The CN(X 2Σ+) + C2H6 reaction: Dynamics study based on an analytical full-dimensional potential energy surface

Abstract

The hydrogen abstraction reaction of the cyano radical with molecules of ethane presents some interesting points 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 the 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 behavior, 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 were 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 analyzed; cold rotational energy distribution was found, associated with a linear and soft transition state, and backward scattering distribution was found, associated with a rebound mechanism.

Kinetic study of the CN + C2H6 hydrogen abstraction reaction based on an analytical potential energy surface

“…, second order Møller–Plesset perturbation theory (MP2) with the 6-31G (d,p) basis set, MP2/6-31G(d,p); Minnesota functional M06-2X/6-311++G(d,p); Minnesota functional M08-HX/6-311++G(3d2f,2p); coupled cluster method with single, double and triple (perturbative) excitations using the correlation consistent triple zeta basis set, CCSD(T)/cc-pVTZ; and the results obtained from the PES-2023 surface. 1 The geometries and vibrational frequencies were obtained using the Gaussian code 51 for the first four electronic structure calculations. The single-point energies of the geometries obtained from the methods mentioned above are determined using a more accurate explicitly correlated coupled cluster method with a larger basis set, i.e.…”

“…, PES-2023. 1 The PES-2023, which describes the nuclear motion within the Born–Oppenheimer approximation, has been constructed by fitting data obtained from highly accurate explicitly correlated CCSD(T)-F12/aug-cc-pVTZ theory to valence-bond/molecular mechanics (VB/MM) functional form. This PES can describe the topology of this polyatomic reactive system in a smooth and continuous way.…”

“…34 Moreover, the low barrier height (+0.23 kcal mol −1 ) and the presence of intermediate complexes in the entrance and exit channels can be identified in PES-2023; the first intermediate, which is moderately stabilised with respect to the reactants (by 0.27 kcal mol −1 ), while the second intermediate has a comparatively larger stabilisation with respect to the products (1.10 kcal mol −1 ). This surface was used in our previous work 1 to perform an exhaustive dynamical study, analysing several dynamical properties, such as the product rotational, vibrational, translational, and angular distributions. The vibrational populations of the three vibrational modes of the product HCN (v 1 ,v 2 ,v 3 ), viz.…”

“…The study of barrierless reactions poses a significant theoretical challenge, particularly for the title reaction, where the complexity is increased due to its polyatomic nature involving 10 bodies and 24 degrees of freedom. We recently developed the first analytical full-dimensional potential energy surface describing the nuclei motion, 1 denoted as PES-2023. These barrierless reactions are typically very fast processes with high exothermicity and proceed with “early” transition states.…”

“…The present paper is organized as follows. In Section 2, the PES-2023 surface 1 is briefly described, together with the computational details of three kinetics approaches used in this work: variational transition-state theory with multidimensional tunnelling corrections (VTST/MT), quasi-classical trajectory calculations (QCT), and ring polymer molecular dynamics (RPMD) method. In Section 3, we provide the details of the theoretical results obtained in this work.…”