Strain-induced bond localization

FIGURE 6.14 Oxidation of PMC-derivatives with different ring strains to mixtures of two possible ortho-qaiaoae methides the oxidation behavior and the ratio of the formed o-QMs agreed fully with the theory of strain-induced bond localization (SIBL). 

Rosenau, T. Ebner, G. Stanger, A. Perl, S. Nuri, L. From a theoretical concept to biochemical reactions strain-induced bond localization (SIBL) in oxidation of vitamin E. Chem. Eur. j. 2005, 11(1), 280-287. 

The idea behind bond localization in annelated benzenes is exemplified by compound 66, tricyclopropabenzene. Of the two standard Kekule structures of 66, one places the double bonds within the three-member ring (endo) and the other places the double bonds outside (exo) of the small ring. Since cyclopropene is much more strained than cyclopropane, the avoidance of strain energy suggests that the double bonds might be localized into the exo positions. Compounds 66 and 67 are the prime test subjects for this idea of strain-induced bond localization (SIBL). ... 

Stanger, A. Strain-induced bond localization. The heteroatom case, 7. Am. Chem. Soc. 1998,120, 12034-12040. 

Cohen and Benson s) group additivity values polysubstituted benzene derivatives density functional theory strain-induced bond localization... 

The Mills-Nixon hypothesis had, as its foundation, certain differences in the chemical behaviour of indan (3) and tetralin (4) from which a localization of the aromatic 7r-bonds was predicted to occur in the direction depicted by la rather lb. The original experimental evidence upon which the effect was based was shown to be erroneous, but calculations at various levels of theory indicated that aromatic bond localization should exist and become more pronounced as the size of the annelated ring decreases In essence one can recognize that the structure of benzene has a symmetry such that both Kekule structures must contribute equally. With Q tetralin (4) (and the lower homologues) no such symmetry requirement exists and ring annelation could induce bond length alternation within the arene nucleus. As the strain imposed by the fused ring increases, the Mills-Nixon effect should increase. The hypothesis has been the subject of considerable discussion and the controversy is far from settled. 

Although a honeycomb lattice theoretically consists of sp2 atoms, the carbon s ability to represent intermediate states of hybridization leads to another kind of defect to counterbalance the strain energy induced by high curvature. This so-called rehybridization results in a higher n-character of the C-C bonds [24]. Furthermore, local sp3 hybridization can be induced though chemical treatment, such as after thermal elimination of functional groups. 

A general theory of dichroism induced by strain in polymeric networks Is developed by adaptation of methods developed earlier for treating strain birefringence. It is generally applicable to dichrolc bands associated with any specified conformation involving sequences of one or more consecutive bonds. The transition dipole moment is introduced in the local framework of the skeletal bonds associated therewith. Possible differences in transition moments for various conformations and repeat units are taken into account. Numerical calculations for PE chains show gauche bonds, rather than trans, to be more favorably oriented with respect to the chain vector r. 

Let us return to Siegel-cyclohexatriene (30) and inspect the relationship between the geometries of the ground state and the twin excited states. The model in Figure 11a (also eq 11) predicts that an attractive 77-curve, displaced to the left of the a-minimum, will reduce the bond alternation induced by the er-strain. Since the 77-curve of the twin excited state is attractive, this state will lose the bond alternation of the ground state and the benzene nucleus will regain its local D%h symmetry. 

This paper presents results from a study of assemblies composed of glass fibre reinforced epoxy composites. First, tests performed to produce mixed mode fracture envelopes are presented. Then results from tests on lap shear and L-stiffener specimens are given. These enabled failure mechanisms to be examined in more detail using an image analysis technique to quantify local strain fields. Finally the application of a fracture-mechanics-based analysis to predict the failure loads of top-hat stiffeners with and without implanted bond-line defects is described. Correlation between test results and predictions is reasonable, but special attention is needed to account for size effects and micro-structural variations induced by the assembly process. 

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