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1.
J Phys Chem A ; 124(24): 4990-4997, 2020 Jun 18.
Artigo em Inglês | MEDLINE | ID: mdl-32515597

RESUMO

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.

2.
J Phys Chem A ; 2020 Feb 12.
Artigo em Inglês | MEDLINE | ID: mdl-31994885

RESUMO

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.

3.
Angew Chem Int Ed Engl ; 58(25): 8504-8509, 2019 06 17.
Artigo em Inglês | MEDLINE | ID: mdl-30985054

RESUMO

One of the fundamental processes in nature, the oxidation of water, is catalyzed by a small CaMn3 O4 ⋅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 H2 18 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.

4.
Phys Chem Chem Phys ; 20(11): 7781-7790, 2018 Mar 14.
Artigo em Inglês | MEDLINE | ID: mdl-29504007

RESUMO

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.

5.
Angew Chem Int Ed Engl ; 56(43): 13406-13410, 2017 10 16.
Artigo em Inglês | MEDLINE | ID: mdl-28869784

RESUMO

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 d 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.

7.
Phys Chem Chem Phys ; 18(23): 15727-37, 2016 Jun 21.
Artigo em Inglês | MEDLINE | ID: mdl-27226138

RESUMO

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.

8.
Sci Adv ; 2(11): e1601609, 2016 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-28138537

RESUMO

Fathoming the principles underpinning the structures of monolayer-coated molecular metal nanoparticles remains an enduring challenge. Notwithstanding recent x-ray determinations, coveted veritable de novo structural predictions are scarce. Building on recent syntheses and de novo structure predictions of M3Au x Ag17-x (TBBT)12, where M is a countercation, x = 0 or 1, and TBBT is 4-tert-butylbenzenethiol, we report an x-ray-determined structure that authenticates an a priori prediction and, in conjunction with first-principles theoretical analysis, lends force to the underlying forecasting methodology. The predicted and verified Ag(SR)3 monomer, together with the recently discovered Ag2(SR)5 dimer and Ag3(SR)6 trimer, establishes a family of unique mount motifs for silver thiolate nanoparticles, expanding knowledge beyond the earlier-known Au-S staples in thiol-capped gold nanoclusters. These findings demonstrate key principles underlying ligand-shell anchoring to the metal core, as well as unique T-like benzene dimer and cyclic benzene trimer ligand bundling configurations, opening vistas for rational design of metal and alloy nanoparticles.

9.
Angew Chem Int Ed Engl ; 54(50): 15113-7, 2015 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-26494552

RESUMO

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.

10.
J Am Chem Soc ; 137(36): 11550-3, 2015 09 16.
Artigo em Inglês | MEDLINE | ID: mdl-26301320

RESUMO

Although silver nanoparticles are of great fundamental and practical interest, only one structure has been determined thus far: M4Ag44(SPh)30, where M is a monocation, and SPh is an aromatic thiolate ligand. This is in part due to the fact that no other molecular silver nanoparticles have been synthesized with aromatic thiolate ligands. Here we report the synthesis of M3Ag17(4-tert-butylbenzene-thiol)12, which has good stability and an unusual optical spectrum. We also present a rational strategy for predicting the structure of this molecule. First-principles calculations support the structural model, predict a HOMO-LUMO energy gap of 1.77 eV, and predict a new "monomer mount" capping motif, Ag(SR)3, for Ag nanoparticles. The calculated optical absorption spectrum is in good correspondence with the measured spectrum. Heteroatom substitution was also used as a structural probe. First-principles calculations based on the structural model predicted a strong preference for a single Au atom substitution in agreement with experiment.


Assuntos
Nanopartículas Metálicas , Prata/química , Cristalografia por Raios X , Modelos Moleculares , Estrutura Molecular , Espectrometria de Massas por Ionização por Electrospray
11.
J Phys Chem A ; 118(37): 8572-82, 2014 Sep 18.
Artigo em Inglês | MEDLINE | ID: mdl-24915185

RESUMO

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.

12.
Nat Mater ; 13(8): 807-11, 2014 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-24705383

RESUMO

Self-assembled nanoparticle superlattices-materials made of inorganic cores capped by organic ligands, of varied structures, and held together by diverse binding motifs-exhibit size-dependent properties as well as tunable collective behaviour arising from couplings between their nanoscale constituents. Here, we report the single-crystal X-ray structure of a superlattice made in the high-yield synthesis of Na(4)Ag(44)(p-MBA)(30) nanoparticles, and find with large-scale quantum-mechanical simulations that its atomically precise structure and cohesion derive from hydrogen bonds between bundledp-MBA ligands. We also find that the superlattice's mechanical response to hydrostatic compression is characterized by a molecular-solid-like bulk modulus B(0) = 16.7 GPa, exhibiting anomalous pressure softening and a compression-induced transition to a soft-solid phase. Such a transition involves ligand flexure, which causes gear-like correlated chiral rotation of the nanoparticles. The interplay of compositional diversity, spatial packing efficiency, hydrogen-bond connectivity, and cooperative response in this system exemplifies the melding of the seemingly contrasting paradigms of emergent behaviour 'small is different' and 'more is different'.

13.
Nano Lett ; 13(11): 5549-55, 2013.
Artigo em Inglês | MEDLINE | ID: mdl-24164444

RESUMO

The interaction of ligand-free manganese oxide nanoclusters with water is investigated, aiming at uncovering phenomena which could aid the design of artificial water-splitting molecular catalysts. Gas phase measurements in an ion trap in conjunction with first-principles calculations provide new mechanistic insight into the water splitting process mediated by bi- and tetra-nuclear singly charged manganese oxide clusters, Mn2O2(+) and Mn4O4(+). In particular, a water-induced dimensionality change of Mn4O4(+) is predicted, entailing transformation from a two-dimensional ring-like ground state structure of the bare cluster to a cuboidal octa-hydroxy-complex for the hydrated one. It is further predicted that the water splitting process is facilitated by the cluster dimensionality crossover. The vibrational spectra calculated for species occurring along the predicted pathways of the reaction of Mn4O4(+) with water provide the impetus for future explorations, including vibrational spectroscopic experiments.


Assuntos
Compostos de Manganês/química , Óxidos/química , Oxigênio/química , Água/química , Catálise , Gases/química , Transição de Fase , Análise Espectral , Vibração
14.
Nature ; 501(7467): 399-402, 2013 Sep 19.
Artigo em Inglês | MEDLINE | ID: mdl-24005327

RESUMO

Noble-metal nanoparticles have had a substantial impact across a diverse range of fields, including catalysis, sensing, photochemistry, optoelectronics, energy conversion and medicine. Although silver has very desirable physical properties, good relative abundance and low cost, gold nanoparticles have been widely favoured owing to their proved stability and ease of use. Unlike gold, silver is notorious for its susceptibility to oxidation (tarnishing), which has limited the development of important silver-based nanomaterials. Despite two decades of synthetic efforts, silver nanoparticles that are inert or have long-term stability remain unrealized. Here we report a simple synthetic protocol for producing ultrastable silver nanoparticles, yielding a single-sized molecular product in very large quantities with quantitative yield and without the need for size sorting. The stability, purity and yield are substantially better than those for other metal nanoparticles, including gold, owing to an effective stabilization mechanism. The particular size and stoichiometry of the product were found to be insensitive to variations in synthesis parameters. The chemical stability and structural, electronic and optical properties can be understood using first-principles electronic structure theory based on an experimental single-crystal X-ray structure. Although several structures have been determined for protected gold nanoclusters, none has been reported so far for silver nanoparticles. The total structure of a thiolate-protected silver nanocluster reported here uncovers the unique structure of the silver thiolate protecting layer, consisting of Ag2S5 capping structures. The outstanding stability of the nanoparticle is attributed to a closed-shell 18-electron configuration with a large energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, an ultrastable 32-silver-atom excavated-dodecahedral core consisting of a hollow 12-silver-atom icosahedron encapsulated by a 20-silver-atom dodecahedron, and the choice of protective coordinating ligands. The straightforward synthesis of large quantities of pure molecular product promises to make this class of materials widely available for further research and technology development.

15.
J Am Chem Soc ; 135(10): 3904-14, 2013 Mar 13.
Artigo em Inglês | MEDLINE | ID: mdl-23421474

RESUMO

One-electron oxidation of A/T-rich DNA leads to mutations at thymine. Experimental investigation of DNA containing methyl-deuterated thymine reveals a large isotope effect establishing that cleavage of this carbon-hydrogen bond is involved in the rate-determining step of the reaction. First-principles quantum calculations reveal that the radical cation (electron hole) generated by DNA oxidation, initially located on adenines, localizes on thymine as the proton is lost from the methyl group, demonstrating the role of proton-coupled electron transfer (PCET) in thymine oxidation. Proton transport by structural diffusion along a segmented "water-wire" culminates in proton solvation in the hydration environment, serving as an entropic reservoir that inhibits reversal of the PCET process. These findings provide insight into mutations in A/T-rich DNA such as replication fork stalling that is implicated in early stage carcinogenesis.


Assuntos
DNA/química , Prótons , Timina/química , Transporte de Elétrons , Elétrons , Modelos Moleculares , Estrutura Molecular , Oxirredução , Água/química
16.
J Am Chem Soc ; 134(51): 20654-9, 2012 Dec 26.
Artigo em Inglês | MEDLINE | ID: mdl-23237307

RESUMO

The palladium oxide cluster Pd(6)O(4)(+) is formed as the sole product upon reaction of a bare palladium cluster Pd(6)(+) with molecular oxygen in an octopole ion trap under multicollision conditions. This oxide cluster is found to be resistant to further oxidation over a large temperature range, and further O(2) molecules merely physisorb on it at cryogenic temperatures. The particular stability of Pd(6)O(4)(+) is confirmed by the observation that the reaction of Pd(7)(+) with O(2) leads to fragmentation resulting in the formation of Pd(6)O(4)(+). However, in an oxygen-rich O(2)/CO mixture, Pd(6)O(4)(+) is identified as the catalytically active species that effectively facilitates the low-temperature oxidation of CO. Gas-phase reaction kinetics measurements in conjunction with first-principles calculations provide detailed molecular level insight into the nano-oxide cluster chemistry and are able to reveal the full catalytic combustion reaction cycle.

17.
Angew Chem Int Ed Engl ; 51(52): 13114-8, 2012 Dec 21.
Artigo em Inglês | MEDLINE | ID: mdl-23154932

RESUMO

A golden opportunity: the total structure of a Au(36)(SR)(24) nanocluster reveals an unexpected face-centered-cubic tetrahedral Au(28) kernel (magenta). The protecting layer exhibits an intriguing combination of binding modes, consisting of four regular arch-like staples and the unprecedented appearance of twelve bridging thiolates (yellow). This unique protecting network and superatom electronic shell structure confer extreme stability and robustness.


Assuntos
Ouro/química , Nanopartículas Metálicas/química , Cristalografia por Raios X , Elétrons , Conformação Molecular , Teoria Quântica
18.
J Phys Chem A ; 115(25): 7378-91, 2011 Jun 30.
Artigo em Inglês | MEDLINE | ID: mdl-21595455

RESUMO

Binding of excess electrons to nanosize water droplets, with a focus on the hitherto largely unexplored properties of doubly-charged clusters, were investigated experimentally using mass spectrometry and theoretically with large-scale first-principles simulations based on spin-density-functional theory, with all the valence electrons (that is, 8e per water molecule) and excess electrons treated quantum mechanically. Singly-charged clusters (H(2)O)(n)(-1) were detected for n = 6-250, and our calculated vertical detachment energies agree with previously measured values in the entire range 15 ≤ n ≤ 105, giving a consistent interpretation in terms of internal, surface and diffuse states of the excess electron. Doubly-charged clusters were measured in the range of 83 ≤ n ≤ 123, with (H(2)O)(n)(-2) clusters found for 83 ≤ n < 105, and mass-shifted peaks corresponding to (H(2)O)(n-2)(OH(-))(2) detected for n ≥ 105. The simulations revealed surface and internal dielectron, e(-)(2), localization modes and elucidated the mechanism of the reaction (H(2)O)(n)(-2) → (H(2)O)(n-2) (OH(-))(2) + H(2) (for n ≥ 105), which was found to occur via concerted approach of a pair of protons belonging to two water molecules located in the first shell of the dielectron internal hydration cavity, culminating in formation of a hydrogen molecule 2H(+) + e(-)(2) → H(2). Instability of the dielectron internal localization impedes the reaction for smaller (n < 105) doubly-charged clusters.

19.
Chemphyschem ; 11(7): 1570-7, 2010 May 17.
Artigo em Inglês | MEDLINE | ID: mdl-20379980

RESUMO

The reactions of small gold cluster cations Au(x)(+) (x=2-6) with CH(4) were studied by joint gas-phase kinetics and first-principles density functional theory calculations. The experimentally obtained temperature-dependent low pressure rate constants were analyzed by employing the Lindemann energy transfer model for association reactions in conjunction with statistical RRKM theory. In this way cluster-size-dependent binding energies of methane to the gold cluster cations were determined from the experimental data for two different transition-state models. The experimental binding energies obtained by employing a "loose" transition-state model are in good agreement with the theoretical values at the optimal adsorption geometries, while a "tight" transition-state model clearly gives a lower limit for the binding energies. Additionally, Kohn-Sham molecular orbitals of Au(x)-CH(4)(+) are presented to gain detailed insight into the cluster-methane bonding mechanism.


Assuntos
Ouro/química , Metano/química , Adsorção , Sítios de Ligação , Simulação por Computador , Cinética , Modelos Químicos , Tamanho da Partícula , Propriedades de Superfície
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