Density Functional Theory Study of the Enantiospecificity of Intrinsically Chiral Surfaces

Percy C. Weintraub

Department of Chemical EngineeringUniversity of Florida

Honors Thesis Submission for Graduating Suma Cum LaudeBachelor of Science in Chemical Engineering, Spring 2010

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Table of Contents

Abstract……………………………………………………………………………………………3

I. Motivation and Background……………………………...……………………………………4

II. Modeling Details……………………………………………………………….………….......7

(i) Chiral Surfaces………..…………………………………….………………...…….…8

(ii) Chiral Molecules……………………………………………………………….....…10

(iii) Details of DFT Calculations………………………………………………………..11

(iv) Procedure for Probing Local Minima........................................................................11

III. Adsorption on Pt (874)……………………………………………………………………...13

(i) AMT…………………………......................................................................................13

(ii) FAM…………………………………………………………………………………14

(iii) AE…………………………………………………………………………………..16

IV. Adsorption on Cu (874)……………………………………………………………………..18

(i) FAM.............................................................................................................................18

(ii) AE…………………………………………………………………………………....21

V. Conclusion……………………………………………………………………………………22

Acknowledgements…………………………………………..…………………………………..23

References……………………………………………………………………………………….24

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Abstract

The search for novel enantio-purification methods is extremely important to the

pharmaceutical industry considering the role of chiral interactions within the human body. One

enantiomer of a drug may exhibit desired properties whereas the other may produce adverse or

even toxic effects despite both having identical physical properties. Many simple metals with

high Miller indices demonstrate enantioselectivity due to intrinsic chirality characterized by kink

sites on the surface. However, the dominance of flat terraces, homogeneity of the metal atoms,

and cost of these materials are several drawbacks to practical implementation for

enantiopurification processes. The use of intrinsically chiral metal oxides as a template for chiral

metal nanostructures offers the potential to circumvent many of these problems. As a first step

towards this goal, this thesis reports a Density Functional Theory study of the enantiospecific

adsorption of small molecules on intrinsically chiral Pt and Cu (874) surfaces. We do find

enantiospecific binding, but the results are not entirely consistent with earlier work by Bhatia and

Sholl. Further studies to resolve these issues and establish the differences between Cu and Pt

enantiospecificity will be the short-term future work. This research serves as an initial step in

probing whether catalytically active metal nanostructures can be grown on chiral metal oxide

surfaces and tailored to enhance enantiospecificity.

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I. Motivation and Background

The term chirality is derived from the Greek

word “” (cheir) meaning hand. The left and

right hand are related by mirror symmetry but are not

super imposable, and a similar relationship holds for

chiral molecules (see Figure 1). Pairs of mirror

image molecules are referred to as enantiomers. The

phenomena of molecular chirality have been of great

interest since Pasteur’s work with tartaric acid in

1849. His observations demonstrated that crystals

derived from tartaric acid via organic sources rotated

polarized light, a property of a pure enantiomeric

species; however, similar crystals synthesized

chemically resulted in no polarization of light despite having identical chemical formulas,

molecular weights, and boiling points. After discovering a mechanism to separate the two types

of crystals, Pasteur was able to show rotation of polarized light in opposite directions. With his

research, Pasteur introduced the molecular origin of chiral molecules and established that

biological systems are homochiral, whereas non-biological chemical synthesis results in racemic

mixtures (50:50 mixtures of the two enantiomers, which as a result do not rotate polarized light)

[1].

Ever since the work of Pasteur there has been a search for efficient routes to obtain

enantiomerically pure chiral molecules from chemical processes. Chiral processing is of extreme

importance in the pharmaceutical industry considering the important functions and consequences

of chiral interactions within the human body. While the physical properties of an enantiomeric

pair are identical, their interactions in biological systems are often immensely different. Drug

interactions with a particular enzyme are heavily dependent on the use of the appropriate pure

enantiomer. Sometimes, one enantiomer of a drug may have desired behavior and potency,

while the other may be inactive or produce adverse and even toxic effects. For example,

thalidomide, a drug prescribed in the 1950’s to treat morning sickness, greatly exploited the

magnitude of using enantiomerically pure pharmaceuticals due to the repercussions of

administering both forms. While one molecule has sedative properties, the enantiomer was

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Figure 1: Illustration of the “handness” of chiral molecules. Isomers of Alanine are shown, and similar to the left and right hands they are mirror image forms that are non-superimposable and are termed enantiomers.

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discovered to cause extreme birth defects, highlighting the importance of developing novel chiral

separation processes [2].

Classical separation methods such as distillation or extraction are not viable routes

towards enantio-purification since pairs of enantiomers have the same physical properties. One

existing method for the production of chiral surfaces includes the binding of organic molecules

to an achiral metal surface (i.e. the surface lacks chiral functionality without the addition of

molecules). If chiral molecules can permanently adsorb to the active metal surface, the resulting

heterogeneous catalyst can offer enantiospecificity due to the chirality of the ligands. The

mechanism for the enantiospecific adsorption of molecules onto this new surface, however, is

not well understood and thus the synthesis of such catalysts is a complex process [3]. Similarly,

while other existing methods can separate chiral molecules effectively, it remains a black art to

tailor the appropriate chiral ligands for specific molecules of interest [4]. Furthermore, such

systems are generally less stable and can become expensive due to patent issues. A potentially

attractive alternate is to use intrinsically or naturally chiral surfaces for enantioselective

separation or catalysis.

Recently, research has focused on the cleaving of

single metal crystal structures like platinum or copper to

form intrinsically chiral surfaces [5]. While these metals

have high symmetry in their bulk crystalline structures (see

Figure 2 for a schematic of a bulk face centered cubic

(FCC) crystal lattice), the proper choice of a cleavage plane

(hkl, where h ≠ k ≠ l) results in a surface face defined by

the presence of monatomic steps with kink sites separated

by terraces with a low-Miller index orientation. Figures

3(a) and 4(a) in Section II show a side and top view of Pt

(874), where the step-kink structure has been highlighted in Figure 4(a). The presence of the

kinked steps prevents the mirror image of the surface to be super-imposable and thus the surface

exhibits chirality. Specific chiral metal surfaces relevant to the research conducted for this thesis

will be discussed in more detail in Section II. Both experimental and theoretical works have

established that metal surfaces are indeed chiral and can distinguish between enantiomers [5-8].

Bhatia and Sholl have used Density Functional Theory (DFT) to examine a range of chiral

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Figure 2: A schematic of the high symmetry bulk FCC structure found for several metals including Pt and Cu.

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molecular surface interactions and enantiospecificity on Cu chiral surfaces [9]. DFT calculations

are very important because they offer another procedure to quantify enantioselective interactions

and some comprehension at the atomic level. This allows for stronger comparisons between the

use of different metals, cleavage planes, and their combinations.

While these chiral metal surfaces are enantioselective, they have several drawbacks.

First, while any (hkl) surface with non-equal Miller indices is chiral, the reality is that all of the

surfaces contain very similar local chiral environments since kinks are constructed from the

intersection of three low-Miller index facets and consist of homogenous metal atoms. Thus,

chiral metal surfaces do not generally provide a diverse range of chiral environments that can be

tuned for specific chiral molecules. Furthermore, although molecular interaction occurs at the

kink sites, the presence of the dominant flat terraces upon the metal surface reduces the overall

enantioselectivity. Finally, large surface areas of single-crystal chiral noble metal surfaces

would be outrageously expensive for practical application. The use of chiral metal oxide

surfaces, however, offers solutions to many of these drawbacks. Metal oxide surfaces can be

structurally similar to metal surfaces, but exhibit differences chemically. Figure 3 (a) and (b) in

Section II show the difference between chiral Pt (874) and a similar surface of SrTiO3 (874).

Clearly the SrTiO3 surface provides a chemically richer surface, which theoretically could offer

more specific molecule-surface interactions. Some previous work has begun to explore this

possibility of increased selectivity on bare metal oxides [10]. This method of creating

intrinsically chiral surfaces can provide cheaper systems that are potentially more robust and

chemically stable than their alternatives. The ultimate aim of this thesis is to explore another

possibility: Can metal films or nanostructures be grown on chiral metal oxide surfaces to

enhance enantioselectivity? The advantage of such systems would include a more efficient use

of the expensive but catalytically active metal (for example platinum) in a manner similar to

standard heterogeneous catalysis. While more research is needed before the above question can

be addressed, the remaining body of work describes research performed by myself in

collaboration with Tim Van Cleve and assistance from Beverly Brooks-Hinojosa, which will

help serve as a primary step.

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II. Modeling Details

The purpose of this work is to use DFT calculations to explore the enhanced

enantiospecific adsorption of chiral molecules on chiral metal oxide surfaces that have been

coated with pure metal nanostructures. First, it is important to establish that these pure metallic

surfaces are in fact intrinsically chiral and demonstrate a preference for a single enantiomer.

Then, the same molecules can be subjected to adsorption upon a surface that is prepared by

depositing the pure metal nanostructures onto chiral metal oxide surfaces which have been

terminated along the same high miller index plane as the pure metal surfaces. The latter surface

is suspected to demonstrate greater enantiospecificities due to both the existence of a kink site

that characterizes its chirality and the heterogeneous electronic structure induced by the

underlying metallic substrate. The use of theoretical modeling (i.e. DFT) can offer more

selective approaches to experimental work by providing insight on the geometries and energetics

of adsorption for different molecules in study.

The research being presented compares DFT calculations of the ground state adsorption

energies between enantiomeric pairs on the select surfaces. Differences in adsorption energies

for different confirmations of R and S enantiomer pairs demonstrate the enantiospecificity of a

surface and more specifically the favored isomer. For the rest of the paper, the difference in

ground state adsorption energies, denoted ΔE0, will be:

ΔE=E0 ( R )−E0(S)

where values for E0 represent the total energy of the enantiomer’s most favoured configuration.

Positive values for ΔE0 indicate that the S-enantiomer is preferred since more negative values for

E0 (eV) represent a more stable confirmation.

The following subsections describe the chiral surfaces and molecules used in this

research, the details of the DFT calculations, and the procedure used to probe for adsorption

minima.

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(i) Chiral Surfaces

The surfaces of interest include platinum and copper surfaces with ideally kinked step

edges defined by their miller indices {Pt (874) and Cu (874)} and the depositing of platinum

nano-particles onto a strontium titanate surface {Pt/SrTiO3 (874)} with the same terminating

miller index. Figure 3 shows platinum, SrTiO3, and platinum coated SrTiO3 surfaces with the

same terminating Miller index (874).

Figure 3: (a) Side view of Pt (874) (b) Side view of SrTiO3 (874) where (c) Side view of Pt (blue) growth onto a SrTiO3 (874) surface.

Pt growth on SrTiO3 is the currently preferred combination because the two metals share

very similar lattice structures and lattice constant, which minimizes the interface strain. Figure 4

demonstrates the chiral similarity between Pt (874) and SrTiO3 (874) while highlighting the step

site.

Figure 4: (a) Overhead view of Pt (874) highlighting step edge (b) Overhead view of SrTiO 3 (874) highlighting step edge. The identical Miller indices give identical step geometries.

Platinum has also been demonstrated to be very catalytically active in many systems. As

shown in Figure 3 (c), the preferred area for depositing platinum is at the kink sites and step

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edges of the metal oxide substrate. This could further promote enantiospecific selectivity by

creating a synergistic effect combining the catalytic activity of platinum with the chiral

selectivity of the step edge given by its termination at high Miller index. Furthermore, previous

experimentation to characterize SrTiO3 has been done in great detail. If larger chiral selectivity

can be demonstrated upon a Pt/SrTiO3 surface, then future research could explore the

enhancement of enantiospecificity with different terminating Miller indices (e.g. adjusting

terrace width) or through different combinations of metal substrates.

Figure 5 shows an overhead schematic of a Pt (874)

unit cell with its high miller index (874) kinked step

edge adjacent to its low miller index (111) oriented

flat terrace. A similar unit cell is used for Cu (874)

since the terminating miller index is the same, and

the two metal surfaces are expected to demonstrate

similar enantiospecificities. Intrinsic chirality is

characterized on these surfaces because of the well

defined step edge kink site that differs from the flat

terrace. Assuming that the 2-D representation in

Figure 5 represents the x-axis (going from left to

right) and the y-axis (going up and down), a

reflection of this surface about a plane perpendicular to Figure 3 (i.e. z-axis) and parallel to the

x-axis would give a surface that is not superimposable upon the original. Therefore, molecular

adsorption at these kink sites yield geometric confirmations specific to the chirality of the

molecule. It can also be noted that these step edges represent an ideally terminated Miller index

and deviations from ideality may exist due to thermal fluctuations [11]. The step edge atoms

within the unit cell have been labeled K, 1, 2 and 3 for ease of discussion.

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Figure 5: Top view of a single surface unit cell for Pt (874) with kink and step atoms labeled. Same unit cell for Cu (874).

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(ii) Chiral Molecules

The chiral molecules studied include Fluoro-Amino-Methoxy (FAM), Amino-Methoxy-

Thiolate (AMT), and Amino-Ethoxy (AE). Schematic representations of these molecules are

shown below in Figure 6, where the chiral carbon center is marked by an asterix (*).

Figure 6: Schematic representations of probe molecules (a) s-FAM (b) s-AMT (c) s-AE

Bhatia and Sholl have previously published extensive DFT studies of these molecules on Cu

surfaces which allowed for comparisons concerning the reproducibility and merits of their

results. This is important for analysis, but also very important considering the large amount of

possible confirmations on the metal surface. Their work also recorded that Amino-Ethoxy

Thiolate (AET) had the largest enantiospecificity on Cu surfaces [9]. AMT was chosen with this

observation in mind; AMT is slightly more simplistic and offers the possibility of demonstrating

enhanced enantiospecificity on the surfaces in study. FAM and AE were chosen based on results

obtained for AMT adsorption on Pt (874), which are discussed later.

For this paper, the R and S nomenclature for enantiomer pairs is adopted. When a carbon

atom in a molecule is bonded to four different groups, a ranking is assigned to each using Cahn-

Ingold-Prelog priority rules. This method of priority is based on the atomic number of the

element bonded to the chiral carbon center. The probe molecules described in this body of work

are all simplistic for ranking purposes in the sense that the first atom bonded to the chiral carbon

is different; therefore, priority is easily assigned to each atom in order of increasing atomic

number. By visualizing the molecule with the atom of lowest priority oriented away from the

viewer, a triangular form of the three remaining atoms is represented and the priorities decreases

in either a clockwise or counterclockwise fashion which is denoted by R and S respectively. For

example, for the case of FAM shown above in Figure 5, the hydrogen atom is of the lowest

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priority and is pointing away from rest of the atoms. Priority decreases from fluorine, to oxygen,

to nitrogen in a counter-clockwise motion giving the S form of the enantiomer accordingly. It is

important to note that this nomenclature does not offer any insight on the rotary direction of

polarized light for the molecule [5].

(iii) Details of DFT Calculations

All DFT calculations reported in subsequent sections were achieved through the Vienna

ab-initio Simulation Package (VASP). The program solves the Schrödinger equation using many

approximations that drastically shorten calculation time. The settings used in the program were

similar to those from the Bhatia and Sholl study [9]. While DFT as implemented in VASP is

rather efficient, the calculations are still time-consuming. The primary objective was to find

relevant adsorption minima for both enantiomers of our chiral probe molecules. The VASP code

takes a prepared initial configuration of the molecule above the surface and relaxes the system

until the forces on all the (free) atoms are less than 0.03 eV/Å.

(iv) Procedure for probing local minima

Computational work was constrained to the adsorption of a single molecule per surface

unit cell, where periodic boundary conditions were applied to simulate a bulk crystalline lattice

for the metal surfaces. Therefore, if calculations gave atom positions beyond the boundary of the

unit cell, its position reappeared on the opposite side of the unit cell with identical energy

criteria. Through the use of the Discovery Studios Visualizer by Accelrys®, output files from

VASP could be transformed to give visual representations of calculations with the unit cells

repeated appropriately. This tool allowed for analysis concerning the geometries of the actual

adsorption configurations obtained. Although probe molecules were only permitted to engage

with one side of the metal surface within a unit cell, an abundance of possible adsorption

confirmations exist due to the spatial dominance of the flat terrace. Previous research indicates

that adsorption of molecules on the flat terrace of the metal lead to smaller binding energies than

configurations along the step edge or kink [12]. This assumption was carried over to this body of

work so that configurations along the flat terrace were not examined which immensely simplified

the search for local energy minima. Therefore, the adsorption geometries of interest were

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realized by using initial configurations of probe molecules with orientations primarily in the

vicinity of the step edge.

Each molecule of interest was formed by relaxing the corresponding atoms in their

appropriate position in a system absent of metal atoms until the relaxation criteria were reached.

Bond distances were verified, and initial position files for calculations were prepared by simply

translating the molecule above different metal atoms along the kink site. This translation was not

performed in VASP, but rather with program scripts written in Fortran. First, the metal atom of

interest and its position were identified. This generally was any atom involved with the kink site

(atoms K, 1, 2, or 3 from Figure 5). Next, the Cartesian coordinates of the molecule were

converted to fractional coordinates (coordinate system where the edges of unit cell describe the

basis vectors) and the particular atom of choice was oriented directly above the identified metal

atom. While this program script allowed for orientation above the kink site, it’s important to

note that adsorptions were also dependent on which particular groups of the molecule resided

over the top or bottom of the step site. Hence, another Fortran code was used to rotate the

molecule appropriately. After designating the chiral carbon as the reference atom and checking

that the fractional coordinates of the system were within the correct boundaries, a value for Ɵ

was input signifying a rotation about the z-axis of the molecule. Next, a value for φ was input

describing a rotation about the y-axis. In this manner, the two program scripts were used to

examine adsorptions upon the metal surface from a variety of initial positions. Often, this was

done in such a way to mimic confirmations of local minima described by previous DFT studies

upon Cu (874). Rotation about the axis signified by a carbon to bonding group was not

attempted under the assumption that rotations would proceed during VASP calculations. After

translation and rotation, the corresponding enantiomer was generated by switching the positions

between two of the molecule’s bonding groups while adjusting the bond lengths accordingly.

While an infinite amount of initial orientations could be made in this fashion, it would not be

desirable to have extremely dependant initial guesses for each adsorption configuration. More

specifically, orientations of the molecules anywhere near the kink site were expected to offer a

fair set of possible favoured local minima.

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III. Adsorption on Pt (874)

(i) AMT

All three probe molecules were subjected to adsorption upon the Pt (874) surface. The

initial choice was to study the adsorption of AMT on Pt because of the molecule’s similarity with

AET, which Bhatia et al. demonstrated to have the largest enantiospecificity on a Cu (874)

surface. This, however, proved to be a poor choice for a model molecule because the majority of

confirmations upon the Pt surface resulted in dissociation of AMT with the loss of its sulfur atom

as shown in Figure 7 (a). Furthermore, the only non-dissociated configuration resulted in the

binding of only the oxygen atom. This configuration is shown in Figure 7 (b) along with an

alternate view in (c) of the metal surface highlighting the oxygen binding orientation.

Figure 7: (a) Top view of final relaxation site for AMT with loss of sulfur atom (b) Top view of final relaxation site for non-dissociated confirmation of AMT (c) Side view of b exploiting oxygen binding to platinum surface. The dotted black line highlights the step edge for all of the above images.

While the sulfur-less model of AMT does not represent the true form of the molecule if it

were to dissociate upon binding with the metal surface, it exploits problems with the method at

hand. Since Bhatia et al. reported tridentate bonding (bonding of three ligands to metal surface)

of AMT on Cu, a few different observations can be made for the difference in results since

similar adsorption geometries would be expected for Pt. First, it is possible that many of their

local minima reported could have similar issues since visual representations were only shown for

the most favoured confirmations. Furthermore, the favoured local minima could be extremely

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dependant on initial position configurations, making it very difficult to reproduce their results.

Many of the initial positions used for this research were tailored in such a way to reproduce the

tridentate bonding. This method was done by orienting the molecule directly above the kink site

with a similar geometry to the one reported by Bhatia et al. Despite very specific initial guesses,

poor results were continually produced for both enantiomers suggesting that the reported

confirmations by Bhatia et al. are not necessarily the most favoured. Finally, since VASP offers

many different algorithms for finding local minima, the particular method chosen for this

research could have used an overly aggressive minimizer in comparison to the work of Bhatia

and Sholl. Regardless, adsorption energies for the R and S enantiomers of AMT were not

reported because these results do not allow for an analysis on AMT’s enantiospecificity on Pt

(874). This prompted choosing the other two probe molecules, FAM and AE, whose results

were much more significant.

(ii) FAM

For the adsorption of FAM on Pt (874), both enantiomers demonstrated preferred

adsorption configurations with both the amino group and oxygen atom bonded to the top of the

step surface as shown below in Figure 8. The R-enantiomer shows oxygen atom binding to

metal atoms K and 1 and the amino group binding to metal atom 2. Alternatively, the S-

enantiomer has its amino group adsorbed onto K and 1 while its oxygen atom is adsorbed onto

metal atom 2.

Figure 8: (a) Top view of favoured final relaxation confirmation for R-FAM with oxygen and amino group adsorbing to metal surface (b) Top view of favoured final relaxation confirmation S-FAM with oxygen and amino group adsorbing to metal surface.

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Significant enantiomeric preference was recorded with the R-enantiomer having a greater

binding energy by 0.24 eV (i.e. ΔE0 = -0.24). The notable difference between the two favoured

adsorption geometries is the position of the fluorine atom relative to the kink site. The R-

enantiomer has its fluorine oriented above the kink site and pointed away, whereas the S-

enantiomer has its fluorine atom much closer to the Pt surface. This suggests that the relative

position of fluorine plays a large role in FAM’s enantioselectivity on Pt. In comparison, Bhatia

et al. report configurations similar to these results for FAM on Cu (874) considering the relative

position of fluorine for both enantiomers. Many other local minima exist for both enantiomers of

FAM and their adsorption energies are reported relative to the most stable energy state in Figure

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Figure 9: Ground state energies of observed minima for enantiomers of FAM on Pt (874). All energies are relative to the most favoured confirmation for R-FAM thus showing the observed enantiospecificity

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(iii) AE

Adsorption of AE on Pt (874) also showed a chiral preference. The stable adsorption

configurations for the R-AE and S-AE are shown in below in Figure 10.

Figure 10: (a) Top view of most favoured final relaxation state for R-AE with oxygen binding to metal surface (b) Top view of most favoured final relaxation state for S-AE with oxygen and amino group binding to metal surface

Both enantiomers adopt adsorption geometries force the hydrogen atom and methyl group to

point away from the metal surface, with the methyl group oriented over the step edge in both

cases. The adsorption geometries between the two enantiomers of AE are very similar in

contrast to the adsorption geometries between the enantiomers of FAM, whose orientation of its

fluorine atom after binding was drastically different depending on chirality. The observed

enantiospecificity was approximately ΔE0 = 0.63 eV, favoring the S-enantiomer, which was

drastically greater than the observed enantiospecificities of between enantiomers of FAM. While

the differences in ground state energy between enantiomers for FAM seem reasonable, the

results for AE pose problems. With AE, the steric hindrance of the bulky methyl group seems to

be the governing force for the observed adsorption confirmations; however, the most favoured

local minimum for S-AE showed a reasonable bi-dentate bonding, whereas the most favoured

configuration for R-AE only had binding between oxygen and the metal surface. Bhatia et al.

reported AE configurations on Cu (874) that demonstrated bi-dentate bonding for both

enantiomers, supporting the expectations for platinum. The bird’s eye view presented in Figure

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10 (a) appears to demonstrate expected adsorption geometry; however, the lack of amino-group

binding to Pt supports the suspicion of these results while raising questions concerning the

demonstrated enantiospecificities for FAM.

The other local minima observed relative to the most favoured confirmation energy are

reported below in Figure 11. The minima reported for R-AE were within a very small energy

range compared to minima reported for S-AE. This further supports the hypothesis that the

majority of favoured local minima were not necessarily obtained by these calculation methods;

however, it’s also important to note that many runs for AE resulted in the dissociation of the

molecule, and the ground state energies for these confirmations were not reported. These results

prompted an attempt to reproduce the results for our probe molecules on Cu (874), which were

previously published by Bhatia et al. The ability to reproduce data via the calculations methods

reported previously in Section II would offer discrimination towards the merit of both the Bhatia

results, and results reported in this paper.

Figure 11: Ground state energies of observed minima for enantiomers of AE on Pt (874). All energies are relative to the most favoured confirmation for R-FAM thus showing the observed enantiospecificity

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IV. Adsorption on Cu (874)

(i) FAM

To avoid calculating an abundance of local

minima to reproduce data, initial position

files for adsorption onto Cu (874) were

prepared by examining the most favoured

relaxation states given by Bhatia et al. as

shown in Figure 12. As stated earlier, their

results are very similar to the results

presented earlier for the adsorption of FAM

on Pt (874), demonstrating bi-dentate

bonding of the oxygen atom and amino group

to the metal surface. The main difference

between enantiomers for these results is the relative position of fluorine, which is oriented over

the kink site for S-FAM and oriented away from the kink site for R-FAM. The observed energy

difference between the favoured configurations of enantiomers was ΔE0 = -0.11 eV. It can be

noted that since energy calculations are relative to a lowest energy state, comparisons can be

made between the range of calculated ground state energies and the differences between ground

state energies of enantiomers.

Overall, the attempt to obtain similar binding confirmations for FAM on Cu was not

successful. The difference between ground state energies for the favored enantiomeric

confirmations was 0.71 eV in favor of R-FAM (ΔE0 = -0.71), almost 7 times greater than the

results published in literature. The preference observed for R-FAM on copper does agree with

literature; however, while the enantiomeric preferences for FAM on both platinum and copper

were consistent, the differences in binding energies were not

Figure 13 shows the initial position of runs for S-FAM obtained from observations from

the configuration geometry given in Figure 12. Comparing Figure 12 (a) and Figure 13 (b), the

favoured adsorption geometries from literature and this paper respectively, an almost identical

configuration was discovered in both. The amino group interacts with metal atom K while the

oxygen atom interacts with atoms 1 and 2. Furthermore, both results show the fluorine atom

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Figure 12: Most favoured relaxation geometries for FAM on Cu (874) reported by Bhatia et al. (a) Top view of favoured minimum for S-FAM (b) Top view of favoured minimum for R-FAM[9]

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hovering over the bottom of the step site. Hence, the results for S-FAM do appear to be

extremely consistent with the results reported in literature.

Figure 13: (a) Initial position file for S-FAM’s favoured local minima oriented in a similar fashion to the relaxed geometry reported by Bhatia et al. Note overhead view makes initial configuration appear to bond to metal surface, but it is in fact oriented above the surface (b) Favoured final relaxation site for S-FAM showing bi-dentate bonding similar to the results produced by Bhatia et al

The R-enantiomer for FAM, on the other hand, did not produce consistent results. Figure

14 below shows the initial position of R-FAM and its favoured final state adsorption

configuration.

Figure 14: (a) Initial position configuration for R-FAM for most favoured binding confirmation. Note that initial position of R-FAM is in fact oriented ABOVE the metal surface (b) Final relaxation configuration for most favoured local minima of R-FAM with only oxygen binding to metal surface (c) Alternate side view of final state for R-FAM highlighting oxygen bonding configuration

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Many initial configurations similar to Figure 14 (a) were attempted, but none resulted in the bi-

dentate bonding expected. The initial configurations for R-FAM were formatted to position the

fluorine atom away from the metal surface while keeping the amino group and oxygen atom in

close proximity to the top of the step edge. These particular adsorption results suggest a few

connections. First, despite the numerous attempts to orient the molecule initially, the abundance

of possible local minima that can be obtained prevents any concrete confirmation that the lowest

ground state energies obtained for a single enantiomer at the kink site represent a global

minimum for the kink site. As stated earlier, it is not desirable to have to have extremely precise

initial molecule positions considering that they reside in the vicinity of the step edge. It can also

be noted that the awkward bonding of only the oxygen atom for R-FAM on Cu (874) was very

similar to the configurations determined for AE on Pt (874). In both cases, only one of the

enantiomers demonstrated the expected binding geometry, and the local minimum observed

seemed reasonable whereas the enantiomer pair did not seem to represent the most favored state.

Thus, the energy difference between the ground state adsorption geometries for the pairs of

enantiomers does not give a promising conclusion for the enantiospecificity of the intrinsically

chiral metal.

Since only a small number of distinct local minima were observed for FAM on Cu (874),

their energy values relative to the most favoured energy confirmation are not shown. The lack of

available local minima in conjunction with the oxygen binding observed for R-FAM seem to

point towards discrepancies with the calculations methods used in this research.

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(ii) AE

In a similar fashion, the results reported by Bhatia et al.

in Figure 15 for molecular adsorption of AE on Cu

(874) were used to prepare initial position files for this

research. The literature results show a ΔE0 = 0.06 eV,

which indicates a lack of enantiospecific preference for

AE on Cu (874). The data reported earlier for AE on Pt

(874) showed a large enantiospecificity due to the

inconsistencies in adsorption geometries between

enantiomers. As postulated earlier, the steric hindrance

of the methyl group on AE seems to suggest a lack of

enantiospecific adsorption on the surface.

Calculations for AE were halted after discovering that the R-enantiomer gave similar

results with oxygen binding. Figure 16 below shows the position configurations for the initial

and final state of the most favored ground state adsorption energy for R-AE.

Figure 16: Most favoured relaxation geometries for R-AE on Cu (874 ) (a) Top view of initial position configuration (b)Top view of final state configuration (c) Alternate side view of b highlighting the binding of only the oxygen atom to the metal surface

With the oxygen atom and amino group close to the top step edge with the methyl group pointing

away from the surface, it seems likely that bi-dentate bonding would occur. However, with yet

another molecule exhibiting mono-atomic bonding, this leads to a reevaluation of the modeling

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Figure 15: Most favoured relaxation geometries for AE on Cu (874) reported by Bhatia et al. (a) Top view of favoured minimum for S-AE (b) Top view of favoured minimum for R-AE [9]

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used for this research due to the inability to reproduce literature results. This leads to the

discussion of future work in the final section (V) of this paper.

V. Conclusion

The research presented in this paper gave a detailed DFT study of the enantioselective

adsorption of chiral molecules on intrinsically chiral metal surfaces; however, the ultimate goal

of the project was to compare these results with the enantioselective adsorption upon a strontium

titanate surface with platinum nano-particles grown at the active chiral sites. Without first

demonstrating promising and consistent results on the pure metal surfaces, the latter objective

could not yet be pursued. Future work on this research would first have to adjust the

computational methods accordingly to reproduce or obtain similar results to those given in

literature. It would be recommended that Bhatia and Sholl are contacted to discuss the complete

details of their computational methods for molecular adsorption on Cu (874) including the

specific initial position files used and algorithms used to obtain their observed local minima.

Although more work needs to be done for the Pt and Cu systems, the results presented do

show that enantioselective adsorption occurs on intrinsically chiral metal surfaces. This work

also conveys that molecular steric factors play an extremely important role in adsorption

geometries, and hence affect enantiospecificity. In the future, demonstrating the enhanced

enantiospecific adsorption on a Pt/SrTiO3 (874) system would offer direction towards the

efficient and economically feasible synthesis of chiral metal nanostructures used for enantio-

purification and catalysis.

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Acknowledgements

I would like to thank Dr. Aravind Asthagiri from the Department of Chemical Engineering for

his time spent advising me for this project and the opportunity to do undergraduate research. I

thank Tim Van Cleve and Beverly Brooks-Hinojosa for their collaborative efforts. Finally, I

would like to thank my honors thesis defense committee for their participation and time.

We gratefully acknowledge financial support for this work provided by the National Science

Foundation under NSF CHE Grant #0911553. We also acknowledge the University of Florida

High-Performance Computing Center (http://hpc.ufl.edu) for providing computational resources

for performing the calculations reported in this paper.

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