# How Adsorption Geometry Dictates Chiroptical Properties: A Computational Study of L-Cysteine on Au(111) Cluster Models

Mary Tabut, Pierpaolo D Antoni, Daniele Toffoli, Mauro Stener, Caroline Salzemann, Monica Calatayud

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Mary Tabut, Pierpaolo D Antoni, Daniele Toffoli, Mauro Stener, Caroline Salzemann, et al. How Adsorption Geometry Dictates Chiroptical Properties: A Computational Study of L-Cysteine on Au(111) Cluster Models. The Journal of physical chemistry, 2026, 130 (1), pp.747-756. 10.1021/acs.jpcc.5c07107. hal-05423279

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# How Adsorption Geometry Dictates Chiroptical Properties: A Computational Study of L-Cysteine on Au(111) Cluster Models

Mary Tabut,*,†,‡ Pierpaolo D'Antoni,¶ Daniele Toffoli,*,¶ Mauro Stener,¶

Caroline Salzemann,‡ and Monica Calatayud‡

†Sorbonne Université, CNRS, Laboratoire de Chimie Théorique, LCT, 4 Place Jussieu, F-75005 Paris, France

‡Sorbonne Université, CNRS, MONARIS, CNRS-UMR 8233, 4 Place Jussieu, F-75005 Paris, France

¶Department of chemical and pharmaceutical sciences, University of Trieste, 34127, Trieste, Italy

E-mail: mary.tabut@sorbonne-universite.fr; toffoli@units.it

## Abstract

Understanding chirality transfer in metal nanoclusters is crucial for developing advanced materials with tailored optical properties. This study investigates the chiroptical properties of gold clusters functionalized with L-cysteine using time-dependent density functional theory (TD-DFT). By exploring different ligand adsorption configurations, molecular orientations, and intermolecular interactions, we systematically analyze their influence on electronic circular dichroism (ECD) spectra. The results reveal that ligand-induced chirality transfer dominates the ECD response, with hydrogen bonding and molecular orientation playing a significant role in modulating the
optical activity. In addition, fragment analysis confirms that metal-ligand interactions strongly contribute to the observed chiroptical effects, demonstrating that controlling ligand arrangement on the gold cluster surface is essential for tuning induced circular dichroism.

Introduction

Chirality plays a fundamental role in numerous chemical and biological processes, and its manifestation in nanoscale materials has accumulated significant attention in recent years. In particular, chirality transfer mechanisms in plasmonic and nanostructured systems have been extensively studied due to their implications in asymmetric catalysis, enantioselective sensing, and optoelectronic applications. 1-3 Among these materials, gold nanoclusters (AuNCs) have emerged as particularly promising candidates for exploring chiroptical effects, as they exhibit size-dependent electronic transitions and strong ligand interactions that can be tuned for specific applications. 4-6

Thiolate-protected gold nanoclusters (RS-AuNCs) are of particular interest for their unique chiroptical properties, which arise from their distinct structural configurations and ligand arrangements. A well-known example is the Au<sub>38</sub>(SR)<sub>24</sub> cluster, which features a bicosahedral core surrounded by a chiral ligand shell.<sup>7-11</sup> Despite the achirality of the metal core and ligands, this structure induces significant circular dichroism (CD) signals, a phenomenon known as induced chirality. This work demonstrates how the chiral information from the ligand shell can influence the electronic structure of the entire nanocluster. The relationship between ligand chirality and the electronic transitions of the gold core is further explored through electronic circular dichroism (ECD) spectroscopy.<sup>12</sup> Research has shown that the adsorption of chiral thiolates onto gold clusters leads to notable ECD signals, indicating a strong coupling between the ligand's chirality and the cluster's electronic transitions.

In this study, we investigate the chiroptical properties of L-cysteine-functionalized gold (111) cluster models, Au<sub>64</sub>(Cys)<sub>2</sub>. L-cysteine, a naturally occurring chiral amino acid,
is an ideal model ligand due to its well-defined stereochemistry and strong affinity for metal surfaces. While numerous studies have explored cysteine derivatives for nanoparticle functionalization, the specific effects of pH and molecular form on optical activity remain less understood. In our work, we focus on the specific acidic form of L-cysteine $[NH_3^+CH(CH_2SH)COOH]$. However, in our study, we adapted our model to represent the chemisorbed state of cysteine on the Au(111) surface, where adsorption typically leads to S-H bond cleavage and formation of a thiolate (Au-S) species. For this reason, the $[NH_3^+CH(CH_2S^-)COOH]$ form of the molecule was chosen as the relevant adsorbed species under acidic conditions. Importantly, this form of the molecule not only allows potential intermolecular interactions between the amino and carboxylic groups of two neighboring molecules but also plays an important role in the way the molecules interact with the gold surface. These effects have been extensively studied in our previous work using periodic surfaces at the density functional theory (DFT) level of theory.¹³

Building on this, we move from periodic models to finite clusters to study the chiroptical response of these systems. Au-thiolate clusters have been extensively studied both experimentally⁶,¹⁴ and theoretically, from small to larger cluster sizes,¹⁵⁻¹⁷ focusing on various ligand effects and structural influences on the chiroptical properties. They have provided valuable insights into chirality transfer mechanisms and the enhancement of optical activity in gold nanoclusters. Notably, Monti et al. demonstrated that solvent interactions, including hydrogen bonding, significantly influence the ECD response of glutathione-protected $[Au_{25}(GSH)_{18}]^{-1}$ nanoclusters, highlighting the active role of the solvent in modulating optical activity.¹⁵

However, a comprehensive analysis of chirality transfer mechanisms and ECD behavior, particularly in relation to molecular orientation, adsorption sites, and intermolecular interactions, is still lacking. To fill these gaps, we employ state of the art time-dependent density functional theory (TD-DFT) to systematically investigate the ECD spectra of L-cysteine-functionalized gold clusters. Our simulations take into account variations in ligand
adsorption, molecular orientation, and hydrogen bonding, providing detailed insights into
how these factors influence the electronic and optical properties of the clusters across the
near UV (250 nm) to near-infrared (800 nm) wavelength range.

The calculated ECD spectra are further analyzed using a fragment projection method, which helps determining the contributions of specific molecular orbitals to the observed transitions. Our findings demonstrate a strong chirality transfer from the L-cysteine ligands to the metal cluster, and emphasize the importance of controlling the surface arrangement of chiral molecules in tuning induced circular dichroism.

## Computational Methods and Cluster model

All cluster geometries presented in this paper were designed based on the corresponding
optimized periodic systems extracted from ref. ¹³ The optimized geometries of the periodic
systems were obtained from Density functional theory (DFT) calculations performed using
the Vienna Ab initio Simulation Package (VASP), version 6.4.0.¹⁸,¹⁹ The Perdew-Burke-
Ernzerhof (PBE) generalized gradient approximation (GGA) was employed as the exchange-
correlation functional,²⁰ with Van der Waals interactions incorporated via the Grimme-D3
method.²¹,²² The periodic systems were constructed for the Au(111) surface, where a slab
model of four atomic layers thick was used, from which the bottom two layers were fixed
to simulate bulk properties, and the top two layers were fully relaxed during optimization
process. A plane-wave basis set with an energy cutoff of 450 eV was applied,¹⁹ and core elec-
trons were treated using projector-augmented wave (PAW) pseudopotentials.²³ The valence
electrons considered in the calculations correspond to 11 for Au, 6 for S, 1 for H, 4 for C, 6
for O, and 5 for N atoms. Periodic boundary conditions were enforced, with a 15 Å vacuum
region along the z-axis to prevent interactions between periodic images. The Au(111) facet
was modeled using a p(4x4) supercell, and the Brillouin zone was sampled with a 3x3x1
k-point mesh generated by the Monkhorst-Pack scheme.²⁴ Self-consistent field calculations
were converged with a threshold of 10<sup>-6</sup> eV.

The cluster model used for this study is presented in Figure 1. All clusters were cut out from
their corresponding optimized slabs using the VESTA program, version 3.9.0,²⁵ see below.

<figure><img src="image_2.png" /><figcaption>Figure 1: Computational model of the functionalized gold cluster: (Left) Top and side views of the optimized slab model used as the foundation for designing the clusters shape. (Right) Top and side views of the L-cysteine-functionalized gold cluster.</figcaption></figure>

This approach ensures a well-defined local environment consistent with surface adsorp-
tion and allows the focus on the molecular surface arrangements rather than in the gold
substrate itself. Our study focuses on the investigation of various factors that can affect
the chirality transfer mechanism. Therefore, in designing these systems, particular attention
was given to the shape of the gold cluster to ensure that it did not introduce any artifacts or
undesired chiral influences on the calculated ECD spectra. The gold cluster is composed of
the first three layers of the Au(111) surface (for a total of 64 gold atoms), and 2 molecules of
L-cysteine adsorbed to the surface, comprising a total of 92 atoms. From the top view of the
slab (Figure 1 (Left)), we can see that a particular attention was paid to place the adsorbed
molecules as centered as possible to avoid side effects that could alter the ECD response.
The optical response of the finite systems was calculated at the polarizable TDDFT (polT-DDFT) level of theory.²⁶,²⁷ For all systems investigated, no further geometry optimization was performed.

The Hybrid Diagonal Approximation (HDA)²⁸ was employed to facilitate the TDDFT calculations using the B3LYP hybrid functional.²⁹,³⁰ This functional was selected based on a systematic comparison against various GGA and hybrid functionals, which identified B3LYP as the most suitable compromise for simultaneously describing the electronic response of both the gold clusters and the organic ligands (see Figure S3 for details). In the present work we employed a much more efficient implementation of the HDA (fitted HDA) which allows the treatment of large systems at a reasonable computational cost.³¹ For organic molecules and gold atoms near cysteine molecules, a Slater Type Orbitals (STO) Triple Zeta plus Polarization (TZP) basis set was used with fitting functions optimized for polTDDFT.³² The remaining gold atoms in the clusters were described using a Double Zeta plus Polarization (DZP) basis set. The simulated ECD spectra were calculated using the Amsterdam Density Functional (ADF) engine within the AMS code. All calculations were carried out at the scalar relativistic level within the Zero-Order Regular Approximation (ZORA) to incorporate relativistic effects, which are crucial for heavy elements such as gold. The ECD spectra were obtained from the imaginary part of the optical rotation tensor, following the formalism detailed in references.²⁶,²⁷ The photon energies were computed with an imaginary part of $\omega_i = 0.060$ eV, resulting in Lorentzian broadening of the profiles with the same half-width at half-maximum (HWHM).

To bring insight into the nature of the interactions, a fragment analysis of the spectra was investigated. Using this methodology we were able to separate the contributions from metallic atoms and organic molecules on the optical response, enabling a fragment projection analysis as described in references.³³,³⁴
Results and discussion

To ensure the accuracy of the model, in Figure 2 we present an example of the calculated
ECD spectrum for the bare gold cluster. The spectrum shows a rather weak signal in the near
UV/visible region of interest (250 nm to 800 nm), with a maximum intensity of 40*M*⁻¹·*cm*⁻¹
in absolute value, showing that the cluster itself does not contribute significantly to any chiral
optical activity within this range of energy. This is a critical observation, as it establishes a
baseline that confirms that any significant ECD activity in the functionalized systems cannot
exist without the transfer of chirality from the ligands to the metallic cluster.

In addition, the ECD spectrum calculated for L-cysteine in the gas phase displays a similar
chiral response (Figure 2), but only in the UV region around 200 nm, which aligns well with
experimental data.³⁵ Notably, there is no significant ECD activity in the rest of the spectral
range explored in this study, further emphasizing the need for a physico-chemical interaction
between the gold cluster and the organic ligand to observe a chiral response for such systems.

<figure><img src="image_3.png" /><figcaption>Figure 2: Calculated ECD spectra of the bare gold cluster (left) and the gas cysteine molecule (right).</figcaption></figure>

In Figure 3, we illustrate the eight configurations selected for this study, labeled HC1 to
HC8. These systems were selected to cover a range of factors that influence their chiroptical
properties, including molecular orientation, adsorption site and the presence of intermolec-
ular interactions between the NH₃⁺ and COO⁻ groups.
<figure><img src="image_4.png" /><figcaption>Figure 3: Top views of all clusters investigated in this study. Only the first layer of the gold atoms is represented for clarity. Au atoms are depicted as dark yellow spheres, while S, C, N, O, and H atoms of the cysteine ligands are represented in yellow, brown, blue, red, and white, respectively.</figcaption></figure>

As previously stated, the configurations were carefully selected from a previous study,¹³ which focused on the stability of self-assembled monolayers of the acidic form of L-cysteine on the gold (111) surface. In that work, we investigated how variations in surface arrangement affect the structural stability of the system. In contrast, the present study shifts the focus toward understanding how these same properties influence the optical activity, with particular emphasis on chiroptical responses. Table 1 summarizes the differences in structural properties for all Au₆₄(cys)₂ conformations, while the selected electronic features, such as total dipole moment and charge transfer are detailed in Table SI 1.
<table><caption>Table 1: Summary of structural properties for the Au<sub>64</sub>(cys)<sub>2</sub> conformations. The table reports hydrogen bond presence (and length), adsorption site, presence of SH bonds, and molecular orientation for all cluster.</caption><thead><tr><th>System</th><th>H bond<br>(Å)</th><th>Adsorption site<br>(S<sub>1</sub>/S<sub>2</sub>)</th><th>S···H bond<br>(S<sub>1</sub>/S<sub>2</sub>)</th><th>Molecular Orientation</th></tr></thead><tbody><tr><td>HC1</td><td>Yes (1.619)</td><td>Top/Top</td><td>Yes/Yes</td><td>Clockwise/Clockwise</td></tr><tr><td>HC2</td><td>Yes (1.779)</td><td>Top/Top</td><td>No/No</td><td>Clockwise/Counterclockwise</td></tr><tr><td>HC3</td><td>No</td><td>Top/Top</td><td>No/No</td><td>Clockwise/Counterclockwise</td></tr><tr><td>HC4</td><td>Yes (1.691)</td><td>Bridge/Top</td><td>No/No</td><td>Clockwise/Clockwise</td></tr><tr><td>HC5</td><td>Yes (1.752)</td><td>Bridge/Top</td><td>No/No</td><td>Counterclockwise/Clockwise</td></tr><tr><td>HC6</td><td>Yes (1.542)</td><td>Bridge/Top</td><td>No/Yes</td><td>Clockwise/Clockwise</td></tr><tr><td>HC7</td><td>No</td><td>Bridge/Top</td><td>No/No</td><td>Clockwise/Counterclockwise</td></tr><tr><td>HC8</td><td>Yes (1.766)</td><td>Bridge/Top</td><td>No/No</td><td>Clockwise/Counterclockwise</td></tr></tbody></table>

Photoabsorption spectra

The calculated UV photoabsorption spectra for the eight gold cluster–ligand complexes are shown in Figure 4. Each spectrum features a prominent absorption band in the UV range (200–400nm), followed by a gradual decrease in oscillator strength toward the visible and near-infrared region (400–800nm). Such behavior is characteristic of small gold nanoclusters, where localized surface plasmon resonances are generally weak and the absorption is dominated by ligand-to-metal charge transfer and metal-centered transitions. The rapid drop in intensity beyond 500nm further supports the notion that purely plasmonic effects do not play a major role in these systems. This finding aligns with expectations, as the ECD technique is highly sensitive to model size, and the dimensions of our clusters are too small to support plasmonic behavior. The fact that the functionalized gold clusters investigated in this study do not show plasmonic resonance behavior permits us to assess other ways of tuning the chiroptical properties of hybrid systems defined by a chiral molecule at the surface of an achiral nanoparticles, or in this study, small metallic clusters.
<figure><img src="image_5.png" /><figcaption>Figure 4: Photo-absorption spectra of all protected gold clusters investigated (HC1 to HC8). Inserts show a zoom between 250 and 500nm.</figcaption></figure>

Among the complexes, HC5 (red curve) shows the strongest absorption at shorter wavelengths (~200-350nm), suggesting a particularly strong charge-transfer interaction between the ligand and the cluster core. HC1 (light blue curve) also show distinct behavior with a stronger absorption at 425 nm. By contrast, the rest of the set of complexes appears more uniform, indicating fewer structural or electronic differences. Subtler variations in peak positions and intensities across the other complexes likely arise from differences in binding mode, or local geometry at the cluster surface.

## Molecular orientation impact

In the present study, we begin by analyzing the influence of the molecular orientation of the ligands. This is examined by comparing the orientation of the ligands onto the surface, which can be defined either in a "clockwise" or "counterclockwise" orientation. The terms "clockwise" and "counterclockwise" refer to the carbon skeleton arrangement of the cysteine molecules relative to their adsorption on the gold surface, where the backbone path is defined
from the SH group to the COOH group from a top view: in the "clockwise" orientation, the
cysteine backbone follows a right-handed turn, while in the "counterclockwise" orientation,
it follows a left-handed turn.

We first compare systems HC5 and HC8, which both feature one cysteine in a clockwise orientation and the other in a counterclockwise orientation (see Table 1). However, the specific cysteine molecules adopting these orientations differ between the two systems. In HC5, the cysteine molecule adsorbed on the bridge site adopts a "counterclockwise" orientation, while the cysteine molecule adsorbed on the top site adopts a clockwise orientation. In contrast, in HC8, the molecular orientations are reversed: the cysteine on the bridge site adopts a clockwise orientation, and the cysteine on the top site adopts an counterclockwise orientation. This results in a complete inversion of the global molecular orientation between HC5 and HC8, creating an enantiomeric molecular arrangement. To further investigate the impact of changes in molecular orientation, we then examine systems HC4 and HC5. These systems differ more subtly: in HC4, one cysteine adopts a clockwise orientation and the other a counterclockwise orientation, while in HC5, both cysteine molecules are oriented in the clockwise direction (see Figure S1).

The calculated spectra of HC5 and HC8 shown in Figure 5 reveal several distinct spectral
features across all the energy range explored (from 250 nm to 800 nm). At first glance
we see a complete mirror image spectra when comparing these 2 systems. In the higher-
energy region (250–400 nm), both spectra display alternating positive and negative peaks
with comparable intensities, but opposite signs. In the intermediate region (400–600 nm),
a dominant feature appears near 500 nm in both systems; once again, its sign is inverted.
Finally, in the lower-energy range (600–800 nm), the signals are less pronounced, yet still
exhibit opposite polarity of the bands. Such sign inversions are closely tied to the molecular
arrangement of the cysteine ligands on the gold cluster. Although HC5 and HC8 share
similar hydrogen-bonding distances (1.75 Å in HC5 vs. 1.77 Å in HC8), and comparable
charge transfer from the ligands to the gold surface (HC5: 0.614 |e|; HC8: 0.605 |e|), their
global ligand orientations differ, creating an enantiomeric molecular arrangement that can be observed by the opposite direction of their permanent dipole moment vectors (see Table S1 available in the Supporting Information).

It is important to note that the permanent dipole moment itself does not directly dictate chiroptical properties; rather, the relevant factors are the electric and magnetic transition dipole moments associated with electronic excitations. However, the permanent dipole moment can serve as an indicator of the global molecular geometry. If changes in orientation lead to a drastic chiral inversion—i.e., producing an enantiomeric configuration—then the ECD signals become inverted accordingly. This phenomenon has been widely discussed in studies on plasmonic and molecular chirality, where even subtle geometric rearrangements can induce significant shifts or sign inversions in ECD spectra. In the present systems, the orientation of cysteine ligands fully changes the chiral environment to the point of inverting the entire ECD spectra.

<figure><img src="image_6.png" /><figcaption>Figure 5: Calculated ECD spectra of gold clusters HC5 (red) and HC8 (black). (a) and (b) illustrate the structural configurations of the cysteine ligands in HC5 and HC8, respectively, highlighting the influence of molecular orientation (backbone chain adopting a clockwise or counterclockwise arrangement) on the observed ECD signals.</figcaption></figure>
A similar observation arises when comparing HC4 and HC5 (Figure S1). In that case, the top-position ligand remains the same, but the bridge-position ligand is inverted and reoriented into a "clockwise" orientation in HC4. Interestingly, in that case, the overall dipole moment does not invert; yet, certain regions of the ECD spectrum (particularly around 400-500 nm) exhibit nearly perfect sign inversion, underscoring how specific transitions can be highly sensitive to local structural changes. Outside this spectral region, the sign does not invert entirely, indicating that only certain electronic states, and their associated transition moments, are significantly perturbed.

## Effect of adsorption site

Next, we investigate the effect of the adsorption site by comparing systems HC1 and HC6. These two configurations feature different adsorption sites (top/top *vs* top/bridge) but exhibit similar types of interactions between the cysteine molecules, including the formation of an SH bond and intermolecular interactions between the COO⁻ and NH₃⁺ groups. Additionally, both systems present the same molecular orientation. The specific adsorption site of the ligands on a metallic surface also modulates local electronic environments, affecting the electric and magnetic transition dipole moments in subtle yet impactful ways. The corresponding ECD spectra of systems HC1 and HC6 is shown in Figure 6.
<figure><img src="image_7.png" /><figcaption>Figure 6: Calculated ECD spectra of gold clusters HC1 (red) and HC6 (black). (a) and (b) illustrate the structural configurations of the cysteine ligands in HC1 and HC6, respectively, highlighting the influence of molecular adsorption (top or bridge) on the observed ECD signals.</figcaption></figure>

In HC1, both cysteine ligands occupy top (T) adsorption sites on the gold cluster (TT), maintaining an intermolecular hydrogen bond of 1.62Å. In contrast, HC6 places one cysteine in a top site and the other in a bridge site (TB), shortening the hydrogen bond distance to 1.54Å. This change in adsorption site disrupts the more symmetrical arrangement found in HC1 and modifies the local electronic environment around each ligand.

A comparison of the ECD spectra (Figure 6) shows that both systems feature multiple peaks in the energetic region of interest, but their intensities and signs differ in several key areas. More pronounced differences appear between 350 and 600nm, where a clear sign inversion occurs but is shifted: HC1 shows a strong positive band around 420nm, while HC6 displays a prominent negative feature near 460nm. This shift in both wavelength and sign suggests that moving one ligand from a top to a bridge site alters the orientation and
energies of the electronic transitions, likely by changing the ligand-metal bonding network and redistributing frontier molecular orbitals. In the 500-600nm region, a clear sign inversion is observed, but with differences in intensities, with the ECD signal arising from system HC6 being twice as intense as for system HC1.

Beyond 600 nm, the rotational strengths gradually decrease, but still showcase shifted sign inversion. Overall, transitioning from a TT (HC1) to a TB (HC6) configuration breaks the higher symmetry present in HC1, thereby reshaping the chiroptical response. Notably, the intense peaks near 420 nm in HC1 and 460 nm in HC6 not only highlight this sensitivity but also differ markedly in magnitude, with the 460 nm peak in HC6 reaching nearly twice the intensity of the 420 nm peak (approximately 250 versus 150 in absolute values). The sign inversion observed between 500 and 600 nm also comes with a significant difference in intensity, with system HC6 showing twice the intensity of HC1. This difference in intensities is likely correlated to the electronic density being more spread out when the ligand is adsorbed into a bridge position rather than a top one.

## Effect of intramolecular interactions.

And finally, we explore the role of intermolecular interactions, specifically hydrogen bonding, on the chiroptical properties by analyzing systems HC2 and HC3 (also HC7 and HC8, see Figure S2). These systems are designed to probe how the presence and strength of hydrogen bonds between the cysteine molecules influence the chiroptical response.

Both HC2 and HC3 systems exhibit similar dipole moment vectors and total energies. However, they differ in the nature of the intermolecular interactions between the ligands. Specifically, HC2 displays a hydrogen bond of 1.78 Å, while HC3 shows no direct interaction between the two ligands, with the minimum distance between them exceeding 3 Å. Both systems exhibit similar absorption profiles, indicating that their electronic transitions involving the metallic core remain largely unchanged; however, their ECD spectra reveals differences, particularly in the wavelength regions between 350-500 nm, where HC2 shows distinct sign inversions with similar intensities in the rotational strength compared to HC3, as shown in
Figure 7.

This inversion suggests that even subtle intermolecular interactions, such as a simple hydrogen bond, can significantly influence the chiroptical response. It is known that hydrogen bonds can highly influence the relative stability of adsorbed layers, in this study we also show that H-bonds can also modulate the electronic structure, thereby shaping the optical response of protected gold clusters.

Systems HC7 and HC8 have also been compared and the corresponding spectra can be found in Figure S2. These 2 systems showcase the same adsorption sites (top-bridge in this case) but HC7 is oriented such that no intermolecular bond is observed between the 2 ligands, in contrast with HC8. Their ECD spectra also present sign inversions between 350nm and 600nm but with significant differences in intensities, with HC8 having a much more intense ECD response than HC7.

<figure><img src="image_8.png" /></figure>

Figure 7: Calculated ECD spectra of gold clusters HC2 (black) and HC3 (red). (a) and (b) illustrate the structural configurations of the cysteine ligands in HC2 and HC3, respectively, highlighting the influence of hydrogen bonding on the observed ECD signals.

Fragment analysis
Overall, in all cases, the most relevant spectral region lies below 600nm, where the most intense peaks and the clearest sign inversions occur. Notably, this behavior can be directly linked to the photo-absorption spectra in Figure 4, where the first absorption signal gradually emerges after around 600 nm. Since an ECD signal is still present beyond this threshold, it is interesting to determine its origin. Therefore, to further evaluate the mechanism of chirality transfer from the organic molecule to the gold cluster, we analyzed the contributions of gold (fragment 1) and cysteine (fragment 2) molecular orbitals to the spectral features. Using the fragment analysis tool in AMS, we identified which molecular orbitals participate in the transitions observed in the overall ECD spectrum. The fragment analysis spectra for all systems are presented in Figure 8.
<figure><img src="image_9.png" /><figcaption>Figure 8: Fragment analysis of the calculated ECD spectrum for conformations HC1–HC8. Contributions from the gold and cysteine molecular orbitals (initial → final states) to the chiroptical response are delineated with distinct colored lines, while the overall spectrum is depicted by a dashed black line.</figcaption></figure>
Across all eight conformations (HC1-HC8), the fragment analysis of the calculated ECD spectra reveals that the dominant contributions generally arise from metal-metal (Au → Au) transitions (yellow lines). By contrast, transitions involving only the cysteine orbitals (Cys → Cys) (green lines) are negligible, which aligns well with the fact that organic molecules typically absorb at higher energies (below 200nm) and their occupied molecular orbitals lie significantly lower than those of gold. Nevertheless, there is clear evidence of chirality transfer between the organic ligands and the metal cluster in every system, as indicated by the Au → Cys (blue lines) and Cys → Au (red lines) components. These transitions correspond to ligand-to-metal charge transfer (LMCT) contributions, in which electron density is transferred from the ligand orbitals to the gold cluster. Conversely, transitions where the electron density remains primarily on the metal atoms are considered metal-centered excitations. Interestingly, these transitions involving both gold and cysteine no longer contribute to the overall ECD spectra after 600nm, leaving only the gold MOs to be at the origin of the optical activity. It is important to keep in mind that while the Au-Au contributions appear dominant in this spectral region, the bare gold cluster itself is intrinsically achiral. Their chiral response arises entirely from the perturbation induced by the attached chiral ligands, representing the effect of chirality transfer from the ligand to the cluster.

In HC3, HC6 and HC8, 3 systems that do not present significant resemblance, the ECD signal is dominated by strong Au → Au transitions, whereas in other systems, the metal-ligand coupling plays a more substantial role. Notably, HC1 exhibits a pronounced ligand-to-metal contribution, particularly around 370nm, where Cys → Au transitions drive an intense positive band. At lower energies, the interplay of Au → Au, Au → Cys, and Cys → Au components can either dampen or enhance the net rotational strength; for instance, at 450nm, these contributions nearly cancel each other out, yielding a near-zero signal. By contrast, HC5 and HC8, which feature a complete inversion of the ECD response compared to other systems, show that this reversal is predominantly due to metal-centered (Au → Au) transitions. The Au → Cys and Cys → Au terms largely cancel one another over
most of the spectral range, resulting in negligible ligand-metal influence beyond 500nm. Finally, comparing HC2 (with a strong hydrogen bond) and HC3 (no H-bond) demonstrates that intermolecular interactions can intensify chirality transfer; in HC2, the ligand orbitals contribute more significantly to the overall ECD than in HC3, highlighting how hydrogen bonding can amplify metal-ligand coupling and thus modulate the chiroptical properties. This is however contrasted by comparing HC7 and HC8 were the H bond seems to have no direct effect on the shape nor intensity of the Au → Cys and Cys → Au contributions.

## Conclusions

In summary, this study provides a comprehensive computational exploration of the chiroptical properties of L-cysteine-functionalized gold(111) cluster models using time-dependent density functional theory (TD-DFT). Our findings highlight the critical role of ligand orientation, adsorption site, and intermolecular interactions in modulating the electronic circular dichroism response. The analysis of molecular configurations confirms that ligand-induced chirality transfer is the primary mechanism governing chiroptical behavior, with structural and electronic factors playing a significant role. Furthermore, fragment projection analysis emphasizes the contributions of ligand-to-metal charge transfer and metal-centered transitions in shaping the overall optical activity.

## Acknowledgement

This research received partial funding from the French National Research Agency (ANR) through the project ANR-22-CE09-0007. We gratefully acknowledge financial support from the Initiative Sciences et Ingénierie Moléculaires (iSiM) at Sorbonne Université. This work was performed using HPC resources from GENCI-IDRIS Grant 2024-A0170802131. Financial support from ICSC—Centro Nazionale di Ricerca in High Performance Computing, Big Data and Quantum Computing, funded by European Union—NextGenerationEU is grate-
fully acknowledged.

**Supporting Information Available**

Characterization data for all systems are provided in the Supporting Information. This
includes the structural and electronic analysis of all systems investigated in this study and
the additional ECD spectra discussed in the text.

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## Supporting Information

# How Adsorption Geometry Dictates Chiroptical Properties: A Computational Study of L-Cysteine on Au(111) Cluster Models

Mary Tabut*,†,‡, Pierpaolo D'Antoni¶, Daniele Toffoli*,¶, Mauro Stener¶,
Caroline Salzemann‡ and Monica Calatayud‡

† Sorbonne Université, CNRS, Laboratoire de Chimie Théorique, LCT, 4 Place Jussieu,
F-75005 Paris, France

‡ Sorbonne Université, CNRS, MONARIS, CNRS-UMR 8233, 4 Place Jussieu, F-75005
Paris, France

¶ Department of chemical and pharmaceutical sciences, University of Trieste, 34127,
Trieste, Italy

E-mail: mary.tabut@sorbonne-universite.fr; toffoli@units.fr

**Table S1** Electronic analyses were performed for all systems investigated in this study. Dipole moment values were obtained directly from polTDDFT calculations, and all charges reported were computed at the DFT level of theory in a previous study for the corresponding periodic systems.¹³ The charge transfer values were obtained from a Bader charge analysis and correspond to ground-state charge distribution between cysteine and the gold surface. Therefore, these values reflect the amount of electron density transferred from cysteine molecules to gold in the ground state, and they are not associated with any specific excitation.
<table><thead><tr><th>Au<sub>64</sub>(cys)<sub>2</sub><br>conformation</th><th>Dipole Moment<br>Vector Components</th><th>Total Dipole<br>Moment (Debye)</th><th>Charge transfer (|e|)<br>(Ligands → Surface)</th></tr></thead><tbody><tr><td>HC1</td><td>-1.45 ; -9.46 ; 5.70</td><td>11.137</td><td>0.363</td></tr><tr><td>HC2</td><td>-4.09 ; 3.21 ; 8.12</td><td>9.643</td><td>0.519</td></tr><tr><td>HC3</td><td>-3.72 ; 1.16 ; 2.58</td><td>4.675</td><td>0.643</td></tr><tr><td>HC4</td><td>-4.99 ; -3.67 ; 9.81</td><td>11.602</td><td>0.649</td></tr><tr><td>HC5</td><td>-2.26 ; -5.57 ; 6.35</td><td>8.744</td><td>0.614</td></tr><tr><td>HC6</td><td>-2.30 ; 2.95 ; 6.10</td><td>7.158</td><td>0.492</td></tr><tr><td>HC7</td><td>-0.59 ; -1.30 ; 4.08</td><td>4.321</td><td>0.542</td></tr><tr><td>HC8</td><td>3.86 ; 5.93 ; 7.00</td><td>9.950</td><td>0.605</td></tr></tbody></table>

**Figure S1** Comparison of ECD spectra of systems HC4 and HC5 to highlight the effect of molecular orientation on the spectral features.

<figure><img src="image_10.png" /></figure>

**Figure S2** Comparison of ECD spectra of systems HC7 and HC8 to highlight the effect of intermolecular interactions on the spectral features.
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>(a) Top view HC7</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>(b) Top view HC8</figcaption></figure>

To determine the most appropriate level of theory, we performed a systematic benchmark using three GGA functionals (PW91, PBE, PBEsol) and four hybrid functionals (B3LYP, B3LYP*, SH12, and PBE0). As shown in Figure S3, while the GGA functionals displayed internal consistency, the trends among the hybrid functionals were highly variable with no straightforward pattern.

In the absence of a unique theoretical trend, our selection criteria prioritized agreement with experimental data. The B3LYP functional was chosen because it yields the best quantitative agreement with experimental spectra for gold clusters and thiolate ligands, as demonstrated in our recent work.⁶

Furthermore, given the variability observed among hybrid functionals, the absolute alignment of electronic levels is less critical than the relative trends between systems. By strictly maintaining the same level of theory (B3LYP), we ensure that the comparative analysis across the different gold-ligand systems remains robust and meaningful.
**Figure S3** Systematic evaluation of exchange-correlation functionals applied to the gold-ligand clusters.

<figure><img src="image_14.png" /></figure>