Structural Characterization of Unprecedented Al₁₄O^– and Al₁₅O₂^–: Photoelectron Spectroscopy and Density Functional Calculations

New aluminum oxide clusters Al₁₄O^– and Al₁₅O₂^– were observed unprecedentedly in the gas-phase reaction of Al_<i>n</i>^– and O₂. Photoelectron spectroscopic measurements and density functional calculations indicated that Al₁₄O^– and Al₁₅O₂^– are composed of an icosahedral Al₁₃ moiety bonded by one and two OAl unit(s), respectively. The preferential formation of Al₁₄O^– and Al₁₅O₂^– is explained in terms of the high stability associated with the Al₁₃ moiety and efficient collisional trapping as intermediates of oxidative etching reactions

Amplification of the Optical Activity of Gold Clusters by the Proximity of BINAP

Despite recent progress in the synthesis and characterization of optically active gold clusters, the factor determining optical rotatory strength has not been clarified due to the lack of structurally resolved, enantiomerically pure Au clusters. We addressed this issue by studying the correlation between the optical activity and geometrical structures of two types of Au clusters that were protected by chiral diphosphines: [Au₁₁(<i>R</i>/<i>S</i>-DIOP)₄Cl₂]⁺ (DIOP = 1,4-bis­(diphenylphosphino)-2,3-<i>o</i>-isopropylidene-2,3-butanediol) and [Au₈(<i>R</i>/<i>S</i>-BINAP)₃(PPh₃)₂]²⁺ (BINAP = 2,2′-bis­(diphenylphosphino)-1,1′-binaphthyl). [Au₈(BINAP)₃(PPh₃)₂]²⁺ showed stronger rotatory strengths than [Au₁₁(DIOP)₄Cl₂]⁺ in the visible region, while the Hausdorff chirality measure calculated from the crystal data indicated that the Au core of the former is less chiral than that of the latter. We propose that the optical activity in the Au core-based transition due to the deformed core is further amplified by chiral arrangement of the binaphthyl moiety near the Au core

Density Functional Theory Study on Stabilization of the Al₁₃ Superatom by Poly(vinylpyrrolidone)

The sequential bonding of <i>N</i>-ethyl-2-pyrrolidone (EP), a monomer unit of poly­(vinylpyrrolidone) (PVP), to an open-shell superatom Al₁₃ was studied by density functional theory calculations. The first three EP ligands prefer to be chemisorbed on the atop sites of Al₁₃ via the carbonyl O atom mainly due to bonding interaction between molecular orbitals of EP and the 1S or 1D superatomic orbital of Al₁₃. The fourth EP ligand, however, prefers to be bound electrostatically to one of the chemisorbed EP ligands rather than to be chemisorbed on Al₁₃. This behavior suggests that the maximum number of PVP that can be chemisorbed on an Al cluster is determined not only by the steric repulsion between adjacent PVP but also by the electronic charge accumulated on the Al cluster. The gross Mulliken charge accumulated on the Al₁₃ moiety increases with the number of EP ligands chemisorbed and reaches nearly −1 e in Al₁₃(EP)₃, suggesting the closure of the electronic shell of Al₁₃ by ligation of three EP ligands. However, the spin density analysis revealed that the superatomic orbital 1F of Al₁₃ remains singly occupied even after chemisorption of three EP ligands. In conclusion, the Al₁₃ moiety stabilized by PVP remains to be an open-shell superatom although it accepts electronic charge through polarized Al–O bonding

Observation and the Origin of Magic Compositions of Co_<i>n</i>O_<i>m</i>^– Formed in Oxidation of Cobalt Cluster Anions

To obtain atomistic insights into the early stage of the oxidation process of free cobalt cluster anions Co_<i>n</i>^–, the reaction of Co_<i>n</i>^– (<i>n</i> ≤ 10) with varied pressure of O₂ was studied experimentally and theoretically. Population analysis of the oxidation products Co_<i>n</i>O_<i>m</i>^– as a function of <i>m</i> revealed two types of magic compositions: the population decreases abruptly upon addition of a single O atom to and removal of a single O atom from the magic compositions. Magic compositions of the former type were further divided into oxygen-rich (<i>n</i>:<i>m</i> ∼ 3:4) and oxygen-poor (<i>n</i>:<i>m</i> ∼ 1:1) series. The oxygen-rich compositions most likely correspond to fully oxidized states, since the compositions are comparable to those of Co₃O₄ in the bulk. Their appearance is ascribed to the significant reduction of binding energies of O atoms to fully oxidized clusters. In contrast, oxygen-poor compositions correspond to the intermediates of the full oxidation states in which only the surface is oxidized on the basis of theoretical prediction that oxidation proceeds by bonding O atoms sequentially on the surface of Co_<i>n</i>^– while retaining its morphology. Their appearance is ascribed to the kinetic bottleneck against internal oxidation owing to significant structural change of the Co_<i>n</i> moiety. In contrast, magic compositions of the latter type are associated with the abrupt increase of survival probability as anionic states during the relaxation of internally hot Co oxide clusters based on the <i>m</i>-dependent behaviors of adiabatic electron affinities determined by photoelectron spectroscopy

Synthesis and the Origin of the Stability of Thiolate-Protected Au₁₃₀ and Au₁₈₇ Clusters

Two stable thiolate-protected gold clusters (Au–SR), Au₁₃₀ and Au₁₈₇ clusters, were synthesized to obtain a better understanding of the size dependence of the origin of the stability of Au–SR clusters. These clusters were synthesized by employing different preparation conditions from those used to synthesize previously reported magic gold clusters; in particular, a lower [RSH] to [AuCl₄^–] molar ratio ([AuCl₄^–]/[RSH] = 1:1) was used than that used to prepare Au₂₅(SR)₁₈, Au₃₈(SR)₂₄, Au₆₈(SR)₃₄, Au₁₀₂(SR)₄₄, and Au₁₄₄(SR)₆₀ (id. = 1:4–12). The two clusters thus synthesized were separated from the mixture by high-performance liquid chromatography with reverse-phase columns. Mass spectrometry of the products revealed the presence of two clusters with chemical compositions of Au₁₃₀(SC₁₂H₂₅)₅₀ and Au₁₈₇(SC₁₂H₂₅)₆₈. The origin of the stability of these two clusters and the size dependence of the origin of the stability of thiolate-protected gold clusters were discussed in terms of the total number of valence electrons

Oxidative Addition of CH₃I to Au^– in the Gas Phase

Reaction of the atomic gold anion (Au^–) with CH₃I under high-pressure helium gas affords the adduct AuCH₃I^–. Photoelectron spectroscopy and density functional theory calculations reveal that in the AuCH₃I^– structure the I and CH₃ fragments of CH₃I are bonded to Au in a linear configuration, which can be viewed as an oxidative addition product. Theoretical studies indicate that oxidative addition proceeds in two steps: nucleophilic attack of Au^– on CH₃I, followed by migration of the leaving I^– to Au. This mechanism is supported by the formation of an ion-neutral complex, [Au^–···<i>t</i>-C₄H₉I], in the reaction of Au^– with <i>t</i>-C₄H₉I because of the activation barrier along the S_N2 pathway resulting from steric effects. Theoretical studies are conducted for the formation mechanism of AuI₂^–, which is observed as a major product. From the thermodynamic and kinetic viewpoints, we propose that AuI₂^– is formed via sequential oxidative addition of two CH₃I molecules to Au^–, followed by reductive elimination of C₂H₆. The results suggest that Au^– acts as a nucleophile to activate C­(sp³)–I bond of CH₃I and induces the C–C coupling reaction of CH₃I

Slow-Reduction Synthesis of a Thiolate-Protected One-Dimensional Gold Cluster Showing an Intense Near-Infrared Absorption

Slow reduction of Au ions in the presence of 4-(2-mercaptoethyl)­benzoic acid (4-MEBA) gave Au₇₆(4-MEBA)₄₄ clusters that exhibited a strong (3 × 10⁵ M^–1 cm^–1) near-infrared absorption band at 1340 nm. Powder X-ray diffraction studies indicated that the Au core has a one-dimensional fcc structure that is elongated along the {100} direction

Structural Model of Ultrathin Gold Nanorods Based on High-Resolution Transmission Electron Microscopy: Twinned 1D Oligomers of Cuboctahedrons

Recently, we have developed a synthetic method of ultrathin gold nanorods (AuUNRs) with a fixed diameter of ∼1.8 nm and variable lengths in the range of 6–400 nm. It was reported that these AuUNRs exhibited intense IR absorption assigned to the longitudinal mode of localized surface plasmon resonance and broke up into spheres owing to Rayleigh-like instability at reduced surfactant concentration and at elevated temperatures. In order to understand the structure–property correlation of AuUNRs, their atomic structures were examined in this work using aberration-corrected high-resolution transmission electron microscopy. Statistical analysis revealed that the most abundant structure observed in the AuUNRs (diameter ≈ 1.8; length ≈ 18 nm) was a multiply twinned crystal, with a periodicity of ∼1.4 nm in length. We propose that the AuUNRs are composed of cuboctahedral Au₁₄₇ units, which are connected one-dimensionally through twin defects

Hydride-Doped Gold Superatom (Au₉H)²⁺: Synthesis, Structure, and Transformation

Doping of a hydride (H^–) into an oblate-shaped gold cluster [Au₉(PPh₃)₈]³⁺ was observed for the first time by mass spectrometry and NMR spectroscopy. Density functional theory calculations for the product [Au₉H­(PPh₃)₈]²⁺ demonstrated that the (Au₉H)²⁺ core can be viewed as a nearly spherical superatom with a closed electronic shell. The hydride-doped superatom (Au₉H)²⁺ was successfully converted to the well-known superatom Au₁₁³⁺, providing a new atomically precise synthesis of Au clusters via a bottom-up approach

Surface Plasmon Resonance in Gold Ultrathin Nanorods and Nanowires

We synthesized and measured optical extinction spectra of Au ultrathin (diameter: ∼1.6 nm) nanowires (UNWs) and nanorods (UNRs) with controlled lengths in the range 20–400 nm. The Au UNWs and UNRs exhibited a broad band in the IR region whose peak position was red-shifted with the length. Polarized extinction spectroscopy for the aligned Au UNWs indicated that the IR band is assigned to the longitudinal mode of the surface plasmon resonance