Abstract

European Journal of Mass Spectrometry
Volume 15 Issue 2, Pages 167–181 (2009)
doi: 10.1255/ejms.953

Energetics and reaction mechanisms for the competitive losses of H₂, CH₄ and C₂H₄ from protonated methylbenzenes—implications to the methanol-to-hydrocarbons (MTH) process^a

Osamu Sekiguchi,^a Verena Meyer,^b Matthias C. Letzel,^b Dietmar Kuck^b,* and Einar Uggerud^a,c,*
aMass Spectrometry Laboratory, Department of Chemistry, University of Oslo, POB, 1033 Blindern, N-0315 Oslo, Norway
^bDepartment of Chemistry, Bielefeld University, POB 100131, D-33501 Bielefeld, Germany
^cCentre for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, University of Oslo, POB 1033 Blindern, N-0315 Oslo, Norway

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.

Keywords: gas-phase ion chemistry, mass spectrometry, methylbenzenes, methanol-to- hydrocarbons, zeolite, alkylbenzenium ions

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