Ring folding

Each of these tin atoms acts as a Lewis acid, with two neighboring nitrogen atoms functioning as electron donors. The ten-membered ring folds up to generate a four-membered cycle bridged at the 1,3- and 2,4-positions. The structure, which can easily be deduced from high-resolution NMR spectra, is displayed in Fig. 14. 

In contrast to the ring folding observed in 1,5-(R2P)2N482, the 7t -7i interaction in [8204] is an intermolecular process involving the association of two species. An analogy in inorganic heterocycles Is provided by the thiatriazinyl... 

Fig. 1. (a) A regular planar benzene ring. (b) The same ring folded about the dotted line through an angle 20. 

Fig. 17. 3,4-Benzophenanthrene a perspective drawing given by Reid (1957) based on the results of Herbstein and Schmidt s analysis. The diagonals across which the benzene rings fold are shown dotted the circles represent the van der Waals radii for hydrogen.

Compounds 16 and 17 were also subjected to X-ray crystallographic analysis <1997JMC381> Both molecules possess a crystallographic inversion center and adopt small chair-like distortions, the dithiole/diselenole rings folding by 7.8 (16) and 7.0° (17) along the S(Se)---S(Se) vectors. Molecules in the crystals are arranged in planar layers with interplanar separations of ca. 3.68 (16) and 3.64 A (17). 

Table 1. Selected bond lengths in Angstroms, bond angles and ring folding a in d rees, and relative energies in kJ/mol for structures 1 to 3 obtained at the B3LYP/6-31+G(d) level.

In these derivatives, the chain folds in such a way that the cationic head becomes placed under the aromatic ring and can establish a typical donor-acceptor interaction with the IT cloud of the aromatic ring (folding effect). ... 

The solid state structures of CD9, CDio, CDh and CD26 have been reported. The structure of CDg exhibits a distorted elliptical boat-like shape, but it retains a similar structure to regular CDs. CDio and CDh also exhibit a more elliptical macrocyclic ring folded in a saddle-like shape. The structure of CD26 has channellike cavities composed of two short V-amylose helices in anti-parallel orientation, and its structure is very different from the regular CDs [1,7] (Fig. 1.3). 

Due to the discovery of the replacement of the cobalt coordinated 5,6-dimethylimidazole (DMB) base by a protein-derived imidazole in several Bi2-dependent enzymes (see later), the analysis of Co -cyano-imidazolylcob-amide (8) [31] was of particular interest. The less bulky and more nucleophilic imidazole base of 8 caused a number of structural differences. The corrin ring fold angle of 8 decreased to 11.3° and the axial Co-N bond shortened (from 2.011 A in 1 to 1.968 A in 8). In addition, the base tilt of 8 (i.e., half the difference between the two Co - N - C angles to the coordinating base) decreased to practically zero, within experimental error. In all crystal structures of 5, 6 -dimethylbenzimidazoyl-cobamides a tilt of about 5° is found [36], which appears to be an inherent property of the cobalt-coordinated DMB. 

When low-ash filter ring is used, an alternative procedure to determine the amount of filler stuck to it can be used. This is to burn the dried filter ring folded over the aggregate. 

In the 14-membered ring, folding to give a cw-complex 3.10 is usually only observed with the trans V configuration... 

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