Theory Finally Agrees with Experiment for the Dynamics of the Cl + C(2)H(6) Reaction

[Image: see text] Since the pioneering reaction dynamics studies of H + H(2) 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 (C(2)H(6)) 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 + CH(4), the Cl + C(2)H(6) reaction is exothermic with an adiabatically submerged transition state, allowing testing of the validity of the Polanyi rules for a negative-barrier reaction.