Mixed Cluster Ions of Magnesium and C60.

Magnesium clusters exhibit a pronounced nonmetal-to-metal transition, and the neutral dimer is exceptionally weakly bound. In the present study, we formed pristine Mgnz+ (n = 1-100, z = 1-3) clusters and mixed (C60)mMgnz+ clusters (m = 1-7, z = 1, 2) upon electron irradiation of neutral helium nanodroplets doped with magnesium or a combination of C60 and magnesium. The mass spectra obtained for pristine magnesium cluster ions exhibit anomalies, consistent with previous reports in the literature. The anomalies observed for C60Mgn+ strongly suggest that Mg atoms tend to wet the surface of the single fullerene positioning itself above the center of a pentagonal or hexagonal face, while, for (C60)mMgnz+, the preference for Mg to position itself within the dimples formed by fullerene cages becomes apparent. Besides doubly charged cluster ions, with the smallest member Mg22+, we also observed the formation of triply charged ions Mgn3+ with n > 24. The ion efficiency curves of singly and multiply charged ions exhibit pronounced differences compared to singly charged ions at higher electron energies. These findings indicate that sequential Penning ionization is essential in the formation of doubly and triply charged ions inside doped helium nanodroplets.

Pentagon, Hexagon, or Bridge? Identifying the Location of a Single Vanadium Cation on Buckminsterfullerene Surface.

Buckminsterfullerene C60 has received extensive research interest since its discovery. In addition to its interesting intrinsic properties of exceptional stability and electron-accepting ability, the broad chemical tunability by decoration or substitution on the C60-fullerene surface makes it a fascinating molecule. However, to date, there is uncertainty about the binding location of such decorations on the C60 surface, even for a single adsorbed metal atom. In this work, we report the gas-phase synthesis of the C60V+ complex and its in situ characterization by mass spectrometry and infrared spectroscopy with the help of quantum chemical calculations and molecular dynamics simulations. We identify the most probable binding position of a vanadium cation on C60 above a pentagon center in an η5-fashion, demonstrate a high thermal stability for this complex, and explore the bonding nature between C60 and the vanadium cation, revealing that large orbital and electrostatic interactions lie at the origin of the stability of the η5-C60V+ complex.

Tuning the degree of CO2 activation by carbon doping Cun- (n = 3-10) clusters: an IR spectroscopic study.

Copper clusters on carbide surfaces have shown a high catalytic activity towards methanol formation. To understand the interaction between CO2 and the catalytically active sites during this process and the role that carbon atoms could play in this, they are modeled by copper clusters, with carbon atoms incorporated. The formed clusters CunCm- (n = 3-10, m = 1-2) are reacted with CO2 and investigated by IR multiple-photon dissociation (IR-MPD) spectroscopy to probe the degree of CO2 activation. IR spectra for the reaction products [CunC·CO2]-, (n = 6-10), and [CunC2·CO2]-, (n = 3-8) are compared to reference spectra recorded for products formed when reacting the same cluster sizes with CO, and with density functional theory (DFT) calculated spectra. The results reveal a size- and carbon load-dependent activation and dissociation of CO2. The complexes [CunC·CO2]- with n = 6 and 10 show predominantly molecular activation of CO2, while those with n = 7-9 show only dissociative adsorption. The addition of the second carbon to the cluster leads to the exclusive molecular activation of the CO2 on all measured cluster sizes, except for Cu5C2- where CO2 dissociates. Combining these findings with DFT calculations leads us to speculate that at lower carbon-to-metal ratios (CMRs), the C can act as an oxygen anchor facilitating the OCO bond rupture, whereas at higher CMRs the carbon atoms increasingly attract negative charge, reducing the Cu cluster's ability to donate electron density to CO2, and consequently its ability to activate CO2.

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

The adsorption forms of NO on Irn+ (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 Irn+ (n = 3-6) is higher than that for Rh6+, which can be explained by the higher metal-oxygen bond strengths for iridium.

Bonding Nature between Noble Gases and Small Gold Clusters.

Noble gases are usually seen as utterly inert, likewise gold, which is typically conceived as the noblest of all metals. While one may expect that noble gases bind to gold via dispersion interactions only, strong bonds can be formed between noble gas atoms and small gold clusters. We combine mass spectrometry, infrared spectroscopy, and density functional theory calculations to address the bonding nature between Aun+ (n ≤ 4) clusters and Ar, Kr, and Xe. We unambiguously determine the geometries and quantitatively uncover the bonding nature in AunNgm+ (Ng = Ar, Kr, Xe) complexes. Each Au cluster can form covalent bonds with atop bound noble gas atoms, with strengths that increase with the noble gas atomic radius. This is demonstrated by calculated adsorption energies, Bader electron charges, and analysis of the electron density. The covalent bonding character, however, is limited to the atop-coordinated Ng atoms.

IR Spectroscopic Characterization of Methane Adsorption on Copper Clusters Cun+ (n = 2-4).

The interaction of CH4 with cationic copper clusters has been studied with infrared-multiple photon dissociation (IRMPD) spectroscopy. Cun+ (n = 2-4) formed by laser ablation were reacted with CH4. The formed complexes were irradiated with the IR light of the free-electron laser for intracavity experiments (FELICE), and the fragments were mass-analyzed with a reflectron time-of-flight mass spectrometer. The structures of the Cun+-CH4 complexes are assigned on the basis of comparison between the resulting IRMPD spectra to spectra of different isomers calculated with density functional theory (DFT). For all sizes n, the structure found is one with molecularly adsorbed CH4. Only slight deformations of the CH4 molecule have been identified upon adsorption on the clusters, which results in redshifts of the spectroscopic bands. This deformation can be explained by charge transfer from the cluster to the adsorbed methane molecule.

Efficient Formation of Size-Selected Clusters upon Pickup of Dopants into Multiply Charged Helium Droplets.

Properties of clusters often depend critically on the exact number of atomic or molecular building blocks, however, most methods of cluster formation lead to a broad, size distribution and cluster intensity anomalies that are often designated as magic numbers. Here we present a novel approach of breeding size-selected clusters via pickup of dopants into multiply charged helium nanodroplets. The size and charge state of the initially undoped droplets and the vapor pressure of the dopant in the pickup region, determines the size of the dopant cluster ions that are extracted from the host droplets, via evaporation of the helium matrix in a collision cell filled with room temperature helium or via surface collisions. Size distributions of the selected dopant cluster ions are determined utilizing a high-resolution time of flight mass spectrometer. The comparison of the experimental data, with simulations taking into consideration the pickup probability into a shrinking He droplet due to evaporation during the pickup process, provides a simple explanation for the emergence of size distributions that are narrower than Poisson.

Adsorption Forms of NO on Iridium-Doped Rhodium Clusters in the Gas Phase Revealed by Infrared Multiple Photon Dissociation Spectroscopy.

The adsorption of an NO molecule on a cationic iridium-doped rhodium cluster, Rh5Ir+, was investigated by infrared multiple photon dissociation spectroscopy (IRMPD) of Rh5IrNO+·Arp complexes in the 300-2000 cm-1 spectral range, where the Ar atoms acted as a messenger signaling IR absorption. Complementary density functional theory (DFT) calculations predicted two near-isoenergetic structures as the putative global minimum: one with NO adsorbed in molecular form in the on-top configuration on the Ir atom in Rh5Ir+, and one where NO is dissociated with the O atom bound to the Ir atom in the on-top configuration and the N atom on a hollow site formed by three Rh atoms. A comparison between the experimental IRMPD spectrum of Rh5IrNO+ and calculated spectra indicated that NO mainly adsorbs molecularly on Rh5Ir+, but evidence was also found for structures with dissociatively adsorbed NO. The estimated fraction of Rh5IrNO+ structures with dissociatively adsorbed NO is approximately 10%, which was higher than that found for Rh6+, but lower than that for Ir6+. The DFT calculations indicated the existence of an energy barrier in the NO dissociation pathway that is exothermic with respect to the reactants, which was considered to prevent NO from dissociating readily on Rh5Ir+. The height of the barrier is lower than that for NO dissociation over Rh6+, which is attributed to the higher binding energy of atomic O to the Ir atom in Rh5Ir+ than to a Rh atom in Rh6+.

The size-dependent influence of palladium doping on the structures of cationic gold clusters.

The physicochemical properties of small metal clusters strongly depend on their precise geometry. Determining such geometries, however, is challenging, particularly for clusters formed by multiple elements. In this work, we combine infrared multiple photon dissociation spectroscopy and density functional theory calculations to investigate the lowest-energy structures of Pd doped gold clusters, PdAu n-1 + (n ≤ 10). The high-quality experimental spectra allow for an unambiguous determination of the structures adopted by the clusters. Our results show that the Pd-Au interaction is so large that the structures of PdAu n-1 + and Au n + are very different. Pd doping induces a 2D to 3D transition at much smaller cluster sizes than for pure Au n + clusters. PdAu n-1 + clusters are three-dimensional from n = 4, whereas for Au n + this transition only takes place at n = 7. Despite the strong Au-Pd interaction, the Au n-1 + cluster geometries remain recognizable in PdAu n-1 + up to n = 7. This is particularly clear for PdAu6 +. In PdAu8 + and PdAu9 +, Pd triggers major rearrangements of the Au clusters, which adopt pyramidal shapes. For PdAu4 + we find a geometry that was not considered in previous studies, and the geometry found for PdAu8 + does not correspond to the lowest-energy structure predicted by DFT, suggesting kinetic trapping during formation. This work demonstrates that even with the continuous improvement of computational methods, unambiguous assignment of cluster geometries still requires a synergistic approach, combining experiment and computational modelling.

Structures of Nitrogen Oxides Attached to Anionic Gold Clusters Au4- Revealed by Infrared Multiple Photon Dissociation Spectroscopy.

The adsorption forms of NO and NO2 on anionic Au4- clusters were investigated by a combination of IR multiple photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) calculations. For all three species investigated (Au4NO-, Au4N2O2-, and Au4NO2-), the spectra were found to be consistent with a Y-shaped Au4- cluster with triangular Au3 and one Au atom sticking out, on which NO and NO2 molecules adsorb molecularly. These species are considered as intermediates of the Au4--mediated disproportionation reaction of NO, Au4(NO)3- → Au4(NO2)(N2O)-. We discuss the reaction path on the basis of the found geometries and energies and conclude that the disproportionation reaction of NO can occur catalytically on the Au4- cluster.

IR spectroscopic characterization of the co-adsorption of CO2 and H2 onto cationic Cun+ clusters.

To understand elementary reaction steps in the hydrogenation of CO2 over copper-based catalysts, we experimentally study the adsorption of CO2 and H2 onto cationic Cun+ clusters. For this, we react Cun+ clusters formed by laser ablation with a mixture of H2 and CO2 in a flow tube-type reaction channel and characterize the products formed by IR multiple-photon dissociation spectroscopy employing the IR free-electron laser FELICE. We analyze the spectra by comparing them to literature spectra of Cun+ clusters reacted with H2 and with new spectra of Cun+ clusters reacted with CO2. The latter indicate that CO2 is physisorbed in an end-on configuration when reacted with the clusters alone. Although the spectra for the co-adsorption products evidence H2 dissociation, no signs for CO2 activation or reduction are observed. This lack of reactivity for CO2 is rationalized by density functional theory calculations, which indicate that CO2 dissociation is hindered by a large reaction barrier. CO2 reduction to formate should energetically be possible, but the lack of formate observation is attributed to kinetic hindering.

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.

IR Spectroscopic Characterization of H2 Adsorption on Cationic Cun+ (n = 4-7) Clusters.

IR spectra of cationic copper clusters Cun+ (n = 4-7) complexed with hydrogen molecules are recorded via IR multiple-photon dissociation (IRMPD) spectroscopy. To this end, the copper clusters are generated via laser ablation and reacted with H2 and D2 in a flow-tube-type reaction channel. The complexes formed are irradiated using IR light provided by the free-electron laser for intracavity experiments (FELICE). The spectra are interpreted by making use of isotope-induced shifts of the vibrational bands and by comparing them to density functional theory calculated spectra for candidate structures. The structural candidates have been obtained from global sampling with the minima hopping method, and spectra are calculated at the semilocal (PBE) and hybrid (PBE0) functional level. The highest-quality spectra have been recorded for [5Cu, 2H/2D]+, and we find that the semilocal functional provides better agreement for the lowest-energy isomers. The interaction of hydrogen with the copper clusters strongly depends on their size. Binding energies are largest for Cu5+, which goes hand in hand with the observed predominantly dissociative adsorption. Due to smaller binding energies for dissociated H2 and D2 for Cu4+, also a significant amount of molecular adsorption is observed as to be expected according to the Evans-Polanyi principle. This is confirmed by transition-state calculations for Cu4+ and Cu5+, which show that hydrogen dissociation is not hindered by an endothermic reaction barrier for Cu5+ and by a slightly endothermic barrier for Cu4+. For Cu6+ and Cu7+, it was difficult to draw clear conclusions because the IR spectra could not be unambiguously assigned to structures.

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.

Carbide Dihydrides: Carbonaceous Species Identified in Ta4 + -Mediated Methane Dehydrogenation.

The products of methane dehydrogenation by gas-phase Ta4 + clusters are structurally characterized using infrared multiple photon dissociation (IRMPD) spectroscopy in conjunction with quantum chemical calculations. The obtained spectra of [4Ta,C,2H]+ reveal a dominance of vibrational bands of a H2 Ta4 C+ carbide dihydride structure over those indicative for a HTa4 CH+ carbyne hydride one, as is unambiguously verified by studies employing various methane isotopologues. Because methane dehydrogenation by metal cations M+ typically leads to the formation of either MCH2 + carbene or HMCH+ carbyne hydride structures, the observation of a H2 MC+ carbide dihydride structure implies that it is imperative to consider this often-neglected class of carbonaceous intermediates in the reaction of metals with hydrocarbons.

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.

The structures of cationic gold clusters probed by far-infrared spectroscopy.

Determining the precise structures of small gold clusters is an essential step towards understanding their chemical and physical properties. Due to the relativistic nature of gold, its clusters remain planar (2D) up to appreciable sizes. Ion mobility experiments have suggested that positively charged gold clusters adopt three-dimensional (3D) structures from n = 8 onward. Computations predict, depending on the level of theory, 2D or 3D structures as putative energy-minimum for n = 8. In this work, far-infrared multiple photon dissociation spectroscopy, using Ar as tagging element, is combined with density-functional theory calculations to determine the structures of Aun+ (n≤ 9) clusters formed by laser ablation. While the Au frameworks in Au6Arm+ and Au7Arm+ complexes are confirmed to be planar and that in Au9Arm+ three-dimensional, we demonstrate the coexistence of 3D and planar Au8Arm+ (m = 1-3) isomers. Thus, it is revealed that at finite temperatures, the formal 2D to 3D transition takes place at n = 8 but is not sharp.

Infrared Spectroscopy of Gold Carbene Cation (AuCH2+): Covalent or Dative Bonding?

The present work explores the structure of the gold carbene cation, AuCH2+, using infrared multiple photon dissociation action spectroscopy and density functional theory (DFT). Unlike several other 5d transition-metal cations (M+ = Ta+, W+, Os+, Ir+, and Pt+) that react with methane by dehydrogenation to form MCH2+ species, gold cations are unreactive with methane at thermal energies. Instead, the metal carbene is formed by reacting atomic gold cations formed in a laser ablation source with ethylene oxide (cC2H4O) pulsed into a reaction channel downstream. The resulting [Au,C,2H]+ product photofragmented by loss of H2 as induced by radiation provided by the free-electron laser for intracavity experiments in the 300-1800 cm-1 range. Comparison of the experimental spectrum, obtained by monitoring the appearance of AuC+, and DFT calculated spectra leads to the identification of the ground-state carbene, AuCH2+ (1A1), as the species formed, as previously postulated theoretically. Unlike the covalent double bonds formed by the lighter, open-shell 5d transition metals, the closed-shell Au+ (1S, 5d10) atom binds to methylene by donation of a pair of electrons from CH2(1A1) into the empty 6s orbital of gold coupled with π back-bonding, i.e., dative bonding, as explored computationally. Contributions to the AuC+ appearance spectrum from larger complexes are also considered, and H2CAu+(c-C2H4O) seems likely to contribute one band observed.

Structures of Cu n+ ( n = 3-10) Clusters Obtained by Infrared Action Spectroscopy.

Coinage metal clusters are of great importance for a wide range of scientific fields, ranging from microscopy to catalysis. Despite their clear fundamental and technological importance, the experimental structural determination of copper clusters has attracted little attention. We fill this gap by elucidating the structure of cationic copper clusters through infrared (IR) photodissociation spectroscopy of Cu n+-Ar m complexes. Structures of Cu n+ ( n = 3-10) are unambiguously assigned based on the comparison of experimental IR spectra in the 70-280 cm-1 spectral range with spectra calculated using density functional theory. Whereas Cu3+ and Cu4+ are planar, starting from n = 5, Cu n+ clusters adopt 3D structures. Each successive cluster size is composed of its predecessor with a single atom adsorbed onto the face, giving evidence of a stepwise growth.

Spectroscopic Identification of the Carbyne Hydride Structure of the Dehydrogenation Product of Methane Activation by Osmium Cations.

The present work explores the structures of species formed by dehydrogenation of methane (CH4) and perdeuterated methane (CD4) by the 5d transition metal cation osmium (Os+). Using infrared multiple photon dissociation (IRMPD) action spectroscopy and density functional theory (DFT), the structures of the [Os,C,2H]+ and [Os,C,2D]+ products are explored. This study complements previous work on the related species formed by dehydrogenation of methane by four other 5d transition metal cations (M+ = Ta+, W+, Ir+, and Pt+). Osmium cations are formed in a laser ablation source, react with methane pulsed into a reaction channel downstream, and the resulting products spectroscopically characterized through photofragmentation using the Free-Electron Laser for IntraCavity Experiments (FELICE) in the 300-1800 cm-1 range. Photofragmentation was monitored by the loss of H2/D2. Comparison of the experimental spectra and DFT calculated spectra leads to identification of the ground state carbyne hydride, HOsCH+ (2A') as the species formed, as previously postulated theoretically. Further, a full description of the systematic spectroscopic shifts observed for deuterium labeling of these complexes, some of the smallest systems to be studied using IRMPD action spectroscopy, is achieved. A full rotational contour analysis explains the observed linewidths as well as the observation of doublet structures in several bands, consistent with previous observations for HIrCH+ (2A'). Graphical Abstract ᅟ.