Benzylium versus Tropylium Ion Dichotomy: Vibrational Spectroscopy of Gaseous C8H9+ Ions

The structural discrimination between C8H9+ benzylium and tropylium ions using IRMPD spectroscopy of the gaseous, unperturbed species is reported. These species, obtained from properly selected precursors, are characterized by their distinct IRMPD spectra. Each spectrum is nicely matched by the calculated IR spectrum for the respective isomer. The so-obtained evidence highlights two major points: (i) the C8H9+ benzylium and tropylium isomers are stable species that do not undergo rearrangement into the time scale of the experiment (several seconds); (ii) IRMPD spectroscopy provides a valuable tool to solve a long debated structural problem.