title: Solvent dependent structural perturbations of chemical reaction intermediates visualized by time-resolved x-ray diffraction.
abstract: 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 CH(2)I(2) 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, CH(2)I(2) dissociates to form CH(2)I* + I*. Iodine radicals remaining within the solvent cage recombine with a nascent CH(2)I* radical to form the transient isomer CH(2)I-I, whereas those which escape the solvent cage ultimately combine to form I(2) 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 CH(2)I-I reaction intermediate. Thus we observe that the transient iodine-iodine bond is 0.07 A+/-0.04 A 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.
authors.affiliation: Department of Photochemistry and Molecular Science, Uppsala University, P.O. Box 523, S-751 20 Uppsala, Sweden.
authors.forename: Jonathan
authors.initials: J
authors.lastname: Vincent
authors.forename: Magnus
authors.initials: M
authors.lastname: Andersson
authors.forename: Mattias
authors.initials: M
authors.lastname: Eklund
authors.forename: Annemarie B
authors.initials: AB
authors.lastname: Wöhri
authors.forename: Michael
authors.initials: M
authors.lastname: Odelius
authors.forename: Erik
authors.initials: E
authors.lastname: Malmerberg
authors.forename: Qingyu
authors.initials: Q
authors.lastname: Kong
authors.forename: Michael
authors.initials: M
authors.lastname: Wulff
authors.forename: Richard
authors.initials: R
authors.lastname: Neutze
authors.forename: Jan
authors.initials: J
authors.lastname: Davidsson