Quantum studies of H2O+Cl→HO+HCl and H2O+Br→HO+HBr reactions. A comparison of two reduced dimensionality approaches

Abstract

3D quantum calculations using both the rotating bond approximation (RBA) and the fixed angle potential averaged (FAPA) approximation have been performed to obtain cross sections for the H₂O+Cl→HO+HCl and H₂O+Br→HO+HBr reactions. The two models mainly differ in the motions that are allowed to the H-spectator atom and to the H-reactive atom. Cross sections for the H₂O(m, n)+Cl→HO+HCl and H₂O(m, n)+Br→HO+HBr reactions have been calculated for a wide range of the bending quantum number m and the local stretching quantum number n. The cross sections, as functions of the translational energy, summed over all the product states, exhibit very interesting features when considering series of H₂O(m, n)+X entrance channels with the value of n fixed within a series. For the H₂O+Cl system, for which the considered total energy range extends from 0 to 1.6 eV above the H₂O(0, 0)+Cl limit, in each n=0, 1 and 2 series, the cross sections for the H₂O(m variable, n fixed)+Cl entrance channels have approximately the same energy threshold and similar shapes above the threshold. These results, obtained with the RBA and FAPA approaches, show that the bending excitation does not significantly enhance the reactivity. On the other hand, the local stretching excitation is found to very efficiently promote reaction. For the H₂O+Br system, for which the considered energy range is wider (0–2.2 eV), the agreement between the results obtained with the two methods is less satisfactory. However, the disagreement concerns highly excited bending states which lie at significantly different energies in the two calculations. The common energy threshold and similar shape are obtained for the n=2 (FAPA) and n=3 (RBA) series and for excited bending states for the n=0 and n=1 series. All these properties are analysed on the basis of potential energy curves related to the motion of the H-atoms, for fixed O–Cl or O–Br distances.