Sp2 bonding

The synthesis of molecular carbon structures in the form of C q and other fullerenes stimulated an intense interest in mesoscopic carbon structures. In this respect, the discovery of carbon nanotubes (CNTs) [1] in the deposit of an arc discharge was a major break through. In the early days, many theoretical efforts have focused on the electronic properties of these novel quasi-one-dimensional structures [2-5]. Like graphite, these mesoscopic systems are essentially sp2 bonded. However, the curvature and the cylindrical symmetry cause important modifications compared with planar graphite. 

A second factor that contributes to alkene stability involves bond strengths. A bond between an sp2 carbon and an sp3 carbon is somewhat stronger than a bond between two sp3 carbons. Thus, in comparing 1-butene and 2-butene, the monosubstituted isomer has one sp -.sp3 bond and one sp3-sp2 bond, while... 

C(sp2)=C(sp2) bond imparts properties which more closely resemble those of the central double bond in butatriene than of the one in a simple tetrasub-stituted ethylene [12]. For this reason, bicyclopropylidene (3) undergoes cycloadditions with 1,3-dienes, and these showed an interesting dependence on the structure of the diene. Whereas cyclopentadiene (6) gave the [4 + 2] cycloadduct 28 exclusively, 1,3-cyclohexadiene (26) and 1,3-butadiene (12) led to mixtures of the [4 + 2] and [2 + 2] cycloadducts, with the proportion of the [2 + 2] adduct increasing respectively [13] (Table 3). 

Amorphous carbon films may be broadly classified as (i) amorphous carbon films, a-C films, deposited from carbon-containing gases with low or zero hydrogen content [72] and (ii) hydrogenated carbon films, a-C H films, formed from hydrocarbon-containing gases [73,74]. Both types of film contain different amounts of sp2 and sp3 bonded carbon. The amount of sp2 bonded carbon can be estimated from X-ray absorption near edge spectroscopy,... 

Separation of covalently bonded atoms into QM and MM regions introduces an unsatisfied valence in the QM region this can be satisfied by several different methods. In the frozen-orbital approach a strictly localized hybrid sp2 bond orbital containing a single electron is used at the QM/MM junction [29]. Fro-... 

All BaCvN nanotubes are made of a hexagonal network of sp2 bonded atoms, with three nearest neighbors to each atom (15, 55, 69, 78, 79). In the case of BC2N nanotubes, two different arrangements of the sheet are possible, leading to two isomers with different structures and with distinct electrical properties (78b). 

Conformations about an sp2-sp2 bond may be similarly designated. It is known that in these cases the stable conformation involves the eclipsing of a double bond by a single bond (25). Therefore, the following symbols are given if A>B>C and X>Y in the Sequence Rule (Scheme 2). 

Conformations about an sp2-sp2 bond can also be designated by the foregoing rule that is, the conformation s-cis in the older designation is sp, whereas the... 

This type of isomerism is possible not only in biphenyls, but also in compounds in which rotation about an sp2-sp2 bond is restricted and the two planes involving the sp2 center are noncoincident and substituted unsymmetrically. In addition to enantiomers, diastereomers are possible. There are some examples reported of restricted rotation about an aromatic ring-to-carbonyl bond or aromatic ring-to-nitrogen bond (28). Since these reports make no mention of diastereomers but only of enantiomers, they will receive no further mention here. 

The sp2 bonding nature of carbon is of great importance for its ability to form the Cgo soccer ball structure. Because Si does not form graphitic structures, which can be explained with standard model total energy calculations, we do not expect Si... 

There are two possible structures for simple alkyl radicals.176 They might have sp2 bonding, in which case the structure would be planar, with the odd electron in a p orbital, or the bonding might be sp3, which would make the structure pyramidal and place the odd electron in an sp3 orbital. Esr spectra of CH3 and other simple alkyl radicals as well as other evidence indicate that these radicals have planar structures.177 This is in accord with the known loss of optical activity when a free radical is generated at a chiral carbon.178 In addition, electronic spectra of the CH3 and CD3 radicals (generated by flash photolysis) in the gas phase have definitely established that under these conditions the radicals are planar or near-planar.179 Ir spectra of CH3 trapped in solid argon led to a similar conclusion.180... 

The most important factor is the type of bonding. Usually, sp3, sp2-vinylic. and sp2-aromatic C-F bonds have markedly different bond strengths. Greater difference is usually observed between sp3 bonds and both types of sp2 bonds than between sp2-vinylic and sp2-aromatic C-F bonds. 

In the reaction of /h/f-difluoro ketones with amines, a,/I-unsaturated / -amino ketones are formed, either by inline formation and imine enaminc tautomerization or via elimination of hydrogen fluoride and subsequent substitution of the other fluorine.22 Geminal sp2-bonded difluoridcs arc converted into ketenimines.12-49,89,90... 

For a simple MO picture of molecular electronic structure, the same procedures can be followed to classify the symmetries spanned by bond and lone-pair orbitals. For instance, we can envisage the electronic structure of BF3 as involving sp2 hybridization of the atoms. We would then have B and three F Is core orbitals, two sp2 and one jr symmetry lone pair on each F atom, and three sp2 bond orbitals for the three BF bonds. Our analysis above shows that the four core orbitals comprise 2 a and a doubly degenerate e orbital the in-plane lone-pairs transform as a, a 2, and 2 e orbitals the out-of-plane lone pairs transform as a"2 and e" and the three BF bonds as a and e. ... 

Formulation of ethene in this way suggests that it should be a planar molecule with H—C—H angles of 120°. Ethene is indeed planar, but its H—C—H angles are found to be 117°, rather than the 120° predicted for sp2 bonds. An explanation of this discrepancy using further electron-repulsion arguments will be discussed later in the chapter. 

---