Aromatic bond-length

Compd Symmetry Basis Set Deformation Angle" a (deg) jS(deg) Aromatic Bond Length (A) Ref. 

In Table 1-9 we have collected only the 7r-bond orders calculated by allvalence-electrons methods and compared their values with those deduced from experimental bond lengths. Both data are indicative of an aromatic molecule with a large dienic character. The 2-3 and 4-5 bonds especially present a large double-bond character, whereas both C-S bonds are relatively simple. 

The sydnones may be represented by structures (123a-d), of which the zwitterionic structure (123a) most clearly implies an aromatic sextet. The diamagnetic susceptibility exaltation for Af-phenylsydnone of ll.Ox 10 cm moP is comparable with the corresponding value for pyrrole (10.2x10 ). 3-p-Bromophenylsydnone (123 R = H, R = p-bromophenyl) is essentially planar however, the O—N bond and 0(1)—C(5) bond lengths are not very different from normal single bond distances. 

Heterocyclics with seven and more ring members display an enormous variety of shapes. Bond lengths are often close to those of open chain counterparts, but bond angles can be greatly different. Aromaticity is possible where the conditions of planarity and Hiickel s rule are met, but the majority of fully unsaturated large heterocycles are not aromatic (see below). 

Oxepin, 4-ethoxycarbonyl-2,3,6,7-tetrahydro-synthesis, 7, 578 Oxepin, 2-methyl-enthalpy of isomerization, 7, 555 Oxepin, 2,3,4,5-tetrahydro-reduction, 7, 563 synthesis, 7, 578 Oxepin, 2,3,4,7-tetrahydro-synthesis, 7, 578 Oxepin, 2,3,6,7-tetrahydro-oxidation, 7, 563 reduction, 7, 563 Oxepin-2,6-dicarboxylic acid stability, 7, 565 Oxepinium ions synthesis, 7, 559 Oxepins, 7, 547-592 antiaromaticity, 4, 535 applications, 7, 590-591 aromatization, 7, 566 bond lengths and angles, 7, 550, 551 cycloaddition reactions, 7, 27, 569 deoxygenation, 7, 570 dipole moment, 7, 553 disubstituted synthesis, 7, 584... 

The NMR spectrum of this compound shows a diamagnetic ring current of the type expected in an aromatic system. X-ray crystal structures of 1 and its carboxylic acid derivative 2 are shown in Fig. 9.2. Both reveal a pattern of bond lengths very similar to that in naphthalene (see p. 534). ... 

---