Alternating bond lengths

Cyclooctatetraene, CgHg, has a D2g tublike structure with four rather isolated double bonds. Cyclooctatetraenes can be the products of the catalytic cyclotetramerization of alkynes, and cyclobutadienes may be the intermediates. The BN homologues of cyclooctatetraenes have been known since 1962 (67). Like cyclooctatetraene, molecules of [(SCN)BNtBu]4 were shown to have a tublike ring structure of S4 symmetry with alternating bond lengths of 140 and 146 pm, the shorter ones perpendicular to the direction of the S4 axis (68). 

The strongly colored planar fused bicyclic system (25) is quinonoid with distinctly alternating bond lengths 1.33-1.35 A and 1.42-1.44 A. A strong 7r-stacking interaction in the molecule (28) between tolyl and phenanthroline planes is evident (interplane distance is 3.3 A), as well as between tolyl and cyclopentadienyl planes (the distance is 3.61 A) which results in columns of stacked molecules. 

Following these ideas, Table 15.10 shows results of 6-3IG calculations of the TV system of normal benzene and benzene distorted to have alternating bond lengths matching standard double and single bonds, which we will call cyclohexatriene. 

Fully unsaturated seven-membered heterocyclics have alternating bond lengths and are normally in boat conformations. Ring inversion barriers are 42.7 kJ mol-1 for 3-methyl-3ff-azepine and 35.6 kJ mol-1 for 3//-azepin-2-one (CHEC 5.16.2.3). The barriers for oxepin and thiepin are somewhat lower. Annulation can introduce large conformational barriers, to the extent of making possible the resolution into enantiomers of a tribenzoxepin (71CB2923). 

In accord with the Hiickel rule of 4/t + 2 electrons, both cyciobutadiene and cyclooctatetraene (cot) are nonaromatic. Cyclooctatetraene contains alternating bond lengths and has a tub-shaped conformation ... 

Fig. 16.30 Eighl-membered sulfur-nitrogen rings (a) molecular structure of S4hl4 (bl diagrammatic structure of illustrating alternating bond lengths. [From Sharmu. 8. D. Donohue, J. Acta Crystalhgr. 1963. 16, 891. Reproduced with permission.]...

The geometric aromaticity criteria are related to the observations of the equalized bond length of benzene, which is in contrast with that of the conjugated polyenes, having alternating bond lengths (and bond orders). The extent of the equalization can be measured statistically, and perhaps the most widely used method is that proposed by Bird [66]. The Bird index (BI), which can also be applied for heteroatoms (including... 

Of the views expressed in the literature, we find two of them to be partly compatible with our findings. The model developed by Shaik, Hiberty and coworkers [55-61] states that the a-system prefers equal bond lengths, while then-system prefers alternating bond lengths. The resonance of the n-system is also a symmetrising influence. 

We can therefore now conclude that there are three contributions that determine the geometry of benzene and cyclobutadiene. The first contribution is that of the a-system which prefers equal bond lengths. The other two contributing factors are the 7l-bonds and the 7l-resonance. The 7i-bonds by themselves give rise to two states with a minimum at one of the geometries with alternating bond lengths. The resonance provides interaction between the two states and stabilises the symmetric structure. 

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