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Small pure-Boorn clusters from B3 to B9 in their uncharged and anionic forms have been studied up to this point. Go to see:
| B3 and B3-; | B4 and B4-; | B5 and B5-; | B6,B6- and B62-; | B7 and B7-. | B8 and B9- neutral and anionic. |
The powerfull combination of ab initio quantum chemical study and photoelectron spectroscopy for anionic species were applied to the considered systems. Surprisingly, all small pure-boron clusters were found to be planar or quasiplanar. That contradicts with all expectations based on the widely known chemistry of boranes and carboranes. We showed that all mentioned species possess either aromatic or antiaromatic character of chemical bonding.
Triangular D3h shape of the global minima smallest considered pure-boron clusters encountered engaged the initial interest to these systems. Anionic 1A1' cluster B3 has calculated photoelectron spectrum perfectly agreeing with the experimentally obtained one. We, thus, enabled to believe that the found species is the real global minimum.
It's electronic structure and chemical bonding were investigated. As was shown by the molecular orbitals' analysis, doubly-aromatic bonding character is responsible for the stability and specific, perfect shape of the cluster. Figure on the right presents the set of molecular orbitals of the species. As one may easily see, HOMO-1 is completely-bonding molecular orbital involving all 3 atoms of the ring. Three-center overlap above and below the molecular plane is tipical pi-overlap. 2 electrons on this MO obey the famous Huckel's rule (4n+2), and, consequently, the system is pi-aromatic.
B4 and B4-.
This cluster of the robius shape has few low-lying rombius isomers with differnet electronic configurations. The global minimum was found to be pi-aromatic but sigma-antiaromatic.
B5 and B5-.
B5 and B5- clusters have very similar C2v structures. Planarity occurs due to the p-aromatic character of the chemical bonding.
B6, B6- and B62-.
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This cluster is planar but distorted hexagon. Anion has an odd number of electrons but nevertheless was prooved to be both pi- and sigma-antiaromatic. |
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Two close in shape strcutres were found to compete for the title "the global minimum of the B7- cluster. Triplet C6v and singlet C2v have insagnificant energy differnce (but the former is actually lower at all aplied levels of theory). The only difference in their chemical bonding is the following:
As one may see from this figure, in the triplet cluster doubly-degenerate HOMO and HOMO-4 belong to the same family - p-molecular orbitals with overlap below and above the molecular plane. 4 electrons on these MOs obey Huckel'l (4n) rule for triplet states, and, thus, form p-aromaticity of the system.
On the other hand, doubly-degenerate HOMO-1 and HOMO-3 perform radial p-overlap. This is the second kind of aromaticity in the system. These MOs contin 6 electrons, which correspond to aromatic character of the chemical bonding for the completely-occupied MOs.
HOOMO-2 itself represents the third type of aromaticity - peripheral. p-electrons on ring-boron atoms gether in peripheral overlap and complete the picture of triple aromatic character of the chemical bonding of the triplet cluster.
Singlet species is closed-shell, of course. One of the former doubly-degenerate HOMO of the C6v structure is now empty, while the other is doubly-occupied. Thus, 4 p-electrons on these MOs result in p-antiaromaticity.
All other MOs have pretty much similar forms and contribute to the stability of the species, it's doubly-aromatic character (peripheral and radial).
Here we see the perfect example of the aromaticity's stabilizing effect. Naturally discomfirting triplet state is here more profitable in comparison with the singlet one due to the additional type of aromaticity it has.
B8 and B9- neutral and anionic.
And how would you like the crown of this project? B8 and B9- having wheel-shapes of the global minima present the first ever observed hepta- and octa-coordinated Boron atom. Photoelectron spectroscopy and our ab initio calculations demonstrated the existence of such clusters.
Perfectly symmetric shape of the species is explained in terms of the triply-aromatic character of the chemical bonding, as it may be seen from the picture on your right.
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How we flattened the well-known B6H6Li2 salt
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