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Cross sections and rate coefficients for sulfur monoxide (SO) + H2 collisions are calculated using a full six-dimensional (6D) potential energy surface (PES). The coupled states (CS) approximation is used to compute fine-structure resolved cross sections for rovibrational transitions between states with v = 0-2, where v is the vibrational quantum number of the SO molecule. The CS calculations for Δv = 1 are benchmarked against close-coupling (CC) results for spin-free interactions. For Δv = 0, the present fine-structure resolved CS results are benchmarked against existing CC results obtained with a rigid rotor approximation. In both cases, the agreement is found to be satisfactory, which suggests that the present results may provide reliable estimates for fine-structure resolved rovibrational transitions. These estimates are the first of their kind based on a full 6D PES. Rate coefficients are reported for temperatures between 10 K and 3000 K for both para- and ortho-H2 colliders. A comparison of the para-H2 rates with mass-scaled results for He shows substantial differences that may be important in astrophysical models.

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Density functional theory was used to study dissociative chemisorption and desorption on Pd x Ni y (x + y = 6) bimetallic clusters. The H2 dissociative chemisorption energies and the H desorption energies at full H saturation were computed. It was found that bimetallic clusters tend to have higher chemisorption energy than pure clusters, and the capacity of Pd3Ni3 and Pd2Ni4 clusters to adsorb H atoms is substantially higher than that of other clusters. The H desorption energies of Pd3Ni3 and Pd2Ni4 are also lower than that of the Pd6 cluster and comparable to that of the Ni6 cluster, indicating that it is easier to pull the H atom out of these bimetallic catalysts. This suggests that the catalytic efficiency for specific Pd x Ni y bimetallic clusters may be superior to bare Ni or Pd clusters and that it may be possible to tune bimetallic nanoparticles to obtain better catalytic performance.

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It is well-known that resonances can serve as a catalyst for molecule formation. Rate constants for resonance-induced molecule formation are phenomenological as they depend upon the mechanism used to populate the resonances. Standard treatments assume tunneling from the continuum is the only available population mechanism, which means long-lived quasibound states are essentially unpopulated. However, if a fast resonance population mechanism exists, the long-lived quasibound states may be populated and give rise to a substantial increase in the molecule formation rate constant. In the present work, we show that the semiclassical formula of Kramers and ter Haar [Bull. Astron. Inst. Neth. 10, 137 (1946)] may be used to compute rate constants for radiative association in the limit of local thermodynamic equilibrium. Comparisons are made with quantum mechanical and standard semiclassical treatments, and results are shown for two limits which provide upper and lower bounds for the six most important radiative association reactions leading to the formation of CO, CN, and SiN. These results may have implications for interstellar chemistry in molecular clouds, where the environmental and thermodynamic conditions often are uncertain.

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A quantum kinetic theory of molecule formation is presented which includes three-body recombination and radiative association for a thermodynamically closed system which may or may not exchange energy with its surrounding at a constant temperature. The theory uses a Sturmian representation of a two-body continuum to achieve a steady-state solution of a governing master equation which is self-consistent in the sense that detailed balance between all bound and unbound states is rigorously enforced. The role of quasibound states in catalyzing the molecule formation is analyzed in complete detail. The theory is used to make three predictions which differ from conventional kinetic models. These predictions suggest significant modifications may be needed to phenomenological rate constants which are currently in wide use. Implications for models of low and high density systems are discussed.

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Collision-induced energy transfer involving H2 molecules plays an important role in many areas of physics. Kinetic models often require a complete set of state-to-state rate coefficients for H2+H2 collisions in order to interpret results from spectroscopic observations or to make quantitative predictions. Recent progress in full-dimensional quantum dynamics using the numerically exact close-coupling (CC) formulation has provided good agreement with existing experimental data for low-lying states of H2 and increased the number of state-to-state cross sections that may be reliably determined over a broad range of energies. Nevertheless, there exist many possible initial states (e.g., states with high rotational excitation) that still remain elusive from a computational standpoint even at relatively low collision energies. In these cases, the coupled-states (CS) approximation offers an alternative full-dimensional formulation. We assess the accuracy of the CS approximation for H2+H2 collisions by comparison with benchmark results obtained using the CC formulation. The results are used to provide insight into the orientation effects of the various internal energy transfer mechanisms. A statistical CS approximation is also investigated and cross sections are reported for transitions which would otherwise be impractical to compute.

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An accurate 2D ab initio potential energy surface of the He-C3 collisional system is calculated using the supermolecular coupled-cluster method with up to perturbative quadruple excitations, CCSDT(Q). This interaction potential is then incorporated in full close-coupling calculations of rotational excitation/de-excitation cross sections in He + C3 collisions for rotational levels j = 0, 2, ..., 10 and collision energies up to 1000 cm(-1). Corresponding rate coefficients are reported for temperature between 1 and 100 K. Results are found to be in excellent agreement with available theoretical data that were restricted to the temperature range of 5-15 K. Implications of the computed rate coefficients to astrophysical models of C3 and carbon clusters in interstellar and circumstellar environments are discussed.

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We show that weakly bound He-containing van der Waals molecules can be produced and magnetically trapped in buffer-gas cooling experiments, and provide a general model for the formation and dynamics of these molecules. Our analysis shows that, at typical experimental parameters, thermodynamics favors the formation of van der Waals complexes composed of a helium atom bound to most open-shell atoms and molecules, and that complex formation occurs quickly enough to ensure chemical equilibrium. For molecular pairs composed of a He atom and an S-state atom, the molecular spin is stable during formation, dissociation, and collisions, and thus these molecules can be magnetically trapped. Collisional spin relaxation is too slow to affect trap lifetimes. However, (3)He-containing complexes can change spin due to adiabatic crossings between trapped and untrapped Zeeman states, mediated by the anisotropic hyperfine interaction, causing trap loss. We provide a detailed model for Ag(3)He molecules, using ab initio calculation of Ag-He interaction potentials and spin interactions, quantum scattering theory, and direct Monte Carlo simulations to describe formation and spin relaxation in this system. The calculated rate of spin-change agrees quantitatively with experimental observations, providing indirect evidence for molecular formation in buffer-gas-cooled magnetic traps. Finally, we discuss the possibilities for spectroscopic detection of these complexes, including a calculation of expected spectra for Ag(3)He, and report on our spectroscopic search for Ag(3)He, which produced a null result.

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Hydrogen dissociative chemisorption and desorption on small lowest energy Ni(n) clusters up to n=13 as a function of H coverage was studied using density functional theory. H adsorption on the clusters was found to be preferentially at edge sites followed by 3-fold hollow sites and on-top sites. The minimum energy path calculations suggest that H(2) dissociative chemisorption is both thermodynamically and kinetically favorable and the H atoms on the clusters are mobile. Calculations on the sequential H(2) dissociative chemisorption on the clusters indicate that the edge sites are populated first and subsequently several on-top sites and hollow sites are also occupied upon full cluster saturation. In all cases, the average hydrogen capacity on Ni(n) clusters is similar to that of Pd(n) clusters but considerably smaller than that of Pt(n) clusters. Comparison of hydrogen dissociative chemisorption energies and H desorption energies at full H-coverage among the Ni family clusters was made.

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Atomic force fields for simulating copper, silver, and gold clusters and nanoparticles are developed. Potential energy functions are obtained for both monatomic and binary metallic systems using an embedded atom method. Many cluster configurations of varying size and shape are used to constrain the parametrization for each system. Binding energies for these training clusters were computed using density functional theory (DFT) with the Perdew-Wang exchange-correlation functional in the generalized gradients approximation. Extensive testing shows that the many-body potentials are able to reproduce the DFT energies for most of the structures that were included in the training set. The force fields were used to calculate surface energies, bulk structures, and thermodynamic properties. The results are in good agreement with the DFT values and consistent with the available experimental data.

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Water dissociation on copper is one of the rate-limiting steps in the water-gas-shift (WGS) reaction. Copper atoms dispersed evenly from freshly made catalyst segregate to form clusters under the WGS operating conditions. Using density functional theory, we have examined water adsorption and dissociation on the smallest stable 3-dimensional copper cluster, Cu(7). Water molecules are adsorbed on the cluster sequentially until full saturation at which no direct water-copper contact is sterically possible. The adsorption is driven mainly by the overlap between the p-orbital of O atom occupied by the lone pair and the 3d-orbitals of copper, from which a fractional charge is promoted to the 4s-orbital to accommodate the charge transfer from water. Water dissociation on the Cu(7) cluster was investigated at both low and high water coverage. It was found that water dissociation into OH and H is exothermic but is inherently a high temperature process at low coverage. At high coverage, the reaction becomes more exothermic with fast kinetics. In both cases, water can catalyze the reaction. It was found that direct dissociation of the OH species is endothermic with a significantly higher barrier at both low and high coverage. However, the OH species can readily react with another adjacent hydroxyl group to form an O adatom and water molecule. Our studies indicate that the basic chemical properties of water dissociative chemisorption may not change significantly with the size of small copper clusters. Similarities between water dissociation on copper clusters and on copper crystalline surfaces are discussed.

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An atomic force field for simulating copper clusters and nanoparticles is developed. More than 2000 cluster configurations of varying size and shape are used to constrain the parametrization of the copper force field. Binding energies for these training clusters were computed using density functional theory. Extensive testing shows that the copper force field is fast and reliable for near-equilibrium structures of clusters, ranging from only a few atoms to large nanoparticles that approach bulk structure. Nonequilibrium dissociation and compression structures that are included in the training set are also well described by the force field. Implications for molecular dynamics simulations and extensions to other metallic and covalent systems are discussed.

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H2 sequential dissociative chemisorption on small palladium clusters was studied using density functional theory. The chosen clusters Pdn (n = 2-9) are of the lowest energy structures for each n. H2 dissociative chemisorption and subsequent H atom migration on the bare Pd clusters were found to be nearly barrierless. The dissociative chemisorption energy of H2 and the desorption energy of H atom in general decrease with the coverage of H atoms and thus the catalytic efficiency decreases as the H loading increases. These energies at full cluster saturation were identified and found to vary in small energy ranges regardless of cluster size. As H loading increases, the clusters gradually change their bonding from metallic character to covalent character. For the selected Pd clusters, the capacity to adsorb H atoms increases almost proportionally with cluster size; however, it was found that the capacity of Pd clusters to adsorb H atoms is, on average, substantially smaller than that of small Pt clusters, suggesting that the catalytic efficiency of Pt nanoparticles is superior to Pd nanoparticles in catalyzing dissociative chemisorption of H2 molecules.

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Platinum is the most widely used catalyst in fuel cell electrodes. Designing improved catalysts with low or no platinum content is one of the grand challenges in fuel cell research. Here, we investigate electronic structures of Pt(4) and Pt(3)Co clusters and report a comparative study of adsorption of H(2), O(2), and CO molecules on the two clusters using density functional theory. The adsorption studies show that H(2) undergoes dissociative chemisorption on the tetrahedral clusters in head on and side on approaches at Pt centers. O(2) dissociation occurs primarily in three and four center coordinations and CO prefers to adsorb on Pt or Co atop atoms. The adsorption energy of O(2) is found to be higher for the Co doped cluster. For CO, the Pt atop orientation is preferred for both Pt(4) and Pt(3)Co tetrahedral clusters. Adsorption of CO molecule on tetrahedral Pt(3)Co in side on approach leads to isomerization to planar rhombus geometry. An analysis of Hirshfeld charge distribution shows that the clusters become more polarized after adsorption of the molecules.

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The structural evolution of small copper clusters of up to 15 atoms and the dissociative chemisorption of H2 on the minimum energy clusters are studied systematically using density functional theory. The preferred copper sites for chemisorption are identified and the transition state structures and activation barriers for clusters four to nine atoms are determined and found to be inconsistent with the empirical Bronsted-Evans-Polanyi relationship. The physicochemical properties of the clusters are computed and compared with the bulk and surface values. The results indicate that a phase transition must occur in the going from cluster to bulk.