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An H/D Isotopic Substitution Study of the H5O2+‚Ar Vibrational Predissociation Spectra: Exploring the Putative Role of Fermi Resonances in the Bridging Proton Fundamentals† Laura R. McCunn, Joseph R. Roscioli, and Mark A. Johnson* Sterling Chemistry Laboratory, Yale UniVersity, P. O. Box 208107, New HaVen, Connecticut 06520
Anne B. McCoy* Department of Chemistry, The Ohio State UniVersity, Columbus, Ohio 43210 ReceiVed: July 6, 2007; In Final Form: August 10, 2007
To clarify the nature of the motions contributing to the observed multiplet structures in the low-energy (9001800 cm-1) vibrational spectrum of the H5O2+ “Zundel” ion, we report the evolution of its vibrational fingerprint with sequential H/D isotopic substitution in a predissociation study of the Ar complexes. Of particular interest is the D4HO2+ complex, which displays a single intense band in the vicinity of the asymmetric OHO stretch of the bridging proton, in contrast to the more complex multiplet observed for both H5O2+ and D5O2+ isotopologues. These intensity patterns are consistent with the recent assignment of the bridging proton band’s doublet in the H5O2+‚Ne spectrum to a 2 × 2 Fermi resonance interaction between the shared proton stretch and a complex background level primarily derived from the O-O stretch together with two quanta of the wagging vibration involving the pyramidal deformations of the flanking H2O groups (Vendrell, O.; Gatti, F.; Meyer, H.-D. Angew. Chem., Int. Ed. 2007, 46, 6918). In addition, the observed trends rule out assignment of the ∼1800 cm-1 feature in H5O2+ to a combination band of the bridging proton vibration with the O-O stretch, providing a secure foundation for the previously reported scheme that attributes this band to the out-of-phase intramolecular bending fundamental. The observed feature occurs at an unusually high energy for typical HOH bends, however, and we explore the participation of the bridging proton in these eigenstates by following how the calculated harmonic spectrum evolves when artificially large masses are assigned to the proton. The empirical assignments are supported by anharmonic estimates of the isotope shifts evaluated by the diffusion Monte Carlo method.
1. Introduction Many chemical and biological processes depend upon proton transfer in relatively hydrophobic environments.2 Developing an accurate molecular description of the speciation of an excess proton surrounded by a few water molecules immediately introduces the role of the H5O2+ “Zundel” ion,3 a robust species that has been observed in the condensed phase (solution and crystals) as well as isolated in the gas phase in the form of a cluster ion.4-11 Much of this literature has focused on the H5O2+ vibrational spectrum, which encodes both its structure and the extent of the coupling between the motions of the proton and those of the flanking water molecules. Of particular interest is the low-frequency region near 1000 cm-1 where we expect to find the vibrational fundamental associated with the bridging proton (νsp for shared proton vibration).12 Early efforts to observe this in both gas-phase and condensed media resulted in rather diffuse bands.7-9 The first two gas-phase studies in the lowfrequency region involved IR multiple photon dissociation spectroscopy (IRMPD), and while they both found a strong absorption near 1750 cm-1 and assigned it to the out-of-phase bends of the flanking water molecules, they recovered rather different patterns at lower energy. Indeed, these authors offer different assignments of the νsp fundamental, with Asmis et al.8 †
Part of the “James T. (Casey) Hynes Festschrift”. * Authors to whom correspondence should be addressed. E-mail:
[email protected] (M.A.J.);
[email protected] (A.B.M.).
attributing it to their band at 1317 cm-1 and Fridgen et al.9 tracing it to the lower-energy feature at 990 cm-1. Subsequent studies using rare gas (Ar and Ne)11,13 predissociation spectroscopy have yielded a dramatically simpler band structure, with a representative spectrum of the Ne complex displayed in the lower trace of Figure 1. Qualitatively, the low-energy bands in the Ne-tagged complex occur in two groups, near 1000 and 1750 cm-1, that were assigned to the νsp and intramolecular H2O bending fundamentals, respectively, in agreement with the scheme forwarded earlier by Fridgen and co-workers.9 Surprisingly, the simple doublet nature of the νsp transition has proven quite difficult to recover even with advanced theoretical methods.13 For example, treatment of the large-amplitude, anharmonic vibrational motion of the bare ion on a fulldimensional potential energy surface (using a vibrational configuration interaction approach with four mode coupling) did not reproduce this doublet.12 Very recent full-dimensional (15D) calculations by Vendrell et al.,1 on the other hand, achieved close agreement with the Ne predissociation spectrum by again considering the bare ion on the same Born-Oppenheimer potential surface.14 Their analysis traced the doublet to a Fermi resonance interaction involving the fundamental of the asymmetric stretching vibration of the bridging proton and a complicated background state (one quantum of O-O stretch and two quanta of -OH2 wag). Their assignment of the motions involved in the higher-energy doublet around 1800 cm-1 is less clear, and there is some ambiguity between the condensed phase
10.1021/jp075289m CCC: $40.75 © 2008 American Chemical Society Published on Web 10/05/2007
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Figure 1. Rare gas predissociation spectra of H5O2+ from 700 to 3800 cm-1. The argon predissociation spectrum (A) displays the spectral shifts and splittings that are due to symmetry breaking introduced by the Ar atom. The spectrum obtained via neon predissociation spectroscopy (B) is significantly less perturbed, exhibiting sharper peaks and a simpler overall pattern. The “νsp” label marks the bands associated with the asymmetric stretch of the bridging proton.
and isolated complex behavior regarding the relative roles of the flanking molecules’ HOH bends and a combination band involving the bridging hydrogen and O-O stretching vibrations.7 Because both the Fermi interaction and the internal bending motions should be sensitive to the mass of the hydrogen isotopes in various sites, we report here the results of an H/D isotopic substitution study of the H5O2+‚Ar complex in order to clarify the assignments of both the coarse band patterns and the fine structure within these features. 2. Experimental Section Vibrational spectra were acquired in an action mode using the rare gas predissociation or “messenger” method, implemented with a double-focusing tandem time-of-flight spectrometer described previously.15 H5O2+‚Ar clusters were formed by passing ∼3 atm of argon over a reservoir of H2O and supersonically expanding the mixture through a pulsed valve into the vacuum chamber. D5O2+‚Ar clusters were made similarly, using a reservoir of D2O. To make the mixed isotopologue, D4HO2+‚Ar, D2O was entrained into the H2O/Ar expansion through another pulsed valve. Nascent neutral clusters were ionized with a pulsed (20 µs width) counterpropagating 1 keV electron beam. Infrared light in the 2200-3800 cm-1 range was generated by a Laser Vision OPO/OPA system pumped by a Nd:YAG laser. For the 600-2700 cm-1 region, the 3 and 1.5 µm beams from the first parametric stage were mixed in AgGaSe2.16,17 Spectra were recorded in a linear action regime and normalized for laser power fluctuations. 3. Results and Discussion 3.1. Analysis of the High-Energy OH(D) Stretching Vibrations. Previous studies established that Ar perturbs the symmetrical structure of the isolated H5O2+ complex10,13 by attaching to one of the dangling H atoms in a quasi-linear bond. This arrangement causes the pattern of two high-energy OH stretching bands in the Ne spectrum (Figure 1B), arising from the in- and out-of-phase symmetric, νs, and asymmetric, νas, OH stretches on the two water molecules (see insets), to split
Figure 2. Argon predissociation spectra of isotopically substituted Zundel ion complexes in the 2200-3800 cm-1 region for (A) D5O2+, (B) D4HO2+, and (C) H5O2+. Bands labeled with an asterisk (/) in (B) are assigned to the shared-D isotopomer (D2O‚D+‚OHD). The bands due to the D2O‚H+‚OD2 isotopomer are empirically assignable, based upon their nearly identical positions in the perdeutero isotopologue. The “Ar” labels denote stretches of OH/OD bonds bound by an argon atom.
into four distinct features in H5O2+‚Ar (right side of Figure 1A). The Ar-bound OH group appears lowest in energy at 3521 cm-1, with its partner H atom oscillating at 3658 cm-1. The latter transition is shifted toward the uncoupled OH stretch in an isolated HOD molecule (3707 cm-1),18 and is situated between the unperturbed νs (3616 cm-1) and νas (3696 cm-1) bands from the free OH stretches on the remote, unsolvated water molecule. Note that we were obliged to carry out this isotopic study with the more perturbative Ar messenger because of complications introduced by mass degeneracies and isotope congestion due to 20Ne and 22Ne. Figure 2 presents the Ar predissociation spectra for three isotopically substituted Zundel ions in the higher-energy range (2200-3800 cm-1) corresponding to the stretching vibrations of free or weakly bound (to an argon atom) OH and OD groups. The evolution of the bands in the isotopologues allows empirical assignment of band locations to transitions derived from H2O, HOD, and D2O in either free or Ar-bound sites, as these positions are reasonably conserved over the series. Note that the D5O2+ spectrum (Figure 2A) displays a pattern of four bands very similar to those found for H5O2+ (Figure 2C), but shifted to lower energy as expected for an approximate (mH/mD)1/2 mass scaling of vibrations involving primarily hydrogen displacements. Among the mixed isotopologues, we focus on the D4HO2+ system both because the fractionation of the light H-atom favors
H5O2+‚Ar Vibrational Predissociation Spectra the interior position (by 170 cm-1)19 and because this arrangement most cleanly addresses the extent of coupling between the bridging proton and the motions of the exterior hydrogen atoms by providing maximal mass asymmetry between the two positions. We have investigated several other isotopologues, but those results are complicated by the presence of two classes of isomers: one derived from unfavorable H/D isotope fractionation at the bridging site and the other giving rise to different spectral patterns according to whether the Ar atom attaches to dangling H or D atoms when both are present. The D4HO2+ stretching spectrum (Figure 2B) is dominated by the intrinsically weaker OD bands,20 indicating that the majority of clusters appear in the D2O‚H+‚OD2 form. This preference occurs in spite of the 4:1 statistical factor that favors H occupation of an exterior site, and is undoubtedly a consequence of its lower zero-point energy, as discussed at length by Devlin et al.19 Conceptually, the energy difference between isotopomers arises because the zero-point energies will be decreased the most when deuterium atoms occupy the higherfrequency (tighter) bonding positions, and at finite temperature, the Boltzmann weighting will favor the isotopologue with the lower zero-point energy. The bridging H-atom arrangement quantitatively accounts for the four observed OD stretching bands of the Ar-solvated D4HO2+ complex, as they appear at almost identical locations and with intensities similar to those in the perdeutero species (Figure 2A). There is, however, a minor contribution from a dangling OH group at 3653 cm-1 in the D4HO2+ spectrum (labeled with an asterisk (/)), signaling the presence of the higher-energy D2O‚ D+‚OHD isomer. Note that the companion ODHOD stretches at 2598 and 2688 cm-1, corresponding to the Ar-bound and free OD (each marked with an asterisk (/) in Figure 2B), appear as sharp shoulders near the bands for the Ar-bound and the free OD components of an Ar-bound D2O. 3.2. Isotope Dependence of the Lower-Energy Bands. The evolution of the low-energy bands upon deuteration is displayed in Figure 3, which establishes that the two prominent H5O2+ absorptions near 1000 and 1750 cm-1 significantly red-shift upon deuteration. This observation is in qualitative agreement with Devlin’s Fourier transform infrared spectra of the crystalline HBr dihydrate.19 There are notable differences, however, as the condensed-phase spectra display a broad peak at 760 cm-1 that emerges with increasing D-enrichment which is significantly higher in energy than the sharper 708 cm-1 peak in the D5O2+‚ Ar predissociation spectrum (Figure 3A). To gauge the magnitude of the expected red-shifts in the shared proton fundamental upon deuteration in this anharmonic system, we turned to the diffusion Monte Carlo (DMC) method to identify the approximate locations of the V ) 1 levels in the fixed node approximation. This method was applied to H5O2+ in earlier studies,13,21-24 and is extended to the other isotopologues here using the same potential surface.14 It is worth noting at the outset that the lowest energy bands in D5O2+ and H5O2+, which are generally thought to derive from the fundamentals corresponding to the asymmetric stretch of the bridging hydrogen mostly parallel to the O-O axis (νsp), do red-shift by roughly 1/(21/2), indicating that the overall anharmonicities are not pathological in a qualitative sense. The DMC results for the expected νsp fundamentals are included in Table 1 and indicated by arrows in Figure 3. Note that, at this level of theory, the νsp fundamental is quite similar in H5O2+ and D4HO2+ (995 ( 5 vs 990 ( 5 cm-1, respectively), suggesting that the bridging proton vibration is not, in fact, dramatically impacted by the masses of the dangling hydrogen atoms. This DMC approach
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Figure 3. Argon predissociation spectra of isotopically substituted Zundel ion complexes in the 600-2000 cm-1 region: (A) D5O2+, (B) D4HO2+, and (C) H5O2+. Bands labeled with an asterisk (/) in (B) are assigned to the shared-D isotopomer (D2O‚D+‚OHD), while the intense transitions near 950 and 1250 cm-1 are associated with the more prevalent D2O‚H+‚OD2 isotopomer. The “νsp” labels mark the bands associated with the asymmetric stretch of the bridging proton or deuteron. The solid arrows indicate νsp frequencies calculated by the diffusion Monte Carlo (DMC) method.
does not (and should not) recover the fine structure splittings. As we are employing a fixed node DMC simulation, the calculated energies should correspond to the centroids of the observed multiplets, thus supporting their assignments to the νsp fundamentals. In the context of the strong multiplet character of the νspbased bands in H5O2+, it is useful to emphasize that the D5O2+ activity close to the calculated νsp value also appears as a strong doublet, with a splitting (110 cm-1) similar to that observed in the H5O2+‚Ne spectrum (120 cm-1; see Figure 1). Interestingly, the intensity distribution of this doublet is inverted relative to that in the H5O2+ spectrum such that the higher-energy component is now weaker. The noise in the D4HO2+ spectrum is somewhat larger than that in the homogeneous isotopologues, but features associated with both the shared D and H isomers are evident, as expected from the appearance of both isomers in the higher-energy stretching bands (Figure 2). Note that both νsp fundamentals in the D2O‚D+‚OHD and D2O‚H+‚OD2 isomers appear as single features in contrast to the close multiplet structures observed in the homogeneous isotopologues. The relative magnitudes of the red-shifts of the two main bands (i.e., 1000 and 1750 cm-1 in H5O2+) in going from H5O2+ to D5O2+ are such that the gap between them closes by roughly 1/(21/2) (750 vs 530 cm-1 for H and D, respectively). This
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TABLE 1: Calculated (DMC) and Experimental (Ar-Tagged) Frequencies (in cm-1) for the Asymmetric Stretch of the Bridging H(D) in Zundel Ion Isotopomers outer atoms
shared atom
ZPE
fundamental (DMC)
experimental
H4 H4 H3D HD3 D4 D4
H D H D H D
12 393 11 837 11 721 9 821 9 706 9 143
995 686 1000 671 990 669
941/1060 768 964/1047 722 930 708/817
TABLE 2: Calculated Harmonic Frequencies (in cm-1) and Intensities of the H2O/D2O Bends, where X Refers to a Bridging Nucleus with Z ) +1 Charge and an Effectively Infinite Mass (mX ) 1010 amu in this case) H5O2+ XH4O2+ D5O2+ XD4O2+
out-of-phase bend
intensity
1769.74 1669.94 1298.38 1231.82
137.5495 34.0019 79.2703 20.7044
immediately rules out the assignment of the upper feature to a combination band based on the O-O stretch and νsp, because this scheme would result in a similar spacing for both isotopes due to the relatively small effect of the oxygens’ substituent masses on this motion (
