Distribution of reaction products (theory). VIII. Cl+HI, Cl+DI

Abstract

The dynamics of the thermal (300°K) reactions $\mathrm{Cl+HI}\to \mathrm{ClH+I}$ and $\mathrm{Cl+DI}\to \mathrm{ClD+I}$ have been examined by the classical trajectory method, in 3D. The Cl+HI reaction has also been studied at an enhanced (6 kcal mole⁻¹) collision energy. The potential‐energy hypersurface was the same as that used earlier [J. Chem. Phys. 49, 5189 (1968)]. Though it is a highly repulsive energy surface it is able to account for the efficient vibrational excitation of the molecular product for the mass combination characteristic of this reaction. The effect of changing the mass combination from H+LH (heavy+light‐heavy; masses m_Cl+m_HI) to L+HH (light+heavy‐heavy; masses m_H+mCl₂) on the Cl+HI surface has been explored using a full 3D set of trajectories at 300°K. The effect is to markedly reduce the ``mixed energy release'' responsible for the efficient vibrational excitation on the repulsive surface. Vibrational and rotational excitation in the reaction products is correspondingly diminished, and translational excitation is enhanced. The efficient vibrational and rotational excitation for H+LH (Cl+HI), and contrasting behavior for L+HH (H+Cl<sub>2</sub>) have been observed in infrared chemiluminescence experiments. The present findings are therefore in accord with earlier proposals that both these reactions involve predominantly ``repulsive'' energy release. The computed product angular distribution for the Cl+HI reaction at 300°K was almost isotropic, in contrast to the exclusively backward‐hemisphere scattering for the L+HH mass combination on the same energy surface. At 6 kcal mole⁻¹ collision energy the computed angular distribution of HCl from the Cl+HI reaction showed exclusively sharply‐forward scattering, in accord with the results of recent molecular beam experiments [J. D. MacDonald and D. R. Herschbach (unpublished)]. Enhanced collision energy gave rise to a small decrease in the computed mean product vibrational excitation, a small increase in mean product rotational excitation and a large increase in product translational excitation. These changes in product energy distribution are in qualitative accord with the findings from infrared chemiluminescence and molecular beam studies at enhanced collision energy. The overall conclusion is that the repulsive LEPS (London, Eyring, Polanyi, Sato) potential‐energy hypersurface used here and in our earlier work, provides an acceptable (though not unique) first approximation to the actual interaction potential.