Optical Eye-Opener

In work that could aid predictions of unknown molecules' spectroscopic and other physical properties, researchers have measured optical rotation by an achiral organic compound.

A prevalent misimpression holds that only chiral organic molecules can rotate the plane of polarized light. Many achiral organic compounds in the crystalline state can also rotate polarized light, so long as their crystal structures satisfy certain symmetry requirements. But optical rotation by such compounds has been extremely difficult to observe.

Bart Kahr, a chemist at the University of Washington, Seattle, for years has been interested in demonstrating experimentally that chirality is not a necessary condition for optical rotation. So Kahr, Kacey Claborn, Werner Kaminsky, and coworkers decided to apply to the problem a highly sensitive polarimetric crystal-analysis technique Kaminsky developed earlier.

The researchers now report that the ability of the simple achiral compound pentaerythritol, C(CH₂OH)₄, to rotate polarized light can be measured in this way (J. Am. Chem. Soc. 2006, 128, 14746). The researchers believe that Kaminsky's technique will also work with other achiral organic compounds.

Theoreticians have for some time been able to calculate polarized-light rotation by solid-phase achiral organic compounds, comments Michael J. Frisch, president of the structure calculation firm Gaussian and a specialist in theoretical prediction of spectra. But until now, those predictions could not be verified because the rotational components could not be observed experimentally.

The new technique measures individual components of the rotational magnitudes and directions instead of averages, Frisch says. "It makes possible careful and detailed comparisons with calculations" and advances the ability to predict spectroscopic and other properties related to a compound's optical rotation.

Chemist Laurence A. Nafie of Syracuse University knows of no other measurements of any form of optical activity of achiral compounds that have been compared with ab initio calculations of a single-crystal sample. "Development of methods for studying optical activity of achiral compounds in solids opens new opportunities to investigate molecular properties and to compare measured to calculated results," he notes.

The ability of achiral compounds to rotate polarized light "is counterintuitive to large numbers of organic chemists," says Jay S. Siegel, a chemist at the University of Zurich. "In the area of crystal and material characterization by chiroptical means, these are frontier results."