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

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.

Reactivity oscillation in the heavy–light–heavy Cl + CH 4 reaction

“…The Cl + CH 4 →HCl + CH 3 reaction has been the subject of extensive experimental and theoretical investigations due to its crucial role in the Cl/O 3 destruction chain mechanism in the stratosphere, and has become a prototype for studying mode specificity and bond selectivity in polyatomic reactions with a late barrier (17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36). As a hydrogen transfer reaction between the heavy Cl atom and relatively heavy CH 3 moieties, it is also an ideal HLH system for dynamics study.…”

Rainbows, supernumerary rainbows and interference effects in the angular scattering of chemical reactions: effect of varying the modulus of the S matrix in the context of Heisenberg’s S matrix programme

“…In addition, our theoretical development applies to reactions treated by many other approximate theoretical techniques such as reduced dimensionality methods [17,18], e.g. the rotating-line-umbrella approximation for reactions of the type…”

Application of the Partial Wave QP Decomposition to the Angular Scattering of the State-to-State F + H2 Reaction at Etrans = 0.04088 eV

“…Now, there is no guarantee that the NF decomposition ()–() is physically meaningful, even though it is mathematically exact. However, it is known that a resummation of the PWS () can significantly improve the physical effectiveness of a NF decomposition, a process known as “cleaning”. ,,− ,,, A detailed account of resummation theory for a Legendre PWS has been given by Totenhofer et al, so we do not repeat this material here. Notice that the numerical values of f (θ R ) at a fixed θ R are independent of resummation.…”

“…The most recent review of NF theory is that of Child, while shorter (earlier) reviews can be found in refs − . Recent applications of NF techniques have provided insights into the dynamics of the CH 4 + Cl → CH 3 + HCl reaction, the S N 2 Cl – + CH 3 Br → ClCH 3 + Br – reaction, four complex-mode reactions, rainbow scattering in reactive collisions, , and the forward-angle scattering of the H + HD → H 2 + D reaction …”