Infrared laser induced decomposition of hexafluorobenzene and some monosubstituted derivatives. Intermediacy of the pentafluorophenyl radical

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The infrared multiphoton laser induced reactions of hexafluorobenzene and related pentafluoro analogues (pentafluorobenzene, pentafluorochlorobenzene, pentafluorobromobenzene, and pentafluoroiodobenzene) have been investigated using a CO 2 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 C 6F 6 (1027.3 cm −1; 0.73.J/cm 2; 10 pulses; 25% decomposition) were C 2F 4(64), C 6F 5CF 3(28), C 2F 6(7), CF 4(1) and that for C 6F 5H (949.4 cm −1; 0.80 J/cm 2; 10 pulses; 25% decomposition) was C 2F 4 and C 6F 5CF 3. Increasing the number of pulses in the reaction with C 6F 6 decreased the amount of C 2F 4 and increased the amount of C 6F 5CF 3 and C 2F 6 indicating secondary and tertiary reactions. Addition of halogen (X 2, X = Cl, Br) to these reactions caused different products to be formed. Thus, the irradiation of a C 6F 6/Cl 2 mixture (7.4/7 Torr; 1027.3 cm −1; 0.7 J/cm 2; 35 pulses; 35% reaction) afforded C 6F 5Cl(46); CF 3Cl(24) and CF 2Cl 2(30). Irradiation of C 6F 5H/X 2 mixtures afforded mainly C 6F 5X + HX. For example, C 6F 5H/Br 2 (10/40 Torr; 949.4 cm −1; 0.93 J/cm 2; 10 pulses; 10% reaction) gave C 6F 5Br and HBr exclusively. Irradiation of C 6F 5-X (X = Cl, Br, I) (977.2 cm −1; ca. 0.74 J/cm 2; 200 pulses, 39–74% reaction) gave C 6F 6 and a minor amount of decafluorobiphenyl [(C 6F 5) 2], a radical combination product of the pentafluorophenyl radical (C 5F 5·). Increasing the fluence in these reactions gave similar produts in most cases but in some instances increased the amount of C 2F 4 formed. The reactions and product distribution of the hydrogen substituted derivative (C 6F 5H) was examined in the presence of Br 2 as a function of laser fluence and halogen concentration. It was found that the threshold for C 6F 5H decomposition was higher for the reaction involving Br 2 (as compared with the reaction involving Cl 2 or neat C 6F 5H). The presence of Br 2 also decreased the amount of C 6F 5H that was decomposed, indicating a quenching process. The decomposition path with the lowest activation energy for these molecules is thought to be C 6F 5X→C 6F 5· + X· and was accessible using a laser pulse with a fluence as low as 0.7 J/cm 2. Using a higher laser fluence ( ca. 1.2 J/cm 2) diand triatomic radicals were defined by spectroscopic identification of the ·CF and :CF 2 species. These reactions are discussed in light of the formation of the C 6F 5· 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.