Crystallographers Complete First Structural Study of An Excited Molecule

BUFFALO, N.Y. -- Nearly every molecule can exist in several states: its ground, or stable, state and its excited states. Chemists have long known that the excited states of molecules may hold some of the secrets of chemical reactions because molecules often pass through these states just before reacting. But attempts to measure excited molecules have not been successful.

Now, University at Buffalo crystallographers have completed the first structural study of an excited molecule, providing scientists with a glimpse of the distortions that a molecule undergoes in the milliseconds or nanoseconds just before it reacts chemically.

"We have learned a lot about the stable state, or ground-state, structure of molecules through X-ray crystallography," explained Philip Coppens, distinguished professor of chemistry at UB and co-investigator with UB postdoctoral fellows Mark R. Pressprich and Mark A. White. "But molecules in an excited state have more energy and so are more reactive. The structures of these molecules may be more important because they tell us how molecules react."

To complete the diffraction study, the UB researchers combined a conventional X-ray diffractometer, used to determine the crystal structure of a molecule, with a laser attachment. The laser sends light through a fiber-optic cable and a series of lenses onto the crystal.

"The laser excites the crystal and the X-rays show its structure," said Coppens. "The scattering pattern of the X-rays changes when the molecule becomes excited."

In most chemical reactions induced by light, when molecules absorb energy from the light, specific changes or distortions in their geometric structures will result.

"There are many cases where two different types of molecules will react only when excited," said Coppens. "When molecules absorb energy and become excited, they change their shape. The distances between the atoms change."

In the experiments conducted at UB, a sodium nitroprusside crystal was studied, primarily because its electronic excited state lasts for hours when the crystal is cooled to very low temperatures. This is extremely long, Coppens explained, compared to the excited states of other molecules.

The researchers found that in its excited state, the molecule underwent a number of changes, including the lengthening of the iron-nitrogen bond, which pushed the atoms in the molecule further apart, and a redistribution of the electrons in the molecule.

Because the work involved a molecule containing an iron atom, it may illuminate reactions related to the conversion of light into energy since excited states of iron are involved in photosynthesis.

The diffraction study was reported by the UB scientists in August in the Journal of the American Chemical Society.

Using novel equipment, some of it designed in the UB laboratory, the group is now beginning diffraction studies of compounds with far less stable excited states.

The work is supported by the American Chemical Society's Petroleum Research Fund and the National Science Foundation