Restrictions to free rotation

Up to this point it has been assumed that there are no restrictions to free rotations ateut bonds. This is physically unrealistic. Both macromolecules and minimolecules display bond rotational potentials that depend strongly upon the respective torsional angle. These restrictions can be incorporated into the expression for the unperturbed dimensions viz 

Here oy(=C, (l +cosT)f/(l —cost) ) is the steric factor, which cleariy also is a measure of the flexibility of the polymer chain. Usually 1-S t/ 2-S and for many carbon backbone polymers ff/ 2-0 (corresponding to C 8). 

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In other cases, the restriction to free rotation is imposed by the formation of a ring. For example, geometric isomerism may be exhibited by the three-membered ring of cyclopropane. 

Hindrances to free rotation of a steric or any other nature may of course restrict the values which the rotation angles 0 may assume. 

Organic dye nanoparticles of DHIA and DHBIA (the chemical structures are shown in Fig. 1) have been synthesized in THF/water mixed solvent by a reprecipitation method [32]. These dye molecules possess a 2-(2-hydroxyphenyl) benzothiazole (HBT) unit, which is known to be more stable as an enol imine form in the ground state and as a keto amine form in the excited state [32, 33] (Fig. 6). The nanoparticles exhibited the AIEE phenomenon mainly due to a restricted intramolecular motion, that is, impediment to free rotation of two end-substituted HBT units around single bonds. It is interesting to note that the... 

The possibility of optical isomerism arising from restricted rotation (atropisomerism) about the 2,2 -bond in 3-substituted 2-(2-naphthyl)-benzo[6]thiophenes has been recognized by Lamberton and McGrail.54 Schuetz and Ciporin164 have interpreted the UV spectra of six 3-aryl-benzo[6]thiophenes according to a theory of steric hindrance to free rotation about the pivot bond of the two aromatic rings, but the results are now invalid, since some of the 3-arylbenzo[6]thiophenes used in this study have been shown to be the corresponding 2-isomers (see Section IV, C). No attempt appears to have been made to resolve compounds of this type. 

From a structural point of view a number of different isomers could be present. In principal the three different hydroxyl groups in the ligand are able to interact with the central metal ion thus giving rise to three different constitutional isomers. The stereochemistry of the different configurational isomers of gadobutrol is best described in view of the stereochemistry of the parent compound Gd-DOTA. In this complex chirality results from the restriction of free rotation around the bonds of the ligand caused by the inclusion of the central metal ion. Thus four stereoisomers are generated upon complexation of Gd(III) by DOTA. 

The random flight chain has the simplest mathematical properties, but, unfortunately, also has the smallest degree of strnctnral similarity to real polymers. A more complex model considering the fixed valency angle (x) and the restrictions of free rotation has been proposed ... 

The apparent contributions of different functional groups and/or bond types to overall binding energies derived from the various studies reviewed above, are summarized in Table 20.1. Also included are corresponding values used or suggested for the overall loss of rotational and translational entropy, TAS, and the loss of conformational entropy resulting from restriction of free rotation, E. ... 

The tricyclic structure of ofloxacin and flumequine was designed to see how antibacterial activity would be affected by restricting the free rotation of the N-1 substituent. When a methyl group is introduced into the 3 position of the system, an asymmetric center is formed and earlier work has shown that the optical isomers of flumequine have markedly different antibacterial potency [54]. In an extension of that work, optical isomers of a new, more potent tricyclic derivative have been reported in 1985 [55, 56] and prepared as shown in Fig. 22. 

Another example of an herbicide where chirality is important is meto-lachlor (Dual , Syngenta), also in class 15 and used for corn and soybeans. Metolachlor has two chiral elements. The bond between the nitrogen and the phenyl group is restricted from free rotation and therefore there is a chiral axis. Chiral axes are not particularly common and it is coincidental that they appear in both this and the previous example. Metolachlor also has an asymmetric carbon at the methine carbon attached to nitrogen. 

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