Bond, sigma

Figure C3.2.7. A series of electron transfer model compounds with the donor and acceptor moieties linked by (from top to bottom) (a) a hydrogen bond bridge (b) all sigma-bond bridge (c) partially unsaturated bridge. Studies with these compounds showed that hydrogen bonds can provide efficient donor-acceptor interactions. From Piotrowiak P 1999 Photoinduced electron transfer in molecular systems recent developments Chem. Soc. Rev. 28 143-50.

MMVB is a hybrid force field, which uses MM to treat the unreactive molecular framework, combined with a valence bond (VB) approach to treat the reactive part. The MM part uses the MM2 force field [58], which is well adapted for organic molecules. The VB part uses a parametrized Heisenberg spin Hamiltonian, which can be illustrated by considering a two orbital, two electron description of a sigma bond described by the VB determinants... 

The H3 and H4 systems were discussed above. Another type of sigma bonds involves a p orbital lying along the reaction coordinate, as, for example, in reaction (15) (Fig. 8). 

A similar situation holds foi a molecule containing a tetrahedral carbon is shown in (Figure 16). The reaction converting one enantiomer to another, is formally equivalent to the exchange of two sigma-bond electr on pair s, and... 

UNSUBSTITUTED BUTADIENE. Butadiene anchors were presented in Figures 1(3) and 13. The basic tetrahedral character of the conical intersection (as for H4) is expected to be maintained, when considering the re-pairing of four electrons. Flowever, the situation is more complicated (and the photochemistiy much richer), since here p electrons are involved rather than s electrons as in H4. It is therefore necessary to consider the consequences of the p-orbital rotation, en route to a new sigma bond. 

Rhodacarborane catalysts have been immobilized by attachment to polystyrene beads with appreciable retention of catalytic activity (227). A 13-vertex /oj iJ-hydridorhodacarborane has also been synthesized and demonstrated to possess catalytic activity similar to that of the icosahedral species (228). Ak-oxidation of closo- >(2- P((Z [) 2 - i- > l[l-Bih(Z, results in a brilliant purple dimer. This compound contains two formal Rh " centers linked by a sigma bond and a pak of Rh—H—B bridge bonds. A number of similar dimer complexes have been characterized and the mechanism of dimer formation in these rhodacarborane clusters have been studied in detail (229). 

The carbon—carbon double bond is the distinguishing feature of the butylenes and as such, controls their chemistry. This bond is formed by sp orbitals (a sigma bond and a weaker pi bond). The two carbon atoms plus the four atoms ia the alpha positions therefore He ia a plane. The pi bond which ties over the plane of the atoms acts as a source of electrons ia addition reactions at the double bond. The carbon—carbon bond, acting as a substitute, affects the reactivity of the carbon atoms at the alpha positions through the formation of the aHyUc resonance stmcture. This stmcture can stabilize both positive and... 

Diphenylcyclopropenone (76) undergoes a true 1,2 cycloaddition with alicyclic enamines followed by the breaking of sigma bonds in the intermediate (35,56). For example, when diphenylcyclopropenone (76) is... 

The key step in this sequence, achieved by exposure of 46 lo a mixture of sulfuric acid and acetic anhydride, involves opening of the cyclopropane ring by migration of a sigma bond from the quaternary center to one terminus of the former cyclo-l>ropane. This complex rearrangement, rather reminiscent of the i enone-phenol reaction, serves to both build the proper carbon. keleton and to provide ring C in the proper oxidation state. 

To answer this question, it is necessary to consider the shape or spatial distribution of the orbitals filled by bonding electrons in molecules. From this point of view, we can distinguish between two types of bonding orbitals. Ihe first of these, and by far the more common, is called a sigma bonding orbital. It consists of a single lobe ... 

Bonding orbitals in ethylene (CH2=CH2) and acetylene (CH=CH). The sigma bond backbones are shown in blue. The pi bonds (one in ethylene and two in acetylene) are shown in red. Note that a pi bonding orbital consists of two lobes. 

You may recall that we discussed the bonding in ethene in Chapter 7. The double bond in ethene and other alkenes consists of a sigma bond and a pi bond. The ethene molecule is planar. There is no rotation about the double bond, since that would require breaking the pi bond. The bond angle in ethene is 120°, corresponding to sp2 hybridization about each carbon atom. The geometries of ethene and the next member of the alkene series, QHg, are shown in Figure 22.6. 

The most important alkyne by far is the first member of the series, commonly called acetylene. Recall from Chapter 7 that the C2H2 molecule is linear, with 180° bond angles. The triple bond consists of a sigma bond and two pi bonds each carbon atom is sp-hybridized. The geometries of acetylene and the next member of the series, C3H4, are shown in Figure 22.7. 

Valence bond theory (Chapter 7) explains the fact that the three N—O bonds are identical by invoking the idea of resonance, with three contributing structures. MO theory, on the other hand, considers that the skeleton of the nitrate ion is established by the three sigma bonds while the electron pair in the pi orbital is delocalized, shared by all of the atoms in the molecule. According to MO theory, a similar interpretation applies with all of the resonance hybrids described in Chapter 7, including SO S03, and C032-. 

Sigma bond A chemical bond in which electron density on the intemudear axis is high, as with all single bonds. In a multiple bond, one and only one of the electron pairs forms a sigma bond, 188-189... 

Single bond A pair of electrons shared between two bonded atoms, 167 Six-coordinate metal complex, 413-414 Skeleton structure A structure of a species in which only sigma bonds are shown, 168... 

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