The carbon-iodine bond cleavage and isomerization of iodoform visualized with femtosecond X-ray liquidography

Iodoform (CHI₃) has garnered significant attention for its unique ability to induce photo-cyclopropanation of olefins by releasing an iodine radical through C-I bond cleavage. However, the detailed mechanism underlying CHI₃ photodissociation is still not fully understood. Here, we elucidate the ultrafast structural dynamics of CHI₃ upon photoexcitation using femtosecond time-resolved X-ray liquidography (fs-TRXL) at an X-ray free-electron laser facility. The fs-TRXL data was decomposed into the isotropic and anisotropic data. The isotropic data reveal that the formation of CHI₂ and I radicals upon photolysis precedes the emergence of iso-CHI₂-I. After a short induction period, two competing geminate recombination pathways of CHI₂ and I radicals take place: one pathway leads to the recovery of CHI₃, while the other results in the formation of iso-CHI₂-I. Additionally, the anisotropic data show how the transient anisotropic distribution of both the species formed upon photoexcitation and the ground-state species depleted upon photoexcitation decays through rotational dephasing. Furthermore, the observed structural dynamics of CHI₃ has distinctive differences with that of BiI₃, which can be attributed to differences in their central moieties, CH and Bi. Our findings provide insights into the photoinduced reaction dynamics of CHI₃, enhancing the understanding of its role in photochemical reactions.