Determining the charge distribution and the direction of bond cleavage with femtosecond anisotropic x-ray liquidography

Abstract

Energy, structure, and charge are fundamental quantities characterizing a molecule. Whereas the energy flow and structure change in chemical reactions are experimentally characterized, determining the atomic charges of a molecule in solution has been elusive, even for a triatomic molecule such as triiodide ion, I3−. Moreover, it remains to be answered how the charge distribution is coupled to the molecular geometry; which I-I bond, if two I-I bonds are unequal, dissociates depending on the electronic state. Here, femtosecond anisotropic x-ray solution scattering allows us to provide the following answers in addition to the overall rich structural dynamics. The analysis unravels that the negative charge of I3− is highly localized on the terminal iodine atom forming the longer bond with the central iodine atom, and the shorter I-I bond dissociates in the excited state, whereas the longer one in the ground state. We anticipate that this work may open a new avenue for studying the atomic charge distribution of molecules in solution and taking advantage of orientational information in anisotropic scattering data for solution-phase structural dynamics.

Reaction dynamics studiedviafemtosecond X-ray liquidography at X-ray free-electron lasers

“…Therefore, such anisotropic structural change of the cage needs to be analyzed properly to extract detailed cage dynamics, as performed in a recent work. 93 In addition, information on changes in nuclear positions was mainly obtained based on the independent atom model from the TRXL signal. In principle, fs-TRXL data contain information on changes in electronic charge distribution, which cannot be retrieved solely by data analysis based on the independent atom model.…”

“…TRXL has the potential to observe the molecular charge distribution, which affects the cage-scattering signal. Indeed, the charge distribution of I 3 − in methanol was recently unraveled using fs-TRXL by Heo et al 93 Although TRXL on I 3 − had been performed at synchrotrons, the charge distribution of I 3 − could not be determined in those studies due to the limited signal-to-noise ratio. The signal-to-noise ratio of fs-TRXL data acquired at an XFEL is much higher than that of the previous TRXL data collected at synchrotrons.…”

“…To reveal the rotational diffusion dynamics upon photoexcitation using fs-TRXL, Heo et al 93 analyzed the anisotropic scattering signal in terms of the rotational diffusion dynamics of molecules in the ground state and excited state. For the analysis, the TRXL data were divided into isotropic and anisotropic signals, which contain information on the radial distribution and the orientation distribution, respectively.…”

“…It was suggested that the negative feature in the low-q region of the difference scattering curve implies an expansion of the structure, which is consistent with the previous theoretical prediction that the 3 MC state has a signicantly elongated metal-ligand bond compared to that of the 3 MLCT state. In particular, the detailed structural analysis showed that the average Fe-ligand bond length of the 3 recently unraveled using fs-TRXL by Heo et al 93 Although TRXL on I 3 À had been performed at synchrotrons, the charge distribution of I 3 À could not be determined in those studies due to the limited signal-to-noise ratio. The signal-to-noise ratio of fs-TRXL data acquired at an XFEL is much higher than that of the previous TRXL data collected at synchrotrons.…”

“…Many studies had attempted to provide evidence of the direction of bond dissociation using computational tools and ultrafast electron diffraction [119][120][121] However, direct evidence for the shorter-bond dissociation from the experimental observation without theoretical support had not been provided. This question was recently addressed via fs-TRXL experiments at XFELs by Heo et al, 93 which allowed a signicantly improved signal-to-noise ratio thanks to the high photon ux. In addition, if the relationship between the charge distribution and the direction of the bond dissociation can be obtained, such results can provide invaluable insights into how the atomic charge distribution is linked to which bond dissociates.…”