Geometric and Electronic Properties of P Atom-Doped Al Nanoclusters: Alkaline-like Superatom of P@Al12

The geometric and electronic characteristics of phosphorus-atom doped aluminum nanoclusters, AlnPm (n = 7-17, m = 1 and 2), were investigated through a combination of experiments and theoretical calculations. The size dependences of the ionization energy (Ei) for AlnPm NCs exhibit a local minimum of 5.37 eV at Al12P1, attributed to an endohedral P@Al12 superatom (SA). This SA originates from an excess electron toward the 2P shell closing (40e), coexisting with an exohedral isomer featuring a vertex P atom. The stability of the endohedral P@Al12 is further enhanced in its cationic state compared to the exohedral isomer, when complexed with a fluorine (F) atom, forming an SA salt denoted as P@Al12+F- with an elevated Ei ranging from 6.42 to 7.90 eV. In contrast, for the anionic Al12P1-, the exohedral form is found to be more stable than the endohedral one using anion photoelectron spectroscopy and calculations. The geometric and electronic robustness of neutral P@Al12 SAs against electron donation and acceptance is discussed in comparison to rare-gas-like Si@Al12 SAs.

A Common Bottleneck for Metal Oxidation by Molecular Oxygen Across Size Regimes: Kinetics of Atomic Lanthanide Cations (La+-Lu+) with O2

Kinetics of the lanthanide cations (Ln+ = La+-Lu+ excluding Pm+) reacting with molecular oxygen were measured in a selected-ion flow tube apparatus from 300 to 600 K. Where exothermic, these reactions occur efficiently, producing LnO+ + O. The reactions display positive temperature dependences consistent with Arrhenius equation behavior and show small activation energies (0-2 kJ mol-1) that are strongly correlated to promotion energies of the Ln+ atoms. Reanalysis of literature data on neutral Ln + O2 reactions show a similar correlation with slightly larger activation energies (0-10 kJ mol-1). The data are explained by a common mechanism controlling oxidation by molecular oxygen in these systems, as well as in gas-phase reactions of transition metal and posttransition metal cluster anions, neutral clusters deposited on surfaces, and for oxygen incident on metal surfaces. It is posited that across these systems, the height of an early barrier along the reaction coordinate is predictable based on knowledge of the electronic states of the reactants and may be used to either promote or inhibit oxygen activation.

Spin-polarized p-block antimony/bismuth single-atom catalysts on defect-free rutile TiO2(110) substrate for highly efficient CO oxidation

Developing high-loading spin-polarized p-block-element-based single-atom catalysts (p-SACs) upon defect-free substrates for various chemical reactions wherein spin selection matters is generally considered a formidable challenge because of the difficulty of creating high densities of underpinning stable defects and the delocalized electronic features of p-block elements. Here our first-principles calculations establish that the defect-free rutile TiO2(110) wide-bandgap semiconducting anchoring support can stabilize and localize the wavefunctions of p-block metal elements (Sb and Bi) via strong ionic bonding, forming spin-polarized p-SACs. Cooperated by the underlying d-block Ti atoms via a delicate spin donation-back-donation mechanism, the p-block single-atom reactive center Sb(Bi) exhibits excellent catalysis for spin-triplet O2 activation and CO oxidation in alignment with Wigner's spin selection rule, with a low rate-limiting reaction barrier of ∼0.6 eV. This work is crucial in establishing high-loading reactive centers of high-performance p-SACs for various important physical processes and chemical reactions, especially wherein the spin degree of freedom matters, i.e., spin catalysis.

Oxidative Activation of Small Aluminum Nanoclusters with Boron Atom Substitution prior to Completing the Endohedral B@Al12- Superatom

Elemental substitution and doping validate the optimization of chemical and physical properties of functional materials, and the composition ratio of the substituting atoms generally determines their properties by changing their geometric and electronic structures. For atomically precise nanoclusters (NCs) consisting of countable atom aggregates, the composition can be controlled accurately to provide an ideal model to study the heteroatom substitution effects. Since aluminum (Al) and boron (B) both belong to group 13 in the periodic table, the effect of B atom substitution on Aln NCs can be investigated while maintaining the total number of valence electrons in AlnBm NCs. In this study, oxidative reactivities of small Al NCs with B atom substitution are studied for AlnBm NCs (m = 1, n = 6-14 and m = 2, n = 11) supported on organic surfaces by using X-ray photoelectron spectroscopy and oxygen molecule (O2) exposure measurements. Before completing the endohedral B@Al12- superatomic NC, one B atom substitution in Al NCs (AlnB) enhances oxidative reactivities 3-20 times compared to those of Aln+1, particularly for n ≤ 11. When one Al atom of Al12B is further substituted by a B atom to form Al11B2, the reactivity drastically increases (6.6 × 102 times), showing that the B atom substitution makes the NC chemically active or inactive geometrically depending on the exohedral or endohedral site for the B atom in the Al NC. In addition, density functional theory calculations show that the electronegative B atom contributes to forming a locally positive Al site to facilitate O2 adsorption except in Al12B, in which the B atom is geometrically shielded by the surface of the Al12 cage in B@Al12.

The s-p Nonhybrid Nature Causes Adaptive Superatomic States of Bismuth Clusters

We report a joint experimental and theoretical study on the stability and reactivity of Bin + (n=5-33) clusters. The alternating odd-even effect on the reaction rates of Bin + clusters with NO is observed, and Bi7 + finds the most inertness. First-principles calculation results reveal that the lowest energy structures of Bi6-9 + exhibit quasi-spherical geometry pertaining to the jellium shell model; however, the Bin + (n≥10) clusters adopt assembly structures. The prominent stability of Bi7 + is associated with its highly symmetric structure and superatomic states with a magic number of 34e closed shell. For the first time, we demonstrate that the unique s-p nonhybrid feature in bismuth rationalizes the stability of Bi6-9 + clusters within the jellium model, by filling the 6s electrons into the superatomic orbitals (forming "s-band"). Interestingly, the stability of 18e "s-band" coincides with the compact structure for Bin + at n≤9 but assembly structures for n≥10, showing an accommodation of the s electrons to the geometric structure. The atomic p-orbitals also allow to form superatomic orbitals at higher energy levels, contributing to the preferable structures of tridentate binding units. We illustrate the s-p nonhybrid nature accommodates the structure and superatomic states of bismuth clusters.

Probing the binding and activation of small molecules by gas-phase transition metal clusters via IR spectroscopy

Isolated transition metal clusters have been established as useful models for extended metal surfaces or deposited metal particles, to improve the understanding of their surface chemistry and of catalytic reactions. For this objective, an important milestone has been the development of experimental methods for the size-specific structural characterization of clusters and cluster complexes in the gas phase. This review focusses on the characterization of molecular ligands, their binding and activation by small transition metal clusters, using cluster-size specific infrared action spectroscopy. A comprehensive overview and a critical discussion of the experimental data available to date is provided, reaching from the initial results obtained using line-tuneable CO₂ lasers to present-day studies applying infrared free electron lasers as well as other intense and broadly tuneable IR laser sources.

Charge Transfer Surface-Enhanced Raman and Absorption Spectra of the Zwitterionic Form of Cysteine Adsorbed on M@Au12 (M = Au, Ag, Pt, and Pd) Nanoclusters

The effect of the core atom type of the M@Au₁₂ nanocluster (M = Au, Ag, Pt, and Pd) on the normal (NR) and charge-transfer surface-enhanced Raman spectroscopy (CT-SERS) of the zwitterion form of l-cysteine (ZWCYS) adsorbed on two different sites (D1 and D2) of the nanocluster is investigated separately in the gas phase and water. Because SERS requires the calculation of the absorption spectrum, the effect of the core atom type on the absorption spectrum of M@Au₁₂ and its complex with the ZWCYS has also been investigated. The vibrational bands that show the intensity enhancement in the CT-SERS of the ZWCYS interacting with the D1 site of M@Au₁₂ nanocluster in water are O─C═O asymmetric stretching (M = Au and Ag), NH₂ bending (M = Ag), S-H stretching (M = Ag, Pt, and Pd), CH₂ bending (M = Pt), and CH₂ symmetric stretching (M = Pt and Pd). The ZWCYS at the D₂ site of the M@Au₁₂ nanocluster in water exhibits intensity enhancement for O─C═O asymmetric stretching (M = Pt), NH₃ wagging (M = Au), and S-H stretching (M = Pd). The intensity of the vibrational bands of ZWCYS does not increase for M = Ag but decreases for O─C═O asymmetric stretching, S-H stretching, CH₂ symmetric stretching, CH₂ asymmetric stretching, and especially NH₂ symmetric stretching.

Conjugation-Modulated Excitonic Coupling Brightens Multiple Triplet Excited States

The design and regulation of multiple room-temperature phosphorescence (RTP) processes are formidably challenging due to the restrictions imposed by Kasha's rule. Here, we report a general design principle for materials that show multiple RTP processes, which is informed by our study of four compounds where there is modulation of the linker hybridization between donor (D) and acceptor (A) groups. Theoretical modeling and photophysical experiments demonstrate that multiple RTP processes can be achieved in sp³ C-linked D-A compounds due to the arrest of intramolecular electronic communication between two triplet states (T₁^H and T₁^L) localized on the donor and acceptor or between two triplet states, one localized on the donor and one delocalized across aggregated acceptors. However, for the sp² C-linked D-A counterparts, RTP from one locally excited T₁ state is observed because of enhanced excitonic coupling between the two triplet states of molecular subunits. Single-crystal and reduced density gradient analyses reveal the influence of molecular packing on the coincident phosphorescence processes and the origin of the observed aggregate phosphorescence. These findings provide insights into higher-lying triplet excited-state dynamics and into a fundamental design principle for designing compounds that show multiple RTP.

Single atom alloy clusters Agn-1X- (X = Cu, Au; n = 7-20) reacting with O2: Symmetry-adapted orbital model

Single atom alloy AgCu catalysts have attracted great attention, since doping the single Cu atom introduces narrow free-atom-like Cu 3d states in the electronic structure. These peculiar electronic states can reduce the activation energies in some reactions and offer valuable guidelines for improving catalytic performance. However, the geometric tuning effect of single Cu atoms in Ag catalysts and the structure-activity relationship of AgCu catalysts remain unclear. Here, we prepared well-resolved pristine Ag_n ^- as well as single atom alloy Ag_n-1Cu^- and Ag_n-1Au^- (n = 7-20) clusters and investigated their reactivity with O₂. We found that replacing an Ag atom in Ag_n ^- (n = 15-18) with a Cu atom significantly increases the reactivity with O₂, while replacement of an Ag with an Au atom has negligible effects. The adsorption of O₂ on Ag_n ^- or Ag_n-1Cu^- clusters follows the single electron transfer mechanism, in which the cluster activity is dependent on two descriptors, the energy level of α-HOMO (strong correlation) and the α-HOMO-LUMO gap (weak correlation). Our calculation demonstrated that the cluster arrangements caused by single Cu atom alloying would affect the above activity descriptors and, therefore, regulates clusters' chemical activity. In addition, the observed reactivity of clusters in the representative sizes with n = 17-19 can also be interpreted using the symmetry-adapted orbital model. Our work provides meaningful information to understand the chemical activities of related single-atom-alloy catalysts.

Water-Gas Shift Catalyzed by Iridium-Vanadium Oxide Clusters IrVO2- with Iridium in a Rare Oxidation State of -II

The discovery of compounds containing transition metals with an unusual and well-established oxidation state is vital to enrich our horizon on formal oxidation state. Herein, benefiting from the study of the water-gas shift reaction (CO + H₂O → CO₂ + H₂) mediated with the iridium-vanadium oxide cluster IrVO₂^-, the missing -II oxidation state of iridium was identified. The reactions were performed by using our newly developed double ion trap reactors that can spatially separate the addition of reactants and are characterized by mass spectrometry and quantum-chemical calculations. This finding makes an important step that all the proposed 13 oxidation states of iridium (+IX to -III) have been known. The iridium atom in the IrVO₂^- cluster features the Ir═V double bond and resembles chemically the coordinated oxygen atom. A reactivity study demonstrated that the flexible role switch of iridium between an oxygen-atom like (Ir^-IIVO₂^-) and a transition-metal-atom like behavior (Ir^+IIVO₃^-) in different species can drive the water-gas shift reaction in the gas phase under ambient conditions. This result parallels and well rationalizes the extraordinary reactivity of oxide-supported iridium single-atom catalysts in related condensed-phase reactions.

Synergetic Catalysis of Magnetic Single-Atom Catalysts Confined in Graphitic-C3N4/CeO2(111) Heterojunction for CO Oxidization

Magnetic single-atom catalysts (MSAC), due to the intrinsic spin degree of freedom, are of particular importance relative to other conventional SAC for applications in various catalytic processes, especially in those cases that involve spin-triplet O₂. However, the bottleneck issue in this field is the clustering of the SAC during the processes. Here using first-principles calculations we predict that Mn atoms can be readily confined in the interface of the porous g-C₃N₄/CeO₂(111) heterostructure, forming high-performance MSAC for O₂ activation via a delicate synergetic mechanism of charge transfer, mainly provided by the p-block g-C₃N₄ overlayer mediated by the d-block Mn active site, and spin selection, preserved mainly through active participation of the f-block Ce atoms and/or g-C₃N₄, which effectively promotes the CO oxidization. Such a recipe is also demonstrated to be valid for V- and Nb-MSACs, which may shed new light on the design of highly efficient MSACs for various important chemical processes wherein spin-selection matters.

Synergetic Charge Transfer and Spin Selection in CO Oxidation at Neighboring Magnetic Single-Atom Catalyst Sites

Deciphering the precise physical mechanism of interaction between an adsorbed species and a reactive site in heterogeneous catalysis is crucial for predictive design of highly efficient catalysts. Here, using first-principles calculations we identify that the two-dimensional ferromagnetic metal organic framework of Mn₂C₁₈H₁₂ can serve as a highly efficient single-atom catalyst for spin-triplet O₂ activation and CO oxidation. The underlying mechanism is via "concerted charge-spin catalysis", involving a delicate synergetic process of charge transfer, provided by the hosting Mn atom, and spin selection, preserved through active participation of its nearest neighboring Mn atoms for the crucial step of O₂ activation. The synergetic mechanism is further found to be broadly applicable in O₂ adsorption on magnetic X₂C₁₈H₁₂ (X = Mn, Fe, Co, and Ni) with a well-defined linear scaling dependence between the chemical activity and spin excitation energy. The present findings provide new insights into chemical reactions wherein spin selection plays a vital role.

Structures and Electron Affinities of Aluminum Hydride Clusters AlnH (n = 3-13)

Low-energy structures and electron affinities (EAs) for aluminum hydride clusters Al_nH (n = 3-13) have been calculated using ab initio and density functional calculations. Geometries were optimized at the PBE0/def-2-TZVPP level of theory, which has been shown to match the currently accepted lowest-energy structures for the all-aluminum clusters Al_n and their anions. Neutral hydride clusters with n = 4, 7, and 9-12 are predicted to adopt terminal structures with the hydrogen atom bound to only one aluminum atom and with only minor alterations of the aluminum atom arrangement from that of the all-aluminum cluster. Clusters with n = 3 and 13 are predicted to adopt "face-centered" geometries, and the n = 6 cluster is predicted to prefer an isomer with the hydrogen atom bridging two aluminum atoms, also with little or no distortion to the aluminum atom arrangement from the all-aluminum cluster. Addition of a hydrogen atom to clusters with n = 5 and 8 is predicted to distort the aluminum atom arrangement significantly from that of the corresponding all-aluminum cluster. In the anionic clusters, terminal clusters are preferred for all cluster sizes except for n = 6 that prefers a face-centered arrangement. Minor distortions in the aluminum scaffolding for Al₁₁ and Al₁₂ were found, while all other anionic clusters adopt structures with little or no deviation in the aluminum atom arrangement from the corresponding all-aluminum cluster. Raw adiabatic electron affinities were computed using CCSD(T)/aug-cc-pVTZ single-point energies for the anionic and neutral hydride clusters at their respective DFT geometries. Isodesmic electron affinities for the hydride clusters were computed relative to their all-aluminum counterparts and show an even-odd alternation with cluster size. Derived EAs alternate in magnitude between even- and odd-numbered clusters, with the even-numbered clusters having relatively larger EAs.

Al13− and B@Al12− superatoms on a molecularly decorated substrate

Aluminum nanoclusters (Al_n NCs), particularly Al₁₃⁻ (n = 13), exhibit superatomic behavior with interplay between electron shell closure and geometrical packing in an anionic state. To fabricate superatom (SA) assemblies, substrates decorated with organic molecules can facilitate the optimization of cluster–surface interactions, because the molecularly local interactions for SAs govern the electronic properties via molecular complexation. In this study, Al_n NCs are soft-landed on organic substrates pre-deposited with n-type fullerene (C₆₀) and p-type hexa-tert-butyl-hexa-peri-hexabenzocoronene (HB-HBC, C₆₆H₆₆), and the electronic states of Al_n are characterized by X-ray photoelectron spectroscopy and chemical oxidative measurements. On the C₆₀ substrate, Al_n is fixed to be cationic but highly oxidative; however, on the HB-HBC substrate, they are stably fixed as anionic Al_n⁻ without any oxidations. The results reveal that the careful selection of organic molecules controls the design of assembled materials containing both Al₁₃⁻ and boron-doped B@Al₁₂⁻ SAs through optimizing the cluster–surface interactions.

Anionic aluminium clusters are promising candidates for the fabrication of superatom-assembled nanomaterials. Here, the authors report enhanced stability for Al₁₃⁻ and boron-doped B@Al₁₂⁻ on a molecularly decorated p-type organic substrate.

Tetrahedral Pt10- Cluster with Unique Beta Aromaticity and Superatomic Feature in Mimicking Methane

Utilizing a customized metal cluster source in tandem with a flow tube reactor and a reflectron time-of-flight mass spectrometer, we have obtained well-resolved pure metal clusters Pt_n^- and observed their gas-phase reactions with a few small gas molecules. Interestingly, the remarkable inertness of Pt₁₀^- was repeatedly observed in different reactions. Meanwhile, we have determined the structure of Pt₁₀^- within a regular tetrahedron. Considering that Pt possesses 5d⁹6s¹ electron configuration, the tetrahedral Pt₁₀^- exhibits unexpected stability at neither a magic number of valence electrons nor a shell closure of geometric structure. Comprehensive theoretical calculations unveil the stability of Pt₁₀^- is significantly associated with the all-metal aromaticity. In addition to the classical total aromaticity, which is mainly due to 6s electrons, there is unique beta-aromaticity ascribed to spin-polarized beta 5d electrons pertaining to singly occupied multicenter bonds. Further, we demonstrate the superatomic feature of such a transition metal cluster Pt₁₀^-, as Pt₆@Pt₄^-, in mimicking methane.

How O2-Binding Affects Structural Evolution of Medium Even-Sized Gold Clusters Aun- (n = 20-34)

We report the first joint anion photoelectron spectroscopy and theoretical study on how O₂-binding affects the structures of medium even-sized gold clusters, Au_n^- (n = 20-34), a special size region that entails a variety of distinct structures. Under the temperature conditions in the current photoelectron spectroscopy experiment, O₂-bound gold clusters were observed only for n = 22-24 and 34. Nevertheless, O₂ binding with the clusters in the size range of n = 20-34 can be still predicted based on the obtained global-minimum structures. Consequently, a series of structural transitions, from the pyramidal to fused-planar to core-shell structures, are either identified or predicted for the Au_nO₂^- clusters, where the O₂-binding is in either superoxo or peroxo fashion. The identified global-minimum structures of Au_nO₂^- (n = 20-34) also allow us to gain improved understanding of why the clusters Au_n^- (n = 26-32) are less reactive with O₂ in comparison to others.

Gas-Phase Anionic Metal Clusters are Model Systems for Surface Oxidation: Kinetics of the Reactions of Mn- with O2 (M = V, Cr, Co, Ni; n = 1-15)

The reactions of anionic metal clusters M_n^- with O₂ (M = V (n = 1-15), Cr (n = 1-15), Co (n = 1-12), and Ni (n = 1-14)) are investigated from 300 to 600 K using a selected-ion flow tube. All rate constants show a positive temperature dependence, well described by an Arrhenius equation. Rate constants exceed (or are extrapolated to exceed at higher temperatures) the Langevin-Gioumousis-Stevenson capture rate constant. Application of a capture model accounting for the finite size of the clusters reproduces the size-dependent trends in reactivity. The assumption that reactivity is further controlled by an energetic barrier early in the reaction coordinate is consistent with the experimental observations. An observed correlation of the derived barrier heights on the electron binding energy of M_n^- suggests the barrier may be formed at an avoided crossing between electronic states correlating to M_n^- + O₂ and M_n + O₂^- reactants, analogous to that previously proposed for Al_n^- + O₂ systems. The mechanism is analogous to that for reactions of O₂ with neutral metal surfaces, indicating that gas-phase reactions of anionic metal clusters can be an appropriate model systems for surface oxidation.

Reactivity of Cobalt Clusters Con±/0 with Dinitrogen: Superatom Co6+ and Superatomic Complex Co5N6. J Phys Chem A 2021;125:2130-2138. [PMID: 33689326 DOI: 10.1021/acs.jpca.1c00483]

We report a joint experimental and theoretical study on the reactions of cobalt clusters (Co_n^±/0) with nitrogen using the customized reflection time-of-flight mass spectrometer combined with a 177.3 nm deep-ultraviolet laser. Comparing to the behaviors of neutral Co_n (n = 2-30) and anionic Co_n^- clusters (n = 7-53) which are relatively inert in reacting with nitrogen in the fast-flow tube, Co_n⁺ clusters readily react with nitrogen resulting in adducts of one or multiple N₂ except Co₆⁺ which stands firm in the reaction with nitrogen. Detailed quantum chemistry calculations, including the energetics, electron occupancy, and orbital analysis, well-explained the reasonable reactivity of Co_n⁺ clusters with nitrogen and unveiled the open-shell superatomic stability of Co₆⁺ within a highly symmetric (D_3d) structure. The D_3d Co₆⁺ bears an electron configuration of a half-filled superatomic 1P orbital (i.e., 1S²1P³||1D⁰), a large α-highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap, symmetric multicenter bonds, and reasonable electron delocalization pertaining to metallic aromaticity. Topology analysis by atom-in-molecule illustrates the interactions between Co_n⁺ and N₂ corresponding to covalent bonds, but the Co-N interactions in cationic Co₂⁺N₂ and Co₆⁺N₂ clusters are apparently weaker than those in the other systems. In addition, we identify a superatomic complex Co₅N₆⁺ which exhibits similar frontier orbitals as the naked Co₅⁺ cluster, but the alpha HOMO-LUMO gap is nearly double-magnified, which is consistent with the high-abundance peak of Co₅N₆⁺ in the experimental observation. The enhanced stability of such a ligand-coordinated superatomic complex Co₅N₆⁺, along with the superatom Co₆⁺ with aromaticity, sheds light on special and general superatoms.

Vanadium Cluster Neutrals Reacting with Water: Superatomic Features and Hydrogen Evolution in a Fishing Mode

Hydrogen evolution reaction (HER) is known as the heart of various energy storage and conversation systems of renewable energy sources. Here we observe the cluster reactions of a light transition metal, vanadium, with water in a gas-phase flow tube reactor. While HER products of V₁ and V₂ were not observed, the effective HER of water on neutral V_n (n ≥ 3) clusters reveals reasonable and size-dependent reactivity of the vanadium clusters. Superatomic features and reaction dynamics of V₁₀, V₁₃, and V₁₆ are highlighted. Among the three typical superatoms, V₁₀ and V₁₆ exhibit an abnormal superatomic orbital energy level order, 1S|2S|1P|1D..., where the energy-reduced 2S orbital helps to accommodate the geometric structure and hence reinforce the cluster stability. In comparison, V₁₃ bears a less symmetrical structure and reacts readily with water, allowing for recombination of a hydroxyl atom with an adsorbed hydrogen atom, akin to a fishing-mode HER process. The joint experimental and theoretical study on neutral V_n clusters clarifies the availability of superatom chemistry for transition metals and appeals further development of cluster theory based on electronic cloud/orbital analysis instead of simply counting the valence electrons. Also, we provide insights into the HER mechanism of metal clusters and propose a strategy to design new materials for portable fuel cells of hydrogen energy.

Gas-phase preparation and the stability of superatomic Nb11O15. Phys Chem Chem Phys 2021;23:15766-15773. [PMID: 34286767 DOI: 10.1039/d1cp02128a]

We report a study of the reactions of pure metal clusters Nbn- with dioxygen in the gas phase. It is found that the presence of low-concentration dioxygen reactants results in oxygen-addition products, whereas sufficient high-concentration dioxygen enables oxygen-etching reactions giving rise to molecular niobium oxides. Interestingly, in the presence of a suitable gas flow rate of an intermediate dioxygen concentration, a highly selective product Nb11O15- shows up in the mass spectra. Utilizing density functional theory (DFT) calculations, we have discussed the reactivities of Nbn- (3 ≤ n ≤ 14) clusters with oxygen, and unveiled the reasonable stability of Nb11O15- pertaining to its unique geometric structure with a D5h Nb@Nb10 core fully protected by 15 bridge-oxygen atoms. The oxygen-passivated Nb@Nb10O15- cluster exhibits a large HOMO-LUMO gap (1.46 eV) and effective multicenter bonds with remarkable superatom orbitals for all the 26 valence electrons of the Nb@Nb10 core corresponding to well-staggered energy levels. We illustrate the superatomic features in the Nb@Nb10 metallic core for which the adaptive natural density partitioning (AdNDP) analysis unveils thirteen 11c-2e bonds. Among them, one of the 11c-2e bonds accounts for the superatomic S orbital, three bonds correspond to superatomic P orbitals, another five display vivid D orbital characteristics, and the remaining four 11c-2e bonds are assigned to F orbital features. In addition, the net atomic charge of the center Nb atom is as high as -0.804 |e| rendering core-shell electrostatic interactions and the shielding effect of the Nb10O15 shell.

Co13O8—metalloxocubes: a new class of perovskite-like neutral clusters with cubic aromaticity

Exploring stable clusters to understand structural evolution from atoms to macroscopic matter and to construct new materials is interesting yet challenging in chemistry. Utilizing our newly developed deep-ultraviolet laser ionization mass spectrometry technique, here we observe the reactions of neutral cobalt clusters with oxygen and find a very stable cluster species of Co₁₃O₈ that dominates the mass distribution in the presence of a large flow rate of oxygen gas. The results of global-minimum structural search reveal a unique cubic structure and distinctive stability of the neutral Co₁₃O₈ cluster that forms a new class of metal oxides that we named as ‘metalloxocubes’. Thermodynamics and kinetics calculations illustrate the structural evolution from icosahedral Co₁₃ to the metalloxocube Co₁₃O₈ with decreased energy, enhanced stability and aromaticity. This class of neutral oxygen-passivated metal clusters may be an ideal candidate for genetic materials because of the cubic nature of the building blocks and the stability due to cubic aromaticity.

Ligand accommodation causes the anti-centrosymmetric structure of Au13Cu4 clusters with near-infrared emission

We synthesized an [Au₁₃Cu₄(PPh₃)₄(SPy)₈]⁺ nanocluster co-capped by phosphine and thiolate ligands. Interestingly, this Au₁₃Cu₄ cluster corresponds to an anti-centrosymmetric structure with the four copper atoms coordinated to the mixed ligands on the same side of the Au₁₃ icosahedron, which is in sharp contrast to the [Au₁₃Cu₄(PPh₂Py)₄(SPhtBu)₈]⁺ and [Au₁₃Cu₂(PPh₃)₆(SPy)₆]⁺ clusters which possess highly symmetric structures with well-separated Cu adatoms. Both [Au₁₃Cu₄(PPh₃)₄(SPy)₈]⁺ and [Au₁₃Cu₂(PPh₃)₆(SPy)₆]⁺ clusters correspond to 8 valence electron superatoms with large HOMO-LUMO gaps, respectively. The difference in structure is rooted in the nature of the mixed ligands, with the bidentate SPy binding strongly to Cu on both binding sites (-N-Cu and Au-SR-Cu) leading to the co-linking of adjacent Cu atoms, while the bidentate PPh₂Py binds Cu on one site and Au on the other giving rise to a separation of the Cu atoms even in the presence of relatively higher monomer concentration. Both [Au₁₃Cu₄(PPh₃)₄(SPy)₈]⁺ and [Au₁₃Cu₂(PPh₃)₆(SPy)₆]⁺ display emissions in the near-IR regions. TD-DFT calculations reproduce the spectroscopic results with specified excited states, shedding light on the geometric and electronic behaviors of the ligand-protected Au₁₃M_x clusters.

Redefining the Mechanism of O2 Etching of Aln- Superatoms: An Early Barrier Controls Reactivity, Analogous to Surface Oxidation

New insights into aluminum anion cluster reactivity with O₂ were obtained through temperature-dependent kinetics measurements. Overall reactivity is controlled by a barrier at an avoided crossing where charge is transferred from the cluster to the O₂, mechanistically similar to what occurs as O₂ approaches a bulk Al surface. Contrary to prior interpretations, spin conservation does not inhibit the reaction of clusters with an odd number of Al atoms. In fact, the only spin constraint in these systems is on the reactivity of even clusters due to repulsive surfaces, not previously recognized. Although the superatom nature of Al₁₃^- is manifest in its high electron binding energy (EBE), the mechanism of its reactivity is not special; it reacts with O₂ as if it were a small piece of bulk Al. These experiments highlight the sensitivity of Al cluster reactivity with O₂ to temperature and EBE, uncovering routes to industrial scale use of aluminum superatom-based materials.

A transition metal–gallium cluster formed via insertion of “GaI”

Synthesis of a new transition metal-group 13 cluster from a low-coordinate diaryl and “GaI”, demonstrates entry into new cluster compounds.

Study of odd-even effects in physisorption and chemisorption of Ar, N2, O2 and NO on open shell Ag11-13+ clusters by means of self-consistent van der Waals density functional calculations

We have studied the adsorption and coadsorption properties of one or more X = Ar, N₂, O₂, and NO adsorbates on cationic silver clusters Ag_11-13⁺, whose sizes are in the open shell region of metal clusters, aiming to understand the observed odd-even effects in the abundance spectra of Ag_11-13⁺·mX complexes. All calculations were performed self-consistently using a non-local van der Waals correlation functional, covering the different nature of the interactions between the silver substrate and the several adsorbates, which range from dispersion (London) forces for Ar, non covalent π-π interactions for N₂, charge-transfer interactions for O₂ and NO, and the covalent Ag-Ag bond in the nude silver cluster. Despite the wide interval of adsorption energies, spanning two orders of magnitude, we have been able to explain the following experimental facts. For X = Ar, N₂, and O₂ reactions with Ag_11-13⁺, it was observed in the mass spectra an abundance peak at n = 12 [M. Schmidt, et al., ChemPhysChem, 2015, 16, 855]. In addition it was observed the competitive adsorption of two or more N₂ molecules, and the cooperative effect of adsorbing N₂ together with O₂ molecules. For X = NO, an abundance peak at n = 12 has been also observed [J. Ma, et al., Phys. Chem. Chem. Phys., 2016, 18, 12819]. We find that the main factors determining these properties are the different core motifs of the cluster geometry (pentagonal bipiramid for Ag₁₁⁺ and Ag₁₃⁺, but triangular prism for Ag₁₂⁺) and, on the other hand, the odd number of valence electrons for Ag₁₂⁺, leading to a smaller HOMO-LUMO gap than those of its neighbours. Further details about the preferred adsorption sites, dipole moments, and dipole polarizabilities are also discussed.

Structure, stability, absorption spectra and aromaticity of the singly and doubly silicon doped aluminum clusters Al n Si m 0/+ with n = 3-16 and m = 1, 2

Structures of the binary Al n Si m clusters in both neutral and cationic states were investigated using DFT and TD-DFT (B3LYP/6-311+G(d)) and (U)CCSD(T)/cc-pvTZ calculations. Silicon-doped aluminum clusters are characterized by low spin ground states. For small sizes, the Si dopant prefers to be located at vertices having many edges. For larger sizes, the Si atom prefers to be endohedrally doped inside an Al n cage. Relative stability, adiabatic ionization energy and dissociation energies of each cluster size were evaluated. A characteristic of most Si doped Al clusters is the energetic degeneracy of two lowest-lying isomers. Calculated results confirm the high stability of the sizes Al4Si2, Al12Si and Al11Si2 + as "magic" clusters, that exhibit 20 or 40 shell electrons and are thermodynamically more stable as compared to their neighbors. Electronic absorption spectra of isoelectronic magic clusters Al13 -, Al12Si, and Al11Si2 + that have two pronounced bands corresponding to blue and violet lights, have been rationalized by using the electron shell model. The magnetically included ring current density (MICD) analyses suggest that they are also aromatic structures as a result of the "magic" 40 shell electrons.

Photoinitiated Charge Transfer in a Triangular Silver(I) Hydride Complex and Its Oxophilicity

The photoexcitation of a triangular silver(I) hydride complex, [Ag₃ (μ₃ -H)(μ₂ -dcpm)₃ ](PF₆ )₂ ([P](PF₆ )₂ , dcpm=bis(dicyclohexylphosphino)methane), designed with "UV-silent" bis-phosphine ligands, provokes hydride-to-Ag₃ single and double electron transfer. The nature of the electronic transitions has been authenticated by absorption and photodissociation spectroscopy in parallel with high-level quantum-chemical computations utilizing the GW method and Bethe-Salpeter equation (GW-BSE). Specific photofragments of mass-selected [P]²⁺ ions testify to charge transfer and competing pathways resulting from the unique [Ag₃ (μ₃ -H)]²⁺ scaffold. This structural motif of [P](PF₆ )₂ has been unequivocally verified by ¹ H NMR spectroscopy in concert with DFT and X-ray diffraction structural analysis, which revealed short equilateral Ag-Ag distances (d_AgAg =3.08 Å) within the range of argentophilic interactions. The reduced radical cation [P]^{. +} exhibits strong oxophilicity, forming [P+O₂ ]^.+ ,which is a model intermediate for silver oxidation catalysis.

Molecular and Dissociative Adsorption of Oxygen on Au-Pd Bimetallic Clusters: Role of Composition and Spin State of the Cluster

Utilization of molecular oxygen as an oxidizing agent in industrially important reactions is the ultimate goal to design environmentally benign processes under ambient conditions. However, the high thermal stability and a large O-O dissociation barrier in O2 molecule pose a great challenge toward its successful application in the oxidative chemistry. To achieve this goal, different catalysts based on monometallic and bimetallic clusters have been developed over the years to promote binding and dissociation of molecular oxygen. The successful design of efficient metal cluster catalysis needs an in-depth knowledge of synergistic effects between different metal atoms and intrinsic catalytic mechanisms for O2 adsorption and dissociation. Here, we present a systematic theoretical investigation of reaction pathways for O2 adsorption and dissociation on Au8, Pd8, and Au8-n Pd n (n = 1-7) nanoclusters in different spin states. The density functional calculations point out that the O2 dissociation barriers can be significantly reduced with the help of certain bimetallic clusters along specific spin channels. Our results particularly indicate that Au5Pd3 and Au1Pd7 show very large O2 binding energies of 1.76 and 1.69 eV, respectively. The enhanced O2 binding subsequently leads to low activation barriers of 0.98 and 1.19 eV along the doublet and quartet spin channels, respectively, without the involvement of any spin flip-over for O2 dissociation. Furthermore, the computed O2 dissociation barriers are significantly low as compared to the already reported barriers (1.95-3.65 eV) on monometallic and bimetallic Au-Ag clusters. The results provide key mechanistic insights into the interaction and dissociation of molecular oxygen with Au-Pd clusters, which can prove informative for the design of efficient catalysts for oxidative chemistry involving molecular oxygen as a reactant.

Thermal Kinetics of Aln- + O2 (n = 2-30): Measurable Reactivity of Al13. J Phys Chem A 2019;123:6123-6129. [PMID: 31251615 DOI: 10.1021/acs.jpca.9b03552]

Mass-selected aluminum anion clusters, Al_n^-, were reacted with O₂. Rate constants (300 K) for 2 < n < 30 and product branching fractions for 2 < n < 17 are reported. Reactivity is strongly anticorrelated to Al_n^- electron binding energy (EBE). Al₁₃^- reacts more slowly than predicted by EBE but notably is not inert, reacting at a measurable 0.05% efficiency (2.5 ± 1.5 × 10^-13 cm³ s^-1). Al₆^- is also an outlier, reacting more slowly than expected after accounting for other factors, suggesting that high symmetry increases stability. Implications of observed Al₁₃^- reactivity, contributions of both electronic shell-closing and geometric homogeneity to Al_n^- resistance to O₂ etching, and future directions to more fully unravel the reaction mechanisms are discussed.

Multiple-Valence Aluminum and the Electronic and Geometric Structure of Al nO m Clusters

Electronic stability in aluminum clusters is typically associated with either closed electronic shells of delocalized electrons or a +3 oxidation state of aluminum. To investigate whether there are alternative routes toward electronic stability in aluminum oxide clusters, we used theoretical methods to examine the geometric and electronic structure of Al _nO _m (2 ≤ n ≤ 7; 1 ≤ m ≤ 10) clusters. Electronically stable clusters with large HOMO-LUMO (highest occupied molecular orbital and lowest unoccupied molecular orbital) gaps were identified and could be grouped into two categories. (1) Al_{2 n}O_{3 n} clusters with a +3 oxidation state on the aluminum and (2) planar clusters including Al₄O₄, Al₅O₃, Al₆O₅, and Al₆O₆. The structures of the planar clusters have external Al atoms bound to a single O atom. Their electronic stability is explained by the multiple-valence Al sites, with the internal Al atoms having an oxidation state of +3, whereas the external Al atoms have an oxidation state of +1.

Neutral Au1-Doped Cluster Catalysts AuTi2O3-6 for CO Oxidation by O2

Oxide supported gold catalysts (e.g., Au/TiO₂) are of great significance in heterogeneous catalysis owing to their extraordinary catalytic activity. Study of heteronuclear metal oxide clusters (HMOCs, e.g., Au _xTi _yO _z ^q) is an important way to uncover the molecular-level mechanisms of gold catalysis in the related heterogeneous catalytic systems. However, the current studies of HMOCs are focused on charged clusters with little attention paid to neutral species. The reactivity study of neutral HMOCs is vital to have a comprehensive understanding of heterogeneous catalysis, but it is experimentally challenging because of the difficulty of cluster ionization and detection without fragmentation. Herein, benefiting from a homemade time-of-flight mass spectrometer coupled with a vacuum ultraviolet laser system, the reactivity of neutral Au₁-doped titanium oxide clusters AuTi₂O_3-6 in catalytic CO oxidation by O₂ has been successfully identified. The mechanistic details of the catalysis have been elucidated by quantum chemistry calculations. The crucial roles of the mobile AuCO species that can facilitate not only the process of CO oxidation but also the process of O₂ activation have been discovered in the cluster catalysis. The fascinating results are of substantial importance to understand the mechanisms of CO oxidation over Au/TiO₂, one type of the best studied gold catalysts.

The stability of binary Al12X nanoclusters (X = Sc and Ti): superatom or Wade's polyhedron

Binary nanoclusters (NCs) exhibit strong potential as building blocks for tailor-made scientific materials based on the precise tuning of their electron countings and spin states along with the synergistic effects that originate from the constituent elements. Herein, we studied the electronic and geometric structures of transition metal (TM) doped aluminum (Al) Al₁₂X NCs (X  =  Sc and Ti), which are binary systems that extend from representative superatom [Formula: see text] anions. On the basis of the photoelectron spectroscopy (PES) and density functional theory (DFT) calculations, Al₁₂X anion and neutral structures are characterized as vertex-replaced icosahedron. The highly stable exohedral Al₁₂X icosahedron is described based on an electron counting rule derived from the coupling of Wade-Mingos' rule and the jellium model.

Aluminum cluster for CO and O2 adsorption

Low temperature oxidation of CO to CO₂ is an important process for the environment. Similarly adsorption of CO from the releasing sources is also of major concern today. Whereas the potential of gold and silver clusters is well proven for the catalysis of the above mentioned reaction, the potential of aluminum (Al) clusters remains unexplored. The present study proves that, similar to the transition metals, Al clusters can also be used for adsorption of gases. We first tested the potential of Al cluster as adsorbents for CO. The high binding energy (BE) values prove that Al clusters can be used for adsorbing both CO and O₂. Since oxygen binding is more facile, we adsorbed oxygen on Al and then checked the effect of this O₂ on the BE of CO. The results were obtained by DFT calculations at M062X/TZVP level of theory. Graphical abstract Activation of carbon monoxide (CO) on oxygen-adsorbed aluminum (Al) cluster.

Photochemistry and spectroscopy of small hydrated magnesium clusters Mg+(H2O)n, n = 1-5

Hydrated singly charged magnesium ions Mg+(H2O)n, n ≤ 5, in the gas phase are ideal model systems to study photochemical hydrogen evolution since atomic hydrogen is formed over a wide range of wavelengths, with a strong cluster size dependence. Mass selected clusters are stored in the cell of an Fourier transform ion cyclotron resonance mass spectrometer at a temperature of 130 K for several seconds, which allows thermal equilibration via blackbody radiation. Tunable laser light is used for photodissociation. Strong transitions to D1-3 states (correlating with the 3s-3px,y,z transitions of Mg+) are observed for all cluster sizes, as well as a second absorption band at 4-5 eV for n = 3-5. Due to the lifted degeneracy of the 3px,y,z energy levels of Mg+, the absorptions are broad and red shifted with increasing coordination number of the Mg+ center, from 4.5 eV for n = 1 to 1.8 eV for n = 5. In all cases, H atom formation is the dominant photochemical reaction channel. Quantum chemical calculations using the full range of methods for excited state calculations reproduce the experimental spectra and explain all observed features. In particular, they show that H atom formation occurs in excited states, where the potential energy surface becomes repulsive along the O⋯H coordinate at relatively small distances. The loss of H2O, although thermochemically favorable, is a minor channel because, at least for the clusters n = 1-3, the conical intersection through which the system could relax to the electronic ground state is too high in energy. In some absorption bands, sequential absorption of multiple photons is required for photodissociation. For n = 1, these multiphoton spectra can be modeled on the basis of quantum chemical calculations.

Mechanistic Study on Water Splitting Reactions by Small Silicon Clusters Si3X, X = Si, Be, Mg, Ca

Interaction, dissociation, and dehydrogenation reactions of water monomer and dimer with pure and mixed tetrameric silicon clusters Si₃X with X = Si, Be, Mg, Ca were investigated using high accuracy quantum chemical calculations. While geometries were optimized using the DFT/B3LYP functional with the aug-cc-pVTZ basis set, reaction energy profiles were constructed making use of the coupled-cluster theory with extrapolation to complete basis set, CCSD(T)/CBS. Cleavage of the O-H bond in water dimer is found to be more favored than that of water monomer in the reaction with Si₄. The water acceptor monomer in water dimer performs as an internal catalyst facilitating H atom transfer to form H₂. Adsorption of water dimer on Si₃X clusters mostly takes place upon interaction of the donor water molecule with Si cluster. Water dimer adsorbs more strongly on Si₃M than on Si₄. The most stable complexes obtained upon interaction of water dimer with Si₃M mainly arise from M-O interaction in preference over a Si-O connection. Substitution of a Si atom in Si₄ by an earth alkaline metal induces a substantial reduction of the energy barrier for the (rate-limiting) first O-H bond cleavage of water dimer. The most remarkable achievement upon doping is a disappearance of the overall energy barrier for the initial O-H bond cleavage in water dimer. Of the three binary Si₃M clusters considered, dehydrogenation of water dimer driven by Si₃Be is the most kinetically and thermodynamically favorable pathway. In comparison to another cluster such as Al₆ and nanoparticles Ru₅₅, energy barriers for water dimer dissociation on Si₃M are much lower. The mixed clusters Si₃M turn out to be as efficient alternative reagents for O-H dissociation and hydrogen production from water dimer. This study proposes further searches for other mixed silicon clusters as realistic gas phase reagents for crucial dehydrogenation processes in such a way they can be prepared and conducted in experiment.

Tuning electronic and magnetic properties of Mn-mullite oxide sub-nanoclusters via MnOn polyhedron units

Ternary oxide nano-clusters compared to unary metallic and binary ones potentially exhibit more remarkable properties due to their higher stoichiometric flexibility in addition to cluster size variations. Herein, by combining with the structural searching scheme CALYPSO, we have built a series of Mn-mullite oxide clusters (SmxMnyOz)n {(xyz) = (125); (115); n = 1-4, 8} prior to investigation of their geometric and electronic structures via first-principles calculations. In small size regime (n < 4), (SmxMnyOz)n prefer nonstoichiometric (Sm1Mn1O5)n phases composed of nonmagnetic MnO4 tetrahedrons. When n ≧ 4, the clusters tend to develop as stoichiometric (Sm1Mn2O5)n species, including magnetic MnOn polyhedrons and Mn-Mn dimers, which contribute 3d-orbitals (dz2 and/or dx2-y2) around the Fermi levels. The different magnetic behaviors of nonstoichiometric and stoichiometric species originate from the distinct couplings of MnOn polyhedronal units, wherein Mn atoms experience different ligand fields and thus display different spin states. Such findings enable the tuning of electronic properties and potential applications in heterogeneous catalysis, electrochemical catalysis, and the related fields via engineering cluster size and stoichiometry.