Energetics and Reaction Mechanisms for the Competitive Losses of H2, CH4 and C2H4 from Protonated Methylbenzenes—Implications to the Methanol-to-Hydrocarbons (MTH) Process:

We report the unimolecular decomposition following collisional activation of protonated mono-, di- and trimethylbenzenes as a function of collision energy. The resulting energy-resolved mass spectra are then used for the quality control of high-level quantum chemical models of the respective potential energy surfaces. Distinction is made between direct dissociation products (CH4 or H2) and indirect products (alkenes), since formation of the latter requires extensive rearrangement of the molecular skeleton. Very good consistency was found between model and experiment. The models thereby provide a solid foundation for discussing the reaction mechanisms of the industrial methanol-to-hydrocarbon process. The losses of CH4, C2H4 and C3H6 from mesitylenium ions have been studied by 13C and 2H labelling and the alkene losses were found to occur via irreversible isomerisation pathways.