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Ultrafast time-resolved wide angle x-ray scattering from chemical reactions in solution has recently emerged as a powerful technique for determining the structural dynamics of transient photochemical species. Here we examine the structural evolution of photoexcited CH2I2 in the nonpolar solvent cyclohexane and draw comparisons with a similar study in the polar solvent methanol. As with earlier spectroscopic studies, our data confirm a common initial reaction pathway in both solvents. After photoexcitation, CH2I2 dissociates to form CH2I·+I·. Iodine radicals remaining within the solvent cage recombine with a nascent CH2I· radical to form the transient isomer CH2I-I, whereas those which escape the solvent cage ultimately combine to form I2 in cyclohexane. Moreover, the transient isomer has a lifetime approximately 30 times longer in the nonpolar solvent. Of greater chemical significance is the property of time-resolved wide angle x-ray diffraction to accurately determine the structure of the of CH2I-I reaction intermediate. Thus we observe that the transient iodine-iodine bond is 0.07 ű0.04 Å shorter in cyclohexane than in methanol. A longer iodine-iodine bond length for the intermediate arises in methanol due to favorable H-bond interaction with the polar solvent. These findings establish that time-resolved x-ray diffraction has sufficient sensitivity to enable solvent dependent structural perturbations of transient chemical species to be accurately resolved. [ABSTRACT FROM AUTHOR]

The photochemistry of halomethanes is fascinating for the complex cascade reactions toward either the parent or newly synthesized molecules. Here, we address the structural rearrangement of photodissociated CH2IBr in methanol and cyclohexane, probed by time-resolved X-ray scattering in liquid solution. Upon selective laser cleavage of the C–I bond, we follow the reaction cascade of the two geminate geometrical isomers, CH2I–Br and CH2Br–I. Both meta-stable isomers decay on different time scales, mediated by solvent interaction, toward the original parent molecule. We observe the internal rearrangement of CH2Br–I to CH2I–Br in cyclohexane by extending the time window up to 3 μs. We track the photoproduct kinetics of CH2Br–I in methanol solution where only one isomer is observed. The effect of the polarity of solvent on the geminate recombination pathways is discussed. [ABSTRACT FROM AUTHOR]