Quantum versus classical unimolecular fragmentation rate constants and activation energies at finite temperature from direct dynamics simulations,Physical Chemistry Chemical Physics

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Quantum versus classical unimolecular fragmentation rate constants and activation energies at finite temperature from direct dynamics simulations
Physical Chemistry Chemical Physics ( IF 2.9 ) Pub Date : 2022-11-23 , DOI: 10.1039/d2cp03809a
Federica Angiolari ₁ , Simon Huppert ₂ , Riccardo Spezia ₁

Affiliation

In the present work, we investigate how nuclear quantum effects modify the temperature dependent rate constants and, consequently, the activation energies in unimolecular reactions. In the reactions under study, nuclear quantum effects mainly stem from the presence of a large zero point energy. Thus, we investigate the behavior of methods compatible with direct dynamics simulations, the quantum thermal bath (QTB) and ring polymer molecular dynamics (RPMD). To this end, we first compare them with quantum reaction theory for a model Morse potential before extending this comparison to molecular models. Our results show that, in particular in the temperature range comparable with or lower than the zero point energy of the system, the RPMD method is able to correctly capture nuclear quantum effects on rate constants and activation energies. On the other hand, although the QTB provides a good description of equilibrium properties including zero-point energy effects, it largely overestimates the rate constants. The origin of the different behaviours is in the different distance distributions provided by the two methods and in particular how they differently describe the tails of such distributions. The comparison with transition state theory shows that RPMD can be used to study fragmentation of complex systems for which it may be difficult to determine the multiple reaction pathways and associated transition states.

中文翻译：

来自直接动力学模拟的有限温度下的量子与经典单分子分裂速率常数和活化能

在目前的工作中，我们研究了核量子效应如何改变依赖于温度的速率常数，从而改变单分子反应中的活化能。在所研究的反应中，核量子效应主要源于大零点能量的存在。因此，我们研究了与直接动力学模拟、量子热浴 (QTB) 和环聚合物分子动力学 (RPMD) 兼容的方法的行为。为此，我们首先将它们与摩尔斯势模型的量子反应理论进行比较，然后再将这种比较扩展到分子模型。我们的结果表明，特别是在与系统的零点能量相当或更低的温度范围内，RPMD 方法能够正确地捕获核量子效应对速率常数和活化能的影响。另一方面，尽管 QTB 很好地描述了包括零点能量效应在内的平衡特性，但它在很大程度上高估了速率常数。不同行为的起源在于两种方法提供的不同距离分布，特别是它们如何不同地描述这种分布的尾部。与过渡态理论的比较表明，RPMD 可用于研究可能难以确定多反应途径和相关过渡态的复杂系统的碎裂。不同行为的起源在于两种方法提供的不同距离分布，特别是它们如何不同地描述这种分布的尾部。与过渡态理论的比较表明，RPMD 可用于研究可能难以确定多反应途径和相关过渡态的复杂系统的碎裂。不同行为的起源在于两种方法提供的不同距离分布，特别是它们如何不同地描述这种分布的尾部。与过渡态理论的比较表明，RPMD 可用于研究可能难以确定多反应途径和相关过渡态的复杂系统的碎裂。

更新日期：2022-11-23

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