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.
Conspectus There exists a special kind of perfectionin symmetry, simplicity, and stabilityattainable 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.
Chirality has been found as a relevant property of nanomaterials, including ligand-protected metal clusters and nanorods. This property not only is crucial in nanotechnology developments related to ...asymmetric catalysis and chiroptical phenomena but also generates fundamental questions on the existence of chirality at the nanoscale. In fact, X-ray total structure determination, electron diffraction studies, NMR, and circular dichroism spectroscopies as well as theoretical calculations performed on gold clusters protected with thiolate or phosphine ligands have confirmed the existence of chiral structures in the size range of 18–144 Au atoms. In this work, we realize a comparative analysis of the degree or amount of chirality existing in chiral ligand-protected gold clusters (LPGC), through a geometric quantification, using the Hausdorff chirality measure (HCM). The calculated HCM values provide a quantitative framework to compare, classify, and gain insight into the origin of chirality. Interestingly, these values are consistent with the current knowledge on the different sources of chirality: achiral cores and chiral arrangement of ligands in, for example, Au102(SR)44 and Au38(SR)24 or intrinsically chiral cores, like in Au52(SR)32 and Au20 protected with phosphine ligands. Our calculations are also helpful to assign an index of chirality and classify as chiral several recently synthesized and structurally solved LPGC that in first instance were not identified as such. The calculated HCM values are used to extract trends on how chirality is spatially distributed in LPGC and correlate them with optical activity measurements. The main trend indicates that the Au–S interface has the dominant role in the chirality of LPGC.
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 Au25(SR)18− cluster that was undertaken to gain insight into the physicochemical origin of the optical activity measured for the glutathione-protected Au25(SG)18− cluster. The calculated CD spectrum of the cysteine-protected cluster, with Rcys = CβH2−CαH(NH2)−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 Au25(SRcys)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.
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.
Experimental and theoretical evidence reveals the resilience and stability of the larger aqueous gold clusters protected with p-mercaptobenzoic acid ligands (pMBA) of composition Au n (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.
The vibrational (phonon) density of states of metal nanoparticles with size between 2 and 6 nm can be measured using nuclear resonant inelastic X-ray or plasmon resonance Raman scattering. In this ...work, we present atomistic calculations, based on a semiempirical tight-binding many-body Gupta potential, of the vibrational density of states (VDOS) for face-centered cubic (FCC), decahedral, and icosahedral (ICO) gold and silver nanoparticles with sizes ∼4 nm (∼2000 atoms). The calculated VDOS are compared with experimental data, recently published for gold and silver nanoparticles of similar size, obtained through plasmon resonance Raman scattering. The best agreement between the calculated and measured VDOS’s is obtained for the ICO morphology for both metal nanoparticles. These results indicate that most of the nanoparticles in the experimental samples should have icosahedral structures. The present study also shows that, as in the case of molecular systems and small clusters, vibrational spectroscopy of metal nanoparticles with few nanometers in size, together with theoretical calculations, are powerful tools for their structure determination.
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.
Revealing chirality′s secrets: Diethynylspiranes have been employed for the construction of all‐carbon double helices as well as chiral flexible shape‐persistent macrocycles. The uncovering of these very diverse chiral scaffolds opens new opportunities for chiroptical applications (see scheme).
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