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Gold nanoparticles are known to be highly versatile oxidation catalysts utilizing molecular oxygen as a feedstock, but the mechanism and species responsible for activating oxygen remain unclear. The reaction between unsupported cationic gold clusters and molecular oxygen has been investigated. The resulting complexes were characterized in the gas phase using IR spectroscopy. A strong red-shift in the observed ν(O-O) stretching frequency indicates the formation of superoxo (O2(-)) moieties. These moieties are seen to form spontaneously in systems, which upon electron transfer attain a closed shell within the spherical jellium model (Au10(+) and Au22(+)), whereas an oxygen induced self-promotion in the activation is observed for other systems (Au4(+), Au12(+), Au21(+)).

Gold nanoparticles and sub-nanoparticles famously act as highly efficient and selective low-temperature oxidation catalysts with molecular oxygen, in stark contrast to the nobility of the bulk phase. The origins of this activity and the nature of the active species remain open questions. Gas-phase studies of isolated gold clusters hold promise for disentangling these problems. Here we address the interaction of neutral gold clusters (Au(n); 4 ≤ n ≤ 21) with molecular oxygen by probing the highly characteristic O-O vibrational stretch frequencies. This reveals that for selected cluster sizes the oxygen is highly activated with respect to the free moiety. Complementary quantum chemical calculations provide evidence for substantial electron transfer to the O(2) unit and concomitant rearrangement of the parent gold cluster structure upon binding and activation. This gives evidence for a model of the interaction between neutral gold clusters and molecular oxygen.

Cationic silver-doped silicon clusters, Si_nAg⁺ (n=6–15), are studied using infrared multiple photon dissociation in combination with density functional theory computations. Candidate structures are identified using a basin-hopping global optimizations method. Based on the comparison of experimental and calculated IR spectra for the identified low-energy isomers, structures are assigned. It is found that all investigated clusters have exohedral structures, that is, the Ag atom is located at the surface. This is a surprising result because many transition-metal dopant atoms have been shown to induce the formation of endohedral silicon clusters. The silicon framework of Si_nAg⁺ (n=7–9) has a pentagonal bipyramidal building block, whereas the larger Si_nAg⁺ (n=10–12, 14, 15) clusters have trigonal prism-based structures. On comparing the structures of Si_nAg⁺ with those of Si_nCu⁺ (for n=6–11) it is found that both Cu and Ag adsorb on a surface site of bare Si_n⁺ clusters. However, the Ag dopant atom takes a lower coordinated site and is more weakly bound to the Si_n⁺ framework than the Cu dopant atom

The structures of neutral cobalt-doped silicon clusters have been assigned by a combined experimental and theoretical study. Size-selective infrared spectra of neutral Si(n)Co (n = 10-12) clusters are measured using a tunable IR-UV two-color ionization scheme. The experimental infrared spectra are compared with calculated spectra of low-energy structures predicted at the B3P86 level of theory. It is shown that the Si(n)Co (n = 10-12) clusters have endohedral caged structures, where the silicon frameworks prefer double-layered structures encapsulating the Co atom. Electronic structure analysis indicates that the clusters are stabilized by an ionic interaction between the Co dopant atom and the silicon cage due to the charge transfer from the silicon valence sp orbitals to the cobalt 3d orbitals. Strong hybridization between the Co dopant atom and the silicon host quenches the local magnetic moment on the encapsulated Co atom.

The ultraviolet photodissociation dynamics of the gold-rare gas atom van der Waals complexes (Au-RG, RG = Ar, Kr, and Xe) have been studied by velocity map imaging. Photofragmentation of Au-Ar and Au-Kr at several wavelengths permits extrapolation to zero of the total kinetic energy release (TKER) spectra as monitored in the Au((2)P(3/2)(o)[5d(10)6p]) fragment channel, facilitating the determination of ground state dissociation energies of D(0)(")(Au-Ar) = 149+/-13 cm(-1) and D(0)(")(Au-Kr) = 240+/-19 cm(-1), respectively. In the same spectral region, transitions to vibrational levels of an Omega(') = 1/2 state of the Au-Xe complex result in predissociation to the lower Au((2)P(1/2)(o)[5d(10)6p])+Xe((1)S(0)[5p(6)]) fragment channel for which TKER extrapolation yields a value of D(0)(")(Au-Xe) = 636+/-27 cm(-1). Asymmetric line shapes for transitions to the v(') = 14 level of this state indicate coupling to the Au((2)P(3/2)(o)[5d(10)6p])+Xe((1)S(0)[5p(6)]) continuum, which allows us to refine this value to D(0)(")(Au-Xe) = 607+/-5 cm(-1). The dissociation dynamics of this vibrational level have been studied at the level of individual isotopologues by fitting the observed excitation spectra to Fano profiles. These fits reveal a remarkable variation in the predissociation dynamics for different Au-Xe isotopologues. For Au-Ar and Au-Xe, the determined ground state dissociation energies are in good agreement with recent theoretical calculations; the agreement of the Au-Kr value with theory is less satisfactory.

The geometric structures of Si_nAu⁺ (n = 2–11, 14, and 15) clusters are investigated using density functional theory computations in combination with infrared multiple-photon dissociation spectra measured on the corresponding cluster·argon and cluster·xenon complexes. The Si_nAu⁺ clusters adopt planar structures for the smallest sizes (n = 2–4) and have three-dimensional geometries for larger sizes (n ≥ 5). All of the investigated Si_nAu⁺ clusters have exohedral structures in which the Au dopant atom is adsorbed on a surface site of the bare Si_n⁺ cluster at a low-coordinated position. The growth mechanism of Si_nAu⁺ clusters is discussed and compared with those of Si_nCu⁺ and Si_nAg⁺. The present results indicate that the filled d shell and the atomic radii of the dopant atoms may play important roles in the cage formation of the transition-metal-doped Si clusters. Moreover, it is found that the localization of charge on the Au dopant atoms in Si_nAu⁺ determines the extent of complex formation with argon and xenon

Gold clusters exhibit strong size and charge state dependent variations in their properties. This is demonstrated by significant changes in their geometric structures and also in their chemical properties. Here we focus on clusters containing up to about 20 gold atoms and briefly review their structural evolution emphasising the role of isomerism and structural fluxionality. The discussion of chemical properties is limited to the interaction of gold clusters with molecular oxygen and carbon monoxide, separately, and their interaction in CO/O₂ co-adsorbates on gold clusters eventually leading to CO oxidation. Whilst highlighting results obtained using different experimental approaches, special attention is given to the insights obtained using infrared multiple photon dissociation (IR-MPD) spectroscop