Shared-Proton Mode Lights up the Infrared Spectrum of Fluxional Cations H5+ and D5+

We report experimental and calculated infrared spectra of the highly fluxional cations H-5(+) and D-5(+). These cations have been postulated to exist in the interstellar medium and to play a central role in the deuterium fractionation. The experiments produce these ions in a pulsed discharge supersonic nozzle ion source and utilize, mass-selected photodissociation spectroscopy in the 2000-4500 cm(-1) region. Vibrational bands apparently broadened by rapid predissociation are detected throughout this region for both isotopologues. The calculated spectra make use of an ab initio potential energy surface and a new dipole moment surface and are based on results from fixed node quantum diffusion Monte Carlo and variational vibrational calculations. The successful assignment of the experimental spectra requires a proper treatment of the delocalized anharmonic shared-proton mode and indicates a major breakdown of the harmonic approximation. Several calculated, intense spectral features associated with this mode in the far infrared region could guide future observational searches of these cations.