A FREAKISH molecule that breaks all the chemical stereotypes has been created with the help of maths. In terms of symmetry, the molecule is in a class of its own.
Since the days of Louis Pasteur, chemists have known that some molecules, described as 鈥渃hiral鈥�, come in distinct left-handed and right-handed varieties. In these molecules there is a chiral centre鈥攁 single atom which has other groups of atoms attached in a 鈥渉anded鈥� way. You can鈥檛 make the mirror image of this arrangement look identical to the original unless you break one of the bonds to the chiral centre.
Molecules that do not have a chiral centre can also momentarily become 鈥渉anded鈥�, as groups of atoms twist themselves around the bonds connecting them to the rest of the molecule. This might happen, for instance, if part of a flat, L-shaped molecule kinks itself into a different plane. But these molecules can always be made symmetrical without breaking any bonds.
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But now, inspired by the mathematics of knot theory, Kurt Mislow of Princeton University in New Jersey and his colleagues have made a nonchiral molecule that always looks asymmetric (Journal of the American Chemical Society, vol 119, p 9558). It contains two rings linked in such a way that its kinks can鈥檛 be flattened out. Every time you try to make it into a molecule that is symmetrical, says Mislow, you end up with either a left-handed or a right-handed one. 鈥淓very single internal motion gives you a chiral molecule.鈥�
鈥淚t鈥檚 a very interesting paper,鈥� comments Jay Siegel of the University of California at San Diego. He adds that the molecule is unlike any natural one. 鈥淚t鈥檚 a challenge nature wouldn鈥檛 offer to us.鈥�