J. M. Weber

Solvent-Driven Reductive Activation of Carbon Dioxide by Gold Anions

Catalytic activation and electrochemical reduction of CO(2) for the formation of chemically usable feedstock and fuel are central goals for establishing a carbon neutral fuel cycle. The role of solvent molecules in catalytic processes is little understood, although solvent-solute interactions can strongly influence activated intermediate species. We use vibrational spectroscopy of mass-selected Au(CO(2))(n)(-) cluster ions to probe the solvation of AuCO(2)(-) as a model for a reactive intermediate in the reductive activation of a CO(2) ligand by a single-atom catalyst. For the first few solvent molecules, solvation of the complex preferentially occurs at the CO(2) moiety, enhancing reductive activation through polarization of the excess charge onto the partially reduced ligand. At higher levels of solvation, direct interaction of additional solvent molecules with the Au atom diminishes reduction. The results show how the solvation environment can enhance or diminish the effects of a catalyst, offering design criteria for single-atom catalyst engineering.

Observation of resonant two-photon photodetachment of water cluster anions via femtosecond photoelectron spectroscopy

Abstract Photoexcitation of the (H 2 O) − n ( n =20–100) clusters with 100 fs pulses at 800 nm results in an increasing propensity for two-photon electron photoejection with increasing cluster size. This increase correlates with the size range ( n ≈30) where the first excited electronic state drops below the electron continuum, and the electronic absorption band approaches the energy of the 800 nm pump photon. No above-threshold, two-photon detachment is observed for n =20. Differences in the shape of the resonant two-photon photoelectron spectrum compared to that arising from direct (high energy) photodetachment are interpreted in terms of the vibrational state selection created in the resonant step.

Solvent-Mediated Reduction of Carbon Dioxide in Anionic Complexes with Silver Atoms

The development of efficient routes toward sustainable fuel sources by electrochemical reduction of CO2 is an important goal for catalysis research. While these processes usually occur in the presence of solvent, solvation effects in catalysis are largely not understood or even characterized. In this work, mass-selected clusters of silver anions with CO2 serve as a model system for reductive activation of CO2 by a catalyst in the presence of a well-controlled number of solvent molecules. Vibrational spectroscopy and electronic structure calculations are used to obtain molecular-level information on the interaction of solvent with the catalyst-CO2 complex and the effects of solvation on one-electron reductive activation of CO2. Charge transfer from the silver catalyst to CO2 increases with increasing cluster size. We observe the coexistence of catalyst-ligand complexes with CO2 monomer and dimer anions, indicating that CO2-based charge carriers can exist in the presence of a silver atom.

Infrared Spectra and Structures of Anionic Complexes of Cobalt with Carbon Dioxide Ligands

We present infrared photodissociation spectra of [Co(CO2)n](-) ions (n = 3-11) in the wavenumber region 1000-2400 cm(-1), interpreted with the aid of density functional theory calculations. The spectra are dominated by the signatures of a core ion showing bidentate interaction of two CO2 ligands with the Co atom, each forming C-Co and O-Co bonds. This structural motif is very robust and is only weakly affected by solvation with additional CO2 solvent molecules. The Co atom is in oxidation state +1, and both CO2 ligands carry a negative charge.

The interaction of negative charge with carbon dioxide – insight into solvation, speciation and reductive activation from cluster studies

The interaction of CO2 with negative charge is of high importance in many natural and industrial processes, since reductive activation is one of the most common and convenient ways to chemically unlock this robust molecule. While free CO2 does not form stable anions, the accessibility of low-lying molecular orbitals is critical for its chemical versatility and allows CO2 to act as solvent as well as a reaction partner for negative ions. Experiments on mass selected cluster ions are highly suitable for the study of the fundamental properties of CO2 and its interaction with excess electrons and anions, since they circumvent many problems associated with experiments in the condensed phase. The combination of mass spectrometry, laser spectroscopy and quantum chemical calculations results in a powerful tool set to address questions of reactivity, ion speciation and solvation, and they can provide key information to understanding the ion chemistry of CO2.

Vibrational Autodetachment-Intramolecular Vibrational Relaxation Translated into Electronic Motion

If a negative ion has vibrational energy in excess of the binding energy of its most weakly bound electron, the anion can undergo vibrational autodetachment, similar to thermionic emission. When this effect occurs after targeted infrared excitation of a specific vibrational mode in the anion, it encodes information on the intramolecular vibrational relaxation processes that take place between excitation and electron emission. We present examples on how vibrational autodetachment can be used to obtain infrared spectra of molecular anions, and we discuss how a vibrational autodetachment photoelectron spectrum can be modeled, using vibrational autodetachment after excitation of CH stretching modes of nitromethane anions as a case study.

Vibrational Feshbach resonances in electron attachment to nitrous oxide clusters: decay into heterogeneous and homogeneous cluster anions

Abstract Using a high-resolution (ΔE≈1 meV) laser photoelectron attachment method, we have studied cluster anion formation in collisions of low-energy electrons (1–180 meV) with (N2O)N clusters. We show that formation of both heterogeneous cluster anions (N2O)qO− (q

The infrared predissociation spectra of Cl−·H2O·Arn (n=1–5): experimental determination of the influence of Ar solvent atoms

Abstract We establish the argon solvent size dependence of the Cl − ·H 2 O·Ar n predissociation spectra, and discuss the discrepancies between previously reported predissociation spectra of the Cl − ·H 2 O and Cl − ·H 2 O·Ar 3 complexes [Choi et al., J. Phys. Chem. A 102 (1998) 503; Ayotte et al., J. Am. Chem. Soc. 120 (1998) 12361]. The argon-induced shift in the ∼3130 cm −1 ionic H-bonded OH stretching band, calculated to be large (>30 cm −1 /Ar red-shift) by Satoh and Iwata [Chem. Phys. Lett. 312 (1999) 522], is found to be quite small (IHB band center=3128±3 cm −1 for 1⩽ n ⩽5). We compare this result with similar behavior displayed by the bare versus argon-solvated bromide monohydrate.

Interaction of Nickel with Carbon Dioxide in [Ni(CO2)n]− Clusters Studied by Infrared Spectroscopy

We present infrared photodissociation spectra of [Ni(CO2)n](-) clusters (n = 2-8) in the wavenumber region of 1000-2400 cm(-1) using the antisymmetric stretching vibrational modes of the CO2 units in the clusters as structural probes. We use density functional theory to aid in the interpretation of our experimental results. The dominant spectral signatures arise from a core ion composed of a nickel atom and two CO2 ligands bound to the Ni atom in a bidentate fashion, while the rest of the CO2 molecules in the cluster play the role of solvent. Other core structures are observed as well but as minor contributors. The results for [Ni(CO2)n](-) clusters are discussed in the context of other anionic transition- metal complexes with CO2.

Photodetachment spectroscopy of PtBr42−: Probing the Coulomb barrier of a doubly charged anion

We probe the repulsive Coulomb barrier of the doubly charged anion PtBr(4) (2-) by photodetachment spectroscopy. The results are discussed in terms of models for the photoemission process, the excitation spectrum of PtBr(4) (2-), and calculations of the energy-dependent tunneling probability for various model potentials.