F和CH3OH多通道反应的速率常数与分支比的研究

F和CH₃OH有两个夺氢反应通道，分别生成HF+CH₃O 和HF+CH₂OH. 尽管这两个通道都没有能垒，但前一个通道即生成HF+CH₃O的反应分支比远远高于期望的统计平均值(四分之一). 不同实验测得的分支比不仅相去甚远，而且定量上与早期由过渡态理论(稳定点信息在MP2以及G2理论水平下计算得到)得到的计算结果也不符合. 此前在CCSD(T)-F12a/AVDZ水平上计算得到了121000个几何构型的能量，采用对易不变多项式结合神经网络的方法拟合得到了该体系的全维高精度势能面. 本文采用该势能面，结合准经典轨线动力学方法，对该反应的反应速率常数和反应分支比进行了理论研究，得到的结果与实验吻合. 由于反应没有能垒，理论计算结果表明反应速率常数随温度升高而有微弱的下降趋势.

Rate Coeffcients and Branching Ratio for Multi-Channel Hydrogen Abstractions from CH3OH by F

The hydrogen abstraction reaction F+CH₃OH has two possible reaction pathways: HF+CH₃O and HF+CH₂OH. Despite the absence of intrinsic barriers for both channels, the former has a branching ratio comparable to the latter, which is far from the statistical limit of 0.25 (one out of four available H atoms). Furthermore, the measured branching ratio of the two abstraction channels spans a large range and is not quantitatively reproduced by previous theoretical predictions based on the transition-state theory with the stationary point information calculated at the levels of M?ller-Plesset perturbation theory and G2. This work reports a theoretical investigation on the kinetics and the associated branching ratio of the two competing channels of the title reaction using a quasi-classical trajectory approach on an accurate full-dimensional potential energy surface (PES) fitted by the permutation invariant polynomial-neural network approach to ca. 1.21x10⁵ points calculated at the explicitly correlated (F12a) version of coupled cluster singles doubles and perturbative triples (CCSD(T)) level with the aug-cc-pVDZ basis set. The calculated room temperature rate coeffcient and branching ratio of the HF+CH₃O channel are in good agreement with the available experimental data. Furthermore, our theory predicts that rate coeffcients have a slightly negative temperature dependence, consistent with barrierless nature of the reaction.