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C: Surfaces, Interfaces, Porous Materials, and Catalysis

A Chiral Surface from Achiral Ingredients: Modification of Cu(110) with Phthalic Acid Chrysanthi Karageorgaki, Pingo Mutombo, Pavel Jelinek, and Karl-Heinz Ernst J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00637 • Publication Date (Web): 13 Mar 2019 Downloaded from http://pubs.acs.org on March 18, 2019

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A Chiral Surface From Achiral Ingredients: Modification of Cu(110) With Phthalic Acid Chrysanthi Karageorgaki,† Pingo Mutombo,§ Pavel Jelinek,§ and Karl-Heinz Ernst*,†,‡,§ †

Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129,

8600 Dübendorf, Switzerland §

Nanosurf Lab, Institute of Physics of the Czech Academy of Sciences, Cukrovarnicka 10,

Prague 6, 162 00, Czech Republic ‡

University of Zurich, Department of Chemistry, Winterthurerstrasse 190, 8057 Zürich,

Switzerland

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ABSTRACT: The adsorption of dicarboxylic acids is a classical model approach for understanding molecular recognition at surfaces. The interaction of achiral phthalic acid with an achiral Cu(110) surface has been investigated in ultrahigh vacuum by means of scanning tunneling microscopy, low energy electron diffraction, X-ray photoelectron spectroscopy, reflection absorption infrared spectroscopy, temperature programmed desorption and density functional theory. Different ordered domains at a length scale of several tens of nanometers are observed, of which three are enantiomorphous and therefore appear in two mirror-symmetric forms. Theoretical considerations suggest that spontaneous mirror-symmetry breaking occurs at the single molecular level, in which the surface becomes also chirally distorted.

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1. INTRODUCTION Dicarboxylic acids are known to be powerful crystal shape modifiers of carbonates such as calcite for example.1 With two terminal –COO– groups in exchange for surface carbonate ions, chiral dicarboxylic acids impose a chiral footprint onto the carbonate mineral surface and induce a chiral shape into the achiral mineral, which might be transmitted even to the macroscopic length scale.2,3 A similar mechanism has been identified for dicarboxylic acids interacting with metal surfaces,4-8 as well as for amino acids9 and Buckminsterfullerene fragment molecules.10 A common observation for 1,4-butanedioic acid derivatives (BADs) on Cu(110) is that these engage in two different adsorption modes, interacting with the surface either with one or both carboxyl groups. This leads to a singly- or doubly-dehydrogenated species (mono- or dicarboxylate), well distinguishable by infrared spectroscopy.11 Several chiral and prochiral BADs have been studied previously on the Cu(110) surface (Figure 1).7,11-16 All of them show both mono- or dicarboxylate adsorption modes. The adsorption of achiral BADs also often induces mirror-symmetry breaking and a chiral modification of the substrate.12,13,17 The equal probability of such prochiral species turning into left- or right-handed adsorbates leads then to both enantiomorphous states coexisting on the surface. Upon additional chiral bias, however, BADs may be engaged into extended homochiral surface systems.18-20 On Cu(110), the chirality of the BADs tartaric acid (TA) and succinic acid (SU) are manifested in a zigzag distortion of the molecular frame.21,22 This occurs either within the rectangular Cu(110) surface unit cell or involves four Cu atoms of an oblique cell. 23,24 In addition, chiral reconstruction of the Cu(110) surface has been discussed, but the direct observation is usually hampered by the fact that the restructured surface is covered by the molecules. Indirect indications, such as different periodicities in diffraction and STM, caused debates if the surface is 3 ACS Paragon Plus Environment

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indeed reconstructed.15,25-27 However, in a few cases the reconstruction has been observed directly by STM.4,5,7,12,13

Figure 1. Structural formulas of 1,4-dicarboxylic acids studied on Cu(110): succinic acid (SU), tartaric acid (TA), malic acid (MA), 2,3-dimethylsuccinic acid (DMSU), fumaric acid (FUA), maleic acid (MAL), aspartic acid (ASP), cyclohexane-1,2-trans-dicarboxylic acid (DCHA) and phthalic acid (PHTA). Motivated by the question if a zigzag distortion of the molecular backbone is a general rule for BADs, the interaction of phthalic acid (PHTA) with the Cu(110) surface has been studied by means of scanning tunneling microscopy (STM), low energy electron diffraction (LEED), reflectionabsorption IR spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and density functional theory (DFT). Due to the rather rigid benzo backbone a zigzag distortion should be difficult to be established. DFT results show for the single adsorbate that a perpendicular adsorption mode in the rectangular unit cell is slightly favored over an oblique arrangement for the diphthalate. However, both cases show a chiral distortion of the molecular adsorbate. Long-range enantiomorphous phases are observed in LEED of which one of these shows in STM striking similarities to three-molecule triplet structures previously reported 4 ACS Paragon Plus Environment

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for TA, malic acid (MA) and dimethyl succinic acid (DMSU) on the same surface.7,11,16 A detailed analysis of the STM appearance of this structure suggests a combination of an oblique and rectangular zigzag distortion, just as recently derived by theoretical means for TA/Cu(110).23 In part the findings on PHTA will be compared here to similar results obtained for cyclohexane-1,2trans-dicarboxylic acid (DCHA, Figure 1).

2. EXPERIMENTAL AND THEORETICAL METHODS The experiments were carried out in an ultrahigh vacuum (UHV) chamber (p = 5·10−10 mbar), equipped with facilities for XPS, LEED, RAIRS, TPD and STM. Experimental set-up and sample preparation have been described in detail previously.28 PHTA (Sigma Aldrich, ≥99%) was evaporated from a differentially turbo-molecular-pumped homemade Knudsen cell, separated from the main chamber by a gate valve. During deposition PHTA and transcyclohexanedicarboxylic acid (CHDA) were heated to 100 °C. With the Cu crystal held at room temperature (RT) the time of exposure was roughly 25 minutes for reaching full monolayer coverage (Figure S1, Supporting Information). In order to observe ordered adsorbate lattices, the deposition was performed at elevated temperatures of the Cu surface and the crystal was cooled back down to RT. STM (in constant current mode) and LEED measurements were performed at room temperature. Coverage calibration has been performed with XPS, assigning the saturation of the C 1s signal to one monolayer (1 ML). The shown STM images were filtered after fast Fourier transformation (FFT) with a first order low pass Butterworth filter to remove the higher frequency noise followed by inverse FFT. Density Functional Theory calculations were performed using the FHI-aims code to find the adsorption geometry of the phthalate on the Cu(110) surface.29 The calculations were performed 5 ACS Paragon Plus Environment

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at the GGA-PBE level, considering the Tkatchenko-Scheffler treatment of the van der Waals interactions.30 The scaled zeroth-order regular approximation was used to include relativistic effects.31 The relaxation of the single molecule on Cu(110) surface was performed using a threelayer slab 10x6 unit cell, in which molecules were placed in three different initial configurations (along, perpendicular and oblique). Note that such large unit cell was chosen to minimize lateral interactions between the molecules. The triplet structure was calculated using the slab with a (1 – 2, 7 3) molecular unit cell. All the atoms of the molecule and of the two uppermost Cu layers were thoroughly relaxed until the remaining atomic forces and the total energy were below 10 -2 eV/Å and 10-5 eV, respectively. The Cu bottom layer was kept fixed at its bulk-like position, which was derived from the optimized bulk lattice vector 3.63 Å using GGA-PBE functional. A single gamma point was used for the integration in the Brillouin zone for the single molecule in the slab (10x6), while for the (1 –2, 7 3) unit cell slab including the molecular triplet structures 12x4x1 k-point mesh was used.

3. RESULTS AND DISCUSSION Upon adsorption of PHTA and annealing of the sample five different long-range ordered structures were observed in LEED, of which two retained the mirror symmetry of the Cu surface and three were enantiomorphous (Figure 2). With the crystal temperature held at 373 K at saturation coverage a (3×2) pattern with missing (0, n) diffraction spots (n = 1/3 , 4/3, …) and missing (n, 0) spots (n = 1/4, 3/4, … ) was observed (Figure 2b). The missing spots suggest the existence of glide planes parallel to the [001] and [1 1 0] directions of the crystal, as previously observed for SU and DMSU.16,24 Deposition onto the crystal surface at 398 K up to a coverage of 2/3 of a complete ML, a (2 –3, 5 3) pattern could be observed. Note that the (2  2) transformation 6 ACS Paragon Plus Environment

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matrix, written here in the form (m11 m12, m21 m22), links the adsorbate lattice vectors (b1, b2) to the substrate lattice vectors (a1, a2) via b1 = m11a1 + m12a2 and b2 = m21a1 + m22a2. Chiral phases are named according to the ‘master matrix’ rules.32 The enantiomorphous (2 –3, 5 3) pattern is based on the superposition of two mirror domains (Figure 2c). With the crystal temperature held at 423 K during evaporation and at saturation coverage, a c(10×2) LEED pattern with missing (0, n) spots, (n = 1/2 , 1, …) and missing (n, 0) spots (n = 1/4, 1/2, …) was observed (Figure 2d). The missing spots again suggest the existence of glide planes parallel to the [001] and [1 1 0] directions. Both structures possessing glide plane symmetries belong to the p2gg plane group. There are also faint streaks at double periodicity along the [001] direction. Considering the very sharp and intense superstructure diffraction spots and these faint features in the same LEED pattern, it is likely that two different structures contribute. For MAL a similar pattern was related to a surface reconstruction, in which the Cu lattice atoms were the origin for the sharp diffraction spots, while faint stripes at ½ periodicity in the pattern originated from a less ordered molecular layer.12 CHDA shows also faint streaks in one LEED pattern at ½ periodicity as well (Figure S2). After deposition of about 0.4 ML of PHTA on the crystal held at 373 K a (1 2, –8 1) structure appeared (Figure 2e). Note that this structure is similar to the (1 2, –8 2) structures reported for (R,R)-TA [originally denoted as (9 0, 1 2)] and for (R)-MA on Cu(110).7,11 A second enantiomorphous structure was obtained when approximately 0.85 ML of PHTA molecules were adsorbed on the substrate with the crystal temperature during evaporation held at 423 K. Analysis of the LEED pattern of this structure reveals a (1 3, –6 1) periodicity (Figure 2f).

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Figure 2. LEED patterns of superstructures of PHTA. (a) Real-space model of the Cu(110) surface aligned to the LEED patterns. (b) p2mg(3×2); (c) (2 –3, 5 3); (d) p(10×2) structure, faint stripes running along the [1 1 0] direction of the crystal indicate the existence of an additional double periodicity; (e) (1 2, –8 2); (f) (1 3, –6 1). The respective primary electron energies are listed near the patterns.

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Figure 3. a) Series of RAIR spectra for PHTA with increasing coverage at room temperature. b) RAIR spectra of two ordered structures. In order to gain information about the chemical binding of the molecules to the surface a series of different amounts of PHTA adsorbed on the crystal at RT were investigated with RAIRS (Figure 3). For all coverages an intense band at around 1420 cm-1 is observed, which describes the symmetric stretching vibration of the carboxylate group. This indicates that at least one carboxylic acid group is deprotonated and bound to the substrate. The fact that the asymmetric component of this vibration at around 1530 cm-1 is not detected suggests that the two oxygen atoms in the carboxylate group are equidistant to the surface, which should result into a small dipole change during vibration. The nature of the second carboxylic acid group is revealed by the band around 1680 cm-1, assigned to the stretching vibration of its C=O group. Hence, the molecule is bound 9 ACS Paragon Plus Environment

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only in a mono-PHTA configuration. This conclusion is further supported by the existence of a band around 3160 cm-1 assigned to the OH group in the carboxylic acid group not bound to the substrate. The fact that the band for the carbonyl group displays a shoulder at around 1720 cm -1 suggests heterogeneity in the molecular layer and the existence of at least two different molecular species, both bound in a mono-PHTA configuration within different chemical environments. The rest of the bands detected are associated with the various vibrations occurring due to the benzene ring, including a weak band around 1460 cm-1, assigned to the ‘ring mode’ (C-C, also denoted as ‘breathing mode’ of the ring). All observed vibrations are listed in Table 1. The respective RAIR spectra of CHDA are shown in Figure S3. RAIRS experiments were also performed with annealed PHTA/Cu(110) samples showing the (1 2, 8 –1) and the (1 3, –6 1) patterns in LEED (Figure 3). For the (1 2, 8 –1) structure, the main bands already observed for the non-tempered samples can be detected. However, a closer inspection of the band for the carbonyl vibration around 1700 cm-1 reveals that its intensity with respect to the carboxylate mode at around 1420 cm-1 is now significant lower. Moreover, the existence of a shoulder at around 1400 cm-1 suggests again that there are two different types of molecules accommodated on the surface. From the intensity of the shoulder with respect to the main peak, the ratio of the two species is estimated as 2:1. The substantially lower intensity of the C=O stretching mode suggests that both carboxyl groups are deprotonated and are bound as carboxylate to the surface. For the (1 3, –6 1) structure a similar scenario has to be concluded, although C=O still shows a strong shoulder. Note that vibrations between 3000 and 3100 cm–1 are also observed for compounds that should not show such bands, they are likely to be due a contamination of the mirrors of the spectrometer.

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Table 1. Frequencies and Assignment of Vibrations Observed at Monolayer Saturation at Room Temperature and for the Long-Range Ordered Structures mode 1ML (1 2, 8 –1) (1 3, –6 1) νsym OH νs CH

νs C=O

3159 (b, m)

3157 (b, m)

3157 (b, m)

3805 (s, m) 3051 (b, w) 2969 (b, m) 2928 (w) 1712 (sh) 1680 (s)

3085 (s, m)

3085 (s, m)

1671 (b, m)

1704 (sh) 1664 (bm) (unidentate) 1574 (w) 1484 (w, s) 1415 (s) 1391 (sh) 1344 (w)

1576 (w)

1574 (w)

1417 (b)

1415 (s) 1396 (sh)

ν C-C (ring) νs OCO ν C-C (ring twist.) 1306 (w)

δ CH sh = shoulder, w = weak, b: broad, s = strong, m = medium.

In order to see whether the oblique or the rectangular footprint of PHTA are favored on Cu(110), DFT calculations for single adsorbates have been performed. Their results (Figure 4) favor the rectangular configuration over the oblique mode only by 11 meV and over a configuration parallel to the [1 1 0] direction (not shown) by 83 meV. If the Cu surface is kept fixed with the Cu atoms not allowed to relax, the oblique footprint is disfavored by 170 meV (Figure S4). For a single TA adsorbate the rectangular footprint was also favored, but in calculations performed for TA triplets a combination of both was found to be lower in energy.23 That is, an oblique TA configuration in the middle was decorated by two rectangular configurations on each side. The oblique configuration was actually found to be slightly lower in energy in case of SU.24 While the oblique adsorbate mode of PHTA leads clearly to a chiral entity, this is not necessarily true for the

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rectangular configuration. However, the DFT calculations show that the adsorbate in the rectangular mode is also slightly chirally distorted (Figures 4a,b). Interestingly, the calculations show also a chiral distortion of the Cu lattice in the topmost layer, in particular for the oblique footprint. Consequently, the long-range chiral structures are likely to be the consequence of spontaneous mirror-symmetry breaking at the single molecule level plus a transmission of chirality from there into extended structures, possibly via the substrate.

Figure 4. Molecular models derived from DFT calculations for two different adsorption modes of PHTA on Cu(110) as dicarboxylate. The Cu crystal directions are indicated. PHTA and the Cu(110) surface are in both modes chirally distorted. Arrows in (d) point to Cu atoms that clearly moved out of their original position of the clean surface. Figure 5 shows STM images of the (1 2, –8 1) structure and its enantiomorphous (1 –2, 7 3) mirror domain. Note that the different notation for mirror domains arises from rules for the proper choice of unit cells.32 The unit cells are indicated in the STM images. The molecules are arranged in triplet lines running along the directions of the crystal. Of the three molecules in each 12 ACS Paragon Plus Environment

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line, the middle one appears to be higher above the surface, coded in the STM image by larger brightness. Note that such triplet lines along with the middle entity at brighter appearance has also been observed for TA, MA and DMSU on Cu(110) by STM.7,11,16 Indicated by red lines in Figure 5, the orientation of the brighter middle feature seems to be oblique with respect to the other two molecules. A tentative model for both enantiomorphous domains is shown in Figure 5c. As suggested by the RAIRS experiments, a dicarboxylate has been chosen as adsorption mode. Similar experiments performed with the (1 2, –8 2) structures of (R)-MA and (R,R)-TA proposed dicarboxylate modes as well.7,11 In order to match the molecular arrangement suggested by STM, an oblique footprint has been combined with rectangular footprints in the model shown in Figure 5c. More principle possibilities for model structures are presented in Figure S5, but they do not entirely fit the STM appearance or have shorter intermolecular distances. The fact that two different frequencies for the symmetric COO stretching vibration at a ratio of 2:1 were observed in RAIRS also supports a model with two different adsorbates in the unit cell. Again, an oblique footprint has been first calculated for SU/Cu(110),24 and a similar combination of rectangular and oblique foot print, as shown in Figure 5c, has been identified by DFT calculations for TA/Cu(110).23 For the latter, ‘stabilizing’ intermolecular hydrogen bonds in the TA-triplets were identified. Intermolecular hydrogen bonds played also an important role in quasi-racemic structures formed by (R)-MA and (S,S)-TA on Cu(110).20 PHTA, however, is quite different in structure to MA and TA and there are no OH groups in surface-bound PHTA in order to establish intermolecular hydrogen bonds. The appearance of the triplet structure is thus likely to be the result of a substrate mechanism induced by the strong interaction of the two carboxylates with the copper surface rather than an intermolecular binding mechanism.

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Figure 5. (a,b) STM images (5.7 nm × 5.7 nm, I = –440 pA, U = –423 mV) of two mirror domains of PHTA on Cu(110). The unit cells are indicated by white parallelograms. (c) The tentative model accounts for the observed position of molecules in the unit cell and a partial oblique alignment of 1/3 of the molecules. The oxygen atoms of the molecules are indicated as red dots, the benzo group as black bar with white dots for the hydrogen atoms. The two (1 –2, 7 3) and (1 2, –8 1) enantiomorphs contain 3 molecules and cover 17 Cu surface atoms. More principle possibilities for model structures are presented in Figure S5.

Triplet motifs for PHTA have also been evaluated by DFT (Figure 6). For the (1 –2, 7 3) enantiomorph the perpendicular-only triplet configuration (Figure 6a) and the triplet structure model shown in Figure 5, that is, an oblique configuration decorated by perpendicular configurations on both sides (Figure 6b), were compared. The actual calculated cell is shown in Figure S6. The structure shown in Figure 6a was found to be preferred by 50 meV. Compared to

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similar triplet DFT modelling performed for TA, this 50 meV difference is rather small and probably due to the absence of hydrogen bonds in the case of PHTA.23

Figure 6. DFT-relaxed structures of the (1 –2, 7 3) structure with triplets entirely built up by perpendicular footprints (a) or by one oblique and two perpendicular footprints per unit cell (b). The structure models in Figures 5 and 6 do not consider any restructuring of the Cu(110) surface. The same applies for the TA triplet structures evaluated recently by DFT.23 However, under the preparation conditions that include mild annealing, other BADs have been shown to reconstruct the Cu(110) surface. Keeping such observations in mind, a surface reconstruction under the triplet motifs of PHTA and TA seems likely. Interestingly, cyclohexanedicarboxylic acid (CHDA) shows LEED patterns that have been observed for other BADs as well (Figure S2). These are the (1 1, –5 2) structure, previously reported for FUA and MAL, and a (1 2, –5 4) structure found for DMSU. In all these cases, chirally arranged copper adatom rows were observed in STM. In addition, the DMSU (1 2, –5 4) structure shows the typical molecular triplet motif.16 Finally, the thermal decomposition of PHTA with respect to other BADs is briefly discussed. Figure 7 shows TPD spectra of PHTA for mass 44, representative for a CO2 fragment released during decomposition. CO2 itself is actually weakly physisorbed on Cu(110) and desorbs below 100 K.33 The thermal decomposition of BADs adsorbed on Cu(110) leads commonly to products 15 ACS Paragon Plus Environment

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such as CO2 H2, H2O and CO.28,34,35 For all coverages two different peaks are observed, one weaker in intensity and shifting from 449 K to 537 K with increasing coverage and a second peak with higher intensity, which shifts from 566 K to 592 K. The presence of two different peaks may suggest that the decomposition mechanism takes place in two steps, in which the first one creates a more stable species, as previously discussed for MA.28 The difference in signal area, however, rather suggests that a small fraction of the molecules is in a less stable configuration. Indications for inhomogeneous layers were also found in RAIRS and STM. The decomposition temperature of almost 600 K here for PHTA is relatively high if compared to TA and MA (510 K), but in the same temperature region as observed for SU.15 This suggests that OH groups lower the thermal stability of BADs on copper surfaces. Interestingly, CHDA is even more stable and has the peak maximum at saturation coverage at 661 K (Figure S7). The shift of the peaks to higher temperature with increasing coverage points to a thermallyinduced decomposition chemistry called ‘surface explosion’.36 Due to dense packing of a molecular layer, a species is stabilized above its usual thermal stability as a single entity. After initiation of decomposition the newly formed free sites catalyze further decomposition, again creating free, catalytically active, surface sites. Hence the decomposition proceeds in an autocatalytic manner in a narrow temperature window.36 However, the TPD peaks here are quite wide (~25 K), as compared to the 1 to 2 K width found for TA and MA surface explosions.34-37 This could be an indication that either the aromatic phenylene unit or the diphthalate adsorbate mode stabilizes the molecules such that the vacancy creation rate is lower than for dehydrationinduced decomposition of OH-containing BADs.

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Figure 7. TPD spectra for PHTA with increasing coverage. The desorption spectrum of the molecule exhibits the presence of two peaks for all coverages, which shift to higher temperatures as the coverage increases.

4. CONCLUSIONS The interaction of achiral phthalic acid with the achiral Cu(110) surface leads to several ordered phases, of which three are chiral. Theoretical considerations propose for the single-molecule adsorbate complex a chiral distortion of molecule and substrate. As also observed for other 1,4dicarboxylic acids on Cu(110), a characteristic triplet-row structure along the surface direction is observed in STM. As previously concluded for tartaric acid, the diphthalate triplet is a combination of an oblique and rectangular complex involving four copper atoms. Comparison with other 1,4-dicarboxylic acid systems on Cu(110) suggest that the surface underneath the phthalate molecular layer is reconstructed in a chiral fashion. It is proposed to perform dynamic electron diffraction studies or photoelectron diffraction in order to determine the exact positions of the topmost Cu atoms. 17 ACS Paragon Plus Environment

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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: Details of coverage calibration, XP spectra and results of DFT calculations. LEED patterns, RAIR spectra and TPD spectra of trans-cyclohexanedicarboxylic acid (CHDA) on Cu(110).

AUTHOR INFORMATION Corresponding Author * e-mail: [email protected] ACKNOWLEDGMENT Funding Sources Swiss National Science Foundation (Supramolecular Chiral Films, 200020_144339 and 200020_163296) CCMX Analytical Platform of the Federal Institute of Technology (ETH) Board University of Zurich Priority Research Program LightChEC Operational Programme Research, Development and Education financed by European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (Project No. SOLID21 CZ.02.1.01/0.0/0.0/16_019/0000760). Financial support by the abovementioned funding sources is gratefully acknowledged. Access to computing and storage facilities of the National Grid Infrastructure MetaCentrum, program "Projects of Large Research, Development, and Innovations Infrastructures" (CESNET 18 ACS Paragon Plus Environment

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LM2015042) is greatly appreciated. P.J. acknowledges support from Praemium Academie of the Academy of Science of the Czech Republic, MEYS LM2015087 and GACR 18-09914S.

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