Chemist Devises Technique to Measure Shape-Shifting Molecules

June 12, 2008 — Brooks Pate, the William R. Kenan Jr. Professor of Chemistry at the University of Virginia, and his colleagues have cast doubt on existing chemistry theory and brought microwave spectroscopy into the Digital Age with a unique new technique that measures the way that molecules change their geometric shapes.

Pate, who won a prestigious MacArthur Foundation fellowship in 2001, is interested in understanding the basic science behind how molecules take in energy and transform into different geometric shapes. He notes that while molecules are held together by specific chemical bonding patterns, the flexibility of these bonds allows them to adopt different three-dimensional configurations. When energy is involved, molecules can move very rapidly through a number of configurations — rapidly folding and unfolding into a number of shapes. Since a molecule's configuration ultimately influences its properties and reactivity, comprehending this dynamic process is important.

Pate finds common research ground with colleagues Bob Jones and Tom Gallagher, both in the physics department. "I study how nuclear configurations or nuclear geometries move around, and they study how the configurations of electrons around individual atoms interact, but the physics at base level is the same," explained Pate. "So there's this common interest in the quantum dynamics of how localized structures interact with one another."

Traditionally, molecular structure has been interpreted through the use of microwave, or rotational, spectroscopy. But this technology is limited when it comes to gauging the dynamic interactions of molecular systems because measurements need to be taken at every possible frequency of the spectrum to try to determine when the molecules are rotating. This is no small task, because the microwave range is measured in megahertz — which occur millions of times per second. "You spend a lot of time making measurements where nothing is happening," notes Pate.

Working with the best available measurement tool, it previously took Pate's students 14 hours to measure the full microwave spectrum and two to three months to get a single measurement completed. But these cumbersome and technically complex experiments, as well as a collaborative National Science Foundation grant with Pate and Gallagher as co-investigators, laid important groundwork for a promising new technique.

Pate and Gallagher's broadband chirped-pulse Fourier transform microwave spectrometer (CP-FTMW) combines an existing chemistry tool with recent advancements in high-speed digital electronics. "The advantage of the new technique that we developed is that instead of taking 14 hours to get through the spectrum, we actually see the whole spectrum all at once, every time," Pate said. A paper published in the May 16 issue of the journal Science details an experiment in which Pate's group completed a full-spectrum measurement in about 10 seconds with the new tool.

This technique opens up many new possibilities in terms of practical applications for quick and definitive chemical analysis. Pate notes that the method could be used to detect chemical warfare agents or find contaminants in a water sample. "This technique has the ability to fingerprint identify hundreds of compounds at the same time — all in one sample — and to take very quantitative measurements of the concentrations," says Pate.

In addition, Pate's Science findings run counter to established chemistry theory. showing molecules converting between their geometries at a much slower rate than was previously predicted. "It does turn out that everything we've measured — and we've measured from 20 to 30 of these reaction rates — have been 10 to 15 times slower than theory would predict," said Pate. "And so that makes it a ripe problem for theoreticians to go back and find out what's going on."