Stability of Aluminum-Doped Copper Cluster Cations and Their Reactivity toward NO and O2

What Determines the Drastic Reactivity of Nbn+ Clusters with Nitric Oxide under Thermalized Conditions?

We report an in-depth study of the adsorption and reaction of NO with cationic Nb_n⁺ (n = 1-20) clusters under thermalized conditions in a laminar flow tube reactor in tandem with a customized triple quadrupole mass spectrometer (FT-TQMS). It is found that the small-sized Nb_n⁺ clusters (2 ≤ n ≤ 7) readily react with NO giving rise to dominant fragmentation products pertaining to the loss of a stable diatomic molecule NbO or NbN. In contrast, the reaction products of larger-sized clusters (n ≥ 10) proceed through diverse channels, including NO adsorption, N₂/N₂O release, and even NO₂ formation. These experimental observations provided the incentive for us to dig deep into the reaction mechanism with the help of DFT calculations. In contrast to the NO-donation coordination in transition metal complexes, here the cationic Nb_n⁺ clusters exhibit dominant electronic donation in initiating the reactions with NO molecules. We fully demonstrated the reaction rate constants, compared the reaction energy diagram of typical Nb_n⁺ clusters, and unveiled the distinct interaction mechanism of niobium clusters available for NO activation and conversion.

Reaction of nitric oxide molecules on transition-metal-doped silver cluster cations: size- and dopant-dependent reaction pathways

We report size- and dopant-dependent reaction pathways as well as reactivity of gas-phase free Ag_nM⁺ (M = Sc-Ni) clusters interacting with NO. The reactivity of Ag_nM⁺, except for M = Cr and Mn, exhibits a minimum at a specific size, where the cluster cation possesses 18 or 20 valence electrons consisting of Ag 5s and dopant's 3d and 4s. The product ions range from NO adducts, Ag_nM(NO)_m⁺, and oxygen adducts, Ag_nMO_m⁺, to NO₂ adducts, Ag_nM(NO₂)_m⁺. At small sizes, Ag_nMO_m⁺ are the major products for M = Sc-V, whereas Ag_nM(NO)_m⁺ dominate the products for M = Cr-Ni in striking contrast. In both cases, these reaction products are reminiscent of those from an atomic transition metal. However, the reaction pathways are different at least for M = Sc and Ti; kinetics measurements reveal that the present oxygen adducts are formed via NO adducts, while, for example, Ti⁺ is known to produce TiO⁺ directly by reaction with a single NO molecule. At larger sizes, on the other hand, Ag_nM(NO₂)_m⁺ are dominantly produced regardless of the dopant element because the dopant atom is encapsulated by the Ag host; the NO₂ formation on the cluster is similar to that reported for undoped Ag_n⁺.

Exploring Cu/Al cluster growth and reactivity: from embryonic building blocks to intermetalloid, open-shell superatoms

Cluster growth reactions in the system [Cu5](Mes)5 + [Al4](Cp*)4 (Mes = mesitylene, Cp* = pentamethylcyclopentadiene) were explored and monitored by in situ LIFDI-MS and 1H-NMR. Feedback into experimental design allowed for an informed choice and precise adjustment of reaction conditions and led to isolation of the intermetallic cluster [Cu4Al4](Cp*)5(Mes) (1). Cluster 1 reacts with excess 3-hexyne to yield the triangular cluster [Cu2Al](Cp*)3 (2). The two embryonic [Cu4Al4](Cp*)5(Mes) and [Cu2Al](Cp*)3 clusters 1 and 2, respectively, were shown to be intermediates in the formation of an inseparable composite of the closely related clusters [Cu7Al6](Cp*)6 (3), [HCu7Al6](Cp*)6 (3H) and [Cu8Al6](Cp*)6 (4), which just differ by one Cu core atom. The radical nature of the open-shell superatomic [Cu7Al6](Cp*)6 cluster 3 is reflected in its reactivity towards addition of one Cu core atom leading to the closed shell superatom [Cu8Al6](Cp*)6 (4), and as well by its ability to undergo σ(C-H) and σ(Si-H) activation reactions of C6H5CH3 (toluene) and (TMS)3SiH (TMS = tris(trimethylsilyl)).

A method for predicting basins in the global optimization of nanoclusters with applications to AlxCuy alloys

The problem of obtaining the geometrical configuration of a molecule that minimizes its potential energy is a very complicated one for a series of applications, ranging from determining the structure of biological macromolecules to nanoclusters of atoms. Global optimization tools are available for this task, and many of them are based in performing successive local optimizations, where the starting geometries for these steps are determined by an intelligent algorithm. Here we develop a method to save computing time in the optimization of nanoclusters by predicting if a given minimum has been previously visited during local optimization steps. Our application to Cu-Al nanoalloys indicates that it is possible to save a substantial amount of computational cost. The application also reveals new promising Al_xCu_y clusters and explain their stabilities in terms of the jellium model.

Multiple-Valence Aluminum and the Electronic and Geometric Structure of Al nO m Clusters

Electronic stability in aluminum clusters is typically associated with either closed electronic shells of delocalized electrons or a +3 oxidation state of aluminum. To investigate whether there are alternative routes toward electronic stability in aluminum oxide clusters, we used theoretical methods to examine the geometric and electronic structure of Al _nO _m (2 ≤ n ≤ 7; 1 ≤ m ≤ 10) clusters. Electronically stable clusters with large HOMO-LUMO (highest occupied molecular orbital and lowest unoccupied molecular orbital) gaps were identified and could be grouped into two categories. (1) Al_{2 n}O_{3 n} clusters with a +3 oxidation state on the aluminum and (2) planar clusters including Al₄O₄, Al₅O₃, Al₆O₅, and Al₆O₆. The structures of the planar clusters have external Al atoms bound to a single O atom. Their electronic stability is explained by the multiple-valence Al sites, with the internal Al atoms having an oxidation state of +3, whereas the external Al atoms have an oxidation state of +1.

Effects of Second-Metal (Al, V, Co) Doping on the NO Reactivity of Small Rhodium Cluster Cations

Reactions of pure and doped rhodium cluster cations, Rh_nX⁺ (n = 2-6; X = Al, V, Co, Rh), with NO molecules were investigated at near-thermal energy using a guided ion beam tandem mass spectrometer. We found that the doping with Al and V increases the total reaction cross section mostly. Under single-collision conditions, Rh₂X⁺ reacts with NO to produce Rh₂N⁺ with release of metal monoxide, XO, whereas Rh_nX⁺ (n = 3-6) adsorb NO. For the specific clusters Rh_nAl⁺ (n = 3 and 4) and Rh_nV⁺ (n = 4-6), the NO adsorption is often accompanied by the release of one Rh atom. In addition, we examined the reactions of Rh₅X⁺ (X = Al, V, Co, Rh) with NO under multiple-collision conditions and observed the cluster dioxide formation and the N₂ release, i.e., NO decomposition. Particularly, the V-doping is most effective for the NO decomposition. One possible explanation for the present results is that the formation of a stable dopant metal-oxygen bond directly leads to the increase of NO dissociative adsorption energy and the reduction of the energy barrier between the molecular and dissociative adsorption, thereby encouraging the NO decomposition on the small Rh_nX⁺ clusters studied.

The adsorption and activation of NO on silver clusters with sizes up to one nanometer: interactions dominated by electron transfer from silver to NO

Evidence for NO unitary adsorption, the formation of (NO)₂ and the reduction to form N₂O is observed on silver clusters with sizes up to one nanometer. The adsorption and activation of NO are enhanced by electron transfer from silver to NO.