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Berlin Brandenburg

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  • 1
    Language: English
    In: Journal of the American Chemical Society, 26 October 2011, Vol.133(42), pp.16930-7
    Description: The oxygen-rich cluster Al(2)O(7)(+) is generated in the gas phase and investigated with respect to both its structure and its reactivity toward small, inert molecules using Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry and DFT-based calculations. Al(2)O(7)(+) reacts with CH(4) under ambient conditions via hydrogen atom transfer (HAT), and with H(2)O a ligand exchange occurs which gives rise to the evaporation of two O(2) molecules. The resulting product ion Al(2)O(4)H(2)(+) is also capable of abstracting a hydrogen atom from both H(2)O and CH(4). As indicated in the H(2)O/2O(2) ligand exchange and supported by collision-induced dissociation (CID) experiments, two O(2) units constitute structural elements of Al(2)O(7)(+). Further insight is provided by DFT calculations, performed at the unrestricted B3LYP/TZVP level, and reaction mechanisms are suggested on the basis of both the experimental and theoretical results.
    Keywords: Aluminum Compounds -- Electric Properties ; Aluminum Compounds -- Structure ; Charge Transfer -- Analysis ; Density Functional Theory -- Usage ; Dissociation Reactions -- Analysis ; Mass Spectrometry -- Usage ; Methane -- Electric Properties ; Methane -- Structure ; Water Chemistry -- Research;
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 2
    Language: English
    In: Proceedings of the National Academy of Sciences of the United States, Nov 13, 2018, Vol.115(46), p.11680(8)
    Description: In a full catalytic cycle, bare [[Ta.sub.2].sup.+] in the highly diluted gas phase is able to mediate the formation of ammonia in a Haber--Bosch-like process starting from [N.sub.2] and [H.sub.2] at ambient temperature. This finding is the result of extensive quantum chemical calculations supported by experiments using Fourier transform ion cyclotron resonance MS. The planar [[Ta.sub.2][N.sub.2].sup.+], consisting of a four-membered ring of alternating Ta and N atoms, proved to be a key intermediate. It is formed in a highly exothermic process either by the reaction of [[Ta.sub.2].sup.+] with [N.sub.2] from the educt side or with two molecules of N[H.sub.3] from the product side. In the thermal reaction of [[Ta.sub.2].sup.+] with [N.sub.2], the N[equivalent to]N triple bond of dinitrogen is entirely broken. A detailed analysis of the frontier orbitals involved in the rate-determining step shows that this unexpected reaction is accomplished by the interplay of vacant and doubly occupied d-orbitals, which serve as both electron acceptors and electron donors during the cleavage of the triple bond of N[equivalent to]N by the ditantalum center. The ability of [[Ta.sub.2].sup.+] to serve as a multipurpose tool is further shown by splitting the single bond of [H.sub.2] in a less exothermic reaction as well. The insight into the microscopic mechanisms obtained may provide guidance for the rational design of polymetallic catalysts to bring about ammonia formation by the activation of molecular nitrogen and hydrogen at ambient conditions. gas-phase catalysis | ammonia synthesis | dinitrogen activation | hydrogen activation | quantum chemical calculation
    Keywords: Ammonia – Chemical Properties ; Ammonia – Thermal Properties ; Chemical Synthesis – Methods ; Tantalum – Chemical Properties ; Tantalum – Thermal Properties ; Nitrogen (Chemical Element) – Chemical Properties ; Nitrogen (Chemical Element) – Thermal Properties ; Hydrogen – Chemical Properties ; Hydrogen – Thermal Properties
    ISSN: 0027-8424
    E-ISSN: 10916490
    Source: Cengage Learning, Inc.
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  • 3
    Language: English
    In: Proceedings of the National Academy of Sciences of the United States of America, 13 November 2018, Vol.115(46), pp.11680-11687
    Description: In a full catalytic cycle, bare Ta in the highly diluted gas phase is able to mediate the formation of ammonia in a Haber-Bosch-like process starting from N and H at ambient temperature. This finding is the result of extensive quantum chemical calculations supported by experiments using Fourier transform ion cyclotron resonance MS. The planar TaN , consisting of a four-membered ring of alternating Ta and N atoms, proved to be a key intermediate. It is formed in a highly exothermic process either by the reaction of Ta with N from the educt side or with two molecules of NH from the product side. In the thermal reaction of Ta with N, the N≡N triple bond of dinitrogen is entirely broken. A detailed analysis of the frontier orbitals involved in the rate-determining step shows that this unexpected reaction is accomplished by the interplay of vacant and doubly occupied d-orbitals, which serve as both electron acceptors and electron donors during the cleavage of the triple bond of N≡N by the ditantalum center. The ability of Ta to serve as a multipurpose tool is further shown by splitting the single bond of H in a less exothermic reaction as well. The insight into the microscopic mechanisms obtained may provide guidance for the rational design of polymetallic catalysts to bring about ammonia formation by the activation of molecular nitrogen and hydrogen at ambient conditions.
    Keywords: Ammonia Synthesis ; Dinitrogen Activation ; Gas-Phase Catalysis ; Hydrogen Activation ; Quantum Chemical Calculation
    E-ISSN: 1091-6490
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  • 4
    Language: English
    In: Journal of the American Chemical Society, 25 July 2018, Vol.140(29), pp.9275-9281
    Description: Mechanistic insight into the thermal O-H bond activation of water by the cubane-like, prototypical heteronuclear oxide cluster [AlMgO] has been derived from a combined experimental/computational study. Experiments in the highly diluted gas phase using Fourier transform ion-cyclotron resonance mass spectrometry show that hydrogen-atom abstraction from water by the cluster cation [AlMgO] occurs at ambient conditions accompanied by the liberation of an OH radical. Because of a complete randomization of all oxygen atoms prior to fragmentation, about 83% of the oxygen atoms of the hydroxyl radical released originate from the oxide cluster itself. The experimental findings are supported by detailed high-level quantum chemical calculations. The theoretical analysis reveals that the transfer of a formal hydrogen atom from water to the metal-oxide cation can proceed mechanistically via proton- or hydrogen-atom transfer exploiting different active sites of the cluster oxide. In addition to the unprecedented oxygen-atom scrambling, one of the more general and quite unexpected findings concerns the role of spin density at the hydrogen-acceptor oxide atom. While this feature is crucial for [M-O]/CH couples, it is much less important in the O-H bond activation of water.
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 5
    Language: English
    In: Journal of the American Chemical Society, 06 December 2018
    Description: Mechanistic aspects of the C-H bond activation of methane by metal-carbide cations MC of the 3d transition-metals Sc - Zn were elucidated by NEVPT2//CASSCF quantum-chemical calculations, and verified experimentally for M = Ti, V, Fe and Cu by using Fourier transform ion-cyclotron resonance mass spectrometry. While MC species with M = Sc, Ti, V, Cr, Cu, and Zn activate CH at ambient temperature, this is prevented with carbide cations of M = Mn, Fe, and Co by high apparent barriers; NiC has a small apparent barrier. Hydrogen-atom transfers from methane to metal-carbide cations were found to proceed via a proton-coupled electron transfer mechanism for M = Sc - Co; wherein the doubly-occupied π-orbitals between metal and carbon at the carbon site serve as electron donors and the corresponding metal-centered vacant π*-orbitals as electron acceptors. Classical hydrogen-atom transfer transpires only in case of NiC, while ZnC follows a mechanistic scenario, in which a formally hydridic hydrogen is transferred. CuC reacts by a synchronous activation of two C-H bonds. While spin density is often so crucial for the reactions of numerous MO/CH couples, it is much less important for the C-H bond activation by carbide cations of the 3d transition-metals, in which one notes large changes in bond dissociation energies, spin states, number of d-electrons, and charge distributions. All these factors jointly affect both the reactivity of the metal carbides and their mechanisms of C-H bond activation.
    Keywords: Methane ; Carbon ; Metal-Carbide ; Cations ; Thermal-Activation ; Electron-Transition ; Electronic-Structure ; Proton ; Fourier-Transforms ; Cyclotrons ; Electron-Acceptor ; Reactivity ; Ambient-Temperature ; Electron-Donor ; Terrain ; Dissociation ; Charge-Distribution ; Methan ; Kohlenstoff ; Metallisches Carbid ; Kation ; Thermische Aktivierung ; Elektronentransfer ; Elektronische Struktur ; Proton ; Fourier-Transformation ; Zyklotron ; Elektronenakzeptor ; Chemisches Reaktionsvermögen ; Umgebungstemperatur ; Elektronendonator ; Gelände ; Dissoziation ; Ladungsverteilung ; Chemistry;
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 6
    Language: English
    In: Journal of the American Chemical Society, 01 February 2017, Vol.139(4), pp.1684-1689
    Description: A mechanistically unique, simultaneous activation of two C-H bonds of methane has been identified during the course of its reaction with the cationic copper carbide, [Cu-C]. Detailed high-level quantum chemical calculations support the experimental findings obtained in the highly diluted gas phase using FT-ICR mass spectrometry. The behavior of [Cu-C]/CH contrasts that of [Au-C]/CH, for which a stepwise bond-activation scenario prevails. An explanation for the distinct mechanistic differences of the two coinage metal complexes is given. It is demonstrated that the coupling of [Cu-C] with methane to form ethylene and Cu is modeled very well by the reaction of a carbon atom with methane mediated by an oriented external electric field of a positive point charge.
    Keywords: Methan ; Kupfercarbid ; Kohlenstoff ; Metallkomplex ; Elektrisches Feld ; Gasphase ; Massenspektrometrie ; Chemistry;
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 7
    Language: English
    In: Journal of the American Chemical Society, 29 June 2016, Vol.138(25), pp.7973-81
    Description: The reactivity of the homo- and heteronuclear oxide clusters [XYO2](+) (X, Y = Al, Si, Mg) toward methane was studied using Fourier transform ion cyclotron resonance mass spectrometry, in conjunction with high-level quantum mechanical calculations. The most reactive cluster by both experiment and theory is [Al2O2](•+). In its favorable pathway, this cluster abstracts a hydrogen atom by means of proton-coupled electron transfer (PCET) instead of following the conventional hydrogen-atom transfer (HAT) route. This mechanistic choice originates in the strong Lewis acidity of the aluminum site of [Al2O2](•+), which cleaves the C-H bond heterolytically to form an Al-CH3 entity, while the proton is transferred to the bridging oxygen atom of the cluster ion. In addition, a comparison of the reactivity of heteronuclear and homonuclear oxide clusters [XYO2](+) (X, Y = Al, Si, Mg) reveals a striking doping effect by aluminum. Thus, the vacant s-p hybrid orbital on Al acts as an acceptor of the electron pair from methyl anion (CH3(-)) and is therefore eminently important for bringing about thermal methane activation by PCET. For the Al-doped cluster ions, the spin density at an oxygen atom, which is crucial for the HAT mechanism, acts here as a spectator during the course of the PCET mediated C-H bond cleavage. A diagnostic plot of the deformation energy vis-à-vis the barrier shows the different HAT/PCET reactivity map for the entire series. This is a strong connection to the recently discussed mechanism of oxidative coupling of methane on magnesium oxide surfaces proceeding through Grignard-type intermediates.
    Keywords: Temperature Effects – Analysis ; Mass Spectrometry – Usage ; Magnesium Oxide – Chemical Properties ; Electron Transport – Analysis;
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 8
    Language: English
    In: Proceedings of the National Academy of Sciences of the United States of America, 18 December 2007, Vol.104(51), pp.20344-9
    Description: The Fra3B locus on chromosome 3p14.2 targeting the fragile histidine triad (Fhit) gene represents one of the most common fragile sites of the human genome and is associated with early preneoplastic and malignant disorders in multiple human tumors. Fhit was classified as a tumor suppressor; however, the molecular mechanisms of its function are not well established. Here, we report that Fhit associates with the lymphoid enhancer-binding factor 1/T cell factor/beta-catenin complex by directly binding to beta-catenin, a major player in the canonical Wnt pathway that is deregulated in numerous forms of human cancer. In binding to the beta-catenin C-terminal domain, Fhit represses transcription of target genes such as cyclin D1, axin2, MMP-14, and survivin. Knockdown of Fhit reversed this effect, whereas this reversal was not detectable when beta-catenin was knocked down simultaneously. The Fhit enzymatic activity as a diadenosine-polyphosphate hydrolase is not required for the down-regulation of beta-catenin-mediated transcription as examined with an enzymatic inactive Fhit-H96N protein. ChIPs revealed recruitment of Fhit/beta-catenin complexes to target gene promoters. In soft agar assays Fhit and beta-catenin are involved in regulation of anchorage-independent growth. These observations assign to the tumor suppressor Fhit an unexpected role in the regulation of beta-catenin-mediated gene transcription.
    Keywords: Gene Expression Regulation, Neoplastic ; Acid Anhydride Hydrolases -- Metabolism ; Neoplasm Proteins -- Metabolism ; Repressor Proteins -- Metabolism ; Tumor Suppressor Proteins -- Metabolism ; Beta Catenin -- Antagonists & Inhibitors
    ISSN: 00278424
    E-ISSN: 1091-6490
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  • 9
    Language: English
    In: Plant and Soil, 2013, Vol.371(1), pp.257-266
    Description: Aims: Nitrification inhibitors (NI) formulated on granulated ammonium sulphate nitrate (ASN) are an option to minimize nitrate leaching into ground waters and emissions of the greenhouse gas N sub(2)O. This paper focuses (a) on the development of an analytic enabling to extract and quantify the NI 3,4-dimethylpyrazolephosphate (DMPP), marketed since 1999. The efficiency of DMPP has been studied in laboratory and field soils. Here the DMPP analytic and the behaviour of a nitrifying bacterial consortium enriched from a field soil and exposed to zero, field applied and a 10 fold higher DMPP concentration than the recommended one for field application are in the focus. Methods: For extracting DMPP quantitatively from soils a method connected to a HPLC analytic has been developed by us and was standardized in laboratory experiment with a silt clay field soil (allochtone Vega). The method is detailed described here. Its reliability has been tested in a 3 years field trial under varying cropping systems and climatic conditions asides the influence of DMPP on CO sub(2)-, CH sub(4)- and N sub(2)O- emissions, measured by the closed chamber method. Parallel a nitrifying bacterial consortium of the silty clay field soil was enriched and subjected to 0, the recommended DMPP concentration for field applications and a 10 times higher one. In incubation experiments the conversion of ammonium to nitrite and nitrate in presence and absence of DMPP was spectrophotometer determined and pH-shifts with a scaled litmus paper. In sacrificed flasks at the end of incubation morphological changes of the bacteria involved were studied by transmission electron microscope (TEM). Results: The ammonium, nitrite and nitrate determinations and the TEM pictures show that in presence of the field applied DMPP concentration the nitrifying activity returned around 30 days later than in the control and the cells were slightly enlarged. In presence of a 10 times higher DMPP concentration a recovery was prevented. DMPP prolongs, compared with dicyandiamide (DCD), the period of nitrifiers' inhibition and reduced N sub(2)O- and CO sub(2)- the emissions (Weiske et al., Biol Fertil Soils 34:109-117, 2001a, Nutr Cycl Agroecosys 60:57-64, b). Conclusions: With the method developed by us the stability of DMPP in agricultural soils can be satisfyingly and reproducible studied down to a detection limit of 0.01 mu g DMPP g super(-1) dry soil. The morphological changes in the nitrifying consortium due to DMPP concentrations are in agreement with the recovery rate found by nitrite and nitrate formation.
    Keywords: Nitrification inhibitor ; 3,4-dimethylpyrazolephosphat (DMPP) ; DMPP quantification ; Dicyandiamide (DCD) ; Transmission electron microscopy ; Microbial soil processes
    ISSN: 0032-079X
    E-ISSN: 1573-5036
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  • 10
    Language: English
    In: Analytical chemistry, 06 October 2015, Vol.87(19), pp.9769-76
    Description: The number of separations and analyses of molecular species using traveling wave ion-mobility spectrometry-mass spectrometry (TWIMS-MS) is increasing, including those extending the technique to analytes containing metal atoms. A critical aspect of such applications of TWIMS-MS is the validity of the collisional cross sections (CCSs) measured and whether they can be accurately calibrated against other ion-mobility spectrometry (IMS) techniques. Many metal containing species have potential reactivity toward molecular nitrogen, which is present in high concentration in the typical Synapt-G2 TWIMS cell. Here, we analyze the effect of nitrogen on the drift time of a series of cationic 1,10-phenanthroline complexes of the late transition metals, [(phen)M](+), (M = Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, and Hg) in order to understand potential deviations from expected drift time behaviors. These metal complexes were chosen for their metal open-coordination site and lack of rotameric species. The target species were generated via electrospray ionization (ESI), analyzed using TWIMS in N2 drift gas, and the observed drift time trends compared. Theoretically derived CCSs for all species (via both the projection approximation and trajectory method) were also compared. The results show that, indeed, for metal containing species in this size regime, reaction with molecular nitrogen has a dramatic effect on measured drift times and must not be ignored when comparing and interpreting TWIMS arrival time distributions. Density-functional theory (DFT) calculations are employed to analyze the periodic differences due to the metal's interaction with nitrogen (and background water) in detail.
    Keywords: Drift ; Traveling Waves ; Cadmium ; Spectroscopy ; Palladium ; Cross Sections ; Spectrometry ; Mathematical Analysis ; Analysis (MD) ; Chemical Analysis (Ep) ; Chemical Analysis (Ed) ; Chemical Analysis (EC);
    ISSN: 00032700
    E-ISSN: 1520-6882
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