ABSTRACT:We investigated various isomers of B 6 , B 6 ϩ , and B 6Ϫ clusters with ab initio [Hartree-Fock (HF), MP2)] and density functional theory (DFT) methods. Ten B 6 isomers, 6 B 6 ϩ isomers, and 6 B 6 Ϫ isomers are determined to be local minima on their potential energy hypersurfaces by the HF, B3LYP, B3PW91, and MP2 methods. Fourteen of these structures are first reported. The most stable neutral B 6 cluster is the capped pentagonal pyramid (C 5v ), in agreement with the results reported previously. Hexagon B 6 ϩ (C 2h ) isomer and fan-shaped B 6 Ϫ (C 2v ) isomer are found to be the most stable on the cationic and anionic energy hypersurfaces, respectively. Natural bond orbital analysis suggests that there are three-centered bonds in the most stable B 6 neutral and ionic clusters. The multicentered bonds are responsible for the special stability of the lowest-energy isomer.
The ground-state geometries, electronic structures, and vibrational frequencies of alkali metal−B5
- MB5 (M
= Li, Na, K, Rb, and Cs) and alkaline earth metal−B5
- MB5
+ (M = Be, Mg, Ca, and Sr) clusters were
investigated using ab initio self-consistent field and density functional theory (DFT) methods. Calculation
results show that planar B5
- anion can coordinate with the metal atom to form metal−polyboron MB5 and
MB5
+ species. On the basis of the molecular orbital (MO) analysis and nucleus-independent chemical shifts
(NICS), we revealed that the planar B5
- anion exhibits characteristic of π-aromaticity with two delocalized
π electrons and maintains its structural and electronic integrity inside the five MB5 and four MB5
+ clusters.
A series of metal−polyboron clusters with the general formula MB6 (M = Be, Mg, Ca, and Sr) were investigated
using ab initio self-consistent field and density functional theory (DFT) methods. Calculation results show
that the two clusters of BeB6 and MgB6 have C
6
v
pyramidal structure with M2+ cation interacting with a
planar hexagonal B6
2- dianion. Whereas the other two CaB6 (C
1) and SrB6 (C
S) clusters possess quasi-pyramidal
structure with M2+ cation interacting with a chairlike B6
2- dianion. Molecular orbital (MO) analysis and
nucleus-independent chemical shifts (NICS) further reveal that the four MB6 species all have three delocalized
π MOs and two delocalized σ MOs and therefore exhibit the multiple-fold aromaticity.
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