Cationic Clusters Research Articles

Water storage capacity of closed-shell silicon oxyhydroxide cluster cations in the gas phase

Abstract Stable compositions and H2O desorption processes of SinO2n+iH2i+1⁠+ (nominally, (SiO2)n(H2O)iH+; n = 3 to 10) clusters have been studied by gas-phase thermal desorption spectrometry coupled with density functional theory (DFT) calculations. Five or six H2O molecules were found to be stored (i = 5, 6) mainly in the clusters at room temperature. The clusters sequentially released H2O molecules upon heating to form the species with i = 2 (n = 3–5, 8) and 3 (n = 6, 7, 9, 10) as the most prominent compositions at 1,000 K. The desorption energies of H2O molecules from the clusters were evaluated from the temperature dependence data for n = 3 to 5. The experimental and theoretical results suggest that (SiO2)n(H2O)iH+ clusters with higher i (e.g. i ≥ 4 for n = 3) involve H2O molecules bound with hydrogen bonds and can easily release the H2O molecules (ΔE < ∼0.5 eV); species with lower i (e.g. i = 2 and 3 for n = 3) form H2O molecules from two hydroxide groups, whose desorption requires higher energies (ca. 1 to 2 eV); and H2O release is hampered if the product species does not maintain the structure with tetrahedrally coordinated Si atoms.

Thermal Reactions of NiAl3O6+ and Al4O6+ with Methane: Reactivity Enhancement by Doping.

Investigation of the reactivity of heteronuclear metal oxide clusters is an important way to uncover the molecular-level mechanisms of the doping effect. Herein, we performed a comparative study on the reactions of CH4 with NiAl3O6+ and Al4O6+ cluster cations at room temperature to understand the role of Ni during the activation and transformation of methane. Mass spectrometric experiments identify that both NiAl3O6+ and Al4O6+ could bring about hydrogen atom abstraction reaction to generate CH3• radical; however, only NiAl3O6+ has the potential to stabilize [CH3] moiety and then transform [CH3] to CH2O. Density functional theory calculations demonstrate that the terminal oxygen radicals (Ot-•) bound to Al act as the reactive sites for the two clusters to activate the first C-H bond. Although the Ni atom cannot directly participate in methane activation, it can manipulate the electronic environment of the surrounding bridging oxygen atoms (Ob) and enable such Ob to function as an electron reservoir to help Ot-• oxidize CH4 to [H-O-CH3]. The facile reduction of Ni3+ to Ni+ also facilitates the subsequent step of activating the second C-H bond by the bridging "lattice oxygen" (Ob2-), finally enabling the oxidation of methane into formaldehyde. The important role of the dopant Ni played in improving the product selectivity of CH2O for methane conversion discovered in this study allows us to have a possible molecule-level understanding of the excellent performance of the catalysts doping with nickel.

Activation and Transformation of Methane on Boron-Doped Cobalt Oxide Cluster Cations CoBO2.

The cleavage of inert C-H bonds in methane at room temperature and the subsequent conversion into value-added products are quite challenging. Herein, the reactivity of boron-doped cobalt oxide cluster cations CoBO2+ toward methane under thermal collision conditions was studied by mass spectrometry experiments and quantum-chemical calculations. In this reaction, one H atom and the CH3 unit of methane were transformed separately to generate the product metaboric acid (HBO2) and one CoCH3+ ion, respectively. Theoretical calculations strongly suggest that a catalytic cycle can be completed by the recovery of CoBO2+ through the reaction of CoCH3+ with sodium perborate (NaBO3), and this reaction generates sodium methoxide (CH3ONa) as the other value-added product. This study shows that boron-doped cobalt oxide species are highly reactive to facilitate thermal methane transformation and may open a way to develop more effective approaches for methane (CH4) activation and conversion under mild conditions.

Size-dependent reactivity of VnO+ (n = 1-9) clusters with ethane.

Cost-effective and readily accessible 3d transition metals (TMs) have been considered as promising candidates for alkane activation while 3d TMs especially the early TMs are usually not very reactive with light alkanes. In this study, the reactivity of Vn+ and VnO+ (n = 1-9) cluster cations towards ethane under thermal collision conditions has been investigated using mass spectrometry and density functional theory calculations. Among Vn+ (n = 1-9) clusters, only V3-5+ can react with C2H6 to generate dehydrogenation products and the reaction rate constants are below 10-13 cm3 molecule-1 s-1. In contrast, the reaction rate constants for all VnO+ (n = 1-9) with C2H6 significantly increase by about 2-4 orders of magnitude. Theoretical analysis evidences that the addition of ligand O affects the charge distribution of the metal centers, resulting in a significant increase in the cluster reactivity. The analysis of frontier orbitals indicates that the agostic interaction determines the size-dependent reactivity of VnO+ cluster cations. This study provides a novel approach for improving the reactivity of early 3d TMs.

Structure of small yttrium monoxide clusters, chemical bonding, and photoionization: threshold photoionization and density functional theory investigations.

The photoionization (PI) spectra of small gas-phase yttrium monoxide clusters, YnO (n = 1-8), are investigated, and the adiabatic ionization energies are determined. The stable structures are obtained from density functional theory (DFT) calculations. The ground state structures are further confirmed by the CCSD(T) method. The PI spectra are calculated for these stable structures and are compared with the experimental PI spectra. The ground-state structures of the neutral and cation clusters are experimentally assigned with confidence on the basis of a favourable agreement between the experimental and calculated PI spectra. New structures are proposed for Y2O, Y6O, and Y8O compared to the previous literature. Y2O is a linear molecule in the ground state that was previously proposed as a C2v bent molecule. The YnO clusters become 3-dimensional from n ≥ 3. The O atom stays outside, bridging a triangular face of yttrium clusters. Chemical bonding between the yttrium and oxygen atoms is mostly ionic. The excess charge on the oxygen atom is around 1.4e-, transferred from the yttrium atoms bonded with it. Yttrium atoms are mostly covalently bonded. However, for the bigger clusters, free charges of both polarities appear on yttrium atoms that are not bonded with oxygen, indicating ionic interactions. Frontier orbitals consist of mainly delocalized 4d electrons with some 5s contributions, forming Y-Y bonding interactions, but with little contribution and zero contribution from the oxygen orbitals, regardless of the cluster size. The lost electron of YnO+ mostly comes from the 5s orbitals of all Y atoms in the cluster up to size n = 4, and then from 4d-5s hybrid orbitals from n ≥ 5, with the d contribution increasing with size. This is contrary to the previous view in the literature that photoionization occurs from a localized 4d orbital.

Magnetocaloric effect in a high-spin ferromagnetic molecular cluster.

The reaction of MnCl2·4H2O with HL ((1-methyl-1H-imidazol-2-yl)methanol) and pdH2 (1, 3 propanediol) in a basic MeCN solution results in the formation of a mixed-valence [Mn20] cationic cluster and two [MnIICl4] counter anions. The metallic skeleton of the cluster describes two geometrically equivalent mixed-valent, linked [MnIII 6MnII 4] supertetrahedra in which nearest-neighbor metal ions have a different oxidation state. Magnetic susceptibility, magnetization data and heat capacity measurements support evidence of predominant ferromagnetic correlations, leading to a s = 22 spin ground state for the [MnIII 6MnII 4] supertetrahedra, which are pair-linked by a weak antiferromagnetic coupling. The properties are discussed in the context of the magnetocaloric effect and the potential application of this compound in cryogenic refrigeration.

Magic cluster sizes of cationic and anionic sodium chloride clusters explained by statistical modeling of the complete phase space.

As one of the main components of sea salt aerosols, sodium chloride is involved in numerous atmospheric processes. Gas-phase clusters are ideal models to study fundamental physical and chemical properties of sodium chloride, which are significantly affected by the cluster size. Of particular interest are magic cluster sizes, which exhibit high intensities in mass spectra. In order to understand the origin of these magic cluster sizes, quantum chemical calculations at the CCSD(T)//DFT level are performed, yielding structures and binding energies of neutral (NaCl)x, anionic (NaCl)xCl- and cationic (NaCl)xNa+ clusters up to x = 8. Our calculations show that the clusters can easily isomerize, enabling dissociation into the lowest-energy isomers of the fragments. Energetics can explain the special stability of (NaCl)4Cl-, but (NaCl)4Na+ actually offers low-lying dissociation channels, despite being a magic cluster size. Collision-induced dissociation experiments reveal that the loss of neutral clusters (NaCl)x, x = 2, 4, is in most cases more favorable than the loss of NaCl or the atomic ion, i.e. sodium chloride clusters actually fragment via the cleavage of the entire cluster, not by evaporating small cluster building blocks. This is rationalized by the calculated high stability of even-numbered neutral clusters (NaCl)x, especially x = 2, 4. Analysis of the density of states and rate constants calculated with a modified Rice-Ramsperger-Kassel-Marcus (RRKM) equation called AWATAR - considering all energetically accessible isomers of reactants and fragments - shows that entropic effects are responsible for the magic cluster character of (NaCl)4Na+. In particular, low-lying vibrational modes provide a high density of states of the near-planar cluster. Together with the small contribution of an atomic ion to the sum of states in a loose transition state for dissociation, this leads to a very small unimolecular rate constant for dissociation into (NaCl)4 and Na+, which is the lowest energy fragmentation pathway. Thus, entropic effects may override energetics for certain magic cluster sizes.

Cationic Al oxo-hydroxide clusters: syntheses, molecular structures, and functional applications

Al oxo-hydroxide clusters, synthesized through the hydrolysis of Al³⁺ solutions, are expected to bridge the gap between metal-aqua complexes and bulk metal oxides/hydroxides. These clusters exhibit remarkable diversity in structure...

Spectroscopic investigation of size-dependent CO2 binding on cationic copper clusters: analysis of the CO2 asymmetric stretch.

Photofragmentation spectroscopy, combined with quantum chemical computations, was employed to investigate the position of the asymmetric CO2 stretch in cold, He-tagged Cun[CO2]+ (n = 1-10) and Cun[CO2][H2O]+ (n = 1-7) complexes. A blue shift in the band position was observed compared to the free CO2 molecule for Cun[CO2]+ complexes. Furthermore, this shift was found to exhibit a notable dependence on cluster size, progressively redshifting with increasing cluster size. The computations revealed that the CO2 binding energy is the highest for Cu+ and continuously decreases with increasing cluster size. This dependency could be explained by highlighting the role of polarization in electronic structure, according to energy decomposition analysis. The introduction of water to this complex amplified the redshift of the asymmetric stretch, showing a similar dependency on the cluster size as observed for Cun[CO2]+ complexes.

Reactivity of cationic silver clusters with O2: a probe of interplay between clusters' geometric and electronic structures.

We explored the size-dependent reactivity of Agn+ (n = 2-22) with O2 under mild conditions and found that only a few sizes of Agn+, with even values of n = 4, 6, 12, 16, 18, and 22, are reactive. Possible structures of Agn+ (n = 2-22) were determined using a genetic algorithm with incomplete local optimizations at the DFT level, and the calculated bonding strengths of O2 on these structures are consistent with experimental observations. Analyses revealed a close relationship between the reactivity of Agn+ with O2 and its HOMO-LUMO gap: cationic silver clusters with a small HOMO-LUMO gap are reactive, which can be rationalized by the covalent character of chemical bonds between Agn+ and O2 involving their frontier orbitals. The peculiar size-dependent HOMO-LUMO gaps and reactivity with O2 correlate with the subtle interplay between the electronic configurations and geometric structures of these silver cluster cations.

CO2 activation by copper oxide clusters: size, composition, and charge state dependence.

The interaction of CO2 with copper oxide clusters of different size, composition, and charge is investigated via infrared multiple-photon dissociation (IR-MPD) spectroscopy and density functional theory (DFT) calculations. Laser ablation of a copper target in the presence of an O2/He mixture leads to the preferred formation of oxygen-rich copper oxide cluster cations, CuxOy+ (y > x; x ≤ 8), while the anionic cluster distribution is dominated by stoichiometric (x = y) and oxygen-deficient (y < x; x ≤ 8) species. Subsequent reaction of the clusters with CO2 in a flow tube reactor results in the preferred formation of near-stoichiometric CuxOy(CO2)+/- complexes. IR-MPD spectroscopy of the formed complexes reveals the non-activated binding of CO2 to all cations while CO2 is activated by all anions. The great resemblance of spectra for all sizes investigated demonstrates that CO2 activation is largely independent of cluster size and Cu/O ratio but mainly determined by the cluster charge state. Comparison of the IR-MPD spectra with DFT calculations of the model systems Cu2O4(CO2)- and Cu3O4(CO2)- shows that CO2 activation exclusively results in the formation of a CO3 unit. Subsequent CO2 dissociation to CO appears to be unfavorable due to the instability of CO on the copper oxide clusters indicating that potential hydrogenation reactions will most likely proceed via formate or bicarbonate intermediates.

Computational insights into the zeolite-supported gold nanocluster-catalyzed ethanol dehydrogenation to acetaldehyde.

Zeolite-supported gold nanoclusters play increasingly important roles in heterogeneous catalysis and exhibit unique catalytic properties for ethanol dehydrogenation to acetaldehyde. Nevertheless, the reaction mechanism and potential roles of the zeolite-encapsulated gold nanoclusters during the catalytic process remain unclear. Herein, computational studies were carried out to gain mechanistic insights into ethanol dehydrogenation to acetaldehyde under both aerobic and anaerobic conditions catalyzed by a silicalite-1 zeolite-encapsulated Au3 cluster cation (Au3+-S1). The presence of O2 can significantly promote the ethanol dehydrogenation catalyzed by Au3+-S1. A feasible mechanistic pathway could be initiated via the O2 induced H-atom transfer (HAT) step from the hydrogen of the hydroxyl group to afford ethoxy and OOH radical species. Subsequently, the OOH induced second HAT from α-C-H of the ethoxy intermediate could follow to afford the acetaldehyde product. Moreover, the possible confinement and stabilization effect of the zeolite channels on the ethanol dehydrogenation reaction was discussed.

Gas-phase electronic spectra of HC2n+1H+ (n = 2-6) chains.

Highly unsaturated carbon chains are generated in combustion processes and electrical discharges, and are confirmed constituents of the interstellar medium. In hydrogen-rich environments smaller carbon clusters tend to exist as linear chains, capped on each end by hydrogen atoms. Although the HC2nH+ polyacetylene chains have been extensively characterized spectroscopically, the corresponding odd HC2n+1H+ chains have received far less attention. Here we use two-colour resonance enhanced photodissociation spectroscopy to measure electronic spectra for HC2n+1H+ (n = 2-6) chains contained in a cryogenically cooled quadrupole ion trap. The HC2n+1H+ chains are formed either top-down by ionizing and fragmenting pyrene molecules using pulsed 266 nm radiation, or bottom-up by reacting cyclic carbon cluster cations with acetylene. Ion mobility measurements confirm that the HC2n+1H+ species are linear, consistent with predictions from electronic structure calculations. The HC2n+1H+ electronic spectra exhibit three band systems in the visible/near infrared spectral range, which each shifts progressively to longer wavelength by ≈90 nm with the addition of each additional CC subunit. The strongest visible HC11H+ band has a wavelength (λ = 545.1 nm) and width (1.5 nm) that match the strong λ 5450 diffuse interstellar band (DIB). However, other weaker HC11H+ bands do not correspond to catalogued DIBs, casting doubt on the role of HC11H+ as a carrier for the λ 5450 DIB. There are no identifiable correspondences between catalogued DIBs and bands for the other HC2n+1H+ chains, allowing upper limits to be established for their column densities in diffuse interstellar clouds.

Reaction of size-selected iron-oxide cluster cations with methane: a model study of rapid methane loss in Mars' atmosphere.

We report gas-phase reactions of free iron-oxide clusters, FenOm+, and their Ar adducts with methane in the context of chemical processes in Mars' atmosphere. Methane activation was observed to produce FenOmCH2+/FenOmCD2+ and FenOmC+, where the reactivity exhibited size and composition dependence. For example, the rate coefficients of methane activation for Fe3O+ and Fe4O+ were estimated to be 1 × 10-13 and 3 × 10-13 cm3 s-1, respectively. Based on these reaction rate coefficients, the presence of iron-oxide clusters/particles with a density as low as 107 cm-3 in Mars' atmosphere would explain the rapid loss of methane observed recently by the Curiosity rover.

Controlling the symmetry of hexamonodentate 3d-transition metal complexes through symmetry propagation from high-symmetry Ti-Mo and Zr-Mo clusters via hydrogen-bonding interactions.

The symmetry of one of the simplest hexamonodentate complexes, [M(H2O)6]2+ (M = Fe, Co, Ni, and Zn), was controlled by tuning interactions in the second coordination sphere. Highly symmetric Ti-Mo or Zr-Mo cluster cations acted as symmetry templates, imposing a crystallographic trigonal coordination geometry in the hexamonodentate complexes through intermolecular hydrogen-bonding interactions. Magnetic measurements revealed that the ideal trigonal symmetry results in weak spin-orbit coupling for high-spin FeII complexes, despite the T-term ground state in the octahedral geometry. In contrast, high-spin CoII analogues with the T-term ground state exhibited strong spin-orbit coupling. DFT studies supported that a d6 FeII ion in the D3 symmetry has a 5A1 ground state while a d7 CoII in the same symmetry has a 4E ground state. Single-ion magnet behavior was observed in the CoII complexes. These results demonstrate that incorporating a diamagnetic, highly symmetric cluster enables precise symmetry control in single-ion magnets containing only monodentate ligands.

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.

CO2 and H2 Activation on Zinc-Doped Copper Clusters.

Here we systematically investigate the CO2 and H2 activation and dissociation on small Cun Zn0/+ (n=3-6) clusters using Density Functional Theory. We show that Cu6 Zn is a superatom, displaying an increased HOMO-LUMO gap and is inert towards CO2 or H2 activation or dissociation. While other neutral clusters weakly activate CO2 , the cationic clusters preferentially bind the CO2 in monodentate nonactivated way. Notably, Cu4 Zn allows for the dissociation of activated CO2 , whereas larger clusters destabilize all activated CO2 binding modes. Conversely, H2 dissociation is favored on all clusters examined, except for Cu6 Zn. Cu3 Zn+ and Cu4 Zn, favor the formation of formate through the H2 dissociation pathway rather than CO2 dissociation. These findings suggest the potential of these clusters as synthetic targets and underscore their significance in the realm of CO2 hydrogenation.

High Coordination Numbers of Actinides (An) in AnC13+ Rings (An = Th and U).

An intriguingly high abundance of both ThC13+ and UC13+ cluster cations was observed in a previous mass spectrometry experiment; however, the structural identification of these cations has not yet been completed. In this study, we determined the lowest lying structures of ThC13+ and UC13+ clusters using an unbiased structural search method. The 13-coordinate planar ring configuration was the most stable for both ThC13+ and UC13+ cluster cations. The C-An bonds in ThC13+ and UC13+ show a small degree of covalency, originating from the overlap of the s, d, and f orbitals of the An atoms with C 2p orbitals of both π and σ characteristics. The infrared and electronic absorption spectra of the most favorable planar ring configurations were theoretically simulated to facilitate the identification of the molecular structures in future experiments. This study provides an in-depth understanding of the experimental mass spectra.

Hydrogenated diblock copoly(norbornene)s bearing triimidazolium anion exchange membranes with enhanced alkaline stability for fuel cells

Overcoming the trade-off relationship between conductivity and alkaline stability is a crucial challenge for the practical application of anion exchange membranes (AEMs) in fuel cells. The cross-linked hydrogenated diblock copoly (norbornene)s AEMs with bulky substituted s-triazine based triimidazolium cation clusters were prepared. The prepared H-rPNB-T-triMPh-diMIm-1.64 has the low swelling ratio (SR less than 20%), reasonable water uptake (WU = 82.9%), and high ionic conductivity (114.8 mS cm−1) at 80 °C. In addition, the membrane is able to maintain 91.3% of the initial conductivity after being exposed to 1 M NaOH solution at 80 °C for 32 days. The single cells assembled with H-rPNB-T-triMPh-diMIm-1.64 exhibit a peak power density of 272 mW cm−2 under a current density of 650 mA cm−2 at 80 °C. The novel development strategy that the imidazolium cation cluster functionalized poly (norbornene) is promised to be an effective approach to develop the high-performance AEMs for application in fuel cells.

Probing adsorption of methane onto vanadium cluster cations via vibrational spectroscopy.

Photofragment spectroscopy is used to measure the vibrational spectra of V2+(CH4)n (n = 1-4), V3+(CH4)n (n = 1-3), and Vx+(CH4) (x = 4-8) in the C-H stretching region (2550-3100 cm-1). Spectra are measured by monitoring loss of CH4. The experimental spectra are compared to simulations at the B3LYP+D3/6-311++G(3df,3pd) level of theory to identify the geometry of the ions. Multi-reference configuration interaction with Davidson correction (MRCI+Q) calculations are also carried out on V2+ and V3+. The methane binding orientation in V2+(CH4)n (n = 1-4) evolves from η3 to η2 as more methane molecules are added. The IR spectra of metal-methane clusters can give information on the structure of metal clusters that may otherwise be hard to obtain from isolated clusters. For example, the V3+(CH4)n (n = 1-3) experimental spectra show an additional peak as the second and third methane molecules are added to V3+, which indicates that the metal atoms are not equivalent. The Vx+(CH4) show a larger red shift in the symmetric C-H stretch for larger clusters with x = 5-8 than for the small clusters with x = 2, 3, indicating increased covalency in the interaction of larger vanadium clusters with methane.