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

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

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

Size-Dependent Polymorphism in Aluminum Carbide Cluster Anions Al_<i>n</i>C₂^–: Formation of Acetylide-Containing Structures

Aluminum carbide cluster anions Al_<i>n</i>C₂^– (<i>n</i> = 5–13) were observed as the most dominant products in the gas-phase reactions of laser-ablated Al_<i>n</i>^– with organic molecules, such as methanol, ethanol, pentane, acetonitrile, or acetone. Density functional theory calculations predicted two possible isomeric structures for Al_<i>n</i>C₂^–: isomers in which two carbons are dissociated (type D) as in the case of the bulk aluminum carbide and novel isomers in which two carbons form an acetylide-like C₂ unit. The latter isomers are further categorized into three types depending on the location of the C₂ unit: the C₂ unit is encapsulated within the Al cage (type I), contained in the surface of Al clusters (type S), or attached to the surface of Al clusters (type O). Size-dependent behavior of the adiabatic electron affinities of Al_<i>n</i>C₂ determined by photoelectron spectroscopy was explained in terms of polymorphism as a function of size (<i>n</i>): type I for <i>n</i> = 5–8, type D for <i>n</i> = 9–11, type D or O for <i>n</i> = 12, and type O for <i>n</i> = 13. The tendency in which the position of the C₂ unit was shifted from the inside to outside with the increase in <i>n</i> was ascribed to the balance between the stabilizations gained by forming Al–C bonds and Al–Al bonds. The smaller Al_<i>n</i>C₂^– clusters (<i>n</i> = 5–8) prefer to surround the acetylide-like C₂ unit with the Al atoms so as to maximize the number of Al–C bonds, whereas larger ones (<i>n</i> = 12 and 13) prefer to attach the C₂ unit onto the surface of the Al clusters so as to maximize the number of Al–Al bonds

Gold Ultrathin Nanorods with Controlled Aspect Ratios and Surface Modifications: Formation Mechanism and Localized Surface Plasmon Resonance

We synthesized gold ultrathin nanorods (AuUNRs) by slow reductions of gold­(I) in the presence of oleylamine (OA) as a surfactant. Transmission electron microscopy revealed that the lengths of AuUNRs were tuned in the range of 5–20 nm while keeping the diameter constant (∼2 nm) by changing the relative concentration of OA and Au­(I). It is proposed on the basis of time-resolved optical spectroscopy that AuUNRs are formed via the formation of small (<2 nm) Au spherical clusters followed by their one-dimensional attachment in OA micelles. The surfactant OA on AuUNRs was successfully replaced with glutathionate or dodecanethiolate by the ligand exchange approach. Optical extinction spectroscopy on a series of AuUNRs with different aspect ratios (ARs) revealed a single intense extinction band in the near-IR (NIR) region due to the longitudinal localized surface plasmon resonance (LSPR), the peak position of which is red-shifted with the AR. The NIR bands of AuUNRs with AR < 5 were blue-shifted upon the ligand exchange from OA to thiolates, in sharp contrast to the red shift observed in the conventional Au nanorods and nanospheres (diameter >10 nm). This behavior suggests that the NIR bands of thiolate-protected AuUNRs with AR < 5 are not plasmonic in nature, but are associated with a single-electron excitation between quantized states. The LSPR band was attenuated by thiolate passivation that can be explained by the direct decay of plasmons into an interfacial charge transfer state (chemical interface damping). The LSPR wavelengths of AuUNRs are remarkably longer than those of the conventional AuNRs with the same AR, demonstrating that the miniaturization of the diameter to below ∼2 nm significantly affects the optical response. The red shift of the LSPR band can be ascribed to the increase in the effective mass of electrons in AuUNRs

Structure Determination of a Water-Soluble 144-Gold Atom Particle at Atomic Resolution by Aberration-Corrected Electron Microscopy

Structure determination by transmission electron microscopy has revealed the long sought 144-gold atom particle. The structure exhibits deviations from face-centered cubic packing of the gold atoms, similar to the solution structure of another gold nanoparticle, and in contrast to a previous X-ray crystal structure. Evidence from analytical methods points to a low number of 3-mercaptobenzoic acid ligands covering the surface of the particle

Collision-Induced Dissociation of Undecagold Clusters Protected by Mixed Ligands [Au₁₁(PPh₃)₈X₂]⁺ (X = Cl, CCPh)

We herein investigated collision-induced dissociation (CID) processes of undecagold clusters protected by mixed ligands [Au₁₁(PPh₃)₈X₂]⁺ (X = Cl, CCPh) using mass spectrometry and density functional theory calculations. The results showed that the CID produced fragment ions [Au_<i>x</i>(PPh₃)_<i>y</i>X_<i>z</i>]⁺ with a formal electron count of eight via sequential loss of PPh₃ ligands and AuX­(PPh₃) units in a competitive manner, indicating that the CID channels are governed by the electronic stability of the fragments. Interestingly, the branching fraction of the loss of the AuX­(PPh₃) units was significantly smaller for X = CCPh than that for X = Cl. We ascribed the effect of X on the branching fractions of dissociations of PPh₃ and AuX­(PPh₃) to the steric difference