Roaming-mediated isomerization is a universal reaction mechanism in photochemistry, yet solvent-dependent pathways of roaming intermediates remain poorly understood, particularly for environmentally relevant halogen compounds involved in ozone depletion. Here, using femtosecond time-resolved X-ray solution scattering, we resolve the solvent-dependent roaming dynamics of CHBr 3 in methanol and methylcyclohexane. By combining multi-method experimental analysis, machine learning-assisted ab initio molecular dynamics simulations, and density functional theory calculations, we uncover distinct solvent-steered reaction pathways. In methanol, roaming enhances solute-solvent interactions, leading to solvolysis before a stable isomer forms. In methylcyclohexane, roaming facilitates isomerization to a long-lived iso-CHBr 2 -Br product. Direct dissociation into CHBr 2 + Br competes with both pathways in either solvent. By tracking bond-length oscillations and angular dynamics in real time, we visualize how the condensed-phase environment governs the branching ratio between competing pathways. Our findings establish solute-solvent interactions as key factors controlling roaming-mediated reactions in CHBr 3 , with broad implications for photochemical outcomes in solution.
Su et al. (Mon,) studied this question.