Sanches-Neto Flávio O, Coutinho Nayara D, Carvalho-Silva Valter H
Grupo de Química Teórica de Anápolis Campus de Ciências Exatas e Tecnológicas, Universidade Estadual de Goiás, Caixa Postal 459, 75001-970, Anápolis, GO, Brazil.
Phys Chem Chem Phys. 2017 Sep 20;19(36):24467-24477. doi: 10.1039/c7cp03806b.
A number of experimental and theoretical papers accounted almost exclusively for two channels in the reaction of atomic hydrogen with methanol: H-abstraction from the methyl (R1) and hydroxyl (R2) functional groups. Recently, several astrochemical studies claimed the importance of another channel for this reaction, which is crucial for kinetic simulations related to the abundance of molecular constituents in planetary atmospheres: methyl radical and water formation (R3 channel). Here, motivated by the lack of and uncertainties about the experimental and theoretical kinetic rate constants for the third channel, we developed first-principles Car-Parrinello molecular dynamics thermalized at two significant temperatures - 300 and 2500 K. Furthermore, the kinetic rate constant of all three channels was calculated using a high-level deformed-transition state theory (d-TST) at a benchmark electronic structure level. d-TST is shown to be suitable for describing the overall rate constant for the CHOH + H reaction (an archetype of the moderate tunnelling regime) with the precision required for practical applications. Considering the experimental ratios at 1000 K, k/k ≈ 0.84 and k/k ≈ 15-40, we provided a better estimate when compared with previous theoretical work: 7.47 and 637, respectively. The combination of these procedures explicitly demonstrates the role of the third channel in a significant range of temperatures and indicates its importance considering the thermodynamic control to estimate methyl radical and water formation. We expect that these results can help to shed new light on the fundamental kinetic rate equations for the CHOH + H reaction.
从甲基（R1）和羟基（R2）官能团上夺取氢。最近，一些天体化学研究表明该反应的另一个通道很重要，这对于与行星大气中分子成分丰度相关的动力学模拟至关重要：甲基自由基和水的形成（R3通道）。在此，由于缺乏关于第三个通道的实验和理论动力学速率常数且存在不确定性，我们在300K和2500K这两个重要温度下开展了第一性原理卡-帕里尼罗分子动力学模拟。此外，在基准电子结构水平上，使用高级变形过渡态理论（d-TST）计算了所有三个通道的动力学速率常数。结果表明，d-TST适用于描述CHOH + H反应（中等隧穿机制的典型反应）的总速率常数，其精度满足实际应用要求。考虑到1000K时的实验比率，k/k≈0.84且k/k≈15 - 40，与之前的理论工作相比，我们给出了更好的估计值：分别为7.47和637。这些方法的结合明确展示了第三个通道在很宽温度范围内的作用，并表明在考虑热力学控制以估计甲基自由基和水的形成时其重要性。我们期望这些结果有助于为CHOH + H反应的基本动力学速率方程提供新的见解。
