Atoms in molecules may be arranged in a variety of complicated and beautiful three-dimensional shapes. The molecule’s specific shape influences its every property, from its color to its toxicity. The same atoms arranged in one way form ethyl alcohol, in another way, ether. A certain molecule may be essential for human metabolism but a molecule which differs from the first one only in being its mirror image will be rejected by the body.

Roald Hoffmann’s work is theoretical; he calculates the preferred three-dimensional shape of molecules and the energies by which a molecule resists deformation from this optimum arrangement. And he tries to understand the bonds that hold molecules together.

Many chemical reactions proceed extremely quickly. Yet there is indirect evidence that these reactions are often not simple one-step changes but involve the fleeting existence of a very unstable molecule called the reaction intermediate. Since these unstable molecules may be present for as short a time as one billionth of a second, it is very difficult to study their properties directly. Roald Hoffmann has been engaged in calculating the geometries, colors and other physical properties of such molecules. He has also studied in detail reactions called isomerizations, in which the connectivity but not the number of atoms changes, and cycloadditions, reactions in which a ring of atoms is formed from two open chains. Dr. Hoffmann has been able to successfully predict when such reactions should go thermally (on heating) and when they should proceed photochemically (by exposing them to light).

Roald Hoffmann has gone on to make sense of the shapes, colors, and reactions of all kinds of molecules, inorganic and organic ones. Lately he has been looking at molecules that extend essentially to infinity in one, two or three dimensions – these include polymers, electronic materials and catalysts.

Chemistry is still an experimental science, with the synthesis and characterization of molecules, both those present in nature, and those first made in the laboratory, far outstripping the theory of these molecules. Dr. Hoffmann’s work is one of the few instances in chemistry in which a theory has been able to predict the outcome of a subsequent experiment. His work has stimulated ingenious tests of the theory in laboratories all over the world. In recent years he has been looking not only at organic reactions but also at the shapes and reactions of molecules containing metal atoms, all across the periodic table. Also he and his coworkers have designed new molecular shapes and networks, which experimental chemists are trying to make.

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