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Letter
Disentangling the Complex Vibrational Spectrum of the Protonated Water Trimer, H(HO), with Two-Color IR-IR Photodissociation of the Bare Ion and Anharmonic VSCF/VCI Theory +
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Chinh H. Duong, Olga Gorlova, Nan Yang, Patrick J. Kelleher, Qi Yu, Anne B. McCoy, Joel M Bowman, and Mark A. Johnson J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01599 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 25, 2017
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Disentangling the Complex Vibrational Spectrum of the Protonated Water Trimer, H+(H2O)3, with Two-Color IR-IR Photodissociation of the Bare Ion and Anharmonic VSCF/VCI Theory Chinh H. Duong,a,* Olga Gorlova,a,* Nan Yang,a Patrick J. Kellehera and Mark A. Johnsona,† a Sterling Chemistry Laboratory, Yale University, New Haven, CT 06520, USA † M. A. Johnson. Tel: +1 203 432 5226, Email:
[email protected] Anne B. McCoyb,§ b Department of Chemistry, University of Washington, Seattle, Washington 98195, USA § A. B. McCoy. Tel: +1 206 543 7464, Email:
[email protected] Qi Yu,c Joel M. Bowmanc,# c Department of Chemistry and Cherry L. Emerson Center for Computational Science, Emory University, Atlanta, GA 30322, USA # J. M. Bowman. Tel: +1 404 727 6592, Email:
[email protected] Abstract: Vibrational spectroscopy of the protonated water trimer provides a stringent constraint on the details of the potential energy surface (PES) and vibrational dynamics governing excess proton motion far from equilibrium. Here we report the linear spectrum of the cold, bare H+(H2O)3 ion using a two-color, IR-IR photofragmentation technique and follow the evolution of the bands with increasing temperature. The key low energy features are insensitive to both D2-tagging and internal energy. The D2-tagged D+(D2O)3 spectrum is reported for the first time, and the isotope dependence of the band pattern is surprisingly complex. These spectra are reproduced by large-scale vibrational configuration interaction calculations, based on a new full-dimensional PES, which treat the anharmonic effects arising from large amplitude motion. The results indicate such extensive mode mixing in both isotopologues that one should be cautious about assigning even the strongest features to particular motions, especially for the absorptions that occur close to the intramolecular bending mode of the water molecule.
1 *Contributed equally.
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The vibrational spectra of the small protonated water clusters, H+(H2O)n, obtained using the messenger “tagging” technique1 over the past three decades,2-28 have yielded a microscopic picture of excess proton speciation within H-bonded water networks. One aspect of these spectra that is currently under discussion is the degree to which even the cold cluster spectra reflect large amplitude motions in both the (zero-point) ground and OH stretching vibrationally excited 29 states. In this regard, the smallest clusters, n = 2 to 6, provide the most intimate view of the interplay between the usual spectroscopic properties of H-bonds (red-shifts and intensity enhancement) and the special features associated with excess proton accommodation (vibrationally driven, inter-molecular Figure 1. Vibrational predissociation spectra of (a) 3H-D2 and proton transfer29). Here we focus on (b) 3D-D2 with the free OH region of 3H-He on top, where the + + the protonated water trimer, 3H and 3D denote the H (H2O)3 and D (D2O)3 isotpologues and 3H-X denotes the complexes with X = He or D2. Colored lines H+(H2O)3, denoted 3H, with the correspond to analogous vibrational modes of the calculated structure displayed in the isotopologues. 3H-He presents the least perturbed position of top of Fig. 1. We are particularly the free hydronium OH (blue). Arrows represent the 57 interested in this cluster because it fundamentals of the bare hydronium umbrella (purple) and 35 34 represents an intermediate case bending (green) modes as well as that of the water bending mode (green). The isolated D2 stretch (pink) is taken from ref. between the n = 2, H+(H2O)2 Zundel 58. Band labels (an, bn) refer to features in the experimental ion,30 which features an equally spectra of 3H-D and 3D-D , respectively. ν and ν refer 2 2 shared proton between two water to the symmetric and antisymmetric stretches of the OH molecules in the global minimum groups on the flanking water molecules. The * band is typical structure, and the Eigen form31 of the for water molecules attached in a single H-bond acceptor motif n = 4 cluster in which the three water and arises from a combination band between the dangling OH(D) stretching fundamentals and the frustrated rotation of molecules largely play the role of the water molecule about the H-bond axis. The † feature is neutral ligands bound to the H3O+ unique to the 3H spectrum and demarcates the upper edge of core ion. As such, the protonated a continuum aborption whose origin is unknown. The trimer presents an interesting case minimum energy structure of 3H, computed at the where two protons in the hydronium CCSD(T)-f12/aVTZ level, is presented in two orientations. A complete list of band positions and assignments is presented in core bind strongly to the two flanking Table 1. water molecules. Note in Fig. 1 that 3 ACS Paragon Plus Environment
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the symmetry axes of these water molecules are bent slightly away from the O-O H-bonding axis by 12°, toward the pyramidal structure of hydronium (31°). Vibrational excitation of these H-bonded protons then occurs in a quasi-covalent regime, leading to a scenario where the ν = 1 OH stretch excited states adopt a partial inter-molecular proton transfer character as the shared protons approach the nearby water molecules. Such mechanics would, for example, be expected to result in strongly anharmonic effects in the vibrational spectra, and indeed, many features in the reported spectra are not recovered at the harmonic level, as indicated by the comparison in Fig. S5.22 We begin this study by extending the previous survey of the spectrum of 3H-Ar22, 32-33 to include the D2-tagged spectra of both the H+(H2O)3 and D+(D2O)3 clusters (denoted 3H-D2 and 3D-D2, respectively), with the results presented in Fig. 1a and 1b. The umbrella mode of the embedded hydronium ion (a11) sharpens considerably upon deuteration (b11), similar to the behaviors found in the 4H and 4D spectra by Wolke et al.12 There are also surprises, however, not anticipated by previous theoretical analysis, as we discuss further below. In particular, the strong band (a8) in the 3H spectrum, long assumed to be assigned to the antisymmetric stretch of the bound OH groups,22 is replaced by a strong doublet (b8a and b8b) in the 3D spectrum. In addition, both spectra display a series of transitions towards higher energies of the dominant transitions, while strong bands (a10 and b10) appear below the Figure 2. D2 vibrational predissociation spectra of (a) 3H-D2 and (f) 3Dintramolecular HOH bend D2 with their corresponding anharmonic calculations. Traces (b) and fundamentals for isolated H2O (e) correspond to 18-mode VSCF/VCI calculations on an updated PES (Ref. 34) and H3O+ (Ref. 35) (see text), which are compared to 17- and 18- mode calculations (d,c) (Fig. 1a green arrows) that are using the previously reported PES in ref. 38. Greek symbols (αn and βn) refer to significant features in the theoretical spectra after presently unassigned. This -1 convolution with a standard Gaussian with FWHM of 15 cm (see behavior is particularly significant Table 1). View Fig. 1 caption for experimental peak labeling scheme. as “extra” features near the 4 ACS Paragon Plus Environment
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bending mode have presented a long-standing puzzle in the spectra of the n = 3-6 clusters, where they have been associated with both the bend and the ionic H-bonded OH (IHB) modes based on calculations at the harmonic level with and without inclusion of anharmonicity with a perturbative (VPT2) treatment.5, 12, 15, 36-37 Very recently, high level vibrational self-consistent field methods combined with vibrational configuration interaction (VSCF/VCI) calculations have been carried out to map the extended potential energy and dipole moment surfaces associated with the 3H system.38 These were benchmarked by comparing the spectrum computed from these quantities, displayed in Fig. 2c, with the experimental 3H-Ar spectrum,22 which is very similar to that obtained here with D2-tagging. This computational approach indeed recovered the large (~400 cm-1) red-shifts in the H-bonded OH stretching fundamentals relative to the CCSD(T)-F12 harmonic values and indicated that, as expected, the dominant band (a8 in Fig. 2a) was primarily due to the antisymmetric stretch of the H-bonded OH groups.38 The progression of bands a5-a7 were then attributed to combination bands of the antisymmetric stretch with various soft modes, while the a10 feature was attributed to an exterior water bend that is strongly coupled to the H-bonded antisymmetric stretch. Given this relatively straightforward assignment scheme, however, the observation of a strong doublet (b8a and b8b) in the 3D isotopologue is unexpected (Fig. 2f), as anharmonic fundamentals in this energy range typically follow an approximate scaling of 1/1.23 upon deuteration.39-40 Such mass-dependent complexities can arise, however, when modes couple as they become close in energy (e.g., Fermi resonances41). To understand the behavior of the 3D-D2 spectrum, we extended these calculations to 3D using the same PES and dipole surfaces as those used for 3H, with the result presented in Fig. 2d. For these calculations, using the 2 old PES, 17 modes were used for Figure 3. Comparison of the IR MPD spectrum of bare 3H at 100 K (b) and 20 K (c) with 3H-D2 (d) and 3H-He (a). The computational efficiency. Calculations 2 level diagram of the IR MPD scheme used to collect the using smaller sets of modes indicate that bare spectra (described in the text), where the spectrum shown is reasonably hν2 = 2900 cm-1, is top left. Temperatures indicate converged, especially with regard to the measurements taken from sensors mounted directly to absence of the doublet feature seen the trap. Refer to Tables 1 and S1 for frequencies and band assignments. See Fig. 1 caption for labeling scheme. experimentally. Surprisingly, the 5 ACS Paragon Plus Environment
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calculated pattern is again rather simple, consistent with a dominant band arising largely from the antisymmetric H-bonded OD (IDB) stretches, in contrast to the dominant doublet character of the observed spectrum. We next address two possibilities for this discrepancy: perturbation by the D2 tag molecules and/or some issues with the 3-body hydronium-water-water (3-b h-ww) potential energy surface that was used in those VSCF/VCI calculations. Because high level calculations of the vibrational quantum structure can only be carried out on the bare protonated trimer, while the experimental results thus far have been reported with the messenger tagging (H2, Ar and He) technique where the tags could perturb the spectrum, we undertook a study to obtain the spectrum of the cold, bare 3H cluster. The main experimental challenge in obtaining a tag-free spectrum is that the binding energy of the trimer (to H+(H2O)2 + H2O) is about 7000 cm-1,42 which is much higher than the
