Caroline Chick Jarrold

Structures of Mo2Oy− and Mo2Oy (y=2, 3, and 4) studied by anion photoelectron spectroscopy and density functional theory calculations

The competitive structural isomers of the Mo(2)O(y) (-)Mo(2)O(y) (y=2, 3, and 4) clusters are investigated using a combination of anion photoelectron (PE) spectroscopy and density functional theory calculations. The PE spectrum and calculations for MoO(3) (-)MoO(3) are also presented to show the level of agreement to be expected between the spectra and calculations. For MoO(3) (-) and MoO(3), the calculations predict symmetric C(3v) structures, an adiabatic electron affinity of 3.34 eV, which is above the observed value 3.17(2) eV. However, there is good agreement between observed and calculated vibrational frequencies and band profiles. The PE spectra of Mo(2)O(2) (-) and Mo(2)O(3) (-) are broad and congested, with partially resolved vibrational structure on the lowest energy bands observed in the spectra. The electron affinities (EA(a)s) of the corresponding clusters are 2.24(2) and 2.33(7) eV, respectively. Based on the calculations, the most stable structure of Mo(2)O(2) (-) is Y shaped, with the two Mo atoms directly bonded. Assignment of the Mo(2)O(3) (-) spectrum is less definitive, but a O-Mo-O-Mo-O structure is more consistent with overall electronic structure observed in the spectrum. The PE spectrum of Mo(2)O(4) (-) shows cleanly resolved vibrational structure and electronic bands, and the EA of the corresponding Mo(2)O(4) is determined to be 2.13(4) eV. The structure most consistent with the observed spectrum has two oxygen bridge bonds between the Mo atoms.

Electronic structures of MoWOy− and MoWOy determined by anion photoelectron spectroscopy and DFT calculations

The anion photoelectron spectra of MoWO(y)(-) (y=2-5) and density functional theory (DFT) calculations on MoWO(y)(-) and MoWO(y) are reported and compared to previous comparable studies on Mo(2)O(y)(-)/Mo(2)O(y) and W(2)O(y)(-)/W(2)O(y). The property governing the structure of the lowest energy MoWO(y) anion and neutral clusters is the stronger W-O bond relative to the Mo-O bond, which results in the stabilization of structures in which the Mo center is in a much lower oxidation state than the W center. Anion PE spectra show a much larger change in structure between anion and neutral states than what was observed in the pure Mo(2)O(y)(-) and W(2)O(y)(-) spectra. DFT calculations show increased single-metal localization of spin with respect to the pure metal oxide clusters.

Water reactivity with tungsten oxides: H2 production and kinetic traps

In a recent mass spectrometry/photoelectron spectroscopy study on the reactions between W(2)O(y) (-) (y=2-6) and water, Jarrold and co-workers [J. Chem. Phys. 130, 124314 (2009)] observed interesting differences in the reactivity of the different cluster ions. Particularly noteworthy is the observation that the only product with the incorporation of hydrogens is a single peak corresponding to W(2)O(6)H(2) (-). As reactions between metal oxide clusters and small molecules such as water have high potential for catalytic applications, we carried out a careful study to obtain a mechanistic understanding of this observed reactivity. Using electronic structure calculations, we identified and characterized multiple modes of reactivity between unsaturated tungsten oxide clusters [W(2)O(y) (-) (y=4-6)] and water. By calculating the free energy corrected reaction profiles, our results provide an explanation for the formation of W(2)O(6)H(2) (-). We propose a mechanism in which water reacts with a metal oxide cluster and eliminates H(2). The results from our calculations show that this is nearly a barrierless process for all suboxide clusters with the exception of W(2)O(5) (-).

Unusual products observed in gas-phase WxOy−+H2O and D2O reactions

Addition of H(2)O and D(2)O to small tungsten suboxide cluster anions W(x)O(y)(-) (x = 1-4; y < or = 3x) was studied using mass spectrometric measurements from a high-pressure fast flow reactor. Within the WO(y)(-) mass manifold, which also includes WO(4)H(-), product masses correspond to the addition of one to three H(2)O or D(2)O molecules. Within the W(2)O(y)(-) cluster series, product distributions suggest that sequential oxidation W(2)O(y)(-) + H(2)O/D(2)O --> W(2)O(y+1)(-) + H(2)/D(2) occurs for y < 5, while for W(2)O(5)(-), W(2)O(6)H(2)(-)/W(2)O(6)D(2)(-) is primarily produced. W(2)O(6)(-) does not appear reactive. For the W(3)O(y)(-) cluster series, sequential oxidation with H(2) and D(2) production occurs for y < 6, while W(3)O(6)(-) and W(3)O(7)(-) produce W(3)O(7)H(2)(-)/W(3)O(7)D(2)(-) and W(3)O(8)H(2)(-)/W(3)O(8)D(2)(-), respectively. Lower mass resolution in the W(4)O(y)(-) mass range prevents definitive product assignments, but intensity patterns suggest that sequential oxidation with H(2)/D(2) evolution occurs for y < 6, while W(4)O(y+1)H(2)(-)/W(4)O(y+1)D(2)(-) products result from addition to W(4)O(6)(-) and W(4)O(7)(-). Based on bond energy arguments, the H(2)/D(2) loss reaction is energetically favored if the new O-W(x)O(y)(-) bond energy is greater than 5.1 eV. The relative magnitude of the rate constants for sequential oxidation and H(2)O/D(2)O addition for the x = 2 series was determined. There are no discernable differences in rate constants for reactions with H(2)O or D(2)O, suggesting that the H(2) and D(2) loss from the lower-oxide/hydroxide intermediates is very fast relative to the addition of H(2)O or D(2)O.

Addition of water to Al5O4- determined by anion photoelectron spectroscopy and quantum chemical calculations.

The anion photoelectron spectra of Al5O4- and Al5O5H2- are presented and interpreted within the context of quantum chemical calculations on these species. Experimentally, the electron affinities of these two molecules are determined to be 3.50(5) eV and 3.10(10) eV for the bare and hydrated cluster, respectively. The spectra show at least three electronic transitions crowded into a 1 eV energy window. Calculations on Al5O4- predict a highly symmetric near-planar structure with a singlet ground state. The neutral structure calculated to be most structurally similar to the ground state structure of the anion is predicted to lie 0.15 eV above the ground state structure of the neutral. The lowest energy neutral isomer does not have significant Franck-Condon overlap with the ground state of the anion. Dissociative addition of water to Al5O4- is energetically favored over physisorption. The ground state structure for the Al5O4- +H(2)O product forms when water adds to the central Al atom in Al5O4- with -H migration to one of the neighboring O atoms. Again, the ground state structures for the anion and neutral are very different, and the PE spectrum represents transitions to a higher-lying neutral structure from the ground state anion structure.

Addition of water and methanol to Al3O3− studied by mass spectrometry and anion photoelectron spectroscopy

The 4.66 eV photoelectron spectra of Al3O3−, Al3O3−⋅solvent and Al3O3−⋅(solvent)2 (solvent=H2O, D2O, and CH3OH) have been obtained and analyzed in the context of existing and preliminary new density functional theory calculations. The structures and vibrational frequencies of the two isomers of Al3O3− and Al3O3 proposed by Ghanty and Davidson [J. Phys. Chem. A 103, 8985 (1999)] agree well with structural information extracted from the Al3O3− spectra using Franck–Condon simulations. Photoelectron spectra of Al3O3−⋅solvent complexes [EA=2.5(1) eV] are broad and congested, and hydroxide formation, multiple structural isomers, and anion photodissociation are suggested as possible sources of this. The photoelectron spectra of Al3O3−⋅(solvent)2 complex spectra [EA=3.05(10) eV] show two distinct electronic transitions, several of which exhibit partially-resolved vibrational structure that are similar to the two electronic bands attributed to the bare rectangular structural isomer of Al3O3−. Possible adsorption s...

Structures of MoxW(3−x)O6 (x=0–3) anion and neutral clusters determined by anion photoelectron spectroscopy and density functional theory calculations

The structures of Mo(3)O(6), Mo(2)WO(6), MoW(2)O(6), and W(3)O(6) and their associated anions were studied using a combination of anion photoelectron (PE) spectroscopy and density functional theory calculations. The 3.49 eV photon energy anion PE spectra of all four species showed broad electronic bands with origins near 2.8 eV. Calculations predict that low-spin, cyclic structures are the lowest energy isomers for both the anion and neutral species. The lowest energy neutral structures for all four species are analogous, C(3v) (Mo(3)O(6) and W(3)O(6)) or C(s) (mixed clusters) symmetry structures in which all three metal atoms are in formally equivalent oxidation states, with singlet ground electronic states. The lowest energy isomers predicted for Mo(3)O(6)(-) and W(3)O(6)(-) are the same with doublet electronic states. The lowest energy structures calculated for the mixed anions are lower symmetry, with the tungsten centers in higher oxidation states than the molybdenum centers. However, C(s) symmetry structures are competitive, and appear to be the primary contributors to the observed spectra. Spectral simulations based on calculated spectroscopic parameters validate the assignments. This series of clusters is strikingly different from the Mo(2)O(4)/MoWO(4)/W(2)O(4) anion and neutral series described recently [Mayhall et al., J. Chem. Phys. 130, 124313 (2009)]. While the average oxidation state is the same for both series, the structures determined for the Mo(2)O(4)/MoWO(4)/W(2)O(4) anions and neutrals were dissimilar and lower symmetry, and high spin states were energetically favored. This difference is attributed to the large stabilizing effect of electronic delocalization in the more symmetric trimetallic cyclic structures that is not available in the bimetallic species.

New insights on photocatalytic H2 liberation from water using transition-metal oxides: lessons from cluster models of molybdenum and tungsten oxides.

Molecular hydrogen (H2) is an excellent alternative fuel. It can be produced from the abundantly present water on earth. Transition-metal oxides are widely used in the environmentally benign photocatalytic generation of H2 from water, thus actively driving scientific research on the mechanisms for this process. In this study, we investigate the chemical reactions of W3O5(-) and Mo3O5(-) clusters with water that shed light on a variety of key factors central to H2 generation. Our computational results explain why experimentally Mo3O5(-) forms a unique kinetic trap in its reaction while W3O5(-) undergoes a facile oxidation to form the lowest-energy isomer of W3O6(-) and liberates H2. Mechanistic insights on the reaction pathways that occur, as well as the reaction pathways that do not occur, are found to be of immense assistance to comprehend the hitherto poorly understood pivotal roles of (a) differing metal-oxygen and metal-hydrogen bond strengths, (b) the initial electrostatic complex formed, (c) the loss of entropy when these TMO clusters react with water, and (d) the geometric factors involved in the liberation of H2.

Disparate product distributions observed in Mo(3-x)WxOy- (x=0-3; y=3-9) reactions with D2O and CO2

Results of gas phase reactivity studies on group six transition metal suboxide clusters, Mo(3)O(y) (-), Mo(2)WO(y) (-), MoW(2)O(y) (-), and W(3)O(y) (-) (Mo((3-x))W(x)O(y) (-), x=0-3; y=ca. 3-9) with both D(2)O and CO(2) are reported. Sequential oxidation for the more reduced species, Mo((3-x))W(x)O(y) (-)+D(2)O/CO(2)-->Mo((3-x))W(x)O(y+1) (-)+D(2)/CO, and dissociative addition for certain species, Mo((3-x))W(x)O(y) (-)+D(2)O/CO(2)-->Mo((3-x))W(x)O(y+1)D(2) (-)/Mo((3-x))W(x)O(y+1)CO(-), is evident in the product distributions observed in mass spectrometric measurements. Reactions with D(2)O proceed at a rate that is on the order of 10(2) higher than for CO(2). The pattern of reaction products reveals composition-dependent chemical properties of these group six unary and binary clusters. At the core of this variation is the difference in Mo-O and W-O bond energies, the latter of which is significantly higher. This results in a larger thermodynamic drive to higher oxidation states in clusters with more tungsten atoms. However, addition products for more oxidized W-rich clusters are not observed, while they are observed for the more Mo-rich clusters. This is attributed to the following: In the higher oxides (e.g., y=8), addition reactions require distortion of local metal-oxygen bonding, and will necessarily have higher activation barriers for W-O bonds, since the vibrational potentials will be narrower. The binary (x=1,2) clusters generally show sequential oxidation to higher values of y. This again is attributed to higher W-O bond energy, the result being that stable binary structures have W atoms in higher oxidation states, and Mo centers both in more reduced states and sterically unhindered. The reduced Mo center provides a locus of higher reactivity. An unusual result that is not readily explained is the chemically inert behavior of Mo(3)O(6) (-).

Termination of the W2Oy−+H2O/D2O→W2Oy+1−+H2/D2 sequential oxidation reaction: An exploration of kinetic versus thermodynamic effects

Several mechanisms proposed and calculated for the sequential oxidation of tungsten suboxide clusters by H(2)O/D(2)O [Mayhall et al., J. Chem. Phys. 131, 144302 (2009)] are evaluated using anion photoelectron spectroscopy of an apparent intermediate, W(2)O(6)D(2) (-). The spectrum of W(2)O(6)D(2) (-) is consistent with the W(2)O(5) (-)+D(2)O-->W(2)O(6) (-)+D(2) intermediate in which the initial water addition involves the interaction of the oxygen from D(2)O with a tungsten atom, approaching from a direction with the least repulsion from the W(2)O(5) (-) oxygen atoms, coupled with the interaction between a deuterium with a tungsten-tungsten bridging oxygen on the cluster. The presence of W(2)O(6)H(2) (-) and W(2)O(6)D(2) (-) suggests that there is insufficient internal energy in the complex to surmount the barrier for rearrangement required for tungsten hydride and hydroxide formation necessary for H(2) or D(2) evolution, which was calculated to be energetically favorable. The quality of the calculations is verified by direct comparison between experimental photoelectron spectra of W(2)O(5) (-) and W(2)O(6) (-) and spectral simulations generated from the lowest energy structures calculated for W(2)O(5) (-), W(2)O(6) (-) and their corresponding neutrals. The results shed light on the importance of repulsion on the pathway a reaction follows under room temperature conditions.