A quasiclassical trajectory study of H+H2O→OH+H2: Angular distributions and OH angular momentum alignment

Abstract

We present a detailed theoretical study of the H+H2O reaction dynamics using quasiclassical trajectories and two potential energy surfaces, one from Walch–Dunning–Schatz–Elgersma (WDSE) and one from Isaacson (I5). Collision energies of 1.0, 1.4, and 2.2 eV are considered, and both scalar and vector properties of the product distributions are presented. The vector properties include polarization-dependent differential cross sections (PDDCS) and angular momentum alignment parameters for both OH and H2. The WDSE and I5 scalar and vector results are in most respects very similar. However, we find that they differ noticeably with respect to angular momentum alignment, with I5 predicting weak OH alignment, while WDSE shows much stronger alignment with the OH angular momentum vector preferentially perpendicular to the scattering plane. The I5 surface is a more recent and more accurate surface for H3O, so it is extremely encouraging that the alignment predicted by I5 is in quantitative agreement with a recent measurement from Brouard and co-workers. In addition, the I5 differential cross section matches the Brouard results quantitatively, while WDSE does not. Detailed mechanistic information underlying the angular distributions, alignment, and PDDCS results is presented, and we find that the differences between I5 and WDSE alignments are connected to different energy release characteristics of the surface in the corner cutting region.

The Dynamics of the H+H2O Reaction

“…Consequently, the two possible reaction pathways that go from H H 2 O to OH H 2 have different barriers. [29] Nevertheless, the I5 PES have been used in QCT calculations, as reviewed below.…”

“…Further theoretical research on the H H 2 O reaction employing the QCT methodology revealed that QCT results can be brought in better agreement with QM results if a zero-point energy constraint on the newly formed H 2 molecule was applied. [21,29] The ZPE constraint has been simply imposed by rejecting all trajectories that lead to H 2 molecules with energy below its ZPE. However, if the ZPE of both OH and H 2 , or just the OH, are constrained the cross-sections obtained are ∫unphysically∫ small.…”

“…This fact is, at first sight, quite surprising because at this collision energy, which is only 0.5 eV above the reaction barrier, a preferentially backward DCS corresponding to rebound-type dynamics was expected. Early QCT calculations of the DCS were carried out by Bradley and Schatz [29] on the old WDSE and I5 PESs. It was found using the I5 PES that the DCS at 1.0 eV was mainly backward, at 1.4 eV was nearly symmetric as observed in the first experiments, [79] and at 2.20 eV scattering in forward hemisphere was dominant.…”

Theoretical studies of product polarization and state distributions of the H+HCl reaction

“…Han and co-workers [21,23] have studied the product rotational polarization of bimolecular reactions with different reagent mass combinations on attractive, mixed and repulsive potential energy surfaces. Schatz and co-workers [24][25][26] have calculated the product angular momentum alignment for the complex four-atom reactions, such as the H þ CO 2 and H þ H 2 O reactions.…”

Stereodynamics of chemical reactions: quasi-classical, quantum and mixed quantum-classical theories

“…A variety of triatomic chemical reactions and their isotopically substituted ones have been systematically studied . Later, Schatz and coworkers extended the QCT method to investigate the stereodynamics of more complex tetra-atomic reaction systems, such as H + H 2 O and H + CO 2 [116][117][118]. Zou and Bowman have done excellent work on the ab initio potential energy surfaces and quantum calculations of many complex molecular systems like HOCl and so on [119][120][121][122][123][124]124].…”