Infrared absorption features of gaseous isopropyl carbocations

C3H7+ ions were formed in the cell of a Fourier transform ion cyclotron resonance mass spectrometer and assayed by their multiphoton dissociation (MPD) behavior, triggered by the absorption of tunable IR radiation from a free-electron loser source providing a high fluence. The derived experimental IRMPD spectrum, which reflects the active vibrational modes of the ion, was compared with the IR spectra calculated for the optimized structures of the most-stable species on the CH,l potential energy surface, namely, a chiral iC(3)H(7)(+) ion of C-2 symmetry and an asymmetric corner-protonoted cyclopropone, cC(3)H(7)(+). The significant features in the IRMPD spectra of both the unlabeled and the perdeuterated ions obtained by ionization and fragmentation of isobutane or 2-chloro[D]propane confirm the presence of the isopropyl cation, the ground-state isomer, whose IR spectroscopic features can thus be comparatively checked in the gas phase and in condensed superacid media. Details of the IRMPD features are suggested to result from the nearly borrierless interconversion of the two C-2 enantiomers.