MNDO/PM3 Study of the Reaction Mechanism between Methane and Complexes Generated in Br2·AlBr3 System

Potential energy surface (PES) of systems AlBr5 and AlBr5 + CH4+ were investigated by MNDO/PM3 method. All the five donor-acceptor complexes Br2·AlBr3 with no barrier add to methane providing multiple adducts with various localization of interactions and with different conformations. However further transformations occur only with adducts of two complexes Br2·AlBr3 possessing considerable ionic character. On the reaction path resulting in CH3Br and HBr as intermediates function bromonium type complexes CH3BrH+·AlBr4- and the intermediates on the path leading to CH2Br2 and H2 are the complexes with 3c-2e bond of H2 quasi-molecule with the C atom of bromomethyl cation H2C(H-H)Br+·AlBr4-. Potential barriers on both reaction paths are about 30 kcal mol-1, and the transition states (TS) are analogous to the classical 3c-2e TS (Olah scheme) with an electrophile attack on a CH bond and to the recently suggested TS with an electrophile attack on an unshared electron pair of the carbon atom in the nonclassical methane H2C(H-H) respectively.