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

TL;DR

This study theoretically investigates the temperature-dependent rate constants and isotope effects of the CN + C2H6 hydrogen abstraction reaction across a broad temperature range using advanced kinetic theories and an accurate potential energy surface.

Contribution

It introduces a full-dimensional analytical potential energy surface and compares three kinetic theories, providing new insights into reaction dynamics from ultra-low to high temperatures.

Findings

Rate constants show a V-shaped temperature dependence with a minimum near 200 K.

QCT and RPMD methods align well with experimental data at low and high temperatures.

The reaction's ultra-low temperature rate increase is due to impact parameter effects, not tunneling.

Abstract

Temperature dependence of the thermal rate constants and kinetic isotope effects (KIE) of the CN + C2H6 gas-phase hydrogen abstraction reaction was theoretically determined within the 25-1000 K temperature range, i.e., from ultra-low to high-temperature regimes. Based on a recently developed full-dimensional analytical potential energy surface fitted to highly accurate explicitly correlated ab initio calculations, three different kinetic theories were used: canonical variational transition state theory (CVT), quasiclassical trajectory theory (QCT), and ring polymer molecular dynamics (RPMD) method for the computation of rate constants. We found that the thermal rate constants obtained with the three theories show a V-shaped temperature dependence, with a pronounced minimum near 200 K, qualitatively reproducing the experimental measurements. Among the three methods used in this work, the QCT and RPMD methods have the best agreement with the experiment at low and high temperatures, respectively. The significant increase in the rate constant at ultra-low temperatures in this very exothermic and practically barrierless reaction can be attributed to the large value of the impact parameter, ruling out the role of the tunneling effect and the intermediate complexes in the entrance channel. The theoretical H/D KIE depicted a normal behaviour, i.e., values greater than unity, emulating the experimental measurements and previous theoretical results. Finally, the discrepancies between theory and experiments were analysed as a function of several factors, such as limitations of the kinetics theories and the potential energy surface, as well as the uncertainties in the experimental measurements.