Double bond distortions

Whereas FMO theory correctly predicts the regioselectivity for cycloadditions in simple alkyl-substituted olefinic systems,51,58 extension of similar calculations for cycloadducts (7a,b-lla,b)120 predicts the formation of regioisomer a, although, except in the case of 7 and 8, the b isomer is the predominant one. The differences between prediction and experiment in stereoselectivity have been attributed primarily to double bond rehybridization arising from double bond distortion in bridgehead olefins,142 which also explains their enhanced reactivity.96,120 Also double-bond deformation that will alter the normal mixing of alkyl substituent orbitals with localized rc-bond orbitals may explain the unexpected formation of 8b.120 Attempts to explain the formation of the b isomers, based on a two-step diradical mechanism, also have failed.120... 

A common example of the Peieds distortion is the linear polyene, polyacetylene. A simple molecular orbital approach would predict S hybddization at each carbon and metallic behavior as a result of a half-filled delocalized TT-orbital along the chain. Uniform bond lengths would be expected (as in benzene) as a result of the delocalization. However, a Peieds distortion leads to alternating single and double bonds (Fig. 3) and the opening up of a band gap. As a result, undoped polyacetylene is a semiconductor. 

For all three diallyl phthalate isomers, gelation occurs at nearly the same conversion DAP prepolymer contains fewer reactive allyl groups than the other isomeric prepolymers (36). More double bonds are lost by cyclisation in DAP polymerisation, but this does not affect gelation. The heat-distortion temperature of cross-linked DAP polymer is influenced by the initiator chosen and its concentration (37). Heat resistance is increased by electron beam irradiation. 

The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffiaction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. ... 

Compare geometries of the cis and trans cycloalkenes. Are the double bonds incorporated into the trans compounds significantly more distorted than those incorporated into the analogous cis cycloalkenes Consider carbon-carbon bond lengths and the twisting and/ or puckering of the double bond. Are any distortions greater in trans-cycloheptene than in trans-cyclooctene ... 

The niobium atom has a slightly distorted octahedral coordination. Interatomic distances between the niobium atom and the two oxygen atoms in trans positions, O-Nb-O are 1.81 and 2.14 A. The niobium atom is shifted from the base plane of the octahedron by 0.23 A, and this shift, in adjacent chains, is in opposite directions. Pakhomov and Kaidalova [204] concluded that the shorter Nb-O bond (1.81 A) is an intermediate between a single and double bond. 

The results of our calculations based on both the static and dynamic theories show that most of the nonbenzenoid cyclic conjugated systems examined exhibit in a greater or lesser degree a marked double-bond fixation. The static theory indicates that even in benzene there exists a hidden tendency to distort into a skewed structure and that such a tendency is actually realized in [4n-f-2] annulenes larger than a certain critical size. In nonalternant hydrocarbons bond distortion is a rather common phenomenon. Fulvenes, fulvalenes and certain peri-condensed nonalternant hydrocarbons undergo a first-order bond distortion, and... 

Multiple bonds can be treated as ring structures with bent bonds. The distortions dealt with in the preceding paragraph must be taken into account. For example, in ethylene every C atom is surrounded tetrahedrally by four electron pairs two pairs mediate the double bond between the C atoms via two bent bonds. The tension in the bent bonds reduces the angle between them and decreases their repulsion toward the C-H bonds, and the HCH bond angle is therefore bigger than 109.5°. 

The one-dimensional chain of hydrogen atoms is merely a model. Flowever, compounds do exist to which the same kind of considerations are applicable and have been confirmed experimentally. These include polyene chains such as poly acetylene. The p orbitals of the C atoms take the place of the lx functions of the H atoms they form one bonding and one antibonding n band. Due to the Peierls distortion the polyacetylene chain is only stable with alternate short and long C-C bonds, that is, in the sense of the valence bond formula with alternate single and double bonds ... 

To sum up, the difference between "triple bonding in the diatomic species SiO or SiS and "double" bonding in Si02 or SiS2 is small with respect to small distortions from the equilibrium distance (f and r), while they differ for large separations of the atoms involved (bond energy). This... 

In the molecule of 4-methylene-3-borahomoadamantane derivative 79, the structure of which was determined by X-ray analysis, the six carbon atoms of the triene system, the two boron and two silicon atoms all lie in one plane within experimental error (mean deviation 1.4 pm). The boron atoms deviate from the trigonal-planar geometry, since the sum of bond angles around the atoms is only 355.8° instead of 360°, as usually encountered in triorganoboranes. Considerable distortions of the bond angles at the terminal C-C double bond occurs in the vicinity of the boron atoms B-C(4)-C(ll) 130.60(19)° and B-C(4)-C(5) 107.38(17)° <2002CEJ1537>. 

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