IR spectroscopy of protonated toluene: Probing ring hydrogen shifts in gaseous arenium ions

Toluenium ions [CH3C6H6]+, formed by protonation of toluene by C2H5+ ions in the cell of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer, have been assayed by IR multiphoton dissociation (IRMPD) spectroscopy. Their IRMPD spectrum compares well with the calculated IR absorption spectra for the ortho-, meta-, and para-toluenium isomers, suggesting a mixture rich in the ortho/para isomers. When the protonation is effected on perdeuterated toluene, the so-formed ions present IRMPD features characteristic for both CD2 and CHD groups in the arenium ring. This finding is spectroscopic evidence for the occurrence of ring hydrogen shifts and provides the first example of differentiation of families of isotopomers (bearing either two D-atoms or one D-atom and one H-atom on the tetrahedral ring carbon) by the powerful tool of IRMPD using the tunable IR radiation of a free electron laser.