Department of Chemistry
Date of this Version
4-21-2008
Abstract
We perform an unbiased search for the lowest-energy structures of Zintl dianions (Si122-, 122-, and 122-), 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 122-, all BH searches (based on independent initial structures) lead to the same lowest-energy structure 12a2-, 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.
Comments
Published in THE JOURNAL OF CHEMICAL PHYSICS 128, 154326 (2008). Copyright © 2008 American Institute of Physics. Used by permission.