Gas-phase Clusters Research Articles

Density functional study of the interaction between small Au clusters, Aun (n=1–7) and the rutile TiO2 surface. II. Adsorption on a partially reduced surface

We use density functional theory to examine the electronic structure of small Au(n) (n=1-7) clusters, supported on a rutile TiO(2)(110) surface having oxygen vacancies on the surface (a partially reduced surface). Except for the monomer, the binding energy of all Au clusters to the partially reduced surface is larger by approximately 0.25 eV than the binding energy to a stoichiometric surface. The bonding site and the orientation of the cluster are controlled by the shape of the highest occupied molecular orbitals (HOMOs) of the free cluster (free cluster means a gas-phase cluster with the same geometry as the supported one). The bond is strong when the lobes of the HOMOs overlap with those of the high-energy states of the clean oxide surface (i.e., with no gold) that have lobes on the bridging and the in-plane oxygen atoms. In other words, the cluster takes a shape and a location that optimizes the contact of its HOMOs with the oxygen atoms. Fivefold coordinated Ti atoms located at a defect site (5c-Ti(*)) participate in the binding only when a protruding lobe of the singly occupied molecular orbital (for odd n) or the lowest unoccupied molecular orbital (for even n) of the free Au(n) cluster points toward a 5c-Ti(*) atom. The oxygen vacancy influences the binding energy of the clusters (except for Au(1)) only when they are in direct contact with the defect. The desorption energy and the total charge on clusters that are close to, but do not overlap with, the vacancy differ little from the values they have when the cluster is adsorbed on a stoichiometric surface. The behavior of Au(1) is rather remarkable. The atom prefers to bind directly to the vacancy site with a binding energy of 1.81 eV. However, it also makes a strong bond (1.21 eV) with any 5c-Ti atom even if that atom is far from the vacancy site. In contrast, the binding of a Au monomer to the 5c-Ti atom of a surface without vacancies is weak (0.45 eV). The presence of the vacancy activates the 5c-Ti atoms by populating states at the bottom of the conduction band. These states are delocalized and have lobes protruding out of the surface at the location of the 5c-Ti atoms. It is the overlap of these lobes with the highest orbital of the Au atom that is the major reason for the bonding to the 5c-Ti atom, no matter how far the latter is from the vacancy. The energy for breaking an adsorbed cluster into two adsorbed fragments is smaller than the kinetic energy of the mass-selected clusters deposited on the surface in experiments. However, this is not sufficient for breaking the cluster upon impact with the surface, since only a fraction of the available energy will go into the reaction coordinate for breakup.

Closed-Cage Clusters in the Gaseous and Condensed Phases Derived from Sonochemically Synthesized MoS 2 Nanoflakes

The Mo(13) clusters we previously reported were derived from MoS(2) flakes prepared from bulk MoS(2), although the nature of the precursor species was not fully understood. The existence of the clusters in the condensed phase was a question. Here we report the preparation of MoS(2) nanoflakes from elemental precursors using the sonochemical method and study the gas-phase clusters derived from them using mass spectrometry. Ultraviolet-visible (UV-vis) spectrum of the precursor is comparable to nano MoS(2) derived from bulk MoS(2). High-resolution transmission electron microscopy (HRTEM) revealed the formation of nanoflakes of MoS(2) with 10- to 30-nm length and 3- to 5-nm thickness. Laser desorption ionization mass spectrometry (LDI-MS) confirmed the formation of Mo(13) clusters from this nanomaterial. Matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) points to the existence of Mo(13) clusters in the condensed phase. The clusters appear to be stable because they do not fragment in the mass spectrometer even at the highest laser intensity. Computational analysis based on generalized Wannier orbitals is used to understand bonding and stability of the clusters. These clusters are highly stable with a rich variety in terms of centricity and multiplicity of Mo-Mo, S-Mo, and S-S bonds.

Photodissociation studies of calcium–coronene and calcium–pyrene cation clusters

Gas-phase cluster cations combining calcium atoms and the polycyclic aromatic hydrocarbons (PAHs) coronene (C 24H 12) and pyrene (C 16H 10) are produced in a molecular beam using laser vaporization in a pulsed nozzle cluster source. Time-of-flight mass spectrometry reveals the formation of clusters of the form Ca x (coronene) y + for up to x = 4 and y = 3 and Ca x (pyrene) y + for up to x = 2 and y = 3. Mass-selected photodissociation studies show that the calcium cation is the most prominent fragment for each system. Photoinduced calcium carbide formation is prominent when two or more calcium atoms are present. Additionally, there is evidence that these clusters can form sandwich structures.

Density functional study of the interaction between small Au clusters, Aun (n=1–7) and the rutile TiO2 surface. I. Adsorption on the stoichiometric surface

This is the first paper in a series of four dealing with the adsorption site, electronic structure, and chemistry of small Au clusters, Au(n) (n=1-7), supported on stoichiometric, partially reduced, or partially hydroxylated rutile TiO(2)(110) surfaces. Analysis of the electronic structure reveals that the main contribution to the binding energy is the overlap between the highest occupied molecular orbitals of Au clusters and the Kohn-Sham orbitals localized on the bridging and the in-plane oxygen of the rutile TiO(2)(110) surface. The structure of adsorbed Au(n) differs from that in the gas phase mostly because the cluster wants to maximize this orbital overlap and to increase the number of Au-O bonds. For example, the equilibrium structures of Au(5) and Au(7) are planar in the gas phase, while the adsorbed Au(5) has a distorted two-dimensional structure and the adsorbed Au(7) is three-dimensional. The dissociation of an adsorbed cluster into two adsorbed fragments is endothermic, for all clusters, by at least 0.8 eV. This does not mean that the gas-phase clusters hitting the surface with kinetic energy greater than 0.8 eV will fragment. To place enough energy in the reaction coordinate for fragmentation, the impact kinetic energy needs to be substantially higher than 0.8 eV. We have also calculated the interaction energy between all pairs of Au clusters. These interactions are small except when a Au monomer is coadsorbed with a Au(n) with odd n. In this case the interaction energy is of the order of 0.7 eV and the two clusters interact through the support even when they are fairly far apart. This happens because the adsorption of a Au(n) cluster places electrons in the states of the bottom of the conduction band and these electrons help the Au monomer to bind to the five-coordinated Ti atoms on the surface.

Adsorption of gold clusters on metal-supported MgO: Correlation to electron affinity of gold

Adsorption of ${\mathrm{Au}}_{N}$ clusters $(N=1\char21{}6)$ on a bulk MgO(001) surface and on an ultrathin (3 ML) MgO(001) film supported by Mo metal is investigated via density-functional theory calculations. Comparison of the two substrates unambiguously shows that the gold clusters adsorbed on $\mathrm{Mg}\mathrm{O}∕\mathrm{Mo}$ turn into singly charged cluster anions ${\mathrm{Au}}_{N}^{\ensuremath{\delta}}$, $\ensuremath{\delta}\ensuremath{\approx}\ensuremath{-}1$. Their structures and internal charging patterns are analogous to singly charged, planar, gas-phase cluster anions ${\mathrm{Au}}_{N}^{\ensuremath{-}}$. The adsorption energy has prominent size-dependent odd-even oscillations, correlating fully with the known electron affinity of gas-phase Au clusters in this size range.

Probing anion–carbon dioxide interactions in room temperature ionic liquids: Gas phase cluster calculations

The interaction of a CO 2 molecule with different anions commonly used in room temperature ionic liquids (RTILs) has been studied under isolated conditions. Minimum energy structures obtained using density functional theory based calculations suggest the dominance of Lewis acid–base interactions between the species. In the optimized configurations, the CO 2 molecule is found to adopt a non-linear geometry. The extent of the consequent split in the CO 2 bending mode calculated from a Hessian analysis agrees qualitatively with vibrational spectroscopic results. The implication of CO 2–anion interaction to the solubility of CO 2 in RTILs is discussed.

Whole Numbers for Half Cells?

CHEMISTRY Electrochemical reduction potentials in solution are generally measured and tabulated relative to a chosen reference reaction at the opposite electrode. Leib et al. have taken a step toward formulation of an absolute scale through gas-phase cluster experiments. They induced collisions

Cluster Phase Chemistry: Collisions of Vibrationally Excited Cationic Dicarboxylic Acid Clusters with Water Molecules Initiate Dissociation of Cluster Components

A homologous series of cationic gas-phase clusters of dicarboxylic acids (oxalic acid, malonic acid, succinic acid, glutaric acid, and adipic acid) generated via electrospray ionization (ESI) are investigated using collision-induced dissociation (CID). Singly charged cationic clusters with the composition (Na(+))(2n+1)(dicarboxylate(2-))(n), where n = 1-5, are observed as major gas-phase species. Significant abundances of singly charged sodiated hydrogen dicarboxylate clusters with the composition (Na(+))(2n)(dicarboxylate(2-))(n)(H+), where n = 1-6, are observed with oxalic acid, malonic acid, and succinic acid. Isolation of the clusters followed by CID results mainly in sequential loss of disodium dicarboxylate moieties for the clusters of succinic acid, glutaric acid, and adipic acid. However, the dimer of sodiated hydrogen succinate, all malonate clusters, and all oxalate clusters, with the exception of the dimer, exhibit complex chemical reactions initiated by the collision of vibrationally excited clusters with water molecules. Generally, water molecules serve as proton donors for reacting dicarboxylate anions in the cluster, initiating dissociation pathways such as the decomposition of the malonate ion to yield an acetate ion and CO(2). The reactivity of several mixed dicarboxylate clusters is also reported. For example, malonate anion is shown to be more reactive than oxalate anion for decarboxylation when both are present in a cluster. The energetics of several representative cluster phase reactions are evaluated using computational modeling. The present results for cationic clusters are compared and contrasted to earlier studies of anionic sodiated dicarboxylic acid clusters.

The Solvation of Cu2+ with Gas-Phase Clusters of Water and Ammonia

A detailed study has been undertaken of the gas-phase chemistry of [Cu(H2O)N]2+ and [Cu(NH3)N]2+ complexes. Ion intensity distributions and fragmentation pathways (unimolecular and collision-induced) have been recorded for both complexes out as far as N=20. Unimolecular fragmentation is dominated by Coulomb explosion (separation into two single charged units) on the part of the smaller ions, but switches to neutral molecule loss for N>7. In contrast, collisional activation promotes extensive electron capture from the collision gas, with the appearance of particular singly charged fragment ions being sensitive to the size and composition of the precursor. The results show clear evidence of the unit [Cu(X)8]2+ being of special significance, and it is proposed that the hydrogen-bonded structure associated with this ion is responsible for stabilizing the dipositive charge on Cu2+ in aqueous solution.

Vacuum-Ultraviolet Photoionization Studies of the Microhydration of DNA Bases (Guanine, Cytosine, Adenine, and Thymine)

In this work, we report on a photoionization study of the microhydration of the four DNA bases. Gas-phase clusters of water with DNA bases [guanine (G), cytosine (C), adenine (A), and thymine (T)] are generated via thermal vaporization of the bases and expansion of the resultant vapor in a continuous supersonic jet expansion of water seeded in Ar. The resulting clusters are investigated by single-photon ionization with tunable vacuum-ultraviolet synchrotron radiation and mass analyzed using reflectron mass spectrometry. Photoionization efficiency (PIE) curves are recorded for the DNA bases and the following water (W) clusters: G, GWn (n = 1-3); C, CWn (n = 1-3); A, AWn (n = 1,2); and T, TWn (n = 1-3). Appearance energies (AE) are derived from the onset of these PIE curves (all energies in eV): G (8.1 +/- 0.1), GW (8.0 +/- 0.1), GW2 (8.0 +/- 0.1), and GW3 (8.0); C (8.65 +/- 0.05), CW (8.45 +/- 0.05), CW2 (8.4 +/- 0.1), and CW3 (8.3 +/- 0.1); A (8.30 +/- 0.05), AW (8.20 +/- 0.05), and AW2 (8.1 +/- 0.1); T (8.90 +/- 0.05); and TW (8.75 +/- 0.05), TW2 (8.6 +/- 0.1), and TW3 (8.6 +/- 0.1). The AEs of the DNA bases decrease slightly with the addition of water molecules (up to three) but do not converge to values found for photoinduced electron removal from DNA bases in solution.

Energy and Structure of Copper Clusters(n=2-70,147,500) Studied by the Monte Carlo Method

The Monte Carlo(MC) and Embedded-Atoms Method(EAM) potential are employed to investigate the structures and binding energies of copper clusters. Three results are obtained: first, the copper clusters are formed by gradually increasing atoms to icosahedral surface when the temperature is 300K and the number of atoms is from 2 to 70; second, the higher the symmetries are, the more stable the structures of copper clusters are; third, copper clusters tend to be stereo construction but not planer construction when the number of atoms is less. The structure of Cu147 is three-fly-icosahedron when the system of 147 atoms reached equilibrium at 300K. And at 700K, all 500 atoms form clusters in gas phase.

Thermochemical stabilities of the gas-phase cluster ions of halide ions with rare gas atoms

Gas-phase clustering reactions of halide ions (X −) with rare gas atoms (Rg) have been investigated using a pulsed-electron beam high-pressure mass spectrometer. The thermochemical stability of the cluster ions X −(Rg) n has been determined. To evaluate bond energies that are difficult to obtain experimentally, ab initio calculations were also carried out. Among the cluster ions investigated, the interaction between the hardest F − ion and the softest Xe atom was found to be strongest (6.3 kcal/mol).

Characterizing the intrinsic stability of gas-phase clusters of transition metal complex dianions with alkali metal counterions: Counterion perturbation of multiply charged anions

The authors report the gas-phase generation and characterization of a series of cation-dianion clusters, e.g., M(+).PtCl(6) (2-), M(+).PtCl(4) (2-), M(+).Pt(CN)(6) (2-), and M(+).Pd(CN)(4) (2-), where M(+)=Na(+),K(+),Rb(+), as model systems for investigating gas-phase contact ionpairs. Low-energy collisional excitation of these systems isolated within a quadrupole ion trap reveals that the fragmentation products are determined by the dianion and are independent of the counterion. This indicates that cation-dianion clusters represent gaseous ion-pair complexes, in line with recent findings for K(+).Pt(CN)(n) (2-), n=4,6 [Burke et al., J. Chem. Phys. 125, 021105 (2006)]. The relative fragmentation energies of several cation-dianion systems are obtained as a function of the counterion to explore the nature of ion-pair binding. For most of the systems studied, e.g., M(+).PtCl(6) (2-), the fragmentation energy increases as the cation size decreases, in line with a simple electrostatic description of the cation-dianion binding. However, the M(+).Pt(CN)(4) (2-) clusters displayed the reverse trend with the fragmentation energy increasing as the cation size increases. Density functional theory calculations of the cation-dianion fragmentation potential energy surfaces reveal the existence of a novel double-minima surface, separated by a repulsive Coulomb barrierlike feature at short range. The experimentally observed trends in the fragmentation energies can be fully understood with reference to the computed surfaces, hence providing strong support for the existence of the double-minima surface.

Gas phase infrared spectra of SO 2 clusters and quantum chemical calculations of some stable isomers

We have observed infrared spectra of the SO 2 clusters in gas phase for the first time by cavity ring-down spectroscopy, and assigned absorption bands of dimer and larger clusters in the O S O asymmetric stretching region. An analysis of the rotational contour showed that the dimeric band can be assigned to the SO 2 vibration in an electron-donor side of the C s-isomer of (SO 2) 2 observed by Fourier-transform microwave spectroscopy [A. Taleb-Bendiab et al., J. Chem. Phys. 94 (1991) 6956.]. The observed band type and wavenumber shift from monomer was consistent with the results of quantum chemical calculations.

Searching for Stable, Five-Coordinate Aquated Al(III) Species. Water Exchange Mechanism and Effect of pH

Density functional theory calculations have been performed for the water exchange mechanism of aquated Al(III). The effect of pH was considered by studying the exchange processes for [Al(H2O)6]3+ and its conjugated base, [Al(H2O)5OH]2+. Both complexes were found to exchange water in a dissociative way with activation energies (EA) of 15.9 and 10.2 kcal/mol, respectively. The influence of solvent molecules on the gas-phase cluster model was considered by the addition of up to four water molecules to the model system. The stabilizing effect of the solvent on the transition state decreases EA to 8.6 (hexa-aqua complex) and 7.6 (monohydroxo complex) kcal/mol, whereas EA for all hydroxo species is consistently significantly lower than those for the related aqua systems, which indicates a much faster water exchange rate. For the hydroxo complex, all calculated five-coordinate intermediates, nH2O.[Al(H2O)4(OH)]2+ (n = 1, 2, 3, 4, 5), are more stable than the corresponding six-coordinate reactants. Our results therefore suggest the presence of a stable five-coordinate species of aquated Al(III), namely, the [Al(H2O)4(OH)]2+ complex.

Gas-Phase Clustering of NO+ with H2S and H2O

The complex NO+·H2S, which is assumed to be an intermediate in acid rain formation, exhibits thermodynamic stability of ∆Hº300 = -76 kJ mol-1, or ∆Gº300 = -47 kJ mol-1. Its further transformation via H-transfer is associated with rather high barriers. One of the conceivable routes to lower the energy of the transition state is the action of additional solvent molecule(s) that can mediate proton transfer. We have studied several NO+·H2S structures with one or two additional water molecule(s) and have found stable structures (local minima), intermediates and saddle points for the three-body NO+·H2S·H2O and four-body NO+·H2S·(H2O)2 clusters. The hydrogen bonds network in the four-body cluster plays a crucial role in its conversion to thionitrous acid.

Single-ion solvation free energies and the normal hydrogen electrode potential in methanol, acetonitrile, and dimethyl sulfoxide.

The division of thermodynamic solvation free energies of electrolytes into contributions from individual ionic constituents is conventionally accomplished by using the single-ion solvation free energy of one reference ion, conventionally the proton, to set the single-ion scales. Thus, the determination of the free energy of solvation of the proton in various solvents is a fundamental issue of central importance in solution chemistry. In the present article, relative solvation free energies of ions and ion-solvent clusters in methanol, acetonitrile, and dimethyl sulfoxide (DMSO) have been determined using a combination of experimental and theoretical gas-phase free energies of formation, solution-phase reduction potentials and acid dissociation constants, and gas-phase clustering free energies. Applying the cluster pair approximation to differences between these relative solvation free energies leads to values of -263.5, -260.2, and -273.3 kcal/mol for the absolute solvation free energy of the proton in methanol, acetonitrile, and DMSO, respectively. The final absolute proton solvation free energies are used to assign absolute values for the normal hydrogen electrode potential and the solvation free energies of other single ions in the solvents mentioned above.

Determination of bond dissociation energies using mass spectrometry

AbstractFourier transform ion cyclotron resonance mass spectrometry (FT‐ICR/MS) offers the opportunity for gas phase cluster formation reactions at very low pressures and at temperatures that are different from room temperature. Reactions take place with single positive‐charge metal ions that are normally +2, +3, +4, etc., charged in solution. The ions formed are detected by measuring the current induced by their cyclotron rotation, but they cannot be physically separated and collected. Collision‐induced dissociation (CID) is widely used for ion‐structure determination via the fragmentation of the excited ions. CID study aims to determine the relationship between the Vpp [peak‐to‐peak voltage of the radiofrequency (rf) pulse] and the mass‐to‐charge (m/z) ratio, which will be used for the calculation of the center‐of‐mass translational kinetic energy (Ekcm) of the excited ion under investigation. CID studies are restricted to stable ions with relatively high abundance. Nevertheless, with the evolution of computational chemistry, such problems can be overcome whereby CID calculations will be used to provide the substantial parameters for computer software, such as the Gaussian 03 program, for the structure determination of the less stable NixS anions. The latter constitutes the core for our current research. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Multiple valence superatoms

We recently demonstrated that, in gas phase clusters containing aluminum and iodine atoms, an Al(13) cluster behaves like a halogen atom, whereas an Al(14) cluster exhibits properties analogous to an alkaline earth atom. These observations, together with our findings that Al(13)(-) is inert like a rare gas atom, have reinforced the idea that chosen clusters can exhibit chemical behaviors reminiscent of atoms in the periodic table, offering the exciting prospect of a new dimension of the periodic table formed by cluster elements, called superatoms. As the behavior of clusters can be controlled by size and composition, the superatoms offer the potential to create unique compounds with tailored properties. In this article, we provide evidence of an additional class of superatoms, namely Al(7)(-), that exhibit multiple valences, like some of the elements in the periodic table, and hence have the potential to form stable compounds when combined with other atoms. These findings support the contention that there should be no limitation in finding clusters, which mimic virtually all members of the periodic table.

Can the pH value of water solutions be estimated by quantum chemical calculations of small water clusters?

In the study, various water clusters were explored from the point of view of the proton transfer between H-bonded neighbors. A relatively modest approach--the MP26-31++G(d,p) level--was chosen as acceptable considering the fact that also larger systems have to be included. The tight ion-pair model (with usually three fixed O-O distances) was adopted for the autodissociation process. First, cluster-estimated pH values rapidly decrease as cluster size increases from 2 to 6. For larger clusters in gas phase, the topology of H bonds plays an important role, varying pH from 7 to 13 in hexamers and from 5 to 15 in octamer clusters. The relationship energy/distance was quantified, too. Enhancing our model with the conductorlike screening model (COSMO) approach brought significant improvement in description of the autodissociation reaction with a stable zwitterionic structure. However, when the O-O restrictions were released, the small barrier for backward reaction disappeared, reforming neutral cluster spontaneously. Also Boltzmann weighting procedure was applied, and for the explored clusters in vacuo, the series of pH 25-18-14-13-10 was obtained for cluster sizes n=2, 3, 4, 6, 8. Using the COSMO approach, the analogous series is 15-14-12-10-9. The limit of the series is still about two to three units above the experimentally known pH. In order to reach the size-independent (bulk) value, larger clusters are needed. However, the situation is far from hopeless since (as it was proven in the study) four-coordinated molecules are not involved in the proton transfer process directly; they can only be a part of the surrounding environment.