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

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, $$k_{{\mathrm{H}}_2}{\text{\hspace{0.17em}/\hspace{0.17em}2k}}_2^{\text{1\hspace{0.17em}/\hspace{0.17em}2}}{\text{\hspace{0.17em}=\hspace{0.17em}9.3\hspace{0.17em}×\hspace{0.17em}10}}^3\exp \text{[(\hspace{0.17em}−\hspace{0.17em}10 660\hspace{0.17em}±\hspace{0.17em}140)\hspace{0.17em}/\hspace{0.17em}RT]}{\mathrm{liter}}^{\text{1\hspace{0.17em}/\hspace{0.17em}2}}{\mathrm{mole}}^{\text{−1\hspace{0.17em}/\hspace{0.17em}2}}·{\sec }^{\text{−1/2}},$$ $$k_{\mathrm{HD}}{\mathrm{\hspace{0.17em}/\hspace{0.17em}k}}_2^{\text{1\hspace{0.17em}/\hspace{0.17em}2}}{\text{\hspace{0.17em}=\hspace{0.17em}7.4\hspace{0.17em}×\hspace{0.17em}10}}^3\exp \text{[(−\hspace{0.17em}10 770\hspace{0.17em}±\hspace{0.17em}150)\hspace{0.17em}/\hspace{0.17em}RT],}$$ $$k_{\mathrm{DH}}{\mathrm{\hspace{0.17em}/\hspace{0.17em}k}}_2^{\text{1\hspace{0.17em}/\hspace{0.17em}2}}{\text{\hspace{0.17em}=\hspace{0.17em}9.6\hspace{0.17em}×\hspace{0.17em}10}}^3\exp \text{[(−\hspace{0.17em}11 630\hspace{0.17em}±\hspace{0.17em}130)\hspace{0.17em}/\hspace{0.17em}RT],}$$ $$k_{{\mathrm{D}}_2}{\text{\hspace{0.17em}/\hspace{0.17em}2k}}_2^{\text{1\hspace{0.17em}/\hspace{0.17em}2}}{\text{\hspace{0.17em}=\hspace{0.17em}6.9\hspace{0.17em}×\hspace{0.17em}10}}^3\exp \text{[(−\hspace{0.17em}11 660\hspace{0.17em}±\hspace{0.17em}110)\hspace{0.17em}/\hspace{0.17em}RT].}$$ 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 $k_{{\mathrm{H}}_2}{\mathrm{\hspace{0.17em}/\hspace{0.17em}k}}_{{\mathrm{D}}_2}\text{\hspace{0.17em}=\hspace{0.17em}1.30}\exp {\text{(1050\hspace{0.17em}/\hspace{0.17em}RT), k}}_{\mathrm{HD}}{\mathrm{\hspace{0.17em}/\hspace{0.17em}k}}_{\mathrm{DH}}\text{\hspace{0.17em}=\hspace{0.17em}0.78}\exp \text{(860\hspace{0.17em}/\hspace{0.17em}RT)}$ . 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.