IR spectroscopic features of gaseous C7H7O+ ions: benzylium versus tropylium ion structures.

Gaseous [C7H7O]+ ions have been formed by protonation of benzaldehyde or tropone (2,4,6-cycloheptatrienone) in the cell of an FT-ICR mass spectrometer using C2H5(+) as a Brønsted acid. The so-formed species have been assayed by infrared multiphoton dissociation (IRMPD) using the free electron laser (FEL) at the CLIO (Centre Laser Infrarouge Orsay) facility. The IRMPD features are quite distinct for ions from the two different precursors, pointing to two different isomers. A number of potential structures for [C7H7O]+ ions have been optimized at the B3LYP/6-31+G(d,p) level of theory, and their relative energies and IR spectra are reported. On this basis, the IRMPD spectra of [C7H7O]+ ions are found to display features characteristic of O-protonated species, with no evidence of any further skeletal rearrangements. The so-formed ions are thus hydroxy-substituted benzylium and tropylium ions, respectively, representative members of the benzylium/tropylium ion family. The IRMPD assay using the FEL laser light has allowed their unambiguous discrimination where other mass spectrometric techniques have yielded a less conclusive answer.