Photochemical Reaction Pathways of Carbon Tetrabromide in Solution Probed by Picosecond X-ray Diffraction

摘要

We report a liquid-phase time-resolved X-ray diffraction study that resolves the molecular structures of the short-lived intermediates formed in the photodissociation of tetrabromomethane in methanol. Time-resolved X-ray diffraction can detect all chemical species simultaneously, and the diffraction signal from each chemical species can be quantitatively calculated from molecular structures and compared with experimental data with high accuracy and precision. The photochemistry of carbon tetrahalides has long been explored to describe their reactions in the natural environment due to its relevance to ozone depletion. Excited with an ultraviolet optical pulse, the complicated photodissociation dynamics of CBr_4 was followed in a wide temporal range from picoseconds up to microseconds and associated rate coefficients were determined by analyzing time-resolved diffraction patterns accumulated from 100 ps X-ray pulses. The homolytic cleavage of one C-Br bond in the parent CBr_4 molecule yields the CBr_3 and Br radicals, which escape from the solvent cage and combine nongeminately to form C_2Br_6 and Br_2, respectively. C_2Br_6 eventually decays to give C_2Br_4 and Br_2 as final stable products. Our diffraction data at the current signal-to-noise ratio could not provide any evidence for the geminate recombination of the CBr_3 and Br radicals to form the Br_2CBr-Br isomer or the solvated ion pair, implying that their formation is a minor channel compared with those observed clearly by time-resolved diffraction in this work.