Solvent environment controls ultrafast bifurcation between dissociation and isomerization in iodoform

Solvent environments can profoundly influence reaction dynamics, yet capturing ultrafast atomic motions during these events remains challenging. Here, we employ femtosecond time-resolved x-ray liquidography to track the atomic motions of iodoform (CHI3) in acetonitrile. We observe an ultrafast bifurcation (<200 fs) into two distinct pathways: complete dissociation to CHI2• + I• radicals and roaming-like isomerization to iso-CHI2-I. Along the dissociation branch, the I⋯I distance increases with time, accompanied by increasing spatial dispersion. In contrast, the isomerization branch exhibits a rapid contraction of the bonding I-I pair to ∼2.8 Å, while non-bonded pairs persist at 3.6-3.9 Å, indicative of reorientation rather than separation. This behavior differs sharply from dynamics in cyclohexane, where sub-picosecond isomer formation is absent. These findings provide an atomically resolved visualization of the bifurcation between dissociation and roaming-like isomerization in solution, demonstrating how solvent environments fundamentally alter the structural dynamics of ultrafast solution photochemistry.