Enhancing Enantiomeric Separation with Strain: The Case of Serine

May 26, 2017 - Structural optimizations based on DFT were carried out with the plane-wave basis set Vienna Ab-initio Simulation Package (VASP) code,(3...
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Enhancing Enantiomeric Separation with Strain: The Case of Serine on Cu(531) Yonghui Wang,† Sha Yang,† Miguel Fuentes-Cabrera,‡ Shuang Li,*,† and Wei Liu*,† †

Nano Structural Materials Center, School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu, China ‡ Center for Nanophase Materials Sciences, Computational Sciences and Engineering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *

ABSTRACT: Serine has two enantiomers, D and L, which exhibit identical physical and chemical properties but have dramatically different physiological effects. For the pharmaceutical industry, it is very important to be able to separate both enantiomers. Here we study the enantioselectivity of the (531) surfaces of Cu, Ag, Au, and Pd using density functional theory with an accurate treatment of the van der Waals interactions. Among these surfaces, it is found that Cu(531) is the most efficient for energetically separating serine enantiomers. This greater efficiency is ultimately related to a conformational strain imposed in serine and most of all in the supporting substrate. Motivated by this, we decorated the step sites of Cu(531) with Ni atoms and showed that serine enantioselectivity increases by 36% as compared to that of pristine Cu(531). These results suggest that efficient enantiomeric separation of small chiral molecules could be achieved with bimetallic stepped surfaces for which strain, both in the surface and the molecule, increases significantly upon deposition.

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INTRODUCTION Serine is a chiral α-amino acid. Its two enantiomers, D- and Lserine, exhibit identical physical and chemical properties but have dramatically different physiological effects. L-Serine plays a critical role in neuronal function and development in the central nervous system. L-Serine treatment has been used to manage schizophrenia, depression, and chronic fatigue syndrome, and also to prevent microcephaly, seizures, and psychomotor retardation in patients with rare congenital defects of L-serine biosynthesis.1 D-Serine is a physiological modulator of many N-methyl D-aspartate receptor (NMDAR)dependent functions, including brain development,2 synaptic transmission, long-term synaptic plasticity,3−7 learning, memory, and social interactions.8,9 Changes in D-serine metabolism and extracellular levels appear to be central to several pathological states; the D-serine level increases greatly in the spinal cord of patients with amyotrophic lateral sclerosis, where it likely mediates motor neuron degeneration.10,11 For the pharmaceutical industry, it is thus very important to be able to separate serine enantiomers effectively. Previous investigations have focused on separating D- and Lserine using high Miller index metal surfaces. Distinct enantioresolution ability has been observed on the Cu(110) surface, where the L- and D-enantiomers aggregate in wellordered islands. Other surfaces, such as Cu(643) and Cu(3,1,17), have also been used for chiral recognition.12−17 Unfortunately, on these types of surfaces, both enantiomers have very similar adsorption energies, which complicates their © 2017 American Chemical Society

separation. Fortunately, the Cu(531) surface can be used to increase the difference in adsorption energies between D- and Lserine. This surface has a smaller unit cell and a higher density of step atoms than other high Miller index surfaces, and this makes Cu(531) an ideal template for enantioselectivity adsorption of small chiral molecules.18−27 Consequently, much of the theoretical and experimental work has focused on exploring the adsorption configurations and enantioselective ability of serine enantiomers on Cu(531).24−27 Despite these efforts, many questions remain open. For example, it is known that serine enantiomers adsorb on Cu(531) in two different adsorption geometries depending on coverage. (At high coverage, covalent bonds between serine and its substrate are formed through the two oxygen atoms of the carboxylate group and the amino group, whereas at low coverage an additional bond is formed through the deprotonated β−OH group). However, it is unknown whether other adsorption geometries are possible. It is also unclear what causes serine enantiomers to have significant adsorption energy differences on Cu(531) as compared to other metallic (531) surfaces. Further complicating matters, the binding configurations of serine enantiomers at the step sites are poorly resolved, and a microscopic picture has not yet emerged. Modeling and simulation can shed light onto the issues above. Density functional theory (DFT), in particular, is the Received: February 3, 2017 Published: May 26, 2017 8167

DOI: 10.1021/jacs.7b01216 J. Am. Chem. Soc. 2017, 139, 8167−8173

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Journal of the American Chemical Society

⎛ p⎞ μ(H 2 , T , p) = TS(H 2 , T , p0 ) − kT ln⎜ 0 ⎟ ⎝p ⎠ 1 (H(H 2 , 0, 0) + ΔH(H 2 , 0 → T , p0 )) − NA

most suitable theoretical approach for doing so. Information on the adsorption geometries and the transition states of serine enantiomers on metal surfaces is readily available with DFT. However, the correct description of molecules adsorbed on metallic surfaces requires treating accurately all of the possible factors that govern adsorption, including covalent bonds, van der Waals (vdW) forces, hydrogen bonds, charge transfer, and Pauli repulsion. Here we use DFT calculations with vdW interactions treated with both screened pairwise28−32 and many-body dispersion (MBD)33−36 methodsto investigate the adsorption of D- and L-serine on Cu/Ag/Au/Pd(531) surfaces. Taking inspiration from the use of bimetallic surfaces in heterogeneous catalysis and the crucial role of step atoms in enantioselectivity,37 we also investigate whether adding Ni and Au atoms at the step edges of Cu(531) increases the enantioselective capability of this surface. Our results reveal that vdW interactions must be taken into account for a correct description of enantioselectivity. The attractive interactions of three functional groups of chiral serine play crucial roles in inducing large enantiomeric differences in the adsorption geometries and energies, and the enantioselectivity of serine increases significantly when Cu(531) is decorated with two monolayers (ML) of Ni atoms.

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(2) where the values for the enthalpy change, ΔH, and entropy, S, were taken from experiments.54 In addition, p0 is 1 atm, T = 298.15 K, and p = 10−10 mbar.

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RESULTS AND DISCUSSION In what follows, the enantioselective capability of Cu/Ag/Au/ Pd(531) surfaces and Cu(531) decorated with Au and Ni atoms is investigated only at low serine coverage. This choice is motivated by the work of Eralp et al., which found that serine enantioselectivity of Cu(531) is larger when the coverage is low.27 The L- and D-enantiomers of serine, the Cu(531) surface, and L-serine adsorbed on the (110) and (311) microfacets of Cu(531) are shown in Figure 1a−d. Four possible config-

COMPUTATIONAL METHODS

Structural optimizations based on DFT were carried out with the plane-wave basis set Vienna Ab-initio Simulation Package (VASP) code,38−40 along with the generalized gradient approximation (GGA) of Perdew−Burke−Ernzernhof (PBE) for the exchange and correlation functional.41−43 Convergence criteria of 10−2 eV/Å for the final force in all structural relaxations, 10−5 electrons per unit volume for charge density, and 10−4 eV for the total energy were used in our calculations. The Brillouin Zone sampling used was 7 × 5 × 1 in the Monkhorst−Pack grid.44 An 11-layer metal (531) slab was used: the uppermost five metal layers were allowed to relax, whereas the bottom six layers were fixed. The vacuum thickness was 25 Å. For computing the adsorption energies, Eads, two recently developed dispersion-inclusive methods, termed as DFT+vdWsurf and DFT +MBD,29,33 were employed to account for the vdW interactions and collective response effects. The DFT+vdWsurf method29 extends pairwise vdW approaches to the modeling of adsorbates on surfaces, employing a synergetic combination of the DFT+vdW method45 for intermolecular interactions with the Lifshitz−Zaremba−Kohn theory46,47 for the nonlocal Coulomb screening within the bulk. This method has been shown to achieve quantitative accuracy for a variety of molecules on transition-metal surfaces, leading to an overall accuracy of 0.1 Å in adsorption heights and 0.1 eV in binding energies with respect to state-of-the-art experiments.48,49 The DFT+MBD method computes the long-range correlation energy through the coupled harmonic oscillator model Hamiltonian and treats dipolar vdW interactions to all orders in perturbation theory beyond the pairwise correction scheme.33−36 This method has been shown to perform very well for molecular crystals, nanoclusters, layered nanostructures, adsorption systems, and so on.50−53 The adsorption energy Eads is defined as

Eads = Esystem +

n μ(H 2 , T , p) − Esubstrate − Eserine 2

Figure 1. (a) Structures of L- and D-serine (red = O, blue = N, gray = C, white = H). (b) Structures of the pristine Cu(531) surface with the unit-cell vectors and the main crystallographic directions; the yellow dashed lines indicate the (1̅1;1̅2̅) unit cell as used in our DFT calculations; the two red triangles indicate the (311) and (110) microfacets, respectively. (c) and (d) Side views of L-serine on (311) and (110) microfacets, respectively. (e) Adsorption energies of Lserine on (311) and (110) microfacets; the inset shows a schematic view of the μ2, μ3, μ4, and μ3′ configurations studied here.

urations of serine adsorbed on Cu(531) were investigated. These are based on the serine dehydrogenation on the (110) and (311) microfacets of Cu(531) and are denoted here as μ2, μ3, μ4, and μ3′, with the subscript (2, 3, and 4) indicating how many atoms of serine are bonded to Cu(531). In μ2, one nitrogen and one oxygen are bonded to the surface. Both μ3 and μ4 were studied by Eralp et al.27 In μ3, the two oxygen atoms of the carboxylate group and the amino group are bonded to Cu(531), whereas in μ4, additional substrate−serine bonds are formed through the deprotonated β−OH group. The μ3′ has one hydrogen atom less than μ4, and in μ3′, serine is bonded to Cu(531) via the nitrogen and two oxygen atoms. The μ3 has a hydroxyl group (−OH), whereas μ3′ has a more stable carbonyl group (-CO). The adsorption energies of L-serine μ2, μ3, μ4, and μ3′ on the (311) and (110) microfacets are shown in Figure 1e. The μ4 is the most stable configuration for both the (311) and (110) microfacets. This result agrees well with that of Eralp et al.27 It can be concluded then that at low coverage serine adsorbs on Cu(531) in the μ4 configuration. From now on, we focus on this configuration only and study the energetic differences between the L- and D-enantiomers adsorbed on Cu(531) and other metallic surfaces.

(1)

where n denotes the number of deprotonation; Esystem, Esubstrate, and Eserine denote the total energy of the system containing the adsorbed deprotonated serine, the bare substrate, and the isolated serine, respectively. The chemical potential, μ, of H2 was included for a more reliable comparison between the different chemical forms: 8168

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Journal of the American Chemical Society Table 1 contains the adsorption energies of μ4 on Cu(531) obtained here and in ref 27; the adsorption structures are

comparable to those obtained with near-edge X-ray-absorption fine-structure (NEXAFS) measurements.27 Defining ΔEenan as Eenan(311) − Eenan(110), where enan = D or L, it is seen that ΔED is −0.15 (PBE), −0.13 (PBE+vdWsurf), and −0.15 (PBE+MBD) eV, whereas ΔEL is −0.33 (PBE), −0.35 (PBE+vdWsurf), and −0.41 (PBE+MBD) eV. These results reveal that both L- and D-enantiomers prefer adsorbing on the (311) microfacet rather than on the (110) one. However, in ref 27 ΔED/L is 0.24/−0.52 eV; the positive ΔED found in ref 27 is most likely a consequence of employing the GGA-PW9155 functional, which, as we show in Table S1 of the Supporting Information, predicts a less stable structure for the adsorbed D-serine. Serine enantioselectivity refers here to the capacity of a metallic surface to energetically separate the L- and Denantiomers. This energetic difference is expressed here as ΔE = ELMS − EDMS, where MS stands for most stable state. As seen above, for both L- and D-serine, MS corresponds to adsorption on the (311) microfacet. Using the energy of this state, we find that ΔE on Cu(531) is about 0.40 eV. Although the calculated adsorption energies obtained with PBE+vdWsurf and PBE+MBD are significantly larger than those obtained with PBE, the ΔE values remain practically the same (in the range of 0.39−0.43 eV; Table 1), no matter whether vdW interactions were included or not. Since PBE is consistently used in these three methods, it can be concluded that the similar ΔE values observed are a consequence of the nearly identical vdW interactions in the L-(311) and D-(311) systems. Figure 2 shows the structures of L- and D-serine adsorbed on Cu(531). An analysis of these structures reveals that the large ΔE reported above is caused by strain. Specifically, for L-(311), the nearest copper atoms provide the necessary support for strong bonds with the oxygen atom of the deprotonated β−OH (Figure 2e). By contrast, the deprotonated β−OH group of Dserine forms bonds with two substrate atoms that are not the nearest ones on the (311) microfacet (Figure 2b); this causes an elongation of 0.1 Å in the O−Cu bond length, and a structural deformation in the molecule and the substrate. Indeed, this structural deformation is so large that it should be visible with STM, as shown in Figure 2c,f. The result above suggests that it might be possible to increase serine enantioselectivity by choosing surfaces that, upon depositing serine, deform themselves and serine significantly. We set out to probe this hypothesis. First, we investigate (531) surfaces that have larger lattice constants than Cu(531) and should therefore cause smaller deformations. Second, we consider whether decorating Cu(531) with other metal atoms can lead to a larger deformation. The face-centered cubic metals Ag, Au, and Pd have larger lattice constants than Cu, and thus are suitable surfaces for probing the first part of our hypothesis (Figure S1 and Figure S2). To quantify the degree of strain in D-serine adsorbed on these metallic substrates, we use the distances d, dA, and dB. Here d is the distance between the two substrate atoms that are bonded with the carboxylate and amino groups of D-serine. Variables dA and dB are the bond lengths of the substrate− amino group and substrate−deprotonated β−OH, respectively (Figure 3a). Figure 3b shows that the adsorption energy difference, ΔE, increases with decreasing d, dA, and dB. Figure 3c−f shows more details: Eads for each metallic substrate is depicted in this figure, and it is clear that only for Cu there exists a sudden jump at D-(311). Therefore, among these surfaces, Cu(531) is the only one for which Eads of L- and D-

Table 1. Adsorption Energies Eads (in eV) and the Orientation Angle α (in Degrees) of D- and L-Serine on the (110) and (311) Microfacets of Cu(531)a Eads Eads Eads Eads(ref) α α α(ref) αexp

methods

D-(110)

D-(311)

L-(110)

L-(311)

ΔE

PBE PBE+vdWsurf PBE+MBD GGA-PW91 PBE PBE+vdWsurf GGA-PW91

−2.01 −2.90 −2.58 −1.98 26.4 21.8 30 24

−2.16 −3.03 −2.73 −1.74 −82.3 −81.7 −78

−2.25 −3.07 −2.75 −1.98 69.6 73.7 80

−2.58 −3.42 −3.16 −2.50 −13.0 −11.6 −10 −14

0.42 0.39 0.43 0.52

The experimental angle (αexp) is obtained with NEXAFS.27 ΔE (in eV) is defined as the adsorption energy difference between D-(311) and L-(311). Eads(ref) and α(ref) are the adsorption energies and orientation angles calculated by GGA-PW91 in ref 27, respectively. a

Figure 2. (a) and (b) Top views of the adsorption geometries and energies of D-serine adsorbed on the (110) and (311) microfacets of Cu(531) by PBE+vdWsurf. (d) and (e) Top views of the adsorption geometries and energies of L-serine on (110) and (311) by PBE +vdWsurf, with the deprotonated β−OH indicated with a circle in (e). (c) and (f) Simulated scanning tunneling microscopy (STM) images (the bias voltage corresponds to 0.5 eV below the Fermi level) of Dand L-serine on the (311) microfacet.

shown in Figure 2a,b,d,e. Table 1 also includes theoretical and experimental values for α, which is defined as the angle between the [11̅2̅] direction and the surface projection of the normal of the carboxylate O−C−O triangle. The results in Table 1 reveal that vdW interactions do affect the adsorption energies and cannot be ignored. The energy of adsorption, Eads, obtained with PBE+vdWsurf (PBE+MBD) is about 0.9 (0.6) eV smaller than that obtained with PBE. With PBE+vdWsurf, Eads is about 0.3 eV smaller than with PBE +MBD, indicating that the many-body effects significantly weaken the adsorption energies. In contrast, the orientation angles, α, are found to be less sensitive to the inclusion of vdW interactions, which is ultimately a result of the nature of the covalent bonding at the interface. As shown in Table 1, PBE and PBE+vdWsurf provide similar α values, which are 8169

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substrates with L-serine. For every case except Au, ΔEmolecule is smaller than ΔEsubstrate, showing that upon molecule deposition, the conformational strain imposed is larger in the substrate than in the molecule, probably because the molecule is more flexible than the substrate. The results in Table 2 indicate that when D- and L-serine are deposited on any of the substrates considered here, D-serine deforms itself and the substrate underneath more than L-serine and the corresponding substrate do. Table 2 also shows that ΔEdef, defined as the sum of ΔEmolecule and ΔEsubstrate, follows the same trend as ΔE. Thus, in untangling why Cu(531) separates D- from L-serine better than Pd, Au, and Ag do, it is found that this is mostly because of the structural deformation caused in Cu(531) upon D-serine adsorption. As seen above, in the (531) metallic surfaces the step atoms play a crucial role in determining serine enantioselectivity. It then seems reasonable to explore methods that enhance enantioselectivity by introducing structural modifications on the (110) and (311) microfacets of these surfaces. In what follows, we explore this possibility by decorating the (110) and (311) microfacets of Cu(531) with Ni or Au atoms. This approach is reminiscent of the use of bimetallic surfaces in heterogeneous catalysis and it is used here to probe the second part of our hypothesis.25,56,57 The adsorption energies, Eads, of serine on Cu(531) decorated with 1 and 2 ML of Ni or Au are shown in Figure 4a, which illustrates that the adsorption strength decreases with

Figure 3. (a) Side view of D-serine on the (311) microfacet of metallic (531). In this plot, d denotes the distance between the two substrate atoms that are bonded with the carboxylate and amino groups of Dserine. The bond lengths are dA and dB of the substrate-amino group and substrate-deprotonated β−OH, respectively. (b) The distances (d, dA, and dB) and the corresponding ΔE values of serine on the (311) surfaces of Cu, Pd, Au, and Ag. (c)−(f) PBE, PBE+vdWsurf, and PBE +MBD adsorption energies of L- and D-serine on Cu/Ag/Au/Pd(531) in two adsorption sites involving the (110) and (311) microfacets.

serine adsorbed on the (311) microfacet differs the most, which leads to the large ΔE observed. To understand the role of strain on enantioselectivity, we computed the deformation energies of L-, D-serine, and the (531) substrates considered here. The deformation energy for the molecule (substrate), E1 (E2), is defined here as the total energy of the deformed molecule (substrate) minus the total energy of the free molecule (clean substrate). Deformed molecule (substrate) stands for the structure that the molecule (substrate) acquired when the molecule was deposited on the substrate and both were relaxed. The values for E1 and E2 are shown in Table 2, where we also included the difference in deformation energies, defined as ΔEmolecule (ΔEsubstrate) = E1D − E1L (E2D − E2L). The E1 and E2 values are positive for every case shown in Table 2. E1 of D-serine is always larger than E1 of L-serine, confirming that upon deposition D-serine is more deformed than L-serine. The same is true for the substrates: E2 is larger for substrates with deposited D-serine than for

Figure 4. (a) PBE+vdWsurf adsorption energies of D- and L-serine on pristine Cu(531), and on Cu(531) decorated with 1 and 2 ML of Ni or Au in two adsorption sites involving the (110) and (311) microfacets. The 1 and 2 ML Ni- or Au-decorated surfaces are shown in the inset. (b) Dependence of d and ΔE on the type and decoration density of decoration atom.

the decreasing Ni coverage in the Ni-decorated Cu(531) surfaces. However, an opposite trend can be seen in the case of Au-decorated surfaces, which is in agreement with the findings of Song et al.25 Compared with pristine Cu(531), ΔE increases

Table 2. Deformation Energy (in eV) of the Molecule (Substrate) E1 (E2)a D-serine

L-serine

systems

E1

E2

E1

E2

ΔEmolecule

ΔEsubstrate

ΔEdef

ΔE

Ag(531) Au(531) Pd(531) Cu(531) 1 ML Ni/Cu(531) 2 ML Ni/Cu(531)

0.187 0.199 0.203 0.169 0.169 0.199

0.073 0.177 0.207 0.213 0.127 0.275

0.184 0.121 0.184 0.161 0.109 0.173

0.059 0.171 0.143 0.101 0.026 0.012

0.003 0.078 0.019 0.008 0.060 0.026

0.014 0.006 0.064 0.112 0.101 0.263

0.02 0.08 0.08 0.12 0.16 0.29

0.08 0.13 0.16 0.39 0.44 0.53

a Deformation energy is defined as the total energy of the deformed molecule (substrate) minus the total energy of the free molecule (clean substrate). The difference in deformation energies (in eV), is determined by ΔEmolecule (ΔEsubstrate) = E1D − E1L (E2D − E2L), and the ΔEdef (in eV), defined as the sum of ΔEmolecule and ΔEsubstrate.

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Journal of the American Chemical Society (decreases) by 0.05 (0.31) and 0.14 (0.37) eV for 1 and 2 ML Ni-(Au-)decorated Cu(531), respectively. ΔE depends then on both the type of decorating atom and the decoration density. The dependence on the type of decorating atom is related to the strength of the interaction between this atom and serine. Ni forms stronger bonds with serine than Au does and, as a consequence, at the same decoration densities, d is smaller for Ni-decorated surfaces than for Au-decorated ones. Smaller d values introduce larger strain, which causes a larger ΔEdef and ΔE. The dependence on the decoration density is more subtle; Figure 4b shows that for Ni, ΔE increases with the decoration density, whereas for Au the opposite is true. For example, Figure 5 shows that for 2 ML Ni-decorated Cu(531), ΔE is

Figure 6. Molecular-orbital density of states (MODOS) of serine on (a) 0.33 ML Ni-decorated Cu(531), (b) pristine Cu(531) surface, and (c) 2 ML Ni-decorated Cu(531). The zero energy corresponds to the Fermi level. The left and middle columns show the results for D- and Lserine. The right column shows ΔMODOS (i.e. the MODOS of Lserine on (311) minus that of D-serine on the same microfacet).

Figure 5. Dependence of ΔE on decoration density for Ni-decorated Cu(531).

that of the D-serine (−2.85 eV) on the same microfacet (Table S7). The larger extent of broadening and the lower energetic positions with respect to the Fermi level for L-serine relative to D-serine, suggest that L-serine is chemically more activated and binds more strongly on the metal surface. Similarly, the broadening of the HOMO and LUMO of L-serine on the 2 ML Ni/Cu(531) surface is more prominent than on the pristine surface (Figure 6c), indicating a larger adsorption energy on the former surface. More importantly, we find that the difference of the projected density of states between L- and D-serine on the same metal substrate directly corresponds to their separation ability. For example, for 0.33 ML Ni/Cu(531) the ΔMODOS shows only small differences, which correlates with a small ΔE, see Figure 6a. However, for the 2 ML Ni/Cu substrate, ΔMODOS is larger, and so is the corresponding ΔE. These results indicate that Ni-decorated Cu(531) is a suitable surface for separating serine enantiomers, suggesting that metal decoration of chiral surfaces could be an effective way to increase enantioselectivity. In conclusion, DFT calculations were used to investigate the adsorption geometries and energetics of D- and L-serine adsorbed on the (531) surface of noble metals, including Cu, Ag, Au, and Pd. It is found that among these surfaces, Cu(531) is the best at energetically separating D- and L-serine. The reason for this is a conformational strain that is imposed on Dserine and most imporantly, the substrate underneath. To further enhance enantioselectivity, decorating the (110) and the (311) microfacets of Cu(531) with Au and Ni atoms was investigated. It is shown that in Ni-decorated Cu(531), enantioselectivity increases by 36% as compared to that of the corresponding pristine surface. These results suggest that bimetallic (531) surfaces could be a viable approach for achieving enantioselectivity of serine and, in general, of similar small chiral molecules. Further, because this result is reminiscent of the use of bimetallic surfaces in heterogeneous catalysis, the body of research in heterogeneous catalysis could be used to guide future enantioselectivity studies of small chiral molecules adsorbed on metallic surfaces.

about 36% larger than the corresponding ΔE for the pristine Cu(531). As shown in Table 2, this is because of the significantly larger ΔEdef value in the former case (0.29 eV), which, as it was for Cu(531), is mostly because of the structural deformation on the substrate. Interestingly, ΔE of 0.33 ML Nidecorated Cu(531) is slightly smaller than that of Cu(531) (Figure S3, Figure S5, and Table S3). To demonstrate that at finite temperature 2 ML Nidecorated Cu(531) is still the best for the enantioselectivity, we further calculated and compared the vibrational free energy for serine on the pristine and Ni-decorated Cu(531) surfaces as a function of temperature. As shown in Table S6, the adsorption energy differences, ΔE, show that thermal vibrations have small effects on the magnitude of ΔE from 0 to 300 K. More importantly, the trend of ΔE for serine enantiomers on pure and Ni-decorated Cu(531) remains unchanged upon including the vibrational effects. Finally, we used the electronic properties, specifically the molecular-orbital density of states (MODOS), to gain more insight on the serine separation ability of pristine and Nidecorated Cu(531). The MODOS was calculated here by projecting the total density of states of the full system (molecule + substrate) onto the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO) of the free molecule, using the same geometry of the molecule adsorbed on the surface.58 Figure 6 shows the MODOS of L- and D-serine adsorbed on pristine and Nidecorated Cu(531). The last column of this figure shows ΔMODOS, which is defined as the difference of the MODOS between L- and D-serine adsorbed on the same metal substrate. Figure 6b shows that for both L- and D-serine, the HOMO and LUMO significantly broaden upon adsorption on Cu(531). However, the width for L-(311) (−5.5 to 0 eV) is larger than that for D-(311) (−5 to 0 eV). Further, the average position of L-serine′s HOMO center (−2.92 eV) is lower in energy than 8171

DOI: 10.1021/jacs.7b01216 J. Am. Chem. Soc. 2017, 139, 8167−8173

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b01216. Adsorption configurations of serine on Cu(531), Ag(531), Au(531), and Pd(531); detailed adsorption structures and energies and ΔE of serine on Ni/Cu(531) at different coverage; the comparison of the PBE+vdWsurf and optB88-vdW functionals for serine on pristine Cu(531) and 2 ML Ni/Cu(531); the vibrational free energies of serine on pure and Ni-decorated Cu(531) including Figures S1−S7 and Tables S1−S7 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Yonghui Wang: 0000-0001-8756-0091 Wei Liu: 0000-0003-3016-7381 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the NSF of China (21403113, 51602155), the NSF of Jiangsu Province (BK20150035, BK20130752), the Fundamental Research Funds for the Central Universities (30917011201), and the Foundation of Jiangsu Specially-Appointed Professor. A portion of this work, interpretation of the data, and writing of the manuscript, was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

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