Bond-formation versus electron transfer: C-C-coupling reactions of hydrocarbon dications with benzene

The bimolecular reactions of several hydrocarbon dications CmHn2+ (m = 6-10, n = 4-9) with neutral benzene are investigated by tandem mass spectrometry using a multipole instrument. Not surprisingly, the major reaction of CmHn2+ with benzene corresponds to electron transfer from the neutral arene to the dication resulting in the pair of monocationic products CmHn+ + C6H6+. In addition, also dissociative electron transfer takes place, whereas proton transfer from the CmHn2+ dication to neutral benzene is almost negligible. Interestingly, the excess energy liberated upon electron transfer from the neutral arene to the CmHn2+ dication is not equally partitioned in the monocationic products in that the cations arising from the dicationic precursor have a higher internal energy content than the monocations formed from the neutral reaction partner. In addition to the reactions leading to monocationic product ions, bond-forming reactions with maintenance of the two-fold charge are observed, which lead to a condensation of the CmHn2+ dications with neutral benzene under formation of intermediate Cm+6Hn+62+ species and then undergo subsequent losses of molecular hydrogen or neutral acetylene. This reaction complements a recently proposed dicationic route for the formation of polycyclic aromatic hydrocarbons under extreme conditions such as they exist in interstellar environments.