Control of Conical Intersection Dynamics in CF ₃ I Photodissociation via Halogen Bonding

The dynamics of conical intersections are critical in governing the diverse pathways of chemical reactions, especially in photoexcited polyatomic systems. However, the systematic control of conical intersection dynamics remains a formidable challenge in reaction dynamics. Here, we demonstrate that halogen bonding, a key type of noncovalent interaction between halogenated molecules and electron-rich species, can control the conical intersection dynamics of halogenated compounds. Using time-sliced ion velocity map imaging with high resolution, we investigate the ultraviolet photodissociation dynamics of CF₃I upon interaction with N₂, CO, and C₂H₄ in supersonic molecular beams. We observe significant changes in the speed and angular distributions of the I-(²P_3/2) products, as well as in the product branching ratios of I-(²P_3/2):(I-(²P_3/2) + I*-(²P_1/2)), which vary from 0.10 to 0.71. These changes arise from halogen bonding of CF₃I with N₂, CO, and C₂H₄ at ultraviolet wavelengths near the conical intersection region. Theoretical calculations reveal that halogen bonding elevates curve-crossing energies at the conical intersections from the ³Q₀₊ state to the ¹Q₁ state in a degree that directly correlates with the strength of the halogen bonding, whether in σ or π halogen bonding. This results in a reduction of product velocities and a controllable enhancement of curve-crossing probabilities to the I-(²P_3/2) channel. These findings highlight the role of halogen bonding in controlling nonadiabatic transition dynamics and offer a "soft chemical control" approach for regulating chemical reactions.