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1.
J Phys Chem Lett ; 11(4): 1215-1221, 2020 Feb 20.
Artigo em Inglês | MEDLINE | ID: mdl-31978303

RESUMO

Site-selective dissociation induced by core photoexcitation of biomolecules is of key importance for the understanding of radiation damage processes and dynamics and for its promising use as "chemical scissors" in various applications. However, identifying products of site-selective dissociation in large molecules is challenging at the carbon, nitrogen, and oxygen edges because of the high recurrence of these atoms and related chemical groups. In this paper, we present the observation of site-selective dissociation at the sulfur L-edge in the gas-phase peptide methionine enkephalin, which contains only a single sulfur atom. Near-edge X-ray absorption mass spectrometry has revealed that the resonant S 2p → σ*C-S excitation of the sulfur contained in the methionine side chain leads to site-selective dissociation, which is not the case after core ionization above the sulfur L-edge. The prospects of such results for the study of charge dynamics in biomolecular systems are discussed.


Assuntos
Gases/química , Peptídeos/química , Enxofre/química , Espectroscopia por Absorção de Raios X , Encefalinas/química , Metionina/química , Prótons , Teoria Quântica
2.
Phys Chem Chem Phys ; 21(12): 6651-6661, 2019 Mar 28.
Artigo em Inglês | MEDLINE | ID: mdl-30855620

RESUMO

We present 2p core-level spectra of size-selected aluminum and silicon cluster cations from soft X-ray photoionization efficiency curves and density functional theory. The experimental and theoretical results are in very good quantitative agreement and allow for geometric structure determination. New ground state geometries for Al12+, Si15+, Si16+, and Si19+ are proposed on this basis. The chemical shifts of the 2p electron binding energies reveal a substantial difference for aluminum and silicon clusters: while in aluminum the 2p electron binding energy decreases with increasing coordination number, no such correlation was observed for silicon. The 2p binding energy shifts in clusters of both elements differ strongly from those of the corresponding bulk matter. For aluminum clusters, the core-level shifts between outer shell atoms and the encapsulated atom are of opposite sign and one order of magnitude larger than the corresponding core-level shift between surface and bulk atoms in the solid. For silicon clusters, the core-level shifts are of the same order of magnitude in clusters and in bulk silicon but no obvious correlation of chemical shift and bond length, as present for reconstructed silicon surfaces, are observed.

3.
Nanoscale ; 10(44): 20821-20827, 2018 Nov 15.
Artigo em Inglês | MEDLINE | ID: mdl-30403214

RESUMO

We present optical absorption spectra from the ultraviolet to the visible for size selected neutral Agn clusters (n = 5-120) embedded in solid Ne. We compare the spectra to time-dependent density functional calculations (TDDFT) that address the influence of the Ne matrix. With increasing size, several highly correlated electron excitations gradually develop into a single surface plasmon. Its energy is situated between 3.9 and 4.1 eV and varies with size according to the spherical electronic shell model. The plasmon energy is highest for clusters with atom numbers fully filling states with the lowest radial quantum number (e.g. 1s, 1p, 1d,...). TDDFT calculations for clusters with several candidate geometrical structures embedded in Ne show excellent agreement with the experimental data, demonstrating that the absorption bands depend only weakly on the exact structure of the cluster.

4.
Nanoscale ; 10(40): 19162-19181, 2018 Oct 18.
Artigo em Inglês | MEDLINE | ID: mdl-30302480

RESUMO

We report the results of a conjoint experimental/theoretical effort to assess the structures of free-standing zinc clusters with up to 73 atoms. Experiment provides photoemission spectra for ZnN- cluster anions, to be used as fingerprints in structural assessment, as well as mass spectra for both anion and cation clusters. Theory provides both a detailed description of a novel protocol to locate global minimum structures of clusters in an efficient and reliable way, and its specific application to neutral and charged zinc clusters. Our methodology is based on the well-known hybrid EP-DFT (empirical potential-density functional theory) approach, in which the approximate potential energy surface generated by an empirical Gupta potential is first sampled with unbiased basin hopping simulations, and then a selection of the isomers so identified is re-optimized at a first-principles DFT level. The novelty introduced in our paper is a simple but efficient new recipe to obtain the best possible EP parameters for a given cluster system, with which the first step of the EP-DFT method is to be performed. Our method is able to reproduce experimental measurements at an excellent level for most cluster sizes, implying its ability to locate the true global minimum structures; meanwhile, if exactly the same method is applied based on the existing Gupta potential (fitted to bulk properties), it leads to wrong predicted structures with energies between 1 and 2 eV above the correct ones. Opposite to what was claimed in the past, our work unequivocally demonstrates that Zn clusters are not amorphous, and they rather adopt high symmetry structures for most sizes. We show that Zn clusters have a number of exotic, unprecedented structural and electronic properties which are not expected for clusters of a metallic element, and describe them in detail.

5.
J Phys Condens Matter ; 30(35): 354002, 2018 Sep 05.
Artigo em Inglês | MEDLINE | ID: mdl-30051877

RESUMO

Size-selected anionic silicon clusters, [Formula: see text] (n = 14-20), have been investigated by photoelectron spectroscopy and density functional theory (DFT) calculations. Low-energy structures of the clusters are globally searched for by using a genetic algorithm based on DFT calculations. The electronic density of states and vertical detachment energies have been simulated by using ten DFT functionals and compared to the experimental results. We systematically evaluated the DFT functionals for the calculation of the energetics of silicon clusters. CCSD(T) single-point energies based on MP2 optimized geometries for selected isomers of [Formula: see text] are also used as benchmark for the energy sequence. The HSE06 functional with aug-cc-pVDZ basis set is found to show the best performance. Our global minimum search corroborates that most of the lowest-energy structures of [Formula: see text] (n = 14-20) clusters can be derived from assembling tricapped trigonal prisms in various ways. For most sizes previous structures are confirmed, whereas for [Formula: see text] a new structure has been found.

6.
J Am Soc Mass Spectrom ; 29(11): 2138-2151, 2018 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-30047073

RESUMO

We have investigated the photoionization and photofragmentation yields of gas-phase multiply protonated melittin cations for photon energies at the K-shell absorption edges of carbon, nitrogen, and oxygen. Two similar experimental approaches were employed. In both experiments, mass selected [melittin+qH]q+ (q=2-4) ions were accumulated in radiofrequency ion traps. The trap content was exposed to intense beams of monochromatic soft X-ray photons from synchrotron beamlines and photoproducts were analyzed by means of time-of-flight mass spectrometry. Mass spectra were recorded for fixed photon energies, and partial ion yield spectra were recorded as a function of photon energy. The combination of mass spectrometry and soft X-ray spectroscopy allows for a direct correlation of protein electronic structure with various photoionization channels. Non-dissociative single and double ionization are used as a reference. The contribution of both channels to various backbone scission channels is quantified and related to activation energies and protonation sites. Soft X-ray absorption mass spectrometry combines fast energy deposition with single and double ionization and could complement established activation techniques. Graphical Abstract ᅟ.

7.
Angew Chem Int Ed Engl ; 57(30): 9310-9314, 2018 Jul 20.
Artigo em Inglês | MEDLINE | ID: mdl-29847000

RESUMO

The synthesis and structural characterization of the hitherto unknown parent Co(bz)2+ (bz=benzene) complex and several of its derivatives are described. Their synthesis starts either from a CoCO5+ salt, or directly from Co2 (CO)8 and a Ag+ salt. Stability and solubility of these complexes was achieved by using the weakly coordinating anions (WCAs) [Al(ORF )4 ]- and [F{Al(ORF )3 }2 ]- {RF =C(CF3 )3 } and the solvent ortho-difluorobenzene (o-DFB). The magnetic properties of Co(bz)2+ were measured and compared in the condensed and gas phases. The weakly bound Co(o-dfb)2+ salts are of particular interest for the preparation of further CoI salts, for example, the structurally characterized low-coordinate 12 valence electron Co(Pt Bu3 )2+ and Co(NHC)2+ salts.

8.
Chemistry ; 24(30): 7631-7636, 2018 May 28.
Artigo em Inglês | MEDLINE | ID: mdl-29637635

RESUMO

Preservation of protein conformation upon transfer into the gas phase is key for structure determination of free single molecules, for example using X-ray free-electron lasers. In the gas phase, the helicity of melittin decreases strongly as the protein's protonation state increases. We demonstrate the sensitivity of soft X-ray spectroscopy to the gas-phase structure of melittin cations ([melittin+qH]q+ , q=2-4) in a cryogenic linear radiofrequency ion trap. With increasing helicity, we observe a decrease of the dominating carbon 1 s-π* transition in the amide C=O bonds for non-dissociative single ionization and an increase for non-dissociative double ionization. As the underlying mechanism we identify inelastic electron scattering. Using an independent atom model, we show that the more compact nature of the helical protein conformation substantially increases the probability for off-site intramolecular ionization by inelastic Auger electron scattering.

9.
J Am Chem Soc ; 137(27): 8676-9, 2015 Jul 15.
Artigo em Inglês | MEDLINE | ID: mdl-26115184

RESUMO

The activation of CO2 and its hydrogenation to methanol are of much interest as a way to utilize captured CO2. Here, we investigate the use of size-selected Cu4 clusters supported on Al2O3 thin films for CO2 reduction in the presence of hydrogen. The catalytic activity was measured under near-atmospheric reaction conditions with a low CO2 partial pressure, and the oxidation state of the clusters was investigated by in situ grazing incidence X-ray absorption spectroscopy. The results indicate that size-selected Cu4 clusters are the most active low-pressure catalyst for catalytic CO2 conversion to CH3OH. Density functional theory calculations reveal that Cu4 clusters have a low activation barrier for conversion of CO2 to CH3OH. This study suggests that small Cu clusters may be excellent and efficient catalysts for the recycling of released CO2.

10.
Angew Chem Int Ed Engl ; 54(15): 4498-501, 2015 Apr 07.
Artigo em Inglês | MEDLINE | ID: mdl-25677238

RESUMO

X-ray magnetic circular dichroism spectroscopy has been used to characterize the electronic structure and magnetic moment of Cr2 (+) . Our results indicate that the removal of a single electron from the 4sσg bonding orbital of Cr2 drastically changes the preferred coupling of the 3d electronic spins. While the neutral molecule has a zero-spin ground state with a very short bond length, the molecular cation exhibits a ferromagnetically coupled ground state with the highest possible spin of S=11/2, and almost twice the bond length of the neutral molecule. This spin configuration can be interpreted as a result of indirect exchange coupling between the 3d electrons of the two atoms that is mediated by the single 4s electron through a strong intraatomic 3d-4s exchange interaction. Our finding allows an estimate of the relative energies of two states that are often discussed as ground-state candidates, the ferromagnetically coupled (12) Σ and the low-spin (2) Σ state.

11.
Angew Chem Int Ed Engl ; 54(7): 2111-5, 2015 Feb 09.
Artigo em Inglês | MEDLINE | ID: mdl-25533454

RESUMO

How many of the several attributes of the bulk metallic state persist in a nanoparticle containing a finite number of atoms of a metallic element? Do all those attributes emerge suddenly at a well-defined cluster size or do they rather evolve at different rates and in a broad size range? These fundamental questions have been addressed through a conjoint experimental/theoretical investigation of zinc clusters. We report the observation of novel coexistence phenomena involving different electronic phases: for some sizes, metallic and insulating electronic states coexist within a single, Janus-like, nanoparticle; for the rest of sizes, we report the coexistence of two weakly interacting metallic phases with different dimensionalities, localized at the shell and the core of the nanoparticle. These fascinating features are due to an anomalously long core-shell separation that equips the shell and core regions with largely independent structural, vibrational, and thermal properties.

12.
Phys Rev Lett ; 113(23): 233001, 2014 Dec 05.
Artigo em Inglês | MEDLINE | ID: mdl-25526125

RESUMO

Extending the range of quantum interferometry to a wider class of composite nanoparticles requires new tools to diffract matter waves. Recently, pulsed photoionization light gratings have demonstrated their suitability for high mass matter-wave physics. Here, we extend quantum interference experiments to a new class of particles by introducing photofragmentation beam splitters into time-domain matter-wave interferometry. We present data that demonstrate this coherent beam splitting mechanism with clusters of hexafluorobenzene and we show single-photon depletion gratings based both on fragmentation and ionization for clusters of vanillin. We propose that photofragmentation gratings can act on a large set of van der Waals clusters and biomolecules which are thermally unstable and often resilient to single-photon ionization.

13.
Nat Nanotechnol ; 5(5): 335-9, 2010 May.
Artigo em Inglês | MEDLINE | ID: mdl-20364134

RESUMO

Metal nanoparticles supported by thin films are important in the fields of molecular electronics, biotechnology and catalysis, among others. Penetration of these nanoparticles through their supporting films can be undesirable in some circumstances but desirable in others, and is often considered to be a diffusive process. Here, we demonstrate a mechanism for the penetration of thin films and other nanoscopic barriers that is different from simple diffusion. Silver clusters that are soft-landed onto a monolayer of C(60) supported by gold sink through the monolayer in a matter of hours. However, the clusters are stable when landed onto two monolayers of C(60) supported on gold, or on one monolayer of C(60) supported on graphite. With backing from atomistic calculations, these results demonstrate that a metallic substrate exerts attractive forces on metallic nanoparticles that are separated from the substrate by a single monolayer.

14.
J Chem Phys ; 131(20): 204313, 2009 Nov 28.
Artigo em Inglês | MEDLINE | ID: mdl-19947686

RESUMO

Oxygen-doped sodium cluster anions Na(n)O(2) (-) with n=41-148 have been studied by low temperature photoelectron spectroscopy and density functional theory (DFT), with a particular emphasis on those sizes where a spherical electron shell closing is expected. The experimental spectra are in good agreement with the electronic density of states of the DFT lowest energy structures. The cluster structures show segregation between an ionically bonded molecular unit located at the cluster surface and a metallic part. The DFT calculations reveal that each oxygen atom removes two electrons from the metallic electron gas in order to become an O(2-) dianion. A jellium model would therefore predict the electron shell closings to be shifted up by four sodium atoms with respect to pure Na(n) (-) cluster anions. The electron shell closings for Na(n)O(2) (-) are located at n=43, 61, 93, and 139, so the expected four-atom shift is observed only for the small clusters of up to n=61, while a two-atom shift is observed for the larger clusters. The DFT calculations explain this departure from jellium model predictions in terms of a structural transition in the ionically bonded molecular unit.

15.
J Chem Phys ; 131(14): 144303, 2009 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-19831437

RESUMO

Photoelectron spectra of cold (10 K) size selected water cluster anions (H(2)O)(n) (-) and (D(2)O)(n) (-) have been measured in the size range n=20-120. A new isomer with a higher binding energy than the so-called isomer I has been identified, which appears in the size range n=25-30 and for (H(2)O)(n) (-) becomes dominant at n=46. Magic numbers observed in the mass spectra of the cluster anions provide evidence that this new isomer class consists of clusters with an internal electron.

16.
Phys Rev Lett ; 102(4): 043401, 2009 Jan 30.
Artigo em Inglês | MEDLINE | ID: mdl-19257415

RESUMO

Caloric curves for sodium clusters with N=139 and 147 atoms show a fine structure near the solid-to-liquid transition. Neither of the two sizes exhibit surface melting. For N=139, diffusion of the surface vacancies is observed, which is not possible in the closed-shell N=147 cluster. A few kelvin above the peak in the heat capacity, N=139 is completely liquid. This is not the case for N=147. Here the inner 13 atoms remain nearly fixed up to several tens of kelvin above the melting temperature of the outer two layers. A simple physical reason is suggested for this unexpected behavior.

17.
Phys Rev Lett ; 98(1): 015701, 2007 Jan 05.
Artigo em Inglês | MEDLINE | ID: mdl-17358487

RESUMO

The thermodynamics of gold cluster anions (AuN-, N=11,...,14) is investigated using quantum molecular dynamics. Our simulations suggest that AuN- may exhibit a novel, freestanding planar liquid phase which dynamically coexists with a normal three-dimensional liquid. Upon cooling with experimentally realizable cooling rates, the entropy-favored three-dimensional liquid clusters often supercool and solidify into the "wrong" dimensionality. This indicates that experimental validation of theoretically predicted AuN- ground states might be more complicated than hitherto expected.

18.
Phys Rev Lett ; 98(4): 043401, 2007 Jan 26.
Artigo em Inglês | MEDLINE | ID: mdl-17358764

RESUMO

Sodium cluster anions Na(n)(-) with n = 39-350 have been studied by low temperature photoelectron spectroscopy and density functional theory (DFT). The highly structured experimental spectra are in excellent agreement with the electronic density of states (DOS) of the DFT lowest energy structures. Even for the largest sizes, a pronounced sensitivity of the DOS on fine geometric details could be observed, allowing for a reliable identification of a specific icosahedral growth motif. The intermediate sizes between the closed-shell Mackay clusters with 55, 147, and 309 atoms form by growth of overlayers, which often exhibit a twist deformation with respect to regular (Mackay-type) ones.

20.
Chemphyschem ; 8(1): 157-61, 2007 Jan 08.
Artigo em Inglês | MEDLINE | ID: mdl-17131433

RESUMO

Ground-state structures and other experimentally relevant isomers of Au(15) (-) to Au(24) (-) clusters are determined through joint first-principles density functional theory and photoelectron spectroscopy measurements. Subsequent calculations of molecular O(2) adsorption to the optimal cluster structures reveal a size-dependent reactivity pattern that agrees well with earlier experiments. A detailed analysis of the underlying electronic structure shows that the chemical reactivity of the gold cluster anions can be elucidated in terms of a partial-jellium picture, where delocalized electrons occupying electronic shells move over the ionic skeleton, whose geometric structure is strongly influenced by the directional bonding associated with the highly localized "d-band" electrons.

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