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The stability of polycyclic aromatic hydrocarbons (PAHs) upon soft X-ray absorption is of crucial relevance for PAH survival in X-ray dominated regions. PAH stability depends on molecular size but also on the degree of hydrogenation that is related to H2 formation in the interstellar medium. In this project, we intend to reveal the changes of electronic structure caused by hydrogenation and the impact of hydrogenation on the stability of the carbon backbone for cationic pyrene and its hydrogenated derivatives by analysis of near C K-edge soft X-ray photoions. In our experiments, the PAH cations were trapped in a cryogenic radiofrequency linear ion trap and exposed to monochromatic X-rays with energies from 279 to 300 eV. The photoproducts were mass-analysed by means of time-of-flight spectroscopy. Partial ion yields were then studied as a function of photon energy. X-ray absorption spectra computed by time-dependent density functional theory (TD-DFT) aided the interpretation of the experimental results. A very good agreement between experimental data and TD-DFT with short-range corrected functionals for all PAH ions was reached. The near-edge X-ray absorption mass spectra exhibit clear peaks due to C 1s transitions to singly occupied molecular orbitals and to low-lying unoccupied molecular orbitals. In contrast to coronene cations, where hydrogen attachment drastically increases photostability of coronene, the influence of hydrogenation on photostability is substantially weaker for pyrene cations. Here, hydrogen attachment even destabilizes the molecular structure. An astrophysical model describes the half-life of PAH ions in interstellar environments.

We present experimental evidence for soft X-ray induced intramolecular hydrogen transfer in the protonated synthetic tri-oligonucleotide d($^{\mbox{\footnotesize{F}}}$UAG) in the gas-phase. The trinucleotide cations were stored in a cryogenic ion trap and exposed to monochromatic synchrotron radiation. Photoionization and photofragmentation product ion yields were recorded as a function of photon energy. Predominanly glycosidic bond cleavage leading to formation of nucleobase-related fragments is observed. In most cases, glycosidic bond cleavage is accompanied by single or double hydrogen transfer. The combination of absorption-site-sensitive soft X-ray spectroscopy with fragment specific mass spectrometry allows to directly relate X-ray absorption site and fragmentation site. We observe pronounced resonant features in the competition between single and double hydrogen transfer towards nucleobases. A direct comparison of experimental data with time-dependent density functional theory calculations, using short range corrected hybrid functionals, reveal that these hydrogen transfer processes are universal and not limited to population of particular excited states localized at the nucleobases. Instead, hydrogen transfer can occur upon X-ray absorption in any nucleobase and in the DNA backbone. Resonances seem to occur because of site-selective suppression of hydrogen transfer channels. Furthermore, non-covalent interactions of the optimized ground state geometries were investigated to identify intramolecular hydrogen bonds along which hydrogen transfer is most likely.

We present a systematic study of the structures and electronic properties of vanadium-doped silicon cluster anions, VSin− (n = 14–20), by combining photoelectron spectroscopy (PES) measurements and density functional theory (DFT) calculations.

We investigated the structural evolution and electronic properties of medium-sized silicon cluster anions doped with two transition metal atoms, TM2Sin− (TM = V, Cr; n = 14–20), by using mass-selective anion photoelectron spectroscopy combined with density functional theory (DFT) calculations.

Oxidation states are integer in number but dn configurations of transition metal centers vary continuously in polar bonds. We quantify the shifts of the iron L3 excitation energy, within the same formal oxidation state, in a systematic L edge X ray absorption spectroscopy study of diatomic gas phase iron II halide cations, [FeX] ,where X F, Cl, Br, I. These shifts correlate with the electronegativity of the halogen, and are attributed exclusively to a fractional increase in population of 3d derived orbitals along the series as supported by charge transfer multiplet simulations and density functional theory calculations. We extract an excitation energy shift of 420 meV 60 meV spanning the full range of possible 3d occupations between the most ionic bond in [FeF] and covalently bonded [FeI]

Photoelectron spectra of large size selected water cluster anions (H2O)n- (n = 100-1100) have been measured at a low cluster temperature (80 K). An extensive peak analysis has been conducted in order to determine average and isomer-resolved vertical detachment energies (VDE) of the hydrated electron. This allows us, in combination with the reevaluated data of the previously reported results on small- and medium-sized water cluster anions ( J. Chem. Phys. 2009, 131, 144303), to draw a comprehensive picture of the size-dependent development of the VDEs of water clusters. This allows for an improved extrapolation of the cluster VDEs to the bulk, which yields a value of 3.60 ± 0.03 eV. The general size dependence of the VDEs is in very good agreement with a standard dielectric model.

Oxidation states are integer in number but d

The local electronic structure of the metal-active site and the deexcitation pathways of metalloporphyrins are crucial for numerous applications but difficult to access by commonly employed techniques. Here, we applied near-edge X-ray absorption mass spectrometry and quantum-mechanical restricted active space calculations to investigate the electronic structure of the metal-active site of the isolated cobalt(iii) protoporphyrin IX cation (CoPPIX+) and its deexcitation pathways upon resonant absorption at the cobalt L-edge. The experiments were carried out in the gas phase, thus allowing for control over the chemical state and molecular environment of the metalloporphyrin. The obtained mass spectra reveal that resonant excitations of CoPPIX+ at the cobalt L3-edge lead predominantly to the formation of the intact radical dication and doubly charged fragments through losses of charged and neutral side chains from the macrocycle. The comparison between experiment and theory shows that CoPPIX+ is in a 3A2g triplet ground state and that competing excitations to metal-centred non-bonding and antibonding σ* molecular orbitals lead to distinct deexcitation pathways., Near-edge X-ray absorption mass spectrometry (NEXAMS) and restricted active space (RAS) quantum mechanical calculations at the metal L-edge reveal the electronic structure and orbital-specific deexcitation pathways of gas-phase metalloporphyrins.

Journal of the American Society for Mass Spectrometry 32(3), 670 - 684 (2021). doi:10.1021/jasms.0c00390, Near-edge X-ray absorption mass spectrometry (NEXAMS) is an action-spectroscopy technique of growing interest for investigations into the spatial and electronic structure of biomolecules. It has been used successfully to give insights into different aspects of the photodissociation of peptides and to probe the conformation of proteins. It is a current question whether the fragmentation pathways are sensitive toward effects of conformational isomerism, tautomerism, and intramolecular interactions in gas-phase peptides. To address this issue, we studied the cationic fragments of cryogenically cooled gas-phase leucine enkephalin ([LeuEnk+H]$^+$) and methionine enkephalin ([MetEnk+H]$^+$) produced upon soft X-ray photon absorption at the carbon, nitrogen, and oxygen K-edges. The interpretation of the experimental ion yield spectra was supported by density-functional theory and restricted-open-shell configuration interaction with singles (DFT/ROCIS) calculations. The analysis revealed several effects that could not be rationalized based on the peptide’s amino acid sequences alone. Clear differences between the partial ion yields measured for both peptides upon C 1s → π*(C═C) excitations in the aromatic amino acid side chains give evidence for a sulfur–aromatic interaction between the methionine and phenylalanine side chain of [MetEnk+H]$^+$. Furthermore, a peak associated with N 1s → π*(C═N) transitions, linked to a tautomeric keto-to-enol conversion of peptide bonds, was only present in the photon energy resolved ion yield spectra of [MetEnk+H]$^+$., Published by ACS Publications, Washington, DC

The conformation and the electronic structure of gas phase oligonucleotides depends strongly on the protonation site. 5 amp; 8242; d FUAG can either be protonated at the A N1 or at the G N7 position. We have stored protonated 5 amp; 8242; d FUAG cations in a cryogenic ion trap held at about 20 K. To identify the protonation site and the corresponding electronic structure, we have employed soft X ray absorption spectroscopy at the nitrogen K edge. The obtained spectra were interpreted by comparison to time dependent density functional theory calculations using a short range exchange correlation functional. Despite the fact that guanine has a significantly higher proton affinity than adenine, the agreement between experiment and theory is better for the A N1 protonated system. Furthermore, an inverse site sensitivity is observed in which the yield of the nucleobase fragments that contain the absorption site appears substantially reduced, which could be explained by non statistical fragmentation processes, localized on the photoabsorbing nucleobase

Although the highest possible oxidation states of all transition elements are rare, they are not only of fundamental interest but also relevant as potentially strong oxidizing agents. In general, the highest oxidation states are found in the electron-rich late transition elements of groups 7-9 of the periodic table. Rhodium is the first element of the 4d transition metal series for which the highest known oxidation state does not equal its group number of 9, but reaches only a significantly lower value of +6 in exceptional cases. Higher oxidation states of rhodium have remained elusive so far. In a combined mass spectrometry, X-ray absorption spectroscopy, and quantum-chemical study of gas-phase

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.

Manganese–oxo species catalyze key reactions, including C–H bond activation or dioxygen formation in natural photosynthesis. To better understand relevant reaction intermediates, we characterize electronic states and geometric structures of [MnOn]+ manganese–oxo complexes that represent a wide range of manganese oxidation states. To this end, we apply soft X-ray spectroscopy in a cryogenic ion trap, combined with multiconfigurational wavefunction calculations. We identify [MnO2]+ as a rare high-spin manganese(V) oxo complex with key similarities to six-coordinated manganese(V) oxo systems that are proposed as reaction intermediates in catalytic dioxygen bond formation.

The phenomenon of single molecule magnet (SMM) behavior of mixed valent Mn 12 coordination clusters of general formula [Mn III 8Mn IV 4O 12(RCOO) 16(H 2O) 4] had been exemplified by bulk samples of the archetypal [Mn III 8Mn IV 4O 12(CH 3COO) 16(H 2O) 4] (4) molecule, and the molecular origin of the observed magnetic behavior has found support from extensive studies on the Mn 12 system within crystalline material or on molecules attached to a variety of surfaces. Here we report the magnetic signature of the isolated cationic species [Mn 12O 12(CH 3COO) 15(CH 3CN)] + (1) by gas phase X-ray Magnetic Circular Dichroism (XMCD) spectroscopy, and we find it closely resembling that of the corresponding bulk samples. Furthermore, we report broken symmetry DFT calculations of spin densities and single ion tensors of the isolated, optimized complexes [Mn 12O 12(CH 3COO) 15(CH 3CN)] + (1), [Mn 12O 12(CH 3COO) 16] (2), [Mn 12O 12(CH 3COO) 16(H 2O) 4] (3), and the complex in bulk geometry [Mn III 8Mn IV 4O 12(CH 3COO) 16(H 2O) 4] (5). The found magnetic fingerprints – experiment and theory alike – are of a remarkable robustness: The Mn IV 4 core bears almost no magnetic anisotropy while the surrounding Mn III 8 ring is highly anisotropic. These signatures are truly intrinsic properties of the Mn 12 core scaffold within all of these complexes and largely void of the environment. This likely holds irrespective of bulk packing effects.

The journal of physical chemistry / A 126(9), 1496 - 1503 (2022). doi:10.1021/acs.jpca.1c10158, We demonstrate site-specific X-ray induced fragmentation across the sulfur L-edge of protonated cystine, the dimer of the amino acid cysteine. Ion yield NEXAFS were performed in the gas phase using electrospray ionization (ESI) in combination with an ion trap. The interpretation of the sulfur L-edge NEXAFS spectrum is supported by Restricted Open-Shell Configuration Interaction (ROCIS) calculations. The fragmentation pathway of triply charged cystine ions was modeled by Molecular Dynamics (MD) simulations. We have deduced a possible pathway of fragmentation upon excitation and ionization of S 2p electrons. The disulfide bridge breaks for resonant excitation at lower photon energies but remains intact upon higher energy resonant excitation and upon ionization of S 2p. The larger fragments initially formed subsequently break into smaller fragments., Published by Soc., Washington, DC

Although the highest possible oxidation states of all transition elements are rare, they are not only of fundamental interest but also relevant as potentially strong oxidizing agents. In general, the highest oxidation states are found in the electron rich late transition elements of groups 7 9 of the periodic table. Rhodium is the first element of the 4d transition metal series for which the highest known oxidation state does not equal its group number of 9, but reaches only a significantly lower value of 6 in exceptional cases. Higher oxidation states of rhodium have remained elusive so far. In a combined mass spectrometry, X ray absorption spectroscopy, and quantum chemical study of gas phaseurn x wiley 14337851 media anie202207688 anie202207688 math 0001 n 1 4 , we identify urn x wiley 14337851 media anie202207688 anie202207688 math 0002 as the urn x wiley 14337851 media anie202207688 anie202207688 math 0003 trioxidorhodium VII cation, the first chemical species to contain rhodium in the 7 oxidation state, which is the third highest oxidation state experimentally verified among all elements in the periodic table

Experimental near edge x ray absorption fine structure NEXAFS spectra of the nitrosonium NO ion are presented and theoretically analyzed. While neutral NO has an open shell, the cation is a closed shell species, which for NEXAFS leads to the simplicity of a closed shell spectrum. Compared to neutral NO, the electrons in the cation experience a stronger Coulomb potential, which introduces a shift of the ionization potential towards higher energies, a depletion of intensity in a large interval above the amp; 960; amp; 8727; resonance, and a shift of the amp; 963; amp; 8727; resonance from the continuum to below the ionization threshold. NEXAFS features at the nitrogen and oxygen K edges of NO are compared, as well as NEXAFS features at the nitrogen edges of the isoelectronic closed shell species NO , N2, and N2H

As an example of symmetry breaking in NEXAFS spectra of protonated species we present a high resolution NEXAFS spectrum of protonated dinitrogen, the diazynium ion N

The phenomenon of single molecule magnet (SMM) behavior of mixed valent Mn

Measured angular distributions of photoelectrons from size-selected copper and sodium cluster anions are demonstrated to exhibit a universal behavior independent of the initial electron state, cluster size, or material, which can be traced back to momentum conservation upon photoemission. Quantum simulations reproduce the universality under the assumption that multielectron dynamics localizes the emission on the cluster surface and renders the cluster opaque to photoelectrons, thereby quenching interference effects that would otherwise obscure this almost classical behavior.

As an example of symmetry breaking in NEXAFS spectra of protonated species we present a high resolution NEXAFS spectrum of protonated dinitrogen, the diazynium ion N2H . By ab initio calculations we show that the spectrum consists of a superposition of two nitrogen 1s absorption spectra, each including a amp; 960; band, and a nitrogen 1s to H charge transfer band followed by a weak irregular progression of high energy excitations. Calculations also show that, as an effect of symmetry breaking by protonation, the amp; 960; transitions are separated by 0.23 eV, only slightly exceeding the difference in the corresponding dark symmetry forbidden and bright symmetry allowed core excitations of neutral N2. By DFT and calculations and vibrational analysis, the complex amp; 960; excitation band of N2H is understood as due to the superposition of the significantly different vibrational progressions of excitations from terminal and central nitrogen atoms, both leading to bent final state geometries. We also show computationally that the electronic structure of the charge transfer excitation smoothly depends on the nitrogen proton distance and that there is a clear extension of the spectra going from infinity to close nitrogen proton distance where fine structures show some, although not fully detailed, similarities. An interesting feature of partial localization of the nitrogen core orbitals, with a strong, non monotonous, variation with nitrogen proton distance could be highlighted. Specific effects could be unraveled when comparing molecular cation NEXAFS spectra, as represented by recently recorded spectra of N2 and CO , and spectra of protonated molecules as represented here by the N2H ion. Both types containing rich physical effects not represented in NEXAFS of neutral molecules because of the positive charge, whereas protonation also breaks the symmetry. The effect of the protonation on dinitrogen can be separated in charge, which extends the high energy part of the spectrum, and symmetry breaking, which is most clearly seen in the low energy amp; 960; transition

Titanium-doped silicon clusters anions, TimSi − (m = 1‒2, n = 14‒20), have been investigated by photoelectron spectroscopy and density functional theory (DFT) calculations. Low-energy structures of TimSi − clusters have been globally searched using a genetic algorithm combined with DFT calculations. The electronic density of states and vertical detachment energies have been computed at the HSE06/aug-cc-pVDZ level and compared to the experimental data. Excellent agreement is found between theory and experiment especially in case of the singly doped clusters. In general, clusters with size m + n ≤ 17 prefer cage structures, while larger sized clusters evolve on a quasi-fullerene Ti@Si14 structural motif. Natural population analysis reveals that the Ti atoms possess negative charges and thus act as electron acceptors. The calculated binding energies and HOMO–LUMO gaps show that the clusters with cage structures have significantly higher stability, particularly Ti1Si16− and Ti2Si15−. One reason is that neutral Ti2Si15 exhibits a closed-shell electronic structure as a superatom, like Ti1Si16.

For C${}_{60}$, the potential energy surface exhibits a driving force toward the ground state that allows it to be found rapidly. However, the potential energy surface of a cluster can also exhibit features that prevent us from finding its global minimum. In the case of decahedral Si${}_{20}$H${}_{20}$, which is a theoretically postulated stable ground state but was never synthesized, there is no driving force towards the global minimum. The system visits a huge number of low energy structures during its evolution without ever falling into the global minimum. This is exactly the scenario described by Levinthal in his famous paradox about protein folding.

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.

Cobalt–benzene cluster ions of the form Co3(bz)n+ (n = 0–3) were produced in the gas phase, mass-selected, and cooled in a cryogenic ion trap held at 3–4 K. To explore ligand effects on cluster magnetic moments, these species were investigated with X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) spectroscopy. XMCD spectra yield both the spin and orbital angular momenta of these clusters. Co3+ has a spin magnetic moment of μS = 6 μB and an orbital magnetic moment of μL = 3 μB. Co3(bz)+ and Co3(bz)2+ complexes were found to have spin and orbital magnetic moments identical to the values for ligand-free Co3+. However, coordination of the third benzene to form Co3(bz)3+ completely quenches the high spin state of the system. Density functional theory calculations elucidate the spin states of the Co3(bz)n+ species as a function of the number of attached benzene ligands, explaining the transition from septet to singlet for n = 0 → 3.

International audience; 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.

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.

X-ray magnetic circular dichroism spectroscopy has been used to characterize the electronic structure and magnetic moment of Cr$_2^+$. Our results indicate that the removal of a single electron from the $4s\sigma_g$ bonding orbital of Cr$_2$ 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 $^{12}\Sigma$ and the low-spin $^2\Sigma$ state., Comment: This is the peer-reviewed, accepted version of the article

Producción Científica, 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 Zn􀀀N 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., Junta de Castilla y León (Project VA124G18)

Journal of the American Society for Mass Spectrometry 29(11), 2138 - 2151 (2018). doi:10.1007/s13361-018-2035-6, 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 iontraps. 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 ionizationare 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.Keywords: Soft X-ray spectroscopy, Soft X-ray absorption, Photodissociation, Melittin, Protonated proteins, Published by Springer, New York [u.a.]

Chemistry - a European journal 24(30), 7631-7636 (2018). doi:10.1002/chem.201801440, 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., Published by Wiley-VCH, Weinheim

Size-selected anionic silicon clusters, Sin- (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 VDEs 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 Sin- 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 Sin- (n=14-20) clusters can be derived from assembling tricapped trigonal prisms (TTP) in various ways. For most sizes previous structures are confirmed, whereas for Si20- a new structure has been found., Comment: 9 pages, 4 figures, 3 tables

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.

We have performed X-ray absorption spectroscopy of size-selected, free, cationic, transition-metal-doped (Sc, Ti, V, Cr) gold clusters in a size range n = 1–9. The electronic configuration of the impurity atom was determined by modeling the X-ray absorption spectrum in a charge-transfer multiplet framework which makes it possible to quantify the amount of localization of the impurity 3d states. Depending on the dopant element and the host cluster size we find a wide variety in the behavior of local electronic structures. ScAun+ clusters show strong hybridization of the scandium 3d states with the host electronic states except for ScAu1+ where we find a completely localized 3d electron. In TiAun+ clusters a pronounced odd–even alternation is present in the local electronic structure of the impurity atom. The 3d occupation number of the titanium dopant is approximately 2 and 1.6 electrons in odd and even-numbered clusters, respectively. In CrAun+ clusters the electronic structure of the dopant is governed b...

The second to fifth ionization energies and the appearance sizes of the higher charge states of silicon clusters with 6–92 atoms are determined by sequential vacuum-ultraviolet multiphoton ionization of size-selected cluster ions in a cryogenic ion trap. The higher ionization energies are well described by the charging energy in a spherical droplet model. In contrast to the electron affinity and the first ionization energy of silicon clusters, the higher ionization energies seem to change less abruptly at the prolate-to-spherical shape transition.

Mithilfe des magnetischen zirkularen Rontgendichroismus wurden die elektronische Struktur und das magnetische Moment von Cr2+ bestimmt. Unsere Ergebnisse zeigen, dass sich die bevorzugte Kopplung der 3d-Spins durch das Entfernen eines einzelnen Elektrons aus dem bindenden 4sσg-Orbital des Cr2 dramatisch umkehrt. Im Unterschied zum neutralen Molekul, das einen Grundzustand mit verschwindendem Spin bei kurzer Bindungslange besitzt, zeigt das Molekulkation einen ferromagnetisch gekoppelten Grundzustand mit dem hochstmoglichen Spin von S=11/2 bei nahezu der doppelten Bindungslange des neutralen Molekuls. Diese besondere Spinkonfiguration ist das Resultat einer indirekten Austauschkopplung der an den Ionenrumpfen lokalisierten 3d-Elektronen durch das delokalisierte 4sσ-Elektron uber den starken intraatomaren 3d-4s-Austausch. Unsere Ergebnisse ermoglichen erstmals eine Abschatzung der relativen Energien des ferromagnetisch gekoppelten 12Σ-Grundzustands und des angeregten 2Σ-Zustands.

A new experimental setup for photoelectron spectroscopy of size-selected cluster ions using synchrotron VUV radiation as generated by the Swiss Light Source is presented. An intense positively charged cluster ion beam is produced in a high-intensity magnetron sputter source. The clusters are subsequently mass selected in a sector magnet. To maximize the residence time of the cluster ions in the ionization region of the velocity map imaging spectrometer, the cluster ion beam is decelerated where it crosses the light beam. First experiments on (MoO3) + (n = 69 and 59) cluster cations show that the approach is capable of delivering photoelectron spectra of size-selected transition metal cluster ions.

We present a photoelectron imaging study of the small sodium cluster anions Na3(-), Na5(-), and Na7(-) at photon energies in the visible and near UV range (hv = 1.64-4.28 eV). The resulting angular distributions are remarkably diverse and exhibit a strong dependence on photon energy; only for hv > 3.5 eV do they evolve into more uniform distributions peaked in the direction of the laser polarization. We show that different energy dependencies of the distributions are related to different angular-momentum characters of the bound states. The jellium s-like character of the lowest single-particle states results in photoelectron emission parallel to the laser polarization at all photon energies, whereas the p-like character of the higher states leads to essentially isotropic distributions at threshold and a strong variation with photon energy. Close to the detachment threshold, the asymptotic angular distributions are attributed to the approximate validity of Wigner's law, which states that the spectrum is dominated by the partial wave with the smallest angular momentum. For the planar cluster Na5(-), we observe characteristically different behavior for electrons detached from the two in-plane p-like states, and we show how this correlates with the molecular symmetry. Our results indicate that a simple jellium-like description of the molecular orbitals is appropriate for the three-dimensional cluster Na7(-), despite the energetic splitting of the normally triply degenerate 1p level.

Cobalt-benzene cluster ions of the form Co

The $^{4}\Phi_{9/2}$ ground state of the Ni$_2^+$ diatomic molecular cation is determined experimentally from temperature and magnetic-field-dependent x-ray magnetic circular dichroism spectroscopy in a cryogenic ion trap, where an electronic and rotational temperature of $7.4 \pm 0.2$ K was reached by buffer gas cooling of the molecular ion. The contribution of the magnetic dipole term to the x-ray magnetic circular dichroism spin sum rule amounts to $7\, T_z = 0.17 \pm 0.06$ $\mu_B$ per atom, approximately 11 % of the spin magnetic moment. We find that, in general, homonuclear diatomic molecular cations of $3d$ transition metals seem to adopt maximum spin magnetic moments in their electronic ground states., Comment: This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in V. Zamudio-Bayer et al., J. Chem. Phys. 145, 194302 (2016) and may be found at https://doi.org/10.1063/1.4967821

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.

Phone: þ00 49 431 880 5098, Fax: þ00 49 431 880 4884Simultaneous time- and energy-resolved two-photon photo-emissionwithnanometerresolutionisdemonstratedforthefirsttime.Wemonitortheenergydependenceofthedecaydynamicsofelectronexcitationsinindividualsilverparticles,whichweredeposited from a gas aggregation cluster source onto a siliconsubstrate. We show furthermore that the near-field enhance-mentduetoplasmon-resonantexcitationisanefficientmeanstoaddress individual nanometer-sized particles using photoem-ission electron microscopy.

Size-selected, ligand-free gold clusters with diameters lessthan 2nm can be routinely generated in the gas phase. Thepronounced size dependence of their physical and chemicalproperties is one of their most important features. Surface-deposited gold clusters are particularly interesting for appli-cations in nanotechnology and heterogeneous catalysis.