Probing the Electronic and Structural Properties of the Niobium Trimer Cluster and Its Mono- and Dioxides: Nb3On− and Nb3On (n = 0−2)

Enhanced stability of the Nb3O6- and Nb4O6+ clusters: the nxcπ rule versus superatomic nature

This study examines the chemical reactivity of niobium clusters with carbon dioxide (CO2), with an emphasis on the analysis of the ensuing products Nb4O6+ and Nb3O6-, which show up in the cationic and anionic mass spectra, respectively. Using density functional theory (DFT) calculations, we demonstrate the reactivity of the Nbn± clusters with CO2 and reveal distinct stabilization mechanisms for the two prominent products. The stability of Nb3O6- is determined by the existence of ten π bonds pertaining to π-electron delocalization, which conforms to the nxcπ electron configuration model. Despite having only a one-atom distinction, Nb4O6+ exhibits superatomic electron shells embodying superatom stability. The divergent stabilizing mechanisms found in Nb4O6+ and Nb3O6- illustrate the intricate nature of cluster chemistry and the significance of electronic structure in governing cluster stability and reactivity.

Advances for Triangular and Sandwich-Shaped All-Metal Aromatics

Much experimental work has been contributed to all-metal σ, π and δ-aromaticity among transition metals, semimetallics and other metals in the past two decades. Before our focused investigations on the properties of triangular and sandwich-shaped all-metal aromatics, A. I. Boldyrev presented general discussions on the concepts of all-metal σ-aromaticity and σ-antiaromaticity for metallo-clusters. Schleyer illustrated that Nucleus-Independent Chemical Shifts (NICS) were among the most authoritative criteria for aromaticity. Ugalde discussed the earlier developments of all-metal aromatic compounds with all possible shapes. Besides the theoretical predictions, many stable all-metal aromatic trinuclear clusters have been isolated as the metallic analogues of either the σ-aromatic molecule's [H3]+ ion or the π-aromatic molecule's [C3H3]+ ion. Different from Hoffman's opinion on all-metal aromaticity, triangular all-metal aromatics were found to hold great potential in applications in coordination chemistry, catalysis, and material science. Triangular all-metal aromatics, which were theoretically proved to conform to the Hückel (4n + 2) rule and possess the smallest aromatic ring, could also play roles as stable ligands during the formation of all-metal sandwiches. The triangular and sandwich-shaped all-metal aromatics have not yet been specifically summarized despite their diversity of existence, puissant developments and various interesting applications. These findings are different from the public opinion that all-metal aromatics would be limited to further applications due to their overstated difficulties in synthesis and uncertain stabilities. Our review will specifically focus on the summarization of theoretical predictions, feasible syntheses and isolations, and multiple applications of triangular and sandwich shaped all-metal aromatics. The appropriateness and necessities of this review will emphasize and disseminate their importance and applications forcefully and in a timely manner.

Carbon Dioxide and Water Activation by Niobium Trioxide Anions in the Gas Phase

Transition metals are important in various industrial applications including catalysis. Due to the current concentration of CO₂ in the atmosphere, various ways for its capture and utilization are investigated. Here, we study the activation of CO₂ and H₂O at [NbO₃]^- in the gas phase using a combination of infrared multiple photon dissociation spectroscopy and density functional theory calculations. In the experiments, Fourier-transform ion cyclotron resonance mass spectrometry is combined with tunable IR laser light provided by the intracavity free-electron laser FELICE or optical parametric oscillator-based table-top laser systems. We present spectra of [NbO₃]^-, [NbO₂(OH)₂]^-, [NbO₂(OH)₂]^-(H₂O) and [NbO(OH)₂(CO₃)]^- in the 240-4000 cm^-1 range. The measured spectra and observed dissociation channels together with quantum chemical calculations confirm that upon interaction with a water molecule, [NbO₃]^- is transformed to [NbO₂(OH)₂]^- via a barrierless reaction. Reaction of this product with CO₂ leads to [NbO(OH)₂(CO₃)]^- with the formation of a [CO₃] moiety.

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.

Exploring the Reaction Mechanism of H2S Decomposition with MS3 (M = Mo, W) Clusters

H₂S is abundantly available in nature, and it is a common byproduct in industries. Molybdenum sulfides have been proved to be active in the catalytic decomposition of hydrogen sulfide (H₂S) to produce hydrogen. In this study, density functional theory (DFT) calculations are carried out to explore the reaction mechanisms of H₂S with MS₃ (M = Mo, W) clusters. The reaction mechanism of H₂S with MoS₃ is roughly the same as that of the reaction with WS₃, and the free-energy profile of the reaction with MoS₃ is slightly higher than that of the reaction with WS₃. The overall driving forces (-ΔG) are positive, and the overall reaction barriers (ΔG _b) are rather small, indicating that such H₂ productions are product-favored. MS₃ (M = Mo, W) clusters have clawlike structures, which have electrophilic metal sites to receive the approaching H₂S molecule. After several hydrogen-atom transfer (HAT) processes, the final MS₄·H₂ (IM-4) complexes are formed, which could desorb H₂ at a relatively low temperature. The singlet product MS₄ clusters contain the singlet S₂ moiety, similar to the adsorbed singlet S₂ on the surface of sulfide catalysts. The theoretical results are compared with the experiments of heterogeneous catalytic decomposition of H₂S by MoS₂ catalysts. Our work may provide some insights into the optimal design of the relevant catalysts.

Compositions and structures of niobium oxide cluster ions, NbmOn±, (m = 2–12), revealed by ion mobility mass spectrometry

The compositions and structures of niobium oxide cluster ions were studied and compared with vanadium oxide.

Beyond organic chemistry: aromaticity in atomic clusters

We describe joint experimental and theoretical studies carried out collaboratively in the authors' labs for understanding the structures and chemical bonding of novel atomic clusters, which exhibit aromaticity.

High reactivity of nanosized niobium oxide cluster cations in methane activation: A comparison with vanadium oxides

The reactions between methane and niobium oxide cluster cations were studied and compared to those employing vanadium oxides. Hydrogen atom abstraction (HAA) reactions were identified over stoichiometric (Nb2O5)N(+) clusters for N as large as 14 with a time-of-flight mass spectrometer. The reactivity of (Nb2O5)N(+) clusters decreases as the N increases, and it is higher than that of (V 2O5)N(+) for N ≥ 4. Theoretical studies were conducted on (Nb2O5)N(+) (N = 2-6) by density functional calculations. HAA reactions on these clusters are all favorable thermodynamically and kinetically. The difference of the reactivity with respect to the cluster size and metal type (Nb vs V) was attributed to thermodynamics, kinetics, the electron capture ability, and the distribution of the unpaired spin density. Nanosized Nb oxide clusters show higher HAA reactivity than V oxides, indicating that niobia may serve as promising catalysts for practical methane conversion.

On the structural and electronic properties of hexanuclear vanadium oxide clusters V6On(-/0) (n=12-15): is V6O12 cluster planar or cage-like?

Density functional theory (DFT) calculations are carried out to investigate the structural and electronic properties of a series of hexanuclear vanadium oxide clusters V6On(-/0) (n=12-15). Generalized Koopmans' theorem is applied to predict the vertical detachment energies (VDEs) and simulate the photoelectron spectra (PES) for V6On(-) (n=12-15) clusters. Extensive DFT calculations are performed in search of the lowest-energy structures for both the anions and neutrals. All of these clusters appear to prefer the polyhedral cage structures, in contrast to the planar star-like structures observed in prior model surface studies for the V6O12 cluster. Molecular orbitals are performed to analyze the chemical bonding in the hexanuclear vanadium oxide clusters and provide insights into the sequential oxidation of V6On(-) (n=12-15) clusters. The V6On(-) (n=12-15) clusters possess well-defined V(5+) and V(3+) sites, and may serve as molecular models for surface defects. Electron spin density analyses show that the unpaired electrons in V6On(-) (n=12-14) clusters are primarily localized on the V(3+) sites rather than on the V(5+) sites. The difference gas phase versus model surface structures of V6O12 hints the critical roles of cluster-substrate interactions in stabilizing the planar V6O12 cluster on model surfaces.

Relevant and unprecedented C-H/σ supramolecular interactions involving σ-aromatic M2X2 cores

A novel type of C-H/σ supramolecular interaction involving σ-aromatic M2X2 (M = Cu, Hg; X = Cl, Br, I, S) cores is reported for the first time. Three new polymeric coordination copper complexes, {[Cu(μ-Cl)(Cl)(μ-L)]2}n (1), {[Cu(μ-I)(μ-L)]2}n (2) and [Cu(Br)2(μ-L)(CH3CN)2]2 (3), have been synthesized with the organic ligand α,ω-bis(benzotriazoloxy)propane system (L) and halides as counterions. A very interesting C-H/σ supramolecular interaction has been observed in the solid state structure of compound 2, similar to a C-Hπ interaction, which has been confirmed by Bader's "atoms-in-molecules" AIM analysis. The Nucleus Independent Chemical Shift (NICs) method was used to evaluate the aromatic character of the different cores in this study. The influence of the nature of the metal ions, bridging atoms, oxidation states, and coordination environments around the metal centers on the strength of the aromaticity of the M2X2 cores was theoretically analyzed and explained. The binding ability of the 1-alkoxy-1,2,3-benzotriazole ring to establish π-π and C-H/π interactions and how its coordination to Cu(I) and Cu(II) ions affects the strength of the aforementioned interactions have been discussed. The electron deficient triazole ring and its π-acidity increases upon coordination of the Cu ion, leading to the formation of a lone pair (lp)-π interaction involving the five-membered ring of the ligand, which has also been analyzed. We have also analyzed the C-H/σ interactions of previously reported X-ray crystal structures of different coordination polymers based on a binuclear copper(I) complex and 2,3-dimethylpyrazine, dithioethers, and benzotriazol-1-yl-based pyridyl units as ligands using I- as a counter-ion. Complex 1 shows antiferromagnetic behavior with a magnetic coupling constant of J = -7.9 cm(-1). Moreover, photoluminescence and TGA studies of the complexes were also carried out.

Theoretical studies on the electronic structures and photoelectron spectra of tri-rhenium oxide clusters: Re3O(n)(-) and Re3O(n) (n=1-6)

Density functional theory (DFT) calculations are performed to study the structural and electronic properties of tri-rhenium oxide clusters Re3On(-/0) (n=1-6). Generalized Koopmans' theorem is applied to predict the vertical detachment energies (VDEs) and simulate the photoelectron spectra (PES). Theoretical calculations at the B3LYP level are carried out to search for the global minima for both the anions and the neutrals. For the anions, the first two O atoms prefer the same corner position of a Re3 triangle. Whereas, Re3O3(-) possesses a C2v symmetry with one bridging and two terminal O atoms. The next three O atoms (n=4-6) are adding sequentially on the basis of Re3O3(-) motif, i.e., adding one terminal O atom for Re3O4(-), one terminal and one bridging O atoms for Re3O5(-), and one terminal and two bridging O atoms for Re3O6(-), respectively. Their corresponding neutral species are similar to the anions in geometry except Re3O4 and Re3O5. Molecular orbital analyses are employed to investigate the chemical bonding and structural evolution in these tri-rhenium oxide clusters.

A combined stochastic search and density functional theory study on the neutral and charged silicon-based clusters MSi6 (M = La, Ce, Yb and Lu)

Structures and simulated photoelectron spectra of MSi₆⁻ (M = La, Ce, Yb and Lu).

Computational study on the molecular structures and photoelectron spectra of bimetallic oxide clusters MW2O9(-/0) (M=V, Nb, Ta)

Density functional theory (DFT) and coupled cluster theory (CCSD(T)) calculations are carried out to investigate the electronic and structural properties of a series of bimetallic oxide clusters MW2O9(-/0) (M=V, Nb, Ta). Generalized Koopmans' theorem is applied to predict the vertical detachment energies (VDEs) and simulate the photoelectron spectra (PES). Theoretical calculations at the B3LYP level yield singlet and doublet ground states for the bimetallic anionic and neutral clusters, respectively. All the clusters present the six-membered ring structures with different symmetries, except that the TaW2O9(-) cluster shows a chained style with a penta-coordinated tantalum atom. Spin density analyses reveal oxygen radical species in all neutral clusters, consistent with their structural characteristics. Moreover, additional calculations are performed to study the oxidation reaction of CO molecule with the W3O9(+) cation and the isoelectronic VW2O9 cluster, and results indicate that the introduction of vanadium at tungsten site can efficiently improve the oxidation reactivity.

Probing the electronic properties of W3O(x)(-/0) (x = 0-2) and W3(2-) clusters: the aromaticity of W3 and W3(2-)

Density functional theory (DFT) calculations are employed to investigate the structural and electronic properties of bare tritungsten clusters (W3, W3(-), W3(2-)) and tritungsten oxide clusters W3Ox(-/0) (x = 1, 2). Generalized Koopmans' theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra (PES) for W3Ox(-) (x = 0-2) clusters. Extensive DFT calculations are performed in search of the lowest energy structures for both the anions and the neutrals. The bare tritungsten clusters are predicted to be triangular structures with D3h ((3)A1'), C2v ((2)A1) and D3h ((1)A1') symmetry for W3, W3(-) and W3(2-), respectively. For W3O(-) and W3O clusters, the oxygen atom occupies the terminal site, while the next added oxygen atom is found to be a bridging one in both W3O2(-) and W3O2 clusters. Molecular orbital analyses are carried out to elucidate the chemical bonding of these clusters and provide insights into the sequential oxidation from W3(-) to W3O2(-). Partial σ- and δ-aromaticity are revealed in the neutral W3 (D3h, (3)A1'), while the anion W3(2-) (D3h, (1)A1') possesses only δ-aromaticity.

Reactions of Nb2 and Nb3 with CO, D2, N2, and O2: reconciling experimental kinetics with density functional theory-calculated reaction profiles

Calculated energy profiles for the reactions of neutral Nb(2) and Nb(3) metal clusters with CO, D(2), N(2), and O(2) are presented. In each reaction path, both a physisorption energy minimum, where the reactant remains intact, and a chemisorption energy minimum, where the reactant has dissociated, are calculated and linked by saddle points. We calculate branching ratios for the forward (dissociative) and reverse reactions which we compare with the experimental kinetic data. It is found that a combination of average thermal energies and barrier heights leads to wide variation in branching ratios which compares favourably to previously determined experimental reaction rates.

Structure characterization of metal oxide clusters by vibrational spectroscopy: possibilities and prospects

This article summarizes the methodological progress that has been made in the vibrational spectroscopy of isolated polynuclear metal oxide clusters, with particular emphasis on free electron laser-based infrared action spectroscopy of gas phase clusters, over the last decade. The possibilities, limitations and prospects of the various experimental approaches are discussed using representative examples from pivotal studies in the field.

Structural, electronic and magnetic effects of Al-doped niobium clusters: a density functional theory study

The application of the ab initio stochastic search procedure with Saunders "kick" method has been carried out for the elucidation of global minimum structures of a series of Al-doped clusters, Nb(n)Al (1 ≤ n ≤ 10). We have studied the structural characters, growth behaviors, electronic and magnetic properties of Nb(n)Al by the density functional theory calculations. Unlike the previous literature reported on Al-doped systems where ground state structures undergo a structural transition from the Al-capped frame to Al-encapsulated structure, we found that Al atom always occupies the surface of Nb(n)Al clusters and structural transition does not take place until n = 10. Note that the fragmentation proceeds preferably by the ejection of an aluminum atom other than niobium atom. According to the natural population analysis, charges always transfer from aluminum to niobium atoms. Furthermore, the magnetic moments of the Nb(n)Al clusters are mainly located on the 4d orbital of niobium atoms, and aluminum atom possesses very small magnetic moments.

Electronic structures, vibrational and thermochemical properties of neutral and charged niobium clusters Nb(n), n = 7-12

Geometric and electronic structures, vibrational properties, and relative stabilities of niobium clusters Nb(n), n = 7-12, are studied using both DFT (BPW91 and M06 functionals) and CCSD(T) calculations with the cc-pVnZ-PP basis set. In each cluster, various lower-lying states are very close in energy in such a way that the ground state cannot be unambiguously established by DFT computations. Nb clusters tend to prefer the lowest possible spin state as the ground state, except for Nb(12) ((3)A(g)). The optimal structure of the cluster at a certain size does not simply grow from that of the smaller one by adding an atom randomly. Instead, the Nb clusters prefer a close-packed growth behavior. Nb(10) has a spherically aromatic character, high chemical hardness and large HOMO-LUMO gap. Electron affinities, ionization energies, binding energy per atom, and the stepwise dissociation energies are evaluated. Energetic properties exhibit odd-even oscillations. Comparison with experimental values shows that both BPW91 and M06 functionals are reliable in predicting the EA and IE values, but the BPW91 is deficient in predicting the binding and dissociation energies. We re-examine in particular the experimental far IR spectra previously recorded using the IR-MPD and free electron laser spectrometric techniques and propose novel assignments for Nb(7) and Nb(9) systems. The IR spectra of the anions are also predicted.

Tetratungsten oxide clusters W4O(n)(-/0) (n = 10-13): structural evolution and chemical bonding

Density functional theory (DFT) calculations are carried out to investigate the electronic and structural properties of a series of tetratungsten oxide clusters, W(4)O(n)(-/0) (n = 10-13). Generalized Koopmans' theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra (PES). A large energy gap (approximately 2.9 eV) is observed for the stoichiometric W(4)O(12) cluster, which reaches the bulk value. The calculations suggest that W(4)O(12)(-/0) have the planar eight-membered ring structures, in which each tungsten atom is tetrahedrally coordinated with two bridging O atoms and two terminal O atoms. W(4)O(10)(-/0) and W(4)O(11)(-) can be viewed as removing two and one terminal O atoms from W(4)O(12)(-/0), respectively. The W(4)O(11) neutral is an interesting species, which possesses the pentabridged structure. We show that W(4)O(11)(-) contains a localized W(3+) site, which can readily react with O(2) to form the W(4)O(13)(-) cluster, whereas the corresponding neutral W(4)O(13) can be viewed as replacing a terminal oxygen in W(4)O(12) by a peroxo O(2) unit. Molecular orbital analyses are performed to analyze the chemical bonding in the tetratungsten oxide clusters and to elucidate their electronic and structural evolution.

Structural evolution, sequential oxidation, and chemical bonding in tritantalum oxide clusters: Ta(3)O(n)(-) and Ta(3)O(n) (n = 1-8)

We report a combined photoelectron spectroscopy (PES) and density functional theory (DFT) study on a series of tritantalum oxide clusters, Ta(3)O(n)(-). Well-resolved PES spectra are obtained for Ta(3)O(n)(-) (n = 1-8) at several detachment photon energies, yielding electronic structure information which is used for comparison with the DFT calculations. A trend of sequential oxidation is observed as a function of O content until Ta(3)O(8)(-), which is a stoichiometric cluster. Extensive DFT calculations are performed in search of the lowest energy structures for both the anions and neutrals. The first three O atoms are shown to successively occupy the bridging sites in the Ta(3) triangle. The next three O atoms each occupy a terminal site, with the seventh and eighth O atoms forming a double-bridge and a double-terminal, respectively. The Ta(3)O(7)(-) anion is found to possess a localized electron pair on a single Ta center, making it an interesting molecular model for Ta(3+) surface sites. Molecular orbital analyses are performed to elucidate the chemical bonding in the Ta(3)O(n)(-) clusters.