Australian Research Council Centre of Excellence for Free Radical Chemistry and Biotechnology, Research School of Chemistry, Australian National University , Canberra, Australian Capital Territory 0200, Australia.
J Am Chem Soc. 2012 Aug 8;134(31):12979-88. doi: 10.1021/ja3006379. Epub 2012 Apr 19.
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 °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 β-hydrogen atom from an alkoxyamine (R(1)R(2)NOCHR(3)R(4)). 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 β-hydrogen.
采用高水平的从头算分子轨道理论计算来确定基于二烷基胺的自由基捕获抗氧化剂表现出非常高的抑制计量因素的起源。我们已经计算了在气相和水溶液中 25、80 和 260°C 时可能涉及胺/氮氧自由基循环的广泛反应的自由能势垒和反应能，以及作为当前工作的一部分提出的几种新途径，包括受阻胺光稳定剂的活化。我们发现，大多数文献中的氮氧自由基再生循环要么由于动力学原因，要么由于热力学原因而被抛弃；有些甚至与现有的实验观察结果不一致。因此，我们提出了一个新的机制循环，该循环依赖于从烷氧基胺（R(1)R(2)NOCHR(3)R(4)）中提取β-氢原子。我们的结果表明，对于一系列底物，该循环在能量上是可行的，并为以前解释不当或未解释的实验结果提供了解释。我们还探索了在烷氧基胺没有可提取的β-氢原子的情况下，胺/氮氧自由基循环的替代机制。
