Neutral clusters electronic properties

Wu and coworkers studied CO adsorption on neutral and charged Au clusters. They found that the charge state influences the geometrical and electronic properties of the adsorption process. The top position is the preferable site for neutral clusters with less than 6 atoms. More recently, Phala and coworkers found that the top site is favored up to a 13 Au atoms substrate, except for 5 atoms, competing with the bridge position. The geometries considered by these authors for more than 6 atoms are extracted from bulk fee gold motifs. 

The electronic properties of Ag4 as well as its ionized forms have been examined in detail by CNDO and EH, as shown in Table IV. Both procedures predict the linear form to be the stable neutral cluster, but as the cluster loses electrons the tetrahedral geometry becomes more stable. This is because the symmetric molecular orbitals are lower in energy for the tetrahedral than linear geometry and only these would be occupied as the cluster loses electrons. These effects are in accord with the electron spin resonance (ESR) experiments of Eachus and Symons (41) on the cationic forms of Ag4 clusters in frozen... 

Born-Oppenheimer molecular dynamics simulations for neutral and ionized phenol-water clusters are reported. The results for [C6H50D-(H20)4],+ illustrate how the PT dynamics is coupled to fluctuations of the solvent. The kinetics of PT/recombination in [C6H50D-(H20)4] + clusters is related to strong fluctuations of the electrostatic field of the water molecules and this relationship points out the relevance of investigating the electronic properties of the HB network for understanding chemical reaction in solution. 

This field has expanded very rapidly in just the last two years with the development of many new experimental techniques. The excitement continues. The nature of these new experimental probes has and will continue to significantly influence the development of the field. The ability to select a specific size cluster ion and study its properties is an important new tool. Photoelectron spectroscopy will provide new insight about the electronic structure of clusters as a function of cluster size. Magnetic deflection and electronic susceptibility experiments on neutral clusters and cluster adducts will also provide important information. Vibrational spectroscopy using a variety of different... 

Theoretical studies of varying sizes of neutral and anionic gold (An) and silver (Ag) clusters indicate that they exhibit an even-odd oscillation in their stability and electronic properties (Fig. 34.5) [17]. Thus, clusters which have an even number of atoms tend to be more stable in the neutral state, while those having an odd number of atoms tend to be more stable in the anionic state. Since the 6s orbital energy of Au is almost as low as 5d orbitals, the strong s-d hybridization in Au favours ID and 2D structures in the case of the gold clusters. 

Chelikowsky JR (1998) Structural and electronic properties of neutral and charged silicalike clusters. Phys Rev B 57 3333-3339... 

Even when cationic and neutral forms share the same basic topology, as in the case of Na4" and Na4 with the rhombic structure, the positions of the most intense transitions are substantially red-shifted for the positively charged species. The transition from planar (2D) to three-dimensional (3D) structures takes place for smaller cluster sizes in the case of cations than for the neutral clusters (Nas as compared with Nav). Distinct structural properties for the same size of neutral and cationic clusters such as for Na6 and Na6" or Nag and Nag give rise to very different absorption patterns. Similarly, clusters with the same number of valence electrons exhibit different optical response features (cf. Nag with NaQ" " or Na2o with Naii ). 

Of the family of nonmagnetic transition metals, niobium clusters are those for which most theoretical investigations have been made, although these were mainly restricted to the structural properties neutral clusters Nb , with n up to 7 (LDA [229]), with n = 8-10 (LDA [230, 231], LDA and B-LYP GC [223]), anionic clusters up to n = 8 (LDA [113]), anionic and cationic clusters with n = 8-10 (LDA and B-LYP GC [223]). Reference [223] (see Appendix C) presents an extensive structural study using CP molecular dynamics as well as local optimization procedures, and is corroborated by electronic and vibrational spectra as well as by an extension to cations and anions, thus allowing comparison... 

The support clearly affects the rate of some Au-catalyzed reactions. The support can play various roles. First, the support can change the nature of the metal particle adhesion to the surface, and thus change the metal particle size that forms, as was discussed above. Second, the support can act to strain the metal-metal bonds, which would significantly change the electronic properties of the metal atoms near the interface and thus their catalytic properties. Third, there can be electron transfer between the metal and the support, which would change the electronic properties of the metal. Neutral and positively and negatively charged Au clusters have all been proposed to be catalytically active for specific reactions in the literature. Lastly, the interface between the metal and the support can act to create unique bifunctional sites which demonstrate enhanced reactivity. We discuss the last three effects below. The effect of particle size on the catalytic performance was discussed in detail in the previous section. 

The optical properties of expanded fluid mercury have inspired a class of models based on electronic excitations from an atomic ground state (6s) to an excited state (6p). The idea is that the electronic dipole moment associated with the excitation is stabilized by interaction with similar dipole moments on other excited atoms. This establishes a state known as an exciton. Since the excitation admixes ground state (6s) character into the excited level, it leads to so-called cooperative hybridization. In a pioneering study of this effect in mercury, Bhatt and Rice (1979) found that the dipole-dipole interactions in a homogeneous fluid were insufficient to stabilize the excitonic state. But they also found that the excitons could be stabilized by neutral clusters and that this could explain certain infrared optical properties of dense mercury vapor. 

A cluster size of 8 is quite low, and the conductivity of such small clusters is difficult to define, let alone measure. However, we can examine a related electronic property directly. The energy required to remove an electron from the metal is given for the neutral atom (the first ionization energy) and for the bulk metal (the work function) in Table 11.1 for several metals. The energy ordering is about the same for the elements whether considering the single atoms or the bulk metal, but the work function tends to be about half of the ionization potential. The electron delocalization of the bulk metal results in a much lower ionization potential (2—3 eV) than in the bare atoms, and much of that reduction is accomplished before the clusters reach ten atoms. 

In recent years, several model complexes have been synthesized and studied to understand the properties of these complexes, for example, the influence of S- or N-ligands or NO-releasing abilities [119]. It is not always easy to determine the electronic character of the NO-ligands in nitrosyliron complexes thus, forms of NO [120], neutral NO, or NO [121] have been postulated depending on each complex. Similarly, it is difficult to determine the oxidation state of Fe therefore, these complexes are categorized in the Enemark-Feltham notation [122], where the number of rf-electrons of Fe is indicated. In studies on the nitrosylation pathway of thiolate complexes, Liaw et al. could show that the nitrosylation of complexes [Fe(SR)4] (R = Ph, Et) led to the formation of air- and light-sensitive mono-nitrosyl complexes [Fe(NO)(SR)3] in which tetrathiolate iron(+3) complexes were reduced to Fe(+2) under formation of (SR)2. Further nitrosylation by NO yields the dinitrosyl complexes [(SR)2Fe(NO)2], while nitrosylation by NO forms the neutral complex [Fe(NO)2(SR)2] and subsequently Roussin s red ester [Fe2(p-SR)2(NO)4] under reductive elimination forming (SR)2. Thus, nitrosylation of biomimetic oxidized- and reduced-form rubredoxin was mimicked [121]. Lip-pard et al. showed that dinuclear Fe-clusters are susceptible to disassembly in the presence of NO [123]. 

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