Letter pubs.acs.org/JPCL
Unveiling the Janus-Like Properties of OH− Yanier Crespo* and Ali Hassanali* The Abdus Salam ICTP, Strada Costiera 11, I-34151 Trieste, Italy ABSTRACT: Using ab initio simulations, we explore the glassy landscape of the OH−(H2O)20 cluster and its infrared spectrum. We show that the OH− has an amphiphilic Janus-type behavior like the hydronium ion induced by the ability of its O−H bond to be buried inside of the cluster or exposed at the surface with different coordination numbers. Recent infrared experiments of aqueous NaOH have found two pronounced peaks at 2000 and 2850 cm−1 [Mandal, A.; et al. J. Chem. Phys. 2014, 140, 1−12]. The microscopic origins of these spectral features remain elusive. Herein, we disentangle the contribution of the spectra between 1700 and 3000 cm−1 in terms of the microscopic solvation structure of OH− and dub this as the amphiphilic band. The delocalized nature of OH− results in a red shift to the O−H stretch, which mixes with bend-vibrations, the extent to which is tuned by the local coordination number. These results have important bearing on understanding the spectroscopic signatures of OH− in environments like the air−water interface. ater’s constituent anion, the hydroxide (OH−), is created by autolysis, a rare but rather important event in liquid water.2−4 Besides its obvious fundamental role in the dynamic equilibrium between neutral and ionized water, which determines the pH, the OH− is an active participant in biochemical processes such as RNA catalysis.5 There is currently an ongoing raging debate regarding the pH of the surface of water. The two competing hypotheses based on different experimental and computational studies advocate contradictory propensities for protons and hydroxide ions at the air−water interface.6−12 At the heart of this controversy is a fundamental gap in our knowledge of the structure and dynamics of water’s constituent ions at the surface of water and the lack of experimental probes that can unambiguously probe the presence of these ions at the interface. The OH− has been the subject of numerous experimental and theoretical studies in both bulk and cluster solvent environments, leading to rather disparate conclusions of both the structural properties of its solvation shell as well as its mobility in liquid water.13−17 Central to this is the question of the ability of OH− to accept or donate hydrogen bonds (HBs). The discussion has typically revolved around whether the ion accepts three or four HBs and if in each of those cases, it donates a weak HB to a surrounding water molecule. Early studies using vibrational spectroscopy of gas-phase hydroxide clusters found that the OH− could not act as a weak HB donor, leading to a low-coordinated species that accept three HBs.18 On the other hand, studies of the anion in bulk water using both neutron and X-ray diffraction indicated the presence of a hypercoordinated species that could in fact be coordinated to five water molecules.19 These results were later reinforced by core-level electron spectroscopy that found that the OH− that accepts four HBs could in fact also donate a weak HB.20 It turns out that the structure of the local solvation environment of the OH− sensitively affects its mobility in bulk water.16 In this regard, identifying and disentangling specific solvation structures around the ion and their coupling
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© 2014 American Chemical Society
to spectroscopic signals becomes quite a daunting task. A common strategy used to circumvent these issues is to focus on smaller water clusters.1,17 In a recent study, for example, Tokmakoff and co-workers have used IR spectroscopy to examine the vibrational spectrum of aqueous NaOH, where they found a broad continuum of absorption occurring between 1700 and 3000 cm−1 with the growth of two characteristic peaks at 2850 and 2000 cm−1.1 By performing a normal-mode analysis on a 17 water molecule cluster with an additional hydroxide ion and focusing on modes near the ion, Tokmakoff and co-workers assigned the shoulder at 2850 cm−1 to the asymmetric stretch vibrations involving both three- and fourcoordinated species but did not make any specific interpretations to the peak at 2000 cm−1. Chandra and co-workers have also examined the solvation and vibrational properties of hydroxide ion clusters and have found several solvation structures for the hydroxide ion.17 Although these previous studies have been instructive, we will see shortly that the sampling of the HB network in these small clusters is critical for interpreting quantities such as the IR spectrum. In this work, we use AIMD to elucidate the amphiphilic character of the OH− induced by its ability to accept and donate different combinations of HBs in a cluster environment. To explore the glassy landscape of the OH−(H2O)20 cluster, we use the enhanced sampling protocol, metadynamics,21 to provide an exhaustive sampling of the HB network and subsequently the solvation landscape of the OH−. An analysis of the IR spectrum using phonon and linear response theory shows that both buried and surface states as well as rather disparate local solvation environments contribute to the broad distribution of IR-active modes between 1700 and 3000 cm−1. This structural heterogeneity gives the hydroxide ion an amphiphilic character very similar to its partner counterion, Received: October 29, 2014 Accepted: December 9, 2014 Published: December 9, 2014 272
DOI: 10.1021/jz502286b J. Phys. Chem. Lett. 2015, 6, 272−278
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The Journal of Physical Chemistry Letters the hydronium.22,23 Water’s constituent ions, the hydronium and the hydroxide, thus appear to be the smallest possible Janus-like molecules. The structural aspects of the amphiphilic character are not sensitive to the system size, exact exchange, or inclusion of nuclear quantum effects. AIMD simulations of the OH−(H2O)20 cluster were conducted using Quickstep, which is part of the CP2K package.24 In these calculations, ab initio Born−Oppenheimer molecular dynamics was used for propagation of the classical nuclei. A convergence criterion of 5 × 10−7 au was used for the optimization of the wave function. Using the Gaussian and plane waves method, the wave function was expanded in the Gaussian TZVP basis set. An auxiliary basis set of plane waves was used to expand the electron density up to a cutoff of 300 Ry. For the simulations, we used the BLYP gradient correction25 to the local density approximation and Goedecker−Teter−Hutter pseudopotentials for treating the core electrons.26 The G3 Grimme dispersion corrections for the Van-der-Waals interactions were used.27 All systems simulated consist of a cubic box of side length 15.0 Å. We have checked that our results are not sensitive to moving to a larger box. The finite temperature simulations were conducted within the NVT ensemble using the canonical sampling velocity-rescaling thermostat.28 The NVT simulations were run for ∼13−15 ps for temperatures of 50 and 200 K, which were used to extract the structural and energetic properties reported later. The infrared spectra were computed from linear response theory using the utilities implemented in the TRAVIS code,29,30 where the simulations were performed using energy-conserving dynamics in the NVE ensemble. These NVE simulations were run for ∼40−60 ps. Exploring the glassy landscape of the hydroxide cluster requires the use of sophisticated sampling techniques.31−33 To achieve this, we performed metadynamics simulations,21 a mature methodology that has been successfully used in sampling systems in aqueous environments.34−36 The collective variable (CV) used for this was the coordination number (CN) of the hydroxide ion with respect to HBs that it can accept or donate. Here, a specific water molecule is tagged and constrained to have the geometry of a OH−. The CN of this species was defined using a Fermi function of the following form: (1 − [r/r0]NN)/(1 − [r/r0]ND), where r0 = 2.5, NN = 16, and ND = 56 with respect to all of the hydrogens in the system to constrain the geometry of the hydroxide to 1 and r0 = 6.5, NN = 6, and ND = 12 with respect to all of the oxygens in the system to bias the solvation environment. A Gaussian hill with a height of 2.0 × 10−4 kcal/mol was added every 10 MD steps. The total simulation time for the metadynamics run was 50 ps, during which the hydroxide ion explored a wide range of solvation environments. The hydroxide ion was identified by determining the oxygen atom that had the lowest CN based on a criterion that was established in an earlier study.4 This CV allows us to explore many different HB network environments of the hydroxide ion. We also performed a phonon analysis of the motifs extracted from our simulations (see Figure 1) using density functional perturbation theory.37 Density functional calculations were performed employing plane wave expansion, the BLYP generalized gradient approximation, and norm-conserving pseudopotentials38 as implemented in the Quantum Espresso code.39 Energy convergence was achieved for cutoff energies of 80 and 320 Ry for the wave functions and charge density respectively as well as a (2 × 2 × 2) grid of k-points generated
Figure 1. Six motifs illustrating the amphiphilic character of the OH− ion: (a) 3A0DS, (b) 4A0DS, (c) 4A0DB, (d) 4A1DB, (e) 5A0DS, and (f) 5A1DB. As a reminder, the number in front of A refers to the number of HBs that the hydroxide accepts, while the number in front of D refers to whether it donates a HB or not. Finally, the B and the S describe whether it is a buried or a surface state.
with the Monkhorst and Pack method.40 Atomic relaxations were performed until forces were smaller than 10−5 au. In addition to the charged clusters, we also performed a vibrational analysis of a neutral water cluster consisting of 21 water molecules. It is well appreciated that standard GGA functionals without dispersion corrections are known to overstructure water compared to the current best-available experiments on the structure of water. There is currently an ongoing debate on the possible origins of these effects.41 In particular, the role of dispersion, exact exchange, and nuclear quantum effects have all been identified as possible origins of this discrepancy. Ongoing investigations indicate that one of the key observations, namely, the amphiphilic character of the hydroxide, is not sensitive to these factors. During our metadynamics simulations, the hydroxide ion explores a broad range of solvation environments with varying propensities for the surface and in its ability to accept or donate HBs. Due to the relatively short simulation times of the AIMD, we cannot construct an accurate free-energy surface. In this work, we use metadynamics as a sampling technique that allows us to generate many configurations with different solvation environments. These are then subsequently used as initial conditions to explore whether they belong to local minima on the potential energy landscape (PEL) at 0 K and also their stability at finite temperature (50−200 K). In order to identify the stable minima on the PEL and to isolate distinct solvation structures, we sampled ∼200 initial configurations from the metadynamics simulation and subsequently performed 0 K optimizations. The resulting PEL is characterized by the OH− interpolating between a wide distribution of CNs resulting from variation in its ability to donate or accept HBs. See Figure 1 and the insets of Figure 2a and b. The small size of the cluster implies that the OH− is neither truly bulk or surface-like but nonetheless takes on a rich variety of combinations involving a different number of donated (D) or accepted (A) HBs. To simplify our analysis, we will now focus our attention on five stable structures of the PEL that are characterized by unique solvation structures. These five motifs are illustrated in Figure 1. In all of these situations, besides the heterogeneity in the number of HBs in which it participates, the O−H bond can orient itself so that it is either buried (B) or 273
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Figure 2. Running CNs as obtained from the radial integral of the OH−−O radial distribution function for the five motifs at 50 (left panel) and 200 K (right panel). The insets in both the left and right panels illustrate the distribution in potential energies for each of the motifs at 50 and 200 K, respectively. The color codes used here correspond to 3A0DS (black circles), 4A0DS (violet stars), 4A0DB (red squares), 4A1DB (blue diamonds), and 5A1DB (green triangles).
that confers its stability in different environments.43,44 A visual inspection of the 3A0DS structure in the work by Chandra indicates a difference in the number of rings in which that the hydroxide ion participates. This indeed highlights again the importance of the sampling that we achieve with our protocol described earlier. For temperatures below 150 K, all five configurations retain their characteristic solvation structures without transforming into any other configurations. This is confirmed in Figure 2a, which shows the CN, defined as the radial integral of the OH−−O radial distribution function. Within the first 3.5 Å, the CN can range between 3 and 6. In Figure 2b, the CNs are shown for the simulations at 200 K, where we begin to see some structural transformations for example between the 4A1DB and 5A1DB motifs. During one of our NVE runs, the 4A0DS motif converted into another stable structure where the OH− accepts five HBs with the O−H bond oriented toward the vacuum, 5A0DS (see Figure 1d). This structure shares very similar properties with the 5A1DB motif and is hence not analyzed separately here for brevity. Regardless of these structural transformations, it is clear that even at these high temperatures (200 K), the hydroxide ion still retains its amphiphilic character. In Figure 2b, we see that despite the effects of enhanced thermal fluctuations, there are still distinct motifs that maintain the hydrogen bonding patterns at 50 K. The amphiphilic properties of the hydroxide ion observed in our simulations mirror those obtained for the proton in its variability to donate or accept HBs and to seek buried versus surface states.22,23,45,46 The proton is a weak acceptor of a HB, and hence, it has been found to be stabilized near hydrophobic interfaces where it less likely to accept any HBs. On the other hand, for the hydroxide, it is the donating side of its O−H bond that tends to be more hydrophobic and can hence be stabilized by orienting its O−H bond toward the vacuum, for example, in surface states like 3A0DS, 4A0DS, and 5A0DS. In these cluster environments, the hydroxide tends to be rather promiscuous, and the donating side can also orient so that it points to water molecules in the center of cluster and at the same time accept four or even five HBs. These properties make water’s constituent ions possibly the smallest molecules exhibiting Janus-like character. Having established that the hydroxide ion in the cluster environment has structural traits giving it an amphiphilic
more surface-like and pointing to the vacuum (S). These features give the ion its amphiphilic character. These five configurations, as illustrated in Figure 1, include situations where the hydroxide ion (a) accepts three and donates zero HBs (3A0DS), (b) accepts four and donates zero HBs in an S state (4A0DS), (c) accepts four and donates zero HBs in a B state (4A0DB), (d) accepts four and donates one HB (4A1DB), and finally (e) accepts five and donates one HB (5A1DB). Each of these motifs involves a distinct 3D network topology that is quite challenging to sample and hence unexplored in previous simulations.1,17 In particular, each motif is characterized by a unique signature in both the number and size of rings that thread the hydroxide ion as well as the type of directional correlations that exist within the closed rings (see Figure 1). As noted in an earlier study, the hydroxide ion is best seen as a topological defect in the HB network.42 Although the motifs identified in Figure 1 represent local minima on the PEL, we wanted to ascertain their stability with respect to finite temperature. Our simulations ranging from 50 to 200 K all confirm that these structural motifs do in fact represent stable structures of the cluster. The insets of Figure 2a and b illustrate the distributions in potential energy for each of the five configurations for the simulations at 50 and 200 K, respectively. The color codes used are described in the caption of Figure 2. The peaks for each motif are very well separated at 50 K, and as we move up in temperature, the distributions tend to merge. In all cases, the distributions in potential energy exhibit large fluctuations with significant overlap from rather disparate solvation structures. A rather curious case is the comparison between 4A1DB and 3A0DS. At 50 K, 4A1DB and 3A0DS are the lowest in potential energy compared to the other configurations. This result is rather striking as the two motifs are structurally very different and the O−H bond is oriented in opposite directions. Our results should be contrasted with that observed by Chandra and co-workers who found that the 3A0DS was over 20 kcal/mol higher in energy than their 4A1DB structure.17 Previous studies have found that the hydroxide ion can be partially stabilized at the air−water interface by undergoing a decrease in the CN.43,44 The similarity in the energetics of 3A0DS and 4A1DB at 50 and 200 K and the change in the strength of the HBs being donated to the hydroxide, as we will show later, provide a molecular mechanism that takes advantage of its amphiphilic character 274
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Figure 3. Calculated IR spectrum at 0 K using a bin width of 40 cm−1 for (a) neat water cluster in black and the cluster with a hydroxide ion in red and (bthe ) spectrum for individual motifs shown between 1500 and 3400 cm−1. The inset shows more clearly the mixture of bands between 2600 and 3000 cm−1. Color codes adopted here are the following: 3A0DS (black and referred to as 3AS), 4A0DB (red and referred to as 4AC), 4A0DS (dashed violet and referred to as 4AS), 4A1DB (dotted blue), and finally 5A1DB (green).
Figure 4. Linear response theory IR spectra (a) between 1500 and 3200 cm−1 and (b) between 3400 and 3800 cm−1. Color codes adopted here are the following: 3A0DS (black), 4A0DB (red), 4A0DS (dashed violet), 4A1DB (dotted blue), and finally 5A1DB (green). The spectra for the neutral water cluster is shown with black dashed lines.
differences. A visual inspection of the normal modes at all of these frequencies for each of the motifs between 2000 and 3000 cm−1 shows that the main contribution comes from O−H stretch mode of the strongest HB that is being donated to the hydroxide ion. Because the hydroxide ion is capable of delocalizing along the HBs in the network, we suspected that both thermal broadening and anharmonic effects could play an important role in tuning the spectra. Figure 4 illustrates the IR spectra obtained from LRD for neutral water and the five motifs. Although many of the qualitative features such as the relative peak positions of the various motifs with respect to each other are conserved, the PH picture differs in several aspects from the LRD spectrum. First, many of the modes associated with the hydroxide in the range of 2000−3000 cm−1 are more red shifted. In fact, for 3A0DS and 4A0DB, the isolated peaks observed in the PHs at 2060 and 2230 cm−1, respectively, merge with the bend-vibration peak of water. The origin of these features will be explored in more detail later in this section. Furthermore, the peak associated with 4A0DS is significantly red-shifted in LRD compared to the PH, giving it a unique signature in the IR spectrum between 2300 and 2500 cm−1. Once again, the regime between 2600 and 3000 cm−1
character, we can now move on to elucidate how these features are manifested in the infrared spectrum. We compared the IR spectrum using an analysis of the phonons (PHs) as well as that obtained from linear response theory using the Fourier transform of the dipole−dipole autocorrelation function (LRD). We begin by examining the spectrum obtained using a purely harmonic approximation. Figure 3a compares the PHs of a neutral water cluster in solid black and to the average over all motifs in dashed red. Consistent with a recent study,1 we observe the presence of several peaks in the region between 2000 and 3000 cm−1 with the hydroxide ion but not in neutral water. In order to better understand the origin of the differences between neat water and the solvated hydroxide ion, Figure 3b shows the IR spectra obtained for the motifs separately. Here, we see clearly that the peak at around 2060 cm−1 is uniquely associated with 3A0DS, while the peak at 2230 cm−1 originates from 4A0DB. We will see shortly that these modes eventually merge with the bend-vibrations of water with LRD. The third peak between 2600 and 2900 cm−1 gets more complicated to interpret because it has contributions coming from all of the motifs. These hypercoordinated species 5A1DB and 5A0DS are curiously very similar to the IR spectrum of neat water despite the obvious electronic and structural 275
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inquiry. Compared to neat water, which also exhibits a peak at 3750 cm−1 coming from the dangling O−H bonds of the water molecules on the surface, the region between 3550 and 3700 cm−1 is uniquely associated with the O−H stretch of the hydroxide ion. Apart from 3A0DS, all other motifs have active modes in this region to varying extents, and there are less obvious correlations here compared to the amphiphilic band. All in all, our results unequivocally illustrate the challenge and complexity involved in the interpretation and correlation of features of the IR spectrum with particular solvation patterns of the hydroxide ion. As we move from 1700 to 3000 cm−1, the overall trend is a decrease in the strength of a HB that is being donated to the hydroxide ion that comes from a wide variety of solvation structures with different local network topologies around the hydroxide ion. In each of these cases, the hydroxide ion can take on both hydrophobic and hydrophilic character. The broad continuum of IR-active modes between 1700 and 3000 cm−1 thus represents the amphiphilic band of the hydroxide ion. We have shown here that an extensive sampling of the cluster is required to explore its glassy landscape, which has important implications on the computed IR spectrum and its subsequent interpretation. Among the motifs that were discovered over the course of our simulations, three of them have a clear distinct signature along the amphiphilic band. These include 3A0DS, 4A0DB, and 4A0DS between ∼1700 and 2400 cm−1, which are indicated by arrows in Figure 4. For the other motifs, there is significant overlap in the absorption, making the mapping more challenging. In particular, 3A0DS, 4A0DS, and 4A1DB all show absorption between ∼2400 and 2550 cm−1, thus making an assignment of these regions of the amphiphilic band to a specific structure rather challenging. 5A1DB and 5A0DS are the least red shifted relative to the IR spectrum of neat water but have an overlapping spectrum with 3A0DS, 4A0DS, and 4A1DB. It is rather interesting that despite having a different solvation structure, 5A1DB and 5A0DS have an indistinguishable IR spectra from neutral water. The IR experiments by Tokmakoff and co-workers strongly suggest that the eigenstates of water and the hydroxide vibrations involve the coupling of bend and intermolecular vibrations. Our results now provide an important starting point for helping to interpret these features in terms of specific microscopic structural features of the amphiphilic character of the hydroxide ion. Although the environment explored by the hydroxide ion in bulk water will naturally be different from that in the clusters especially in the surface states, the solvation structures explored and corresponding IR spectra provide welldefined limiting cases that are not irrelevant for hydroxide ion transport in liquid water. For example, the undercoordinated three-accepting hydroxide in a recent ab initio simulation was shown to be stabilized if its O−H bond was pointing to a small cavity in water.42 Similar types of solvation structures are likely to exist for four- and five-accepting hydroxides depending on the local topology of the HB network. Furthermore, temperature-dependent studies of the IR spectra will also likely change the relative population of three-, four-, and five-coordinated species, and our results provide essential ingredients for understanding how the amphiphilic band changes as a function of finite temperature. Besides bulk water, our results also have important implications on looking for vibrational signatures of the hydroxide ion at hydrophobic interfaces such as the air− water interface using surface-specific techniques. Mundy and co-workers, for example, have recently shown that the CN of
involves a complicated mixture of IR-active modes coming from motifs with very different local solvation environments. The comparison of the PH and LRD spectra clearly suggests that thermal fluctuations result in a mixing of different modes of the spectra. In particular, one might suspect that the red shift in the O−H stretch and in some cases the merging with bendvibrations are somehow related to the variation in the strength of the HBs in the immediate vicinity of the hydroxide ion. In this spirit, we examined the distribution of the proton-transfer coordinate, δ, defined as rOM−H−rOD−H, where OM is the oxygen atom identified to be the hydroxide, OD corresponds to the oxygen that is donating a HB to the OM, and H is the candidate proton that is sandwiched between OM and OD atoms. On the time scales of the current simulations and the temperature of the cluster, there is typically one HB that is particularly stronger than the others. In Figure 5, we focus on
Figure 5. Distribution of the proton-transfer coordinate δ for the strongest HB that is being donated to the hydroxide ion. Color codes used here refer to 3A0DS (black circles), 4A0DB (red squares), 4A0DS (violet star), 4A1DB (blue diamond), and 5A1DB (green triangle).
the strongest HB that is donated to the hydroxide ion for each of the motifs. With the current definition of the HB, values closer to zero correspond to a proton that is more shared and consequently a stronger HB. The results are quite striking; there is a clear correlation between the strength of the HB, the local CN of the motif, and finally its corresponding position along the amphiphilic band in Figure 4. For 3A0DS and 4A0DB, the merging of the O−H stretch with the bendvibrations is clearly related to the presence of the hydroxide being more delocalized, which in turn causes bigger differences between the PH and LRD. The motifs 4A0DS and 4A1DB are quite similar to the former, having a slightly more pronounced shoulder toward shorter distances. Furthermore, the second strongest HB is also characterized by shorter distances in δ in 4A0DS compared to 4A1DB. The IR experiments by Tokmakoff and co-workers1 also show the development of a small shoulder in aqueous NaOH at frequencies above 3500 cm−1, which has been interpreted as originating from the dangling O−H bond of OH−. Our AIMD simulations provide the unique opportunity to determine how the solvation structure, as reflected in our different motifs, is reflected in this region of the IR spectrum. In this highfrequency region, we observe two regimes, one between 3550 and 3700 cm−1 and another at 3750 cm−1, that render some 276
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The Journal of Physical Chemistry Letters the hydroxide decreases near the air−water interface.44 It would in fact be very interesting to examine how the IR amphiphilic band that we observe changes at the air−water interface using both experimental and theoretical approaches.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected](Y.C.). *E-mail:
[email protected] (A.H.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Sandro Scandolo for fruitful discussions. Y.C.’s position was partly sponsored by the grant “ERC Advanced Grant 320796 − MODPHYSFRICT”.
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