Ultrafast X-ray Diffraction of Transient Molecular Structures in Solution

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 [approximately]0.01 angstrom spatial resolution and [approximately]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.