Electron bond, sigma

Gassman, P.G. Fentiman Jr., A.F. J. Am. Chem. Soc., 1969, 91, 1545 1970, 92, 2549. For a discussion of the use of the tool of increasing electron demand to probe neighboring-group activity by double bonds, sigma bonds, and aryl rings, see Lambert, J.B. Mark,... 

Electrocyclic reactions are examples of cases where n-electron bonds transform to sigma ones [32,49,55]. A prototype is the cyclization of butadiene to cyclobutene (Fig. 8, lower panel). In this four electron system, phase inversion occurs if no new nodes are formed along the reaction coordinate. Therefore, when the ring closure is disrotatory, the system is Hiickel type, and the reaction a phase-inverting one. If, however, the motion is conrotatory, a new node is formed along the reaction coordinate just as in the HC1 + H system. The reaction is now Mobius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mobius-Hiickel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

In the case of two valence electrons there is hardly any difference between the localized orbital and the canonical valence orbital, except for the fact that the localization has separated the valence shell somewhat from the other shells. — In the case of four valence electrons, the sigma bonding and the sigma antibonding canonical orbitals yield two equivalent localized orbitals which resemble distorted atomic (2s) orbitals on each of the two atoms. They are precursors of what will be seen to be sigma lone pairs and are denoted by oC and ok . The absence of a bond can be ascribed to the nonbonded repulsion between these orbitals. This corresponds to the case of the unstable Be2 molecule. —... 

The protonated paraffin can be envisioned as a mixture of sigma-protonated species containing three-center two-electron bonds that can be conveniently represented as a penta-coordinated carboniun ion [9]. Products are formed by collapse of the latter. Since... 

A e-bond (sigma-bond) forms when the bonding pair of electrons are localized directly between the two bonding atonts. Since the electrons in a o-bond are as close as possible to the two sources of positive charge (the two nuclei), a o-boml has the lowest energy and is the most stable form of covalent bond. Thus o-bonds are strong. A o-bond is always the first type of covalent bond to be formed between any two atoms a single bond must be a c-bond. 

Alkanes, also called paraffins or aliphatic hydrocarbons, are hydrocarbons in which the C atoms are joined by single covalent bonds (sigma bonds) consisting of two shared electrons (see... 

The carbon-hydrogen and carbon-carbon bonds have the same general electron distribution, being cylindrically symmetrical about a line joining the atomic nuclei (see Fig. 3.1) because of this similarity in shape, the bonds are given the same name, a bonds sigma bonds). 

For clarity, electrons in sigma bonds are not shown, (a) carbon and oxygen are both sp hybridized... 

Valence electron sets (lone pairs and electrons in sigma bonds) are housed, at least in part, in hybrid orbitals. This means that an atom surrounded by three electron sets uses three hybrid orbitals, as in formaldehyde. There, the central carbon atom uses hybrid orbitals in forming the C-H single bonds and the sigma portion of the C=0 double bond. The... 

Figure 2. Molecular orbital energy diagrams for (a) FI2 showing both electrons in the bonding sigma MO and (b) Fle2 in which two of the electrons are in the antibonding sigma-starred MO...

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