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Peripheral dynamics of the Cl + CH4 HCl + CH3 reaction. A classical trajectory computation

Abstract

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 essential features of the dynamics are not sensitive to the level of detail with which the CH3 is described, showing that the intramolecular dynamics of the radical do not significantly affect the dynamics of the reactive event. The reaction is found to proceed by two distinct mechanisms: for trajectories with a large impact parameter, a very short lived complex is formed and dissociates to a rotationally cold HCl product, scattered into the forward direction. At smaller impact parameters, the reaction proceeds via a direct mechanism with a rotationally hot HCl which is scattered backward. The computed angular distribution is in agreement with the experiment, which detects HCl in the j = 1, 3 states and suggests that higher rotational states of HCl, which were not probed in the experiment, will also be scattered backward. The role of the initial vibrational excitation of CH4 is discussed.

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