Theory Finally Agrees with Experiment for the Dynamics of the Cl + C₂H₆ Reaction

Abstract

Since the pioneering reaction dynamics studies of H + H2 in the 1970s, theory increased the system size by one atom in every decade arriving to six-atom reactions in the early 2010s. Here, we take a significant step forward by reporting accurate dynamics simulations for the nine-atom Cl + ethane (C2H6) reaction using a new high-quality spin–orbit–ground-state ab initio potential energy surface. Quasi-classical trajectory simulations on this surface cool the rotational distribution of the HCl product molecules, thereby providing unprecedented agreement with experiment after several previous failed attempts of theory. Unlike Cl + CH4, the Cl + C2H6 reaction is exothermic with an adiabatically submerged transition state, allowing testing of the validity of the Polanyi rules for a negative-barrier reaction.

First-Principles Reaction Dynamics beyond Six-Atom Systems

“… 64 The former is more efficient but currently implemented for a limited number of system types beyond six atoms, whereas the latter uses automatic code generation to be able to handle arbitrary systems; thus, we used the MSA program 64 to fit PESs for the F/Cl + C 2 H 6 reactions. 37 , 38 While we used PIP exclusively for our PESs to date and thus have no direct experience with other fitting methods, it is also important to note that in the recent past promising neural-network-based fitting strategies started to become widespread. 33 , 65 , 66 The above three steps are usually carried out multiple times, thereby iteratively improving the PES.…”

“…We demonstrate the success of the first-principles methodology described in section II for post-six-atom systems by briefly discussing recent applications and results from our group on the Cl + C 2 H 6 , F + C 2 H 6 , and OH – + CH 3 I reactions. 37 , 38 , 41 As highlighted below, the three systems pose different challenges, whose solutions may be found useful in similar future investigations. In all cases we used Robosurfer ( 18 ) to automatically develop the full-dimensional PESs and the dynamics was studied by the QCT method.…”

“…The full-dimensional PES for the Cl( 2 P 3/2 ) + C 2 H 6 reaction was obtained by fitting 11 701 energy points with a fifth-order polynomial of Morse-like variables resulting in 3234 coefficients. 38 The ab initio energies were computed at the UCCSD(T)-F12b/aug-cc-pVDZ + RMP2-F12/aug-cc-pVTZ – RMP2/aug-cc-pVDZ + Δ SO (MRCI+Q/aug-cc-pVDZ) composite level of theory, thereby obtaining spin–orbit-corrected UCCSD(T)-F12b/aug-cc-pVTZ-quality results at a significantly lower computational cost. As Figure 2 shows, the Cl( 2 P 3/2 ) + C 2 H 6 → HCl + C 2 H 5 reaction has a small classical barrier (Δ E TS = 2.21 kcal/mol) and is endothermic (Δ E = 2.04 kcal/mol) without ZPE correction, whereas the vibrationally adiabatic reaction pathway is exothermic (Δ H 0 = −3.06 kcal/mol) with a submerged transition state (Δ E TS = −2.12 kcal/mol).…”

“… Schematic classical (blue lines without ZPE) and adiabatic (green lines with ZPE) potential energy surface of the Cl( 2 P 3/2 ) + C 2 H 6 → HCl + C 2 H 5 reaction showing the relative energies of the stationary points corresponding to the analytical PES (ref ( 38 )) compared with benchmark relativistic all-electron CCSDT(Q)/complete-basis-set-quality reference data (taken from ref ( 69 ) and shown in parentheses) and experiment (ATcT). Adapted with permission from ref ( 38 ). Copyright 2020 American Chemical Society.…”

“…37 , 38 , 41 The three systems represent different challenges during the PES developments from the electronic structure point of view. Cl + C 2 H 6 is a less complicated case, 38 for F + C 2 H 6 the Hartree–Fock method fails in the entrance channel, 37 and OH – + CH 3 I suffers from a serious breakdown of the gold-standard CCSD(T) method. 41 We show how to solve these problems and provide comparisons with experiments 47 − 49 demonstrating the power and accuracy of first-principles reaction dynamics for post-six-atom systems.…”