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MARGINALIA

Why Think Up New Molecules?

Adding to the world of known chemical structures is a wonderful mental experiment

Roald Hoffmann


Endless Possibilities

Since chemistry is a semi-infinite macrocosm of structure, there are many interesting molecules waiting to be made. And still many more that might as well wait a while longer. Few of the 355 dodecanes (C 12 H 26 ) are extant, for good reasons—new principles and properties are most unlikely to be found among them, because they're too similar to those that already exist.

So it's not just a matter of predicting any molecule that does not exist, it's predicting one that's in some way "interesting." That loose word has both cognitive and emotional sides to it, and is definitely subjective. Nevertheless, I find "interesting" works very well in evoking the psychological mix that makes the intelligent graduate student's mind hop to it. Some examples follow.

With Timothy R. Hughbanks of Texas A&M University, Miklos Kertesz of Georgetown (who were both at Cornell at the time) and Peter Bird of Concordia University in Montreal, we designed a carbon allotrope that, if it is made (no, when it is made!), will be metallic (see the first figure, in the opening paragraph) . Now that would be interesting.

Theoretical%20moleculeClick to Enlarge Image In another piece of work, Musiri M. Balakrishnarajan, in my group at the time and now at Pondicherry University in India, thought up a kind of three-dimensional analogue to one of the very best oxidation/reduction couples in organic chemistry, quinone/hydroquinone. Polyhedral boron cages, such as the octahedron shown in the second figure, can go through the process twice over, accepting two and four electrons (the intermediate stage of oxidation/reduction is shown) with correlated changes in geometry. The molecule will "breathe" as it sops up electrons.

Isomers%20of%20a%20simple%20molecule%20of%20silicon%20and%20hydrogenClick to Enlarge Image To move away from my work, wonderful predictions were made of two variants of H 2 Si 2 , a simple molecule that is not likely to fill any glass bottles, but is nonetheless detectable. Wonderful, because they were completely unexpected—the molecule was calculated by Hans Lischka of the University of Vienna and Hans-Joachim Köhler of the Karl Marx University in Leipzig, not to have the expected acetylenic, or linear, H-Si-Si-H connectivity, but instead to feature two bridging hydrogens and a folded geometry. And it does! Then Brenda T. Colegrove and H. F. Schaefer III of the University of Georgia predicted a second "isomer" with a different shape to be metastable (see the third figure, at right) . And this too was found, by Michel Cordonnier, Marcel Bogey, Claire Demuynck, and Jean-Luc Destombes of the Université des Sciences et Technologies de Lille in France, in 1992.


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