A Simple Twist

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Science  22 May 2009:
Vol. 324, Issue 5930, pp. 991
DOI: 10.1126/science.324_991a

The topologically fascinating Möbius strip exhibits a single face and a single edge; it has no distinguishable back or front. At the molecular scale, Möbius strips intrigue chemists for the additional reason that they change the rules of aromaticity. Ring structures such as benzene and naphthalene have particular electronic stability on account of the atoms along the periphery sharing a certain number of pi-bonded electrons—specifically, even non-multiples of 4. If a twist is introduced into the ring framework, p orbitals overlap in an orientation opposite to that in a flat ring, and aromatic stabilization occurs instead when the pi electrons sum to multiples of 4. A few molecules exhibiting such Möbius aromaticity have been prepared, but the routes were complex. Tokuji et al. found that simple treatment of a hexaphyrin precursor with acetic acid yields an aromatic Möbius compound directly. The twist, confirmed crystallographically, ensues from the introduction of an oxygen atom that links a pendant perfluorophenyl group to the pyrrole backbone.

J. Am. Chem. Soc. 131, 10.1021/ja902836x (2009).

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