Photoinduced Reaction Dynamics of 1,2-Diiodoethane Probed by Time-resolved X-ray Solution Scattering

초록

The iodine elimination reaction of C2H4I2 is a two-step, non-concerted process to form ethylene (C2H4) through a bridged C2H4I radical intermediate. This reaction has attracted much interest as a prototypical photochemical reaction in the reaction dynamics studies, particularly due to the formation of an intermediate that has a unique bridged structure [1, 2]. The iodine elimination reaction of C2H4I2 is initiated by UV photoexcitation, resulting in rapid loss of the first iodine atom and the formation of the transient C2H4I radical intermediate that was identified to be of a bridged structure, rather than an anti structure. Subsequently, an iodine atom is recombined to form a C2H4I-I isomer, which subsequently is transformed to the final C2H4 product. Despite the studies of the photoreaction dynamics of C2H4I2 using femtosecond optical spectroscopy and X-ray solution scattering, the mechanism and the stereochemistry for the formation of bridged C2H4I radical intermediate still remains an open question. While femtosecond optical spectroscopy can measure the dynamics of such ultrafast processes, it is not sensitive to the change in global molecular structure. In contrast, femtosecond X-ray solution scattering can be used to directly measure the structural dynamics of primary iodine dissociation with atomic structural sensitivity and ~100 fs time resolution (at XFEL) [3]. Here we investigate the formation mechanism of C2H4I radical using femtosecond X-ray solution scattering (fs-XSS) at XFEL. From the analysis of fs-XSS data with the help of DFT calculation and molecular dynamics simulation, we found that anti form of C2H4I radical is formed on sub-100 fs time scale and is transformed into the bridged C2H4I radical in tens of picoseconds. In other words, the bridged form of C2H4I radical is formed by a sequential mechanism through the anti form.