Energetics and reaction mechanisms for the competitive losses of H-2, 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 H-2) 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 tosses of CH4, C2H4 and C3H6 from mesitylenium ions have been studied by C-13 and H-2 labelling and the alkene tosses were found to occur via irreversible isomerisation pathways.