Vibrational mode-specificity in the dynamics of the Cl + C2H6 → HCl + C2H5 reaction

Abstract

We report a detailed dynamics study on the mode-specificity of the Cl + C 2 H 6 → HCl + C 2 H5 H-abstraction reaction. We perform quasiclassical trajectory simulations using a recently developed high-level ab initio full-dimensional potential energy surface by exciting five different vibrational modes of ethane at four collision energies. We find that all the studied vibrational excitations, except that of the CCstretching mode, clearly promote the title reaction, and the vibrational enhancements are consistent with the predictions of the Sudden Vector Projection (SVP) model, with the largest effect caused by the CH-stretching excitations. Intramolecular vibrational redistribution is also monitored for the differently excited ethane molecule. Our results indicate that the mechanism of the reaction changes with increasing collision energy, with no mode-specificity at high energies. The initial translational energy mostly converts into product recoil, while a significant part of the excess vibrational energy remains in the ethyl radical. An interesting competition between translational and vibrational energies is observed for the HCl vibrational distribution: the effect of exciting the low-frequency ethane modes, having small SVP values, is suppressed by translational excitation, whereas a part of the excess vibrational energy pumped into the CH-stretching modes (larger SVP values) efficiently flows into the HCl vibration.

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

“…However, (i) these methods present a high computational demand because a huge amount of high-level electronic structure calculations is necessary to describe the whole reactive system, today ~100,000 points; (ii) in addition, the points calculated for a given reactive system cannot be used for another reaction, and therefore independent sets of points need to be calculated for each particular reaction, and finally, (iii) the absence of unphysical anomalies in areas far from the sampled regions cannot be ruled out. By using MO-based surfaces, in the last fourto-five years and parallel with our research, Czako et al [57][58][59][60][61][62][63][64] performed an impressive work analyzing reactions with ethane, X + C 2 H 6 → HX + C 2 H 5 ; X ≡ F( 2 P), Cl( 2 P), Br( 2 P), I( 2 P) and OH. These authors performed benchmark calculations at very sophisticated ab initio levels to develop full-dimensional potential energy surfaces and from them to characterize stationary points and calculate dynamics properties.…”

Rotational Mode-Specificity in the Cl + C2H6 → HCl + C2H5 Reaction

“…Due to the smaller moment of inertia in the K = J case, based on the expression, the same J quantum number results in larger angular velocity ( Ω ) and therefore larger rotational energy. The K = J energies are commeasurable with some of the fundamental vibrational energies of ethane; thus, their effect on the reaction can be readily compared.…”

“…ICSs as a function of total energy for the Cl + C 2 H 6 → HCl + C 2 H 5 reaction computed on our PES from ref with C 2 H 6 ( v = 0) and C 2 H 6 ( v x = 1) [ x = 1, 3, 5, 6, 7] vibrationally and C 2 H 6 ( J , K = J ) [ J = 0, 10, 20, 30, 40] rotationally excited reactants. The unexcited ICS value at a 20 kcal/mol collision energy in the lower panel is taken from ref . The upper panel is adapted with permission from ref .…”

“…Or, even more specifically, how can one reach the selective cleavage of a chemical bond? These issues have also been extensively investigated, − involving even 7- and 9-atomic systems, as well. − …”

Vibrational mode-specific dynamics of the F− + CH3CH2Cl multi-channel reaction

“…Mode specificity has been widely studied for atom + molecule reactions, 1–23 however, the effect of vibrational excitations is less known for ion–molecule reactions, 24–37 such as the bimolecular nucleophilic substitution (S N 2). A typical S N 2 reaction in the gas phase has a submerged transition state as well as deep pre- and post-reaction minima supporting long-lived complex formations, which may undermine the mode-specific behavior.…”