Comparison of structures and energies of CH₅^2+• with CH₄^+• and their possible role in superacidic methane activation

Abstract

Contrary to previous theoretical studies at the UHF/6-31G* level, the methonium radical dication CH₅ ²⁺ is not a C_s symmetrical structure with a 2e—3c bond but a C_2v symmetrical structure 1 with two 2e—3c bonds (at the UHF/6-31G**, UMP2/6-31G**, and UQCISD(T)/6-311G** levels). The C_s symmetrical structure is not even a minimum at the higher level of calculations. The four hydrogen atoms in 1 are bonded to the carbon atom by two 2e—3c bonds and the fifth hydrogen atom by a 2e—2c bond. The unpaired electron of 1 is located in a formal p-orbital (of the sp²-hybridized carbon atom) perpendicular to the plane of the molecule. Hydrogen scrambling in 1 is however extremely facile, as is in other C₁ cations. It is found that the protonation of methane to CH₅ ⁺ decreases the energy for subsequent homolytic cleavage resulting in the exothermic (24.1 kcal/mol) formation of CH₄ ^+•. Subsequent reaction with neutral methane while reforming CH₅ ⁺ gives the methyl radical enabling reaction with excess methane to ethane and H₂. The overall reaction is endothermic by 11.4 kcal/mol, but offers under conditions of oxidative removal of H₂ an alternative to the more energetic carbocationic conversion of methane.