Adsorption and Structure of Chiral Epoxides on Pd(111): Propylene

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Adsorption and Structure of Chiral Epoxides on Pd(111): Propylene Oxide and Glycidol Mausumi Mahapatra†,‡ and Wilfred T. Tysoe*,† †

Department of Chemistry and Laboratory for Surface Studies, University of WisconsinMilwaukee, Milwaukee, Wisconsin 53211, United States ‡ Chemistry Division, Brookhaven National Laboratory, Upton, New York 11973-5000, United States ABSTRACT: The adsorption of enantiopure versus racemic propylene oxide (PO) on Pd(111) is studied by temperature-programmed desorption (TPD) to explore possible differences in their saturation coverage. It is found that that the saturation coverage of enantiopure PO on Pd(111) is identical to that of racemic PO, in contrast to results on Pt(111) where significant coverage differences were found. The surface structures of enantiopure PO on Pd(111) were characterized by scanning tunneling microscopy (STM), which shows the formation of linear chains and hexagonal structures proposed to be due to freely rotating PO, in contrast to the relatively disordered PO overlayers found on Pt(111). STM experiments were carried out for enantiopure glycidol, which contains the same epoxy ring as PO, but where the methyl group of propylene oxide is replaced by a −CH2OH group to provide a hydrogen-bonding sites. Glycidol STM images show the formation of completely different surface structures; at low coverages, glycidol forms pseudohexagonal structures which assemble from glycidol dimers, while at high coverages the surface shows extensive hydrogen-bonded networks. Density functional theory (DFT) calculations were carried out to model the enantiopure PO linear chain and the glycidol dimers that are observed by STM. Similar calculations were carried out for racemic PO and glycidol structures. The calculated interaction energies for the enantiopure and the racemic pairs reveal that there is no difference for homochiral versus heterochiral structures for both PO and glycidol on Pd(111).

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affinity for pairing with a homo- or a heteroenantiomer6 to form a pseudoracemate. The differences in interaction energies between the enantiomers is reflected in the variation of melting temperature3,4 or vapor pressure2 as a function of the concentration of each enantiomer in the mixture. In the case of a conglomerate, the melting point of equimolar proportions of each enantiomer is lower than the enantiopure compounds, while the reverse is true for a racemic compound. In the case of a pseudoracemate, the melting temperature is independent of the proportion of enantiomer in the compound. However, in addition to thermodynamic effects, the interfacial growth kinetics can also influence the crystal structure.7 Such effects can be explored for the growth of two-dimensional chiral assemblies on surfaces from the adsorbate sticking coefficients so that both the kinetics and resulting structures can be investigated for the same systems. The adsorption of PO on Pt(111) provides an example of a system for which the kinetics appear to control the surface structure.8 In this case, scanning tunneling microscope (STM) images of PO on Pt(111) reveal neither racemate formation

INTRODUCTION

Two enantiomers of a chiral compound have identical physical and chemical properties in an achiral environment. However, when the two enantiomers interact with each other, their difference in handedness can lead to different physical and chemical properties. For example, the normal melting point of pure D- or L-menthol is 316 K, whereas a DL-menthol mixture melts at 307 K1 because racemic menthol forms a 1:1 compound in the solid phase. The equilibrium structures formed from mixtures of two enantiomers of any chiral compound can be classified based on the interaction energies between the enantiomers.2 In the majority of cases, opposite enantiomers interact more strongly than the same enantiomers to form racemic compounds that crystallize in a lattice that includes both; 90 to 95% of the chiral compounds crystallize by forming racemic compounds.3,4 Less common is the situation in which there is a higher affinity between the same enantiomers. In this case, separate enantiopure crystals, referred to as solid conglomerates, are formed, where the same enantiomers interact strongly to form homochiral domains.4 This occurs with ammonium sodium tartrate and allowed Pasteur to separate their enantiomers and investigate the molecular origins of chirality.5 A third, and much rarer (∼1%) possibility occurs when there is an equal © XXXX American Chemical Society

Received: November 2, 2017 Revised: December 14, 2017

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DOI: 10.1021/acs.jpcc.7b10852 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

could be cooled to 80 K by thermal contact to a liquidnitrogen-filled reservoir and resistively heated to ∼1200 K. The Pd(111) sample was cleaned using a standard procedure that consisted of cycles of argon ion bombardment followed by heating at 800 K in ∼4 × 10−8 Torr of oxygen and then annealing at ∼1100 K in vacuo to remove any remaining oxygen.

nor long-range order, while a saturated monolayer of a racemic mixture of PO on a Pt(111) was found to be ∼20% less dense than an enantiopure overlayer. The coverage difference was attributed to adsorbate-assisted adsorption kinetics where different sticking probabilities were found for PO adsorption adjacent to preadsorbed PO (of 0.99) and on the clean surface (of 0.01). While such adsorbate-assisted adsorption has been found previously,9,10 it is relatively rare. In order to attempt to gain insights into this unusual adsorption-controlled effect, we have studied closely related PO on Pd(111) since the adsorption of PO on both surfaces is very similar; a saturated PO overlayer desorbs molecularly in a temperature-programmed desorption (TPD) experiment with a peak temperature of ∼193 K for Pt(111)8 and ∼160 K for Pd(111).11 The desorption peak temperature on Pd(111) decreases with increasing coverage and this effect has been traced to a change in adsorbate structure with coverage; the PO methyl group interacts with the surface at low coverages, thereby leading to stronger binding, while it binds only via the epoxide oxygen at higher coverages, to desorb at a lower temperature.11 The following paper studies the adsorption of enantiopure and racemic PO on Pd(111) using TPD and STM to compare with the structures found on Pt(111) and to compare PO adsorption on palladium and platinum. The influence of intermolecular interactions on the surface structures is also investigated using glycidol, in which the −CH3 in PO is replaced by a −CH2OH group thereby allowing the possibility of intermolecular hydrogen-bonding interactions. These have been found to control the enantioselectivity of glycidol on tartaric-acid modified Pd(111) surfaces.12 Propylene oxide (PO) and glycidol have been used extensively as chiral probe molecules to measure the enantioselectivity of chirally modified surfaces.12−20 The general aspects of propylene oxide adsorption have been studied previously on both Pd(111) and Pt(111)20 single crystal surfaces, where it was observed that molecular adsorption occurs at low temperatures. The molecules desorb intact in two different states from Pd(111) upon heating: the monolayer desorbs at ∼160 K, while the multilayer desorbs at ∼140 K. The adsorption of glycidol has been studied by TPD and infrared spectroscopy on Pd(111),21 where it is found to undergo a small amount of decomposition on the surface.

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THEORETICAL METHODS Density functional theory (DFT) calculations were performed with the projector augmented wave (PAW) method24,25 as implemented in the Vienna ab initio simulation package, VASP.26,27 The exchange-correlation potential was described using the generalized gradient approximation (GGA) of Perdue, Burke, and Ernzehof.28 van der Waals correction were included by using the method of Tkatchenko and Scheffler.29 Hydrogen-bonding interactions are well reproduced (within ∼4 kJ/mol) using this functional, although the accuracy deteriorates as the hydrogen bonds deviate from linear.30 A cutoff of 400 eV was used for the plane-wave basis set, and the wave functions and electron density were converged to within 1 × 10−5 eV. The first Brillouin zone was sampled with a 4 × 4 × 1 Γ-centered k-point mesh. Geometric relaxations were considered to be converged when the force was less than 0.02 eV/Å on all unrestricted atoms. Periodic calculations were carried on palladium slabs that were three layers thick with a 2-nm vacuum gap between them, where the bottom two layers were fixed and the top layer was allowed to relax.

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RESULTS Desorption of Propylene Oxide (PO) from Pd(111). Previous work has shown that enantiopure and racemic PO desorb from Pt(111) at identical temperatures (of ∼193 K), but that the coverage of the saturated enantiopure overlayer was ∼20% higher than for a racemic mixture.8 To compare these results with those on Pd(111), TPD experiments were carried out as a function of exposure for S-PO adsorbed at a sample temperature of ∼120 K using a linear heating rate of ∼2 K/s by monitoring the 58 amu signal, which corresponds to molecular PO (Figure 1). Collecting TPD profiles at other masses confirms the desorption of molecular PO, and no other desorption products were found at masses not due to PO, indicating that no gas-phase decomposition products were detected. PO desorbs in a single state at ∼175 K at low exposures, due to desorption from adsorbed monolayer. This peak increases in intensity as the exposure increases and the peak temperature moves to lower values, as found previously,11 with a peak temperature of ∼160 K at saturation. An additional peak grows at ∼140 K at higher exposures following saturation of the adsorbed monolayer, due to desorption of PO from the multilayer.14 Adsorption of PO on a surface after completing a previous TPD experiment, without cleaning the sample in between, showed exactly identical profiles. This, along with the absence of gas-phase decomposition products, indicates that no PO has decomposed during the desorption sweep so that the integrated area under the desorption profile provides an accurate measure of the adsorbate coverage. In order to compare the behavior of PO on Pd(111) and Pt(111), where in the latter case the saturation coverage of a racemic mixture of PO was found to be 20% less than enantiopure PO on Pt(111),8 TPD experiments were carried

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EXPERIMENTAL METHODS All experiments were carried out in ultrahigh vacuum (UHV) at base pressures of ∼1 × 10−10 Torr following bakeout. STM experiments were performed using a UHV STM (RHK UHV 350) and acquired using an electrochemically etched tip made from recrystallized tungsten wire.22 This was conditioned by a controlled interaction with a clean Au(111) single crystal. This is expected to result in a gold-terminated tip. PO and glycidol (Aldrich Chemicals, 99% purity) were introduced into the UHV chamber via gas-handling manifolds attached to the respective UHV chambers. They were further purified by several freeze−pump−thaw cycles before dosing onto the sample surface and were introduced into vacuum through precision leak valves attached to a tube (0.25 in. in diameter) directed toward the sample to enhance the dosing rate. TPD data were collected using a Dichor quadrupole mass spectrometer interfaced to a computer that allowed up to five masses to be monitored in a single experiment.23 The sample B


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imaged. The results are displayed in Figure 3 where onedimensional linear chain structures are observed. As a

Figure 1. TPD uptake profiles following S-propylene oxide (PO) adsorption on a Pd(111) surface at ∼125 K, using exposures of 0.4 (black line), 1.0 (red line), 1.6 (green line), 2.2 (purple line), 2.8 (blue line) and 3.4 (pink line) L (1 L (Langmuir) = 1 × 10−6 Torr·s). The spectra were collected at 58 amu by using a linear heating rate of ∼2 K/s.

out to compare the saturation coverages of racemic and enantiopure PO on Pd(111). Figure 2 shows the superimposed Figure 3. STM images for a low coverage of propylene oxide adsorbed and imaged on a Pd(111) surface at 120 K: (a) wide-scan image and (b) higher magnification image. The ⟨11̅0⟩ close-packed directions of the underlying Pd(111) lattice are indicated and orientations of the linear features with respect to the close-packed directions are indicated on the figures (Vb = −0.2 mV; It = 30 pA).

reference, the figure also indicates the close-packed ⟨11̅0⟩ directions of the underlying Pd(111) surface. Two types of linear-row structures are observed, either oriented at ∼30° to the close-packed directions, along ⟨12̅1⟩ (labeled P), or at ∼10° to ⟨12̅1⟩ (labeled 10°). An example of a row oriented parallel to the ⟨12̅1⟩ direction is shown at the top of Figure 3, parts a and b, while rows aligned at ∼10° to the ⟨12̅1⟩ direction are shown at the bottom of the images. A highermagnification image in Figure 3b resolves the individual molecules within the row. The intermolecular spacing depends on whether the rows are oriented along the ⟨12̅1⟩ directions, or at ±10° with respect to these directions; the intermolecular spacings for rows aligned along the ⟨12̅1⟩ directions (labeled P) are 0.49 ± 0.02 nm, and the spacing between rows is ∼0.96 nm, while the intermolecular spacing for rows aligned at ±10° (labeled 10°) to these directions is 0.39 ± 0.02 nm. More complex structures are found for higher coverages of PO adsorbed on Pd(111), an example of which is shown in Figure 4. Figure 4a shows the linear-row structures that are observed at lower coverages (Figure 3), along with less welldefined close-packed lines that appear between the PO rows. Figure 4b shows a higher-magnification image of a region that contains only the close-packed line structures. Some structure can be seen within the lines, which are not well resolved preventing the periodicity from being measured. However, the close-packed lines are oriented along the ⟨11̅0⟩ directions (Figure 4b) and the spacing between the lines is 0.44 ± 0.02 nm. The average length of the linear structures is ∼3 to 4 nm. Very occasionally, addition hexagonal features were also observed at high PO coverages and Figure 5 shows an STM image of these structures. The top of the scan area consists of

Figure 2. TPD profiles following a saturated dose, by using 3 L of Rpropylene oxide (PO) and racemic propylene oxide on a Pd(111) surface at ∼140 K. The spectra were collected for 58 amu by using a linear heating rate of ∼6 K/s.

desorption profiles, collected after dosing PO at a sample temperature of ∼140 K to avoid populating the multilayer, of saturated monolayers of R-PO and racemic PO (color coded by red and green respectively) on Pd(111). The peak temperatures in these profiles are shifted slightly from those in Figure 1 due to different heating rates used in each experiment. In contrast to the behavior on Pt(111), the desorption profiles for R-PO and racemic PO overlap on Pd(111); there are no differences in either the desorption temperatures or the saturation coverages. STM experiments were also carried out to investigate the structures formed from PO and glycidol on Pd(111) as a basis for understanding the differences. STM of S-Propylene Oxide on Pd(111). STM images were acquired following adsorption of a low coverage of S-PO on Pd(111) at ∼120 K, at which temperature the surface was C


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different from those seen on Pt(111), where the PO is relatively disordered. The unit cell of the ring structures in matric notation is 13 22̅ . STM of R-Glycidol on Pd(111). In order to explore the influence of intermolecular hydrogen-bonding interactions, the surface structures formed by the adsorption of R-glycidol on Pd(111) was also investigated by STM. Low coverages of Rglycidol were dosed onto Pd(111) at ∼150 K and the sample then cooled to 120 K for imaging, where it is found to selforganize to form pseudohexagonal structures (Figure 6). A

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Figure 4. STM images for a high coverage of PO adsorbed and imaged on a Pd(111) surface at 120 K. (a) Shows the presence of propylene oxide linear chains seen in Figure 3, where the periodicity depends on the growth directions of the chains and a series of closepacked rows occurring between these structures (Vb = −0.8 V, It = 83 pA) (b) Shows a region that only consists of only of close-packed rows of propylene oxide which order on the surface in lamellae (Vb = −0.16 V, It = 80 pA). The orientation of the close packed ⟨11̅0⟩ directions of the underlying lattice are shown as red lines illustrating that the close-packed rows grow along these directions.

Figure 6. Low coverage STM image of glycidol on Pd(111) surface collected at ∼120 K which shows the formation of hexamers (Vb = −254 mV; It = 40 pA). Line profiles are measured along A, giving an intermolecular distance of ∼0.75 nm, and along B, giving an intermolecular distance of ∼0.48 nm. The red lines indicate the presence of dimer pairs.

closer examination of the structures reveals that they are not perfectly hexagonal and consist of different intermolecular distances where the intermolecular distance indicated by profile A is ∼0.756 nm and closer distances of ∼0.47 nm are found as indicated by profile B. As will be shown below, the smaller intermolecular distances are due to the formation of homochiral R-glycidol dimers. The structures are rather dynamic as indicated by streaks that appear adjacent to the hexamers during scanning, suggesting some mobility on the time scale of one scan. This is further illustrated in parts a−c of Figure 7, which show three consecutive, time-lapse images, collected at 8 min intervals, which clearly show molecular diffusion. The areas highlighted in white dotted squares show how the hexamer arrays coalesce to form larger units, and red circles are shown to indicate identical positions on sequential images. Figure 8 shows an STM image with higher glycidol coverages. Unlike PO, glycidol forms close-packed regions of predominantly dimeric units, presumably due to the presence of −OH groups that can form hydrogen bonds. Figure 8 shows regions of the surface that are densely packed and other lessdense areas that consist of a mixture of glycidol dimers

Figure 5. STM image of structures formed from a higher coverage of PO on Pd(111) at 120 K. The top portion of the image consists of ordered domains which consist of patches of linear rows and the bottom of the image consists of hexagonal features ordered in a hexagonal pattern (Vb = −1 V; It = 179 pA).

domains of close-packed lines with different orientations, as seen in Figure 4, while the bottom of the scanned area consists of ring-like features, which are ordered in a hexagonal pattern on the surface. The ring-like features are aligned along vectors that are oriented at ∼40° to the ⟨11̅0⟩ directions. Line profile measurements show that the spacing between two hexagonal features is ∼0.74 nm (profile C) and the distance between the highest points of the ring structures is ∼0.3 nm (profile D). These results indicate that the surface structures formed on Pd(111), where significant order is observed, are completely D


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Pd(111) (Figure 1) and ∼175 K on Pt(111),8 the behavior of enantiopure and racemic mixtures of PO on Pd(111) and Pt(111) is substantially different. The most significant difference is that the saturation coverages of enantiopure and racemic PO are identical on Pd(111) (Figure 2), while a saturated monolayer of a racemic mixture of PO on a Pt(111) is ∼20% less dense than an enantiopure overlayer. The adsorbate structures of PO on Pd(111) and on Pt(111) are also different, where a number of ordered structures are observed on Pd(111) at ∼120 K, while, at the same temperature, PO is distributed almost randomly on Pt(111), with only some local ordering. We first address the nature of the adsorbed structures on Pd(111), and the possible interactions that lead to them. Previous DFT calculations have shown that that PO binds to a Pd atop site through the lone-pair electrons of the epoxy oxygen,11 where the orientation is proposed to change with coverage. At low coverages, the methyl group of PO also interacts with the surface, leading to stronger binding by about 16 kJ/mol, but PO changes orientation at higher coverages due to crowding, leading to a reduction in the heat of adsorption, thereby causing the desorption temperature to decrease with increasing coverage as seen in Figure 1. Gas-phase PO has a dipole moment of ∼2 D,31 so that intermolecular dipole−dipole interactions may influence the surface structures. This appears to be the case on Pd(111), where PO forms equally spaced, one-dimensional linear chain structures at low coverages (Figure 3). The spacing between adsorbed PO molecules in chains that are oriented along the ⟨12̅1⟩ directions is 0.49 ± 0.02 nm. This distance agrees well with the periodicity of a Pd(111) surface along this direction (of 0.476 nm for a face-centered cubic (fcc) lattice constant of 0.389 nm32). Consequently, DFT calculations were carried out for chains in which the epoxide oxygens are placed at adjacent Pd atop sites aligned along a ⟨12̅1⟩ direction. The rows were placed four Pd−Pd nearest neighbors apart (to yield an interchain distance of ∼0.83 nm, similar to that found in the STM images, see Figure 3). The calculations were carried out by including van der Waals’ interactions and the resulting most-stable homochiral structure yields a modest attractive interaction energy of ∼2 kJ/mol. The DFT-optimized structure is shown in Figure 9A where molecules are aligned perpendicular to the chain directions. Similar DFT calculations were carried out for opposite enantiomers of PO and the resulting structure is shown in Figure 9B which yield an attractive interaction energy of ∼1 kJ/mol. The results indicate that the interactions between PO molecules of same and opposite chiralities are identical and relatively weak. This is in accord with the TPD results where no coverage differences between the enantiopure versus the racemic propylene oxide was observed (Figure 2). Alternative linear structures appear aligned at ±10° with respect to the ⟨12̅1⟩ directions with a smaller periodicity of 0.39 ± 0.02 nm. Adjacent atop sites along this direction on the surface are spaced by ∼0.81 nm, twice the observed intermolecular spacing, suggesting that other, less stable adsorption sites are occupied in this structure. More complex structures appear at higher PO coverages consisting of linear (Figure 4) and hexagonal features (Figure 5). It is difficult to resolve any detail in the images of the linear structures, but they are generally found some time after dosing, which could either imply some tip-induced effects or be due to the relatively slow kinetics for the adsorbates to diffuse and form these self-assembled structures.

Figure 7. Three sequential STM images of glycidol on a Pd(111) surface which shows the mobility of the molecules. The images are collected in 8 min intervals. The area highlighted in white dotted squares shows the growth of hexamers with time. (Vb = −254 mV; It = 40 pA).

Figure 8. STM image of a high coverage of glycidol on Pd(111) surface. The surface is closely packed due to extended hydrogenbonding interactions. (Vb = −250 mV; It = 50 pA). Examples of dimers are highlighted in blue, tetramers in red, and dimer rows in green.

(examples of which are highlighted in blue), tetramers (highlighted in red), and longer dimer rows (highlighted in green). The dimers also interact to form extended structures.

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DISCUSSION Propylene Oxide on Pd(111). Despite the similarity between the surface chemistry of PO on Pt(111) and Pd(111), where PO desorbs at ∼160 K at a saturation coverage on E


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experimental value of heat of adsorption at low PO coverages of 56 kJ/mol.11 The average diffusion distance ∼ Γt lattice spacings, so that the above jump frequencies yield diffusion distances per second between ∼400 and 2 × 105 lattice spacings for barriers of 5 and 20% the heat of adsorption, respectively. This indicates that PO is relatively mobile on Pd(111), in accord with the appearance of ordered surface structures. However, using the slightly larger values of the heat of adsorption of PO on Pt(111) does not substantially influence the jump frequencies. Unfortunately, these results provide no clear explanation for the lack of order of PO on Pt(111), except to imply that PO diffusion on that surface is anomalously low, so that any difference in the sticking coefficient controls the final adsorbate structure and leads to a relatively random distribution of PO in the surface. Glycidol on Pd(111). Stronger intermolecular hydrogenbonding is expected to occur for glycidol, and gas-phase homochiral glycidol dimers have been identified due to complementary hydrogen bonds between the hydroxyl group and the epoxide oxygen.36 Similar strong intermolecular interactions in adsorbed glycidol lead to the formation of ordered two-dimensional structures as found experimentally (Figures 6−8). The surface interactions were investigated using DFT calculations where the two most stable dimer structures are shown in Figure 10. Previous DFT studies of

Figure 9. Proposed structural models for the observed STM images of linear chains of PO along the ⟨12̅1⟩ directions, indicated as P in Figure 3. The DFT calculations were carried out by placing PO on adjacent atop sites along the ⟨12̅1⟩ directions on the Pd(111) surface: (A) structure of a homochiral linear chain; (B) structure of a heterochiral structure.

The hexagonal features (Figure 5) contain a clearly defined central dark spot with a distance across the ring of ∼0.3 nm. This value is too small to be due to a trigonal arrangement of three PO molecules, suggesting that they could be due to PO dimers. DFT calculations were therefore carried out for two PO molecules placed on adjacent Pd atop sites by orienting them antiparallel to each other, but this structure was found to be unstable and the PO molecules repelled each other. An alternative possibility is that they images are due to relatively freely rotating PO, where the ring traces the trajectory of the rotating methyl group. Such effects have been seen for chiral molecules on gold imaged at low temperatures.33 At low coverages, adsorbed PO is stabilized by the methyl group interacting with the surface, which would inhibit free rotation. The orientation was proposed to change at higher coverages so that the PO is only bound through the epoxide oxygen, thereby allowing it to rotate freely.11 The observation that the ring structures are observed at higher PO coverages (Figure 3) but not at low coverages (Figure 3) is in accord with this proposal. These results indicate that there are substantial differences between the surface structures of PO on Pd(111) and Pt(111) despite their similar heats of adsorption on both surfaces. The presence of ordered structures on Pd(111) suggests that that PO is sufficiently mobile to allow ordering to occur. Surface diffusion barriers are generally proportional to the heat of adsorption and are generally taken to be between 5% and 20% of the adsorption energy, where the barrier is likely to be closer to 5% on relatively flat (111) surfaces.34,35 The jump frequency, Γ = ν e−Ediff / RT , where Edif f is the energy barrier for diffusion and ν is an attempt frequency. Taking ν ∼ 1 × 1010 s−1 gives Γ ∼ 1.4 × 105 s−1 for a diffusion barrier of 20% of the heat of PO adsorption on Pd(111), and ∼6 × 108 s−1 when the barrier is 5% of the heat of adsorption, using the

Figure 10. DFT optimized glycidol dimer structures: (A) two glycidol molecules of the same chirality are placed such that there is a possible hydrogen-bonding interaction between the −OH group and the epoxy oxygen,; (B) the most-stable structure of the heterochiral dimer.

glycidol on Pd(111)37,38 found that it binds to a palladium atop site through the epoxy oxygen and the adsorption energy of the most stable structure, calculated from the difference in energy of the glycidol adsorbed on Pd(111) and the sum of the energies of gas-phase glycidol and a clean Pd(111) slab, was found to be −22 kJ/mol, increasing to −44 kJ/mol when van der Waals’ corrections were included. DFT calculations were performed by placing two glycidol molecules in a Pd(111) slab for a number of different geometries which allowed hydrogenF


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The Journal of Physical Chemistry C bonding interactions between two − OH groups or the epoxy oxygen of one glycidol with the − OH group of other. Figure 10A shows the most stable homochiral dimer structure which allows − OH···−OH hydrogen-bonding interactions and yields an interaction energy of −51 kJ/mol when van der Waals interactions are included, indicating that glycidol dimers are significantly stabilized by − OH··· − OH interactions. Similar dimer calculations for opposite enantiomers of glycidol have been carried out and the optimized geometry is shown in Figure 10B, yielding an interaction energy of the heterochiral dimer of −48 kJ/mol. The STM images of R-glycidol on Pd(111) show the formation of ordered pseudo hexagonal structures in which there are relatively closely spaced molecules that are ∼0.47 nm apart, and while others are more distant from each other (Figure 6). The most stable glycidol dimer structure shown in Figure 10A indicates that the methylene groups of the CH2− OH group are the highest above the surface and will therefore image the brightest, and have a distance between them of ∼0.49 nm. This is close to the measured distance of the closely spaced molecules in the ordered structure in Figure 6, suggesting that they are due to the presence of homochiral dimer pairs, implying that the ordering observed for R-glycidol on Pd(111) arises from interaction between these dimer pairs. Adjacent, closely spaced molecules are highlighted on the image by red lines. This shows that the pseudo hexagonal structures are not formed by the assembly of three dimer pairs but that they tend to assemble in structures in which one dimer bonds almost perpendicularly to an adjacent one in a distorted “T” geometry. In some cases, steric constraints appear to result in the presence of some isolated R-glycidol molecules, with larger spacings between them. However, as shown in Figure 7, the relatively weak interaction between dimer pairs results in a quite dynamic surface structure. Increasing the R-glycidol coverage (Figure 8) produces a crowded surface that inhibits the mobility of the dimer pairs and causes them to coalesce into somewhat ordered structures, while in some areas isolated dimers (some of which are highlighted in blue), tetramers (highlighted in red), and dimer rows (highlighted in green) are observed.

the same coverages on Pd(111). Estimates of diffusion distances for PO on Pd(111) suggest that PO is sufficiently mobile to be able to form ordered structures on the surface, as found by STM experiments. However, similar estimates for PO on Pt(111) suggest that it should also be similarly mobile on this surface, and should therefore presumably also form ordered structures. This implies that the mobility of PO on Pt(111) is anomalously low. Analogous experiments were carried out for glycidol on Pd(111) to explore the influence of replacing the methyl group on PO with a −CH2OH group. This forms dynamic pseudohexagonal structures on the surface, suggested to assemble from relatively stable glycidol dimers. DFT calculations of glycidol dimers reveals that the most stable dimers interact quite strongly due to the formation of intermolecular hydrogen bonds and the intermolecular spacing found by DFT for the most stable structure agrees with the intermolecular spacings in the STM images. Interestingly, homo- and heterochiral glycidol pairs have essentially identical interaction energies.

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AUTHOR INFORMATION

Corresponding Author

*(W.T.T.) Telephone: (414) 229-5222. Fax: (414) 229-5036. E-mail: [email protected]. ORCID

Wilfred T. Tysoe: 0000-0002-9295-448X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based on work supported by the U.S. Department of Energy, Office of Science, Office of Chemical Sciences, Office of Basic Energy Sciences, under Award Number DE-SC008703.

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CONCLUSIONS The adsorption of PO was studied on Pd(111) surfaces to compare with previous results found on Pt(111), where different saturation coverages, and relatively disordered surface structures were found for racemic and enantiopure PO. In contrast, saturated overlayers of racemic and enantiopure PO have exactly identical coverages on Pd(111), and the surface shows a number of order structures consisting of linear rows at lower coverages and a hexagonal array of apparently freely rotating molecules at high coverage. The observation of free rotation at high coverages is in accord with previous DFT results and Monte Carlo simulations that show that the methyl group in PO interacts with the surface at lower coverages, presumably inhibiting molecular rotation. At higher coverages, the methyl group is suggested to be remote from the surface, thereby allowing freer rotation on the molecule. DFT calculation of the energies of the most stable PO linear chain structures indicate that there is a slight attractive interaction between PO molecules in the chain, but that there is essentially no difference in the energy of homochiral and heterochiral chains. This result is in accord with the observation that homoand heterochiral PO desorb at identical temperatures, and have G


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Adsorption and Structure of Chiral Epoxides on Pd(111): Propylene

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