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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.
Highlights
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
The reactions of a chlorine atom (Cl) with organic molecules such as methane (CH4), ethane (C2H6), methanol (CH3OH), etc. have become benchmark systems to uncover the dynamics of hydrogen-abstraction processes forming hydrogen chloride (HCl) molecules.[1−11] These experiments provided deep insights into the state-to-state dynamics and mode-specific energy transfer of polyatomic reactions, thereby extending and modifying the fundamental rules[12] of chemical reactivity
The former is done quantum mechanically resulting in a potential energy surface (PES), which governs the latter via classical or quantum methods
Summary
The construction of the PES starts from randomly displaced geometries of the stationary points[28] of the Cl + C2H6 reaction and utilizes the ROBOSURFER program system,[34] which enables the automated development of the surface by the iterative and selective addition of new geometries extracted from QCT simulations, subjected to ab initio quantum chemical computations, and fitted by using the monomial symmetrization approach[35] of the permutationally invariant polynomial method.[36] The fitting function is a polynomial expansion of the yij = exp(−rij/a) Morse-like variables of the rij interatomic distances, where a = 1.5 bohr is applied and the highest total order of the polynomials is 5. In the first round of the PES development we apply the RMP2/aug-cc-pVDZ level of theory for quantum chemical computations and perform 368 ROBOSURFER iterations. The second (and final) round of the PES development uses the above-defined composite ab initio level of theory and consists of 140 ROBOSURFER iterations.
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