Theoretical study on the formation mechanisms of CH₃⁺ from doubly charged benzene

The theoretical investigations on the two-body dissociation pathway CH₃⁺+C₅H₃⁺ from doubly charged benzene are reported by scanning the potential energy surfaces with the structures and energies of the relevant transition states and intermediate states clarified. Two different composition modes of H atoms in CH₃⁺ fragment are identified, and they are deduced to play a non-negligible role in CH₃⁺+C₅H₃⁺ generation. Both two formation mechanisms were not disclosed in previous theoretical works, and they can further explain the previous experimental observation. In detail, besides the hydrogen atom originally attached to the carbon atom who forms CH₃⁺ fragment, the other two hydrogen atoms in CH₃⁺ fragment can come from two ortho-position carbon atoms, or one from the ortho-position carbon atom and the other from the meta-position carbon atom located on the other side. Furthermore, the competition between the ring-opening and proton migration process in the dissociation of polycyclic aromatic hydrocarbons is taken into account by comparing the energy barriers of trajectories with proton migration or ring opening as the first step, and it is concluded that proton migration and ring-opening are quite competitive in the dissociation of PAHs.