W. Hunter Woodward

Complementary Active Sites Cause Size-Selective Reactivity of Aluminum Cluster Anions with Water

The reactions of metal clusters with small molecules often depend on cluster size. The selectivity of oxygen reactions with aluminum cluster anions can be well described within an electronic shell model; however, not all reactions are subject to the same fundamental constraints. We observed the size selectivity of aluminum cluster anion reactions with water, which can be attributed to the dissociative chemisorption of water at specific surface sites. The reactivity depends on geometric rather than electronic shell structure. Identical arrangements of multiple active sites in Al16–, Al17–, and Al18– result in the production of H2 from water.

Reactivity of Aluminum Cluster Anions with Water: Origins of Reactivity and Mechanisms for H2 Release

The reactivity of aluminum anion clusters with water was found to exhibit variations with size, with some clusters exhibiting negligible reactivity, others absorbing one or more water, while even others releasing H(2) with addition of multiple waters. (Roach, P.J., Woodward, W.H. et al. Science, 2009, 323, 492). Herein, we provide further details on the role of complementary active sites in the breaking of the O-H bond on the cluster. We examine the reactions of Al(n)(-) + H(2)O where n = 7-18, and show how the complementary active sites may be best identified. The clusters with active sites are found to be reactive, and clusters with barriers to reactivity have an absence of paired active sites. The role of charge in the reactivity is considered, which could account for the observed increase in reactivity at large sizes. The H(2) release in the reactivity of Al(17)(-) with multiple water molecules is also studied by comparing multiple reaction pathways, and the selective H(2) production is explained by the first water inducing a new active site. A mechanism for transferring hydroxyl groups on the surface of the cluster is also discussed.

The growth of ionic crystals based on the halogenation of copper cluster anions.

We investigated the halogenation reactivity of copper cluster anions produced via a magnetron-sputter source after introduction into a fast-flow tube reaction apparatus simultaneously with chlorine gas. Interesting cluster products corresponding to [Cu(n)Cl(n+1)](-) (n = 1-6) were observed with notable stability, and the mass distribution of these clusters exhibits an exponential decay with increasing values of n. Reaction kinetics analysis is provided on the gas-phase reactivity of copper cluster anions with chlorine. First-principle calculations suggest a series of cubic-like structures for these species similar to the structure of alkali halide clusters due to their similar electronic configurations. These structures act as a starting point in the formation of ionic crystals.