The [12]Annulene Global Minimum

How do the Hückel and Baird Rules Fade away in Annulenes?

Two of the most popular rules to characterize the aromaticity of molecules are those due to Hückel and Baird, which govern the aromaticity of singlet and triplet states. In this work, we study how these rules fade away as the ring structure increases and an optimal overlap between p orbitals is no longer possible due to geometrical restrictions. To this end, we study the lowest-lying singlet and triplet states of neutral annulenes with an even number of carbon atoms between four and eighteen. First of all, we analyze these rules from the Hückel molecular orbital method and, afterwards, we perform a geometry optimization of the annulenes with several density functional approximations in order to analyze the effect that the distortions from planarity produce on the aromaticity of annulenes. Finally, we analyze the performance of three density functional approximations that employ different percentages of Hartree-Fock exchange (B3LYP, CAM-B3LYP and M06-2X) and Hartree-Fock. Our results reveal that functionals with a low percentage of Hartree-Fock exchange at long ranges suffer from severe delocalization errors that result in wrong geometrical structures and the overestimation of the aromatic character of annulenes.

Design and synthesis of the first triply twisted Möbius annulene

As long as 50 years ago theoretical calculations predicted that Möbius annulenes with only one π surface and one edge would exhibit peculiar electronic properties and violate the Hückel rules. Numerous synthetic attempts notwithstanding, the first singly twisted Möbius annulene was not prepared until 2003. Here we present a general, rational strategy to synthesize triply or even more highly twisted cyclic π systems. We apply this strategy to the preparation of a triply twisted [24]dehydroannulene, the structure of which was confirmed by X-ray analysis. Our strategy is based on the topological transformation of 'twist' into 'writhe'. The advantage is twofold: the product exhibits a lower degree of strain and precursors can be designed that inherently include the writhe, which, after cyclization, ends up in the Möbius product. With our strategy, triply twisted systems are easier to prepare than their singly twisted counterparts.

Möbius molecules with twists and writhes

The Möbius topology inspired science and arts for many centuries. In chemistry interest was stimulated by Heilbronner almost 50 years ago when he predicted that Möbius annulenes would violate the Hückel rule. A number of theoretical papers followed but it was not until 2003 when the first synthesis of an aromatic Möbius structure was accomplished. Inspired by the recent progress, the concept of Möbius topology has gained further momentum in synthesis as well as theory. We present a topological description of Möbius molecules, and strategies for the design and synthesis of these structures.

Jailbreaking benzene dimers

We suggest four new benzene dimers, (C(6)H(6))(2), all featuring one or more cyclohexadiene rings trans-fused to 4- or 6-membered rings. These hypothetical dimers are 50-99 kcal/mol less stable than two benzenes, but have computed activation energies to fragmentation ≥27 kcal/mol. A thorough search of potential escape routes was undertaken, through cyclobutane ring cleavage to 12-annulenes, sigmatropic 1,5-H-shifts, electrocyclic ring-openings of the 6-membered rings, and Diels-Alder dimerizations. Some channels for reaction emerge, but there is a reasonable chance that some of these new benzene dimers can be made.

Möbius aromaticity and antiaromaticity in expanded porphyrins

Aromaticity is a key concept in chemistry, dating back to Faraday's discovery of benzene in 1825 and Kekulé's famous alternating-double-bond structure of 1865. In 1858, the Möbius strip was discovered by Möbius and Listing. The Hückel rules for predicting aromaticity, stating that [4n + 2] π electrons result in an aromatic system, work for planar molecules. Although molecules with Möbius geometry are not found in nature, chemists have tried to synthesize such molecules since the first theoretical prediction by Heilbronner in 1964 and the prediction of Möbius aromaticity for suitable compounds with [4n] π electrons. However, Möbius-aromatic molecules have proved difficult to synthesize, and sometimes even to identify. Here we summarize recent contributions of several research groups that have succeeded in synthesizing Möbius-type molecules. The results of this survey lead us to suggest that the generation of Möbius topologies in expanded porphyrins is easier than hitherto appreciated.