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.

It is shown that spin accommodation plays a determining role in the reactivity of aluminum based anion clusters with oxygen. Experimental reactivity studies on aluminum and aluminum-hydrogen clusters show variable reactivity in even electron systems and rapid etching in odd electron systems. The reactivity of even electron clusters is governed by a spin transfer to the singlet cluster through filling of the spin down antibonding orbitals on triplet oxygen. Theoretical investigations show that when the spin transfer cannot occur, the species is unreactive. When spin accommodation is possible, more subtle effects appear, such as the required spin excitation energy, which raises the total energy of the system, and the filling of the antibonding levels of the O2 molecule, which is stabilized by becoming an aluminum oxygen pi bond. This explanation is consistent with observed behavior in oxygen etching reactions with a variety of clusters including AlnHm-, Aln-, AlnIm-, and AlnC-. The proposed reaction mechanism lends a physical interpretation as to why the HOMO-LUMO gap successfully predicts oxygen etching behavior of the considered systems.

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 limits and useful modifications of the jellium model are of great interest in understanding the properties of metallic clusters, especially involving bimetallic systems. We have measured the relative reactivity of CuAl n − clusters ͑n =11-34͒ with O 2. An odd-even alternation is observed that is in accordance with spin-dependant etching, and CuAl 22 − is observed as a "magic peak." The etching resistance of CuAl 22 − is explained by an unusually large splitting of the 2D 10 subshell that occurs because of a geometric distortion of the cluster that may also be understood as a crystal field splitting of the superatomic orbitals.

]. An intuitive model that can predict the existence of stable hydrogenated cluster species is proposed. The potential synthetic utility of the superatom assemblies built on these units is addressed.alanes ͉ cluster-assembled materials ͉ magic clusters ͉ oxygen etching S mall metal clusters have great synthetic potential in novel material applications because of the atom-by-atom tunability of their electronic structure and chemical activity (1-8). Cluster tunability is currently being studied rigorously to discern suitable applications in which novel cluster assemblies may be realized and exploited. One such application is hydrogen storage (9-10). The recent discovery of a diverse series of aluminum-hydride clusters, or alanes, by Li and colleagues (11, 12) has given further credence to the existing idea that aluminum clusters might serve as hydrogen storage media with an outstanding capacity (13). Alanes have also been considered for use in the high-energydensity application of solid rocket fuels (14), wherein both aluminum and hydrogen are burned as fuel. Although the potential of small aluminum clusters in future fuel applications is apparent, questions about their properties and behavior remain.Through a synergistic effort that combines gas phase reactivity experiments and first-principles electronic structure studies, we have investigated the formation and oxidation of the Al 4 H n Ϫ series to determine how and why they are formed, if they are amenable to storage without degradation due to oxidation, and how they may oxidize during combustion. Al 4 H 7 Ϫ , in particular, is shown to be an extremely robust building block that is readily formed and is resistant to reaction with oxygen, making it a good candidate for further assembly. Guided by electronic structure studies we propose chemical principles that supplement the Wade-Mingos rules generally used to predict geometries in boron hydrogen systems (11,12,15,16). These principles describe the reactivity of the Al 4 H n Ϫ series and further enable the identification of species that are stable in an oxygen-rich environment. The present developments share similarities with Grimm's hydrogen-displacement theorem (17) for Hass's concept of pseudoelements (18).The identification of stable aluminum-hydride clusters began with flow reactor experiments wherein cluster anions were formed by the laser-induced plasma technique (19) in a fast-flow reactor as described in Experimental Methods. (19,20) The abundances of specific species produced by the laser-induced plasma technique are governed partially by kinetic restrictions. However, distinct differences in the abundances of compositionally similar species are representative of thermodynamic differences in formation processes. Furthermore, thermalized clusters react with other molecular species present in the fast-flow reactor. Thus, relative thermodynamic information is obtained by examining the differences and changes in abundance, resultant from the chemical interactions of the clusters with oxygen or other molecu...

Understanding the emergence of properties from the size-selective cluster regime to larger nanoparticles is one of the principal goals of nanoscience. We have measured the size-selective reactivity of aluminum cluster anions with alcohols. All clusters with more than 20 atoms are found to be reactive, while Al11(-), Al13(-), and Al20(-) show enhanced resistance to oxidation at smaller sizes. The reactivity of aluminum cluster anions with water, methanol, and tert-butyl alcohol all exhibit patterns that require complementary active sites (Lewis acid, Lewis base) on adjacent atoms. Theoretical investigations reveal that at small sizes, the location of reactive pairs occurs on specific active sites, but at larger sizes the reactive pairs begin to accumulate on the edges between facets, marking the transition from the nonscalable size-dependent regime to the scalable regime where the nanoparticles are universally reactive.

An in-depth investigation is presented on the hydrogen evolution reaction of aluminum clusters with water and methanol/isopropanol. Aluminum clusters were found to undertake an etching effect in the presence of methanol, but also resulted in an addition reaction with isopropanol. Such reactivity without producing hydrogen is different than water, although they all contain an OH group. Further, we studied the competition of water versus alcohols reacting with Al clusters by simultaneously introducing them into a fast-flow tube reactor. Water dominates the competitive reaction with Al clusters, and the O-H bond in water is readily activated to form aluminum hydroxide cluster products. Also found is that water functions as a catalyst in the activation of the O-H bond in alcohol molecules.

Recently, it was discovered that specific aluminum clusters (e.g., Al 13 À ) that demonstrate enhanced resistance to reactivity with oxygen may do so not only because of a closed electronic jellium shell as originally supposed but also because of a forbidden spin-flip in the transition state of the reaction. Herein, we discuss an experiment using a multiple-species laminar flow reaction vessel coupled to a singlet oxygen generator. The present results suggest that all clusters react with singlet oxygen. Additionally, we observe Al 9 À , a cluster previously unidentified as having any notable stability, as being resistant to reaction with triplet oxygen. Furthermore, we discuss a means of estimating rate constants in a multiple-species flow tube where the products and reactants do not allow the use of traditional methods.