Manfred P. Irion

Size effects in metal cluster-ion chemistry

Abstract In a special Fourier-transform ion cyclotron resonance mass spectrometer, the chemical reactions of different metal cluster ions with a variety of reactive gases have been observed at room temperature as a function of cluster size. The cluster ions generated by sputtering with Xe+ primary ions are transported via ion lenses to the analyzer cell, where they can be stored for up to 100s. For noble metals, mere adsorption of 1–4 molecules is the prevalent reaction type. In the case of the more reactive transition metals, addition to the cluster is typically accompanied by dehydrogenation. Several drastic size-specific effects are discussed, which depend not only on the metal alone but also on the complete system of metal cluster ion and reactive gas. For some systems, only a few sizes are reactive with the majority being inert. For others, the reverse is true and the largest number of sizes is maximized. In addition, there are systems where reactivity either oscillates with increasing size or varies smoothly. For some ion/molecule reactions, the absolute rate constants have been measured. Reactions that do not proceed spontaneously (oxidation of copper clusters, methane activation) are induced by resonantly exciting the ions to a higher kinetic energy. The Fe+4 ion is distinguished by a strong reactivity towards NH3 as well as towards C2H4, etc., whereas Ni+4 proves totally inert. This suggests that the tetramer cannot have the same structure in both cases. For the first time, the catalytic activity of a naked gas-phase metal cluster could be proven. Fe+4 was shown by a complex (MS)5 experiment to synthesize benzene from three adsorbed ethene molecules.

Secondary ion fourier transform mass spectrometry: a new approach towards the study of metal cluster ion chemistry

Abstract By combining an external Xe + ion secondary ion mass spectrometry (SIMS) source with a Fourier transform mass spectrometer, a new and versatile instrument has been created that allows the study of sputtered metal cluster ions in great detail. They are transported through the magnetic stray fields via an electrostatic lens arrangement and injected into the ion cyclotron resonance (ICR) cell to be trapped for time spans of the order of seconds. They are thermalized by interaction with a collision gas added through a piezoelectric pulsed valve close to the ICR cell. Typical examples include the mass spectra of In + n , Cu + n and Ni + n which extend up to n = 37, 31 and 25 respectively and contain the expected isotope patterns. The first two spectra are characterized by intensity anomalies confirming that the ions detected are exclusively the most stable net results of all earlier decay processes. Another pulsed valve serves to start chemical reactions with O 2 , C 2 H 4 and N 2 . In the case of Cu + n ions, collision-induced dissociation (CID) with O 2 as the target gas has been investigated, and a method for selective activation of a single isotope out of the complete pattern is presented. Preliminary results on the chemistry of Ni + n ions with C 2 H 4 indicate a predominance of dehydrogenation.

Fourier transform ion cyclotron resonance studies of sputtered metal cluster ions: The chemistry of Cun+ with O2

Abstract In a new apparatus, combining an external sputtering ion source with an ion storage cell, we have studied the chemical reactions of mass-selected copper cluster cations with oxygen. Trapped and thermalized, they show no tendency to react, but when additionally excited they undergo low-energy collision-induced dissociation. Different parent ions fragment in a similar pattern, with Cu 5 O 2 + and Cu 3 O + as common reaction products that display extraordinary stability, in analogy to the well known hydrated protons H 2 n +1 O n + . The oxygen does not simply adsorb to the original Cu n + cluster, but causes its complete restructuring

Reactions of Fe+n clusters (n = 2–11) with C6H6 and C6D6. Ligand isomerization in the benzene precursor ion Fe4(C2H2)+3

Abstract Even though Fe+4 clusters in the gas phase have been demonstrated to be able to induce the cylotrimerization of added C2H2 ligands to benzene, the precursor ion Fe4(C2H2)+3 has not yet been structurally characterized. To improve our understanding of this type of ion, we compare the products obtained by the direct reaction of benzene with bare Fe+n clusters (n = 2–11) to those obtained by dehydrogenative addition of ethane. Additional mass spectrometric studies such as collision-induced dissociation (CID) and thermoneutral ligand exchange with C6D6 suggest that the reaction of C2H4 with Fe+4 produces at least two types of structural isomers, with a performed benzene molecule existing only in one of them (Fe4C6H+6).

Absolute rate constants for some reactions of ethylene and cyclopropane with iron-cluster ions

Abstract The absolute rate constants for the consecutive binding of ethylene and cyclopropane to Fe + 4 ions under hydrogen abstraction are reported. It is evident from these data that the formation of cluster ions with Fe 4 C 6 H + 6 stoichiometry takes place at an unexpected high reaction rate. In addition, we report on chemical reactions of Fe 4 (C 2 H 2 ) + m ions ( m =0–3) with ammonia. These experiments show that cluster ions with m =0–2 bind NH, releasing simultaneously H 2 , while cluster ions with m =3 adsorb intact ammonia molecules.

FTMS studies of sputtered metal cluster ions (IV): size-selective effects in the chemistry of Fe n + with NH3 and Pd n + with D2 or C2H4

Fen+ and Pdn+ clusters up ton=19 andn=25, respectively, are produced in an external ion source by sputtering of the respective metal foils with Xe+ primary ions at 20 keV. They are transferred to the ICR cell of a home-built Fourier transform mass spectrometer, where they are thermalized to nearly room temperature and stored for several tens of seconds. During this time, their reactions with a gas leaked in at low level are studied. Thus in the presence of ammonia, most Fen+ clusters react by simply adsorbing intact NH3 molecules. Only Fe4+ ions show dehydrogenation/adsorption to Fe4(NH)m+ intermediates (m=1, 2) that in a complex scheme go on adsorbing complete NH3 units. To clarify the reaction scheme, one has to isolate each species in the ion cell, which often requires the ejection of ions very close in mass. This led to the development of a special isolation technique that avoids the use of isotopically pure metal samples. Pdn+ cluster ions (n=2...9) dehydrogenate C2H4 in general to yield Pdn(C2H2)+, yet Pd6+ appear totally unreactive. Towards D2, Pd7+ ions seem inert, whereas Pd8+ adsorb up to two molecules.

23Na and 35Cl nuclear quadrupole interaction in Na2ZnCl4·3H2O. A single-crystal NMR study

Abstract The 35Cl and 23Na nuclear quadrupole interaction in single crystals of Na2ZnCl4·3H2O has been investigated at room temperature. With the high-field method (H0 ≈ 2·5 T) from the angular dependence of ν( 35 Cl ) (m = + 1 2 ⇌ m = − 1 2 ) and ν( 23 Na ) (m = ± 1 2 ⇌ ± 3 2 ) the coupling constant and η found are (number of positions; point symmetry; e2qQh−1/MHz; η; T (K)): 35ClI (1; 3m; 15.3676 ± 0.0010; 0.000 ± 0.002; 294); 35ClII (3; m; 18.308 ± 0.002; 0.0005 ± 0.0020; 294); 23Na (2; 3; 0.2617 ± 0.0003; 0.0000 ± 0.0005; 295.5). The direction cosines of the electric field gradient tensors were determined. The results are discussed with respect to possible hudrogenbonds Cl … HOH.