Infrared spectra and structures of diamantyl and triamantyl carbocations

Gas-phase infrared spectra have been recorded for the diamantyl and triamantyl carbocations by infrared multiple photon dissociation (IRMPD) spectroscopy using the free electron laser FELIX. Chemical ionization of the neutral parent diamondoid molecule in an rf ion trap is accompanied by H-atom loss, forming a stable closed-shell carbocation. Comparing the IRMPD spectra with harmonic DFT calculations permits to determine the structures of the species trapped in our set-up. Comparison of experimental and calculated spectra suggests that H-atom abstraction occurs on a tertiary carbon (CH group) rather than on a secondary carbon (CH2 group), as also indicated by the calculated relative energies of the various isomers. Combining experimental results and DFT calculations, we compare the spectra of neutral and carbocationic forms of adamantane, diamantane and triamantane. Substantial differences are observed between the IR spectra of neutral and ionic species. In the ions, the 3 mu m CH stretching modes become much weaker, while the other mid-IR modes strongly gain in intensity. An intense band centered at 1200 cm(-1) due to a CH/CH2 bending mode appears to be characteristic of the dehydrogenated cationic species. Finally, the intensities of the low frequency modes (