Kinetic Isotope Effect in Photochlorination of H2, CH4, CHCl3, and C2H6

Abstract

The relative rates of hydrogen and deuterium abstraction by chlorine atoms from H₂–D₂, CH₄–CD₄, and C₂H₆–C₂D₆ in direct competition have been studied by a mass spectrometer technique permitting direct determinations of rates. The pair CHCl₃–CDCl₃ was investigated by a substitution—addition competition with C₂Cl₄. Data were obtained between 300° and 475°K, and values of k_H/k_D ranged between 3.64 and 10.9 for CH₄/CD₄, 2.37 to 2.69 for C₂H₆/C₂D₆, 1.58 to 2.26 for CHCl₃/CDCl₃, and 4.7 to 8.9 for H₂/D₂. For the family of reactions involving C–H–Cl, an extensive series of model calculations were made by the ``bond‐energy—bond‐order'' method. The expected variation of activation energy with heat of reaction was computed and compared with experiment. The expected variation of kinetic isotope effect with both temperature and heat of reaction was computed for a range of conditions much wider than these experiments. Simple activated complex theory predicts several strong trends of kinetic isotope effect with heat of reaction. The data, in a general semiquantitative way, follow these predicted trends, but in terms of fine details the data do not fit the theoretical lines. In view of the known simplifications in activated complex theory, this agreement with broad trends is all that anyone should expect.