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Originally published in Science Express on 14 July 2005
Science 19 August 2005:
Vol. 309. no. 5738, pp. 1223 - 1227
DOI: 10.1126/science.1114782
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Reports

Ultrafast X-ray Diffraction of Transient Molecular Structures in Solution

H. Ihee,1* M. Lorenc,2 T. K. Kim,1 Q. Y. Kong,2 M. Cammarata,2,3 J. H. Lee,1 S. Bratos,4 M. Wulff2

We report direct structural evidence of the bridged radical (CH2ICH2·) in a polar solution, obtained using time-resolved liquid-phase x-ray diffraction. This transient intermediate has long been hypothesized to explain stereo-chemical control in many association and/or dissociation reactions involving haloalkanes. Ultrashort optical pulses were used to dissociate an iodine atom from the haloethane molecule (C2H4I2) dissolved in methanol, and the diffraction of picosecond x-ray pulses from a synchrotron supports the following structural dynamics, with ~0.01 angstrom spatial resolution and ~100 picosecond time resolution: The loss of one iodine atom from C2H4I2 leads to the C-I-C triangular geometry of CH2ICH2·. This transient C2H4I then binds to an iodine atom to form a new species, the C2H4I-I isomer, which eventually decays into C2H4 + I2. Solvent dynamics were also extracted from the data, revealing a change in the solvent cage geometry, heating, and thermal expansion.

1 Department of Chemistry and School of Molecular Science (BK21), Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 305-701, Republic of Korea.
2 European Synchrotron Radiation Facility, Grenoble Cedex 38043, Boîte Postal 220, France.
3 National Institute for the Physics of Matter and Department of Physical and Astronomical Sciences, via Archirafi 36, 90123, Palermo, Italy.
4 Laboratoire de Physique Théorique des Liquides, Université Pierre et Marie Curie, Case courrier 121, 4, Place Jussieu, Paris Cedex 75252, France.

* To whom correspondence should be addressed. E-mail: hyotcherl.ihee{at}kaist.ac.kr

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THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Imaging Atomic Structure and Dynamics with Ultrafast X-ray Scattering.
K. J. Gaffney and H. N. Chapman (2007)
Science 316, 1444-1448
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Spatiotemporal reaction kinetics of an ultrafast photoreaction pathway visualized by time-resolved liquid x-ray diffraction.
T. K. Kim, M. Lorenc, J. H. Lee, M. Lo Russo, J. Kim, M. Cammarata, Q. Kong, S. Noel, A. Plech, M. Wulff, et al. (2006)
PNAS 103, 9410-9415
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Science. ISSN 0036-8075 (print), 1095-9203 (online)