State-to-state dynamics of the Cl(2P) + C2H6(ν5, ν1 = 0, 1) → HCl(v′, j′) + C2H5 hydrogen abstraction reactions

Abstract

Using quasi-classical trajectory calculations on a recently developed full-dimensional potential energy surface, the effects of ethane reactant vibrational excitation, ν5 = 1, on the dynamics of the title reaction were analyzed. By analyzing the classical results from a quantum-mechanical point of view, we performed state-to-state calculations to show the influence of vibrational excitation on the vibrational populations of the two coproducts, as well as rotational distributions of the diatomic HCl(v′, j′) product. We found that excitation of the C2H6(ν5 = 1) mode by one quantum increases reactivity by 9% with respect to the ground-state reaction, where the ethyl radical coproduct receives an important internal energy, ~ 30% of the available energy, There are many populated vibrational states in the ethyl radical, although the population in each is very low, 1–5%. In fact, the most populated vibrational state, C2H5 ground state, presents a population of only ~ 10%. The HCl(v′ = 0, 1) product shows non-inverted vibrational populations, 88:12%, with relatively hot rotational distributions, and where the HCl(v′ = 1) product is forward-scattered. These theoretical results qualitatively reproduce the only experimental study by Zare et al., 20 years ago, taking into account that the authors themselves recognized that their results were not quantitative. In addition, we analyze two related issues: mode specificity and translational versus vibrational efficiency to promote reactivity, which have not been experimentally studied. We found that independent excitation of the ethane C–H stretching ν5 and ν1 modes, which differ by only 15 cm−1, shows similar dynamics behavior, which discards mode specificity. Finally, vibrational energy promotes reactivity only slightly more effectively than an equal amount of energy as translation. This result was rationalized by the sudden vector projection model for this “central” barrier reaction, to which Polanyi’s rules cannot be applied.