The reaction rates of O2 with closed-shell and open-shell Al(x)⁻ and Ga(x)⁻ clusters under single-collision conditions: experimental and theoretical investigations toward a generally valid model for the hindered reactions of O2 with metal atom clusters.

In order to characterize the oxidation of metallic surfaces, the reactions of O2 with a number of Al(x)(-) and, for the first time, Ga(x)(-) clusters as molecular models have been investigated, and the results are presented here for x = 9-14. The rate coefficients were determined with FT-ICR mass spectrometry under single-collision conditions at O2 pressures of ~10(-8) mbar. In this way, the qualitatively known differences in the reactivities of the even- and odd-numbered clusters toward O2 could be quantified experimentally. To obtain information about the elementary steps, we additionally performed density functional theory calculations. The results show that for both even- and odd-numbered clusters the formation of the most stable dioxide species, [M(x)O2](-), proceeds via the less stable peroxo species, [M(x)(+)···O2(2-)](-), which contains M-O-O-M moieties. We conclude that the formation of these peroxo intermediates may be a reason for the decreased reactivity of the metal clusters toward O2. This could be one of the main reasons why O2 reactions with metal surfaces proceed more slowly than Cl2 reactions with such surfaces, even though O2 reactions with both Al metal and Al clusters are more exothermic than are reactions of Cl2 with them. Furthermore, our results indicate that the spin-forbidden reactions of (3)O2 with closed-shell clusters and the spin-allowed reactions with open-shell clusters to give singlet [M(x)(+)···O2(2-)](-) are the root cause for the observed even/odd differences in reactivity.

On the kinetics of the Al(13) (-)+Cl(2) reaction: Cluster degradation in consecutive steps.

The kinetics of the reaction system initiated by the Al(13) (-)+Cl(2) reaction was experimentally studied in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The Al(13) (-) clusters were produced by laser desorption/ionization of LiAlH(4), then transferred into the ICR cell, cooled by collisions with Ar, and exposed to an excess of Cl(2) with a concentration of approximately 10(8) cm(-3). Relative concentration-time profiles of Al(n) (-) clusters with n=13, 11, 9, and 7 as well as profiles of Cl(-) ions have been recorded. Other ionic species, besides traces of Al(12)Cl(-), were not found, which indicates a double-step degradation mechanism via the odd-numbered Al(n) (-) clusters. From a kinetic analysis of the experimental results, a rate coefficient of (5+/-2)x10(-10) cm(3) s(-1) for the Al(13) (-)+Cl(2) reaction was obtained. Furthermore, it is inferred from a simultaneous fit of all concentration-time profiles that the Al(n) (-)+Cl(2) reactions for n=13, 11, 9, and 7 occur with rate coefficients near the Langevin limit in the range k(bim) approximately (5+/-4)x10(-10) cm(3) s(-1). The branching ratios between the Al(n-2) (-)-producing and Cl(-)-producing channels of a given cluster Al(n)Cl(2) (-) indicate an increasing contribution of the Cl(-)-producing channels with decreasing cluster size. Statistical rate theory calculations on the basis of molecular data from quantum chemical calculations show that the experimental Al(n) (-) profiles are compatible with a sequence of association-elimination reactions proceeding via the formation of highly excited Al(n)Cl(2) (-) adducts followed by a sequential elimination of two AlCl molecules. Rate coefficients for these reactions were calculated, and the production of Cl(-) was shown probably not to proceed via these Al(n)Cl(2) (-) intermediates.

Monitoring the dissolution process of metals in the gas phase: reactions of nanoscale Al and Ga metal atom clusters and their relationship to similar metalloid clusters.

Formation and dissolution of metals are two of the oldest technical chemical processes. On the atomic scale, these processes are based on the formation and cleavage of metal-metal bonds. During the past 15 years we have studied intensively the intermediates during the formation process of metals, i.e. the formation of compounds containing many metal-metal bonds between naked metal atoms in the center and ligand-bearing metal atoms at the surface. We have called the clusters metalloid or, more generally, elementoid clusters. Via a retrosynthetic route, the many different Al and Ga metalloid clusters which have been structurally characterized allow us to understand also the dissolution process; i.e. the cleavage of metal-metal (M-M) bonds. However, this process can be detected much more directly by the reaction of single metal atom clusters in the gas phase under high vacuum conditions. A suitable tool to monitor the dissolution process of a metal cluster in the gas phase is FT-ICR (Fourier transform ion cyclotron resonance) mass spectrometry. Snapshots during these cleavage processes are possible because only every 1-10 s is there a contact between a cluster molecule and an oxidizing molecule (e.g. Cl2). This period is long, i.e. the formation of the primary product (a smaller metal atom cluster) is finished before the next collision happens. We have studied three different types of reaction:(1) Step-by-step fragmentation of a structurally known metalloid cluster allows us to understand the bonding principle of these clusters because in every step only the weakest bond is broken.(2) There are three oxidation reactions of an Al13(-) cluster molecule with Cl2, HCl and O2 central to this review. These three reactions represent three different reaction types, (a) an exothermic reaction (Cl2), (b) an endothermic reaction (HCl), and (c) a kinetically limited reaction based on spin conservation rules (O2).(3) Finally, we present the reaction of a metalloid cluster with Cl2 in order to show that in this cluster only the central naked metal atoms are oxidized, and a smaller metalloid cluster results containing the entire protecting shell as the primary cluster. All the experimental results, supported by quantum chemical calculations, give a rough idea about the complex reaction cascades which occur during the dissolution and formation of metals. Furthermore, these results cast a critical light on many simplifying and generalizing rules in order to understand the bonding and structure of metal clusters. Finally, the experiments and some recent results provided by physical measurements on a crystalline Ga(84) compound build a bridge to nanoscience; i.e. they may be a challenge for chemistry in the next decades, since it has been shown that only with a perfect orientation of nanoscale metal clusters, e.g. in a crystal, can novel, unexpected properties (e.g. superconducting nanoscale materials) be obtained.

Primary reaction steps of Al13- clusters in an HCl atmosphere: snapshots of the dissolution of a base metal.

Recently, the icosahedral Al13- cluster has been shown to possess some unusual characteristics due to its special stability (Bergeron, D. E.; et al. Science 2004, 304, 84-87; 2005, 307, 231-235). Here we present reactions of isolated Al13- clusters with hydrogen chloride, following their oxidation through the application of Fourier transform ion cyclotron resonance mass spectrometry. Due to the ultra-low-pressure conditions, the reaction time can be expanded to make one intermediate after another come into view. The following intermediates are generated sequentially, releasing AlCl and H2: Al13HCl-, Al12H-, Al12H2Cl-, Al11H2-, Al12Cl-, and Al11-. The resulting reaction scheme proves to be a molecular model for the dissolution of a metal in an acid, revealing the initial steps of a heretofore unknown fundamental heterogeneous reaction.