Femtosecond concerted elimination of halogen molecules from halogenated alkanes

The concerted dynamics involved in the molecular detachment of halogenated alkanes, i.e., CX2YZ→CX2+YZ (where X=H, F or Cl and Y, Z=I, Br or Cl) have been studied by femtosecond pump–probe spectroscopy. Particular emphasis has been placed on exploring the role of symmetry in the parent molecule. For experiments carried out on CH2ICl, product fluorescence was observed in two regions of the spectrum: at 430 nm, corresponding to the D′→A′ transition, and at 340 nm, attributed to the G→A transition. Differences in the dissociation time for the two pathways can be understood by considering the energy available for fragment recoil. The elimination process was found to be slower for methylene bromide than for methylene iodide, possibly because of the difference in the enthalpy of reaction. When three gem-dibromo compounds (Y, Z=Br, X=H, F or Cl) were compared, they were found to have significantly different dissociation times: 29.7 fs for the fluorinated species, 58.6 fs for the hydrogenated species, and 80.6 fs for the chlorinated species. The difference in the transition state lifetime between the hydrogenated and chlorinated species can be rationalized in terms of changes in the reduced mass, the thermodynamics of the reactions, and the energy partitioning. The fluorinated compound was found to dissociate much faster than predicted by thermodynamic and reduced mass arguments.