Boron molecular orbital model

The effecfs of boron addition were also calculated based on a semiempiri-cal molecular-orbital model. Results show that the introduction of boron is favorable for lithium intercalation. When a layer of BQ is coated onto the surface of natural graphite, the performance improves considerably. In contradiction, another theoretical study based also on a semiempirical molecular orbital method concludes that the substitution of the carbon by boron is not effective for lithium storage. This illustrates the complexity of the carbon structure. These results suggest that the exact bonding states of boron may markedly influence the properties of the carbon materials. 

Boranes are typical species with electron-deficient bonds, where a chemical bond has more centers than electrons. The smallest molecule showing this property is diborane. Each of the two B-H-B bonds (shown in Figure 2-60a) contains only two electrons, while the molecular orbital extends over three atoms. A correct representation has to represent the delocalization of the two electrons over three atom centers as shown in Figure 2-60b. Figure 2-60c shows another type of electron-deficient bond. In boron cage compounds, boron-boron bonds share their electron pair with the unoccupied atom orbital of a third boron atom [86]. These types of bonds cannot be accommodated in a single VB model of two-electron/ two-centered bonds. 

The TEC model developed by Teo also has been successfully applied to rationalize the geometries of a large number of cluster compounds. The TEC model combines Lauher s rule with Euler s theorem and adds an adjustable parameter This parameter X is equal to the number of electron pairs present in excess of that predicted by the 18-electron rule. " X has also been interpreted in terms of the number of missing antibonding orbitals. Given a value for X, determined by the shape of the cluster, an equation predicts the electron count for a cluster. Theoretical justification of the parameter X is based largely upon the classical molecular orbital calculations performed by Hoffmann and Lipscomb via the extended Hiickel method on the corresponding polyhedral boron hydride clusters The values... 

The bond models for the boron-rich borides such as MBe have a common feature with the ionic carbides discussed in Section 6.4.2, in that a mechanism exists for a favorable transfer of electrons from the metal atoms to the boron framework to add a considerable ionic character to the overall bond character. Molecular orbital calculations for isolated units such as Bg, using 2s and 2p boron orbitals, reveal that the molecular orbitals can be divided into two sets one group Of six outward pointing at an energy... 

The initial reaction is a Prins reaction, catalysed by the boron trifluoride complex. In order to achieve maximum overlap of the -orbitals of the olefin and aldehyde groups, the aldehyde must approach the olefin from below, as will easily be seen using molecular models. This means that the resultant alcohol function is located on the downward side of the molecule as shown in Figure S21. A 1,2-carbon shift followed by a transannular bond formation with concomitant loss of a proton, provides the skeletal rearrangement to the product. It may not be too obvious in the figure, but an experiment with molecular models will soon... 

Boron is a group IIIA element. Open the molecular model for boron trifluoride from the 3D Molecular Models section of the book s website. Near the boron atom, above and below the plane of the atoms in BF3, are two relatively large lobes. Considering the position of boron in the periodic table and the three-dimensional and electronic structure of BF3, what type of orbital does this lobe represent Is it a hybridized orbital or not ... 

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