How chemical reactions are influenced by reactant vibrational excitation is a long-standing question at the core of chemical reaction dynamics. In reactions of polyatomic molecules, where the Polanyi rules are not directly applicable, certain vibrational modes can act as spectators. In nucleophilic substitution reactions, CH stretching vibrations have been considered to be such spectators. While this picture has been challenged by some theoretical studies, experimental insight has been lacking. We show that the nucleophilic substitution reaction of F^- with CH₃I is minimally influenced by an excitation of the symmetric CH stretching vibration. This contrasts with the strong vibrational enhancement of the proton transfer reaction measured in parallel. The spectator behavior of the stretching mode is supported by both quasi-classical trajectory simulations and the Sudden Vector Projection model.

中文翻译：

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拉伸振动是亲核取代中的一个旁观者。

化学反应动力学如何影响化学反应是一个长期存在的问题。在多原子分子的反应中，波兰尼规则不能直接应用，某些振动模式可以充当旁观者。在亲核取代反应中，CH拉伸振动被认为是这种旁观者。尽管此图片受到了一些理论研究的挑战，但仍缺乏实验见识。我们表明，F的亲核取代反应^-用CH ₃对称CH拉伸振动的激发对I的影响最小。这与平行测量的质子转移反应的强烈振动增强相反。准经典轨迹模拟和“突然矢量投影”模型都支持拉伸模式的旁观者行为。