Ultraviolet photodissociation and thermochemistry of CH2BrCH2I, CF2BrCF2I, and CF2ICF2I

Abstract

Using photofragment translational spectroscopy, we have monitored the dissociation of CH₂BrCH₂I at 248, 266, and 308 nm, and CF₂BrCF₂I and CF₂ICF₂I at 308 nm. The primary fragments are I(²P_3/2) and I(²P_1/2) and the corresponding haloethyl radicals. The I(²P_3/2) contribution decreases upon fluorination, but it is dominant for CH₂BrCH₂I at 308 nm. The electronic absorption dipole lies roughly along the C–I bond axis in every case. Stable CF₂CF₂Br and CF₂CF₂I radicals can be readily generated through photodissociation of the parent compounds, while stable CH₂CH₂Br could not be unambiguously observed. Upper limits to the reaction enthalpy at 0 K for CF₂ICF₂Br(I)→C₂F₄+I+Br(I) are 75±1(59±1) kcal/mol. The TOF spectra and related data suggest that there is a barrier to decomposition for CF₂CF₂I→C₂F₄+I that exceeds the C–I bond energy in the radical.