Probing the structures of gas-phase rhodium cluster cations by far-infrared spectroscopy

Multifacets of Fullerene-Metal Clusters: From Fundamental to Application

ConspectusBuckminsterfullerene, C60, was discovered through a prominent mass peak containing 60 atoms produced from laser vaporization of graphite, driven by Kroto's interest in understanding the formation mechanisms of carbon-containing molecules in space. Inspired by the geodesic dome-shaped architecture designed by Richard Buckminster Fuller, after whom the particle was named, C60 was found to have a football-shaped structure comprising 20 hexagons and 12 pentagons. It sparked worldwide interest in understanding this new carbon allotrope, resulting in the awarding of the Noble Prize in Chemistry to Smalley, Kroto, and Curl in 1996.Intrinsically, C60 is an exceptional species because of its high stability and electron-accepting ability and its structural tunability by decorating or substituting either on its exterior surface or interior hollow cavity. For example, metal-decorated fullerene complexes have found important applications ranging from superconductivity, nanoscale electronic devices, and organic photovoltaic cells to catalysis and biomedicine. Compared to the large body of studies on atoms and molecules encapsulated by C60, studies on the exteriorly modified fullerenes, i.e., exohedral fullerenes, are scarcer. Surprisingly, to date, uncertainty exists about a fundamental question: what is the preferable exterior binding site of different kinds of single atoms on the C60 surface?In recent years, we have developed an experimental protocol to synthesize the desired fullerene-metal clusters and to record their infrared spectra via messenger-tagged infrared multiple photon dissociation spectroscopy. With complementary quantum chemical calculations and molecular dynamics simulations, we determined that the most probable binding site of a metal, specifically a vanadium cation, on C60 is above a pentagonal center in an η5 fashion. We explored the bonding nature between C60 and V+ and revealed that the high thermal stability of this cluster originates from large orbital and electrostatic interactions. Through comparing the measured infrared spectra of [C60-Metal]+ with the observational Spitzer data of several fullerene-rich planetary nebulae, we proposed that the complexes formed by fullerene and cosmically abundant metals, for example, iron, are promising carriers of astronomical unidentified spectroscopic features. This opens the door for a real consideration of Kroto's 30-year-old hypothesis that complexes involving cosmically abundant elements and C60 exhibit strong charge-transfer bands, similar to those of certain unidentified astrophysical spectroscopic features. We compiled a VibFullerene database and extracted a set of vibrational frequencies and intensities for fullerene derivatives to facilitate their potential detection by the James Webb Space Telescope. In addition, we showed that upon infrared irradiation C60V+ can efficiently catalyze water splitting to generate H2. This finding is attributed to the novel geometric-electronic effects of C60, acting as "hydrogen shuttle" and "electron sponge", which illustrates the important role of carbon-based supports in single-atom catalysts. Our work not only unveils the basic structures and bonding nature of fullerene-metal clusters but also elucidates their potential importance in astrophysics, astrochemistry, and catalysis, showing the multifaceted character of this class of clusters. More exciting and interesting aspects of the fullerene-metal clusters, such as ultrafast charge-transfer dynamics between fullerene and metal and their relevance to designing hybrid fullerene-metal junctions for electronic devices, are awaiting exploration.

Size-dependent reactivity of Rh cationic clusters to reduce NO by CO in the gas phase at high temperatures

The reactivity of the reduction of NO pre-adsorbed on Rh2-9+ clusters by CO was investigated using a combination of an alternate on-off gas injection method and thermal desorption spectrometry. The reduction of RhnNxOy+ clusters by CO was evaluated by varying the CO concentration at T = 903 K. Among the RhnNxOx+ clusters, the Rh3N2O2+ cluster exhibited the highest reduction activity, whereas the other clusters, Rh2,4-9NxOx+, showed lower reactivity. Density functional theory (DFT) calculations for Rh3+ and Rh6+ revealed that the rate-determining step for NO reduction in the presence of CO was NO bond dissociation through the kinetics analysis using the RRKM theory. The reduction of Rh3N2O2+ is kinetically preferable to that of Rh6N2O2+. The DFT results were in qualitative agreement with the experimental results.

To What Extent Do Iodomethane and Bromomethane Undergo Analogous Reactions?

Iodomethane and bromomethane (CH3I/CH3Br) are common chemicals, but their chemistry on nanometals is not fully understood. Here, we analyze the reactivity of Rhn+ (n = 3-30) clusters with halomethanes and unveil the spin effect and concentration dependence in the C-H and C-X bond activation. It is found that the reactions under halomethane-rich conditions differ from those under metal-rich conditions. Both CH3I and CH3Br undergo similar dehydrogenation on the Rhn+ clusters in the presence of small quantity reactants; however, different reactions are observed in the presence of sufficient CH3I/CH3Br, showing dominant Rh(CH3Br)x+ (x = 1-4) products but a series of RhnCxHyIz+ species (x = 1-4, y = 1-12, and z = 1-5) pertaining to H2, HI, or CH4 removal. Density functional theory calculations reveal that the dehydrogenation and demethanation of CH3Br are relatively less exothermic and will be deactivated by sufficient gas collisions if helium cooling takes away energy immediately; instead, the successive adsorption of CH3Br gives rise to a series of Rh(CH3Br)x+ species with accidental C-Br bond dissociation.

Reactions of N2O and CO on neutral Rh10On clusters: a density functional study

Density functional theory calculations were performed to identify product, reactant and intermediate dissociative/associative structures for the oxygen abstraction and addition reactions: Rh10On + CO → Rh10On-1 + CO2, n = 1-5 and Rh10On + N2O → Rh10On+1 + N2, n = 0-4 reactions. In the case of the oxygen abstraction reactions, the energetics of the reaction path were very similar in energy regardless of the number of oxygen atoms on the Rh10On cluster, whereas for the addition of oxygen to the Rh10On cluster, the reaction was found to become significantly less exothermic with each successive addition of oxygen.

How important is the amount of exact exchange for spin-state energy ordering in DFT? Case study of molybdenum carbide cluster, Mo4C2

Since the form of the exact functional in density functional theory is unknown, we must rely on density functional approximations (DFAs). In the past, very promising results have been reported by combining semi-local DFAs with exact, i.e. Hartree-Fock, exchange. However, the spin-state energy ordering and the predictions of global minima structures are particularly sensitive to the choice of the hybrid functional and to the amount of exact exchange. This has been already qualitatively described for single conformations, reactions, and a limited number of conformations. Here, we have analyzed the mixing of exact exchange in exchange functionals for a set of several hundred isomers of the transition metal carbide, Mo4C2. The analysis of the calculated energies and charges using PBE0-type functional with varying amounts of exact exchange yields the following insights: (1) The sensitivity of spin-energy splitting is strongly correlated with the amount of exact exchange mixing. (2) Spin contamination is exacerbated when correlation is omitted from the exchange-correlation functional. (3) There is not one ideal value for the exact exchange mixing which can be used to parametrize or choose among the functionals. Calculated energies and electronic structures are influenced by exact exchange at a different magnitude within a given distribution; therefore, to extend the application range of hybrid functionals to the full periodic table the spin-energy splitting energies should be investigated.

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.

Miniaturized infrared spectrometer based on the tunable graphene plasmonic filter

Miniaturization of a conventional spectrometer is challenging because of the tradeoffs of size, cost, signal-to-noise ratio, and spectral resolution, etc. Here, a new type of miniaturized infrared spectrometer based on the integration of tunable graphene plasmonic filters and infrared detectors is proposed. The transmittance spectrum of a graphene plasmonic filter can be tuned by varying the Fermi energy of the graphene, allowing light incident on the graphene plasmonic filter to be dynamically modulated in a way that encodes its spectral information in the receiving infrared detector. The incident spectrum can then be reconstructed by using decoding algorithms such as ridge regression and neural networks. The factors that influence spectrometer performance are investigated in detail. It is found that the graphene carrier mobility and the signal-to-noise ratio are two key parameters in determining the resolution and precision of the spectrum reconstruction. The mechanism behind our observations can be well understood in the framework of the Wiener deconvolution theory. Moreover, a hybrid decoding (or recovery) algorithm that combines ridge regression and a neural network is proposed that demonstrates a better spectral recovery performance than either the ridge regression or a deep neural network alone, being able to achieve a sub-hundred nanometer spectral resolution across the 8∼14 µm wavelength range. The size of the proposed spectrometer is comparable to a microchip and has the potential to be integrated within portable devices for infrared spectral imaging applications.

NO Bond Cleavage on Gas-Phase Irn+ Clusters Investigated by Infrared Multiple Photon Dissociation Spectroscopy

The adsorption forms of NO on Ir_n⁺ (n = 3-6) clusters were investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) calculations. Spectral features indicative both for molecular NO adsorption (the NO stretching vibration in the 1800-1900 cm^-1 range) and for dissociative NO adsorption (the terminal Ir-O vibration around 940 cm^-1) were observed, elucidating the co-existence of molecular and dissociative adsorption of NO. In all calculated structures for molecular adsorption, NO is adsorbed via the N atom on on-top sites. For dissociative adsorption, the O atom adsorbs exclusively on on-top sites (μ₁) of the clusters, whereas the N atom is found on either a bridge (μ₂) or a hollow (μ₃) site. For Ir₅⁺ and Ir₆⁺, the N atom is also found on the on-top sites. The observed propensity for NO dissociation on Ir_n⁺ (n = 3-6) is higher than that for Rh₆⁺, which can be explained by the higher metal-oxygen bond strengths for iridium.

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.

Ladder Oxygenation of Group VIII Metal Clusters and the Formation of Metalloxocubes M13O8. J Phys Chem Lett 2022;13:733-739. [PMID: 35025527 DOI: 10.1021/acs.jpclett.1c04098]

The diversity of valence and bonding of transition metals makes their oxidation processes perplexing at reduced sizes. Here we report a comprehensive study on the oxidation reactions of rhodium clusters Rh_n^± (n = 3-30) and find that Rh_3,4O₄⁺, Rh_5-7O₆⁺, and Rh_8-13O₈⁺ always dominate the mass distributions showing size-dependent ladder oxygenation which is closely associated with the O-binding modes. While the Rh_8-13O₈⁺ clusters display a μ₃-O binding mode (hollow site adsorption), Rh_3-4O₄⁺ and Rh_5-7O₆⁺ favor the μ₂-O binding mode (edge-site adsorption) or a mixture of the two modes. The μ₃-O binding mode is inclined to yield a cubic Rh₁₃O₈, while the μ₂-O binding mode gives rise to oxygen-bridge protection for the metal clusters. Such ladder oxidation was also observed for Pt_n⁺, Fe_n⁺, Co_n⁺, and Ni_n⁺ clusters. We propose a three-dimensional diagram for the oxidation states and O-binding modes of metals, and highlight the metalloxocubes M₁₃O₈⁺ for cluster-genetic materials.

Cryo infrared spectroscopy of N2 adsorption onto bimetallic rhodium-iron clusters in isolation

We investigated the N₂ adsorption behavior of bimetallic rhodium-iron cluster cations [Rh_iFe_j(N₂)_m]⁺ by means of InfraRed MultiplePhotoDissociation (IR-MPD) spectroscopy in comparison with density functional theory (DFT) modeling. This approach allows us to refine our kinetic results [Ehrhard et al., J. Chem. Phys. (in press)] to enhance our conclusions. We focus on a selection of cluster adsorbate complexes within the ranges of i = j = 3-8 and m = 1-10. For i = j = 3, 4, DFT suggests alloy structures in the case of i = j = 4 of high (D_2d) symmetry: Rh-Fe bonds are preferred instead of Fe-Fe bonds or Rh-Rh bonds. N₂ adsorption and IR-MPD studies reveal strong evidence for preferential adsorption to Rh sites and mere secondary adsorption to Fe. In some cases, we observe adsorption isomers. With the help of modeling the cluster adsorbate complex [Rh₃Fe₃(N₂)₇]⁺, we find clear evidence that the position of IR bands allows for an element specific assignment of an adsorption site. We transfer these findings to the [Rh₄Fe₄(N₂)_m]⁺ cluster adsorbate complex where the first four N₂ molecules are exclusively adsorbed to the Rh atoms. The spectra of the larger adsorbates reveal N₂ adsorption onto the Fe atoms. Thus, the spectroscopic findings are well interpreted for the smaller clusters in terms of computed structures, and both compare well to those of our accompanying kinetic study [Ehrhard et al., J. Chem. Phys. (in press)]. In contrast to our previous studies of bare rhodium clusters, the present investigations do not provide any indication for a spin quench in [Rh_iFe_j(N₂)_m]⁺ upon stepwise N₂ adsorption.

Decomposition of nitric oxide by rhodium cluster cations at high temperatures

Decomposition reactions of NO molecules on gas-phase Rh_n⁺ (n = 6-9) clusters were investigated by gas-phase thermal desorption spectrometry and density functional theory calculations. We found that NO adsorbs on the clusters, forming Rh_nN_xO_x⁺ at room temperature. Upon heating, NO desorption was observed below 800 K. Above 800 K, while for n = 7 and 8, each of Rh₇N₃O₃⁺, Rh₇N₄O₄⁺, and Rh₈N₃O₃⁺ was found to release an N₂ molecule, no N₂ formation was clearly observed for Rh_6,9N_xO_y⁺. We considered that both Rh₇N₃O₃⁺ and Rh₈N₃O₃⁺ have at least two dissociated NO molecules, while Rh₆N_xO_x⁺ (x = 1-3) has one or less. Our computational results for Rh₈N₃O₃⁺ suggested that the formation of an N-N bond in the Rh₈N₃O₃⁺ structure must overcome an energy barrier of ∼2 eV, which is the highest among the suggested possible reaction pathways. These findings suggested that the size-dependent activity of NO decomposition is governed primarily by how NO molecules are adsorbed on Rh_n⁺ clusters, i.e. whether two or more N atoms from dissociated NO molecules exist in the NO adsorbed clusters, and secondly, by the readiness of the N-N bond formation.

Assessment of the van der Waals, Hubbard U parameter and spin-orbit coupling corrections on the 2D/3D structures from metal gold congeners clusters

The coinage-metal clusters possess a natural complexity in their theoretical treatment that may be accompanied by inherent shortcomings in the methodological approach. Herein, we performed a scalar-relativistic density functional theory study, considering Perdew, Burke, and Ernzerhof (PBE) with (empirical and semi empirical) van der Waals (vdW), spin-orbit coupling (SOC), +U (Hubbard term), and their combinations, to treat the Cu 13 , Ag 13 , and Au 13 clusters in different structural motifs. The energetic scenario is given by the confirmation of the 3D lowest energy configurations for Cu 13 and Ag 13 within all approaches, while for Au 13 there is a 2D/3D competition, depending on the applied correction. The 2D geometry is 0.43 eV more stable with plain PBE than the 3D one, the SOC, +U, and/or vdW inclusion decreases the overestimated stability of the planar configurations, where the most surprising result is found by the D3 and D3BJ vdW corrections, for which the 3D configuration is 0.29 and 0.11 eV, respectively, more stable than the 2D geometry (with even higher values when SOC and/or +U are added). The D3 dispersion correction represents 7.9% (4.4%) of the total binding energy for the 3D (2D) configuration, (not) being enough to change the sd hybridization and the position of the occupied d -states. Our predictions are in agreement with experimental results and in line with the best results obtained for bulk systems, as well as with hybrid functionals within D3 corrections. The properties description undergoes small corrections with the different approaches, where general trends are maintained, that is, the average bond length is smaller (larger) for lower (higher)-coordinated structures, since a same number of electrons are shared by a smaller (larger) number of bonds, consequently, the bonds are stronger (weaker) and shorter (longer) and the sd hybridization index is larger (smaller). Thus, Au has a distinct behavior in relation to its lighter congeners, with a complex potential energy surface, where in addition to the relevant relativistic effects, correlation and dispersion effects must also be considered.

Reactivity of Cobalt Clusters Con±/0 with Dinitrogen: Superatom Co6+ and Superatomic Complex Co5N6. J Phys Chem A 2021;125:2130-2138. [PMID: 33689326 DOI: 10.1021/acs.jpca.1c00483]

We report a joint experimental and theoretical study on the reactions of cobalt clusters (Co_n^±/0) with nitrogen using the customized reflection time-of-flight mass spectrometer combined with a 177.3 nm deep-ultraviolet laser. Comparing to the behaviors of neutral Co_n (n = 2-30) and anionic Co_n^- clusters (n = 7-53) which are relatively inert in reacting with nitrogen in the fast-flow tube, Co_n⁺ clusters readily react with nitrogen resulting in adducts of one or multiple N₂ except Co₆⁺ which stands firm in the reaction with nitrogen. Detailed quantum chemistry calculations, including the energetics, electron occupancy, and orbital analysis, well-explained the reasonable reactivity of Co_n⁺ clusters with nitrogen and unveiled the open-shell superatomic stability of Co₆⁺ within a highly symmetric (D_3d) structure. The D_3d Co₆⁺ bears an electron configuration of a half-filled superatomic 1P orbital (i.e., 1S²1P³||1D⁰), a large α-highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap, symmetric multicenter bonds, and reasonable electron delocalization pertaining to metallic aromaticity. Topology analysis by atom-in-molecule illustrates the interactions between Co_n⁺ and N₂ corresponding to covalent bonds, but the Co-N interactions in cationic Co₂⁺N₂ and Co₆⁺N₂ clusters are apparently weaker than those in the other systems. In addition, we identify a superatomic complex Co₅N₆⁺ which exhibits similar frontier orbitals as the naked Co₅⁺ cluster, but the alpha HOMO-LUMO gap is nearly double-magnified, which is consistent with the high-abundance peak of Co₅N₆⁺ in the experimental observation. The enhanced stability of such a ligand-coordinated superatomic complex Co₅N₆⁺, along with the superatom Co₆⁺ with aromaticity, sheds light on special and general superatoms.

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.

A density functional study on the electronic structure, nature of bonding and reactivity of NO adsorbing Rh0/±n (n = 2–8) clusters

Systematic investigations on lowest energy NO adsorbing neutral and ionic Rh_n (n = 2–8) clusters in the gas phase are executed with an all electron relativistic method using density functional theory (DFT) within the generalized gradient approximation.

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.

Evolution of the structural, energetic, and electronic properties of the 3d, 4d, and 5d transition-metal clusters (30 TMn systems for n = 2–15): a density functional theory investigation

Subnanometric transition-metal (TM) clusters have attracted great attention due to their unexpected physical and chemical properties, leastwise compared to their bulk counterparts.

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.

Ion-molecule clustering in differential mobility spectrometry: lessons learned from tetraalkylammonium cations and their isomers

Differential mobility spectrometry (DMS) can distinguish ions based upon the differences in their high- and low-field ion mobilities as they experience the asymmetric waveform applied to the DMS cell. These mobilities are known to be influenced by the ions' structure, m/z, and charge distribution (i.e., resonance structures) within the ions themselves, as well as by the gas-phase environment of the DMS cell. While these associations have been developed over time through empirical observations, the exact role of ion structures or their interactions with clustering molecules remains generally unknown. In this study, that relationship is explored by observing the DMS behaviors of a series of tetraalkylammonium ions as a function of their structures and the gas-phase environment of the DMS cell. To support the DMS experiments, the basin-hopping search strategy was employed to identify candidate cluster structures for density functional theory treatment. More than a million cluster structures distributed across 72 different ion-molecule cluster systems were sampled to determine global minimum structures and cluster binding energies. This joint computational and experimental approach suggests that cluster geometry, in particular ion-molecule intermolecular separation, plays a critical role in DMS.

Structural and electronic study of neutral, positive, and negative small rhodium clusters [Rh(n), Rh(n)(+), Rh(n)(-) ; n = 10-13]

We have carried out a systematic study for the determination of the structure and the fundamental state of neutral and ionic small rhodium clusters [Rhn, Rhn(+), Rhn(-); n = 10-13] using ab initio Hartree-Fock methods with a LANL2DZ basis set. A range of spin multiplicities is investigated for each cluster. We present the bond lengths, angles, and geometric configuration adopted by the clusters in its minimum energy conformation showing the differences when the clusters have different number of unpaired electrons. Also we report the vertical ionization potential and the adiabatic one calculated by the Koopmans' theorem.

Invited review article: laser vaporization cluster sources

The laser vaporization cluster source has been used for the production of gas phase atomic clusters and metal-molecular complexes for 30 years. Numerous experiments in the chemistry and physics of clusters have employed this source. Its operation is simple in principle, but there are many subtle design features that influence the number and size of clusters produced, as well as their composition, charge state, and temperature. This article examines all aspects of the design of these cluster sources, discussing the relevant chemistry, physics, and mechanical aspects of experimental configurations employed by different labs. The principles detailed here provide a framework for the design and implementation of this source for new applications.