Möbius Aromaticity in [12]Annulene:  Cis−Trans Isomerization via Twist-Coupled Bond Shifting

Abstract

Density functional and coupled cluster calculations show that facile thermal configuration change in [12]annulene occurs via a twist-coupled bond-shifting mechanism. The transition state for this process is highly aromatic with Möbius topology. At the CCSD(T)/cc-pVDZ//BH&HLYP/6-311+G** level, the isomerization of tri-trans-[12]annulene 1a (CTCTCT) to its di-trans isomer 2 (CCCTCT) via such a mechanism has a barrier of 18.0 kcal/mol, in good agreement with earlier experiments. Two other aromatic Möbius bond-shifting transition states were located that result in configuration change for other [12]annulene conformers. This mechanism contrasts sharply with diradical configuration change for acyclic polyenes and with planar bond-shifting mechanisms generally assumed for annulenes. This constitutes evidence that neutral Möbius aromatic annulenes play a role in the dynamic processes of neutral [4n]annulenes.

Figure Eights, Möbius Bands, and More: Conformation and Aromaticity of Porphyrinoids

“…As a consequence, Möbius aromaticity and antiaromaticity are defined respectively by a doubly even (4n) and singly even electron count (4n + 2). [11] Möbius aromaticity was subsequently invoked in several theoretical discussions, and proposed, sometimes controversially, as possible transition states [128][129][130] or reactive intermediates. [131] However, the first example of a stable, neutral Möbius molecule was experimentally characterized in 2003.…”

Figure‐Eight‐Strukturen, Möbius‐Bänder und mehr: Konformation und Aromatizität von Porphyrinoiden

“…einfach-gerade (4n + 2) Elektronenzählung definiert sein. [11] Verschiedene theoretische Arbeiten befassten sich später teilweise kontrovers mit der Möbius-Aromatizität und schlugen sie als möglichen Übergangszustand [128][129][130] oder reaktive Zwischenstufe [131] vor. Dann jedoch wurde 2003 das erste stabile, neutrale Möbius-Molekül charakterisiert, [87,92] ein Annulen-Bianthrochinondimethan-Hybrid, das eine kombinierte planare und In-plane-p-Konjugation aufweist (siehe Abbildung 3).…”

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

“…For instance, benzene has represented along the years one of the preferred reference systems in order to validate new ab initio theoretical procedures. Higher annulenes, on the other hand, have allowed to investigate about isomeric rearrangements by means of advanced experimental techniques [6] and theoretical methods [7][8][9]. Comprehensive treatments of annulene preparation, structure and properties are available in textbooks [4,10,11], thematic issues [12,13] and review articles [14,15].…”