Centric rings

The structure of 51-(THF)4 consists of two ort/io-phenylene indium dichloride moieties linked through a central Hg atom, giving rise to a pseudo-centric core (Fig. 16). The mercury atom is, as expected, linearly coordinated. Each indium atom is penta-coordinated in a trigonal-bipyramidal fashion with two THF molecules at the axial positions and two chloride ligands as well as a phenylene ring at the equatorial sites. 

Valves Centric Valves arranged in a ring or circle. 

Figure 16 (a) Diatom morphologies as depicted by Haeckel (1899-1904a). (b) Whole cell of the centric diatom Stephanodiscus showing the mineralized box and cover structure with a protruding ring of spines, 1,000X (source... 

According to the former the quinones still contain the true benzene ring with either three double or six centric bonds. The two oxygen atoms are only singly united with the benzene-carbon atoms, and are united to each other as in the peroxides. According to the second formula, the quinones do not contain the true benzene ring, but they are derived from a dihydrobenzene,... 

The crystal structures of (39) and a related iron complex have been determined.In each case the M—B bonds are longer than the M—C (ring) bonds. This non-centric co-ordination appeal s to be characteristic of... 

In the formula of Kekule the carbon atoms are joined alternately by double bonds. According to Claus, each carbon atom is in combination with the carbon atom opposite to it in the molecule. The so-called centric formula of Armstrong and Baeyer is meant to indicate that the combining powers of the carbon atoms which are not utilized in the ring formation and... 

These syntheses indicate that the relation between the structure of quinoline and that of naphthalene is similar to that between the structures of pyridine and benzene in both cases one CH group may be considered as replaced by a nitrogen atom. On account of the aromatic properties of quinoline the centric formula is usually assigned to it. The substitution-products are indicated, by assigning a number to each atom in the ring —... 

Baeyer showed that reduced benzene rings acquire aliphatic properties. At first he adopted a centric formula for benzene in place of Kekule s (see p. 804), but as a result of this mass of work he announced in 1892 that no conclusion as to the structure of benzene derivatives can be drawn from their behaviour on reduction . In the course of this work he prepared cis- and mw -derivatives of hexahydrophthalic acid and introduced the names cis-and tram-. 

A centric formula for benzene was proposed by Lothar Meyer, who says The model of Kekule is almost identical with the assumption of a ring-shaped linking and a free affinity on each atom ... 

As tools for measuring electron delocalization, we opt for the visual inspection of the occupied orbitals and especially the so-called domain-averaged Fermi-hole (DAFH) analysis and MCI. Both of these require availability of atomic overlap matrices, where the overlap between two molecular orbitals is obtained in atom condensed form. This requires the definition of an atom in the molecule and in this study we chose the Ftirshfeld-I method. Finally, we introduce ring current maps in the ipso-centric approach. 

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