Electron Distribution in Partially Reduced Mixed Metal Oxide Systems: Infrared Spectroscopy of CemVnOo+ Gas-Phase Clusters

Lanthanide Oxides: From Diatomics to High-Spin, Strongly Correlated Homo- and Heterometallic Clusters

Small lanthanide (Ln) oxide clusters present both experimental and theoretical challenges because of their partially filled, core-like 4f ⁿ orbitals, a feature that results in a plethora of close-lying and fundamentally similar electronic states. These clusters provide a bottom-up approach toward understanding the electronic structure of defective or doped bulk material but also can offer a challenge to the theorists to find a method robust enough to capture electronic structure patterns that emerge from within the 4f ⁿ (0 < n < 14) series. In this Feature Article, we explore the electronic structures of small lanthanide oxide clusters that deviate from bulk stoichiometry using anion photoelectron spectroscopy and supporting density functional theory calculations. We will describe the evolution of electronic structure with oxidation and how Ln_xO_y^- cluster reactivities can be correlated with specific Ln-local orbital occupancies. These strongly correlated systems offer additional insights into how interactions between electrons and electronically complex neutrals can lead to detachment transitions that lie outside of the sudden one-electron detachment approximation generally assumed in anion photoelectron spectroscopy. With a better understanding of how we can control nominally forbidden transitions to sample an array of spin states, we suggest that more in-depth studies on the magnetic states of these systems can be explored. Extending these studies to other Ln-based materials with hidden magnetic phases, along with sequentially ligated single molecule magnets, could advance current understanding of these systems.

Identification of Active Sites and Structural Characterization of Reactive Ionic Intermediates by Cryogenic Ion Trap Vibrational Spectroscopy

Cryogenic ion trap vibrational spectroscopy paired with quantum chemistry currently represents the most generally applicable approach for the structural investigation of gaseous cluster ions that are not amenable to direct absorption spectroscopy. Here, we give an overview of the most popular variants of infrared action spectroscopy and describe the advantages of using cryogenic ion traps in combination with messenger tagging and vibrational predissociation spectroscopy. We then highlight a few recent studies that apply this technique to identify highly reactive ionic intermediates and to characterize their reactive sites. We conclude by commenting on future challenges and potential developments in the field.

Heterogeneous catalysis

A heterogeneous catalyst is a functional material that continually creates active sites with its reactants under reaction conditions. These sites change the rates of chemical reactions of the reactants localized on them without changing the thermodynamic equilibrium between the materials.

Gas-phase reaction of CeVO5+ cluster ions with C2H4: the reactivity of cluster bonded peroxides

The reactivity of the peroxide unit with hydrocarbon molecules on transition metal oxide clusters with a closed-shell electronic structure has been identified for the first time.

Gas-phase reactions of cationic vanadium-phosphorus oxide clusters with C2H(x) (x=4, 6): a DFT-based analysis of reactivity patterns

The reactivities of the adamantane-like heteronuclear vanadium-phosphorus oxygen cluster ions [V(x)P(4-x)O(10)](.+) (x=0, 2-4) towards hydrocarbons strongly depend on the V/P ratio of the clusters. Possible mechanisms for the gas-phase reactions of these heteronuclear cations with ethene and ethane have been elucidated by means of DFT-based calculations; homolytic C-H bond activation constitutes the initial step, and for all systems the P-O(.) unit of the clusters serves as the reactive site. More complex oxidation processes, such as oxygen-atom transfer to, or oxidative dehydrogenation of the hydrocarbons require the presence of a vanadium atom to provide the electronic prerequisites which are necessary to bring about the 2e(-) reduction of the cationic clusters.

Gas-Phase Neutral Binary Oxide Clusters: Distribution, Structure, and Reactivity toward CO

Neutral binary (vanadium-cobalt) oxide clusters are generated and detected in the gas phase for the first time. Their reactivities toward carbon monoxide (CO) are studied both experimentally and theoretically. Experimental results suggest that neutral VCoO4 can react with CO to generate VCoO3 and CO2. Density functional theory studies show parallel results as well as provide detailed reaction mechanisms.

Structure characterization of metal oxide clusters by vibrational spectroscopy: possibilities and prospects

This article summarizes the methodological progress that has been made in the vibrational spectroscopy of isolated polynuclear metal oxide clusters, with particular emphasis on free electron laser-based infrared action spectroscopy of gas phase clusters, over the last decade. The possibilities, limitations and prospects of the various experimental approaches are discussed using representative examples from pivotal studies in the field.