Sarah E Waller

Indiana University Bloomington, Bloomington, IN, United States

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Publications (15)41.94 Total impact

  • Sarah E Waller, Caroline Chick Jarrold
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    ABSTRACT: To test recent computational studies on the mechanism of metal oxide cluster anion reactions with water [R. O. Ramabhadran et al., J. Phys. Chem. Lett. 1, 3066 (2010); R. O. Ramabhadran et al., J. Am. Chem. Soc. 135, 17039 (2013)], reactivity of molybdenum oxo-cluster anions, MoxOy- (x = 1 - 4; y ≤ 3x), toward both methanol (MeOH) and ethanol (EtOH) has been studied using mass spectrometric analysis of products formed in a high-pressure, fast-flow reactor. The size-dependent product distributions are compared to previous MoxOy- + H2O/D2O reactivity studies, with particular emphasis on the Mo2Oy- and Mo3Oy- series. In general, sequential oxidation, MoxOy- + ROH --> MoxOy+1- + RH, and addition reactions, MoxOy- + ROH --> MoxOy+1RH-, largely corresponded with previously studied MoxOy- + H2O/D2O reactions [D. W. Rothgeb, J. E. Mann, and C. C. Jarrold, J. Chem. Phys. 133, 054305 (2010)], though with much lower rate constants than those determined for MoxOy- + H2O/D2O reactions. This finding is consistent with the computational studies that suggested -H mobility on the cluster-water complex was important feature in overall reactivity. There were several notable differences between cluster/ROH and cluster/water reactions associated with lower R-OH bond dissociation energies relative to the HO-H dissociation energy.
    The Journal of Physical Chemistry A 03/2014; · 2.77 Impact Factor
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    ABSTRACT: 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.
    Journal of the American Chemical Society 10/2013; · 10.68 Impact Factor
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    ABSTRACT: The photoelectron spectra of WO3H- and WO2F- are presented and analyzed in the context of a series of previous similar measurements on MOy- (M = Mo, W; y = 0 - 3), MO4H- and AlMOy- (y ≤ 4) complexes. The electronic structures of the WO3H and WO2F anion and neutral complexes were investigated using the B3LYP hybrid density functional method. The spectra of WO3H-, WO2F- and previously measured AlWO3- photoelectron spectra show that the corresponding neutrals, in which the transition metal centers are all in a +5 oxidation state, have comparable electron affinities. In addition, the electron affinities fit the trend of monotonically increasing electron affinity with oxidation state, in spite of the WO3H-, WO2F- and AlWO3- having closed shell ground states, suggesting that the oxidation state of the metal atom has more influence than shell closing on the electron affinity of these transition metal-oxo complexes. Results of DFT calculations suggest that the neutrals are pyramidal while the anions are planar. However, the barriers for inversion on the neutral surface are low, and attempts to generate simple Franck-Condon simulations based on simple normal coordinate displacement, ignoring the effects of inversion, are inadequate.
    The Journal of Physical Chemistry A 10/2013; · 2.77 Impact Factor
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    ABSTRACT: The electronic and molecular structures of Al2MoOy (y = 2 - 4) anion and neutral complexes were studied using anion photoelectron spectroscopy and density functional theory calculations. The spectra are broad, reflecting significant structural changes in the transition from anion to neutral, and the neutral electron affinities determined from the spectra are similar for all three species. The calculations suggest that the lowest energy isomers of the neutral clusters can be described as predominantly (Al+)2[MoOy-2] ionic complexes, in which the Al+ cations bond with O-2 anions in a way that minimizes repulsion with the positively charged Mo-center. The anion structures for all three complexes favor closer Mo-Al and Al-Al internuclear distances, with the extra negative charge distributed more evenly among all three metal centers. Energetically, the fully occupied 3s orbitals on the Al centers are lower than the Mo-local 4d-like orbitals and above the O-local 2p-like orbitals. In the case of Al2MoO2-, there is direct Al-Al covalent bonding. The calculated spectroscopic parameters for these species are consistent with the observed spectra, though definitive assignments are not possible due to the broad, unresolved spectra observed and predicted.
    The Journal of Physical Chemistry A 07/2013; · 2.77 Impact Factor
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    ABSTRACT: While high-power laser ablation of metal alloys indiscriminately produces gas-phase atomic ions in proportion to the abundance of the various metals in the alloy, gas-phase ions produced by moderate-power laser ablation sources coupled with molecular beams are formed by more complicated mechanisms. A mass spectrometric study that directly compares the mass distributions of cluster anions and cations generated from laser ablation of pure aluminum, an aluminum/molybdenum mixed target, and an aluminum/tungsten mixed target is detailed. Mass spectra of anionic species generated from the mixed targets showed that both tungsten and molybdenum were in higher abundance in the negatively charged species than in the target material. Mass spectra of the cationic species showed primarily Al and aluminum oxide and hydroxide cluster cations. No molybdenum- or tungsten-containing cluster cations were definitively assigned. The asymmetric distribution of aluminum and Group 6 transition metals in cation and anion cluster composition is attributed to the low ionization energy of atomic aluminum and aluminum suboxide clusters. In addition, the propensity of both molybdenum and tungsten to form metal oxide cluster anions under the same conditions that favor metallic aluminum cluster anions is attributed to differences in the optical properties of the surface oxide that is present in the metal powders used to prepare the ablation targets. Mechanisms of mixed metal oxide clusters are considered.
    The Journal of Physical Chemistry A 02/2013; 117(8):1765-72. · 2.77 Impact Factor
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    ABSTRACT: Results of a study combining anion photoelectron spectroscopy and density functional theory calculations on the heteronuclear MoNbO(y)(-) (y = 2-5) transition metal suboxide cluster series are reported and analyzed. The photoelectron spectra, which exhibit broad electronic bands with partially resolved vibrational structure, were compared to spectral simulations generated from calculated spectroscopic parameters for all computationally determined energetically competitive structures. Although computational results on the less oxidized clusters could not be satisfactorily reconciled with experimental spectra, possibly because of heavy spin contamination found in a large portion of the computational results, the results suggest that (1) neutral cluster electron affinity is a strong indicator of whether O-atoms are bound in M-O-M bridge positions or M═O terminal positions, (2) MoNbO(y) anions and neutrals have structures that can be described as intermediate with respect to the unary (homonuclear) Mo(2)O(y) and Nb(2)O(y) clusters, and (3) structures in which O-atoms preferentially bind to the Nb center are slightly more stable than alternative structures. Several challenges associated with the calculations are considered, including spin contamination, which appears to cause spurious single point calculations used to determine vertical detachment energies.
    The Journal of Physical Chemistry A 09/2012; 116(39):9639-52. · 2.77 Impact Factor
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    ABSTRACT: The anion photoelectron spectra of WAlO(y)(-) (y = 2-4) are presented and assigned based on results of density functional theory calculations. The WAlO(2)(-) and WAlO(3)(-) spectra are both broad, with partially resolved vibrational structure. In contrast, the WAlO(4)(-) spectrum features well-resolved vibrational structure with contributions from three modes. There is reasonable agreement between experiment and theory for all oxides, and calculations are in particular validated by the near perfect agreement between the WAlO(4)(-) photoelectron spectrum and a Franck-Condon simulation based on computationally determined spectroscopic parameters. The structures determined from this study suggest strong preferential W-O bond formation, and ionic bonding between Al(+) and WO(y)(-2) for all anions. Neutral species are similarly ionic, with WAlO(2) and WAlO(3) having electronic structure that suggests Al(+) ionically bound to WO(y)(-) and WAlO(4) being described as Al(+2) ionically bound to WO(4)(-2). The doubly-occupied 3sp hybrid orbital localized on the Al center is energetically situated between the bonding O-local molecular orbitals and the anti- or non-bonding W-local molecular orbitals. The structures determined in this study are very similar to structures recently determined for the analogous MoAlO(y)(-)/MoAlO(y) cluster series, with subtle differences found in the electronic structures [S. E. Waller, J. E. Mann, E. Hossain, M. Troyer, and C. C. Jarrold, J. Chem. Phys. 137, 024302 (2012)].
    The Journal of Chemical Physics 07/2012; 137(4):044301. · 3.12 Impact Factor
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    ABSTRACT: Vibrationally-resolved photoelectron spectra of AlMoO(y)(-) (y = 1-4) are presented and analyzed in conjunction with density functional theory computational results. The structures determined for the AlMoO(y) anion and neutral clusters suggest ionic bonding between Al(+) and a MoO(y)(-) or MoO(y)(-2) moiety, and point to the relative stability of Mo=O versus Al=O bonds. The highest occupied and partially occupied orbitals in the anions and neutrals can be described as Mo atomic-like orbitals, so while the Mo is in a higher oxidation state than Al, the most energetically accessible electrons are localized on the molybdenum center.
    The Journal of Chemical Physics 07/2012; 137(2):024302. · 3.12 Impact Factor
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    ABSTRACT: Vibrationally-resolved photoelectron spectra of MoAlOy- (y = 1 - 4) are presented and analyzed within the context of Density Functional Theory computational results. The structures reflect the relative stability of the Mo{-}O versus Al{-}O bond, with the Mo center in a higher oxidation state than Al. The highest occupied and partially occupied orbitals in the anions and neutrals can be described as Mo atomic-like orbitals, so while the Mo center is in a higher oxidation state, the most energetically accessible electrons are localized on the molybdenum center.
    06/2012;
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    ABSTRACT: Anion photoelectron spectra of Mo(3)O(y)(-) and W(3)O(y)(-) (y = 3-6) are reported and analyzed using density functional theory results in an attempt to determine whether electronic and structural trends in the less oxidized clusters (y = 3, 4) could elucidate the disparate chemical properties of the M(3)O(y)(-) (M = Mo, W, y = 5, 6) species. In general, cyclic structures are calculated to be more stable by at least 1 eV than extended structures, and the lowest energy structures calculated for the most reduced species favor M = O terminal bonds. While the numerous low-energy structures found for Mo(3)O(y)(-)/Mo(3)O(y) and W(3)O(y)(-)/W(3)O(y) were, in general, similar, various structures of W(3)O(y)(-)/W(3)O(y) were found to be energetically closer lying than analogous structures of Mo(3)O(y)(-)/Mo(3)O(y). Additionally, the Mo-O-Mo bridge bond was found to be a more stabilizing structural motif than the W-O-W bridge bond, with the oxygen center in the former having the highest negative charge. Based on this, the observation of trapped intermediates in reactions between Mo(3)O(y)(-) and water or CO(2) that are not observed in analogous W(3)O(y)(-) reactivity studies may be partially attributed to the role of bridge bond fluxionality.
    The Journal of Chemical Physics 09/2011; 135(10):104312. · 3.12 Impact Factor
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    ABSTRACT: A study combining anion photoelectron spectroscopy and density functional theory calculations on the transition metal suboxide series, Nb(2)O(y)(-) (y = 2-5), is described. Photoelectron spectra of the clusters are obtained, and Franck-Condon simulations using calculated anion and neutral structures and frequencies are used to evaluate the calculations and assign transitions observed in the spectra. The spectra, several of which exhibit partially resolved vibrational structure, show an increase in electron affinity with increasing cluster oxidation state. Hole-burning experiments suggest that the photoelectron spectra of both Nb(2)O(2)(-) and Nb(2)O(3)(-) have contributions from more than one structural isomer. Reasonable agreement between experiment and computational results is found among all oxides.
    The Journal of Chemical Physics 09/2011; 135(10):104317. · 3.12 Impact Factor
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    ABSTRACT: Photoelectron spectroscopy and DFT calculations are used to study the evolution of the molecular properties of metal suboxide clusters as they are sequentially oxidized toward bulk stoichiometry. The spectra of group 6 tri-metal suboxide anions with three to six oxygen atoms reflect a shift from direct metal-metal bonding to metal-oxo-metal bonding motifs. Additionally, multiple energetically competitive structures are predicted for the most reduced species.
    06/2011;
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    ABSTRACT: The resonant two-photon detachment spectrum of WO2− in the 1.75–2.41eV range exhibits at least two electronic transitions near the detachment continuum of WO2−. The states are assigned to valence-bound states of the anion, rather than dipole bound states, in part because the observed bending frequencies are considerably lower than those in the neutral. One band exhibits doublets, which is attributed to spin–orbit splitting in the two E1/2 sub-states of a quartet anion state. A qualitative assignment of the spectrum is made based on comparison with the photoelectron spectrum of WO2−, as well as density functional theory calculations.
    Chemical Physics Letters 01/2011; 506(1):31-36. · 2.15 Impact Factor
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    ABSTRACT: The vibrationally resolved anion photoelectron (PE) spectra of MoVO(y)(-) (y = 2 - 5) metal suboxide clusters are presented and analyzed in the context of density functional theory (DFT) calculations. The electronically congested spectra reflect an increase in cluster electron affinity with increasing oxidation state. Ion beam hole-burning results reveal the features in the PE spectra of MoVO(2)(-) and MoVO(4)(-) are a result of only one anion isomer, while at least two isomers contribute to electronic structure observed in the PE spectrum of MoVO(3)(-). Spectral features of the binary systems are compared to their pure analogs, Mo(2)O(y) and V(2)O(y). An attempt to characterize the anion and neutral electronic and molecular structures is made by comparison with results from DFT calculations. However, reconciliation between the cluster spectra and the calculated spectroscopic parameters is not as straightforward as in previous studies on similar systems (Yoder, B. L.; Maze, J. T.; Raghavachari, K.; Jarrold, C. C. J. Chem. Phys. 2005, 122, 094313 and Mayhall, N. J.; Rothgeb, D. W.; Hossain, E.; Raghavachari, K.; Jarrold, C. C. J. Chem. Phys. 2009, 130, 124313).
    The Journal of Physical Chemistry A 10/2010; 114(42):11312-21. · 2.77 Impact Factor
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    ABSTRACT: Vibrationally-resolved photoelectron spectra of molybdenum vanadium oxo cluster anions with 2 to 5 oxygen atoms and measured using 2.33 eV, 3.49 eV and 4.66 eV photon energies generally exhibit broad and overlapping electronic states. The adiabatic electron affinities for the series are 1.68(3) eV, 1.73(3) eV, 2.89(1) eV, and 3.4(1) eV for two through five oxygen atoms, respectively. Vibrational structure observed in the spectra can be reconciled with the lowest energy structural isomers of the anions determined in DFT calculations: The lowest energy isomers have low symmetry, with the Mo center in a higher oxidation state than the V center, and high spin states are favored.
    06/2010;