Chemistry: Ultrafast X-ray diffraction of transient molecular structures in solution

We report direct structural evidence of the bridged radical (CH ₂ICH₂•) in a polar solution, obtained using time-resolved liquid-phase x-ray diffraction. This transient intermediate has long been hypothesized to explain stereochemical control in many association and/or dissociation reactions involving haloalkanes. Ultrashort optical pulses were used to dissociate an iodine atom from the haloethane molecule (C ₂H₄I₂) 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 C₂H ₄I₂ leads to the C-I-C triangular geometry of CH ₂ICH₂•. This transient C₂H₄I then binds to an iodine atom to form a new species, the C₂H ₄I-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.