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Langmuir 1998, 14, 862-867
Adsorption of Chiral Hydrocarbons on Chiral Platinum Surfaces David S. Sholl* Department of Chemistry, Yale University, PO Box 208107, New Haven, Connecticut 06520-8107 Received August 1, 1997. In Final Form: November 25, 1997 Binding energies and configurations of chiral hydrocarbons adsorbed on stepped Pt surfaces are calculated using Monte Carlo simulations. McFadden et al. have recently shown that many stepped single-crystal surfaces have chiral characteristics and that chiral molecules adsorbed on these surfaces should display enantiospecific properties [McFadden, C. F.; Cremer, P. S.; Gellman, A. J. Langmuir 1996, 12, 2483]. The results presented here provide the first theoretical predictions of this phenomenon by measuring enantiomeric shifts in the binding energies of chiral hydrocarbons on chiral Pt surfaces. Multiple examples of enantiomeric shifts that should be readily detectable using temperature-programmed desorption experiments are identified. The role of adsorbate size and surface structure on the magnitude of enantiomeric shifts in binding energies is discussed.
1. Introduction The enormous number of chemicals whose useful properties are enantiospecific has driven efforts to perform enantioselective separations and chemistry, since the 1920s.1 Techniques used to perform enantioselective separations1-6 or catalysis7-10 often involve adsorbing chiral modifiers on an achiral substrate. The resulting material interacts differently with different enantiomers of the compound of interest, allowing enantiospecific processes to occur. Despite the success of many of these processes, the precise mechanisms that allow enantioselectivity are not generally well understood. Recently, McFadden et al. described a class of physical systems where the techniques of surface science should be able to provide detailed characterizations of enantiospecific adsorption properties.11 They pointed out that many single-crystal surfaces include kinked surface steps where the step lengths on either side of the kinks are unequal. If a surface with this property is reflected through a plane normal to the surface, the new surface cannot be superimposed on the original surface. Hence, single-crystal surfaces with asymmetric steps are chiral. In principle, different enantiomers of chiral compounds may exhibit different adsorption properties when they are adsorbed on these surfaces. These systems promise the possibility of studying enantiospecific adsorption under highly characterized and controlled circumstances.11 McFadden et al. also defined a nomenclature that explicitly distinguishes between nonsuperimposable surfaces that are related by a planar reflection.11 * Permanent address: Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890. E-mail:
[email protected]. (1) Pirkle, W. H.; Pochapsky, T. C. Chem. Rev. 1989, 89, 347. (2) Gu¨bitz, G. Chromatographia 1990, 30, 555. (3) Armstrong, D. W.; Tang, Y.; Ward, T. Anal. Chem. 1993, 65, 1114. (4) Armstrong, D. W.; Tang, Y.; Chen, S. Anal. Chem. 1994, 66, 1473. (5) Whatley, J. A. J. Chromatogr. 1995, 697, 257. (6) Richards, A.; McGague, R. Chem. Ind. 1997, 422. (7) Fu, L.; Kung, H. H.; Sachtler, W. M. H. J. Mol. Catal. 1987, 42, 29. (8) Sutherland, I. M.; Ibbotson, A.; Moyes, R. B.; Wells, P. B. J. Catal. 1990, 125, 77. (9) Minder, B.; et al. J. Catal. 1995, 154, 371. (10) Singh, U. K.; et al. J. Catal. 1995, 154, 91. (11) McFadden, C. F.; Cremer, P. S.; Gellman, A. J. Langmuir 1996, 12, 2483.
To attempt to demonstrate the enantiospecific adsorption of molecules on chiral single-crystal surfaces, McFadden et al. used temperature-programmed desorption (TPD) to measure the binding energy of several chiral alcohols on Ag(643) and Ag(6 4 3).11 With these experiments, it should have been possible to resolve differences in binding energy as small as 0.1 kcal/mol. Unfortunately, no measurable differences between the binding energies of (R)- and (S)-2-butanol on Ag(643) and Ag(6 4 3) were detected.11 McFadden et al. were also unable to measure any enantiospecific effects in the kinetics of alkoxide decomposition on the same surfaces.11 Thus, although the possibility of enantiospecific adsorption properties involving chiral single-crystal surfaces is clear, no demonstration of this phenomenon has been made. In this paper, theoretical calculations of the adsorption properties of a number of chiral hydrocarbons on chiral Pt surfaces are presented. The principle prediction of these calculations is that chiral single-crystal surfaces can exhibit significant enantiospecific adsorption properties. Multiple examples of differences between binding energies of enantiomeric pairs that should be readily detectable experimentally are given. In addition to providing the first predictions of this type of enantiospecificity, the results below also allow an examination of the roles of adsorbate size and surface structure on the properties of chiral adsorbates. The methods used to model the adsorption properties of hydrocarbons on singlecrystal surfaces are described in section 2. In section 3, the first examples of enantiospecific adsorption on singlecrystal surfaces are presented by investigating the adsorption of various chiral hydrocarbons on Pt(643). The role of surface structure on enantiomeric differences in adsorption properties is examined in section 4, where the results of section 3 are extended to include several other chiral Pt surfaces. The prospects for identifying other adsorbates and substrates with significant enantiospecific properties as well as the results of the paper are discussed in section 5. 2. Model and Methods The binding energies of molecules on single-crystal surfaces can be computed using Monte Carlo (MC) methods once the interatomic forces between atoms of the adsorbate and substrate have been specified. An important restriction on the adsorbates
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Figure 1. Structures of (a) (R)-(-)-1,3-dimethylallene and (b) (R)-(+)-limonene. and substrates that can be investigated with these methods is that a reliable interatomic potential must be available, or developed, before any further progress can be made. One particularly useful interatomic potential for our purposes has been developed by Stinnett, Madix and co-workers12-14 to describe the adsorption of hydrocarbons on Pt surfaces. This potential was parametrized to fit an extensive experimental data set for the interaction of ethane with Pt(111).12 The chosen parametrization provided a good description of the dependence of the ethane-trapping probability on incident energy and angle, as well as the angular distribution of ethane scattered from the surface.12 Subsequent simulations using this potential provided accurate quantitative predictions of the complex dependence on incident energy and the angle of ethane’s trapping probability on Pt(110)-(1 × 2).13 The potential has also been used to describe methane and propane scattering from Pt(111) and Pt(110)-(1 × 2).14 Within this potential, hydrocarbons are described as collections of united atoms (UAs). That is, each carbon atom and any hydrogen atoms bonded to it are represented by a single point mass. The potential energy of an adsorbed molecule is then
V)
∑∑ V k
UA(rk
- rj )
(1)
j
where rk and rj are the coordinates of united atoms in the adsorbed molecule and atoms in the surface, respectively. The UA surface potential is taken to be a Morse potential.12-14 The same Morse potential is used for all UAs, regardless of the number of hydrogen atoms bonded to them.14 To speed up the calculation of V, the UA surface potential was set to zero at separations larger than 9 Å and the potential was shifted to ensure its continuity.12,15 The adsorption properties of four chiral hydrocarbons have been examined: trans-1,2-dimethylcyclopropane,16 trans-1,2dimethylcyclobutane,16 1,3-dimethylallene,16,17 and limonene.18,19 The first two of these compounds are the smallest in a large family of chiral disubstituted cycloalkanes.16 In the remainder of this paper, the trans designation of these molecules will be omitted for brevity. Although the disubstituted cycloalkanes are interesting examples for theoretical studies of adsorption because of their small size and nonplanar geometries, they are unfortunately not available in enantiomerically resolved forms. In contrast, 1,3-dimethylallene16,17 and limonene18 can be enantiomerically resolved and therefore are feasible candidates for future experimental examination. The structure of these molecules is shown in Figure 1. From the point of view of ease of availability, limonene is a particularly attractive candidate, because pure samples of either enantiomer are inexpensive and readily available.18 Fortuitously, limonene is predicted to have (12) Stinnett, J. A.; Madix, R. J.; Tully, J. C. J. Chem. Phys. 1996, 104, 3134. (13) Stinnett, J. A.; McMaster, M. C.; Madix, R. J. Surf. Sci. 1996, 365, 683. (14) Stinnett, J. A.; Madix, R. J. J. Chem. Phys. 1996, 105, 1609. (15) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Clarendon Press: Oxford, 1987. (16) Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry, 3rd ed.; Plenum Press: New York, 1990. (17) Waters, W. L.; Linn, W. S.; Caserio, M. C. J. Am. Chem. Soc. 1968, 90, 6741. (18) Catalog Handbook of Fine Chemicals; Aldrich Chemical Co.: Milwaukee, WI, 1994. (19) Egˇe, S. N. Organic Chemistry, 2nd ed.; D. C. Heath and Company: Lexington, 1989.
the largest enantiospecific adsorption properties of the four compounds listed above, as will be shown in section 3. It is important to note that the potential of Stinnett et al.12-14 was developed for saturated hydrocarbons, so applying it to unsaturated hydrocarbons such as allene and limonene must be regarded as a significant approximation. The geometries of each of the four hydrocarbons listed above were optimized within the MM2 potential20 using a commercial software package.21 The united atom positions within each molecule were taken to be the carbon positions of the optimized structure. Throughout the remainder of this paper, the intramolecular geometries of the adsorbates are assumed to be rigid; no internal vibrations or torsions are considered. While fixing the adsorbate’s internal coordinates is certainly an accurate approximation for rigid molecules such as 1,2-dimethylcyclopropane, this approximation may be less accurate for larger molecules such as limonene. In either case, the importance of intramolecular flexibility can be minimized experimentally by examining adsorbates at low temperatures. A more subtle issue that is not addressed by this approximation is that the optimal geometries of a molecule in a vacuum and adsorbed on a surface are generally slightly different because of the heterogeneous environment presented by the surface. Previous examinations of hydrocarbons adsorbed on Pt surfaces indicate that the influence of these surfaces on the optimal geometries of their adsorbates is small.22,23 The coordinates of the chiral surfaces were generated by truncating a bulk Pt crystal along the appropriate plane. All of the surface atoms were held in their bulk positions. This approach implicitly assumes that the single-crystal surfaces examined do not undergo reconstructions. All of the surface atoms within the cutoff distance of the UA surface potential were used to compute the contribution of each UA to the potential energy of the adsorbate. Using the model and potential described above, Monte Carlo (MC) simulations were used to compute the canonically averaged binding energy of single adsorbates on Pt surfaces, Eb ) 〈|V|〉T. By restricting attention to the adsorption properties of single adsorbates, the complications and computational effort inherent in simulating ensembles of interacting adsorbates can be avoided. Since the strength of adsorbate-adsorbate interactions between hydrocarbons adsorbed on Pt surfaces is generally weak relative to the adsorbate-surface interactions,23 the isolated adsorbate limit can be studied experimentally by using low surface coverages. In these simulations, new adsorbate configurations were generated using either rigid body rotations or translations of the adsorbate. The acceptance probability of a new configuration was determined using the usual Metropolis algorithm.15 The convergence of these calculations was substantially enhanced by using umbrella sampling.15,24 Specifically, Metropolis MC simulations were performed at temperatures 2-3 times higher than the target temperature, and the results of those simulations were reweighted to derive canonically averaged quantities at the target temperature. It must be emphasized that this procedure is merely a useful mathematical device for accurately sampling the canonical phase space distributions at the target temperature. In all of the results reported below, each MC trajectory began by choosing the adsorbate’s orientation randomly. The adsorbate’s position was also chosen randomly within a plane lying parallel to the surface. The configuration of the adsorbate was then followed for 105 MC moves before data collection was begun. The adsorbate’s orientation and energy were then averaged over 5 × 105 moves. To compute the final canonical averages, this procedure was repeated 30-50 times. The uncertainties in binding energies were taken to be the standard deviations of the data from these trajectories taken (20) Burkert, U.; Allinger, N. L. Molecular Mechanics; American Chemical Society: Washington, DC, 1982. (21) CSChem3D; Cambridge Scientific Computing Inc. (22) Huang, D.; Chen, Y.; Fichthorn, K. A. Mol. Simul. 1994, 13, 285. (23) Raut, J. S.; Sholl, D. S.; Fichthorn, K. A. Surf. Sci. 1997, 389, 88. (24) Torrie, G. M.; Valleau, J. P. J. Comput. Phys. 1977, 23, 187.
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Figure 2. View of the united-atom model of (S)-1,2-dimethylcyclopropane. one at a time. All other averaged quantities were averaged over all of the individual trajectories. The steps taken to ensure accurate convergence of this method to the true canonical averages are described in section 3.
3. Enantiomeric Shifts on Pt(643) Using the methods described in section 2, the adsorption energetics of four chiral hydrocarbons on a variety of Pt surfaces have been examined. In this section, the enantiomeric shifts in adsorption energies on Pt(643) are presented and discussed. The (643) face of an fcc crystal is chiral because of the asymmetric placement of kinks along its step edges.11 Before examining the enantiomeric shifts in adsorption energy between different enantiomers adsorbed on Pt(643), it is instructive to briefly examine the difference between the adsorption of hydrocarbons on flat and stepped Pt surfaces. One way to study this issue is to characterize the orientation of adsorbed molecules. As a specific example, consider the orientation of 1,2-dimethylcyclopropane on Pt(643) and Pt(111). This choice of surfaces is useful because the terraces between steps on Pt(643) have the same structure as Pt(111). One view of the united-atom model of (S)-1,2-dimethylcyclopropane is shown in Figure 2. To fully specify the orientation of a rigid adsorbate, two vectors are needed. For clarity, just one orientation vector will be discussed: a vector oriented along the bond between the methyl-substituted C atoms (see Figure 2). The orientation of this vector is described by the polar and azimuthal angles it defines (θ and φ, respectively) in a coordinate system where a vector normal to the surface has φ ) 0. Figure 3 shows the normalized angular distributions of φ and θ for 1,2dimethylcyclopropane adsorbed on Pt(643) and Pt(111) at 200 K as measured from MC simulations. This molecule is invariant under 180° rotations about an axis orthogonal to the orientation vector considered in Figure 3, so P(φ) is symmetric about φ ) 90°, and P(θ+π) ) P(θ). Note that because Pt(111) is an achiral surface, the results for Pt(111) in Figure 3 are independent of the enantiomer used in the simulations. The differences between the flat and stepped surface results are most evident in the distribution of polar angles (Figure 3b). On Pt(111), the polar angles of the adsorbates are almost equally distributed across all possible angles, although a small effect from the surface’s 3-fold symmetry can be seen. In contrast, the polar angle distributions on Pt(643) have a great deal of structure, indicating that the surface steps play an important role in the orientation of the adsorbates. It is useful to note that the multiple peaks in the angular distributions can be used to verify the convergence of the MC simulations used to measure these distributions. In
Figure 3. Angular distributions of (a) φ and (b) θ for 1,2dimethylcyclopropane/Pt(111) (solid curve), (S)-1,2-dimethylcyclopropane/Pt(643) (dotted curve), and (R)-1,2-dimethylcyclopropane/Pt(643) (dashed curve) at 200 K.
Figure 4. Energetically favorable configurations for (S)-1,2dimethylcyclopropane (top) and (R)-1,2-dimethylcyclopropane (bottom) adsorbed on Pt(643).
all of the simulations reported in this paper, the parameters controlling the MC simulations were adjusted so that each individual MC trajectory sampled all of the peaks in the angular distributions. The data in Figure 3 also indicate that the orientations of the two enantiomers of 1,2-dimethylcyclopropane adsorbed on Pt(643) are substantially different. Qualitatively, the angular distributions of the (R) enantiomer are more peaked and exhibit less structure than the (S) enantiomer orientations. The peaks of P(θ) for the (R) enantiomer correspond to orientation vectors approximately parallel to the Pt(643) surface steps, and the simple form of the (R) distributions suggests that there is essentially one type of favorable orientation for this molecule. In contrast, the (S) distributions appear to be made up of contributions from several qualitatively different adsorption orientations. To give another perspective on the orientations of the adsorbed molecules, the minimum energy configurations observed during the MC simulations used to generate Figure 3 are shown in Figures 4 and 5. The two enantiomers are shown on the same surface for conven-
Chiral Hydrocarbons on Chiral Platinum Surfaces
Langmuir, Vol. 14, No. 4, 1998 865 Table 2. Binding Energies and Enantiomeric Shifts of Chiral Hydrocarbons on Pt(643)a adsorbate
E(R) b
E(S) b
∆Eb
limonene 1,2-dimethylcyclopropane 1,2-dimethylcyclobutane 1,3-dimethylallene
33.19 19.08 20.95 19.98
35.30 18.00 20.24 19.96
-2.11 ( 0.22 1.08 ( 0.06 0.71 ( 0.29 0.02 ( 0.22
a
Figure 5. Side view of the same adsorption configurations shown in Figure 4. Table 1. Binding Energies of 1,2-Dimethylcyclopropane on Pt(643) and Pt(111) at 200 K enantiomer
surface
Eb (kcal/mol)
(R) (S)
Pt(111) Pt(643) Pt(643)
13.31 19.08 18.00
ience; the adsorbates did not interact with each other during the MC simulations. The (S)-1,2-dimethylcyclopropane configuration in Figures 4 and 5 has φ ) 41.2° and θ ) 8.6°, while the (R) enantiomer has φ ) 127.9° and θ ) 52.5°. Both enantiomers are oriented so that they occupy the kink formed by the surface step. In these orientations, the (S) enantiomer extends considerably further across the terrace toward the next step than the (R) enantiomer. Figures 4 and 5 suggest that the (R) enantiomer “fits” into the kink site more closely than the (S) enantiomer, which is consistent with the fact that the (R) enantiomer is more tightly bound and occupies a smaller range of orientations than the (S) enantiomer (cf. Figure 3 and Table 1). The results above are the first quantitative calculations showing how the chirality of metal surfaces can affect the adsorption properties of chiral adsorbates. Unfortunately, the experimental determination of adsorbate orientation is a difficult task (although significant progress in this area is being made25-29). Thus, it seems more useful to characterize properties of chiral adsorbates that can be readily measured experimentally. One such property is the adsorbate binding energy, which can be measured experimentally using TPD.11 The binding energies of the adsorbates discussed above were measured as described in section 2, and the results are summarized in Table 1. The statistical uncertainty in each of the binding energies in Table 1 is approximately 0.03 kcal/mol. As mentioned above, the Pt(111) results are independent of the enantiomer simulated because Pt(111) is achiral. Two important observations can be made from Table 1. First, the binding energy of the adsorbates is approximately 40% greater on the stepped surface than on Pt(111). The binding energy of each united atom in an adsorbate is largely determined by the number of surface atoms in its immediate vicinity. United atoms below a step edge, or, in particular, in a “corner” formed by a step kink, gain binding energy from the surface atoms forming the step in addition to the binding energy due to the terrace atoms. Thus, adsorbate binding energies on stepped surfaces will in general be larger than those on flat surfaces. The second observation to be made from Table (25) Solomon, J. L.; Madix, R. J.; Sto¨hr, J. J. Chem. Phys. 1991, 94, 4012. (26) Woodruff, D. P.; Bradshaw, A. M. Rep. Prog. Phys. 1994, 57, 1029. (27) Woodruff, D. P.; et al. Surf. Sci. 1996, 357/358, 19. (28) Samant, M. G.; et al. Macromolecules 1996, 29, 8334. (29) Street, S. C.; Gellman, A. J. J. Chem. Phys. 1996, 105, 7158.
All energies are in kcal/mol.
1 is that there is a substantial difference between the binding energies of the two enantiomers on Pt(643). These results predict an enantiomeric shift of ∆Eb ) E(R) b E(S) ) 1.08 ( 0.06 kcal/mol for 1,2-dimethylcyclopropane b adsorbed on Pt(643). A shift of this magnitude should be readily detectable using the experiments described in ref 11. The binding energies of the other three chiral hydrocarbons listed in section 2 adsorbed on Pt(643) have also been calculated. The results of these calculations, as well as the enantiomeric shifts in binding energy for each compound, are summarized in Table 2. All of the data in Table 2 were taken at a temperature of 200 K except for the limonene data, which was recorded at 250 K to aid the convergence of the simulations. These results lend strong support to the idea that chiral surfaces will exhibit enantiospecific adsorption properties under quite general circumstances. Three of the four adsorbates examined, limonene, 1,2-dimethylcyclopropane, and 1,2-dimethylcyclobutane, have predicted values of ∆EB that far exceed the shifts detectable in current experiments.11 Table 2 demonstrates that while increasing molecular weight does in general lead to increased binding energies, the size of the enantiomeric shift in binding energies does not necessarily increase with adsorbate size (cf. ∆Eb for 1,2dimethylcyclopropane and 1,2-dimethylcyclobutane). The results in Table 2 for 1,2-dimethylallene, together with the experimental measurements of McFadden et al.,11 indicate that not every chiral compound exhibits a significant enantiomeric shift when adsorbed on a chiral surface. 4. Enantiomeric Shifts on Other Chiral Pt Surfaces The (643) surface examined above and in ref 11 is just one of many chiral surfaces that can be formed from fcc crystals, since any stepped surface whose step structure is asymmetric is a chiral surface.11 In this section, the question of how enantiomeric shifts in adsorption energies vary as a function of the surface structure is examined by comparing the adsorption properties of limonene and 1,2dimethylcyclopropane on several chiral Pt surfaces. These two adsorbates showed the largest enantiomeric shifts of the compounds whose adsorption properties on Pt(643) were described in section 2. The number of chiral surface faces that could be generated from an fcc crystal is vast. To simplify the comparison of different surfaces, the adsorption properties of adsorbates on Pt(532), Pt(643), and Pt(754) were examined. These three surfaces have the same step structure; the step edges are comprised of steps alternately one and two atoms long (see Figures 2 and 3). Furthermore, all three surfaces have the same stereochemical designation, (S), in the nomenclature defined by McFadden et al.11 The three surfaces differ only in the width of the terraces between steps. Using the methods described above, the binding energies and enantiomeric shifts of limonene and 1,2-dimethylcyclopropane were calculated on each surface.
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Table 3. Binding Energies and Enantiomeric Shifts of Limonene Enantiomers Adsorbed on Pt Surfaces at 250 Ka
Table 4. Binding Energies and Enantiomeric Shifts of 1,2-Dimethylcyclopropane Enantiomers Adsorbed on Pt Surfaces at 200 Ka
surface
Aterrace (Å2)
E(R) b
E(S) b
∆Eb
surface
Aterrace (Å2)
E(R) b
E(S) b
∆Eb
Pt(532) Pt(643) Pt(754) Pt(111)
52.95 60.00 72.88 ∞
31.49 33.19 33.56 27.83
34.20 35.30 35.27 27.83
-2.71 ( 0.23 -2.11 ( 0.22 -1.71 ( 0.49 0
Pt(532) Pt(643) Pt(754) Pt(111)
52.95 60.00 72.88 ∞
18.91 19.08 19.09 13.31
17.81 18.00 18.04 13.31
1.10 ( 0.06 1.08 ( 0.06 1.05 ( 0.06 0
a
All energies are in kcal/mol.
Figure 6. Energetically favorable configuration for (S)limonene adsorbed on Pt(532).
Figure 7. Energetically favorable configuration for (S)limonene adsorbed on Pt(754).
The binding energies and enantiomeric shifts for limonene adsorbed on Pt(532), Pt(643), Pt(754), and Pt(111) are summarized in Table 3. The area of the surface’s unit cell, Aterrace, is also listed in the table, and the surfaces are listed in order of increasing Aterrace. All of the binding energies on the stepped surfaces are higher than those on Pt(111) due to the binding of the adsorbates at the step edge. One useful way to interpret the fact that ∆Eb vanishes on Pt(111) is that the enantiomeric shift per kink site approaches a constant value as the terrace area increases and the density of kink sites goes to zero in the limit of a (111) surface. The most important result in Table 3 is that the enantiomeric shift in binding energy is quite strongly dependent on the surface terrace size. The magnitude of ∆Eb increases as the surface terrace size is decreased. The shift of -2.71 kcal/mol seen on Pt(532) is the largest enantiomeric shift that has been predicted to date. The change in ∆EB as a function of the surface terrace area can be understood by examining the configurations of (S)-limonene on Pt(532) and Pt(754). Figures 6 and 7 show the minimum energy configuration of these adsorbate/substrate pairs observed during the calculation of the binding energies of these systems. In both cases the adsorbate adsorbs immediately adjacent to an ascending step. Not surprisingly, the adsorbates’ orientations with respect to the steps are very similar. The differences between the two systems arise because of the substrate atoms underneath the portions of the adsorbate that are furthest from the step edge. Figure 6 shows that, on Pt(532), the adsorbate is approximately as wide as the surface terrace, so part of the adsorbate lies close to a kink site of a descending step. In contrast, the adsorbed molecule does not closely approach the descend-
a
All energies are in kcal/mol.
ing step on Pt(754) (see Figure 7) because the surface terraces on this surface are considerably wider than the adsorbate. Thus, the binding energy of (S)-limonene is less on Pt(532) than on Pt(754). More significantly, (S)limonene molecules on Pt(532) interact with two kink sites, while on Pt(754) the adsorbate only interacts with one kink site. It is these interactions that are the origin of the changes in ∆Eb as the terrace area is changed. This observation suggests that a useful strategy for finding adsorbate/substrate pairs that exhibit large enantiomeric shifts in binding energy may be to seek systems where adsorbates can interact with multiple kink sites simultaneously. This type of interaction can be achieved either by using surfaces with narrow terraces [such as the (532) surface] or by using adsorbates that must span multiple unit cells. The calculated adsorption energetics of 1,2-dimethylcyclopropane on Pt(532), Pt(643), Pt(754), and Pt(111) are summarized in Table 4. Unlike the case for limonene, the enantiomeric shifts of 1,2-dimethylcyclopropane show only a weak dependence on the substrate terrace size. As with limonene, the magnitude of the enantiomeric shift increases as the terrace area decreases due to the interactions between the adsorbed molecules and the descending step on the opposite side of the terrace from the kink site where they adsorb (see Figures 4 and 5). Because the overall interactions between the 1,2-dimethylcyclopropane molecules and the descending step edge are much weaker than the analogous interactions experienced by limonene molecules, the terrace area dependence of ∆Eb is much weaker for 1,2-dimethylcyclopropane than for limonene. The results presented in this section demonstrate that enantiospecific adsorption properties can, in principle, be optimized by examining a variety of chiral substrates. In particular, by using a surface with a terrace width comparable to the adsorbate size, the magnitude of the enantiomeric shift in binding energies can be significantly increased for limonene molecules adsorbed on Pt surfaces. Although the results above have only characterized three chiral surfaces, they indicate that the study of how terrace width and step structure affect the adsorption of chiral molecules will be a productive direction for future work. 5. Conclusion The results presented above are the first theoretical calculations to examine the adsorption of chiral molecules on chiral single-crystal surfaces, and they provide the first predictions of enantiospecific adsorption properties on these surfaces. These results suggest that the study of chiral adsorption on single-crystal surfaces may be a fruitful area for developing a fundamental understanding of the mechanisms and capabilities of enantioselective heterogeneous catalysis. Multiple examples of adsorbates whose enantiomeric shifts in binding energy should be detectable using TPD experiments have been identified. Perhaps the most promising candidate for experimental examination is limonene, which is predicted to have an
Chiral Hydrocarbons on Chiral Platinum Surfaces
enantiomeric shift in excess of 2 kcal/mol on Pt(532) and Pt(643) and is readily available in enantiomerically pure form.18 Theoretical simulations of chiral adsorption possess a number of strengths that make them an extremely useful complement to experimental investigations. For example, information about the orientation of adsorbed molecules is straightforward to extract from simulations such as those used here. It is generally quite difficult to determine similar information experimentally.25-29 One of the greatest strengths of the simulations reported here is the ease with which multiple substrates may be examined. This ability has been used to measure the enantiospecific properties of limonene and 1,2-dimethylcyclopropane adsorbed on various chiral Pt surfaces. The results indicate that enantiospecific adsorption properties can be substantially enhanced by using a chiral surface with a structure appropriate for the adsorbate of interest. For example, limonene’s enantiomeric shift in binding energy is significantly larger on Pt(532) than on Pt(643) or Pt(754). Each of these surfaces has the same step structure, but their terrace widths differ. On the Pt(532) surface the terraces are sufficiently narrow for the adsorbed molecules to interact closely with two distinct surface kink sites. The results presented in this paper have examined only a few of the vast number of possible chiral adsorbate/ substrate combinations. There are several areas that will be important to explore in future theoretical studies of chiral adsorption as a fundamental and general understanding of this phenomenon is sought. One such direction is to assess the importance of the assumptions underlying the models used here. The calculations reported here have all dealt with rigid adsorbates on rigid surfaces in the limit of zero coverage. While this situation can in principle
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be approached experimentally by using very low exposures at low temperatures, it would be very useful to extend the methods used here to include intramolecular strain and relaxation and adsorbate-adsorbate interactions. Although these effects can be naturally incorporated into the types of models used above, they will in general greatly increase the computational effort needed to study adsorption. Inclusion of intramolecular degrees of freedom will be particularly important if direct comparisons between TPD experiments and theoretical calculations are to be made, since conformer interconversions may be important at elevated temperatures. Similarly, the possible relaxation or mobility of surface step edges will need to be assessed before the high-temperature behavior of these systems can be accurately modeled. A second direction that would increase the applicability of theoretical methods to chiral adsorption is the continued development of interatomic potentials that can describe adsorbed species. For example, a potential describing the adsorption of alcohols on metal surfaces would greatly increase the variety of chiral adsorbates that could be examined theoretically. Similarly, a potential that explicitly accounted for differences between the adsorption of saturated and unsaturated hydrocarbons would also be very useful. Finally, it would also be interesting to use methods such as those described above to examine the adsorption of chiral molecules on chiral surfaces in the presence of chiral or achiral coadsorbates. Acknowledgment. The assistance of M. Plount and D. Price in determining the optimal structure of limonene is greatly appreciated, as are helpful discussions with J. Kindt and A. Gellman. LA9708546
