IR spectroscopic characterization of 3d transition metal carbene cations, FeCH2+ and CoCH2+: periodic trends and a challenge for DFT approaches.

A combination of IR multiple-photon dissociation (IRMPD) action spectroscopy and quantum chemical calculations was employed to investigate the [M,C,2H]+ (M = Fe and Co) species. These were formed by reacting laser ablated M+ ions with oxirane (ethylene oxide, c-C2H4O) in a room temperature ion trap. IRMPD spectra for the Fe and Co species are very similar and exhibit one major band. Comparison with density functional theory (DFT) and coupled cluster with single and double excitations (CCSD) calculations allows assignment of the spectra to MCH2+ carbene structures. For these 3d transition metal systems, experimental IRMPD spectra compare relatively poorly with DFT calculated IR spectra, but CCSD calculated spectra are a much better match primarily because the M-C stretch gains significant intensity. The origins of this behavior are explored in some detail. The present results are also compared to previous results for the 4d and 5d congeners and the periodic trends in these structures are evaluated.

IR spectroscopic characterization of [M,C,2H]+ (M = Ru and Rh) products formed by reacting 4d transition metal cations with oxirane: Spectroscopic evidence for multireference character in RhCH2.

A combination of infrared multiple-photon dissociation (IRMPD) action spectroscopy and quantum chemical calculations was employed to investigate the [M,C,2H]+ (M = Ru and Rh) species. These ions were formed by reacting laser ablated M+ ions with oxirane (ethylene oxide, c-C2H4O) in a room-temperature ion trap. IRMPD spectra for the Ru species exhibit one major band and two side bands, whereas spectra for the Rh species contain more distinct bands. Comparison with density functional theory (DFT), coupled-cluster (CCSD), and equation-of-motion spin-flip CCSD (EOM-SF-CCSD) calculations allows assignment of the [M,C,2H]+ structures. For the spectrum of [Ru,C,2H]+, a combination of HRuCH+ and RuCH2+ structures reproduces the observed spectrum at all levels of theory. The well-resolved spectrum of [Rh,C,2H]+ could not be assigned unambiguously to any calculated structure using DFT approaches. The EOM-SF-CCSD calculations showed that the ground-state surface has multireference electronic character, and symmetric carbenes in both the 1A1 and 3A2 states are needed to reproduce the observed spectrum.

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.

Ion spectroscopy in methane activation.

This review is devoted to ion spectroscopy studies of complexes relevant for the understanding of methane activation with metal ions and clusters. Methane activation starts with the formation of a complex with a metal ion. The degree of the interaction between an intact methane molecule and the ion can be monitored by the perturbations of C-H stretch vibrations in the methane molecule. Binding mediated by the electrostatic interaction results in a η3 type coordination of methane. In contrast, binding governed by orbital interactions results in a η2 type coordination of methane. We further review the spectroscopic characterization of activation products of metal-methane reactions, such as the metal-carbene and carbyne products resulting from the interaction of selected 5d metals with methane. The focus of recent research in the field has shifted towards the investigation of interactions between methane and metal clusters. We show examples highlighting that metal clusters can be more reactive in methane activation reactions.

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.

Cluster size dependent coordination of formate to free manganese oxide clusters.

The interaction of free manganese oxide clusters, MnxOy+ (x = 1-9, y = 0-12), with formic acid was studied via infrared multiple-photon dissociation (IR-MPD) spectroscopy together with calculations using density functional theory (DFT). Clusters containing only one Mn atom, such as MnO2+ and MnO4+, bind formic acid as an intact molecule in both the cis- and trans-configuration. In contrast, all clusters containing two or more manganese atoms deprotonate the acid's hydroxyl group. The coordination of the resulting formate group is strongly cluster-size-dependent according to supporting DFT calculations for selected model systems. For Mn2O2+ the co-existence of two isomers with the formate bound in a bidentate bridging and chelating configurations, respectively, is found, whereas for Mn2O4+ the bidentate chelating configuration is preferred. In contrast, the bidentate bridging structure is energetically considerably more favorable for Mn4O4+. This binding motif stabilizes the 2D ring structure of the core of the Mn4O4+ cluster with respect to the 3D cubic geometry of the Mn4O4+ cluster core.

Carbon Dioxide and Water Activation by Niobium Trioxide Anions in the Gas Phase.

Transition metals are important in various industrial applications including catalysis. Due to the current concentration of CO2 in the atmosphere, various ways for its capture and utilization are investigated. Here, we study the activation of CO2 and H2O at [NbO3]- in the gas phase using a combination of infrared multiple photon dissociation spectroscopy and density functional theory calculations. In the experiments, Fourier-transform ion cyclotron resonance mass spectrometry is combined with tunable IR laser light provided by the intracavity free-electron laser FELICE or optical parametric oscillator-based table-top laser systems. We present spectra of [NbO3]-, [NbO2(OH)2]-, [NbO2(OH)2]-(H2O) and [NbO(OH)2(CO3)]- in the 240-4000 cm-1 range. The measured spectra and observed dissociation channels together with quantum chemical calculations confirm that upon interaction with a water molecule, [NbO3]- is transformed to [NbO2(OH)2]- via a barrierless reaction. Reaction of this product with CO2 leads to [NbO(OH)2(CO3)]- with the formation of a [CO3] moiety.

C-H Bond Activation and C-C Coupling of Methane on a Single Cationic Platinum Center: A Spectroscopic and Theoretical Study.

We spectroscopically investigated the activation products resulting from reacting one and multiple methane molecules with Pt+ ions. Pt+ ions were formed by laser ablation of a metal target and were cooled to the electronic ground state in a supersonic expansion. The ions were then transferred to a room temperature ion trap, where they were reacted with methane at various partial pressures in an argon buffer gas. Product masses observed were [Pt,C,2H]+, [Pt,2C,4H]+, [Pt,4C,8H]+, and [Pt,2C,O,6H]+, which were mass-isolated and characterized using infrared multiple-photon dissociation (IRMPD) spectroscopy employing the free electron laser for intra-cavity experiments (FELICE). The spectra for [Pt,2C,4H]+ and [Pt,4C,8H]+ have several well-defined bands and, when compared to density functional theory-calculated spectra for several possible product structures, lead to unambiguous assignments to species with ethene ligands, proving Pt+-mediated C-C coupling involving up to four methane molecules. These findings contrast with earlier experiments where Pt+ ions were reacted in a flow-tube type reaction channel at significantly higher pressures of helium buffer gas, resulting in the formation of a Pt(CH3)2+ product. Our DFT calculations show a reaction barrier of +0.16 eV relative to the PtCH2+ + CH4 reactants that are required for C-C coupling. The different outcomes in the two experiments suggest that the higher pressure in the earlier work could kinetically trap the dimethyl product, whereas the lower pressure and longer residence times in the ion trap permit the reaction to proceed, resulting in ethene formation and dihydrogen elimination.

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.

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.

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+.

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.

IR photofragmentation of the phenyl cation: spectroscopy and fragmentation pathways.

We present the gas-phase infrared spectra of the phenyl cation, phenylium, in its perprotio (C6H5+) and perdeutero (C6D5+) forms, in the 260-1925 cm-1 (5.2-38 µm) spectral range, and investigate the observed photofragmentation. The spectral and fragmentation data were obtained using Infrared Multiple Photon Dissociation (IRMPD) spectroscopy within a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FTICR MS) located inside the cavity of the free electron laser FELICE (Free Electron Laser for Intra-Cavity Experiments). The 1A1 singlet nature of the phenylium ion is ascertained by comparison of the observed IR spectrum with DFT calculations, using both harmonic and anharmonic frequency calculations. To investigate the observed loss of predominantly [2C,nH] (n = 2-4) fragments, we explored the potential energy surface (PES) to unravel possible isomerization and fragmentation reaction pathways. The lowest energy pathways toward fragmentation include direct H elimination, and a combination of facile ring-opening mechanisms (≤2.4 eV), followed by elimination of H or CCH2. Energetically, all H-loss channels found are more easily accessible than CCH2-loss. Calculations of the vibrational density of states for the various intermediates show that at high internal energies, ring opening is thermodynamically the most advantageous, eliminating direct H-loss as a competing process. The observed loss of primarily [2C,2H] can be explained through entropy calculations that show favored loss of [2C,2H] at higher internal energies.

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 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.

Cluster Size Dependent Interaction of Free Manganese Oxide Clusters with Acetic Acid and Methyl Acetate.

We have employed infrared multiple-photon dissociation (IR-MPD) spectroscopy together with density functional theory (DFT) calculations to study the interaction of series of subnanometer sized manganese oxide clusters, MnxOy+ (x = 1-6, y = 0-9) with acetic acid (HOAc) and methyl acetate (MeOAc). Reaction with HOAc leads to strongly cluster size and composition dependent IR-MPD spectra, indicating molecular adsorption on MnOx+ clusters and thermodynamically favorable but kinetically hampered HOAc dissociation (deprotonation) on Mn2O4+ and Mn3O5+. Other cluster sizes exhibit the preferred formation of a dissociative bidentate chelating structure. In contrast to HOAc, all clusters bind MeOAc via the carbonyl group as an intact molecule, and dissociation appears to be kinetically hindered under the given experimental conditions.

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.

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.

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.

Energetic Stabilization of Carboxylic Acid Conformers by Manganese Atoms and Clusters.

Free cationic manganese atoms and clusters Mnx+ (x = 1-3) have been reacted with small carboxylic acids (formic, acetic, and propionic acids) and methyl acetate in a flow tube reactor held at room temperature. The geometry of the thus formed complexes has subsequently been studied via infrared multiple-photon dissociation (IR-MPD) spectroscopy and density-functional theory (DFT) calculations. The IR-MPD spectra of the acid complexes show two signals in the C═O stretch region indicating the coexistence of two conformers. In agreement, the DFT calculations reveal that the-intrinsically less stable-cis-conformer of the carboxylic acids binds more strongly to Mn+ than the trans-conformer, which leads to the energetic stabilization of the former. This stronger binding is attributed to a stronger electrostatic interaction with the manganese cation. A similar stabilization is also predicted for the cis-conformer of methyl acetate; however, the resulting change of the C═O stretch eigenfrequency is too small to be resolved in the experiment. This finding can open up completely new routes for the future room-temperature preparation of the cis-conformers of carboxylic acids and their derivatives.