Stretched bond isomerism

As discussed previously, such species are more tangibly understood to be intermediates on passing from covalent species to carbenium ions, rather than chemical entities, because their lifetime should be comparable to a bond vibration (Eq. 79). Stretched bond isomerism does not exist unless it is associated with a change in spin number, as in some inorganic compounds [266,267]. Since ionization is accompanied by a change in hybridization from sp3 to sp2, it is very unlikely that species with stretched covalent bonds could be considered real individual chemical species. Moreover, the chemoselectivity of reactions with alleged stretched covalent bonds is not significantly different than that in con-... 

Different types of chemical reactions involve different types of vibrational modes, e.g. dissociation reactions may be controlled by stretching vibrations, isomerizations by skeletal modes, and so on. The argument that infrared quanta are relatively energy-poor and infrared transitions generally have low absorption cross sections, especially if multiphoton excitation is required, limits the choice of suitable molecular transitions. With respect to these constraints the type of reaction chosen and described below was dissociation, involving molecules with maximal transition dipole moments, comparatively weak bonds to be broken, and vibrational excitation in the mid-infrared spectral range. 

Bond-Stretch Isomerism and the Reactivity of some Highly Strained Tricyclic Hydrocarbons towards Acids and Bases... 

This isomerization is a symmetry-forbidden one, since a level crossing such as the ones found in trimethylene and tetramethylene is found (22) (Fig. 13). It should be noted that, in all these cases, the two-minima structure due to level crossing might, in principle, be swept out by configuration interaction. If the barrier survives, however, the two isomers [4] and [5] can be called bond-stretch isomers. Another example is the isomerization [6] [7] (23). 

A transition complex which is consistent with the low energy of activation of the isomerization involves the simultaneous deformation (twisting) of the cyclobutene ring with the stretching of the carbon-carbon bond opposite the double bond ... 

As in the case of the bicyclo-octane the isomerizations must occur by a disrotatory process. It is clear that, owing to the rigid nature of these bicyclobutenes, considerable stretching of the bridgehead bond is necessary before appreciable twisting of the cyclobutene ring can... 

In the case where the macromolecular backbone is flexible, the axial tension may affect the molecular conformation. Flexibility of the main chain can be realized by bond rotational isomerism and minimizes the entropic penalty caused by the stretching of the main chain. As depicted in Fig. 21 on the right ... 

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