Angular Scattering Dynamics of the CH4 + Cl → CH3 + HCl Reaction Using Nearside-Farside, Local Angular Momentum, and Resummation Theories.

A J Totenhofer,Gunnar Nyman,J N L Connor

doi:10.1021/acs.jpcb.5b10189

A J Totenhofer, Gunnar Nyman + Show 1 more

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https://doi.org/10.1021/acs.jpcb.5b10189

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Abstract

The differential cross section (DCS) for the CH4 + Cl → CH3 + HCl reaction is studied at six total energies where all of the species are in their ground states. The scattering (S) matrix elements have been calculated by the rotating line umbrella method for a dual-level ab initio analytic potential energy surface. We make the first application to this reaction of nearside-farside (NF) and local angular momentum (LAM) techniques, including resummation orders (r) of 0, 1, 2, and 3 for the partial-wave series representation of the full scattering amplitude. We find that resummation usually cleans the NF r = 0 DCSs of unphysical oscillations, especially at small angles. This cleaning effect is typically most pronounced when changing from no resummation (r = 0) to r = 1; further resummations from r = 1 to r = 2 and from r = 2 to r = 3 have smaller effects. The NF DCS analyses show that the reaction is N-dominated at sideward and large angles, whereas at small angles there are oscillations caused by NF interference. The NF LAM analysis provides consistent and complementary information, in particular for the total angular momenta that contribute to the reaction at different scattering angles. The NF analyses also provide justification for simpler N-dominant dynamical theories such as the semiclassical optical model, which provides an explanation for the distorted mirror image effect for the moduli of the S matrix elements and the DCSs, as well as the use of a hard-sphere DCS over limited angular ranges.

Highlights

The reaction of CH4 and its isotopologues with Cl is of fundamental importance in atmospheric and combustion chemistry
NF theory is a simple yet powerful method for analyzing structures in the differential cross section (DCS) of elastic, inelastic, and reactive collisions. It does this by exactly decomposing the scattering amplitude into the sum of N and F subamplitudes, each having simpler properties than the full scattering amplitude.[46−48] Resummations of the partial-wave series (PWS) for the full scattering amplitude are included in our analyses before making an NF decomposition, since such resummations usually clean the NF DCSs of unphysical oscillations.[49−51] In addition, we carry out NF analyses of the local angular momentum (LAM),[49−57] including PWS resummations
We have studied the DCSs of the CH4(si = 0) + Cl → CH3(sf = 0) + HCl(vf = 0, jf = 0) reaction at six total energies

Summary

Introduction

The reaction of CH4 and its isotopologues with Cl is of fundamental importance in atmospheric and combustion chemistry. It is important to analyze and understand the detailed dynamical information contained within plots of the differential cross sections (DCSs) versus reactive scattering angle. NF theory is a simple yet powerful method for analyzing structures in the DCSs of elastic, inelastic, and reactive collisions It does this by exactly decomposing the scattering amplitude into the sum of N and F subamplitudes, each having simpler properties than the full scattering amplitude.[46−48] Resummations of the partial-wave series (PWS) for the full scattering amplitude are included in our analyses before making an NF decomposition, since such resummations usually clean the NF DCSs of unphysical oscillations.[49−51] In addition, we carry out NF analyses of the local angular momentum (LAM),[49−57] including PWS resummations. An NF LAM analysis provides information on the full and NF local angular momenta that contribute to the reaction at a given scattering angle under semiclassical conditions.[49−56]

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