/6-31G** level, besides the C-isobutonium cation (5), the 2-H-isobutonium cation (6), and the 1-H-isobutonium cation (7), two additional structures, representing the van der Wads complex between methane and isopropyl. cation (8) and hydrogen plus tert-butyl cation (9), could also be characterized. The,energy increases in the order 9 < 8 < 5 < 6 < 7, indicating the lower energy of the van der Waals complexes. The experimental proton affinity of isobutane is in good agreement with the calculated values for the van der Waals complexes 8 and 9, indicating the facility of rupture of the three center bond in 5 and 6. On the other hand, the relative order of stability of the isobutonium cations can explain the experimental gas phase protonation of isobutane by small electrophiles, such as H-3(+) and H3O+, as well as the H-D exchange in liquid superacid.