PERIPHERAL DYNAMICS OF THE CL-]HCL+CH3 REACTION - A CLASSICAL TRAJECTORY COMPUTATION(CH4)

The Cl + CH4 --> HCl + CH3 reaction is expected to provide a prototype of a peripheral mechanism. This proposal is examined via a classical trajectory computation using a number of model potentials in which the degrees of freedom which do not take part in the net reaction are, or are not, frozen. The models include a full six-atom potential. The es sential features of the dynamics are not sensitive to the level of det ail with which the CH3 is described, showing that the intramolecular d ynamics of the radical do not significantly affect the dynamics of the reactive event. The reaction is found to proceed by two distinct mech anisms: for trajectories with a large impact parameter, a very short l ived complex is formed and dissociates to a rotationally cold HCl prod uct, scattered into the forward direction. At smaller impact parameter s, the reaction proceeds via a direct mechanism with a rotationally ho t HCl which is scattered backward. The computed angular distribution i s in agreement with the experiment, which detects HCl in the j = 1, 3 states and suggests that higher rotational states of HCl, which were n ot probed in the experiment, will also be scattered backward. The role of the initial vibrational excitation of CH4 is discussed.