Boron-stabilized calculations

Heating 7 for 1 h at 130°C affords an equilibrium mixture of 7 and tricyclic borane II in a 5 95 ratio. The greater thermodynamic stability of 11 is due to the vinylic position of the boron atom. Calculations show that 11 is 1.8 kcal moF1 more stable than 7. The... 

In the same way as for Ceo and C70 fullerenes, boron clusters with magic numbers B5+ and Bi3+ appear as prominent peaks in mass spectra. These boron clusters were calculated for various geometries by several authors and were considered to present 3D aromatic stabilization.297-299... 

We would like to remind that all tubular structures are composed of 96 boron atoms, the same number of atoms in the elemental a-boron unit cell of boron crystals. The purpose is simply having clusters of the same size, in order to be energetically comparable. On one hand, a-boron is a real component existing in nature, and on the other hand, single-wall nanotubes so far have been predicted and also synthesized [15]. In this case we calculated the total B3LYP energies and determined the structure stability as follows = (nEi - E ) / n = Ei - E / n, where E is the... 

The seven-membered fully unsaturated boron heterocycle is named l//-borepin or, more frequently, borepin. It has six 7r-electrons and is therefore a potential Hiickel aromatic, isoelectronic with the tropylium ion. This similarity has induced several groups to search for a synthesis. Recent theoretical calculations have predicted the -stabilization to be less than that of the tropylium ion. On the other hand, borepins are organoboranes and therefore expected to be sensitive to oxygen. 

Our group has devoted two articles for the study of systems where the chiral subunits are bonded to a boron or to a metallic atom (Scheme 3.28). In both cases, the weakness of the bonds formed can be easily broken. In the first case, a series of bisdiphenylborates and the effect of the fluoro substitution on the relative stability of the homo vs. heterochiral complexes have been studied by means of DFT calculations (B3LYP/6-31G ) [29]. In addition, the corresponding isoelectronic structures, in which the boron has been substituted by a carbon or a positively charged nitrogen, have been considered. In all the cases, the homo and heterochiral complexes present D2 and S4 symmetries, respectively. 

It also seems of interest to compare the stabilities of tetrahedral states of aluminum and boron in terms of this simple qualitative picture. The calculations have revealed that the planar structure is preferential for B(OH)3 as well. The value of r for B(OH)3 is 30.1 kcal/mol, that is, it exceeds substantially the value for Al(OH)3. This is a manifestation of the difference in the covalent radii of B and Al. The value of Ec for the B(OH)3 + HzO system is no longer sufficient to compensate Er, that is, in this system the tetrahedral coordination of the boron atom is unfavorable. However, in the B(OH)3 + OH" system, Ec = 88.4 kcal/mol, that is, the presence of the highly electron-donating oxygen atom of OH- should make the tetrahedral state of boron preferential. Actually, the tetrahedral boron is known to exist in a number of mixed oxides. 

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