Ring-Opening Competes with Peroxidation in Fenchone Low-Temperature Autoignition

We report an atypical competition between fenchyl radical
β-scission
and peroxidation at low temperatures and unravel the impacts of strain
energy and ring substituent location on their respective contributions.
Our RRKM modeling reveals that radicals positioned on secondary carbons
are the fastest-scission ones, exhibiting maximum local ring relief.
Dimethyl substituents contribute to increased local strain compared
to norbornane, hindering bridge scission and leading to cyclopentene
and isoprene products. The dimethyl corset generates extra torsional
strain during HO2 elimination from QOOH, while ether formation
is favored by electron donation from the carbonyl group. The falloff
extent is also affected by steric hindrance, insofar as it increases
bridge stiffness, leading to a lower vibrational partition function
and low-pressure rate constant. Furthermore, methyl-induced restrictions
on reactant reorganization are found to modulate an enthalpy–entropy
compensation in the Korcek reaction of fenchyl hydroperoxide. Unlike
in our previous stirred reactor experiments, the impact of fenchyl
peroxidation on reactivity is notable under our new rapid compression
machine (RCM) experiments. The present model predicts contrasted fenchyl
selectivities with radical position, β-scission and peroxidation
prevailing respectively for F1/F2/F3/F4 and F5/F6 radicals. The kinetic
mechanism accurately predicts the experimental IDT but indicates a
slight first-stage pressure inflection point at the lower experimental
temperature, which could not be confirmed experimentally. This new
insight into fenchone ring-opening and -closing mechanisms under high-pressure
oxidation can be useful for other polycyclic ketones.

本研究报道了低温下莰基自由基（fenchyl radical）的β-断裂（β-scission）与过氧反应（peroxidation）之间的非典型竞争关系，并阐明了应变能（strain energy）与环取代位点（ring substituent location）对二者各自贡献的影响。我们的RRKM模型（RRKM modeling）显示，连接在仲碳上的自由基是断裂速率最快的物种，其表现出最大的局部环松弛效应。相较于降冰片烷（norbornane），二甲基取代基会加剧局部应变，阻碍桥位断裂，最终生成环戊烯（cyclopentene）与异戊二烯（isoprene）产物。二甲基取代结构会在QOOH消除HO2的过程中产生额外的扭转应变，而羰基（carbonyl group）的给电子效应则有利于醚键的生成。压力跌落效应的程度同样受位阻（steric hindrance）影响——位阻会提升桥位刚性，进而导致振动配分函数（vibrational partition function）降低与低压速率常数（low-pressure rate constant）减小。此外，研究发现甲基诱导的反应物重组受限，会调控莰基氢过氧化物（fenchyl hydroperoxide）的科尔谢克反应（Korcek reaction）中的焓-熵补偿（enthalpy–entropy compensation）效应。与此前的搅拌反应器实验结果不同，在本次的快速压缩机（rapid compression machine, RCM）实验中，莰基过氧反应对反应活性的影响十分显著。本模型预测，莰基自由基的选择性会随自由基位点不同而呈现显著差异：β-断裂与过氧反应分别在F1/F2/F3/F4型自由基与F5/F6型自由基中占主导地位。该动力学模型能够精准预测实验测得的点火延迟时间（ignition delay time, IDT），但在较低实验温度下预测到了第一阶段的微小压力拐点，这一现象尚未通过实验得到证实。这项针对高压氧化条件下莰酮（fenchone）开环与闭环机制的新见解，对其他多环酮类（polycyclic ketones）化合物的相关研究具有参考价值。