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.

Chirality is a fundamental property of matter with profound impact in physics, chemistry, biology, and medicine. It is present at several scales going from elementary particles, to molecules, to macroscopic materials, and even to astronomical objects. During the last 30 years, chirality has also been investigated at the nanoscale, being a hot research topic in nanoscience. The importance of chirality at the nanoscale is due, in part, to the potential applications that chiral nanomaterials could have in nanotechnology. Great interest exists nowadays in the study of chirality in bare and ligand-protected metal nanoclusters. These are aggregates of n metal atoms (n~10-300) that can be in gas phase or stabilized by organic ligands, covering the cluster surface. Chirality in bare and thiolate-protected gold clusters (TPGC) has received special attention because of the important progress achieved in their synthesis, size separation, and precise structural characterization. Here, we review the recent experimental and theoretical developments on the origin and physicochemical manifestations of chirality in bare and TPGC. Since chirality is a geometrical property, we also discuss the proposal for its quantification, and the correlation of this geometric measure with the chiroptical response, like the circular dichroism spectrum, calculated from quantum mechanical methods.

The ground state, GS, geometries for Fe(1,2)(benzene)(1,2) clusters were determined by means of all-electron calculations done with the density functional BPW91/6311++G(2d,2p) method. The stability of Fe(C(6)H(6))(1,2) is accomplished by the formation of Fe-C eta(6) coordinations in the half-sandwich and sandwich GS structures, which are of lower spin, 2S = 2 (S is the total spin) than the Fe atom, 2S = 4. Departures from eta(6) bonding occur on [Fe(C(6)H(6))(2)](-), since the GS of this anion, of less symmetric sandwich geometry, presents eta(6) and eta(2) coordination, which is mainly due to the enhanced repulsion of the adsorbed benzene units. On Fe(2)(C(6)H(6))(1,2) the stronger Fe(2) bond, compared to the Fe-C ones, produce rice-ball geometries, where the Fe(2) molecule, although with a longer bond length, is preserved. For example, in Fe(2)(C(6)H(6)), Fe(2) lies perpendicular or parallel to the benzene ring depending on the charge of the complex, and in [Fe(2)(C(6)H(6))(2)](+/-0, +/-1) the benzene ligands are placed above and beneath the molecular axis of Fe(2), producing highly compact structures. Multiple decker sandwich states, where Fe(2) is not retained, are located more than 20 kcal mol(-1) above the GS levels. Electron affinities, agreeing well with experimental results, ionization and binding energies, and vibrational frequencies were also determined, providing insight on the complexes.

The interactions of one and two benzene molecules with the superparamagnetic Fe(6) cluster were studied by means of gradient-corrected density functional theory. The ground state, GS, of bare Fe(6) presents a distorted octahedral structure with 2S = 20; S is the total spin. For the calculated 2S = 16 GS of the neutral Fe(6)-C(6)H(6) complex, as well as in the positive and negative ions both with 2S = 15, the benzene unit is adsorbed on one axial Fe(a) atom. The 2S = 14 GS for Fe(6)-(C(6)H(6))(2) resembles a sandwich structure, with the metal Fe(6) cluster separating the benzene rings that are bonded symmetrically on the two axial sites of Fe(6). The binding is accounted for by electrostatic interactions and by 3d-pi bonds, as revealed by the molecular orbitals. Though each C-Fe bond is weak, eta(6) coordinations were indicated by the topology of the electronic density. The 3d-pi bonding is reflected by the adiabatic ionization energies and electron affinities, which are smaller than those of bare Fe(6). The computed IR spectra show vibrational bands near those of bare benzene; some forbidden IR modes in benzene and in Fe(6) become IR active in Fe(6)-(C(6)H(6))(1,2). The results show a strong perturbation of the electronic structure of Fe(6). The decrease of its magnetic moment implies that the magnetic effects play an important role in the adsorption of benzene.

The interaction between high-spin Fe7 clusters and a benzene molecule was studied using the BPW91/6-311++G(2d,2p) method. The Fe7-C6H6 ground state has a T-shaped structure, similar to that of the benzene dimer, and a multiplicity M = 2S + 1 = 19 (S = total spin). The carbon atoms are bonded to a single equatorial iron atom, which experiences a dramatic decrease in its magnetic moment, from 3.1 to -0.8 mu(B); the magnetic moments of other Fe atoms are larger than those in the ground-state Fe7 cluster. Such unexpected magnetic behavior of the cluster is crucial for adsorption of benzene.