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 center dot+I center dot. Iodine radicals remaining within the solvent cage recombine with a nascent CH2I center dot radical to form the transient isomer CH2I-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 CH2I-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.

Ämnesord

NATURVETENSKAP  -- Kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences (hsv//eng)

NATURVETENSKAP  -- Data- och informationsvetenskap (hsv//swe)

NATURAL SCIENCES  -- Computer and Information Sciences (hsv//eng)

NATURVETENSKAP  -- Fysik -- Subatomär fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Subatomic Physics (hsv//eng)

Nyckelord

free radical reactions

hydrogen bonds

iodine

liquid structure

organic compounds

perturbation theory

photochemistry

photodissociation

photoexcitation

photolysis

solvent effects

spectrochemical analysis

time resolved spectra

X-ray diffraction

Chemistry

Kemi

Elementary particle physics

kvantkemi

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