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）与过氧反应之间的非典型竞争关系，并阐明了应变能与环取代基位置对二者各自贡献的影响。本研究的RRKM建模（RRKM modeling）结果表明，连接在仲碳上的自由基具有最快的断裂速率，可实现最大程度的局部环张力释放。相较于降冰片烷（norbornane），二甲基取代基会增大局部应变，阻碍桥头断裂反应，最终生成环戊烯（cyclopentene）与异戊二烯（isoprene）产物。二甲基束缚结构在QOOH消除HO2的过程中会引入额外的扭转应变，而醚键生成反应则可通过羰基（carbonyl group）的给电子效应得到促进。跌落效应（falloff extent）的程度同样受空间位阻（steric hindrance）影响：空间位阻会增大桥头刚度，进而导致更低的振动配分函数（vibrational partition function）与低压速率常数（low-pressure rate constant）。此外，甲基诱导的反应物重组限制效应，可调控莰基氢过氧化物（fenchyl hydroperoxide）的Korcek反应（Korcek reaction）中的焓熵补偿（enthalpy–entropy compensation）行为。与本团队此前的搅拌反应器实验不同，在本次全新的快速压缩机（RCM）实验中，莰基过氧反应对反应活性的影响十分显著。本模型预测了莰基自由基的选择性随自由基位置变化呈现显著差异：β-断裂与过氧反应分别在F1/F2/F3/F4与F5/F6类自由基中占主导地位。该动力学模型可准确预测实验测得的点火延迟时间（IDT），但在较低实验温度下，模型预测存在一处微弱的第一阶段压力拐点，这一现象尚未通过实验得到证实。本研究针对高压氧化条件下莰酮（fenchone）的开环与闭环机制所获得的新认识，可为其他多环酮类（polycyclic ketones）的相关研究提供参考。