Annulene and Larger Annulenes

The properties of [18]annulene are consistent with its being aromatic. The X-ray crystal stmcture shows the molecule to be close to planarity, with the maximum deviation from the plane being 0.085 A. The bond lengths are in the range 1.385-1.405 A, and 

There are also examples of [18]annulene systems constructed around a saturated central core, such as in compound 4 ° In this compound, the internal protons are at very high field (-6 to -8 ppm), whereas the external protons are far downfield ( 9.5ppm). 

Calder andF. Sondheimer, J. Chem. Soc., Chem. Commun., 904 (1966). 

Theoretical calculations indicate that the tendency to be aromatic decreases as ring size increases.For example, the NICS value decreases from -15.0 and -14.7 for [14] and [18]annulene, respectively, to -5.6 at [42]annulene and -1.2 at [66]annulene. The delocalized structures are computed to be more stable, however, leveling off at 22-23 kcal/mol at [30]annulene. ° Of course, on a per electron basis, this means a decrease in relative stability. 

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