A density functional study of small neutral and cationic vanadium clusters Vn and Vn+ (n=2–9)

Probing adsorption of methane onto vanadium cluster cations via vibrational spectroscopy

Photofragment spectroscopy is used to measure the vibrational spectra of V2+(CH4)n (n = 1-4), V3+(CH4)n (n = 1-3), and Vx+(CH4) (x = 4-8) in the C-H stretching region (2550-3100 cm-1). Spectra are measured by monitoring loss of CH4. The experimental spectra are compared to simulations at the B3LYP+D3/6-311++G(3df,3pd) level of theory to identify the geometry of the ions. Multi-reference configuration interaction with Davidson correction (MRCI+Q) calculations are also carried out on V2+ and V3+. The methane binding orientation in V2+(CH4)n (n = 1-4) evolves from η3 to η2 as more methane molecules are added. The IR spectra of metal-methane clusters can give information on the structure of metal clusters that may otherwise be hard to obtain from isolated clusters. For example, the V3+(CH4)n (n = 1-3) experimental spectra show an additional peak as the second and third methane molecules are added to V3+, which indicates that the metal atoms are not equivalent. The Vx+(CH4) show a larger red shift in the symmetric C-H stretch for larger clusters with x = 5-8 than for the small clusters with x = 2, 3, indicating increased covalency in the interaction of larger vanadium clusters with methane.

Size-Specific Infrared Spectroscopic Study of the Reactions between Water Molecules and Neutral Vanadium Dimer: Evidence for Water Splitting

Investigation of the reactions between water molecules and neutral metal clusters is important in water splitting but is very challenging due to the inherent difficulty of size selection. Here, we report a size-specific infrared-vacuum ultraviolet spectroscopic study on the reactions of water with neutral vanadium dimer. The V₂O₃H₄ and V₂O₄H₆ products were characterized to have unexpected V₂(μ₂-OH)(μ₂-H)(η¹-OH)₂ and V₂(μ₂-OH)₂(η¹-H)₂(η¹-OH)₂ structures, indicative of a water decomposition. A combination of theory and experiment reveals that the water splitting by V₂ is both thermodynamically exothermic and kinetically facile in the gas phase. The present system serves as a model for clarifying the pivotal roles played by neutral metal clusters in water decomposition and also opens new avenues toward systematic understanding of water splitting by a large variety of single-cluster catalysts.

Ionization Energies and Ground-State Structures of Neutral Lan (n = 2-14) Clusters: A Combined Experimental and Theoretical Investigation

Neutral lanthanum clusters are studied by photoionization time-of-flight mass spectroscopy, laser threshold photoionization spectroscopy, and density functional theory (DFT). Mass abundance spectra (MS) registered at multiple photoionization wavelengths in the range of 195-230 nm by single photon ionization reveal the production of all sizes, La_n (n ≥ 50), in good abundance, nullifying previously predicted low abundances for certain sizes in the 3-14 size range. Also, the MS do not reveal the extraordinary stability of any specific size, as one would expect, from previous theoretical predictions of 7- and 13-atom clusters as magic. Ionization energies (IEs) are measured for La_n (n = 2-14) clusters. DFT has been used to determine the stable geometric isomers for 2- to 10-atom clusters and to calculate their IEs. The theoretical IEs of 2-7 atom clusters are in decent agreement with their experimental values; however, the theoretical IEs are somewhat lower by ∼0.4 eV for n ≥ 8 than their experimental IEs.

Hydrogen release from a single water molecule on Vn+ (3 ≤ n ≤ 30)

Water and its interactions with metals are closely bound up with human life, and the reactivity of metal clusters with water is of fundamental importance for the understanding of hydrogen generation. Here a prominent hydrogen evolution reaction (HER) of single water molecule on vanadium clusters V_n⁺ (3 ≤ n ≤ 30) is observed in the reaction of cationic vanadium clusters with water at room temperature. The combined experimental and theoretical studies reveal that the wagging vibrations of a V-OH group give rise to readily formed V-O-V intermediate states on V_n⁺ (n ≥ 3) clusters and allow the terminal hydrogen to interact with an adsorbed hydrogen atom, enabling hydrogen release. The presence of three metal atoms reduces the energy barrier of the rate-determining step, giving rise to an effective production of hydrogen from single water molecules. This mechanism differs from dissociative chemisorption of multiple water molecules on aluminium cluster anions, which usually proceeds by dissociative chemisorption of at least two water molecules at multiple surface sites followed by a recombination of the adsorbed hydrogen atoms.

Furthering the reaction mechanism of cationic vanadium clusters towards oxygen

We prepared well-resolved V_n⁺ clusters and clarified the reactivity with oxygen involving both etching effect and building block addition.

Probing the Structural, Electronic, and Magnetic Properties of Ag n V (n = 1-12) Clusters

The structural, electronic, and magnetic properties of Ag _n V (n = 1-12) clusters have been studied using density functional theory and CALYPSO structure searching method. Geometry optimizations manifest that a vanadium atom in low-energy Ag_nV clusters favors the most highly coordinated location. The substitution of one V atom for an Ag atom in Ag _n + 1 (n ≥ 5) cluster modifies the lowest energy structure of the host cluster. The infrared spectra, Raman spectra, and photoelectron spectra of Ag _n V (n = 1-12) clusters are simulated and can be used to determine the most stable structure in the future. The relative stability, dissociation channel, and chemical activity of the ground states are analyzed through atomic averaged binding energy, dissociation energy, and energy gap. It is found that V atom can improve the stability of the host cluster, Ag₂ excepted. The most possible dissociation channels are Ag _n V = Ag + Ag _n - 1V for n = 1 and 4-12 and Ag _n V = Ag₂ + Ag _n - 2V for n = 2 and 3. The energy gap of Ag _n V cluster with odd n is much smaller than that of Ag _n + 1 cluster. Analyses of magnetic property indicate that the total magnetic moment of Ag _n V cluster mostly comes from V atom and varies from 1 to 5 μ _B. The charge transfer between V and Ag atoms should be responsible for the change of magnetic moment.

Exploration of the Structural, Electronic and Tunable Magnetic Properties of Cu₄M (M = Sc-Ni) Clusters

The structural, electronic and magnetic properties of Cu₄M (M = Sc-Ni) clusters have been studied by using density functional theory, together with an unbiased CALYPSO structure searching method. Geometry optimizations indicate that M atoms in the ground state Cu₄M clusters favor the most highly coordinated position. The geometry of Cu₄M clusters is similar to that of the Cu₅ cluster. The infrared spectra, Raman spectra and photoelectron spectra are predicted and can be used to identify the ground state in the future. The relative stability and chemical activity are investigated by means of the averaged binding energy, dissociation energy and energy level gap. It is found that the dopant atoms except for Cr and Mn can enhance the stability of the host cluster. The chemical activity of all Cu₄M clusters is lower than that of Cu₅ cluster whose energy level gap is in agreement with available experimental finding. The magnetism calculations show that the total magnetic moment of Cu₄M cluster mainly come from M atom and vary from 1 to 5 μ_B by substituting a Cu atom in Cu₅ cluster with different transition-metal atoms.

Consecutive reactions of small, free tantalum clusters with dioxygen controlled by relaxation dynamics

The reaction of small cationic tantalum clusters (Ta_n⁺, n = 4–8) with molecular oxygen is studied under multi-collision conditions in the gas phase, and the reaction kinetics are analyzed in order to elucidate underlying mechanisms.

Insights into the structural, electronic and magnetic properties of V-doped copper clusters: comparison with pure copper clusters

The structural, electronic and magnetic properties of Cu_n+1 and Cu_nV (n = 1–12) clusters have been investigated by using density functional theory. The growth behaviors reveal that V atom in low-energy Cu_nV isomer favors the most highly coordinated position and changes the geometry of the three-dimensional host clusters. The vibrational spectra are predicted and can be used to identify the ground state. The relative stability and chemical activity of the ground states are analyzed through the binding energy per atom, energy second-order difference and energy gap. It is found that that the stability of Cu_nV (n ≥ 8) is higher than that of Cu_n+1. The substitution of a V atom for a Cu atom in copper clusters alters the odd-even oscillations of stability and activity of the host clusters. The vertical ionization potential, electron affinity and photoelectron spectrum are calculated and simulated for all of the most stable clusters. Compare with the experimental data, we determine the ground states of pure copper clusters. The magnetism analyses show that the magnetic moments of Cu_nV clusters are mainly localized on the V atom and decease with the increase of cluster size. The magnetic change is closely related to the charge transfer between V and Cu atoms.

Low-energy excited states of divanadium: a matrix isolation and MRCI study

The ground and excited electronic states of the vanadium dimer (V2) have been studied using Ne matrix isolation experiments and quantum chemical calculations (multireference configuration interaction based on complete active space self-consistent orbitals). In the near infrared absorption spectrum, two vibrational progressions of a new electronic term with a large number of members have been observed with the origin at 1.08 eV and a fundamental vibrational quantum of 475 cm(-1). With the aid of calculations, it has been assigned to a (3)Πu electronic term. The calculations yield potential energy curves for a large number of singlet, triplet, and quintet electronic terms.

Gas-phase perspective on the thermodynamics and kinetics of heterogeneous catalysis

Gas-phase studies of small transition metal cluster cations provide thermochemistry of utility to surface science and heterogeneous catalysis.

Structures and magnetism of multinuclear vanadium-pentacene sandwich clusters and their 1D molecular wires

Two types of multinuclear sandwich clusters, (V(3))(n)Pen(n+1), (V(4))(n)Pen(n+1) (Pen = Pentacene; n = 1, 2), and their corresponding infinite one-dimensional (1D) molecular wires ([V(3)Pen](∞), [V(4)Pen](∞)) are investigated theoretically, especially on their magnetic coupling mechanism. These sandwich clusters and molecular wires are found to be of high stability and exhibit intriguing magnetic properties. The intra-layered V atoms in (V(3))(n)Pen(n+1) clusters prefer antiferromagnetic (AFM) coupling, while they can be either ferromagnetic (FM) or AFM coupling in (V(4))(n)Pen(n+1) depending on the intra-layered V-V distances via direct exchange or superexchange mechanism. The inter-layered V atoms favor FM coupling in (V(3))(2)Pen(3), whereas they are AFM coupled in (V(4))(2)Pen(3). Such magnetic behaviors are the consequence of the competition between direct exchange and superexchange interactions among inter-layered V atoms. In contrast, the 1D molecular wires, [V(3)Pen](∞) and [V(4)Pen](∞), appear to be FM metallic with ultra high magnetic moments of 6.8 and 4.0 μ(B) per unit cell respectively, suggesting that they can be served as good candidates for molecular magnets.

Reactivity of group 5 bielement clusters with H2

Bielement metal clusters composed of the group 5 elements, A(n)B(m) (A, B = V, Nb, Ta), were prepared in the gas phase over the wide n and m ranges. We measured reactivities of the attachment reaction of the neutral clusters to H(2), which were plotted as a function of a map with n and m. The clusters with n + m = 4 and 5 were found to be more reactive than the other clusters. The measurement of the ionization energies reveals that there is no strong correlation between the reactivity and the ionization energy. In addition, the reactivities of the cations, A(n)B(m)(+), were also highest at n + m = 4 and 5. These findings suggest that the electronic structure does not totally determine the reactivity of the neutral clusters. On the other hand, theoretical calculations for A(n)B(m) (n + m = 4) reported by Metha et al. showed that the optimized geometrical structures of the congener group 5 bielement clusters do not change significantly by changing n and m within n + m = 4. Hence, it is highly likely that the pyramidal (n + m = 4) and bipyramidal (n + m = 5) structures of A(n)B(m) determine the high reactivity of the clusters.

Dynamics of Clusters Initiated by Photon and Surface Impact

Clusters of atoms/molecules show dynamics characteristic of the method of excitation. Two contrasted processes are discussed: (1) electronic excitation via single-photon absorption and (2) impulsive excitation of nuclear motions by surface impact. Process 1 is exemplified by photodissociation dynamics of size-selected metal cluster ions. The electronic energy is converted most likely to vibrational energy of internal modes; dissociation follows via statistical mechanism to produce energetically favored fragments. Exceptionally, a silver cluster ion, Ag4(+), is shown to undergo nonstatistical dissociation along the potential-energy surface of the excited state. Energy partitioning to translational and vibrational modes of fragments is analyzed as well as bond dissociation energies. Furthermore, the spectrum of the photodissociation yield provides electronic and geometrical structures of a cluster with the aid of ab initio calculations; manganese, Mn(N)(+), and chromium, Cr(N)(+), cluster ions are discussed, where the importance of magnetic interactions is manifested. On the other hand, momentum transfer upon surface impact plays a role in process 2. An impulsive mechanical force triggers extraordinary chemical processes distinct from those initiated by atomic collision as well as photoexcitation. Experiments on aluminum, Al(N)(-), silicon, SiN(-), and solvated, I(2)(-)(CO2)(N), cluster anions provide evidence for reactions proceeding under extremely high temperatures, such as pickup of surface atoms, annealing of products, and mechanical splitting of chemical bonds. In addition, a model experiment to visualize and time-resolve the cluster impact process is performed by using a micrometer-sized liquid droplet. Multiphoton absorption initiates superheating of the droplet surface followed by a shock wave and disintegration into a number of small fragments (shattering). These studies further reveal how the nature of chemical bonds influences the dynamics of clusters.

Coexistence of ferroelectricity and ferromagnetism in tantalum clusters

The atomic and electronic structures of Ta(N) (N=2-23) clusters have been determined in the framework of pseudopotential density-functional calculations, based upon an unbiased global search with guided simulated annealing to an empirical potential. It is found that the ground-state structures of Ta(N) are very similar to those of Nb(N), showing no preference for the icosahedral growth. Also, a size- and structure-dependent ferroelectricity is found in these tantalum clusters. More importantly, it is found that the ferroelectricity and ferromagnetism can coexist in the homogeneous transition-metal cluster, offering a possibility to obtain a new type of "multiferroic" materials composed of the clusters. Finally, the far-infrared spectroscopy is suggested to be an efficient tool to distinguish the ferroelectric clusters.

Geometrical and electronic structures of AumAgn (2⩽m+n⩽8)

The structural and electronic properties of Au(m)Ag(n) binary clusters (2 < or = m + n < or = 8) have been investigated by density functional theory with relativistic effective core potentials. The results indicate that Au atoms tend to occupy the surface of Au(m)Ag(n) clusters (n > or = 2 and m > or = 2). As a result, segregation of small or big bimetallic clusters can be explained according to the atomic mass. The binding energies of the most stable Au(m)Ag(n) clusters increase with increasing m+n. The vertical ionization potentials of the most stable Au(m)Ag(n) clusters show odd-even oscillations with changing m+n. The possible dissociation channels of the clusters considered are also discussed.

Preparing transition-metal clusters in known structural forms: The mass-analyzed threshold ionization spectrum of V3

The first results are presented of a new experiment designed both to generate and characterize spectroscopically individual isomers of transition-metal cluster cations. As a proof of concept the one-photon mass-analyzed threshold ionization (MATI) spectrum of V3 has been recorded in the region of 44,000-45,000 cm-1. This study extends the range of a previous zero-kinetic-energy (ZEKE) photoelectron study of Yang et al. [Chem. Phys. Lett. 231, 177 (1994)] with which the current results are compared. The MATI spectra reported here exhibit surprisingly high resolution (0.2 cm-1) for this technique despite the use of large discrimination and extraction fields. Analysis of the rotational profile of the origin band allows assignment of the V3 ground state as and the V3+ ground state as , both with D3h geometry, in agreement with the density-functional theory study of the V3 ZEKE spectrum by Calaminici et al. [J. Chem. Phys. 114, 4036 (2001)]. There is also some evidence in the spectrum of transitions to the low-lying excited state of the ion. The vibrational structure observed in the MATI spectrum is, however, significantly different to and less extensive than that predicted in the density-functional theory study. Possible reasons for the discrepancies are discussed and an alternative assignment is proposed which results in revised values for the vibrational wave numbers of both the neutral and ionic states. These studies demonstrate the efficient generation of cluster ions in known structural (isomeric) forms and pave the way for the study of cluster reactivity as a function of geometrical structure.

Homonuclear transition-metal trimers

Density-functional theory has been used to determine the ground-state geometries and electronic states for homonuclear transition-metal trimers constrained to equilateral triangle geometries. This represents the first application of consistent theoretical methods to all of the ten 3d block transition-metal trimers, from scandium to zinc. A search of the potential surfaces yields the following electronic ground states and bond lengths: Sc3(2A1',2.83 A), Ti3(7E',2.32 A), V3(2E",2.06 A), Cr3(17E',2.92 A), Mn3(16A2',2.73 A), Fe3(11E",2.24 A), Co3(6E",2.18 A), Ni3(3A2",2.23 A), Cu3(2E',2.37 A), and Zn3(1A1',2.93 A). Vibrational frequencies, several low-lying electronic states, and trends in bond lengths and atomization energies are discussed. The predicted dissociation energies DeltaE(M3-->M2+M) are 49.4 kcal mol(-1)(Sc3), 64.3 kcal mol(-1)(Ti3), 60.7 kcal mol(-1)(V3), 11.5 kcal mol(-1)(Cr3), 32.4 kcal mol(-1)(Mn3), 61.5 kcal mol(-1)(Fe3), 78.0 kcal mol(-1)(Co3), 86.1 kcal mol(-1)(Ni3), 26.8 kcal mol(-1)(Cu3), and 4.5 kcal mol(-1)(Zn3).

Identification, Structure, and Spectroscopy of Neutral Vanadium Oxide Clusters

Neutral vanadium oxide clusters are studied by photoionization time-of-flight (TOF) mass spectroscopy, electronic spectroscopy, and density functional theory (DFT) calculations. Mass spectra of vanadium oxide clusters are observed by photoionization with lasers of three different wavelengths: 118, 193, and 355 nm. Mechanisms of 118 nm single photon ionization and 193 and 355 nm multiphoton ionization/fragmentation of vanadium oxide clusters are discussed on the basis of observed mass spectral patterns and line widths of the mass spectral features. Only the 118 nm laser light can ionize vanadium oxide neutral species by single photon ionization without fragmentation. The stable vanadium oxide neutral clusters under saturated oxygen growth conditions are found to be of the form (VO2)x(V2O5)y. Structures of the first few members of this series of clusters are determined through high level DFT calculations. Fragmentation of this series of clusters through 355 and 193 nm multiphoton ionization processes is discussed in light of these calculated structures. The B(2)B2 <-- X(2)A1 transition is observed for the VO2 neutral species, and nu1 and nu2 vibrations are assigned for both electronic states. From this spectrum, the VO2 rotational and vibrational temperatures are found to be approximately 50 and approximately 700 K, respectively.

Structure determination of small vanadium clusters by density-functional theory in comparison with experimental far-infrared spectra

The far-infrared vibrational spectra for charged vanadium clusters with sizes of 3-15 atoms have been measured using infrared multiple photon dissociation of Vn+Ar-->Vn(+)+Ar. Using density-functional theory calculations, we calculated the ground state energy and vibrational spectra for a large number of stable and metastable geometries of such clusters. Comparison of the calculated vibrational spectra with those obtained in the experiment allows us to deduce the cluster size specific atomic structures. In several cases, a unique atomic structure can be identified, while in other cases our calculations suggest the presence of multiple isomers.

Ab initio calculations for the photoelectron spectra of vanadium clusters

We report ab initio calculations for the electronic and structural properties of V(n), V(n) (-), and V(n) (+) clusters up to n=8. We performed the calculations using a real-space pseudopotential method based on the local spin density approximation for exchange and correlation. This method assumes no explicit basis. Wave functions are evaluated on a uniform grid; only one parameter, the grid spacing, is used to control convergence of the electronic properties. Charged states are easily handled in real space, in contrast to methods based on supercells where Coulombic divergences require special handling. For each size and charge state, we find the lowest energy structure. Our results for the photoelectron spectra, using the optimized structure, agree well with those obtained by experiment. We also obtain satisfactory agreement with the measured ionization potential and electron affinity, and compare our results to calculations using an explicit basis.

Structure determination of isolated metal clusters via far-infrared spectroscopy

We present a new method for the size selective structure determination of small isolated metal clusters in the gas phase. The technique is applied to cationic vanadium clusters containing 6 to 23 atoms, whose far infrared absorption spectra are measured in the 140-450 cm(-1) spectral range. The spectra are unique for each cluster size and are true fingerprints of the cluster's structure. By comparing the experimental spectra to spectra obtained from density-functional theory, the geometric cluster structure can be identified.

A density-functional study of Al-doped Ti clusters: TinAl (n=1–13)

Equilibrium geometries, stabilities, and electronic properties of TinAl (n = 1-13) clusters have been studied by using density-functional theory with local spin density approximation and generalized gradient approximation. The ground-state structures of TinAl clusters have been obtained. The resulting geometries show that the aluminum atom remains on the surface of clusters for n < 9, but is slowly getting trapped beyond n = 9, meanwhile, the Al atom exhibits a valent transition from monovalent to trivalent. The geometric effects and electronic effects clearly demonstrate the Ti4Al cluster to be endowed with special stability. The studies on the bonds indicate the change from ionic to metalliclike.

DFT Calculations on Group 5 Mixed Metal Tetramers: TaxNbyVz (x + y + z = 4)

We report Density Functional Theory (DFT) calculations on mixed-metal tetramers comprised of the Group 5 (Vb) elements V, Nb, and Ta. Our results show that the lowest energy structures for Nb4 and Ta4 are regular tetrahedra with Td symmetry and singlet multiplicity whereas V4 is a triplet state with C2v symmetry. The monosubstituted isomers, A3B, all have C3v symmetry but several higher energy Cs structures have been found that are approximately 100 kJ mol−1 higher in energy. The disubstituted isomers all posses arachno-butterfly structures; the A2B2 types with C2v symmetry and the A2BC types with Cs symmetry. However, the relative openness of the arachno structures is found to be specific to the composition of the mixed-metal cluster.

Reactions and thermochemistry of small transition metal cluster ions

This review discusses the reactivities and thermodynamics of small-size-specific transition metal clusters and focuses on thermodynamic information, which has not been comprehensively discussed before. Because of this focus, guided-ion-beam mass spectrometry was used to acquire much of the data. The details of this technique and the associated data analysis methods are provided. Results on the stabilities of bare transition metal clusters are provided for neutral, cationic, and anionic species. Implications for the electronic and geometrical structures are discussed, as well as the extrapolation of these values to bulk phase behavior. Detailed results for reactions of transition metal clusters with D2 and the oxygen donors O2 and CO2 are reviewed. Available bond energies between size-specific clusters and one D atom and one and two O atoms are compiled, and their implications are evaluated and favorably compared with bulk phase analogs. Several additional thermodynamic studies of various cluster systems are also discussed.