Ultrafast X-ray Diffraction of Transient Molecular Structures in Solution

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Science  19 Aug 2005:
Vol. 309, Issue 5738, pp. 1223-1227
DOI: 10.1126/science.1114782

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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.

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