Structural Determination of a Transient Isomer of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>CH</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">I</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>by Picosecond X-Ray Diffraction

Ultrafast time-resolved spectroscopic studies of complex chemical reactions in solution are frequently hindered by difficulties in recovering accurate structural models for transient photochemical species. Time-resolved x-ray and electron diffraction have recently emerged as techniques for probing the structural dynamics of short lived photointermediates. Here we determine the structure of a transient isomer of photoexcited ${\mathrm{CH}}_{2}{\mathrm{I}}_{2}$ in solution and observe the downstream reactions of the initial photoproducts. Our results illustrate how geminate recombination proceeds via the formation of a transient covalent bond onto the iodine atom remaining with the parent molecule. Further intramolecular rearrangements are thus required for the ${\mathrm{CH}}_{2}\mathrm{I}\mathrm{\text{\ensuremath{-}}}\mathrm{I}$ isomer to return to ${\mathrm{CH}}_{2}{\mathrm{I}}_{2}$. The generation of $\mathrm{I}_{3}{}^{\ensuremath{-}}$ from those iodine radicals escaping the solvent cage is also followed with time.