Your browser doesn't support javascript.
Mostrar: 20 | 50 | 100
Resultados 1 - 20 de 66
Filtrar
Mais filtros










Base de dados
Intervalo de ano de publicação
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.
Phys Chem Chem Phys ; 21(43): 23922-23930, 2019 Nov 21.
Artigo em Inglês | MEDLINE | ID: mdl-31661104

RESUMO

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+.

4.
J Am Soc Mass Spectrom ; 30(10): 1895-1905, 2019 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-31300975

RESUMO

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.

5.
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.

6.
J Phys Condens Matter ; 30(50): 504001, 2018 Dec 19.
Artigo em Inglês | MEDLINE | ID: mdl-30465551

RESUMO

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.

7.
Acc Chem Res ; 51(12): 3104-3113, 2018 12 18.
Artigo em Inglês | MEDLINE | ID: mdl-30462479

RESUMO

Silver and gold molecular nanoparticles (mNPs) are a relatively new class of molecular materials of fundamental interest. They are high-nuclearity metal-organic compounds, with ligated metal cores, where the different character of bonding in the ligand shell and metal core gives rise to many of the unique properties of these materials. Research has primarily focused on gold mNPs, due to their good stability and the ease with which they may be synthesized and processed. To understand these materials as a general class, however, it will be necessary to broaden research efforts to other metals. Gold and silver are isoelectronic and have the same atomic radius, making the comparison of gold and silver mNPs attractive. The optical and chemical differences of the two metals provide useful contrasts, however, as well as a means to access a wider range of properties. In this Account, we focus on the synthesis, structure, and reactivity of silver mNPs. First, we review the origins and history of the field, from the ill-defined gas-phase metal clusters of the 1980s to the precisely defined mNPs of 1996 and onward. Next, we discuss the role of silver as a complement to gold mNPs in the effort to generalize lessons learned from either material and extend them into new metals. The synthesis of silver mNPs is covered in some detail, noting the choices made as the chemistry and the materials were developed. The importance of coordinating solvents and thermodynamic stability are also noted. The need to reduce solvent use is discussed and a new approach to achieving this goal is presented. Next, the structures of silver mNPs are discussed, including the Ag44 and Ag17 archetypes, and focusing on the successful de novo structure prediction of the latter. Structure and prediction of ligand shell motifs are also discussed. Finally, the postsynthetic chemistry and reactivity of silver mNPs are presented, including some of the first efforts to elucidate reaction mechanisms, beginning in 2012. Silver nanoparticles are gaining in popularity, particularly compared with gold, as the potential for silver to make a technological and economic impact is recognized. The superior optical properties of silver already make it a valuable material for plasmonics, but this may also translate to molecular species for nonlinear optics, sensors, and optoelectronics. The higher reactivity may also lead to a greater diversity of chemistry for silver compared to gold, including as an important broad-spectrum antimicrobial. Conversely, the "ultrastability" of the Ag44 archetype has already enabled unprecedented scale up with molecular precision, and may lead to the first industrial-scale production of metal mNPs. Clearly, silver mNPs are one of the most promising and significant new materials being studied today.

8.
Acta Crystallogr E Crystallogr Commun ; 74(Pt 7): 987-993, 2018 Jul 01.
Artigo em Inglês | MEDLINE | ID: mdl-30002900

RESUMO

Crystals of M4Au12Ag32(p-MBA)30 bimetallic monolayer-protected clusters (MPCs), where p-MBA is p-mercapto-benzoic acid and M+ is a counter-cation (M = Na, Cs) have been grown and their structure determined. The mol-ecular structure of triacontakis[(4-carboxylatophenyl)sulfanido]dodecagolddotriacontasilver, Au12Ag32(C7H5O2S)30 or C210H150Ag32Au12O60S30, exhib-its point group symmetry at 100 K. The overall diameter of the MPC is approximately 28 Å, while the diameter of the Au12Ag20 metallic core is 9 Å. The structure displays ligand bundling and inter-molecular hydrogen bonding, which gives rise to a framework structure with 52% solvent-filled void space. The positions of the M+ cations and the DMF solvent mol-ecules within the void space of the crystal could not be determined. Three out of the five crystallographically independent ligands in the asymmetric unit cell are disordered over two sets of sites. Comparisons are made to the all-silver M4Ag44(p-MBA)30 MPCs and to expectations based on density functional theory.

9.
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.

10.
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.

11.
Angew Chem Int Ed Engl ; 55(31): 8953-7, 2016 07 25.
Artigo em Inglês | MEDLINE | ID: mdl-27356301

RESUMO

Ethylene hydrogenation was investigated on size-selected Pt13 clusters supported on three amorphous silica (a-SiO2 ) thin films with different stoichiometries. Activity measurements of the reaction at 300 K revealed that on a silicon-rich and a stoichiometric film, Pt13 exhibits a similar activity to that of Pt(111), in line with the known structure insensitivity of the reaction. On an oxygen-rich film, a threefold increased rate was measured. Pulsing ethylene at 400 K, then measuring the activity at 300 K, resulted in complete loss of activity on the silicon-rich surface compared to only marginal losses on the other surfaces. The measured reactivity trends correlate with charging characteristics of a Pt13 cluster on the SiO2 films, predicted through first-principle calculations. The results reveal that the stoichiometry-dependent charging by the support can be used to tune the selectivity of reaction pathways during a catalytic hydrogenation reaction.

12.
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.

14.
Nat Commun ; 7: 10389, 2016 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-26817713

RESUMO

The sensitivity, or insensitivity, of catalysed reactions to catalyst structure is a commonly employed fundamental concept. Here we report on the nature of nano-catalysed ethylene hydrogenation, investigated through experiments on size-selected Ptn (n=8-15) clusters soft-landed on magnesia and first-principles simulations, yielding benchmark information about the validity of structure sensitivity/insensitivity at the bottom of the catalyst size range. Both ethylene-hydrogenation-to-ethane and the parallel hydrogenation-dehydrogenation ethylidyne-producing route are considered, uncovering that at the <1 nm size-scale the reaction exhibits characteristics consistent with structure sensitivity, in contrast to structure insensitivity found for larger particles. The onset of catalysed hydrogenation occurs for Ptn (n ≥ 10) clusters at T>150 K, with maximum room temperature reactivity observed for Pt13. Structure insensitivity, inherent for specific cluster sizes, is induced in the more active Pt13 by a temperature increase up to 400 K leading to ethylidyne formation. Control of sub-nanometre particle size may be used for tuning catalysed hydrogenation activity and selectivity.

15.
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.

16.
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.

17.
Nano Lett ; 15(10): 7105-11, 2015 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-26352520

RESUMO

"Bottom-up" approaches to the many-body physics of fermions have recently demonstrated precise number and site-resolved preparations with tunability of interparticle interactions in single-well, SW, and double-well, DW, nanoscale confinements created by manipulating ultracold fermionic atoms with optical tweezers. These experiments emulate an analogue-simulator mapping onto the requisite microscopic Hamiltonian, approaching realization of Feynmans' vision of quantum simulators that "will do exactly the same as nature". Here we report on exact benchmark configuration-interaction computational microscopy solutions of the Hamiltonian, uncovering the spectral evolution, wave function anatomy, and entanglement properties of the interacting fermions in the entire parameter range, including crossover from an SW to a DW confinement and a controllable energy imbalance between the wells. We demonstrate attractive pairing and formation of repulsive, highly correlated, ultracold Wigner molecules, well-described in the natural orbital representation. The agreement with the measurements affirms the henceforth gained deep insights into ultracold molecules and opens access to the size-dependent evolution of nanoclustered and condensed-matter phases and ultracold-atoms quantum information.

18.
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
19.
Sci Rep ; 5: 7893, 2015 Jan 20.
Artigo em Inglês | MEDLINE | ID: mdl-25599915

RESUMO

Graphene's isolation launched explorations of fundamental relativistic physics originating from the planar honeycomb lattice arrangement of the carbon atoms, and of potential technological applications in nanoscale electronics. Bottom-up fabricated atomically-precise segmented graphene nanoribbons, SGNRs, open avenues for studies of electrical transport, coherence, and interference effects in metallic, semiconducting, and mixed GNRs, with different edge terminations. Conceptual and practical understanding of electric transport through SGNRs is gained through nonequilibrium Green's function (NEGF) conductance calculations and a Dirac continuum model that absorbs the valence-to-conductance energy gaps as position-dependent masses, including topological-in-origin mass-barriers at the contacts between segments. The continuum model reproduces the NEGF results, including optical Dirac Fabry-Prot (FP) equidistant oscillations for massless relativistic carriers in metallic armchair SGNRs, and an unequally-spaced FP pattern for mixed armchair-zigzag SGNRs where carriers transit from a relativistic (armchair) to a nonrelativistic (zigzag) regime. This provides a unifying framework for analysis of coherent transport phenomena and interpretation of forthcoming experiments in SGNRs.

20.
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.

SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA