Benzene Nonplanar structure

Do you see a pattern forming The important point is not the number of conjugated atoms but the number of electrons in the n system. When they have 4 or 8 7t electrons, both benzene and cyclooctatetraene adopt nonplanar structures when they have 6 or 10 7t electrons, a planar structure is preferred. 

In this situation one might foresee either a strained planar structure with strong a bonding such as in benzene or an unstrained nonplanar structure with more or less isolated double bonds. In any case, we do not expect cyclooctatetra-ene to behave like benzene. Actually cyclob ctatetraene behaves like an unsaturated compound and possesses the "tub" configuration with alternating single and double bonds as shown below ... 

Spectroscopic methods have been successfully applied to the elucidation of some details of the fine structure of isoxazole derivatives. Thus IR spectra revealed steric hindrance in the case of some 3,4,5-trisubstituted isoxazoles for phenylisoxazoles this results in the nonplanarity of the benzene and isoxazole rings and decreasing mutual interaction. 

Polynuclear aromatic hydrocarbons formed by the angular fusion of benzene rings may adopt either planar or nOnplanar minimum energy conformations, as demonstrated by the circulenes 8-10. [6]Circulene (coronene 9), the earliest known member of the family, prefers a planar structure of symmetry since the presence... 

Since a calculation of the resonance energy of benzene by the valence bond method shows that the greater part of it is a result of the resonance between the two Kekule structures shown, we might suppose that its homologs would also have significant resonance stabilization energies. Such conclusions are at variance with experimental fact, however, since cyclobutadiene appears to be too unstable to have any permanent existence, and cyclooctatetraene exists as a nonplanar tetraolefin, having no resonance stabilization of the sort considered. 

In two recent works the phosphorus analogues of pyridones 109-111 (Scheme 58) have been studied computationally [317, 318], 110 is planar, while for 109 and 111 the planar structures are transition states with 11.0 and 19.8 kcal moH above the nonplanar minimum, respectively. The low barriers to inversion indicate that the planar structures have considerable aromaticity, which is shown by the NICS values exhibiting 67%, 51%, and 33% of the benzene value for 109,110, and 111, respectively [317], ELF analysis also indicates a certain aromaticity in these compounds [317], The bonding situation in 109 is similar to the cyclic phosphinocarbene isomer of phosphinine 112 [319] (Scheme 59), which was shown to be planar and aromatic. Stabilization of this carbene can be achieved by incorporating it to an anne-lated ring system 113, and also by the electron donor effect of an ylide formed with an additional o4,l5-P (114) [319],... 

The trigonal pyramidal molecular structure has been subjected to numerous interpretations. In Sc(btsa)3 the pyramidal ScN3 fragment was ascribed to crystal packing because the nonplanarity is lost in the gas phase [114a]. Electric dipole moment measurements confirmed a planarity in dilute benzene solutions [106a]. Zero dipole moments are also obtained for the larger lanthanide metals... 

On the other hand, cyclohexane, unlike benzene, is saturated, and its structure can readily be explained on the basis of a tetrahedral carbon atom. Hermann Sachse in 1890 deduced that cyclohexane is a cyclic hexamer. It is, however, nonplanar with several forms for the puckered rings existing in solution in an equilibrium. These ideas, surprisingly, lay dormant for many years until Ernst Mohr ° reintroduced them in 1915. 

The annellation of a benzene ring to the 5 and 6 positions of the benzopyran moiety in CH2, instead of the pyridine ring in CHI, does not essentially affect the structure of the naphthopyran fragment. The pyran rings in CHI and CH2 are nonplanar with folding along O- -C(2) and O- -0(3) in a similar manner as in SPs and SOs. The Cspiro-0 bond in CHI and CH2 is elongated, and the 0-CPh bond is shortened (Table 5). 

Compounds in which E = Si, Ge, or Sn and M = Si are similar in structure in that the heterocyclic ring has a boat conformation and the molecules on the whole are nonplanar <840M255, 840M1755). In fact, the silanthrene (E = M = Si in Table 7) retains this structure when it forms a cationic chromium(I) complex in 9,9,10,10-tetramethyl-9,10-disiladihydroanthracene(benzene)chromium(I) tetraphenylborate <88X41627-02). The chromium atom is coordinated in a sandwich fashion between... 

For this reason, the bond parameters for oxepin and other monocyclic derivatives have not been determined. The spectroscopic data show that oxepins have a polyolefinic structure with localized C=C bonds. Oxepins exist in a nonplanar boat conformation that equilibrates with inversion as in la and lb [2]. This process is slowed down by fusion with benzene rings. 13-Methyltribenzoxepin-l 1-carboxylic acid 6 can be separated into its enantiomers which racemize with an energy barrier AG 20.s 86.9 kJ mol-i [3]. 

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