Chemical reaction dynamics are studied to follow and understand the concerted motion of several atoms while they rearrange from reactants to products. With the number of atoms growing, the number of pathways, transition states, and product channels also increases and rapidly presents a challenge to experiment and theory. Here, we disentangle the competition between bimolecular nucleophilic substitution (S N 2) and base-induced elimination (E2) in the polyatomic reaction F - + CH 3 CH 2 Cl. We find quantitative agreement for the energy- and angle-differential reactive scattering cross sections between ion imaging experiments and quasi-classical trajectory simulations on a 21-dimensional potential energy hypersurface. The anti-E2 pathway is most important, but the S N 2 pathway becomes more relevant as the collision energy is increased. In both cases the reaction is dominated by direct dynamics. Our study presents atomic level dynamics of a major benchmark reaction in physical organic chemistry, thereby pushing the number of atoms for detailed reaction dynamics studies to a size that allows applications in many areas of complex chemical networks and environments.

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.

We compute benchmark structures, frequencies, and relative energies for the stationary points of the potential energy surface of the F + CHCHCl reaction using explicitly correlated ab initio levels of theory. CCSD(T)-F12b geometries and harmonic vibrational frequencies are obtained with the aug-cc-pVTZ and aug-cc-pVDZ basis sets, respectively. The benchmark relative energies are determined using a high-level composite method based on CCSD(T)-F12b/aug-cc-pVQZ frozen-core energies, CCSD(T)-F12b/cc-pCVTZ-F12 core electron correlation effects, and CCSD(T)-F12b/aug-cc-pVDZ zero-point energy corrections. The S2 channel leading to Cl + CHCHF (-33.2) can proceed via back-side (-11.5), front-side (29.1), and double-inversion (18.0) transition states, whereas the bimolecular elimination (E2) products, Cl + HF + CH (-19.3), can be formed via anti (-15.0) and syn (-7.3) saddle points, whose best adiabatic energies relative to F + CHCHCl are shown in parentheses in kcal/mol. Besides the S2 and E2 channels, the 0 K reaction enthalpies of the HF + HC-CHCl (29.4), H + HC-CHClF (46.2), H + FHC-CHCl (51.1), and FCl + CHCH (49.7) product channels are determined. Utilizing the new benchmark data, the performance of the DF-MP2, MP2, MP2-F12, CCSD(T), and CCSD(T)-F12b methods with aug-cc-pVDZ and aug-cc-pVTZ basis sets is tested.

We report a full-dimensional spin-orbit-corrected analytical potential energy surface (PES) for the HBr + C2H5 → Br + C2H6 reaction and a quasi-classical dynamics study on the new PES. For...

Quasi-classical trajectory computations on a high-level analytical potential energy surface reveal the mode-specific dynamics of the F− + CH3CH2Cl reaction.