Analyzing solution-phase time-resolved x-ray diffraction data by isolated-solute models

Extracting transient structural information of a solute from time-resolved x-ray diffraction (TRXD) data is not trivial because the signal from a solution contains not only the solute-only term as in the gas phase, but also solvent-related terms. To obtain structural insights, the diffraction signal in q space is often Fourier sine transformed (FT) into r space, and molecular dynamics (MD) simulation-aided signal decomposition into the solute, cage, and solvent terms has so far been indispensable for a clear-cut assignment of structural features. Here we present a convenient method of comparative structural analysis without involving MD simulations by incorporating only isolated-species models for the solute. FT is applied to both the experimental data and candidate isolated-solute models, and comparison of the correlation factors between the experimental FT and the model FTs can distinguish the best candidate among isolated-solute models for the reaction intermediates. The low q region whose influence by solvent-related terms is relatively high can be further excluded, and this mode of truncated Fourier transform (TFT) improves the correlation factors and facilitates the comparison. TFT analysis has been applied to TRXD data on the photodissociation of C2 H4 I2 in two different solvents (methanol and cyclohexane), Hg I2 in methanol, and I3- in methanol excited at 267 nm. The results are consistent with previous conclusions for C2 H4 I2 in methanol and Hg I2 in methanol, and the new TRXD data reveal that the C2 H4 I transient radical has a bridged structure in cyclohexane and I3- in methanol decomposes into I+ I2- upon irradiation at 267 nm. This TFT method should greatly simplify the analysis because it bypasses MD simulations.