Spectroscopic and Computational Studies of the Laser Photolysis of Matrix Isolated 1,2-Dibromoethanes: Formation and Fate of the Bromoethyl Radicals

We report experimental and computational studies of the photolysis of atmospherically important 1,2-dibromoethanes (1,2-C2X4Br2; X = H, F) in Ar matrixes at 5 K. Using the pulsed deposition method, we find that significant conformational relaxation occurs for 1,2-C2H4Br2 (EDB; observed anti/gauche ratio =30:1) but not for 1,2-C2F4Br2 (TFEDB; anti/gauche = 3:1), which is traced to a larger barrier to rotation about the C−C bond in the latter. Laser photolysis of matrix-isolated EDB at 220 nm reveals the growth of infrared bands assigned to the gauche conformer and C2H4−Br2 charge transfer complex (both as major products), and the C2H4Br radical and C2H3Br−HBr complex as minor (trace) products. The presence of the C2H4−Br2 complex is confirmed in the UV/visible spectrum, which shows an intense charge transfer band at 237 nm that grows in intensity upon annealing. In contrast to previous reports, our experimental and computational results do not support a bridged structure for the C2H4Br radical in either the gas phase or matrix environments. We also report on the laser photolysis of matrix-isolated TFEDB at 220 nm. Here, the dominant photoproducts are the anti and gauche conformers of the C2F4Br radical, the vibrational and electronic spectra of which are characterized here for the first time. The increase in yield of radical for TFEDB vs EDB is consistent with the stronger C−Br bond in the fluoro-substituted radical species. The photochemistry of the C2F4Br radical following excitation at 266 nm was investigated and found to lead C−Br bond cleavage and formation of C2F4. The implications of this work for the atmospheric and condensed phase photochemistry of the alkyl halides is emphasized.