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CO Hydration Shell Structure and Transformation Samual R Zukowski, Pavlin D. Mitev, Kersti Hermansson, and Dor Ben-Amotz J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 09 Jun 2017 Downloaded from http://pubs.acs.org on June 13, 2017
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CO2 Hydration Shell Structure and Transformation Samual R. Zukowski,1Pavlin D. Mitev ,2 Kersti Hermansson,2 and Dor Ben-Amotz*,1 1
Department of Chemistry, Purdue University, West Lafayette, IN 47907, United States Department of Chemistry-Ångström, Uppsala University, Box 538, S-75121 Uppsala, Sweden
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ABSTRACT: The hydration-shell of CO2 is characterized using Raman multivariate curve resolution (Raman-MCR) spectroscopy combined with ab initio molecular dynamics (AIMD) vibrational density of states simulations to validate our assignment of the experimentally observed high-frequency OH band to a weak hydrogen bond between water and CO2. Our results reveal that while the hydration-shell of CO2 is highly tetrahedral, it is also occasionally disrupted by the presence of entropically stabilized defects associated with the CO2-water hydrogen bond. Moreover, we find that the hydration-shell of CO2 undergoes a temperature dependent structural transformation to a highly disordered (less tetrahedral) structure, reminiscent of the transformation that takes place at higher temperatures around much larger oily molecules. The biological significance of the CO2 hydration shell structural transformation is suggested by the fact that it takes place near physiological temperatures.
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Although much of the carbon dioxide (CO2) on Earth is sequestered in the oceans and H2O/CO2 is widely used as a green organic synthesis medium,1 remarkably little is known regarding the structure of the hydration-shell surrounding CO2 dissolved in water. Here we report the first experimental confirmation that an exceptionally weak hydrogen bond (H-bond) is formed between water and CO2, whose assignment is corroborated by ab initio molecular dynamics (AIMD) simulations of the vibrational density of states of water surrounding CO2 performed in both H2O and D2O at ambient temperature and pressure. Our additional temperature dependent Raman multivariate curve resolution (Raman-MCR)2-5 measurements demonstrate that the H-bond between water and CO2 is entropically stabilized, and further reveal that the hydration shell of CO2 is remarkably fragile, as it undergoes a temperature dependent structural transformation near physiological temperatures, reminiscent of that which occurs at higher temperatures around larger oily molecules dissolved in water.3-4 Carbonated water contains predominantly molecular CO2, as less than 0.3% of the dissolved CO2 is converted to carbonic acid (H CO ), bicarbonate (HCO ), and carbonate (CO ) under ambient
conditions (see supporting information, SI).6-8 Recent microfluidic X-ray absorption measurements9 have detected hydrated CO2 produced by the reaction of aqueous NaHCO3 with HCl. Prior Raman10 and infrared11 vibrational spectroscopic measurements implied that CO2 interacts weakly with water, as the symmetric and asymmetric stretch vibrations of CO2 in water are close to those in the gas phase (see SI). Quantum,12-16 classical9, 17 and QM/MM18 calculations predicted that CO2 has little propensity to accept a hydrogen-bond (H-bond) from water and the hydration-shell of CO2 is reminiscent of that around small hydrophobic solutes. However, neither the formation of an H-bond between water and CO2 nor its hydrophobic hydration shell structure have previously been experimentally detected. Here we do so, and compare the temperature dependent Raman-MCR results obtained from aqueous solutions of CO2 and ethanol to reveal the exceptional fragility of the hydration-shell of CO2.
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Figure 1A shows the Raman spectrum of pure water (dashed blue), along with the Raman-MCR solute-correlated (SC) spectra of aqueous CO2 (blue) and ethanol (gray), all obtained at 20°C, and scaled to the same solute concentration. Note that SC spectra contain features arising from the solute itself as well as hydration-shell water molecules whose vibration spectrum is perturbed by the solute.2-5 The experimental hydration-shell spectra of CO2 and ethanol both contain a low frequency OH sub-band peaked near 3200 cm-1, which is more pronounced than in pure water, indicating that the hydration shells of these two solutes are more tetrahedrally ordered than pure water,3 as supported by the observation that both ice19 and clathrate hydrates,20 as well as cold liquid water,3 have an enhanced OH sub-band peaked at ~3200 cm-1. The hydration-shell spectra of CO2 and ethanol also both contain relatively sharp high frequency OH peaks with maxima at 3654 ± 1 cm-1 and 3667 ± 2 cm-1, respectively. These bands are similar to those of dangling (non-H-bonded) OH groups observed in the hydration-shells of other aqueous alcohol2, 2122 solutions, and at macroscopic air-water and oil-water interfaces.23-24 The high frequency OH band in the hydration shell of CO2 is both slightly red-shifted and significantly more intense than the dangling OH band in the hydration shell of ethanol, and thus is reminiscent an OH band associated with the formation of pi-H-bond between water and benzene,5 although the latter band has a somewhat lower frequency, ~3610 cm-1, and is about 3 times more intense than that in the hydrationshell of CO2 (see SI). Both the frequency and intensity of the high frequency OH stretch band around CO2 suggest that it arises from water OH groups that are weakly H-bonded to CO2, as corroborated by our AIMD simulation predictions (described below).
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(B) CO2 in D2O
(A) CO2 and Ethanol-d5 in H2O AIMD
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Figure 1. Raman-MCR and AIMD pure water and solute-correlated (SC) spectra in H2O and D2O. (A) Comparison of the hydration shell spectrum of CO2 (blue) with the hydration shell spectrum of ethanol-d5 (gray). Bulk water (dashed blue) is shown for reference. The inset panel in (A) shows predictions obtained using AIMD, including the vibrational band of pure water and the hydration-shell of CO2 obtained using the same MCR used to obtain the corresponding experimental SC spectra. (B) Comparison of experimental and AIMD SC spectra for CO2 dissolved in in D2O. (C)-(E) Temperature dependent SC spectra of aqueous ethanol (C) and carbon dioxide in H2O (D) and D2O (E), revealing the temperature-dependent structural transformation in the hydrationshell of CO2.
In order to critically test the assignment of the high frequency OH band we performed AIMD (as described in the Experimental and Theoretical Methods section). We chose to perform AIMD simulations rather than classical force-field simulations in order to more realistically represent the interaction between water and CO2. Our AIMD vibrational density of state predictions clearly reveal the formation of a high frequency OH feature arising from the hydration-shell of CO2, as shown in the inset panels of
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Figures 1A (in H2O) and 1B (in D2O). Both the experimental and AIMD hydration-shell spectra were obtained using exactly the same MCR procedure to extract the SC spectrum of CO2 from spectra of pure water and CO2 in water. Although our vibrational density of states predictions are not Raman-weighted, they are expected to accurately represent the population of water molecules that give rise to the experimentally observed high frequency OH band, since the Raman cross-section of a water OH stretch is not strongly dependent on H-bond strength.25 Note that theoretically reproducing the experimental Raman and infrared vibrational spectra of water and aqueous solutions remains a considerable challenge,26-28 although calculations of Raman spectra from AIMD simulations have recently been reported,28-30 including very recent pure water IR and Raman spectral predictions obtained from simulations that included both electronic and nuclear quantum effects and reproduced the experimental spectra very well.30 The latter results illustrate the rapid current progress in this area, setting the stage for future extensions of such calculations to Raman-weighted spectra of aqueous CO2. To further confirm our assignment of the high frequency OH band arising from the hydrationshell of CO2 we used the AIMD trajectories to identify water molecules whose H-atoms are within 2.5 Å of any CO2 atom. The vibrational density of states of these hydrogen atoms is shown in in Figure 2B (purple curves). In this analysis, the averaging was carried out over the (continuous) residence times of the hydrogen atoms in the selected region. The resulting narrow high-frequency OH vibrational band peaks at 3647±2 cm-1 is very near the experimental high frequency OH peak at 3654±1 cm-1. Moreover, the g(r) results shown in Figure 2A confirm that these H-atoms are primarily adjacent to the O-atoms (and not the C-atom) of CO2, thus definitively corroborating our assignment of the high frequency OH peak to water molecules that are H-bonded to CO2 (see Figure 2).
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Figure 2. Results from AIMD simulations. (A) The radial distribution of water H atoms around each of the CO2 atoms. (B) Three vibrational density of states spectra: pure water (dashed blue), the total aqueous CO2 solution (solid green), and the spectrum arising from only those H atoms that reside within 2.5 Å of any one of the CO2 atoms (purple). The spectra in (B) are all normalized to the same maximum OH stretch band intensity. Although the pure water and the total aqueous CO2 solution spectra look virtually identical, their difference (obtained using MCR) yields the SC spectrum shown in the inset panel of Figure 1A.
We have used our temperature dependent measurements (between 00C and 400C) to quantify the enthalpic and entropic contributions to the free energy of forming the high frequency OH structure in the hydration shell of CO2.2 More specifically, we consider the equilibrium between hydration shells with and without this high frequency feature, as illustrated schematically in Figure 3A. The area of the high frequency OH peak shown in Figure 3B is used to obtain the probability that the hydration-shell of CO2 contains such a high frequency structure, as previously described.2 Briefly, the observed area is converted to an average number, , of high frequency OH structures per CO2 hydration-shell, as follows, Ω 〈 〉 = Ω where ΩS/ΩOH is the Raman cross-section ratio of the solute CO (for CO2) or CD (for ethanol-d5) stretch bands to the water OH band of pure water (see SI), and IOH/IS is the measured ratio of the high frequency
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OH band area to the solute CO or CD area in the corresponding SC spectrum. A finite lattice statistical analysis31 is used to convert to the equilibrium constant, K, for the process illustrated in Figure 3A, as previously described,2 and confirm that K ~ whenever
