Loosen rearrangement

Let us first consider kinetic energy of translation for the sake of simplicity. At the moment of collision most of this kinetic energy may be transferred to the interior of the molecules, and the result may be a sufficient loosening of the two molecules of HI to permit their rearrangement to H2 + I2. Activated reaction products are thus produced, which in due season lose their excess energy by collision or otherwise. 

Fig. 10.37. Changes in bond length at the TS for [3,3]-sigmatropic rearrangement of phenyl-substituted 1,3,5-hexatrienes. (a) Top cumulative loosening of the TS with one, two, and four phenyl groups in active positions, (b) Center unsubstituted system, (c) Lower Left cumulative tightening of TS by 2- and 2,5-phenyl substitution, (d) Lower Right, unmatched 2- and 4-substituents are competitive, leading to little structural change. Reproduced from J. Am. Chem. Soc., 122, 7456 (2000), by permission of the American Chemical Society.

In terms of the discussed approach to the quantitative description of carbocation rearrangements it proved possible to cover by general correlations the MT of different migrants and its dependence on the electronic structure of the MCR of the rearranging carbocation The basis of these correlations is a simple model and the corresponding terms Ej which describes the loosening of the O —R bond, and E2 which describes the affinity of the migrant to the carbocation centre. 

AB corresponds to an immobile fixed bed, the pressure drop across which increases as the velocity increases. From B to C, across the blunt maximum caused by interlocking of particles followed by their rearrangement, the bed, if it has been densely packed, loosens up with each incremental velocity increase Anally, at and beyond C, it continues to expand in the mobile fluidized state, during which the pressure drop remains constant. The constant frictional pressure drop is then given by... 

On the basis of these assignments and of his X-ray studies Sikka proposes that during fatigue weaker Van der Waal s and possibly some backbone bonds are broken. This has a twofold effect. The loosening of the structure permits on one hand the PS chains to locally rearrange themselves in a more perfect manner and to reduce the average intraphenyl and interchain distances. The local rearrangements are, on the other hand, accompanied by an increase in free volume in interstitial areas between the domains of improved order. 

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