TOLUENIUM AND OTHER GASEOUS METHYLBENZENIUM IONS: COMPLEX INTERPLAY OF PROTONATED ARENES AND CYCLO-OLEFINS

The development of the current knowledge of the gas-phase chemistry of protonated methylbenzenes, such as toluenium, xylenium and mesitylenium ions, their higher congeners as well as of their mostly cyclo-olefinic isomers by mass spectrometric methodology is presented. Starting from the observation of the characteristic expulsion of dihydrogen from metastable C7H9+ ions, which is associated with the release of large amounts of kinetic energy, and the composite C- and H-scrambling prior to the loss of methane, in particular, insights into the isomerization scenario of various isomeric C7H9+, C8H11+, and C9H13+ ions, based on a large variety of independent techniques, are discussed. Besides isotope labeling and metastable ion methodology, these include flowing afterglow mass spectrometry, gas-phase titration and infrared spectroscopy of mass-selected ions. The particularly complex energy hypersurface of isomerizing and fragmenting toluenium ions, which has been elaborated in various reports over the years, is presented in a combined way to assess the role of protonated cycloheptatriene, norbornadiene, and 6-methylfulvene as well as a number of further C7H9+ isomers. The formation and nature of C7H9+ ions generated by fragmentation of various hydrocarbon precursors, such as monoterpenes and adamantane, is also addressed. The contribution of infrared multiphoton dissociation spectroscopy (IRMPD) and tagged-ion infrared photodissociation (IRPD) of the gaseous C7H9+ ions as compared to the wealth of previous understanding of their chemistry is commented on as well. Finally, remarkable parallels of the gas-phase chemistry of methylbenzenium ions and the role of such species within the cavities of acidic zeolite catalysts in the course of the industrially important methanol-to-hydrocarbon reaction are discussed. (c) 2020 John Wiley & Sons Ltd. Mass Spec Rev

Automated summary

This article presents the development of gas-phase chemistry of protonated methylbenzenes and their higher congeners by mass spectrometric methodology, focusing on isomerization and fragmentation mechanisms of different C7H9+ ions and the application of techniques such as infrared spectroscopy. The complex energy hypersurface of toluenium ions and the role of such species in acidic zeolite catalysts are also discussed.