New Insights into the Mechanism of Amine/Nitroxide Cycling during the Hindered Amine Light Stabilizer Inhibited Oxidative Degradation of Polymers
New Insights into the Mechanism of Amine/Nitroxide Cycling during the Hindered Amine Light Stabilizer Inhibited Oxidative Degradation of Polymers
复制标题
DOI：
10.1021/ja3006379
复制
发表时间：
2012-08-08
影响因子：
15
通讯作者：
Coote, Michelle L.
中科院分区：
文献类型：
--
作者：
Gryn'ova, Ganna;Ingold, K. U.;Coote, Michelle L.
高层次从头计算分子轨道理论计算用于确定显着的高抑制化学计量因子所表现出的基于二烷基胺的自由基捕获抗氧化剂的起源。我们已经计算了在25，80，和260摄氏度的气相和水溶液中的自由能势垒和反应能的反应范围广泛，可能，潜在地，参与胺/氮氧化物循环，以及几个新的途径提出作为本工作的一部分，包括N-烷基受阻胺光稳定剂活化。我们发现，大多数的文献氮氧化物再生循环应被丢弃的动力学或热力学的理由，有些甚至是不符合现有的实验观察。因此，我们提出了一种新的机理循环，其依赖于从烷氧基胺（（RRNOCHRR 4）-R-1-N-2-R-3）中提取β-氢原子。我们的研究结果表明，这种循环是积极可行的一系列基板，并提供了一个解释以前误解或无法解释的实验结果。我们还探讨了胺/氮氧化合物循环的情况下，烷氧基胺不具有抽象的β-氢的替代机制。
High-level ab initio molecular orbital theory calculations are used to identify the origin of the remarkably high inhibition stoichiometric factors exhibited by dialkylamine-based radical-trapping antioxidants. We have calculated the free energy barriers and reaction energies at 25, 80, and 260 degrees C in the gas phase and in aqueous solution for a broad range of reactions that might, potentially, be involved in amine/nitroxide cycling, as well as several novel pathways proposed as part of the present work, including that of N-alkyl hindered amine light stabilizer activation. We find that most of the literature nitroxide regeneration cycles should be discarded on either kinetic or thermodynamic grounds; some are even inconsistent with existing experimental observations. We therefore propose a new mechanistic cycle that relies on abstraction of a beta-hydrogen atom from an alkoxyamine ((RRNOCHRR4)-R-1-N-2-R-3). Our results suggest that this cycle is energetically feasible for a range of substrates and provides an explanation for previously misinterpreted or unexplained experimental results. We also explore alternative mechanisms for amine/nitroxide cycling for cases where the alkoxyamines do not possess an abstractable beta-hydrogen.