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.
Monomer- and polymer radicals of vinyl compounds: EPR and DFT studies of geometric and electronic structures in the adsorbed state
作者:Anders Lund、Marek Danilczuk
DOI:10.1016/j.saa.2012.08.054
日期:2012.12
paramagnetic resonance (EPR) spectroscopy was applied to study the paramagnetic species formed from styrene, 1,1-diphenylethylene, α-methylstyrene, β-methylstyrene and methylmethacrylate adsorbed on amorphous silica gel after γ-irradiation at 77K. Radicals formed by the hydrogenatomaddition at the vinyl group of the monomers were observed in all samples. The hydrogenatoms were shown to originate to
EPR Investigation of Persistent Radicals Produced from the Photolysis of Dibenzyl Ketones Adsorbed on ZSM-5 Zeolites
作者:Nicholas J. Turro、Xue-gong Lei、Steffen Jockusch、Wei Li、Zhiqiang Liu、Lloyd Abrams、M. Francesca Ottaviani
DOI:10.1021/jo011047l
日期:2002.4.1
reactions. Radicals that are produced on the external surface and whose molecular structure prevents diffusion into the internal surface are transient because radical-radical reactions occur rapidly on the external surface. The reactions of the persistentradicals with oxygen and nitric oxide were directly studied in situ by EPR analysis. In the case of reaction with oxygen, persistent peroxy radicals are
The reactivity of ketyl and alkyl radicals in reactions with carbonyl compounds
作者:E. T. Denisov
DOI:10.1007/bf02494263
日期:1998.11
parabolic model of bimolecular radical reactions was used for analysis of the hydrogen transfer reactions of ketyl radicals: >C·OH+R1COR2→>C=O+R1R2C·OH. The parameters describing the reactivity of the reagents were calculated from the experimental data. The parameters that characterize the reactions of ketyl and alkyl radicals as hydrogen donors with olefins and with carbonyl compounds were obtained: >C·OH
The decomposition rate constants kdec as well as the activation rate constants kact of some alkoxyamines were determined and compared with regard to their structures. The results showed that decomposition depends predominantly on steric factors, while both steric and polar factors affect kact. Bulky nitroxides are less susceptible to decomposition probably due to the sterically congested transition
Kinetics of nitroxide radical trapping. 2. Structural effects
作者:V. W. Bowry、K. U. Ingold
DOI:10.1021/ja00039a006
日期:1992.6
Laserflashphotolysis and kinetic competition product studies have demonstrated that in isooctane at ambient temperatures the rate constant for coupling of carbon-centered radicals with persistent nitroxides, k T , depends upon the degree of steric hindrance to coupling and upon the extent of resonance stabilization of the carbon radical. Sterically induced reductions in the magnitude of k T are observed
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.