The electronic spectroscopy of the two most stable C₇H₇⁺ species has now been studied in detail thanks to the efforts of John Maier and his team from the University of Basel, Switzerland. Electronic absorption spectra of benzylium (Bz⁺; see picture) and tropylium (Tr⁺) cations were recorded after mass-selected trapping of the cations into a neon matrix. Measurements in different regions of the spectra revealed both weak and strong transitions for Bz⁺ (C_2v), along with the lowest dipole-allowed transition for Tr⁺ (D_7h). Density functional calculations were performed to support interpretations of the ground-state energy and vibrational frequencies of Bz⁺ and Tr⁺.

Physical organic chemists have been debating the mass spectrometry and spectroscopic data with respect to the formation and automerization pathways of Bz⁺ and Tr⁺, for a long time. The spectroscopic information from this study should provide a reliable starting point for further gas-phase detection and investigation of benzylium and tropylium cations.

• On the Benzylium/Tropylium Ion Dichotomy: Electronic Absorption Spectra in Neon Matrices
  A. Nagy, J. Fulara, I. Garkusha, J. P. Maier,
  Angew. Chem. Int. Ed. 2011.
  DOI: 10.1002/anie.201008036
• A. Nagy, J. Fulara, I. Garkusha, J. P. Maier,
  Angew. Chem. 2011.
  DOI: 10.1002/ange.201008036