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(2P3/2) and I(2P1/2) and the corresponding haloethyl radicals. The I(2P3/2) 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.