Barcaro Giovanni, Aprà Edoardo, Fortunelli Alessandro
Molecular Modeling Laboratory, IPCF-CNR, Via G. Moruzzi 1, Pisa, 56124, Italy.
Chemistry. 2007;13(22):6408-18. doi: 10.1002/chem.200601796.
The structure of AgN clusters (N=1-4, 6, 8, 10), both in the gas phase and grown on the MgO(1 0 0) surface containing Fs-defects, has been investigated by a density functional basin-hopping (DF-BH) approach. In analogy with what observed in the case of gold clusters, it is found that the presence of the defect implies a double frustration and a cylindrical invariance of the metal-surface interaction, causing small Ag clusters growing around the Fs defect to be highly fluxional. Nevertheless, two different structural crossovers are found to be induced by the metal-defect interaction for the adsorbed clusters such that: 1) planar structures prevail for N<or=4 (as in the gas phase); 2) noncrystalline (fivefold symmetric) structures, which are the lowest energy ones in the gas phase for medium sized AgN clusters (N>or=7), prevail for N=6 and N=8; 3) distorted face-centered cubic (fcc) structures grown pseudomorphically on the defected surface prevail for N=10. The transition from fivefold to fcc motifs is rationalized in terms of the double-frustration effect, which increases the bond strain of the noncrystalline structures. Detrapping energies from the defect were also calculated; the lowest energy pathway corresponds to the detachment of a dimer.
采用密度泛函盆地跳跃（DF-BH）方法研究了气相中以及生长在含氟缺陷的MgO(1 0 0)表面上的AgN团簇（N = 1 - 4、6、8、10）的结构。与在金团簇中观察到的情况类似，发现缺陷的存在意味着双重受挫以及金属-表面相互作用的圆柱对称性，导致在氟缺陷周围生长的小Ag团簇具有高度的流动性。然而，对于吸附团簇，发现金属-缺陷相互作用会引发两种不同的结构转变，具体如下：1）对于N≤4（与气相中情况相同），平面结构占主导；2）非晶态（五重对称）结构，在气相中是中等尺寸AgN团簇（N≥7）能量最低的结构，对于N = 6和N = 8占主导；3）在缺陷表面上以赝晶形式生长的扭曲面心立方（fcc）结构对于N = 10占主导。从五重到fcc结构单元的转变可以用双重受挫效应来解释，该效应增加了非晶态结构的键应变。还计算了从缺陷处的脱阱能；能量最低的路径对应于二聚体的脱离。
