The CN + C2H6 reaction. Dynamics study based on an analytical full-dimensional potential energy surface.

The hydrogen abstraction reaction of the cyano radical with molecules of ethane presents interest in the chemistry from ultra-cold to combustion environments, especially with regard to HCN(v) product vibrational distribution. In order to understand its dynamics a new analytical full-dimensional potential energy surface was developed, named PES-2023. It uses a combination of valence bond and mechanic molecular terms as functional form, fitted to high-level ab initio calculations at the explicitly correlated CCSD(T)-F12/aug-cc-pVTZ level on a reduced and selected number of points describing the reactive process. The new surface showed a continuous and smooth behaviour, describing reasonably the topology of the reaction: high exothermicity, low barrier and presence of intermediate complexes in the entrance and exit channels. Using quasi-classical trajectory calculations (QCT) on the new PES-2023, a dynamics study was performed at room temperature, with special emphasis on the HCN(v1,v2,v3) product stretching and bending vibrational excitations, and the results compared with the experimental evidence, which presented discrepancies in the bending excitation. The available energy was mostly deposited as HCN(v) vibrational energy, with the vibrational population inverted in the CH stretching mode and not inverted in the CN stretching and bending modes, thus simulating the experimental evidence. Other dynamics properties at room temperature were also analysed; cold rotational energy distribution was found, associated with a linear and soft transition state, and backward scattering distribution, associated with a rebound mechanism.