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in the geometry of the gold cluster caused by the adsorption of the reactant molecules, mainly the peripherially adsorbed O2. This structural change, occurring during adsorption (or in the course of reaction), is a manifestation of the "structural fluxtionality" of clusters (5). The system shown in Fig. 2C possesses two IR-active 13CO molecules (CO(1) and CO(3)), as we had seen experimentally (18).

Because the bonding characteristics, and other properties of the system, with a single adsorbed CO molecule (Fig. 2B) are similar to the ones associated with the CO-saturated cluster (Fig. 2C), we focus next on the former (which is more convenient to analyze and illustrate). The energetics of the adsorption of the O2 and CO molecules to the defect-free and F-center clusters, corresponding in each case to two possible spin states (S = 0 or 1), together with the calculated amount of charge transferred to the deposited complex (gold cluster and adsorbed molecules), the bond length d(CO), and the calculated vibrational frequency are given in Table 2. Stronger binding of O2 to the cluster occurs in the presence of the F center (A and B in Table 2).

Adsorption is accompanied by a significant degree of charge transfer, which in the case of the defective surface is directed from the oxygen vacancy into the deposited gold cluster and adsorbed molecules. Comparison of the charge-difference isosurfaces for the bare cluster (Fig. 2E) and for the cluster with adsorbed O2 and CO molecules (Fig. 2F), bonded to MgO(FC), reveals a significant amount of charge excess on the adsorbed molecules. The consequent weakening of intramolecular bonds manifests itself in increased bond lengths and lower vibration-al frequencies of the adsorbed molecules (Table 2) (28).

The correlation diagram (Fig. 3, A to C) depicts the local density of states (LDOS) for the two reactants, CO (Fig. 3A) and the Au8/ O2/MgO(FC) complex (Fig. 3C), as well a8s fo2r the product complex Au8/O2/CO/MgO(FC) with the CO adsorbed on the deposited cluster (Fig. 3B). The LDOS of Au8/O2/CO/ MgO(FC) is reproduced in Fig. 4 along with images of the corresponding molecular orbitals, which aid visualization of the bonding mechanism discussed below. As expected, the 3o and the nonbonding 4s (oxygen lone-pair) molecular orbitals of the isolated CO are not involved in the bonding to the cluster complex, and the nonbonding 5o (carbon lone-pair) orbital is stabilized by the interaction (by about 3eV) and thus it contributes to CO chemisorption (29). The 1 p level is slightly pushed upward in energy (<1eV).

The LDOS of the complex projected on the adsorbed CO also reveals contributions of the 2p* orbitals that are spread over the g09*ptlBK CO

g09*ptlBK CO

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