Signatures of Large-Amplitude Vibrations in the Spectra of H5+ and D5+

H-5(+) is a weakly bound molecular ion, which is formed from the reaction of H-3(+) and H-2 and that has a very rich vibrational spectrum. In this work, diffusion Monte Carlo (DMC) approaches are used to explore the nature of vibrationally excited states of the proton-transfer mode in H-5(+). On the basis of these calculations, alternative assignments of the recently reported infrared multiphoton dissociation spectra of H-5(+) and D-5(+) [J. Phys. Chem. Lett. 2012, 3, 3160-3166] are suggested. In the proposed assignments, progressions of transitions in the proton-transfer mode with up to nine quanta of excitation are invoked. Reduced dimensional calculations of the spectra of H-5(+) and D-5(+). are used to provide an understanding of why such high overtones should be observable through absorption spectroscopy. Implications of how excitations of this mode can provide insights into the H-3(+) + H-2 reaction are also discussed.