Infrared Induced Reactivity on the Surface of Isolated Size-Selected Clusters: Dissociation of N2O on Rhodium Clusters

A Combined Infrared and Computational Study of Gas-Phase Mixed-Ligand Rhodium Complexes: Rh(CO)n(N2O)m+ (n = 1-5, m = 1-4)

In this study, mixed carbonyl and nitrous oxide complexes with Rh+ were studied by mass-selective infrared photodissociation spectroscopy in a molecular beam. The infrared spectra, recorded in the region of the CO and N2O N═N stretches, were assigned and interpreted with the aid of simulated spectra of low-energy structural isomers. Clear evidence of an inner coordination shell of four ligands is observed. The observed vibrational structure can be understood on the basis of local mode vibrations in the two ligands. However, there is also evidence of multiple low-lying isomers and cooperative binding effects between the two ligands. In particular, σ donation from directly coordinated nitrous oxide ligands drives more classical carbonyl bonding than has been observed in pure carbonyl complexes. The observed fragmentation branching ratios following resonant infrared absorption are explained by simple statistical and energetic arguments, providing a contrast with those of equivalent Au+ complexes.

Probing the binding and activation of small molecules by gas-phase transition metal clusters via IR spectroscopy

Isolated transition metal clusters have been established as useful models for extended metal surfaces or deposited metal particles, to improve the understanding of their surface chemistry and of catalytic reactions. For this objective, an important milestone has been the development of experimental methods for the size-specific structural characterization of clusters and cluster complexes in the gas phase. This review focusses on the characterization of molecular ligands, their binding and activation by small transition metal clusters, using cluster-size specific infrared action spectroscopy. A comprehensive overview and a critical discussion of the experimental data available to date is provided, reaching from the initial results obtained using line-tuneable CO₂ lasers to present-day studies applying infrared free electron lasers as well as other intense and broadly tuneable IR laser sources.

Zooming in on the initial steps of catalytic NO reduction using metal clusters

The study of reactions relevant to heterogeneous catalysis on the surface of well-defined metal clusters with full control over the number of consituent atoms and elemental composition can lead to a detailed insight into the interactions between metal and reactants. We here review experimental and theoretical studies involving the adsorption of NO molecules on mostly rhodium-based clusters under near-thermal conditions in a molecular beam. We show how IR spectrosopic characterization can give information on the binding nature of NO to the clusters for at least the first three NO molecules. The complementary technique of thermal desorption spectrometry reveals at what temperatures multiple NO molecules on the cluster surface desorb or combine to form rhodium oxides followed by N₂ elimination. Variation of the cluster elemental composition can be a powerful method to identify how the propensity of the critical first step of NO dissociation can be increased. The testing of such concepts with atomic detail can be of great help in guiding the choices in rational catalyst design.

The study of reactions relevant to heterogeneous catalysis on metal clusters with full control over the number of constituent atoms and elemental composition can lead to a detailed insight into the interactions governing catalytic functionality.

Water Splitting by C60 -Supported Vanadium Single Atoms

Water splitting is an important source of hydrogen, a promising future carrier for clean and renewable energy. A detailed understanding of the mechanisms of water splitting, catalyzed by supported metal atoms or nanoparticles, is essential to improve the design of efficient catalysts. Here, we report an infrared spectroscopic study of such a water splitting process, assisted by a C₆₀ supported vanadium atom, C₆₀ V⁺ +H₂ O→C₆₀ VO⁺ +H₂ . We probe both the entrance channel complex C₆₀ V⁺ (H₂ O) and the end product C₆₀ VO⁺ , and observe the formation of H₂ as a result from resonant infrared absorption. Density functional theory calculations exploring the detailed reaction pathway reveal that a quintet-to-triplet spin crossing facilitates the water splitting reaction by C₆₀ -supported V⁺ , whereas this reaction is kinetically hindered on the isolated V⁺ ion by a high energy barrier. The C₆₀ support has an important role in lowering the reaction barrier with more than 70 kJ mol^-1 due to a large orbital overlap of one water hydrogen atom with one carbon atom of the C₆₀ support. This fundamental insight in the water splitting reaction by a C₆₀ -supported single vanadium atom showcases the importance of supports in single atom catalysts by modifying the reaction potential energy surface.

Spectroscopy and Infrared Photofragmentation Dynamics of Mixed Ligand Ion-Molecule Complexes: Au(CO)x(N2O)y. J Phys Chem A 2021;125:7266-7277. [PMID: 34433267 DOI: 10.1021/acs.jpca.1c05800]

We report a combined experimental and computational study of the structure and fragmentation dynamics of mixed ligand gas-phase ion-molecule complexes. Specifically, we have studied the infrared spectroscopy and vibrationally induced photofragmentation dynamics of mass-selected Au(CO)_x(N₂O)_y⁺ complexes. The structures can be understood on the basis of local CO and N₂O chromophores in different solvation shells with CO found preferentially in the core. Rich fragmentation dynamics are observed as a function of complex composition and the vibrational mode excited. The dynamics are characterized in terms of branching ratios for different ligand loss channels in light of calculated internal energy distributions. Intramolecular vibrational redistribution appears to be rapid, and dissociation is observed into all energetically accessible channels with little or no evidence for preferential breaking of the weakest intermolecular interactions.

Synergic effects between boron and nitrogen atoms in BN-codoped C59-n BN n fullerenes (n = 1-3) for metal-free reduction of greenhouse N2O gas

The geometries, electronic structures, and catalytic properties of BN-codoped fullerenes C59-n BN n (n = 1-3) are studied using first-principles computations. The results showed that BN-codoping can significantly modify the properties of C60 fullerene by breaking local charge neutrality and creating active sites. The codoping of B and N enhances the formation energy of fullerenes, indicating that the synergistic effects of these atoms helps to stabilize the C59-n BN n structures. The stepwise addition of N atoms around the B atom improves catalytic activities of C59-n BN n in N2O reduction. The reduction of N2O over C58BN and C57BN2 begins with its chemisorption on the B-C bond of the fullerene, followed by the concerted interaction of CO with N2O and the release of N2. The resulting OCO intermediate is subsequently transformed into a CO2 molecule, which is weakly adsorbed on the B atom of the fullerene. On the contrary, nitrogen-rich C56BN3 fullerene is found to decompose N2O into N2 and O* species without the requirement for activation energy. The CO molecule then removes the O* species with a low activation barrier. The activation barrier of the N2O reduction on C56BN3 fullerene is just 0.28 eV, which is lower than that of noble metals.

The efficient separation of N2O/CO2 using unsaturated Fe2+ sites in MIL-100Fe

It is a big challenge to separate N2O from CO2 using adsorption because they have similar physical properties. The Fe3+-F- site in MIL-100Fe transforms to an unsaturated Fe2+ site under high-temperature activation (300 °C), and the target sorbent MIL-100Fe-300 exhibits the biggest difference in CO2 and N2O adsorption capacity, and the selectivity of N2O/CO2 (50%/50%) is up to 3.00 at 298 K. According to DFT calculations, the original Fe3+-F- site has strong interaction with CO2, but the open Fe2+ site has a stronger interaction with N2O. Through a breakthrough experiment, it was confirmed that MIL-100Fe-300 has the best N2O/CO2 separation performance, making it potentially a useful material in industry.

Infrared action spectroscopy of nitrous oxide on cationic gold and cobalt clusters

Understanding the catalytic decomposition of nitrous oxide on finely divided transition metals is an important environmental issue. In this study, we present the results of a combined infrared action spectroscopy and quantum chemical investigation of molecular N2O binding to isolated Aun+ (n ≤ 7) and Con+ (n ≤ 5) clusters. Infrared multiple-photon dissociation spectra have been recorded in the regions of both the N[double bond, length as m-dash]O (1000-1400 cm-1) and N[double bond, length as m-dash]N (2100-2450 cm-1) stretching modes of nitrous oxide. In the case of Aun+ clusters only the ground electronic state plays a role, while the involvement of energetically low-lying excited states in binding to the Con+ clusters cannot be ruled out. There is a clear preference for N-binding to clusters of both metals but some O-bound isomers are observed in the case of smaller Con(N2O)+ clusters.

Not Completely Innocent: How Argon Binding Perturbs Cationic Copper Clusters

Argon is often considered as an innocent probe that can be attached and detached to study the structure of a particular species without perturbing the species too much. We have investigated whether this assumption also holds for small copper cationic clusters and demonstrated that small but significant charge transfer from argon to metal changes the remaining binding positions, leading in general, to weaker binding of other argon atoms. The exception is binding to just one copper ion, where the binding of the first argon facilitates the binding of the second.

Oxophilicity as a Descriptor for NO Cleavage Efficiency over Group IX Metal Clusters

Iridium and rhodium are group IX elements that can both catalytically reduce NO. To understand the difference in their reactivity toward NO, the adsorption forms of NO onto clusters of Ir and Rh are compared using vibrational spectra, recorded via infrared multiple-photon dissociation spectroscopy. The spectra give evidence for the existence of at least two specific adsorption forms. The main Ir₆⁺NO isomer is one in which NO is dissociated, whereas one other is a local minimum structure in the reaction pathway leading to dissociative adsorption. In contrast to adsorption onto Rh₆⁺, where less than 10% of the isomeric population was found in the global minimum associated with dissociative adsorption, a substantial fraction (about 50%) of NO dissociates on Ir₆⁺. This higher efficiency is attributed to a considerably reduced activation barrier for dissociation on Ir₆⁺. The key chemical property identified for dissociation efficiency is the cluster's affinity to atomic oxygen.

Free electron laser infrared action spectroscopy of nitrous oxide binding to platinum clusters, Ptn(N2O)+

Infrared multiple-photon dissociation spectroscopy has been applied to study Pt_n(N₂O)⁺ (n = 1–8) clusters which represent entrance-channel complexes on the reactive potential energy surface for nitrous oxide decomposition on platinum.

Spectroscopic properties and activated mechanism of NO on isolated cationic tantalum clusters: A first-principles study

The adsorption and dissociation of NO on the cationic Ta₁₅⁺ cluster were investigated using the density-functional theory (DFT) calculations, and the Ta-centered bicapped hexagonal antiprism (BHA) structure of cationic Ta₁₅⁺ cluster can be identified as the global minimum, which reproduces well the infrared multiple photo dissociation (IR-MPD) spectrum. Our results show that the cationic BHATa₁₅⁺ cluster provides the hollow region for NO to interact effectively, and possess larger adsorption strength on the region than other sites. The density of states, charge density differences and frontier molecular orbitals were analyzed to understand the electronic properties of the stable NO-adsorbed isomers. The characteristic IR peaks of the firstly two low-lying isomers are properly assigned, in which the strongest IR peak originates from the N - O stretching vibration. For the dissociation of NO on the BHATa₁₅⁺ cluster, it is found that the reaction path II easily occurs rather than path I due to small reaction barrier, and the cluster may possess the great catalytic behavior to dissociate NO molecule. The present results will inevitably stimulate future theoretical and experimental studies for the design of novel Ta-based catalytic materials for the NO dissociation.

Infrared multiple photon dissociation of cesium iodide clusters doped with mono-, di- and triglycine

Charged cesium iodide clusters doped with mono-, di- and triglycine serve as a model system for sea salt aerosols containing biological molecules. Here, we investigate reactions of these complexes under infrared irradiation, with spectra obtained by infrared multiple photon dissociation. The cluster ions are generated via electrospray ionization and analyzed in the cell of a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer. Depending on the cluster size and peptide length, loss of HI or loss of a glycine unit is observed. The experimental measurements are supported by quantum chemical calculations. We show that N-H and O-H stretching modes dominate the spectrum, with large shifts depending on local interactions, namely due to interaction with iodide anions or intramolecular hydrogen bonding. Both experiment and theory indicate that several isomers are present in the experimental mixture, with different infrared fingerprints as well as dissociation pathways.

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.

Account of chemical bonding and enhanced reactivity of vanadium-doped rhodium clusters toward C–H activation: a DFT investigation

The chemical bonding and enhanced reactivity of vanadium-doped rhodium clusters toward C–H activation were investigated using DFT.

Structural isomers and low-lying electronic states of gas-phase M+(N2O)n (M = Co, Rh, Ir) ion–molecule complexes

The structures of gas-phase group nine cation–nitrous oxide metal–ligand complexes, M⁺(N₂O)_n (M = Co, Rh, Ir; n = 2–7) have been determined by a combination of infrared photodissociation spectroscopy and density functional theory.

MIL-100Cr with open Cr sites for a record N2O capture

Nitrous oxide (N2O) is considered as the third most important greenhouse gas after carbon dioxide and methane and needs to be removed from air. Herein, we reported the metal-organic framework MIL-100Cr with open Cr sites for record N2O capture capacities of 5.78 mmol g-1 at 298 K and 8.25 mmol g-1 at 273 K, respectively. DFT calculations showed that the static binding energy of N2O on the open-Cr site is notably higher than that of N2, 72.5 kJ mol-1vs. 51.6 kJ mol-1, which enforces MIL-100Cr to exhibit extremely high N2O/N2 ideal adsorbed solution theory (IAST) gas separation selectivity up to 1000.

Infrared Spectroscopy of Au+(CH4) n Complexes and Vibrationally-Enhanced C-H Activation Reactions

A combined spectroscopic and computational study of gas-phase Au⁺(CH₄)_n (n = 3–8) complexes reveals a strongly-bound linear Au⁺(CH₄)₂ core structure to which up to four additional ligands bind in a secondary coordination shell. Infrared resonance-enhanced photodissociation spectroscopy in the region of the CH₄a₁ and t₂ fundamental transitions reveals essentially free internal rotation of the core ligands about the H₄C–Au⁺–CH₄ axis, with sharp spectral features assigned by comparison with spectral simulations based on density functional theory. In separate experiments, vibrationally-enhanced dehydrogenation is observed when the t₂ vibrational normal mode in methane is excited prior to complexation. Clear infrared-induced enhancement is observed in the mass spectrum for peaks corresponding 4u below the mass of the Au⁺(CH₄)_n=2,3 complexes corresponding, presumably, to the loss of two H₂ molecules.

Ab initio and anion photoelectron study of AunRhm (n = 1–7, m = 1–2) clusters

Anion photoelectron spectroscopy and DFT calculations study on Au_nRh_m (n = 1–7 and m = 1–2). PES spectra, vertical and adiabatic detachment energies, are compared. The characteristic planarity for gold clusters is preserved for many of the bimetallic clusters.

Catalytic reduction of NO by CO on Rh4+ clusters: a density functional theory study

An extensive study was conducted to explore the catalytic reduction of NO by CO on Rh₄⁺ clusters at the ground and first excited states at the B3LYP/6-311+G(2d), SDD level.

Nitric oxide adsorption and reduction reaction mechanism on the Rh7(+) cluster: a density functional theory study

The transition metal rhodium has been proved the effective catalyst to convert from NO(x) to N(2.) In the present work, we are mainly focused on the NO adsorption and decomposition reaction mechanism on the surface of the Rh(7)(+) cluster, and the calculated results suggest that the reaction can proceed via three steps. First, the NO can adsorb on the surface of the Rh(7)(+) cluster; second, the NO decomposes to N and O atoms; finally, the N atom reacts with the second adsorbed NO and reduces to a N(2) molecule. The N-O bond breaks to yield N and O atoms in the second step, which is the rate-limiting step of the whole catalytic cycle. This step goes over a relatively high barrier (TS(12)) of 39.6 kcal/mol and is strongly driven by a large exothermicity of 55.1 kcal/mol during the formation of stable compound 3, accompanied by the N and O atoms dispersed on the different Rh atoms of the Rh(7)(+) cluster. In addition, the last step is very complex due to the different possibilities of reaction mechanism. On the basis of the calculations, in contrast to the reaction path II that generates N(2) from two nitrogen atoms coupling, the reaction path I for the formation of intermediate N(2)O is found to be energetically more favorable. Present work would provide some valuable fundamental insights into the behavior of the nitric oxide adsorption and reduction reaction mechanism on the Rh(7)(+) cluster.

Chemistry with methane: concepts rather than recipes

Four seemingly simple transformations related to the chemistry of methane will be addressed from mechanistic and conceptual points of view: 1) metal-mediated dehydrogenation to form metal carbene complexes, 2) the hydrogen-atom abstraction step in the oxidative dimerization of methane, 3) the mechanisms of the CH(4)→CH(3)OH conversion, and 4) the initial bond scission (C-H vs. O-H) as well as the rate-limiting step in the selective CH(3)OH→CH(2)O oxidation. State-of-the-art gas-phase experiments, in conjunction with electronic-structure calculations, permit identification of the elementary reactions at a molecular level and thus allow us to unravel detailed mechanistic aspects. Where appropriate, these results are compared with findings from related studies in solution or on surfaces.