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

Abstract

This paper is a detailed review of the chemistry of medium-size reactive systems using the following hydrogen abstraction reactions with ethane, X + C2H6 ® HX + C2H5; X ≡ H, F(2P), Cl(2P), O(3P) and OH, and focusing attention mainly on the theoretical developments. These bimolecular reactions range from exothermic to endothermic systems and from barrierless to high classical barriers of activation. Thus, the topography of the reactive systems changes from reaction to reaction with the presence or not of stabilized intermediate complexes in the entrance and exit channels. The review begins with some reflections on the inherent problems in the theory/experiment comparison. When one compares kinetics or dynamics theoretical results with experimental measures, one is testing both the potential energy surface describing the nuclei motion and the kinetics or dynamics method used. Discrepancies in the comparison may be due to inaccuracies of the surface, limitations of the kinetics or dynamics methods, and experimental uncertainties that also cannot be ruled out. The paper continues with a detailed review of some bimolecular reactions with ethane, beginning with the reactions with hydrogen atoms. The reactions with halogens present a challenge owing to the presence of stabilized intermediate complexes in the entrance and exit channels and the influence of the spin-orbit states on reactivity. Reactions with O(3P) atoms lead to three surfaces, which is an additional difficulty in the theoretical study. Finally, the reactions with the hydroxyl radical correspond to a reactive system with ten atoms and twenty-four degrees of freedom. Throughout this review, different strategies in the development of analytical potential energy surfaces describing these bimolecular reactions have been critically analyzed, showing their advantages and limitations. These surfaces are fitted to a large number of ab initio calculations, and we found that a huge number of calculations leads to accurate surfaces, but this information does not guarantee that the kinetics and dynamics results match the experimental measurements.

Direct and steady state rate constants of C2H6 + X (X = H, Cl, OH): influence of the van der Waals well

“…22,23 In particular, most of these reactions possess van der Waals wells in the reactant and product valleys, and the depth of the wells varies with the reactions. [24][25][26][27] Considering that the depth the of van der Waals well may have a critical impact on the reaction rate constant, three different reactions (i.e. H + C 2 H 6 , Cl + C 2 H 6 and OH + C 2 H 6 ) are chosen in the present paper, which differ in the depth of the van der Waals well.…”

Global potential energy surface and dynamics for the OH + CH3OH reaction

“…In the case of polyatomic systems this goal is far from being achieved. Recently, our group has analysed this problem 1 by studying the current status of the X + C2H6 [X H, F( 2 P), Cl( 2 P), O( 3 P), OH] hydrogen abstraction reactions. We concluded that the experiment/theory comparison worsens when we go from macroscopic properties (average properties, such as the rate constants) to microscopic properties (state-to-state properties, such as product vibrational distribution).…”