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 provide benchmark relative energies for the stationary points of three different channels of the halogen atom + ethane reactions.

We report a detailed quasi-classical dynamics study on a new full-dimensional multireference spin-orbit-corrected potential energy surface (PES) for the F( 2 P3/2) + C2H6 → HF + C2H5 reaction. For the PES development the ROBOSURFER program package is applied and the MRCI-F12+Q(5,3)/aug-cc-pVDZ energy points are fitted using the monomial symmetrization approach of the permutationally invariant polynomial method. Our simulations provide substantial reaction probabilities and sharply increasing cross sections with increasing collision energy for this early-and negative-barrier reaction. Direct rebound/stripping mechanism is preferred at low/high collision energies, and the initial translational energy turns out to convert mostly into product recoil, whereas the reaction energy excites the HF vibration. Vibrational and vibrationally-resolved rotational state distributions of the HF product obtained from our computations agree well with single-collision experimental data for the vHF = 1, 2, 3 states.

We report a detailed dynamics study on the mode-specificity of the Cl + C 2 H 6 → HCl + C 2 H5 H-abstraction reaction. We perform quasiclassical trajectory simulations using a recently developed high-level ab initio full-dimensional potential energy surface by exciting five different vibrational modes of ethane at four collision energies. We find that all the studied vibrational excitations, except that of the CCstretching mode, clearly promote the title reaction, and the vibrational enhancements are consistent with the predictions of the Sudden Vector Projection (SVP) model, with the largest effect caused by the CH-stretching excitations. Intramolecular vibrational redistribution is also monitored for the differently excited ethane molecule. Our results indicate that the mechanism of the reaction changes with increasing collision energy, with no mode-specificity at high energies. The initial translational energy mostly converts into product recoil, while a significant part of the excess vibrational energy remains in the ethyl radical. An interesting competition between translational and vibrational energies is observed for the HCl vibrational distribution: the effect of exciting the low-frequency ethane modes, having small SVP values, is suppressed by translational excitation, whereas a part of the excess vibrational energy pumped into the CH-stretching modes (larger SVP values) efficiently flows into the HCl vibration.

Multiple-inversion, the analogue of the double-inversion pathway recently revealed for SN2@C, is the key mechanism in SN2 at N center undermining stereospecificity.