Chemists have synthesized the first stable molecule twisted into the shape of the famous one-sided Möbius strip, proving a prediction made nearly 40 years ago.
The trademark stability of ring-shaped molecules such as benzene arises because some electrons are shared by all the atoms in the ring. This feature, known as aromaticity, binds the atoms together more tightly than if the electrons were standoffish. Dumbbell-shaped regions, or orbitals, where electrons roam, project up and down from each atom, perpendicular to the plane of the ring. As long as the orbitals stay nearly parallel to each other, they overlap and electrons can hop from one to another.
A twist would seem like the last thing an aromatic molecule should have. But in 1964 a chemist predicted that a ring-shaped molecule with a single twist in it, better known as a Möbius strip, could indeed be aromatic. Researchers discovered 2 years later that this possibility was more than theoretical--some reactions proceeded as if they formed transition states with the Möbius shape. Nobody had managed to synthesize a stable Möbius aromatic molecule, however, because the rings could always wriggle back into their regular shape.
The new trick is stitching together one rigid molecule and one that's more flexible, reports a group from the University of Kiel, Germany, in the 18 December issue of Nature. The rigid molecule (TDDA), which prevents the final product from untwisting itself, contains a central ring and four rings branching from it. The second molecule (TCOD) is structured like a short ladder. By irradiating a mixture of the two, the group got the central ring of TDDA to open up. Then TCOD joined the loose ends, and all but one of its rungs dissolved, leaving a flexible strand that completes the loop. The reaction produced five different products, one of which was a stable Möbius strip.
"It is a wonderful contribution to organic chemistry, a real breakthrough," says Henning Hopf, an organic chemist at the University of Braunschweig, Germany. The result will have a strong influence on organic chemistry theory, he predicts, adding that he expects chemists to explore its optical and structural properties and start looking for applications. The molecule is only moderately aromatic, meaning its electrons are not shared as much as they could be, but more dramatic examples should be possible in the future, says organic chemist David Lemal of Dartmouth College in Hanover, New Hampshire. Rainer Herges, leader of the Kiel group, agrees: "We are working on that."
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