Classical trajectory study of the unimolecular dissociation of ammonia

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Abstract
An analytical potential‐energy surface based on the Varandas–Murrell potential for equilibrium NH3 has been formulated using the results of scaled CI/6‐31G* calculations to adjust the potential parameters to give the correct energies and geometries for equilibrium NH3, the inversion, and the NH2+H and NH+H2 dissociation channels. Microcanonical unimolecular decay coefficients have been calculated for both channels over the energy range 5.25–7.0 eV from classical trajectories. The overall dissociation mechanism is found to consist of two parallel first‐order decay processes. Although the reaction thresholds for both channels are nearly identical on our potential‐energy surface, it is found that dissociation to NH2+H is the major decomposition pathway at all energies. The computed product translational energy distributions for NH2+H are peaked at energies near zero, as expected for dissociation processes which have no barrier to the back reaction. In contrast, the corresponding distributions for the NH+H2 product are found to be shifted toward higher energies due to the presence of a 15 kcal/mol back‐reaction barrier. Examination of the mechanistic details of individual trajectories shows that dissociation to NH+H2 occurs via a concerted elimination. Deuterium isotope effects are reported for the dissociation of ND3, NH2D, and NHD2.