Courtesy of Philip Coppens

X-ray crystallography typically is used to capture static images of molecular structure. But Philip Coppens of the University of Buffalo and colleagues recently employed a variation called time-resolved X-ray crystallography to study contraction of the metal-metal bond lengths in a complex ion containing two rhodium atoms.1 These changes occur on the microsecond time scale and represent highly reactive, transient excited states that are impossible to capture using standard crystallographic techniques.

"We're measuring the average change in the geometry of these molecules within a crystal, a very short time after they've been excited, and that gives us the molecule's excited-state structure," says coauthor Tim Graber of the University of Chicago. "It's like taking a snapshot of a molecule with a camera at a particular point in time."

Time-resolved crystallography describes any crystallographic technique that follows a reaction in time, says Keith Moffat of the University...

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