Thermal Decomposition of Azomethane. II. The Unimolecular Reaction on the Quantum Harmonic Model

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Abstract
The decomposition of azomethane is a complex process, but previous work has shown that the residual, fully NO-inhibited reaction has all the features of a unimolecular reaction. This residual reaction is now treated by the quantum harmonic version of the Marcus—Rice theory of unimolecular reactions. Extrapolation to infinite pressure gives log10k∞=17.32 — 55 500/2.303 RT. In accordance with the high pre-exponential factor (and hence high entropy of activation) a very loose activated complex is postulated involving free rotations of the two methyls and the central nitrogen. Two models of this complex are considered: one having a planar methyl and the other a tetrahedral methyl. From the pre-exponential factor there are deduced six more active rotations in the complex than in the molecule. Exact vibrational—rotational energy sums are evaluated on a computer, as is the rate constant integral itself. Within the relatively large experimental error, and subject to the uncertainty in collision diameter and collisional deactivation efficiency, good pressure fit between theory and experiment in the rate constant pressure fall-off is obtained for a theoretical model having nine or more active rotations in the complex and three or more in the molecule. It turns out that the vibrational frequency pattern of the complex is so much less important than the number of active rotations that both models for the complex, tetrahedral or planar, lead to identical fall-off behavior. Theory also predicts an activation energy fall-off with pressure that is, on the whole, in accord with the rather limited experimental data.

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