PROTONATED ISOBUTANE - A THEORETICAL AB-INITIO STUDY OF THE ISOBUTONIUM CATIONS

The structure and energy of the isobutonium cations, protonated isobut ane, were studied by ab initio methods. At MP2(full)/6-31G* level, be sides the C-isobutonium cation (5), the 2-H-isobutonium cation (6), an d the 1-H-isobutonium cation (7), two additional structures, represent ing 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 l ower energy of the van der Waals complexes. The experimental proton af finity of isobutane is in good agreement with the calculated values fo r the van der Waals complexes 8 and 9, indicating the facility of rupt ure of the three center bond in 5 and 6. On the other hand, the relati ve order of stability of the isobutonium cations can explain the exper imental gas phase protonation of isobutane by small electrophiles, suc h as H-3(+) and H3O+, as well as the H-D exchange in liquid superacid.