Probing Iodine Atom Interactions in 4-Iodo-L-phenylalanine Crystals by X-Ray Absorption Near-Edge Structure Spectroscopy

Abstract

Iodine L-edge X-ray absorption near-edge structure (XANES) measurements were performed to characterize each crystal form of the compounds containing iodine atoms. 4-Iode-l-phenylalanine (4ILP) was used as a model compound. Each crystalline form had a distinctive XANES spectral shape. Complementary single-crystal X-ray structure analyses revealed diverse atomic interactions surrounding the iodine atoms, including C···I contacts and I···I halogen bonds. These different interactions influence the electronic states of the iodine atoms of 4ILP, resulting in distinct features in the XANES spectra. Notably, the spectral changes in the L₁ absorption edge of iodine atoms were found to be sensitive to van der Waals contacts. On the other hand, those in L₂ and L₃ were sensitive to the presence or absence of halogen bonds. This research highlights the crucial impact of weak nonconventional atomic interactions on the XANES spectra, providing valuable insights for the development and evaluation of crystalline materials.