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 Waals 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 H 3 O +, as well as the H−D exchange in liquid superacid.