INFRARED-LASER INDUCED DECOMPOSITION OF HEXAFLUOROBENZENE AND SOME MONOSUBSTITUTED DERIVATIVES - INTERMEDIACY OF THE PENTAFLUOROPHENYL RADICAL

The infrared multiphoton laser induced reactions of hexafluorobenzene and related pentafluoro analogues (pentafluorobenzene, pentafluorochlorobenzene, pentafluorobromobenzene, and pentafluoroiodobenzene) have been investigated using a CO2 TEA laser. The study was carried out in order to define the decomposition products and to attempt to clarify their mode of formation. Thus, the products (relative yield, %) of the irradiation of C6F6 (1027.3 cm-1; 0.73.J/cm2; 10 pulses; 25% decomposition) were C2F4(64), C6F5CF3(28), C2F6(7), CF4(1) and that for C6F5H (949.4 cm-1; 0.80 J/cm2; 10 pulses; 25% decomposition) was C2F4 and C6F5CF3. Increasing the number of pulses in the reaction with C6F6 decreased the amount of C2F4 and increased the amount of C6F5CF3 and C2F6 indicating secondary and tertiary reactions. Addition of halogen (X2, X = Cl, Br) to these reactions caused different products to be formed. Thus, the irradiation of a C6F6/Cl2 mixture (7.4/7 Torr; 1027.3 cm-1; 0.7 J/cm2; 35 pulses; 35% reaction) afforded C6F5Cl(46); CF3Cl(24) and CF2Cl2(30). Irradiation of C6F5H/X2 mixtures afforded mainly C6F5X + HX. For example, C6F5H/Br2 (10/40 Torr; 949.4 cm-1; 0.93 J/cm2; 10 pulses; 10% reaction) gave C6F5Br and HBr exclusively. Irradiation of C6F5-X (X = Cl, Br, I) (977.2 cm-1; ca. 0.74 J/cm2; 200 pulses, 39-74% reaction) gave C6F6 and a minor amount of decafluorobiphenyl [(C6F5)2], a radical combination product of the pentafluorophenyl radical (C5F5·). Increasing the fluence in these reactions gave similar produts in most cases but in some instances increased the amount of C2F4 formed. The reactions and product distribution of the hydrogen substituted derivative (C6F5H) was examined in the presence of Br2 as a function of laser fluence and halogen concentration. It was found that the threshold for C6F5H decomposition was higher for the reaction involving Br2 (as compared with the reaction involving Cl2 or neat C6F5H). The presence of Br2 also decreased the amount of C6F5H that was decomposed, indicating a quenching process. The decomposition path with the lowest activation energy for these molecules is thought to be C6F5X→C6F5· + X· and was accessible using a laser pulse with a fluence as low as 0.7 J/cm2. Using a higher laser fluence (ca. 1.2 J/cm2) diand triatomic radicals were defined by spectroscopic identification of the ·CF and :CF2 species. These reactions are discussed in light of the formation of the C6F5· radical during a primary, larser induced, process. Subsequent decomposition to smaller fragments, combination with other radicals or scavenging by added reagents also takes place depending on the reaction conditions. © 1990.