Ultrafast X-ray Diffraction of Transient Molecular Structures in Solution

Abstract

We report direct structural evidence of the bridged radical (CH 2 ICH 2 ·) 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 (C 2 H 4 I 2 ) 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 C 2 H 4 I 2 leads to the C-I-C triangular geometry of CH 2 ICH 2 ·. This transient C 2 H 4 I then binds to an iodine atom to form a new species, the C 2 H 4 I-I isomer, which eventually decays into C 2 H 4 + I 2 . Solvent dynamics were also extracted from the data, revealing a change in the solvent cage geometry, heating, and thermal expansion.