MgO $^+$热活化甲烷：温度相关动力学、反应分子动力学模拟和统计模型
Thermal Activation of Methane by MgO$^+$: Temperature Dependent Kinetics, Reactive Molecular Dynamics Simulations and Statistical Modeling
摘要：
采用可变离子源、温度可调的选择离子流管（VISTA-SIFT）装置，从300 K到600 K对MgO $^+$ + CH $_4$的动力学进行了实验研究，并通过运行和分析反应原子模拟进行了计算。速率和产物支化分数被确定为温度的函数。反应以$k = 5.9 \pm 1.5 10^{-10}(T/300 $ K $)^{-0.5 \pm 0.2}$ cm $^3$ s $^{-1}$的速率进行。MgOH $^+$是在所有温度下的主要产物，但观察到Mg $^+$，氧原子转移形成甲醇的副产物，产物支化分数为$0.08 \pm 0.03 (T / 300 $ K $)^{-0.8 \pm 0.7}$。反应分子动力学模拟使用反应力场，以及神经网络产量率系数约一个数量级低。这种低估的速率可以追溯到过渡态[MgOCH $_4$ ] $^+$的多参考字符。统计建模的温度依赖性动力学提供了进一步的洞察力的反应电位表面。发现速率限制步骤与C-H键的四中心活化一致，与先前的计算一致。产物支化被建模为直接在限速过渡态之后的插入中间体的解离与穿过对应于甲基迁移的过渡态之间的竞争，所述甲基迁移导致Mg-CH $_3$ OH $^+$复合物，尽管仅当该过渡态相对于解离的MgOH $^+$ + CH $_3$产物通道显著稳定时。另一种非统计机制进行了讨论，从而在潜在的表面后过渡态分叉可以允许反应直接进行从四中心TS的Mg-CH $_3$ OH $^+$复合物，从而允许更强大的产品通道之间的竞争。
英文摘要：
The kinetics of MgO$^+$ + CH$_4$ was studied experimentally using the variable ion source, temperature adjustable selected ion flow tube (VISTA-SIFT) apparatus from 300 $-$ 600 K and computationally by running and analyzing reactive atomistic simulations. Rates and product branching fractions were determined as a function of temperature. The reaction proceeded with a rate of $k = 5.9 \pm 1.5 10^{-10}(T/300 $ K$)^{-0.5 \pm 0.2}$ cm$^3$ s$^{-1}$. MgOH$^+$ was the dominant product at all temperatures, but Mg$^+$, the co-product of oxygen-atom transfer to form methanol, was observed with a product branching fraction of $0.08 \pm 0.03 (T / 300 $ K$)^{-0.8 \pm 0.7}$. Reactive molecular dynamics simulations using a reactive force field, as well as a neural network yield rate coefficients about one order of magnitude lower. This underestimation of the rates is traced back to the multireference character of the transition state [MgOCH$_4$]$^+$. Statistical modeling of the temperature-dependent kinetics provides further insight into the reactive potential surface. The rate limiting step was found to be consistent with a four-centered activation of the C-H bond, consistent with previous calculations. The product branching was modeled as a competition between dissociation of an insertion intermediate directly after the rate-limiting transition state, and traversing a transition state corresponding to a methyl migration leading to a Mg-CH$_3$OH$^+$ complex, though only if this transition state is stabilized significantly relative to the dissociated MgOH$^+$ + CH$_3$ product channel. An alternative non-statistical mechanism is discussed, whereby a post-transition state bifurcation in the potential surface could allow the reaction to proceed directly from the four-centered TS to the Mg-CH$_3$OH$^+$ complex thereby allowing a more robust competition between the product channels.
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