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Science 16 May 2008:
Vol. 320. no. 5878, pp. 924 - 928
DOI: 10.1126/science.1155736
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Reports

Measuring Picosecond Isomerization Kinetics via Broadband Microwave Spectroscopy

Brian C. Dian,* Gordon G. Brown, Kevin O. Douglass, Brooks H. Pate{dagger}

The rotational spectrum of a highly excited molecule is qualitatively different from its pure rotational spectrum and contains information about the intramolecular dynamics. We have developed a broadband Fourier transform microwave spectrometer that uses chirped-pulse excitation to measure a rotational spectrum in the 7.5- to 18.5-gigahertz range in a single shot and thereby reduces acquisition time sufficiently to couple molecular rotational spectroscopy with tunable laser excitation. After vibrationally exciting a single molecular conformation of cyclopropane carboxaldehyde above the barrier to C–C single-bond isomerization, we applied line-shape analysis of the dynamic rotational spectrum to reveal a product yield and picosecond reaction rate that were significantly different from statistical predictions. The technique should be widely applicable to dynamical studies of radical intermediates, molecular complexes, and conformationally flexible molecules with biological interest.

Department of Chemistry, University of Virginia, McCormick Road, Charlottesville, VA 22904–4319, USA.

* Present address: Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907–2084, USA.

{dagger} To whom correspondence should be addressed. E-mail: brookspate{at}virginia.edu

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Science. ISSN 0036-8075 (print), 1095-9203 (online)