Ultra fast X-ray diffraction - watching the birth and death of exotic molecules

Some very interesting structural studies have been performed recently at the European Synchrotron Radiation Facility (ESRF), Grenoble, France, on photo-chemically generated, short-lived (<10^-6 sec) iodo radicals. The research team dissolved a molecule of C₂H₄I₂ in liquid methanol and then subjected it to a short laser pulse. This excited the molecule, which then cooled down while releasing heat into the surrounding liquid. As a consequence, the temperature rose and the liquid started to expand in response to the increase in temperature. The absorption of light triggered a chemical reaction, which the researchers studied with picosecond time resolution. The research team measured the change in shape and composition as early as 100 picoseconds after the initial explosion, then at an interval of 10 nanoseconds, then 1 microsecond and so on. From these measurements, the team obtained direct structural evidence of the bridged radical (CH₂ICH₂) in a polar solution. This transient intermediate has long been hypothesized to explain stereo-chemical control in many association and/or dissociation reactions involving haloalkanes. For more information, see the paper, "Ultrafast X-ray Diffraction of Transient Molecular Structures in Solution", H. Ihee et al., Science, 309, 1223-1227, (2005).