N.M.R.-Studies of Bond Order in Azulene, Biphenylene and 1,6-Methano[10]annulene

Abstract

The ⁴J(H-C^-C-Me) coupling constants of methyl-substituted derivatives, probes of bond order, have been used to examine the ground-state π- electron distributions in azulene (1), biphenylene (2) and 1,6- methano [10] annulene (3). The experimental data obtained are in good agreement with theoretical predictions for biphenylene (2) and provide some evidence for π-electron disproportionation towards the five-membered ring in azulene (1). The bond orders in 1,6-methano [10] annulene (3) obtained in this work are at variance with those predicted on the grounds of bond lengths.