Jennifer E. Mann

The Role of Excited-State Topology in Three-Body Dissociation of sym-Triazine

Molecular fragmentation into three products poses an analytical challenge to theory and experiment alike. We used translational spectroscopy and high-level ab initio calculations to explore the highly debated three-body dissociation of sym-triazine to three hydrogen cyanide molecules. Dissociation was induced by charge exchange between the sym-triazine radical cation and cesium. Calculated state energies and electronic couplings suggest that reduction initially produces a population of sym-triazine partitioned between the 3s Rydberg and π* ← n electronically excited manifolds. Analysis of the topology of these manifolds, along with momentum correlation in the dissociation products, suggests that a conical intersection of two potential energy surfaces in the 3s Rydberg manifold leads to stepwise dissociation, whereas a four-fold glancing intersection in the π* ← n manifold leads to a symmetric concerted reaction.

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.

Asymmetric Partitioning of Metals among Cluster Anions and Cations Generated via Laser Ablation of Mixed Aluminum/Group 6 Transition Metal Targets

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.

Study of MoVOy(y= 2−5) Anion and Neutral Clusters using Anion Photoelectron Spectroscopy and Density Functional Theory Calculations†

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).

Study of MoNbOy (y = 2–5) Anion and Neutral Clusters Using Photoelectron Spectroscopy and Density Functional Theory Calculations: Impact of Spin Contamination on Single Point Calculations

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.

Probing the Structure of CH5+ by Dissociative Charge Exchange

Dissociative charge exchange of CH5+ with Cs, coupled with quasiclassical trajectory calculations on an ab initio PES for CH5, has been used to probe the structure of the CH5+ cation. Product kinetic energy release distributions and branching ratios for CH5 --> CH4 + H and CH5 --> CH3 + H2 have been compared. The agreement of the product branching ratios provides evidence for the fluxional nature of CH5+.

Conical for stepwise, glancing for concerted: the role of the excited-state topology in the three-body dissociation of sym-triazine.

The highly debated three-body dissociation of sym-triazine to three HCN products has been investigated by translational spectroscopy and high-level ab initio calculations. Dissociation was induced by charge exchange between the sym-triazine radical cation and cesium. Calculated state energies and electronic couplings suggest that sym-triazine is produced in the 3s Rydberg and pi* <-- n manifolds. Analysis of the topology of these manifolds along with momentum correlation in the dissociation products suggest that the 3s Rydberg manifold characterized by a conical intersection of two potential energy surfaces leads to stepwise dissociation, while the pi* <-- n manifold consisting of a four-fold glancing intersection leads to a symmetric concerted reaction.

Electronic structures of WAlOy and WAlOy− (y = 2–4) determined by anion photoelectron spectroscopy and density functional theory calculations

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)].

Three-body dissociative charge exchange dynamics of sym-triazine.

Translational spectroscopy coupled with coincidence detection techniques has been used to investigate the two-body dissociation of sym-triazine to HCN + (HCN)2 upon electronic excitation from charge exchange between the sym-triazine cation and cesium. This dissociation mechanism was determined to occur after excitation of sym-triazine into the 3s Rydberg electronic manifold, and the observed dynamics suggest that the mechanism competes with a stepwise three-body dissociation mechanism to three HCN products that occurs from this same electronic excitation. On the basis of reported stabilization energies of several isomers of the HCN dimer, possible structures of the metastable (HCN)2 species are discussed in light of the current measurements.

Dissociation Dynamics of Isotopologs of CH5 Studied by Charge Exchange of CH5+ with Cs and Quasiclassical Trajectory Calculations

The product branching fractions and dissociation dynamics for the dissociation of CH₅, CH₂D₃, CHD₄, and CD₅ have been experimentally measured following charge exchange of the parent cations with Cs. These results are compared with quasiclassical trajectory calculations run on an ab initio potential energy surface for CH₅. In all cases it is found that dissociation channels involving the ejection of a single atom (H or D) are dominant. The statistically weighted branching fractions show that in the mixed isotopologs, ejection of a hydrogen atom is favored over a deuterium atom. The results are consistent with the fluxional nature of CH₅(+) isotopologs--no evidence was found for quantum localization upon deuteration in the comparison of the experimental and theoretical branching fractions, possibly as a result of the vibrational temperature of the cations (estimated to be as high as 1660 K for high frequency modes). This comparison of experiment and theory provides a test of the CH₅ potential energy surface at high levels of excitation.