O2 activation by subnanometer Re-Pt clusters supported on TiO2(110): exploring adsorption sites.

Activation of O2 by subnanometer metal clusters is a fundamental step in the reactivity and oxidation processes of single-cluster catalysts. In this work, we examine the adsorption and dissociation of O2 on RenPtm (n + m = 5) clusters supported on rutile TiO2(110) using DFT calculations. The adhesion energies of RenPtm clusters on the support are high, indicating significant stability of the supported clusters. Furthermore, the bimetallic Re-Pt clusters attach to the surface through the Re atoms. The oxygen molecule was adsorbed on three sites of the supported systems: the metal cluster, the surface, and the interface. At the metal cluster site, the O2 molecule binds strongly to RenPtm clusters, especially on the Re-rich clusters. O2 activation occurs by charge transfer from the metal atoms to the molecule. The dissociation of O2 on the RenPtm clusters is an exothermic process with low barriers. As a result, sub-nanometer Re-Pt clusters can be susceptible to oxidation. Similar results are obtained at the metal-support interface, where both the surface and cluster transfer charge to O2. To surface sites, molecular oxygen is adsorbed onto the Ti5c atoms with moderate adsorption energies. The polarons, which are produced by the interaction between the metal cluster and the surface, participate in the activation of the molecule. However, dissociating O2 in these sites is challenging due to the endothermic nature of the process and the high energy barriers involved. Our findings provide novel insights into the reactivity of supported clusters, specifically regarding the O2 activation by Re-Pt clusters on rutile TiO2(110).

A chiral metal cluster triggers enantiospecific electronic transport.

Chirality is a geometric property of matter that can be present at different scales, especially at the nanoscale. Here, we investigate the manifestation of chirality in electronic transport through a molecular junction. Spinless electronic transport through a chiral molecular junction is not enantiospecific. However, when a chiral metal cluster, C3-Au34, is attached to the source electrode, a different response is obtained in spinless electronic transport between R and L systems: this indicates the crucial role of chiral clusters in triggering enantiospecific spinless electronic transport. In contrast, when an achiral metal cluster, C3v-Au34, is attached, no change in conductance occurs between enantiomeric systems. Using the non-equilibrium green's function method, we characterized this phenomenon by calculating the transmission and conductance of spin-unpolarized electrons. Our theoretical results highlight the importance of metal clusters with specific sizes and chiral structures in electronic transport and support previously published experimental results that exhibited enantiospecific scanning tunneling measurements with intrinsically chiral tips.

Interaction Mechanisms and Interface Configuration of Cysteine Adsorbed on Gold, Silver, and Copper Nanoparticles.

Cysteine-protected metal nanoparticles (NPs) have shown interesting physicochemical properties of potential utility in biomedical applications and in the understanding of protein folding. Herein, cysteine interaction with gold, silver, and copper NPs is characterized by Raman spectroscopy and density functional theory calculations to elucidate the molecular conformation and adsorption sites for each metal. The experimental analysis of Raman spectra upon adsorption with respect to free cysteine indicates that while the C-S bond and carboxyl group are similarly affected by adsorption on the three metal NPs, the amino group is sterically influenced by the electronegativity of each metal, causing a greater modification in the case of gold NPs. A theoretical approach that takes into consideration intermolecular interactions using two cysteine molecules is proposed using a S-metal-S interface motif anchored to the metal surface. These interactions generate the stabilization of an organo-metallic complex that combines gauche (PH) and anti (PC) rotameric conformers of cysteine on the surface of all three metals. Similarities between the calculated Raman spectra and experimental data confirm the thiol and carboxyl as adsorption groups for gold, silver, and copper NPs and suggest the formation of monomeric "staple motifs" that have been found in the protecting monolayer of atomic-precise thiolate-capped metal nanoclusters.

Effect of the Metal-Ligand Interface on the Chiroptical Activity of Cysteine-Protected Nanoparticles.

Gold, silver, and copper small nanoparticles (NPs), with average size ≈2 nm, are synthesized and afterward protected with l- and d-cysteine, demonstrating emergence of chiroptical activity in the wavelength range of 250-400 nm for all three metals with respect to the bare nanoparticles and ligands alone. Silver-cysteine (Ag-Cys) NPs display the higher anisotropy factor, whereas gold-cysteine (Au-Cys) NPs show optical and chiroptical signatures slightly more displaced to the visible range. A larger number of circular dichroism (CD) bands with smaller intensity, as compared to gold and silver, is observed for the first time for copper-cysteine (Cu-Cys) NPs. The manifestation of optical and chiroptical responses upon cysteine adsorption and the differences between the spectra corresponding to each metal are mainly dictated by the metal-ligand interface, as supported by a comparison with calculations of the oscillatory and rotatory strengths based on time-dependent density functional theory, using a metal-ligand interface motif model, which closely resembles the experimental absorption and CD spectra. These results are useful to demonstrate the relevance of the interface between chiral ligands and the metal surfaces of Au, Ag, and Cu NPs, and provide evidence and further insights into the origin of the transfer mechanisms and induction of extrinsic chirality.

On the forbidden graphene's ZO (out-of-plane optic) phononic band-analog vibrational modes in fullerenes.

The study of nanostructures' vibrational properties is at the core of nanoscience research. They are known to represent a fingerprint of the system as well as to hint the underlying nature of chemical bonds. In this work, we focus on addressing how the vibrational density of states (VDOS) of the carbon fullerene family (Cn: n = 20 → 720 atoms) evolves from the molecular to the bulk material (graphene) behavior using density functional theory. We find that the fullerene's VDOS smoothly converges to the graphene characteristic line-shape, with the only noticeable discrepancy in the frequency range of the out-of-plane optic (ZO) phonon band. From a comparison of both systems we obtain as main results that: (1) The pentagonal faces in the fullerenes impede the existence of the analog of the high frequency graphene's ZO phonons, (2) which in the context of phonons could be interpreted as a compression (by 43%) of the ZO phonon band by decreasing its maximum allowed radial-optic vibration frequency. And 3) as a result, the deviation of fullerene's VDOS relative to graphene may hold important thermodynamical implications, such as larger heat capacities compared to graphene at room-temperature. These results provide insights that can be extrapolated to other nanostructures containing pentagonal rings or pentagonal defects.

Head to Tail Distortion Wave Characterizes the Enantiomerization of Helicenes.

A fresh look on helicenes' enantiomerization process with a focus on ring conformation reveals that it can be described as a step-by-step mechanism in which maximal distortion is consecutively transferred along the helicene skeleton, head to tail. Density functional theory methods were used to compute the enantiomerization pathway, and continuous symmetry measures were applied to quantify the distortion of even-number helicenes with 8-14 rings. Our findings show that the distortion wave is additive-the process always starts from one edge of the helicene and progresses along the rings until the other edge is reached. As more rings are added to the helicene, extra steps are appended to the distortion wave. Implications of this fundamental process are discussed in light of similar natural phenomena from polymer dynamics to snake locomotion.

CeO2(111) electronic reducibility tuned by ultra-small supported bimetallic Pt-Cu clusters.

Controlling Ce4+ to Ce3+ electronic reducibility in a rare-earth binary oxide such as CeO2 has enormous applications in heterogeneous catalysis, where a profound understanding of reactivity and selectivity at the atomic level is yet to be reached. Thus, in this work we report an extensive DFT-based Basin Hopping global optimization study to find the most stable bimetallic Pt-Cu clusters supported on the CeO2(111) oxide surface, involving up to 5 atoms in size for all compositions. Our PBE+U global optimization calculations indicate a preference for Pt-Cu clusters to adopt 2D planar geometries parallel to the oxide surface, due to the formation of strong metal bonds to oxygen surface sites and charge transfer effects. The calculated adsorption energy values (Eads) for both mono- and bimetallic systems are of the order of 1.79 up to 4.07 eV, implying a strong metal cluster interaction with the oxide surface. Our calculations indicate that at such sub-nanometer sizes, the number of Ce4+ surface atoms reduced to Ce3+ cations is mediated by the amount of Cu atoms within the cluster, reaching a maximum of three Ce3+ for a supported Cu5 cluster. Our computational results have critical implications on the continuous understanding of the strong metal-support interactions over reducible oxides such as CeO2, as well as the advancement of frontier research areas such as heterogeneous single-atom catalysts (SAC) and single-cluster catalysts (SCC).

Chiral-Icosahedral ( I) Symmetry in Ubiquitous Metallic Cluster Compounds (145A,60X): Structure and Bonding Principles.

There exists a special kind of perfection-in symmetry, simplicity, and stability-attainable for structures generated from precisely 60 ligands (all of a single type) that protect 145 metal-atom sites. The symmetry in question is icosahedral ( Ih), generally, and chiral icosahedral ( I) in particular. A 60-fold equivalence of the ligands is the smallest number to allow this kind of perfection. Known cluster compounds that approximate this structural ideal include palladium-carbonyls, Ih-Pd145(CO)60; gold-thiolates, I-Au144(SR)60; and gold-alkynyls, I-Au144(C2R)60. Many other variants are suspected. The Pd145 compound established the basic achiral structure-type. However, the Au144-thiolate archetype is prominent, historically in its abundance and ease of preparation and handling, in its proliferation in many laboratories and application areas, and ultimately in the intrinsic chirality of its geometrical structure and organization of its bonding network or connectivity. As discovered by mass spectrometry (the "30-k anomaly") in 1995, it appeared as a broad single peak, as solitary and symmetrical as Mount Fuji, centered near 30 kDa (∼150 Au atoms), provoking these thoughts: Surely this phenomenon requires a unique explanation. It appears to be the Buckminsterfullerene (carbon-60) of gold-cluster chemistry. Herein we provide an elementary account of the unexpected discovery, in which the Pd145-structure played a critical role, that led to the identification and prediction, in 2008, of a fascinating new molecular structure-type, evidently the first one of chiral icosahedral symmetry. Rigorous confirmation of this prediction occurred in early spring 2018, when two single-crystal X-ray crystallography reports were submitted, each one distinguishing both enantiomeric structures and noting profound chirality for the surface (ligand) layer. The emphasis here is on the structure and bonding principles and how these have been elucidated. Our aim has been to present this story in simplest terms, consistent with the radical simplicity of the structure itself. Because it combines intrinsic profound chirality, at several levels, with the highest possible symmetry-type (icosahedral), the structure may attract broader interest also from educators, especially if studied in tandem with the analysis of hollow (shell) metallic systems that exhibit the same chirality and symmetry. Because the shortest (stiffest) bonds follow the chiral 3-way weave pattern of the traditional South-Asian reed football, this cultural artifact may be used to introduce chiral-icosahedral symmetry in a pleasant and memorable way. One may also appreciate easily the bonding and excitations in I-symmetry metallic nanostructures via the golden fullerenes, that is, the proposed hollow Au60,72 spheres. Beyond any aesthetic or pedagogical value, we aim that our Account may provide a firm foundation upon which others may address open questions and the opportunities they present. This Account can scarcely hint at the prospects for further fundamental understanding of these compounds, as well as a widening sphere of applications (chemical, electronic, imaging). The compounds remain crucial to a wider field presently under intense development.

Mechanical Vibrations of Atomically Defined Metal Clusters: From Nano- to Molecular-Size Oscillators.

Acoustic vibrations of small nanoparticles are still ruled by continuum mechanics laws down to diameters of a few nanometers. The elastic behavior at lower sizes (<1-2 nm), where nanoparticles become molecular clusters made by few tens to few atoms, is still little explored. The question remains to which extent the transition from small continuous-mass solids to discrete-atom molecular clusters affects their specific low-frequency vibrational modes, whose period is classically expected to linearly scale with diameter. Here, we investigate experimentally by ultrafast time-resolved optical spectroscopy the acoustic response of atomically defined ligand-protected metal clusters Au n(SR) m with a number n of atoms ranging from 10 to 102 (0.5-1.5 nm diameter range). Two periods, corresponding to fundamental breathing- and quadrupolar-like acoustic modes, are detected, with the latter scaling linearly with cluster diameters and the former taking a constant value. Theoretical calculations based on density functional theory (DFT) predict in the case of bare clusters vibrational periods scaling with size down to diatomic molecules. For ligand-protected clusters, they show a pronounced effect of the ligand molecules on the breathing-like mode vibrational period at the origin of its constant value. This deviation from classical elasticity predictions results from mechanical mass-loading effects due to the protecting layer. This study shows that clusters characteristic vibrational frequencies are compatible with extrapolation of continuum mechanics model down to few atoms, which is in agreement with DFT computations.

CO2 adsorption on gas-phase Cu4-xPtx (x = 0-4) clusters: a DFT study.

Transition and noble metal clusters have proven to be critical novel materials, potentially offering major advantages over conventional catalysts in a range of value-added catalytic processess such as carbon dioxide transformation to methanol. In this work, a systematic computational study of CO2 adsorption on gas-phase Cu4-xPtx (x = 0-4) clusters is performed. An exhaustive potential energy surface exploration is initially performed using our recent density functional theory basin-hopping global optimization implementation. Ground-state and low-lying energy isomers are identified for Cu4-xPtx clusters. Secondly, a CO2 molecule adsorption process is analyzed on the ground-state Cu4-xPtx configurations, as a function of cluster composition. Our results show that the gas-phase linear CO2 molecule is deformed upon adsorption, with its bend angle varying from about 132° to 139°. Cu4-xPtx cluster geometries remain unchanged after CO2 adsorption, with the exception of Cu3Pt1 and Pt4 clusters. For these particular cases, a structural conversion between the ground-state geometry and the corresponding first isomer configurations is found to be assisted by the CO2 adsorption. For all clusters, the energy barriers between the ground-state and first isomer structures are explored. Our calculated CO2 adsorption energies are found to be larger for Pt-rich clusters, exhibiting a volcano-type plot. The overall effect of a hybrid functional including dispersion forces is also discussed.

Tetrahedral (T) Closed-Shell Cluster of 29 Silver Atoms & 12 Lipoate Ligands, [Ag29(R-α-LA)12](3-): Antibacterial and Antifungal Activity.

Here we report on the identification and applications of an aqueous 29-atom silver cluster stabilized with 12 lipoate ligands, i.e. Ag29(R-α-LA)12 or (29,12), wherein R-α-LA = R-α-lipoic acid, a natural dithiolate. Its uniformity is checked by HPLC-ESI-MS and analytical ultracentrifugation, which confirms its small dimension (~3 nm hydrodynamic diameter). For the first time, this cluster has been detected intact via electrospray ionization mass spectrometry, allowing one to confirm its composition, its [3-] charge-state, and the 8-electron shell configuration of its metallic silver core. Its electronic structure and bonding, including T-symmetry and profound chirality in the outer shell, have been analyzed by DFT quantum-chemical calculations, starting from the known structure of a nonaqueous homologue. The cluster is effective against Methicillin-Resistant Staphylococcus aureus bacteria (MRSA) at a minimum inhibitory concentration (MIC) of 0.6 mg-Ag/mL. A preformed Candida albicans fungal biofilm, impermeable to other antifungal agents, was also inhibited by aqueous solutions of this cluster, in a dose-response manner, with a half-maximal inhibitory concentration (IC50) of 0.94 mg-Ag/mL. Scanning electron micrographs showed the post-treatment ultrastructural changes on both MRSA and C. albicans that are characteristic of those displayed after treatment by larger silver nanoparticles.

Diverse Chiral Scaffolds from Diethynylspiranes: All-Carbon Double Helices and Flexible Shape-Persistent Macrocycles.

State-of-the-art chiroptical spectroscopies are valuable tools for structural elucidation. However, the potential of these spectroscopies for everyday applications has not been exploited to date partially due to the lack of sufficiently stable and efficient chiroptical systems. To this end, the development of suitable chiroptical structures is essential. Herein, we present the synthesis of spiro-compounds (P2 )-1 and (P4 )-2 as well as (M2 )-1 and (M4 )-2 exhibiting remarkable chiroptical responses. Theoretical simulations show that (P2 )-1, constituted by two (P)-configured spiranic chiral axes, presents an all-carbon double helix structure with (M)-helicity. On the other hand, molecular dynamic simulations reveal (P4 )-2 to have a single path for geometry-modification along its flat conformational space, certifying it as a chiral flexible shape-persistent macrocycle. Geometric quantification of chirality has been used to compare the spiranic derivatives presented herein.

2D-3D structural transition in sub-nanometer PtN clusters supported on CeO2(111).

Transition metal particles dispersed on oxide supports are used as heterogeneous catalysts in numerous applications. One example is platinum clusters supported on ceria which is used in automotive catalysis. Although control at the nm-scale is desirable to open new technological possibilities, there is limited knowledge both experimentally and theoretically regarding the geometrical structure and stability of sub-nanometer platinum clusters supported on ceria. Here we report a systematic, Density Functional Theory (DFT) study on the growth trends of CeO2(111) supported PtN clusters (N = 1-10). Using a global optimization methodology as a guidance tool to locate putative global minima, our results show a clear preference for 2D planar structures up to size Pt8. It is followed by a structural transition to 3D configurations at larger sizes. This remarkable trend is explained by the subtle competition between the formation of strong Pt-O bonds and the cluster internal Pt-Pt bonds. Our calculations show that the reducibility of CeO2(111) provides a mechanism to anchor PtN clusters where they become oxidized in a two-way charge transfer mechanism: (a) an oxidation process, where Osurface atoms withdraw charge from Pt atoms forming Pt-O bonds, (b) surface Ce4+ atoms are reduced, leading to Ce3+. The active role of the CeO2(111) support in modifying the structural and eventually the chemical properties of sub-nanometer PtN clusters is computationally demonstrated.

Hidden Components in Aqueous "Gold-144" Fractionated by PAGE: High-Resolution Orbitrap ESI-MS Identifies the Gold-102 and Higher All-Aromatic Au-pMBA Cluster Compounds.

Experimental and theoretical evidence reveals the resilience and stability of the larger aqueous gold clusters protected with p-mercaptobenzoic acid ligands (pMBA) of composition Aun(pMBA)p or (n, p). The Au144(pMBA)60, (144, 60), or gold-144 aqueous gold cluster is considered special because of its high symmetry, abundance, and icosahedral structure as well as its many potential uses in material and biological sciences. Yet, to this date, direct confirmation of its precise composition and total structure remains elusive. Results presented here from characterization via high-resolution electrospray ionization mass spectrometry on an Orbitrap instrument confirm Au102(pMBA)44 at isotopic resolution. Further, what usually appears as a single band for (144, 60) in electrophoresis (PAGE) is shown to also contain the (130, 50), recently determined to have a truncated-decahedral structure, and a (137, 56) component in addition to the dominant (144, 60) compound of chiral-icosahedral structure. This finding is significant in that it reveals the existence of structures never before observed in all-aromatic water-soluble species while pointing out the path toward elucidation of the thermodynamic control of protected gold nanocrystal formation.

Vibrational properties and specific heat of core-shell Ag-Au icosahedral nanoparticles.

The vibrational density of states (VDOS) of metal nanoparticles can be a fingerprint of their geometrical structure and determine their low-temperature thermal properties. Theoretical and experimental methods are available nowadays to calculate and measure it over a size range of 1-4 nm. In this work, we present theoretical results regarding the VDOS of Ag-Au icosahedral nanoparticles with a core-shell structure in that size range (147-923 atoms). The results are obtained by changing the size and type of atoms in the core-shell structure. For all sizes investigated, a smooth and monotonic variation of the VDOSs from Ag to Au is obtained by increasing the number of core Au atoms, and vice versa. Nevertheless, the Ag561Au362 nanoparticle, with a Ag core, shows an anomalous enhancement at low frequencies. An analysis of the calculated VDOSs indicates that as a general trend the low-frequency region is mainly due to the shell contribution, whereas at high frequencies the core effect would be dominant. A linear variation with size is obtained for the period of quasi-breathing mode (QBM), in agreement with the behaviour obtained for pure Ag and Au nanoparticles. A non-monotonic variation is obtained for the QBM frequency as a function of the Ag concentration for all nanoparticles investigated. The calculated specific heat at low temperatures of the Ag-Au nanoparticles is smaller (larger) than the corresponding one calculated for the pure Au (Ag) nanoparticles of same size. Nevertheless, the enhancement of VDOS at low frequencies of the Ag561Au362 nanoparticle with a Ag core induced larger values of specific heat than those of the pure Au923 nanoparticle in the temperature range of 5-15 K.

Structures and chiroptical properties of the BINAS-monosubstituted Au38(SCH3)24 cluster.

The structure and optical properties of a set of R-1,1'-binaphthyl-2,2'-dithiol (R-BINAS) monosubstituted A-Au38(SCH3)24 clusters are studied by means of time dependent density functional theory (TD-DFT). While it was proposed earlier that BINAS selectively binds to monomer motifs (SR-Au-SR) covering the Au23 core, our calculations suggest a binding mode that bridges two dimer (SR-Au-SR-Au-RS) motifs. The more stable isomers show a negligible distortion induced by BINAS adsorption on the Au38(SCH3)24 cluster which is reflected by similar optical and Circular Dichroism (CD) spectra to those found for the parent cluster. The results furthermore show that BINAS adsorption does not enhance the CD signals of the Au38(SCH3)24 cluster.

Structural, electronic, optical, and chiroptical properties of small thiolated gold clusters: the case of Au6 and Au8 cores protected with dimer [Au2(SR)3] and trimer [Au3(SR)4)] motifs.

We report results of a theoretical study, based on density functional theory (DFT), on the structural, electronic, optical, and chiroptical properties of small thiolated gold clusters, [Au(n)(SR)(m) (n = 12-15, 16-20; m = 9-12, 12-16)]. Some of these clusters correspond to those recently synthesized with the surfactant-free method. To study the cluster physical properties, we consider two cluster families with Au(6) and Au(8) cores, respectively, covered with dimer [Au(2)(SR)(3)] and trimer [Au(3)(SR)(4)] (CH(3) being the R group) motifs or their combinations. Our DFT calculations show, by comparing the relaxed structures of the [Au(6)[Au(2)(SR)(3)](3)](+), [Au(6)[Au(2)(SR)(3)](2)[Au(3)(SR)(4)]](+), [Au(6)[Au(2)(SR)(3)][Au(3)(SR)(4)](2)](+), and [Au(6)[Au(3)(SR)(4)](3)](+) cationic clusters, that there is an increasing distortion in the Au(6) core as each dimer is replaced by a longer trimer motif. For the clusters in the second family, Au(8)[Au(3)(SR)(4)](4), Au(8)[Au(2)(SR)(3)][Au(3)(SR)(4)](3), Au(8)[Au(2)(SR)(3)](2)[Au(3)(SR)(4)](2), Au(8)[Au(2)(SR)(3)](3)[Au(3)(SR)(4)], and Au(8)[Au(2)(SR)(3)](4), a smaller distortion of the Au(8) core is observed as dimer motifs are substituted by trimer ones. An interesting trend emerging from the present calculations shows that as the number of trimer motifs increases in the protecting layer of both Au(6) and Au(8) cores, the average of the interatomic Au(core)-S distances reduces. This shrinkage in the Au(core)-S distances is correlated with an increase of the cluster HOMO-LUMO (H-L) gap. From these results, it is predicted that a larger number of trimer motifs in the cluster protecting layer would induce larger H-L gaps. By analyzing the electronic transitions that characterize the optical absorption and circular dichroism spectra of the clusters under study, it is observed that the molecular orbitals involved are composed of comparable proportions of orbitals corresponding to atoms forming the cluster core and the protecting dimer and trimer motifs.

On the structure of the Au18(SR)14 cluster.

First principles calculations are used for a systematic search of the lowest-energy (most-stable) structure of the recently synthesized Au(18)(SR)(14) cluster. A comparison of the calculated optical absorption and electronic circular dichroism spectra, which are highly sensitive to the cluster structure and chirality, with the experimental spectra of the glutathione-protected gold cluster, Au(18)(SG)(14), is used to discriminate between low-energy isomers of the Au(18)(SR)(14) (R = CH(3)) cluster. From the good agreement between calculated and measured spectra, it is predicted that the structure of the Au(18)(SR)(14) cluster consists of a prolate Au(8) core covered with two dimer (SR-Au-SR-Au-SR) and two trimer (SR-Au-SR-Au-SR-Au-SR) motifs. These results provide additional evidence on the existence of longer trimer motifs as protecting units of small thiolated gold clusters.

Vibrational circular dichroism and IR absorption spectra of amino acids: a density functional study.

With density functional theory, vibrational circular dichroism (VCD) and infrared absorption (IR) spectra are obtained at the B3LYP/CC-pVTZ level of theory for 20 alpha-amino acids. The contribution of different vibration modes to the IR and VCD spectra is analyzed. Overall agreement between calculated results for amino acids in gas phase with the available experimental VCD data for matrix-assisted amino acid films is found. The analysis of the calculated IR and VCD spectra indicates that the functional groups in the backbones and side chains of amino acids contribute differently to the spectra line shape. It is obtained that molecular torsions are the characteristic vibrations of the amino acids at the low-frequency regime, whereas the bending of bond angles, the out-of-plane wagging of individual atoms, and some stretching modes dominate the intermediate frequency range. Specific modes like NH(2) scissoring, CO bond stretching, and the (symmetric and asymmetric) stretching of the hydrogen atoms in the NH(2) and OH groups characterize the high-frequency regime. A general trend emerging from these calculations indicates that the rho(OH) rocking and nu(C=O) stretching modes have the highest intensity in the VCD spectra of most amino acids.

On the origin of the optical activity displayed by chiral-ligand-protected metallic nanoclusters.

Ligand-protected metallic clusters exhibit optical activity when chiral molecules are used as protecting units. Various mechanisms, such as the inherently chiral metallic cluster core, the dissymmetric field effect, and the chiral footprint model, have been proposed as possible explanations of the nonzero circular dichroism (CD) spectra found for these nanoscale materials. This communication presents a first-principles theoretical study of the CD spectrum of the [Au(25)(SR)(18)](-) cluster that was undertaken to gain insight into the physicochemical origin of the optical activity measured for the glutathione-protected [Au(25)(SG)(18)](-) cluster. The calculated CD spectrum of the cysteine-protected cluster, with R(cys) = C(beta)H(2)-C(alpha)H(NH(2))-COOH, shows good agreement with the experimental data obtained for the glutathione-protected cluster. Analysis of the calculated CD spectra of the peculiar two-shell metallic core and the two distinct thiolate-Au binding modes existing in the [Au(25)(SR(cys))(18)](-) cluster showed that the weak CD signal due to the slight distortion of cluster core is enhanced by the dissymmetric location of the ligands forming the Au-S binding modes. This result shows that the mechanisms proposed to explain the optical activity of chiral-ligand-protected metallic clusters cannot be differentiated but are acting concurrently. It is also predicted that the CD line shape should be highly sensitive to the orientation of the thiolate ligands forming the cluster protecting layer and to the stability of the thiolate-Au binding modes.