Topological isomer

In order to prolong the pot life of the system, a reduction in polydispersity is quite important. Shape polydispersity of the polymer, which is determined by the distribution of topological isomers, is expressed through a quantity Frechet and coworkers originally termed degree of branching [78], which reaches unity for... 

Figure 2.1 A square scheme illustrating a redox-driven intramolecular motion. Species with an asterisk (Ox and Red ) are metastable and tend to rearrange to their stable topological isomer (Ox and Red).

Naturally occurring or artificially prepared supercoiled DNA molecules can often be separated by electrophoresis into about ten forms, each differing from the other by one supercoiled turn and by ALA = 1 (Fig. 5-20). The relative amounts of these topological isomers form an approximately Gaussian distribution. The isomers apparently arise as a result of thermal fluctuations in the degree of supercoiling at the time that the circles were enzymatically closed.202... 

Scheme 11 Topological isomers possible for linpen and 2,3,2,3,2-hex. The R,S designation is applicable to linpen and the...

The topologies of various cryptands are shown in Fig. 1. Considering the macrobicyclic ligands, each bridgehead may be turned either inward or outward with respect to the molecular cavity (11,12,22). This leads to three topological isomers exo-exo, exo-endo, and endo-endo. Crystal structures of one representative of each have been described. [2.2.2] adopts the endo-endo conformation with both nitrogen lone pairs directed into the cavity, while its bis-borane derivative is exo-exo (23) and the mono-borane derivative of 11.1.11 has the exo-endo form (24) (Fig. 2), with the —BH 1 group outside the cavity. 

When short, flexible diamine a was used as a subcomponent in helicate formation, as shown on the left side of Scheme 1.7, only one topological isomer of product was observed twisted macrocycle 5. This diamine is not long enough to loop around the phenanthroline to form a catenated structure [28]. 

Note that this same part of the canonical name will be valid whether or not this 108 atom chain has a topologically restraining loop or is straight. Graph theory can not distinguish between such topological isomers. Also note that there are no a bonds between one part of the chain and any other part of it. How the nomenclature is able to deal with such ideas will be addressed in a follow-on article. 

We believe it important to stress out that the expression topological isomers should never be used instead of constitutional isomers , since, as we will see later, the expression topological stereoisomers is used only for stereoisomers differing by extrinsic topology... 

Let us consider another way of connecting molecules without chemical bond formation. If through a macrocy-clic molecule passes a linear one with bulky terminal groups, the system thus obtained represents a topological isomer of the type dumbbell in a ring (Fig. 31 ). Such... 

Also known are other types of topological isomers which draw the researchers attention, but they exist only on paper so far. Catenanes and rotaxanes are the only topological structure classes which have been investigated experimentally. 

Syntheses of tomorrow. The search for various topological isomers in nature and the efforts of their laboratory synthesis continue. Many hypotheses and reaction charts... 

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