Revisiting Julg's Structural Approach to Aromaticity

Abstract

Julg's classical formula for aromaticity is updated so as to involve bond indices. A simple CNDO/2 calculation is shown to account satisfactorily for heterocyclic typical rings and other mis­cellaneous systems. Results are compared with a multicenter MO bond index recently introduced as an aromaticity measure.

Isolation of an Antiaromatic Singlet Cyclopentadienyl Zwitterion

“…While structural (as well as all other) criteria have been criticized to be distinctive for aromatic compounds, we expect that an aromatic state of the same molecule shows much less bond alternation than an antiaromatic states with large Jahn–Teller distortion. The variations of bond lengths in the cyclopentadienyl units of T- 1e , S- 1e , and S- 1e ···BF 3 can be correlated to the A parameter as defined by Julg and François. , For T- 1e , S- 1e , and S- 1e ···BF 3 the calculated A values are 0.99, 0.29, and 0.36, respectively, in perfect agreement with the values determined for the corresponding electronic states of the parent cyclopentadienyl cation 1a by Wright and Lee . On the basis of these results, T- 1e has an aromatic character, while S- 1e and S- 1e ···BF 3 are clearly antiaromatic molecules.…”

Energetic Aspects of Cyclic Pi-Electron Delocalization: Evaluation of the Methods of Estimating Aromatic Stabilization Energies

“…In the case of [6]radialene all CC bond lengths are significantly elongated and equal to ∼1.503 Å . Both the Bird I 6 model and the Julg A J model incorrectly predict a fully aromatic character ( I 6 = 100 and A J = 1.0, respectively). The reason for this shortcoming is that these geometry-based models are defined solely on the basis of the variance of bond lengths 82 or equivalent bond orders. , No alternation of bond lengths (orders) implies fully aromatic character, as in the case of benzene.…”

Two faces of the structural aspects of aromaticity

“…26 The model has also been revisited recently by de Giambiagi. 27 It is important to note that using only the perimeter bonds may have also some advantages. All internal bonds in a p-electron system may be treated as a perturbation of the p-electron structure of the perimeter -hence these bonds may be topologically recognized as stabilizing (or destabilizing) the molecule as a whole.…”