Illustration from Scientific Research

The shortest double bond is indicated by red arrows and the longest single bond by blue arrows for the (homo)aromatic part of methano[10]annulene (17) 41, 51, barbaralane 4 (refs. 36, 37, 38), triasterane 14a, homoannulene ester 15 and homoannulene carboxylic acid 16 (refs. 6, 7, 8). Please note that in 4 and 14a, a potential homoaromaticity relies on through-bond homoconjugation, whereas 15 – 17 hinge on through-space homoconjugation. Therefore, the HOMA values of 4 and 14a differ significantly from those of 15 – 17. Supplementary Discussion Section 1.4 provides calculations and further discussion. Compounds 15 and 16 display significant bond-length equalization, reaching values associated with aromatic (cyclic conjugated) molecules.

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