Infrared Spectroscopy of Fluorenyl Cations at Cryogenic Temperatures

The notion of (anti)aromaticity
is a successful concept in chemistry
to explain the structure and stability of polycyclic hydrocarbons.
Cyclopentadienyl and fluorenyl cations are among the most studied
classical antiaromatic systems. In this work, fluorenyl cations are
investigated by high-resolution gas-phase infrared spectroscopy in
helium droplets. Bare fluorenyl cations are generated in the gas phase
by electrospray ionization. After mass-to-charge selection, ions are
captured in ultracold helium nanodroplets and probed by infrared spectroscopy
using a widely tunable free-electron laser in the 600–1700
cm–1 range. The highly resolved cryogenic infrared
spectra confirm, in combination with DFT computations, that all cations
are present in their singlet states.