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, I-3(-). 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 I-3(-) 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. Tracking the flow of charge in reacting molecules may provide key insight into reaction mechanisms, but is particularly challenging in liquid solutions. Here the authors, by analyzing the isotropic and anisotropic scattering signal in femtosecond time resolved X-ray liquidography, determine the charge localization and structural changes during photodissociation of the triiodide anion I-3(-)