Burke Michael P, Meng Qinghui, Sabaitis Christopher
Department of Mechanical Engineering, Columbia University, USA.
Department of Chemical Engineering & Data Science Institute, Columbia University, USA.
Faraday Discuss. 2022 Oct 21;238(0):355-379. doi: 10.1039/d2fd00054g.
In 1922, Lindemann proposed the now-well-known mechanism for pressure-dependent rate constants for unimolecular reactions: reactant molecules with sufficiently high energies dissociate more quickly than collisions can reestablish the Boltzmann distribution of the internal energies of the molecule during its dissociation at low pressures - yielding pressure-dependent rate constants for unimolecular reactions due to the preferential depletion of the high energy states capable of dissociation. In the last century, incredible progress has been made in achieving a far greater understanding of and quantitative predictions for unimolecular and association reactions. In the modern era, pressure-dependent phenomenological rate constants are now nearly universally used to describe the rates of unimolecular and associative reactions in phenomenological kinetic modeling. However, there is a second, more indirect, implication of Lindemann's mechanism that relates to how these dissociation-induced non-equilibrium distributions impact bimolecular reactions, including non-associative bimolecular reactions - which are generally not considered to have pressure-dependent rate constants. Yet, as we show herein, the same high energy states depleted due to dissociation would otherwise react most rapidly in high-activation-energy bimolecular reactions - yielding a mechanism for pressure-dependent rate constants for bimolecular reactions (including non-associative reactions). Here, we present results from a case study for CHO dissociation, isomerization, and bimolecular reaction with O to explore this question. Results from our master equation calculations indicate that the effect of dissociation-induced non-equilibrium distributions on bimolecular reactions can be substantial - even when chemical timescales are well separated from internal energy relaxational timescales ( when the traditional rate constant description would be thought to apply). This effect is found to be more pronounced - and more complex - for bimolecular reactions involving molecular entities whose chemical timescales are merged with the internal energy relaxational timescales. Finally, we present some ideas for discussion regarding what should be considered as "chemical species" in phenomenological kinetic models.
1922年，林德曼提出了如今广为人知的单分子反应压力依赖速率常数机制：能量足够高的反应物分子解离速度比碰撞在其低压解离过程中重新建立分子内能的玻尔兹曼分布的速度更快——由于能够解离的高能态优先耗尽，从而产生单分子反应的压力依赖速率常数。在上个世纪，在对单分子反应和缔合反应有更深入的理解和定量预测方面取得了令人难以置信的进展。在现代，压力依赖的唯象速率常数现在几乎普遍用于描述唯象动力学模型中的单分子反应和缔合反应速率。然而，林德曼机制还有第二个更间接的含义，它涉及这些解离诱导的非平衡分布如何影响双分子反应，包括非缔合双分子反应——这些反应通常不被认为具有压力依赖速率常数。然而，正如我们在此所示，由于解离而耗尽的相同高能态，否则将在高活化能双分子反应中反应最快——从而产生双分子反应（包括非缔合反应）的压力依赖速率常数机制。在这里，我们展示了一个关于CHO解离、异构化以及与O的双分子反应的案例研究结果，以探讨这个问题。我们主方程计算的结果表明，解离诱导的非平衡分布对双分子反应的影响可能很大——即使化学时间尺度与内能弛豫时间尺度很好地分开（当传统速率常数描述被认为适用时）。对于涉及化学时间尺度与内能弛豫时间尺度合并的分子实体的双分子反应，发现这种影响更为显著且更复杂。最后，我们提出了一些关于在唯象动力学模型中应将什么视为“化学物种”的讨论思路。
