Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA.
Angewandte Chemie International Edition (Impact Factor: 11.34). 05/2004; 43(16):2132-4. DOI: 10.1002/anie.200353287
Source: PubMed

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    ABSTRACT: Here we report a systematic theoretical study of the structure and electronic properties of Snn-1Pb and Pbn-1Sn (n = 2-13) clusters and compare these results with pure Snn and Pbn to understand the influence of the dopant elements. The calculations were carried out using the density functional theory with generalized gradient approximation for the exchange-correlation potential. Extensive search based on large number of initial configurations has been carried out to locate the stable isomers of Snn-1Pb and Pbn-1Sn (n = 2-13) clusters. The relative stability of Snn-1Pb and Pbn-1Sn (n = 2-13) clusters is analyzed based on the calculated binding energies and second difference in energy. The stability analysis of these clusters suggests that, while the substitution of Sn by Pb lowers the stability of Snn clusters, presence of Sn enhances the stability of the Pbn clusters. The results suggest that while for Snn-1Pb, n=4, 7, 10, 12 clusters are more stable than their respective neighbors, Pbn-1Sn clusters with n = 4, 7 and 9 are found to be more stable. Based on the fragmentation pattern it is seen that for Snn-1Pb and Pbn-1Sn clusters favor monomer evaporation of the Pb atom up to n =11 and n =12, respectively. Unlike this trend, the Sn11Pb undergoes fission type fragment into Sn5Pb and Sn6 clusters. A comparison between our theoretical results and surface induced dissociation experiment shows good agreement, which gives confidence on the prediction of the ground state geometries.
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    ABSTRACT: We perform an unbiased search for the lowest-energy structures of Zintl dianions (Si122−, Ge122−, and Sn122−), by using the basin-hopping (BH) global optimization method combined with density functional theory geometric optimization. High-level ab initio calculation at the coupled-cluster level is used to determine relative stabilities and energy ranking among competitive low-lying isomers of the dianions obtained from the BH search. For Si122−, all BH searches (based on independent initial structures) lead to the same lowest-energy structure Si12a2−, a tricapped trigonal prism (TTP) with Cs group symmetry. Coupled-cluster calculation, however, suggests that another TTP isomer of Si12c2− is nearly isoenergetic with Si12a2−. For Sn122−, all BH searches lead to the icosahedral structure Ih-Sn12a2−, i.e., the stannaspherene. For Ge122−, however, most BH searches lead to the TTP-containing Ge12b2−, while a few BH searches lead to the empty-cage icosahedral structure Ih-Ge12a2− (named as germaniaspherene). High-level ab initio calculation indicates that Ih-Ge12a2− and TTP-containing Ge12b2− are almost isoenergetic and, thus, both may be considered as candidates for the lowest-energy structure at 0 K. Ge12a2− has a much larger energy gap (2.04 eV) between highest occupied molecular orbital and lowest unoccupied molecular orbital than Ge12b2− (1.29 eV), while Ge12b2− has a lower free energy than Ih-Ge12a2− at elevated temperature (>980 K). The TTP-containing Si12a2− and Ge12b2− exhibit large negative nuclear independent chemical shift (NICS) value ( ∼ −44) at the center of TTP, indicating aromatic character. In contrast, germaniaspherene Ih-Ge12a2− and stannaspherene Ih-Sn12a2− exhibit modest positive NICS values, ∼ 12 and 3, respectively, at the center of the empty cage, indicating weakly antiaromatic character.
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    ABSTRACT: Medium-sized double magic metal clusters, Al@Ag(54) (-) and Al@Cu(54) (-), are predicted based on unbiased global search and density functional calculation. Both bimetallic core-shell clusters have icosahedral symmetry, and they are much lower in energies than all other low-lying isomers. In contrast, the icosahedral cluster Al@Au(54) (-) is a high-energy isomer. Both Al@Ag(54) (-) and Al@Cu(54) (-) exhibit appreciable gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, and strong spherical aromaticity, which provide two additional evidences for the likelihood of their high stability. The simulated anion photoelectron spectra and optical absorption spectra are readily compared with future experiments.
    The Journal of Chemical Physics 08/2008; 129(8):084703. · 3.12 Impact Factor