Solvent dependent structural perturbations of chemical reaction intermediates visualized by time-resolved x-ray diffraction

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 $0.07ű0.04Å$ 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.