Solvent-dependent complex reaction pathways of bromoform revealed by time-resolved X-ray solution scattering and X-ray transient absorption spectroscopy

Abstract

The photochemical reaction pathways of CHBr₃ in solution were unveiled using two complementary X-ray techniques, time-resolved X-ray solution scattering (TRXSS) and X-ray transient absorption spectroscopy, in a wide temporal range from 100 ps to tens of microseconds. By performing comparative measurements in protic (methanol) and aprotic (methylcyclohexane) solvents, we found that the reaction pathways depend significantly on the solvent properties. In methanol, the major photoproducts are CH₃OCHBr₂ and HBr generated by rapid solvolysis of <i>iso-</i>CHBr₂-Br, an isomer of CHBr₃. In contrast, in methylcyclohexane, <i>iso</i>-CHBr₂-Br returns to CHBr₃ without solvolysis. In both solvents, the formation of CHBr₂ and Br is a competing reaction channel. From the structural analysis of TRXSS data, we determined the structures of key intermediate species, CH₃OCHBr₂ and <i>iso</i>-CHBr₂-Br in methanol and methylcyclohexane, respectively, which are consistent with the structures from density functional theory calculations.