Planar rings

Fig. 5. Schematic representation of arrays of carbon nanotubes with a common tubule axial direction in the (a) tetragonal, (b) hexagonal I, and (c) hexagonal II arrangements. The reference nanotube is generated using a planar ring of twelve carbon atoms arranged in six pairs with the symmetry [16,17,30].

Perhaps the most notable difference between S-N and N-O compounds is the existence of a wide range of cyclic compounds for the former. As indicated by the examples illustrated below, these range from four- to ten-membered ring systems and include cations and anions as well as neutral systems (1.14-1.18) (Sections 5.2-5.4). Interestingly, the most stable systems conform to the well known Htickel (4n -1- 2) r-electron rule. By using a simple electron-counting procedure (each S atom contributes two electrons and each N atom provides one electron to the r-system in these planar rings) it can be seen that stable entities include species with n = 1, 2 and 3. 

The [S3N3]" cation is of interest as an example of an antiaromatic eight TT-electron system (Section 4.4). Ab initio molecular orbital calculations indicate that a triplet cation, with a planar ring, is more stable than the singlet cation. The [S3N3]" cation has been obtained as the norbomene adduct 5.16, but salts of the free cation have not been isolated. ... 

Consistent with its aromatic character, the planar ring 12.20 (R = Ph) has high thermal stability and is chemically unreactive. For example, it does not react with n-butyllithium, m-chlorobenzoic acid or N204. ° ... 

The Haworth representation implies a planar ring. However, monosaccharides assume conformations that are not planar these may be represented by Haworth conformational formulae. The nomenclature of conformations is described in 2-Carb-7. For example, (5-D-glucopyranose assumes a chair conformation ... 

The planar cyclic Pj anion isoelectronic with cyclopentadienyl anion has been prepared in the form of M Pj" salts (M = Li, Na) by Handler et al. [49], The penta-phosphole anion Pj" favors planar geometry [50] while the most stable structure of P/ is square-pyramidal [51], The negatively charged pentamers Sb and Bi are planar rings [52, 53],... 

Plastoquinone, whose line structure appears in the margin, has ten double bonds. Unlike the bonds in the tail of retinal, however, the bonds in the long tail of plastoquinone are not delocalized because s p -hybridized carbon atoms separate them. The delocalized 7t system of plastoquinone is its planar ring of six carbon atoms, with two of the carbon atoms double-bonded to outer oxygen atoms. 

Symmetrical hexacoordinated Pcs with planar rings have been the focus of by several groups in the past 5 years [26-35], In order to display clearly the influence of... 

The number of X-ray structures published since the publication of CHEC-II(1996) has increased, underlining the importance of this technique in structure elucidation. The structure of a number of 1,2,4-thiadiazoles and 1,2,4-thiadiazolidines has been determined by X-ray techniques and they are listed in Table 1. The first preparation of an A7-oxide derivative of a 1,2,4-thiadiazole 2 has been reported. The X-ray structure of compound 2 shows that it has a nearly planar ring this conformation is stabilized by hydrogen bonding with the carboxamide group <1999J(P1)2243>. 

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