Signatures of Large-Amplitude Vibrations in the Spectra of H 5 + and D 5 + Zhou Lin and Anne B. McCoy* Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States * S Supporting Information ABSTRACT: 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. SECTION: Spectroscopy, Photochemistry, and Excited States H 5 + is a molecular ion that has been of continuing interest for experiment, theory, and observation since its first laboratory observation in 1962. 1 Much of this interest derived from observations of H 3 + in the interstellar medium. 2 From studies of the relative abundances of the ortho and para forms of H 3 + and H 2 in the interstellar medium, it was found that the rotational distributions of these two molecules predict different temperatures. 3,4 In addition, the relative abundances of the partially deuterated forms of H 3 + do not follow the natural abundance of deuterium. 5 On the basis of this, it seems that the low-temperature equilibrium abundances of various forms of H 3 + and H 2 are determined by more than their energies. The exchange of a proton or a deuteron between H 2 and H 3 + occurs through a bound H 5 + intermediate. The vibrational mode in H 5 + that is most closely related to the exchange of a proton between H 3 + and H 2 is the H 2 -H + -H 2 asymmetric stretch (R 1 - R 2 in Figure 1). Near the equilibrium geometry, shown in Figure 1, this corresponds to a chattering of the shared proton between the two H 2 groups, while at higher energy, it is better described as a large-amplitude vibration of a H 2 ·H 3 + complex. While H 5 + has not been identified in the interstellar medium, the results of several studies of the vibrational spectrum of this molecule have been reported. The earliest of these was a low- resolution action spectrum reported by Okumura, Yeh, and Lee. 6 In this experiment, H 3 + was detected following fragmentation of vibrationally excited H 5 + into H 3 + and H 2 . As such, the experiment was limited to transitions to states that are higher in energy than the dissociation threshold of H 5 + . On the basis of this work, three broad features were reported in the 3500-4350 cm -1 region. More recently, Duncan and co-workers revisited this spectrum at higher resolution and over a larger spectral range. 7 They also reported the spectrum for D 5 + . In addition to observing the three peaks in the spectrum for H 5 + that were reported by Lee and co-workers, they found a fourth peak at 2603 cm -1 . The spectrum of D 5 + consisted of a similar progression of three transitions, shifted to the red by the expected factor of roughly 2 -1/2 . The fourth peak was not observed for D 5 + as its energy is below the dissociation energy of D 5 + . Very recently, the lower-energy spectrum (below 2200 cm -1 ) was recorded using the FELIX free electron laser. 8 Again, infrared absorption was detected by monitoring H 3 + or D 3 + generation from dissociation of H 5 + or D 5 + , respectively. Because the energies that were probed in this study are well below the dissociation threshold of these ions, the signal reflects multiphoton processes. Consequently, sources of the intensities are more complicated than those for single-photon absorption experiments. Also, the bands may be broadened and their Received: November 1, 2012 Accepted: November 26, 2012 Figure 1. Equilibrium structure of H 5 + . The two H 2 groups are composed of atoms 1 and 2 and of atoms 4 and 5, while the central proton is atom 3. The two vectors R 1 and R 2 connect the central proton to the center of mass of one of the H 2 groups. The coordinates of interest in this study are R 1 and R 2 . Letter pubs.acs.org/JPCL © XXXX American Chemical Society 3690 dx.doi.org/10.1021/jz3017683 | J. Phys. Chem. Lett. 2012, 3, 3690-3696