Superaromaticity

In 1991, Cioslowski and collaborators performed ab initio molecular orbital calculations on kekulene.^[7] They concluded that the molecule was stabilized not only by the aromaticity of its individual benzene rings but also by conjugation within the "super-ring" of those rings.^[10] They estimated this extra "superaromatic stabilization energy" to be significant, on the order of 25.4–32.9 kcal/mol.^[4] This view corresponds to the annulenoid resonance structure of kekulene, where two concentric annulenes ([18]annulene on the inside and [30]annulene on the outside) both satisfy Hückel's rule.^[4]

In 1993, Jun-ichi Aihara challenged this conclusion using a method based on chemical graph theory.^[1] He defined superaromaticity as the stabilization energy arising from "type-II circuits"—electron pathways that enclose the central cavity of the molecule.^[11] By calculating a "superaromatic stabilization energy" (SSE), he found the value for kekulene to be negligibly small and concluded that it was "essentially non-superaromatic."^[12] This supported the benzenoid model, where kekulene's stability is almost entirely derived from its six Clar-type aromatic sextets.^[13]

In a 1996 paper in Angewandte Chemie, Haijun Jiao and Paul von Ragué Schleyer revisited the question using what were then more advanced computational methods, including density functional theory (DFT) and magnetic criteria like nucleus-independent chemical shift (NICS).^[14] Their analysis provided multiple lines of evidence against superaromaticity in kekulene:

• Energetic Criteria: Using homodesmotic reactions, they calculated the extra aromatic stabilization energy (ASE) and found it to be negligible (2.5 kcal/mol) or slightly negative, in stark contrast to Cioslowski's earlier estimates.^[14]
• Criterios magnéticos: Su evidencia más concluyente provino de cálculos de NICS. Los sistemas aromáticos mantienen una corriente de anillo diatrópica , lo que resulta en un valor de NICS negativo en el centro del anillo. Mientras que los anillos individuales tipo benceno del kekuleno mostraron valores de NICS negativos (por ejemplo, -10,8 ppm para los anillos orientados hacia el interior), el valor de NICS en el centro mismo de la gran cavidad fue positivo (+5,0 ppm). ^{[ 6 ]} Este valor positivo indica una influencia paratrópica (antiaromática), lo que contradice directamente la idea de una corriente diatrópica global requerida para la superaromaticidad. ^{[ 6 ]}

Basándose en la falta de energía de estabilización adicional y en los criterios magnéticos decisivos, Jiao y Schleyer concluyeron que "el kekuleno no es superaromático", sino un "hidrocarburo bencenoide normal". ^{[ 6 ]} Esta opinión es ahora el consenso científico ampliamente aceptado, "[aunque no se puede descartar por completo la influencia superaromática]". ^{[ 15 ]}