QCT study of the vibrational and translational role in the H + C₂H₆(ν₁, ν₂, ν₅, ν₇, ν₉ and ν₁₀) reactions

Abstract

Two important issues were analysed in the title reaction: the effects of vibrational excitation, associated with mode selectivity, and the role of translational energy, associated with Polanyi’s rules. Based on a global analytical potential energy surface, PES-2018, recently developed in our group, quasi-classical trajectory (QCT) calculations were performed at total energy of 35 kcal mol⁻¹, either as translation or as a combination of translation and vibration energy. Independent vibrational excitation by one quantum of any of the CH₃ stretching modes in ethane leads to similar dynamics pictures of reaction cross sections and H₂(v′, j′) rotovibrational and scattering distributions, ruling out mode selectivity. Normal mode analysis showed a cold, non-inverted, H₂(v′) product vibrational distribution, while the C₂H₅(v′) co-product presented many vibrational states, all of them with a low population, practically simulating a classical behaviour. An equivalent amount of energy as translation raises reactivity somewhat less effective than vibrational energy, contrary to that found for the O(³P) + CH₄ reaction. Both reactions present “central” barriers, so this opposite behaviour shows the difficulties for a straightforward application of the Polanyi′s rules. The role of vibrational and translational energy on dynamics has been rationalized by the coupling between vibrational modes, which makes analysis of vibrational excitation difficult in polyatomic systems. Finally, the role of the total energy on reactivity and mode selectivity was analysed, concluding that at lower energy, 15 kcal mol⁻¹, translational energy is much more effective than vibrational energy to enhance reactivity, while at intermediate energy, 20 kcal mol⁻¹, the situation is more confusing and strongly dependent on the counting methods used in the QCT calculations. Therefore, very small mode selectivity is found, and translation seems to be more effective in enhancing reactivity than vibration at low collision energies, while this behaviour is reversed as we increase the collision energy, being the turning point around 20 kcal mol⁻¹.