Quantum mechanical electronic structure calculations were carried out to determine equilibrium geometries, energetics, and normal-mode frequencies for stationary points along the minimum energy reaction path for the reaction of methyl radicals with chlorine molecules. The results are used to calculate the rate coefficient, employing both extended RRKM theory and quasi-classical trajectory techniques. The results of both methods agree well with each other and with the experimental measurements. The reactivity is investigated and is shown to be greatly enhanced by increasing the initial translational energy of the reactants. A large part of the total available energy is shown to be directed into product vibration, in good consistency with the experimental observations.