LETTER pubs.acs.org/JPCL
Hydroxide Hydrogen Bonding: Probing the Solvation Structure through Ultrafast Time Domain Raman Spectroscopy Ismael A. Heisler, Kamila Mazur, and Stephen R. Meech* School of Chemistry, University of East Anglia, Norwich NR4 7TJ, U.K.
bS Supporting Information ABSTRACT: The mechanism of charge transport in aqueous media is critical in molecular, materials, and life sciences. The structure of the solvated hydroxide ion has been an area of some controversy. Polarization-resolved ultrafast time domain polarizability relaxation is used here to resolve the terahertz frequency Raman spectrum of hydroxide solutions. The measurements reveal the totally symmetric hydrogen-bond stretching (HO 3 3 3 HOH) mode of the solvated hydroxide, permitting an experimental measurement of the bond force constant. The observed polarized Raman spectra are compared with those obtained from DFT calculations performed on HO(H2O)n clusters. Good agreement between the observed frequency and the polarization dependence is found for the n = 3 or 4 clusters, particularly for those in which the solvating water molecules adopt a planar structure. The frequency of the symmetric stretch increases with concentration, consistent with an effect of ionic strength on either the H-bond or the structure of the cluster. SECTION: Statistical Mechanics, Thermodynamics, Medium Effects
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harge transport is of fundamental importance in the chemistry of aqueous solutions and plays a crucial role in fuel cells and in a wide variety of cellular processes. The prototypical example of charge transport is provided by simple aqueous solutions of acids and bases. Such solutions have long been known to display anomalously high ion mobilities.1 For the case of proton transport, this was explained through the Grotthuss mechanism which allows for the transport of charge without the need for diffusion of the molecular ion.2 In this mechanism, rapid interconversion between H9O4þ and H2O 3 3 3 Hþ 3 3 3 OH2 complexes occurs through H-bond reorganization, leading to efficient charge transport with minimal structural change. The transport mechanism in aqueous hydroxide solutions is less well established and has been the subject of intense debate for a number of years. A historical perspective is included in a recent and comprehensive review.3 Initially, it was supposed that OH transport occurred as the mirror image of proton transport, the so-called proton hole mechanism.1,4 In this mechanism, H7O4 and H3O2 were assumed to be the dominant interconverting structures. In the past 10 years, this view has been challenged, on the basis that the solvation structure of OH is markedly different from that of Hþ, suggesting that an intrinsic asymmetry must exist between their respective transport mechanisms. Tuckerman and co-workers proposed an alternative “presolvation” picture in which the OH is hypercoordinated through H-bonding to four water molecules, with the hydroxide hydrogen atom only weakly and transiently bonded to a fifth water.5 Rupture of one of the four H-bonds allows completion of the charge transfer, again with minimum structural reorganization. This mechanism was shown to be consistent with a number of r 2011 American Chemical Society
quantum and molecular dynamics calculation.68 It has also been supported by neutron scattering experiments that suggest coordination numbers between 3.5 and 5.9,10 In addition, CarParrinello molecular dynamics (CPMD) calculations support the hypercoordination picture, suggesting a concentration and counteriondependent distribution of 35 water molecules bonded to the hydroxide oxygen.11,12 Such a distribution is consistent with the inhomogeneous distribution of H-bonded structures suggested by transient IR hole-burning experiments.13 Detailed structural information on the dominant species in hydroxide ion solutions should be accessible through vibrational spectroscopy. A number of measurements of IR and Raman spectra in the high-frequency OH stretch region have been reported.4,1316 The well-known broad high-frequency OH stretch of liquid water is broadened further on the low-energy side by increasing concentrations of OH, while the high-energy edge develops a weak shoulder in the IR that corresponds with an intense narrow band in Raman. These spectra have most frequently been discussed in terms of a stable H3O2 structure.4,15 In particular, the high-frequency band is taken to be characteristic of the non-H-bonded OH stretch, and the relative intensity in Raman and IR are indicative of the symmetry of the H3O2 anion.4 While it has been shown both in calculation and measurement that there is a degree of ambiguity in this assignment,17 it is striking that the principal spectroscopic measurements of the structure of the solvated hydroxide ion are at odds with the prevailing calculations and molecular dynamics Received: March 15, 2011 Accepted: April 20, 2011 Published: April 27, 2011 1155
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The Journal of Physical Chemistry Letters
LETTER
Figure 1. Time and polarization-resolved study of the polarizability relaxation for the three polarization conditions studied in both (a) D2O and (b) 10.7 M NaOD in D2O solution. The corresponding spectral densities and depolarization ratios (black lines) are shown in (c) for D2O and in (d) for a 10.7 M NaOD in D2O solution. For details and the meaning of Rijkl, see the Experimental Section and Supporting Information.
view of the transport mechanism. In this work, we apply ultrafast polarization-resolved time domain Raman spectroscopy to probe directly the very low frequency (Terahertz, THz) spectral region in which the relatively weak H-bond modes are expected to appear. This method has previously been shown to probe directly the H-bond structure of solvated halide anions.18 Here, measurements of hydrogen-bond spectra in aqueous solutions of alkali hydroxides are reported as a function of concentration and counterion and assigned on the basis of DFT calculations on solvated hydroxide ion clusters. In time domain measurements, a nonresonant ultrafast linearly polarized pump pulse polarizes the sample, the relaxation of which is measured by polarization analysis of a transmitted timedelayed probe pulse. In Figure 1a and b, polarization-resolved data for pure D2O are contrasted with measurements for a 10.7 M solution of NaOD in D2O. The polarizability relaxation in the time domain yields the sample’s Raman spectral density in the frequency domain via a Fourier transform; the frequency domain representations are shown in Figure 1c and d.19 These spectral densities are directly related to Raman spectra but are unaffected by the thermal effects that distort conventional THz Raman measurements. Thus, the complex oscillatory response observed in the time domain corresponds with Raman-active vibrational modes in the THz frequency range. Pairs of polarization-resolved measurements can be combined to yield separately the isotropic and anisotropic Raman spectra (Figure 1; see the Experimental Section and Supporting Information).20 The anisotropic response is comprised of contributions from depolarized Ramanactive vibrational modes, molecular orientational motion, and intermolecular relaxation. The isotropic response contains contributions from polarized Raman-active modes and some intermolecular effects.21 From these data, several pieces of information can be extracted, including the symmetry of the Raman-active
mode (from its polarization dependence), the wavenumber of the mode, the dependence of the wavenumber on the concentration and counterion, and the relaxation dynamics associated with reorganization of the H-bonded network in solution.1921 There is a striking difference between the responses of pure D2O and the NaOD solution. For NaOD, a temporal oscillation dominates in the isotropic response, while in bulk water, the response is largely anisotropic (Figure 1a and b), with only a very small isotropic component. The amplitude of the oscillation in the isotropic response of the NaOD solution grows linearly with solute concentration between 0 and 6 M (Figure S2, Supporting Information). When D2O is exchanged for H2O, the frequency of the mode shifts by 13 cm1 (see Supporting Information, Figures S3 and S2b). In contrast, when the mass of the cation is changed by a factor of 5, the frequency of the mode in relatively dilute solution shifts by 6 M), the frequency associated with NaOH is significantly higher than that for KOH. A linear extrapolation of the concentration dependence (Figure 3) yields the wavenumber of the HO 3 3 3 H2O mode at infinite dilution. Converting this measured frequency into a force constant for the H-bond, k, requires some assumptions. At the simplest level, a diatomic harmonic oscillator approximation treats OH and H2O as two H-bonded “atoms”. This yields k = 33 ( 2 N m1. Any more sophisticated analyses require additional parameters (Supporting Information).23 Approximating the structure as a linear triatomic O 3 3 3 H 3 3 3 OH2 yields k = 34 ( 2 N m1, while assuming a trigonal pyramid with three waters represented as mass 18 atoms H-bonded to the OH oxygen, HO(H2O)3, yields k = 31 ( 2 N m1. Evidently, the precise model does not greatly alter the H-bond force constant recovered. Conducting similar calculations for the waterwater H-bond stretch (∼180 cm1, Figure 1a) yields k = 15 ( 2 N m1, in good agreement with literature measurements, which lie in the range of k = 1020 N m1.24 Thus, the force constant for the HO 3 3 3 H2O mode is approximately a factor of 2 larger than that for the water 3 3 3 water H-bond and also larger than that for the H-bond formed between halide ions and water.18 The data of Figure 1 also yield structural information, through their polarization dependence. The strongly polarized (F < 0.2) response of the hydroxide solution shows that a totally symmetric mode is responsible for the observed oscillation. The experimental data have been compared with the mode frequency and depolarization ratio obtained in DFT calculations for a series of hydroxide ion water clusters. DFT calculations were performed for H3O2 and HO(H2O)n, where n = 16. For the H3O2, calculations were made for both the nonplanar equilibrium structure and with the condition of planarity imposed (in which case, one imaginary frequency was calculated, suggesting that the structure is a transition state between two minimumenergy nonplanar structures). For the HO(H2O)n clusters, properly minimized symmetrical structures were recovered for n = 3 and 4, with all of the water OH bonds pointing toward the hydroxide ion oxygen atom, as expected for a H-bond. The structures obtained here by DFT are consistent with the stable structures reported by Lee et al. at the MP2 level.25 Additional water molecules (n = 5, 6) H-bond to this first solvation shell, creating a set of less symmetric isomers. Examples are shown in 1157
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Figure 4. DFT calculated structures for two HO(H2O)n clusters, with the calculated Raman spectra. The isotropic Raman intensity is calculated by using the equation Iiso = [(3 4F)/(3(1 þ F))]/Itot, where the total Raman intensity, Itot, and depolarization ratio, F, are retrieved from the DFT calculations. (a) HO(H2O)4 with the displacements associated with the totally symmetric mode marked (arrows). The strongly polarized symmetric stretch at 333 cm1 is shown in red. (b) HO(H2O)5 isomer showing the more planar arrangement of the solvating water molecules for this particular structure. The totally symmetric mode has shifted to 278 cm1 (displacements indicated), and an additional polarized mode is calculated at 316 cm1.
Figure 4 and in Figure S7 of the Supporting Information. The hydrogen atom of the hydroxide ion does not form a H-bond with water until at least 17 water molecules are included in the calculation.26 Calculations for hydrated OH clusters are clearly not an exact model for condensed-phase measurements but do permit the determination of both IR and polarized Raman spectra and, critically for the present assignments, the associated Raman polarization. In an earlier study of solvated halide ions, DFT calculations on clusters reproduced the symmetry of the vibration, while the frequency calculated was typically within 10% of that determined experimentally in aqueous alkali halide solution.18 H3O2 has been proposed as a stable ground-state structure in hydroxide solutions, partly on the basis of the spectra of the OH stretch observed in IR absorption and Raman.4,15 The OH stretches calculated at 36003700 cm1 for H3O2 reproduce the observed weak IR, intense Raman transitions observed experimentally. However, in the low-wavenumber region (
