Aromaticity: The Alternating CC Bond Length Structures of [14]‐, [18]‐, and [22]Annulene

Abstract

[18]Annulene (C 18 H 18 ) [1] is the critical signpost along the 4n+2 p electron Hückel [2] route from benzene to the larger aromatic (CH) k rings. Although Mislow [1a] pointed out the steric problems of the planar D 6h form very early, [18]annu-

Direct Assessment of Electron Delocalization Using NMR Chemical Shifts

“…Much larger direct ring current effects are illustrated by the 1 H chemical shifts of [18]-annulene (5; computed in D 6h symmetry [28] ) and 1,6-methano [10]annulene (6). The large (ca.…”

Concerning the Structure of [18]Annulene

“…3 kcal mol À1 energetically lower than the non-alternating D 6h form, reported as a transition state with the high imaginary frequency of ca. 1100i cm À1 [1]. Schleyer and co-workers conclude from their results that the X-ray geometry of [18]annulene is not correct, and attribute this putative crystallographic failure to supposedly insurmountable disorder problems similar to those indicated previously for the related fundamental case of benzene [4].…”

“…A second point of consideration concerns the rather intriguing question of whether [18]annulene indeed has the localized alternating structure in the crystal as tacitly supposed by Schleyer and co-workers [1]. Of course, their comparison of computed and experimental 1 H-NMR chemical shifts refers to the gaseous and solution states, respectively.…”

“…From a comparison of calculated and observed 1 H-NMR chemical shifts, it has recently been firmly concluded by Schleyer and co-workers that [18]annulene prefers a structure with alternating (−localized×) C,C single and double bonds, rather than a nonalternating (−delocalized×) structure of maximum symmetry D 6h with more nearly equalized, but ± for obvious symmetry reasons ± not exactly equal, C,C lengths [1] 1 ). The latter had previously been deduced from single-crystal X-ray-diffraction data [2] [3].…”

On the Importance of Clar Structures of Polybenzenoid Hydrocarbons as Revealed by the π-Contribution to the Electron Localization Function

“…With regard to sexiphenyl (12) and [18]annulene (13), these compounds are segments of several of the larger PBHs. Their features at D 6h symmetric structures are therefore interesting, although these structures are either a higher-order saddle point (12) or a conformer that is most likely not the lowest in energy (13) even though the relative energies of the conformers of 13 depend very much on the level of theory used [69]. As seen in Figure 3, biphenyl (8) has a nonzero BV(ELF  ) and a rather small BV(ELF  ) for the single bond interconnecting the two phenyl groups, indicating a weak conjugation.…”