Photolysis of Hexafluoroacetone in the Presence of H2, D2, and HD. Kinetic Isotope Effects in the Reaction of CF3 with Molecular Hydrogen

Abstract

The photolysis of hexafluoroacetone has been used as a source of CF3 radicals which either react with hydrogen to produce fluoroform or recombine to form hexafluoroethane. Relative rates of these reactions approximately obey the Arrhenius expressions over the temperature range from 333° to 870°K, kH2 / 2k21 / 2 = 9.3 × 103exp[( − 10 660 ± 140) / RT] liter1 / 2mole−1 / 2·sec−1/2, kHD / k21 / 2 = 7.4 × 103exp[(− 10 770 ± 150) / RT], kDH / k21 / 2 = 9.6 × 103exp[(− 11 630 ± 130) / RT], kD2 / 2k21 / 2 = 6.9 × 103exp[(− 11 660 ± 110) / RT]. Direct measurements of isotope effects were obtained by mass spectrometric analysis of the fluoroform produced during photolysis with added H2/D2 mixtures or with HD. Rate constant ratios are kH2 / kD2 = 1.30 exp(1050 / RT), kHD / kDH = 0.78 exp(860 / RT). The observed isotope effects are compared with those calculated on the basis of three- and six-atom LEPS and BEBO models for the activated complex.