Protonation Sites of Isolated Fluorobenzene Revealed by IR Spectroscopy in the Fingerprint Range

Protonated fluorobenzene ions (C6H6F+) are produced by chemical ionization of C6H5F in the cell of a FT-ICR mass spectrometer using either CH5 + or C2H5 +. The resulting protonation sites are probed by IR multiphoton dissociation (IRMPD) spectroscopy in the 600−1700 cm-1 fingerprint range employing the free electron laser at CLIO (Centre Laser Infrarouge Orsay). Comparison with quantum chemical calculations reveals that the IRMPD spectra are consistent with protonation in para and/or ortho position, which are the thermodynamically favored protonation sites. The lack of observation of protonation at the F substituent, when CH5 + is used as protonating agent, is attributed to the low-pressure conditions in the ICR cell where the ions are produced. Comparison of the C6H6F+ spectrum with IR spectra of C6H5F and C6H7 + reveals the effects of both protonation and H → F substitution on the structural properties of these fundamental aromatic molecules.