Dynamical Bifurcation in Gas-Phase XH⁻ + CH₃Y S_N2 Reactions: The Role of Energy Flow and Redistribution in Avoiding the Minimum Energy Path

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• 作者：Yaicel G. Proenza ; Prof. Miguel A. F. de Souza and Prof. Ricardo L. Longo
• 刊名：Chemistry - A European Journal
• 出版年：2016
• 出版时间：November 2, 2016
• 年：2016
• 卷：22
• 期：45
• 页码：16220-16229
• 全文大小：1578K
• ISSN：1521-3765

文摘

The gas-phase reactions of XH⁻ (X=O, S) + CH₃Y (Y=F, Cl, Br) span nearly the whole range of S_N2 pathways, and show an intrinsic reaction coordinate (IRC) (minimum energy path) with a deep well owing to the CH₃XH⋅⋅⋅Y⁻ (or CH₃S⁻⋅⋅⋅HF) hydrogen-bonded postreaction complex. MP2 quasiclassical-type direct dynamics starting at the [HX⋅⋅⋅CH₃⋅⋅⋅Y]⁻ transition-state (TS) structure reveal distinct mechanistic behaviors. Trajectories that yield the separated CH₃XH+Y⁻ (or CH₃S⁻+HF) products directly are non-IRC, whereas those that sample the CH₃XH⋅⋅⋅Y⁻ (or CH₃S⁻⋅⋅⋅HF) complex are IRC. The IRCIRC/non-IRC ratios of 90:10, 40:60, 25:75, 2:98, 0:100, and 0:100 are obtained for (X, Y)=(S, F), (O, F), (S, Cl), (S, Br), (O, Cl), and (O, Br), respectively. The properties of the energy profiles after the TS cannot provide a rationalization of these results. Analysis of the energy flow in dynamics shows that the trajectories cross a dynamical bifurcation, and that the inability to follow the minimum energy path arises from long vibration periods of the X−C⋅⋅⋅Y bending mode. The partition of the available energy to the products into vibrational, rotational, and translational energies reveals that if the vibrational contribution is more than 80 %, non-IRC behavior dominates, unless the relative fraction of the rotational and translational components is similar, in which case a richer dynamical mechanism is shown, with an IRC/non-IRC ratio that correlates to this relative fraction.