Search for lowest-energy structure of Zintl dianion Si₁₂^2-, Ge₁₂^2-, and Sn₁₂^2-

Authors

• Nan Shao, University of Nebraska-Lincoln
• Satya S. Bulusu, University of Nebraska-LincolnFollow
• Xiao Cheng Zeng, University of Nebraska-LincolnFollow

Document Type

Article

Date of this Version

4-21-2008

Comments

Published in THE JOURNAL OF CHEMICAL PHYSICS 128, 154326 (2008). Copyright © 2008 American Institute of Physics. Used by permission.

Abstract

We perform an unbiased search for the lowest-energy structures of Zintl dianions (Si₁₂^2-, ₁₂^2-, and ₁₂^2-), 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 ₁₂^2-, all BH searches (based on independent initial structures) lead to the same lowest-energy structure _12a^2-, a tricapped trigonal prism (TTP) with C_s group symmetry. Coupled-cluster calculation, however, suggests that another TTP isomer of Si_12c^2- is nearly isoenergetic with Si_12a^2-. For Sn₁₂^2-, all BH searches lead to the icosahedral structure I_h-Sn_12a^2-, i.e., the stannaspherene. For Ge₁₂^2-, however, most BH searches lead to the TTP-containing Ge_12b^2-, while a few BH searches lead to the empty-cage icosahedral structure I_h-Ge_12a^2- (named as germaniaspherene). High-level ab initio calculation indicates that I_h-Ge_12a^2- and TTP-containing Ge_12b^2- are almost isoenergetic and, thus, both may be considered as candidates for the lowest-energy structure at 0 K. Ge_12a^2- has a much larger energy gap (2.04 eV) between highest occupied molecular orbital and lowest unoccupied molecular orbital than Ge_12b^2-(1.29 eV), while Ge_12b^2- has a lower free energy than I_h-Ge_12a^2- at elevated temperature (>980 K). The TTP-containing Si_12a^2- and Ge_12b^2- exhibit large negative nuclear independent chemical shift (NICS) value (~−44) at the center of TTP, indicating aromatic character. In contrast, germaniaspherene I_h-Ge_12a^2- and stannaspherene I_h-Sn_12a^2- exhibit modest positive NICS values, ~12 and 3, respectively, at the center of the empty cage, indicating weakly antiaromatic character.