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Issue 19, 2003
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Aromaticity and antiaromaticity in fulvenes, ketocyclopolyenes, fulvenones, and diazocyclopolyenes

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Abstract

The structures, energies, natural charges, and magnetic properties of 3-, 5-, 7-, and 9-membered cyclic polyenes 1–4, respectively, with exocyclic methylene, keto, ketenyl, and diazo substituents (ad, respectively) were computed at the B3LYP/6-311G+**//B3LYP/6-311+G** level to elucidate their aromatic and antiaromatic properties. The corresponding conjugated cyclic cations 1e and 3e were also studied. The criteria used are isomerization energies (ISE), magnetic susceptibility exaltations (Λ), aromatic stabilization energies (ASE), nucleus independent chemical shifts (NICS), and bond length alternation (ΔR). Planar C2v structures were found to be the lowest energy minima with the exceptions of diazocyclopropene (1d), cycloheptafulvenone (3c), diazocycloheptatriene (3d), and all of the cyclononatetraene derivatives (4). The fulvenes (1a–4a) have modest aromatic or antiaromatic character, and are used as standards for comparison. By these criteria the ketenylidene and diazo cyclopropenes and cycloheptatrienes 1,3-c,d and oxo cyclopentadiene and cyclononatetraene 2,4b are antiaromatic, while the 5- and 9-ring ketenyl and diazo compounds and 3- and 7-ring ketones are aromatic. The degree of aromatic/antiaromatic character decreases with ring size. The consistent agreement with Hückel rule predictions for all the criteria shows their utility for the evaluation of the elusive properties of aromaticity and antiaromaticity.

Graphical abstract: Aromaticity and antiaromaticity in fulvenes, ketocyclopolyenes, fulvenones, and diazocyclopolyenes

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Publication details

The article was received on 02 May 2003, accepted on 07 Jul 2003 and first published on 26 Aug 2003


Article type: Paper
DOI: 10.1039/B304718K
Org. Biomol. Chem., 2003,1, 3410-3417

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    Aromaticity and antiaromaticity in fulvenes, ketocyclopolyenes, fulvenones, and diazocyclopolyenes

    K. Najafian, P. von Ragué Schleyer and T. T. Tidwell, Org. Biomol. Chem., 2003, 1, 3410
    DOI: 10.1039/B304718K

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