84 DELOCALIZED CHEMICAL BONDING
molecule undergoes aromatic substitution276 and is diatropic.280 The outer protons are found at 8.14–8.67 d, while the CH3 protons are at 4.25 d. Other nonplanar aromatic dihydropyrenes are known.281 Annulenes 111 and 112 are also diatro- pic,282 although X-ray crystallography indicates that the p periphery in at least 111 is not quite planar.283 However, 113, in which the geometry of the molecule greatly reduces the overlap of the p orbitals at the bridgehead positions with adja- cent p orbitals, is definitely not aromatic,284 as shown by NMR spectra278 and X-ray crystallography, from which bond distances of 1.33–1.36 A ˚ for the double bonds and 1.44–1.49 A ˚ for the single bonds have been obtained.285 In contrast, all the bond distances in 111 are
1.38–1.40 A ˚ .283 Another way of eliminating the hydrogen interferences of [14]annulene is to intro- duce one or more triple bonds into the system, as in dehydro[14]annulene (114).286 All five known dehydro[14]annulenes are diatropic,and 87can benitrated orsulfonated.287 The extra electrons of the triplebond do notform partofthearomaticsystem, butsimply
84 DELOCALIZED CHEMICAL BONDING
molecule undergoes aromatic substitution276 and is diatropic.280 The outer protons are found at 8.14–8.67 d, while the CH3 protons are at 4.25 d. Other nonplanar aromatic dihydropyrenes are known.281 Annulenes 111 and 112 are also diatro- pic,282 although X-ray crystallography indicates that the p periphery in at least 111 is not quite planar.283 However, 113, in which the geometry of the molecule greatly reduces the overlap of the p orbitals at the bridgehead positions with adja- cent p orbitals, is definitely not aromatic,284 as shown by NMR spectra278 and X-ray crystallography, from which bond distances of 1.33–1.36 A ˚ for the double bonds and 1.44–1.49 A ˚ for the single bonds have been obtained.285 In contrast, all the bond distances in 111 are
1.38–1.40 A ˚ .283 Another way of eliminating the hydrogen interferences of [14]annulene is to intro- duce one or more triple bonds into the system, as in dehydro[14]annulene (114).286 All five known dehydro[14]annulenes are diatropic,and 87can benitrated orsulfonated.287 The extra electrons of the triplebond do notform partofthearomaticsystem, butsimply
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