Full-dimensional analytical potential energy surface describing the gas-phase Cl + C₂H₆reaction and kinetics study of rate constants and kinetic isotope effects

Abstract

Within the Born-Oppenheimer approximation a full-dimensional analytical potential energy surface, PES-2017, was developed for the gas-phase hydrogen abstraction reaction between the chlorine atom and ethane, which is a nine body system. This surface presents a valence-bond/molecular mechanics functional form dependent on 60 parameters and is fitted to high-level ab initio calculations. This reaction presents little exothermicity, -2.30 kcal mol, with a low height barrier, 2.44 kcal mol, and intermediate complexes in the entrance and exit channels. We found that the energetic description was strongly dependent on the ab initio level used and it presented a very flat topology in the entrance channel, which represents a theoretical challenge in the fitting process. In general, PES-2017 reproduces the ab initio information used as input, which is merely a test of self-consistency. As a first test of the quality of the PES-2017, a theoretical kinetics study was performed in the temperature range 200-1400 K using two approaches, i.e. the variational transition-state theory and quasi-classical trajectory calculations, with spin-orbit effects. The rate constants show reasonable agreement with experiments in the whole temperature range, with the largest differences at the lowest temperatures, and this behaviour agrees with previous theoretical studies, thus indicating the inherent difficulties in the theoretical simulation of the kinetics of the title reaction. Different sources of error were analysed, such as the limitations of the PES and theoretical methods, recrossing effects, and the tunnelling effect, which is negligible in this reaction, and the manner in which the spin-orbit effects were included in this non-relativistic study. We found that the variation of spin-orbit coupling along the reaction path, and the influence of the reactivity of the excited Cl(P) state, have relative importance, but do not explain the whole discrepancy. Finally, the activation energy and the kinetics isotope effects reproduce the experimental information.

Detailed benchmark ab initio mapping of the potential energy surfaces of the X + C2H6 [X = F, Cl, Br, I] reactions

“…Comparing our new benchmark data to previous results 14,16,17,22,25,[28][29][30]33,35,[37][38][39][40][41] of the literature, where only the main H-abstraction channel was investigated for the X + C 2 H 6 [X = F, Cl, Br, I] reactions, we find considerable inaccuracies regarding the barrier heights, even their signs for X = F and Cl.…”

“…Besides the main H-abstraction reaction path and the higher-energy H-substitution studied also for halogen + methane reactions, in the X + C 2 H 6 [F, Cl, Br, I] reactive system methyl-substitution appears as a new mechanism and is expected to occur at lower energies then H-substitution. For these reactions so far only the H-abstraction channel (X = F, Cl) [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] or its reverse reaction (X = Br, I) [34][35][36][37][38][39][40][41] have been investigated.…”

“…[26][27][28][29][30] In a recent paper Rangel and Espinosa-Garcia have published an analytical PES including valence-bond and molecular-mechanic terms for this abstraction reaction. 33 They also characterized the stationary points using the CCSD(T) method with different basis sets and implied that high-level quantum chemical computations are needed to determine the exact energetics of the reaction.…”

Global potential energy surface and product pair-correlated distributions for the F(2P) + SiH4 reaction – comparison with experiments

“…In the first paper of the series 2 we developed the analytical potential energy surface for the Cl( 2 P) + SiH 4 gas-phase reaction, which is also very exothermic and barrierless. That PES was based on previous knowledge of the H + SiH 4 and Cl( 2 P) + CH 4 reactions, 5,19,20 and a term-by-term detailed description can be found in ref. 21 (ESI).…”

Current Status of the X + C2H6 [X ≡ H, F(2P), Cl(2P), O(3P), OH] Hydrogen Abstraction Reactions: A Theoretical Review

“…Note that a detailed term-by-term description of the PESs can be found in Ref. [65] and so is not repeated here. The analytical functional form was developed as a valence bond (VB) function augmented with molecular mechanics (MM) terms, in brief, VB/MM.…”