Restricting Solvation to Two Dimensions: Soft-landing of

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Restricting Solvation to Two Dimensions: SoftLanding of Microsolvated Ions on Inert Surfaces Janos Daru, Prashant Kumar Gupta, and Dominik Marx J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03801 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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The Journal of Physical Chemistry Letters

Restricting Solvation to Two Dimensions: Soft-landing of Microsolvated Ions on Inert Surfaces János Daru ,∗ Prashant Kumar Gupta,∗ and Dominik Marx∗ Lehrstuhl für Theoretische Chemie, Ruhr–Universität Bochum, 44780 Bochum, Germany E-mail: [email protected]; [email protected]; [email protected]

Abstract

Water on solid interfaces has been the focus of intense research efforts for decades 1–10 due to outstanding relevance in a myriad of important processes such as surface wetting, precipitation, corrosion and heterogeneous (electro-)chemical reactions. 11,12 Neat 7,11,13–17 and ion-doped 18–20 water clusters on metal, 11,17,19 metal-oxide 7 or salt 13–16,18,20 surfaces have been intensely studied not only computationally, but in particular via high-resolution real-space imaging techniques. 13,14,21 Such sophisticated surface probe techniques have even been utilized to study proton transfer processes up to collective manybody tunneling effects in H-bonded structures on such surfaces. 1,15,22,23 Common to all afore-mentioned and a plethora of similar studies are rather strong and directional surface-induced interactions, which often exert an overriding impact on both, the internal structure and the surface arrangement of these non-covalently interacting water networks. Thus, the highly directional water-water and the surface-specific water-surface interactions 7 as well as the generic spatial restriction effects on the clusters due to the excluded volume of the surface compete with each other and, thus, are intricately intermingled. In stark contrast to previous work, our intention here is to disclose – as much as possible – the generic behavior of ion-doped water clusters deposited on surfaces resulting from geometric

In an effort to scrutinize dimensional restriction effects on finite hydrogen-bonded networks, we deposit ion doped water clusters by computational soft-landing on a chemically inert supported xenon surface. In stark contrast to the much studied metal or metal oxide surfaces, the rare gas surface interacts only rather weakly and non-directionally with these networks. Surprisingly, the strongly-bound Na+ doped networks undergo very significant plastic deformations, whereas the weakly-bound Cl− counterparts barely change upon surface deposition. This counterintuitive finding is traced back to the significantly less favorable water-water interactions enforced by the cation, which results in an easier adaption to geometric restrictions, whereas H-bonding stabilizes the anionic clusters.

Graphical TOC Entry

∗

To whom correspondence should be addressed

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dimensionality-restriction effects in the absence of overriding surface-specific chemical interaction effects. This is realized by introducing a class of systems that has been designed to disentangle the chemical effects, such as bonding interactions, from physical effects due to spatial hindrance imprinted by restricting the space that is available to the H-bond network. Moreover, we aimed to construct systems that can be handled not only computationally, but also experimentally using for instance usual scanning probe or surface-sensitive spectroscopic techniques in the future. Our specific composite surface of choice is built up using two monolayers of xenon deposited on a copper (111) surface, Xe(2 ML)/Cu(111) 24 . It exhibits only week adhesive interactions towards the doped water clusters (the binding energy between Xe and a water molecule is -0.59 kcal/mol 25 ) , while providing them with an asymmetric environment which imposes the geometric restriction. For doping our water clusters, we have chosen the ions of rock salt (Na+ and Cl− ), which are not only generic to both electrolyte solutions and seawater, but are also well understood in the unrestricted solvation limit of isotropic aqueous bulk solutions. 26,27 The other limit of unrestricted solvation, being microsolvation in the gas phase, is simultaneously established within this study as to provide the ideal cluster reference in the absence of any external perturbations. As a basis for unraveling genuine restriction effects on solvation, we have performed ab initio (AIMD) and hybrid quantum/classical (QM/MM) molecular dynamics simulations 28 at low temperature (formally corresponding to 50 K according to the kinetic energy) to probe restricted versus unrestricted solvation of the Na+ and Cl− doped water clusters (see Scheme 1 and the Supporting Information, SI, Section 1 for the computational aspects). The initial structures for the surface simulations have been obtained from soft-landing trajectories in order to mimic typical experimental methods for depositing charged species at surfaces 29,30 as detailed in the SI. In order to enhance statistics, our strategy is to generate three statistically independent trajectories

per solvatomer resulting in altogether 120 000 configurations for each solvatomer. The corresponding full ensemble has been utilized for the calculation of distribution functions for each solvatomer, while the structures shown in Figs. 1-3 are representative snapshot configurations chosen to illustrate the key points. Following a from-simple-to-complex approach we have first investigated the surface effects on the sodium doped water clusters, since we expected only minor structural changes based on the stronger total binding energies in comparison to their Cl− counterparts (see supporting Fig. 13). In this spirit, we considered first the most stable, tetrahedral [Na(H2 O)4 ]+ solvatomer (4a), see Scheme 1(b) In this cluster the water molecules are not H-bonded to each other since their partially negatively charged oxygen sites are strongly attracted by the cationic core of this cluster as visualized by the top-right snapshot in Fig. 1. Despite of our expectations, already this complex reveals upon soft-landing structural differences with respect to the gas phase simulations as follows. First of all, restricting solvation from three to two dimensions breaks the tetrahedral symmetry of the heavy atom skeleton observed in the gas phase, see Scheme 1(b). In order to proceed, we classify the four water molecules in two sets: The H2 O that remains disconnected from the Xe adlayer (called “top H2 O”) and the remaining three H2 O that are touching the surface (“bottom H2 O”) as visualized by the snapshots in Fig. 1. Since such a structural distinction is ambiguous in the isotropic gas phase reference situation, we have considered all four possible combinations for choosing the “top” and the remaining three “bottom” waters in that case. In an effort to disclose the pronounced surface effect, the minimal intermolecular H· · · O distance between the “top” H2 O and the “bottom” ones (denoted with min {d(Ht · · · Ob )}) and the distance between the Na+ ion and the geometric center of the “bottom” oxygen atoms (denoted with d(Na · · · Ob )) have been introduced as order parameters. The resulting probability density for the gas phase and the deposited system are directly compared in Fig. 1.


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While in the gas phase all four water molecules are structurally equivalent and provide a typical minimal H· · · O distance of 3.8 Å, the two dimensional solvation restriction imposed by the surface imprints distortions. While the “bottom” molecules that establish the interface with the surface feature a rather similar arrangement as all water molecules in the gas phase, the water molecule which is far from the surface, in the “top” position, shows remarkably different characteristics. This water molecule can either stay in a distorted tetrahedral (gas phase like) arrangement (as shown in the top-right representative snapshot), or it can bend over toward one of the three “bottom” molecules (reducing the H· · · O distance typical value to 2.6 Å). The latter distortion allows the microsolvated cluster to establish energetically favorable enhanced non-covalent interactions leading to the top-left snapshot. Simultaneously, the sodium ion also moves closer to the center of the three “bottom” water molecules by about 0.2 Å, which can even reach 0.5 Å with respect to the typical gas phase value in strongly distorted cases (see Fig. 1, top left). Is the discovered effect specific to the n = 4 cluster, or rather a general phenomenon that can be observed for microsolvated Na+ ions on chemically inert surfaces, and thus a generic solvation restriction effect? In a first step, we considered solvatomer 4b with again n = 4 water molecules but a distinctly different shape. In this case, the Na+ cation has only three close H2 O neighbors (constituting the first solvation shell), whereas the fourth water (being in the second solvation shell) accepts two Hbonds from two of these first shell waters, see Scheme 1(b). In the unrestrained gas phase limit, the specific first shell water molecule (depicted using large spheres) that is opposite to the second shell water can easily rotate with respect to the plane defined by all three first shell waters (marked by a gray triangle) thus yielding a bimodal distribution P (ϕ), see black line in Fig. 2. After having soft-landed this solvatomer on the xenon surface, again two different scenarios are found. One option is that the behavior found in the gas phase is only slightly modified

Scheme 1: Panel (a): Hybrid quantum/classical (QM/MM) setup of the Xe(2 ML)/Cu(111) surface on which the microsolvated ions, [Cl(H2 O)n ]− and [Na(H2 O)n ]+ , are deposited as a result of soft-landing simulations. The Xe atoms described by a force field (MM) are represented in green while the quantum-mechanical (QM) atoms Xe, Na, O and H are depicted in purple, blue, red and gray, respectively. The Cu atoms of the supporting (111)-terminated copper surface (shown in orange for the purpose of visualization) are represented in terms of an external corrugation potential acting on all Xe atoms and additional mirror charge interactions with the entire QM subsystem (as explained in the SI). Panels (b) and (c): Optimized gas phase structures and labeling of all [Na(H2 O)n ]+ and [Cl(H2 O)n ]− solvatomers, respectively, that have been soft-landed on the surface depicted in (a)


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(blue line and top-left snapshot). In stark contrast, the motion of that water molecule can be completely frozen on the surface and, in addition, arrested is an arrangement that used to be the least stable in the gas phase and in the aforementioned dynamical state on the surface. Thereby, the perpendicular arrangement of the specific water molecule transmutes from a saddle point, and thus from a transition state for dynamical rearrangements, into the global minimum due to solvation restriction. The corresponding distribution function P (ϕ) is therefore unimodal and has its maximum right at the former minimum at ϕ ≈ 90◦ , see red line and topright snapshot in Fig. 2. Unexpectedly, this implies that geometric solvation restriction effects are able to stabilize high-symmetry cluster structures that are unknown in the gas phase. Encouraged by these findings, we probed the behavior of solvatomers with five water molecules interacting with Na+ . Interestingly, the most stable five water solvatomer contains the structural elements of both, the most stable four water cluster (central tetrahedral arrangement) and the H-bonded square-shaped ring of the previous example. However, in contrast to the latter, it features two dangling water molecules. These two water molecules are arranged symmetrically in the gas phase (in accordance with the tetrahedrality of the first shell water molecules), while they feature remarkable steric effects on the surface (see Fig. 3). The distortion of the structure is disclosed by the joint distribution function of two angles (denoted with ϕ1 and ϕ2 and defined in Fig. 3) that are equivalent in the gas phase. The water closer to the surface is well localized due to its interaction with the surface, while the top-most water molecule shows favorable interactions with other first-shell water molecules (see the change in intermolecular H· · · O distance distribution in supporting Fig. 18). The overall distribution is significantly more localized on the surface than in the gas phase. The soft-landing trajectory of another [Na(H2 O)5 ]+ solvatomer, namely 5b from Scheme 1(b), also resulted in unexpectedly rich dynamical transformations (see SI, Section 5 for supporting analyses).

Figure 1: Symmetry breaking by restricted solvation of the tetrahedral [Na(H2 O)4 ]+ solvatomer 4a, see Scheme 1(b), as a result of soft-landing on the Xe(2 ML)/Cu(111) surface as visualized in Scheme 1. Top: Representative snapshots depicting structures (sampled from QM/MM simulations) that are strongly distorted, distorted or unperturbed with respect to the gas phase ensemble from left to right, respectively. Bottom: Joint probability distribution in the gas phase reference (left) and after deposition on the surface (right) of the minimal inter-molecular H· · · O distances between the “top” (t) and “bottom” (b) waters molecules (see text) and the distance between the Na+ ion and the center of the “bottom” oxygen atoms.


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Figure 2: Symmetrized probability distribution (bottom panel) of the dynamical (blue) and arrested (red) states of a higher-lying [Na(H2 O)4 ]+ solvatomer 4b, see Scheme 1(b), as a result of soft-landing on the Xe(2 ML)/Cu(111) surface as visualized in Scheme 1 in comparison to the gas phase reference (black). The angle ϕ is that between the plane (gray triangle) defined by all three first shell water molecules and the molecular plane spanned by the specific first shell water molecule (highlighted using larger spheres) that is opposite to this single second shell water molecule. Representative configurations corresponding to the the dynamical and arrested states are shown by the top-left and topright snapshots, respectively, sampled from the QM/MM simulations.

Figure 3: Quantifying surface steric hindrance effects of the most stable [Na(H2 O)5 ]+ solvatomer 5a, see Scheme 1(b), after soft-landing on the Xe(2 ML)/Cu(111) surface in terms of the joint distribution function depending on the angles ϕ1 and ϕ2 as defined in the top panel. Top: Representative configuration snapshot sampled from the QM/MM simulations. Bottom: Joint probability distribution in the gas phase (left) versus on the surface (right). ferent behavior of cationic versus anionic solvatomers after deposition, we have decomposed their normalized total binding energy into that of the ion with respect to the water cluster in a given solvatomer structure, Eibind and the binding energy of that exact water cluster (in the absence of the ion), Ewbind as defined in the caption of Fig. 4. We note in passing that the impact of both, the ions and the surface, on Ewbind has been accurately quantified (in supporting Sec. VII using the rigorous electronic-structure-based local energy decomposition (LED) technique relying on advanced DLPNO-CCSD(T) as explained in the SI) and found to be rather small. The correlation due to this Ewbind versus Eibind decomposition makes clear that the energetic fingerprints of these two solvatomer classes are vastly different, see

Since the normalized total binding energy for chloride solvatomers is significantly lower than for the sodium ones (see supporting Fig. 13), we have expected much more dramatic structural changes as a result of soft-landing them. Surprisingly, however, each and every structure of the chloride solvatomers remained intact. Even utmost careful analysis of the structures did reveal remarkable structural similarity not only for the heavy atoms, but including all hydrogens and thus the H-bonding topology as well (this is illustrated in supporting Fig. 9). How can this apparent puzzle be solved? In an effort to explain the distinctly dif-


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Fig. 4. While all cationic solvatomers are characterized by rather weak, sometimes even repulsive water-water interactions and strongly attractive ion-water binding energies, the opposite is true for all anionic species. This implies that strong H-bonds are established among the water molecules that microsolvate the Cl− ion, whereas cationic microsolvation largely prevents such favorable water-water arrangements since the oxygen atoms are forced to point toward Na+ . Clearly, the weakly bound H-bond network of the cationic solvatomers can be easily deformed as a result of soft-landing, thus leading to the observed “plastic deformations”. In stark contrast, the microsolvation shells of the anionic clusters are strongly H-bonded and, thus, undergo only slight “elastic deformations” within the same rigid H-bonded network as in the gas phase. In conclusion, our soft-landing simulations of water clusters doped by cations and anions, namely [Na(H2 O)n ]+ and [Cl(H2 O)n ]− , disclose a coherent picture, which is however counterintuitive at first sight: The more strongly bound cationic species feature pronounced structural distortions, whereas the energetically less stable anionic clusters are barely affected upon surface deposition. Our detailed energy correlation analyses have shown that the particular arrangements of water molecules around the cation, pointing with their partially negatively charged oxygen site toward the positive ionic charge, prohibit favorable water-water interactions such as H-bonding. These weakly interacting water networks are, therefore, prone to perturbations leading to large-amplitude “plastic deformations” as a result of deposition up to the extent of imprinting exotic structures. In stark contrast, the anionic clusters are characterized by strongly attractive water-water interactions, which allow only for insignificant “elastic deformations” upon soft-landing. What remains open at this stage is to find out if nuclear quantum effects at very low temperatures could lead to non-trivial quantum phenomena beyond the expected usual broadening effects. Given its fundamental nature, the discovered phenomenon is expected to be general rather than specific to the particular system classes

+

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[Na(H2 O)n ]

bind

Ew /kcal · mol

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0 4 [Cl(H2 O)n ]

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Figure 4: Confidence regions (at two standard deviations) of the joint probability distributions the normalized binding energy of the water cluster structure (i.e. Ewbind =(EWn − n EW1 )/n) and the corresponding normalized binding energy of the ion to that water structure (i.e. Eibind = (Eion−Wn − EWn − Eion )/n ) as computed for ion-water clusters sampled from the reference simulations in the gas phase (ellipses filled with light colors) and after soft-landing on the Xe(2 ML)/Cu(111) surface (ellipses using colored dashed lines without any filling); see SI, Section 7 for rigorous definitions of the binding energies. The gray dashed lines separate the sectors populated by the [Na(H2 O)n ]+ (upper left corner) and [Cl(H2 O)n ]− (bottom right corner) clusters; red, green, blue and black colors denote species 4a, 4b, 5a and 5b, respectively, according to Scheme 1(b) and (c). Note that the binding energy distributions of the anionic clusters 5a and 5b are superimposing. The data represented by ellipses and the change of these joint probabilities upon cluster deposition is highlighted in supporting Fig. 14. investigated.


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Acknowledgments

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We are grateful to Stefan Grimme for having provided the QMDFF code to parameterize the QM/MM embedding and to Marcella Iannuzzi for insightful discussions concerning the image charge scheme implemented in CP2k. Our research is partially funded by the DFG Cluster of Excellence “RESOLV” (EXC 2033) and the calculations were carried out using HPC-RESOLV, HPC@ZEMOS, BOVILAB@RUB, and RV-NRW.

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Restricting Solvation to Two Dimensions: Soft-landing of

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