Trajectory Studies of Hot-Atom Reactions. I. Tritium and Methane

We have studied the reactions of T+CH4 and T+CD4, treating these as six distinct particles, using a variety of potential energy surfaces subject to the restriction that only one methane hydrogen at a time is reactive. Our principal findings are: (1) This trial assumption about the potential is unjustified. Substitution (products CH3T+H and CD3T+D) involves strong interactions between at least four atoms. (2) There were no inertial isotope effects of any kind when CH4 was replaced by CD4. (3) From (2) and the details of the trajectories, there is suggestive but not conclusive evidence that substitution in CH4 proceeds by Walden inversion. (4) Abstraction (products CH3+HT and CD3+DT) is direct and concerted and occurs at relatively low energy. In our calculations it had a maximum cross section of 3.5 Å2 for a reactant translation energy of 65 kcal. At sufficiently high energy it is a stripping reaction. (5) About half the abstraction product energy is translational; the remainder appears as internal energy of both HT and CH3. When substitution occurs as a three-centered process, it deposits about 70 kcal in CH3T. Fragmentation is an important process above 100-kcal reactant energy.