Del Mazo-Sevillano Pablo, Aguado Alfredo, Jiménez Elena, Suleimanov Yury V, Roncero Octavio
Unidad Asociada UAM-CSIC, Departamento de Química Física Aplicada, Facultad de Ciencias M-14 , Universidad Autónoma de Madrid , 28049 Madrid , Spain.
Departamento de Química Física, Facultad de Ciencias y Tecnologías Químicas , Universidad de Castilla La Mancha , Avda. Camilo José Cela 1B , 13071 Ciudad Real , Spain.
J Phys Chem Lett. 2019 Apr 18;10(8):1900-1907. doi: 10.1021/acs.jpclett.9b00555. Epub 2019 Apr 4.
The quantum dynamics of the title reactions are studied using the ring polymer molecular dynamics (RPMD) method from 20 to 1200 K using recently proposed full dimensional potential energy surfaces which include long-range dipole-dipole interactions. A V-shaped dependence of the reaction rate constants is found with a minimum at 200-300 K, in rather good agreement with the current experimental data. For temperatures above 300 K the reaction proceeds following a direct H-abstraction mechanism. However, below 100 K the reaction proceeds via organic-molecule···OH collision complexes, with very long lifetimes, longer than 10 s, associated with quantum roaming arising from the inclusion of quantum effects by the use of RPMD. The long lifetimes of these complexes are comparable to the time scale of the tunnelling to form reaction products. These complexes are formed at zero pressure because of quantum effects and not only at high pressure as suggested by transition state theory (TST) calculations for OH + methanol and other OH reactions. The zero-pressure rate constants reproduce quite well measured ones below 200 K, and this agreement opens the question of how important the pressure effects on the reaction rate constants are, as implied in TST-like formalisms. The zero-pressure mechanism is applicable only to very low gas density environments, such as the interstellar medium, which are not repeatable by experiments.
使用环聚合物分子动力学（RPMD）方法，在20至1200 K的温度范围内，利用最近提出的包含长程偶极 - 偶极相互作用的全维势能面，研究了上述反应的量子动力学。发现反应速率常数呈V形依赖关系，在200 - 300 K处有最小值，这与当前的实验数据相当吻合。对于温度高于300 K的情况，反应遵循直接氢提取机制进行。然而，在100 K以下，反应通过有机分子···OH碰撞复合物进行，其寿命非常长，超过10 s，这与通过使用RPMD纳入量子效应而产生的量子漫游有关。这些复合物的长寿命与形成反应产物的隧穿时间尺度相当。这些复合物在零压力下由于量子效应而形成，而不仅仅是如过渡态理论（TST）对OH + 甲醇及其他OH反应计算所表明的那样在高压下形成。零压力速率常数在200 K以下能很好地重现测量值，这种一致性引发了一个问题，即如类TST形式体系所暗示的，压力对反应速率常数的影响有多重要。零压力机制仅适用于非常低气体密度的环境，如星际介质，而这种环境无法通过实验重现。
