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The interaction of free cationic silicon oxide clusters, Si x O y + (x = 2-5, y ≥ x), with dilute water vapor, was investigated in a flow tube reactor. Product mass distributions indicate cluster size-dependent dissociative water adsorption. To probe the structure and vibrational spectra of the resulting Si x O y H2 + (x = 2-4) clusters, we employed infrared multiple photon dissociation spectroscopy and density functional theory calculations. The planar rhombic cluster core of the disilicon oxides (x = 2) appears to be retained upon dissociative adsorption of one H2O unit, whereas a significant structural transformation of the tri- and tetra-silicon oxides (x = 3 and 4) is induced, resulting in an increased coordination of the Si atoms and more 3D cluster structures. In an astronomical context, we discuss the potential relevance of Si x O y H z + clusters as seeds for dust nucleation and catalysts for carbon-based chemistry in diffuse or translucent interstellar clouds, where all the necessary conditions for producing these species are found. In the produced clusters, the frequency of the isolated silanol Si-OH stretching vibrational mode is considerably blue-shifted compared to that in hydroxylated bulk silica and small inorganic compounds. This mode has a characteristic frequency range between 1200 cm-1 (8.3 µm) and 1090 cm-1 (9.2 µm) and is associated with the anomalously small Si-OH bond lengths in these ionised species. In infrared observations such high frequency Si-O stretching modes are usually associated with a pure bulk silica component of silicate cosmic dust. The presence of Si x O y H2 + clusters in low silica astrophysical environments could thus potentially be detected via their signature Si-O band using the James Webb space telescope.
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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.
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Silicates are ubiquitously found as small dust grains throughout the universe. These particles are frequently subject to high-energy processes and subsequent condensation in the interstellar medium (ISM), where they are broken up into many ultrasmall silicate fragments. These abundant molecular-sized silicates likely play an important role in astrochemistry. By approximately mimicking silicate dust grain processing occurring in the diffuse ISM by ablation/cooling of a Mg/Si source material in the presence of O2, we observed the creation of stable clusters based on discrete pyroxene monomers (MgSiO3 +), which traditionally have only been considered possible as constituents of bulk silicate materials. Our study suggests that such pyroxene monomer-based clusters could be highly abundant in the ISM from the processing of larger silicate dust grains. A detailed analysis, by infrared multiple-photon dissociation (IR-MPD) spectroscopy and density functional theory (DFT) calculations, reveals the structures and properties of these monomeric silicate species. We find that the clusters interact strongly with oxygen, with some stable cluster isomers having a silicate monomeric core bound to an ozone-like moiety. The general high tendency of these monomeric silicate species to strongly adsorb O2 molecules also suggests that they could be relevant to the observed and unexplained depletion of oxygen in the ISM. We further find clusters where a Mg atom is bound to the MgSiO3 monomer core. These species can be considered as the simplest initial step in monomer-initiated nucleation, indicating that small ionized pyroxenic clusters could also assist in the reformation of larger silicate dust grains in the ISM.
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We report on the first preparation of isolated ligand-free CaMn4O5+ gas-phase clusters, as well as other pentameric CaxMn5-xO5+ (x = 0-4) clusters with varying Ca contents, which serve as molecular models of the natural CaMn4O5 inorganic cluster in photosystem II. Ion trap reactivity studies with D2O and H218O reveal a pronounced cluster composition-dependent ability to mediate the oxidation of water to hydrogen peroxide. First-principles density functional theory simulations elucidate the mechanism of water oxidation, proceeding via formation of a terminal oxyl radical followed by oxyl/hydroxy (O/OH) coupling. The critical coupling reaction step entails a single electron transfer from the oxyl radical to the accommodating cluster core with a concurrent O/OH coupling forming an adsorbed OOH intermediate group. The spin-conserving electron transfer step takes place when the spin of the transferred electron is aligned with the spins of the d-electrons of the Mn atoms in the cuboidal high-spin cluster isomer. The d-electrons provide a ferromagnetically ordered environment that facilitates the spin-gated selective electron transfer process, resulting in parallel-spin-exchange stabilization and a lowered transition state barrier for the coupling reaction involving the frontier orbitals of the oxyl and hydroxy reactant intermediates.
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Manganeso , Agua , Espectroscopía de Resonancia por Spin del Electrón/métodos , Oxidación-Reducción , Oxígeno , Complejo de Proteína del Fotosistema II/metabolismoRESUMEN
The detection of intermediate species and the correlation of their ultrafast dynamics with the morphology and electronic structure of a surface is crucial to fully understand and control heterogeneous photoinduced and photocatalytic reactions. In this work, the ultrafast photodissociation dynamics of CH3Br molecules adsorbed on variable-size Au clusters on MgO/Mo(100) is investigated by monitoring the CH3+ transient evolution using a pump-probe technique in conjunction with surface mass spectrometry. Furthermore, extreme-UV photoemission spectroscopy in combination with theoretical calculations is employed to study the electronic structure of the Au clusters on MgO/Mo(100). Changes in the ultrafast dynamics of the CH3+ fragment are correlated with the electronic structure of Au as it evolves from monomers to small nonmetallic clusters to larger nanoparticles with a metallic character. This work provides a new avenue to a detailed understanding of how surface-photoinduced chemical reactions are influenced by the composition and electronic structure of the surface.
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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.
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Gas-phase ion-trap reactivity experiments and density functional simulations reveal that water oxidation to H2O2 mediated by (calcium) manganese oxide clusters proceeds via formation of a terminal oxyl radical followed by oxyl/hydroxy O-O coupling. This mechanism is predicted to be energetically feasible for Mn2Oy+ (y = 2-4) and the binary CaMn3O4+, in agreement with the experimental observations. In contrast, the reaction does not proceed for the tetramanganese oxides Mn4Oy+ (y = 4-6) under these experimental conditions. This is attributed to the high fluxionality of the tetramanganese clusters, resulting in the instability of the terminal oxyl radical as well as an energetically unfavorable change of the spin state required for H2O2 formation. Ca doping, yielding a symmetry-broken lower-symmetry three-dimensional (3D) CaMn3O4+ cluster, results in structural stabilization of the oxyl radical configuration, accompanied by a favorable coupling between potential energy surfaces with different spin states, thus enabling the cluster-mediated water oxidation reaction and H2O2 formation.
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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 CâO 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 CâO 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.
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The interaction of manganese oxide clusters MnxOy+ (x = 2-5, y ≥ x) with CO2 is studied via infrared multiple-photon dissociation spectroscopy (IR-MPD) in the spectral region of 630-1860 cm-1. Along with vibrational modes of the manganese oxide cluster core, two bands are observed around 1200-1450 cm-1 and they are assigned to the characteristic Fermi resonance of CO2 arising from anharmonic coupling between the symmetric stretch vibration and the overtone of the bending mode. The spectral position of the lower frequency band depends on the cluster size and the number of adsorbed CO2 molecules, whereas the higher frequency band is largely unaffected. Despite these effects, the observation of the Fermi dyad indicates only a small perturbation of the CO2 molecule. This finding is confirmed by the theoretical investigation of Mn2O2(CO2)+ revealing only small orbital mixing between the dimanganese oxide cluster and CO2, indicative of mainly electrostatic interaction.
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Infrared multiple-photon dissociation (IR-MPD) spectroscopy and density functional theory (DFT) calculations have been employed to elucidate the geometric structure of a series of di-manganese oxide clusters Mn2Ox+ (x = 4-7). The theoretical exploration predicts that all investigated clusters contain a rhombus-like Mn2O2 core with up to four, terminally bound, oxygen atoms. The short Mn-O bond length of the terminal oxygen atoms of ≤1.58 Å indicates triple bond character instead of oxyl radical formation. However, the IR-MPD spectra reveal that higher energy isomers with up to two O2 molecules η2-coordinated to the cluster core can be kinetically trapped under the given experimental conditions. In these complexes, all O2 units are activated to superoxide species. In addition, the sequential increase of the oxygen content in the cluster allows for a controlled increase of the positive charge localized on the Mn atoms reaching a maximum for Mn2O7+.
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Infrared multiple photon dissociation (IR-MPD) spectroscopy in conjunction with density functional theory (DFT) calculations has been employed to study the activation of molecular oxygen and ethylene co-adsorbed on a free gold dimer cation Au2+. Both studied complexes, Au2O2(C2H4)+ and Au2O2(C2H4)2+, show distinct features of both intact O2 and ethylene co-adsorbed on the cluster. However, the ethylene C=C double bond is activated, increasing in length by up to 0.07 Å compared with the free molecule, and the red shift of the O-O vibration frequency increases with the number of adsorbed ethylene molecules, indicating a small but increasing activation of the O-O bond. The small O2 activation and the rather weak interaction between O2 and C2H4 are also reflected in the calculated electronic structure of the co-adsorption complexes which shows only a small occupation of the empty anti-bonding O2 2π*2p orbital as well as the localization of most of the Kohn-Sham orbitals on O2 and C2H4, respectively, with only limited mixing between O2 and C2H4 orbitals. The results are compared with theoretical studies on neutral AuxO2(C2H4) (x = 3, 5, 7, 9) complexes.
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One of the fundamental processes in nature, the oxidation of water, is catalyzed by a small CaMn3 O4 â MnO cluster located in photosystem II (PS II). Now, the first successful preparation of a series of isolated ligand-free tetrameric Can Mn4-n O4+ (n=0-4) cluster ions is reported, which are employed as structural models for the catalytically active site of PS II. Gas-phase reactivity experiments with D2 O and H218 O in an ion trap reveal the facile deprotonation of multiple water molecules via hydroxylation of the cluster oxo bridges for all investigated clusters. However, only the mono-calcium cluster CaMn3 O4+ is observed to oxidize water via elimination of hydrogen peroxide. First-principles density functional theory (DFT) calculations elucidate mechanistic details of the deprotonation and oxidation reactions mediated by CaMn3 O4+ as well as the role of calcium.
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Calcio/metabolismo , Manganeso/metabolismo , Oxígeno/metabolismo , Complejo de Proteína del Fotosistema II/metabolismo , Calcio/química , Gases/química , Gases/metabolismo , Manganeso/química , Oxígeno/química , Complejo de Proteína del Fotosistema II/química , Teoría CuánticaRESUMEN
The bound unoccupied electronic state structure of an Ir(111)/graphene surface covered by differently sized and spaced Ir clusters was investigated by means of two-photon photoemission spectroscopy. The cluster lattice was found to affect the image potential states of the substrate to a surprisingly large extent. This effect can be related to the influence of the cluster lattice on the screening of the image state electron trapped in front of the surface. The symmetric arrangement of Ir clusters considerably reduces the lateral extension of graphene areas with a homogeneous local work function, and from a certain minimum area size, the excitation of an electron to a stable state in a Coulomb-like potential is not possible anymore. Furthermore, lateral confinement effects could be observed due to the decreasing extension of bare graphene areas.
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The interaction of ethylene with free gold clusters of different sizes and charge states has been previously shown theoretically to involve two different adsorption modes of the C2H4 molecule, namely: the di-σ- and π-bonded ethylene adsorption configurations. Here, we present the first experimental investigation of the structure of a series of gas-phase gold-ethylene complexes, [Formula: see text]. By employing infrared multiple-photon dissociation spectroscopy in conjunction with first-principles calculations it is revealed that up to three C2H4 molecules preferably bind to gold cations in a π-bonded configuration. The binding of all ethylene molecules is found to be dominated by partial electron donation from the ethylene molecules to the gold clusters leading to an activation of the C-C bond. The cooperative action of multiple coadsorbed C2H4 on [Formula: see text] is shown to enable additional charge back-donation and an enhanced C-C bond activation. In contrast, the strong C-H bond is not weakened and the experimental spectra do not give any indication for C-H bond dissociation. The possible correlations of the C-C bond stretch vibration with the C-C bond length and the net charge transfer are discussed.
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The thermal decomposition of free cationic iron-sulfur clusters FexSy+ (x = 0-7, y = 0-9) is investigated by collisional post-heating in the temperature range between 300 and 1000 K. With increasing temperature the preferential formation of stoichiometric FexSy+ (y = x) or near stoichiometric FexSy+ (y = x ± 1) clusters is observed. In particular, Fe4S4+ represents the most abundant product up to 600 K, Fe3S3+ and Fe3S2+ are preferably formed between 600 K and 800 K, and Fe2S2+ clearly dominates the cluster distribution above 800 K. These temperature dependent fragment distributions suggest a sequential fragmentation mechanism, which involves the loss of sulfur and iron atoms as well as FeS units, and indicate the particular stability of Fe2S2+. The potential fragmentation pathways are discussed based on first principles calculations and a mechanism involving the isomerization of the cluster prior to fragmentation is proposed. The fragmentation behavior of the iron-sulfur clusters is in marked contrast to the previously reported thermal dissociation of analogous iron-oxide clusters, which resulted in the release of O2 molecules only, without loss of metal atoms and without any tendency to form particular prominent and stable FexOy+ clusters at high temperatures.
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Methane represents the major constituent of natural gas. It is primarily used only as a source of energy by means of combustion, but could also serve as an abundant hydrocarbon feedstock for high quality chemicals. One of the major challenges in catalysis research nowadays is therefore the development of materials that selectively cleave one of the four C-H bonds of methane and thus make it amenable for further chemical conversion into valuable compounds. By employing infrared spectroscopy and first-principles calculations it is uncovered herein that the interaction of methane with small gold cluster cations leads to selective C-H bond dissociation and the formation of hydrido methyl complexes, H-Aux+ -CH3 . The distinctive selectivity offered by these gold clusters originates from a fine interplay between the closed-shell nature of the dâ states and relativistic effects in gold. Such fine balance in fundamental interactions could prove to be a tunable feature in the rational design of a catalyst.
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In the quest for cheap and earth abundant but highly effective and energy efficient water splitting catalysts, manganese oxide represents one of the materials of choice. In the framework of a new hierarchical modeling strategy we employ free non-ligated manganese oxide clusters MnxOx+y(+) (x = 2-5, y = -1, 0, 1, 2) as simplified molecular models to probe the interaction of water with nano-scale manganese oxide materials. Infrared multiple-photon dissociation (IR-MPD) spectroscopy in conjunction with first-principles spin density functional theory calculations is applied to study several series of MnxOx+y(H2O)n(+) complexes and reveal that the reaction of water with MnxOx+y(+) leads to the deprotonation of the water molecules via hydroxylation of the cluster oxo-bridges. This process is independent of the formal Mn oxidation state and occurs already for the first adsorbed water molecule and it proceeds until all oxo-bridges are hydroxylated. Additional water molecules are bound intact and favorably form H3O2 units with the hydroxylated oxo-bridges. Water adsorption and deprotonation is also found to induce structural transformations of the cluster core, including dimensionality crossover. Furthermore, the IR-MPD measurements reveal that clusters with one oxygen atom in excess MnxOx+1(+) contain a terminal O atom while clusters with two oxygen atoms in excess MnxOx+2(+) contain an intact O2 molecule which, however, dissociates upon adsorption of a minimum number of water molecules. These basic concepts could aid the future design of artificial water-splitting molecular catalysts.
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As the biological activation and oxidation of water takes place at an inorganic cluster of the stoichiometry CaMn4 O5 , manganese oxide is one of the materials of choice in the quest for versatile, earth-abundant water splitting catalysts. To probe basic concepts and aid the design of artificial water-splitting molecular catalysts, a hierarchical modeling strategy was employed that explores clusters of increasing complexity, starting from the tetramanganese oxide cluster Mn4 O4 (+) as a molecular model system for catalyzed water activation. First-principles calculations in conjunction with IR spectroscopy provide fundamental insight into the interaction of water with Mn4 O4 (+) , one water molecule at a time. All of the investigated complexes Mn4 O4 (H2 O)n (+) (n=1-7) contain deprotonated water with a maximum of four dissociatively bound water molecules, and they exhibit structural fluxionality upon water adsorption, inducing dimensional and structural transformations of the cluster core.
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The reactions of ruthenium clusters, Rux(+) (x = 2-5), and ruthenium oxide clusters, RuxOy(+) (x = 2-5, y = 1-2), with water molecules have been investigated by gas phase ion trap mass spectrometry and first principle density functional calculations. The joint experimental and theoretical study reveals that the reactions of the ruthenium oxide clusters with water are considerably more efficient. This is assigned theoretically to the stronger binding of the water molecules to RuxOy(+) and, more importantly, to water activation leading to an efficient hydrogen transfer reaction from the water molecules to the oxygen atoms of the ruthenium oxide clusters. The theoretically predicted hydrogen shift reaction has been confirmed experimentally through (16)O/(18)O isotope exchange experiments. Calculated energy profiles for the reactions of selected oxide clusters with water illustrate that the oxygen isotope exchange relies on the facile transfer of hydrogen atoms via [1,3] shift reactions between the oxygen atoms of the complexes due to the relatively low barriers involved. These findings might open perspectives for the future realization of water oxidation driven by ruthenium oxide clusters.
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Temperature-dependent gas phase ion trap experiments performed under multicollision conditions reveal a strongly size-dependent reactivity of Pd(x)(+) (x = 2-7) in the reaction with molecular oxygen. Yet, a particular stability and resistance to further oxidation is generally observed for reaction products with two oxygen molecules, Pd(x)O4(+). Complementary first-principles density functional theory simulations elucidate the details of the size-dependent bonding of oxygen to the small palladium clusters and are able to assign the pronounced occurrence of Pd(x)O4(+) complexes to a dissociatively chemisorbed bridging oxygen atomic structure which impedes the chemisorption of further oxygen molecules. The molecular physisorption of additional O2 is only observed at cryogenic temperatures. Additional experiments and simulations employing preoxidized clusters Pd(x)O(+) (x = 2-8) and Pd(x)O2(+) (x = 4-7) confirm the formation of the two different oxygen species.