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.