Entropy Control of the Cross-Reaction between Carbon-Centered and Nitroxide Radicals
摘要:
Absolute rate constants for the cross-coupling reaction of several carbon-centered radicals with various nitroxides and their temperature dependence have been determined in liquids by kinetic absorption spectroscopy. The rate constants range from <2 x 10(5) M-1 s(-1) to 2.3 x 10(9) M-1 s(-1) and depend strongly on the structure of the nitroxide and the carbon-centered radical. Grossly, they decrease with increasing rate constant of the cleavage of the corresponding alkoxyamine. In many cases, the temperature dependence shows a non-Arrhenius behavior. A model assuming a short-lived intermediate that is hindered to form the coupling product by an unfavorable activation entropy leads to a satisfactory analytic description. However, the behavior is more likely due to a barrierless single-step reaction with a low exothermicity where the free energy of activation is dominated by a large negative entropy term.
激光闪光光解和动力学竞争产物研究表明,在环境温度下,在异辛烷中,以碳为中心的自由基与持久性氮氧化合物的偶联速率常数 k T 取决于偶联的空间位阻程度和共振稳定的程度。碳自由基。对于氮氧化合物结构和碳自由基结构的变化,观察到 k T 大小的空间诱导减少。因此,对于任何特定的碳基,k T 对于 Bredt 规则保护的氮氧化物、9-氮杂双环 [3.3.1] 壬烷-N-氧基 (ABNO) 和去甲托烷-N-氧基而言是最大的,而对于通常的'二叔-烷基氮氧化物 k T 沿系列递减,1,1,3,3-四甲基异碘啉-2-氧基≥2,2,5,5-四甲基哌啶-1-氧基(Tempo)>二叔丁基氮氧化物,i.
The synthesis of several new O-acyl selenophosphates were investigated. The stability and reactivity of the products were studied and related to their structure.
Compounds of formula I
wherein X is a group of formula Ia or Ib,
R₁ and R₂ are independently e.g. hydrogen, alkyl, alkenyl, aryl or phenylalkyl, and R₃ to R₇ are independently e.g. hydrogen, alkyl or substituted alkyl are very effective process stabilizers for polymeric systems providing melt flow stabilization and resistance to discoloration during processing at elevated temperatures as well as effective stabilizers for lubricant compositions.
式 I 的化合物
其中 X 是式 Ia 或 Ib 的基团、
R₁ 和 R₂ 独立地为氢、烷基、烯基、芳基或苯基烷基,以及 R₃ 至 R₇ 独立地为氢、烷基或取代烷基,是非常有效的聚合物体系加工稳定剂,可在高温加工过程中提供熔体流动稳定和抗变色性能,同时也是润滑剂组合物的有效稳定剂。