-
[show abstract]
[hide abstract]
ABSTRACT: The disproportionation reaction of the subvalent metastable halide SnBr proved to be a powerful synthetic method for the synthesis of metalloid cluster compounds of tin. Hence, the neutral metalloid cluster compound Sn(10)[Si(SiMe(3))(3)](6) (3) was synthesized from the reaction of SnBr with LiSi(SiMe(3))(3). In the course of the reaction anionic clusters might also be present, and we now present the first anionic cluster compound {Sn(8)E[Si(SiMe(3))(3)](3)}(-) (E = Si, Sn), where one position in the cluster core is occupied by a silicon or a tin atom, giving further insight into structural variations of E(9) cages in metalloid group 14 cluster compounds.
Inorganic Chemistry 03/2012; 51(7):3989-95. · 4.60 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The reaction of GeBr with LiSi(SiMe(3))(3) leads to the metalloid cluster compound [(THF)(2)Li](3)Ge(14)[Si(SiMe(3))(3)](5) (1). After the introduction of a first cluster of this type, in which 14 germanium atoms form an empty polyhedron, [(THF)(2)Li](3)Ge(14)[Ge(SiMe(3))(3)](5) (2), we present here further investigations on 1 to obtain preliminary insight into its chemical and bonding properties. The molecular structure of 1 is determined via X-ray crystal structure solution using synchrotron radiation. The electronic structure of the Ge(14) polyhedron is further examined by quantum chemical calculations, which indicate that three singlet biradicaloid entities formally combine to yield the singlet hexaradicaloid character of 1. Moreover, the initial reactions of 1 after elimination of the [Li(THF)(2)](+) groups by chelating ligands (e.g., TMEDA or 12-crown-4) are presented. Collision induced dissociation experiments in the gas phase, employing FT-ICR mass spectrometry, lead to the elimination of the singlet biradicaloid Ge(5)H(2)[Si(SiMe(3))(3)](2) cluster. The unique multiradicaloid bonding character of the metalloid cluster 1 might be used as a model for reactions and properties in the field of surface science and nanotechnology.
Journal of the American Chemical Society 02/2011; 133(8):2518-24. · 9.91 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The cluster anion {Ge9[Si(SiMe3)3]3}– (1) is transferred intact into the gas phase via the electro spray method. Subsequently the fragmentation of 1 after resonant excitation as well as the oxidation reaction with O2 and Cl2 are investigated in an FT-ICR mass spectrometer (Fourier Transform Ion Cyclotron Resonance). Unlike former results with off-resonant excitation the fragmentation leads mainly to the end-product Ge9–. Moreover, applying an on-resonant excitation the dissociation experiment can be quantified; 2.0 ± 0.15 eV (193 ± 15kJ·mol–1) for the elimination of the first two ligands and 2.7 ± 0.15 eV (261 ± 15 kJ·mol–1) for all ligands, respectively. Particular attention is turned on the first step, where sterically encumbered Si2(SiMe3)6 molecules are formed in a concerted reaction. This result, which is also important for elemental reactions on metal surfaces in catalyses, is based on experimentally determined threshold energies, DFT calculations and calculations on the lifetime of the involved species., In contrast to the high reactivity of crystalline 1·Li(THF)4, gaseous 1 is inert against oxygen. The analogy to recently published spin forbidden reactions of Al13– with O2 hints to a general importance of spin conversion during gas phase reactions of larger cluster molecules. The oxidation of 1 with Cl2 proceeds through different reaction channels. DFT calculations give a first insight on the complex primary oxidation steps. These calculations also reveal that the delocalized bonding situation in the Ge9 core is distorted upon oxidation. This result together with the dissociation experiments shed more light on differences and similarities between metalloid clusters and Zintl ions.
Zeitschrift für anorganische Chemie 05/2010; 636(7):1173 - 1182.
-
[show abstract]
[hide abstract]
ABSTRACT: Marco Neumaier, Ralf Köppe, Hansgeorg SchnöckelImmer noch sind die atomaren Prozesse, die bei der Oxidation von Metalloberflächen stattfinden, weitgehend unverstanden. Massenspektrometrische und quantenchemische Methoden zeigen, dass in der Reaktion der Modellverbindung Al13− mit Sauerstoff der Spinzustand des O2-Moleküls für die Reaktivität eine entscheidende Rolle spielt.
Nachrichten aus der Chemie 09/2008; 56(10):999 - 1004. · 0.19 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Electron detachment from fullerene dianions Cm2- (m=76,84) stored in a room temperature Penning trap was probed upon pulsed laser excitation at wavelengths of 355 and 532 nm. The fraction of Cm(2-) surviving trapping times exceeding tens of milliseconds under UHV conditions, as well as the fraction of singly charged anions Cm(-) generated were recorded as a function of the laser fluence. Analysis by means of Poisson statistics yields absolute absorption cross sections and the number of photons necessary to induce the detachment. The cross sections obtained are in good agreement with the literature values. By describing the electron detachment as a statistical unimolecular process, we deduce effective activation energies from the number of photons required. These energies are compared to the sum of the second electron affinity and the Coulomb barrier height as calculated from an electrostatic charging model.
The Journal of Chemical Physics 05/2008; 128(13):134306. · 3.33 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We revisit the reactivity of trapped pure gold (Au(n)+, n < 26) and silver gold alloy cluster cations (Ag(m)Au(n)+, m + n < 7) with carbon monoxide as studied in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The experimental results are discussed in terms of ab initio computations which provide a comprehensive picture of the chemical binding behaviour (like binding energy, adsorption sites, associated vibrational frequencies) of CO to the noble metal as a function of cluster size and composition. Starting from results for pure gold cluster cations for which an overall decrease of CO binding energy with increasing cluster size was experimentally observed--from about 1.09 +/- 0.1 eV (for n = 6) to below 0.65 +/- 0.1 eV (for n > 26) we demonstrate that metal--CO bond energies correlate with the total electron density and with the energy of the lowest unoccupied molecular orbital (LUMO) on the bare metal cluster cation as obtained by density functional theory (DFT) computations. This is a consequence of the predominantly sigma-donating character of the CO-M bond. Further support for this concept is found by contrasting the predictions of binding energies to the experimental results for small alloy cluster cations (Ag(m)Au(n)+, 4 < m + n < 7) as a function of composition. Here, binding energy drops with increasing silver content, while CO still binds always in a head-on fashion to a gold atom. Finally we show how the CO stretch frequency of Ag(m)Au(n)CO+ may be used to identify possible adsorption sites and pre-screen favorable isomers.
Faraday Discussions 02/2008; 138:393-406; discussion 421-34. · 5.00 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Near thermal energy reactive collisions of small mixed metal cluster cations Ag(m)Au(n) (+) (m+n=4, 5, and 6) with carbon monoxide have been studied in the room temperature Penning trap of a Fourier transform ion-cyclotron-resonance mass spectrometer as a function of cluster size and composition. The tetrameric species AgAu(3) (+) and Ag(2)Au(2) (+) are found to react dissociatively by way of Au or Ag atom loss, respectively, to form the cluster carbonyl AgAu(2)CO(+). In contrast, measurements on a selection of pentamers and hexamers show that CO is added with absolute rate constants that decrease with increasing silver content. Experimentally determined absolute rate constants for CO adsorption were analyzed using the radiative association kinetics model to obtain cluster cation-CO binding energies ranging from 0.77 to 1.09 eV. High-level ab initio density functional theory (DFT) computations identifying the lowest-energy cluster isomers and the respective CO adsorption energies are in good agreement with the experimental findings clearly showing that CO binds in a "head-on" fashion to a gold atom in the mixed clusters. DFT exploration of reaction pathways in the case of Ag(2)Au(2) (+) suggests that exoergicities are high enough to access the minimum energy products for all reactive clusters probed.
The Journal of Chemical Physics 10/2006; 125(10):104308. · 3.33 Impact Factor
-
The Journal of Chemical Physics 12/2005; 123(19):199902. · 3.33 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Electron transfer collisions of trapped doubly charged fullerene anions C76(2-), C(78)2-, and C84(2-) with SF6 are studied in a Fourier transform ion cyclotron resonance mass spectrometer at center-of-mass collisional energies ranging from thermal energy to 77 eV. Collision energy dependencies manifest threshold energies for (nominally exoergic) single electron transfer onto SF6 of 1.46+/-0.3 eV, 1.56+/-0.3 eV, and 1.63+/-0.3 eV for C(76)2-, C78(2-), and C(84)2-, respectively. Kinetics studies reveal charge-transfer cross sections of up to 430+/-200 A2 for C84(2-) at a collision energy of 77 eV. The mechanism and the energetics are discussed in terms of classical electrostatic model calculations. Additionally, we rationalize the collision energy dependencies of the charge-transfer cross sections using the two-state Landau-Zener formalism to describe the associated resonant electron tunneling probability.
The Journal of Chemical Physics 09/2005; 123(7):074318. · 3.33 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Room temperature CO adsorption on isolated gold cluster cations is studied over a wide size range (Au(n) (+),1<n<66) in a Fourier transform ion cyclotron resonance mass spectrometer. Kinetic data which yield absolute bimolecular rate constants are analyzed by means of a radiative association model to give adsorption energies for the first CO molecule binding to the gold cluster ions. The results show that the binding energies decrease with increasing cluster size from about 1.09+/-0.1 eV (n=6) to below 0.65+/-0.1 eV (for n>26), with notable exceptions at n=30, 31 and 48, 49 which manifest local binding energy maxima. For the smallest sizes (3<n<9 and n=20), the experimental CO-Au(n) (+) binding energies are compared with those found for the most stable isomers by density functional theory based ab initio computations. While the overall size trend can be understood in terms of classical electrostatics, the atom by atom variations are likely due to differently coordinated binding sites available on the gold clusters.
The Journal of Chemical Physics 03/2005; 122(10):104702. · 3.33 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Intense fullerene anion beams were generated by electrospray of dichlorobenzene solutions of fullerenes and tetrakis(dimethylamino)ethylene (TDAE). Ions were probed with Fourier transform mass spectrometry (FTMS) about one second after the electrospray step. In addition to monoanions, large quantities of fullerene dianions were detected. The efficiency of dianion formation and detection was found to increase dramatically with cage size. While C602− could not be observed, dianions of trace fullerenes up to C1242− were readily accessed. Among all fullerene dianions probed only C702− was found to be metastable under collision-free conditions on the measurement timescale with a half-life of about 80 s towards electron autodetachment. Observation and stability of fullerene dianions are discussed in terms of rough estimates of their second electron affinities.
Chemical Physics Letters.
-
[show abstract]
[hide abstract]
ABSTRACT: Singly and doubly charged fullerene anions were introduced into gas phase by means of electrospray starting from a solution containing a very efficient electron donor compound. Following isolation and trapping, mass selected anions were irradiated with a ns-pulse from a Nd:YAG laser at the third harmonic (hν = 3.5 eV) in order to study (delayed) photoemission. We present results on the laser-fluence dependence of photoemission for singly charged C76− and C84− as well as for doubly charged C762−. Results are discussed in terms of statistical electron emission as opposed to direct detachment. They are further analyzed to give information on the number of photons necessary to induce delayed electron emission on the experimental time scale. The averaged absolute absorption cross-sections obtained were about 0.6–0.8 Å2 for all species probed. The agreement between experimental data and statistical modelling shows that electronic excitations which rapidly relax to vibrationally hot ions provide the dominant contributions to the 355 nm absorption cross-sections. As a corollary, we show that direct single-electron detachment is a minor process for the singly charged fullerene anions having a cross-section <0.02 Å2. For C762−, electron emission follows an absorption law in which single-photon excitation is the leading contribution. From the data we deduce a Coulomb barrier height for C762− of 1.20 eV.
International Journal of Mass Spectrometry.
