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 multi-photon dissociation (MPD) behavior, triggered by the absorption of tunable IR radiation from a free-electron laser 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 C3H7+ potential energy surface, namely, a chiral iC3H7+ ion of C2 symmetry and an asymmetric corner-protonated cyclopropane, cC3H7+. The significant features in the IRMPD spectra of both the unlabeled and the perdeuterated ions obtained by ionization and fragmentation of isobutane or 2-chloroD7]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 barrierless interconversion of the two C2 enantiomers.