Christian Berg

Reactions of simple hydrocarbons with Nb n : Chemisorption and physisorption on ionized niobium clusters

In a Fourier transform‐ion cyclotron resonance mass spectrometer the gas phase reactivities of niobium clusters Nb+n (n=1–28) with molecular hydrogen, water, methane, ethane, n‐propane, n‐heptane, cyclohexane, acetylene, ethylene, allene, benzene, propene, toluene, xylene, and acetonitrile were investigated under single collision conditions as well as the reactivities of oxidized niobium cluster cations with ethylene and benzene. The reactions of larger clusters with a variety of unsaturated hydrocarbons are believed to proceed via long lived ‘‘physisorbed’’ addition intermediate complexes, which subsequently rearrange to form ‘‘chemisorbed,’’ extensively dehydrogenated final products. The overall reaction seems to proceed with near collision rates, almost independent of cluster size. In some cases also the physisorbed primary products are stabilized and detected. Their yields depend sensitively on the specific nature of the reactant, and on the niobium cluster size n. Fully saturated hydrocarbons unable ...

Protonated water clusters and their black body radiation induced fragmentation

Abstract Protonated water clusters H+(H2O)n, n = 5, …, 65 and their perdeuterated analogues were produced in a discharge sour and stored in an electromagnetic ICR-ion trap under collision-free conditions. The rates of their fragmentation or ‘evaporation’ due to absorption of the 300 K black body background radiation exhibit an overall τ ∼ 1 n dependence. Local deviations of some clusters (e.g. n = 21 or 55) from the overall trend are attributed to their higher stabilities. The fragmentation is modeled by extrapolating macroscopic water droplet evaporation to the microscopic clusters.

Methane activation by platinum cluster ions in the gas phase: effects of cluster charge on the Pt4 tetramer

Abstract The reactions of cationic and anionic platinum clusters Pt ± n , n =1−9, with methane CH 4 are investigated under single collision conditions in a Fourier-Transform Ion Cyclotron Resonance Mass Spectrometer. The reaction of the platinum clusters proceeds through the activation of C–H bonds of methane and leads to the subsequent elimination of molecular hydrogen H 2 to form the final metal–carbene complex Pt ± n CH 2 . The cation cluster reactions proceed in general with collision rate whereas the anion cluster reactions are more than an order of magnitude slower. The platinum tetramer anion is unique among all the clusters studied, reacting more efficiently than the corresponding cation. Tentative interpretation in terms of electronic and geometric effects is performed.

STABILITY AND REACTIVITY OF HYDRATED MAGNESIUM CATIONS

Abstract Unimolecular fragmentation and bimolecular reactions with HCl of water clusters which nominally contain Mg + cations were studied in an FT-ICR spectrometer. A cluster fragmentation and successive evaporation of single water molecules occurring on a millisecond timescale and driven by ambient black body radiation is triggering interesting intracluster reactions. Below a certain critical size (∼17 water molecules) MgOH + forms, and a hydrogen atom is ejected. Similarly bimolecular reactions of Mg aq + clusters with HCl result in a release of H atom and formation of MgCl aq + . Both findings can be rationalized by assuming that the solvated Mg + cations actually detach an additional electron forming a Mg aq 2+ and e aq − within clusters with more than 17 water molecules. Mg + formed by recombination when not enough solvent is available to stabilize the separate charged species then reacts with a water molecule resulting in H-atom formation. Detailed studies of the ion reactions and fragmentation provide additional insights into the structure and stability of solvated magnesium cations.

Methane activation by rhodium cluster argon complexes

Abstract Rhodium cluster argon complexes Rh n + Ar m are produced by laser vaporization followed by supersonic expansion, stored in an FT-ICR mass spectrometer, and their reactions with methane investigated. Ligand exchange reactions are observed, in which up to three argon atoms are replaced by methane. In addition, the solvated rhodium dimer and trimer cations are found to dehydrogenate methane. The efficiency of the dehydrogenation depends on the number of argons, with only the dimer exhibiting this reaction without ligands. This dependence of methane activation on the size of the cluster and number of “solvent” argon atoms is discussed, and compared with heterogenous catalysis on bulk surfaces, where activity and selectivity are controlled by pressure and temperature.

Effect of charge upon metal cluster chemistry: Reactions of Nbn and Rhn anions and cations with benzene

The reactions of anionic niobium and rhodium clusters Mn−, M=Nb, Rh, n=3–28, with C6H6 are investigated under single collision conditions in a Fourier-transform ion-cyclotron-resonance mass spectrometer and compared with the results of previous studies on corresponding cationic species. This reveals strong effects of the cluster charge state on hydrocarbon activation as a function of cluster size. Both differences and parallels are observed for reactions of anions and cations. Niobium clusters with a given number of atoms react quite differently than those with a single atom more or less. The fact that almost identical such effects are in the present work found for anion clusters, as for cations with the same number of atoms but two less electrons, suggests that the observed reactivity patterns are more a function of the cluster shape and geometry, than of the details of their electronic structure. The variety of interesting trends and effects observed is interpreted in terms of simple physical models.

Chemistry and charge transfer phenomena in water cluster cations

Abstract The present paper discusses three case studies of water cluster cation stability and reactivity by FT-ICR technique: (1) HCl reactions with protonated water clusters that reveal mechanisms of ionic solvation and evaporative recombination, (2) the black body radiation induced fragmentation of hydrated magnesium, Mg + (H 2 O) n , that leads to mono-hydroxide formation and evaporation of a hydrogen atom once that a critical size of n = 17 is reached, and (3) the fragmentation of hydrated aluminum cations, Al + (H 2 O) n , which leads to the formation of aluminum di-hydroxide and evaporation of a hydrogen molecule at a critical size of about 22 water molecules. Solvation shell effects and possible clathrate-like structures are discussed together with the liquid- versus solid-like character of the clusters. Future investigations are proposed.

Reactions of homonuclear and heteronuclear group Vb clusters with ethylene: evidence for structural isomers

Abstract Reactions of Nb n + and Nb n −1 V + ( n =2–15) with C 2 H 4 were investigated in the gas phase under single collision conditions using a Fourier-transform ion cyclotron resonance mass spectrometer. Biexponential decays deviating from simple pseudo-first-order kinetics were observed for Nb 10 + , Nb 9 V + and Nb 12 + . They suggest the presence of at least two structural isomers of these clusters with one of these being unreactive. The abundance ratios of the reactive to the unreactive isomers for these three clusters can be determined from the kinetic fits to be 1/4, 2/1 and 1/8, respectively.

Gas-phase reactions of rhenium-oxo species ReOn+, n= 0, 2–6, 8, with O2, N2O, CO, H2O, H2, CH4 and C2H4

The reactions of ReOn+, n = 0, 2–6, 8, with the small molecules O2, N2O, CO, H2O, H2 , CH4 and C2H4, are studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry under single collision conditions on the timescale of seconds. The ReOn+ species are produced by laser vaporization of solid rhenium and pulsed supersonic expansion in a helium–oxygen mixture into high vacuum. A wide variety of reactions are observed, including methane activation and epoxidation reactions. Re+ reacts with ethylene by sequential dehydrogenation. ReO2+ and ReO4+ exhibit the most diverse reaction pathways, while ReO5+ almost exclusively undergoes ligand exchange. ReO6+ and ReO8+ are largely unreactive, the only efficient reactions observed are with ethylene and water. Both molecules seem to be able to directly attack the dioxygen ligands. The observed chemistry is governed by a fine interplay between available coordination sites and thermochemistry.

Solvation of hydrochloric acid in protonated water clusters

Abstract The reactions of protonated water clusters H + (H 2 O) n n ≈ 1–50, have been investigated in an FT-ICR instrument under single collision conditions. Collisions with ambient temperature argon result in fragmentation and eventually formation of the n = 4, hydrated hydroxonium cation. With hydrochloric acid the smaller clusters fragment, larger ones also exchange ligand. Clusters having n ⩾ 11 can exchange one, and n ⩾ 17 two, water molecules for HCl. Implications for stratospheric chemistry are discussed.