SUBSTITUENT EFFECTS ON BENZYL RADICAL ELECTRON-SPIN-RESONANCE HYPERFINE COUPLING-CONSTANTS - THE SIGMA-ALPHA.SCALE BASED UPON SPIN DELOCALIZATION
SUBSTITUENT EFFECTS ON BENZYL RADICAL ELECTRON-SPIN-RESONANCE HYPERFINE COUPLING-CONSTANTS - THE SIGMA-ALPHA.SCALE BASED UPON SPIN DELOCALIZATION
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DOI：
10.1021/ja00343a024
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发表时间：
1983-01-01
影响因子：
15
通讯作者：
ARNOLD, DR
中科院分区：
文献类型：
--
作者：
DUST, JM;ARNOLD, DR
本文分析了21个对位和间位取代苄基自由基的电子自旋共振谱。由苄基-氢超精细偶合常数定义了取代基常数·。·常数反映了与未取代的自由基相比，取代的苄基自由基中可能归因于自旋离域的能量分量。对于研究的衍生物，帕拉取代是稳定的，除了当氟是取代基; Meta取代不稳定的苄基自由基。用推广的Hammett关系重新考察了一系列自由基反应。自旋离域效应与极性效应的相对重要性是通过p·与p的比值来评估的。选择取代基的重要性，对于这种评估，·和相差很大。应用线性自由能关系来解释和预测涉及离子的反应速率是现代物理有机化学的一个主要成就。[1]相比之下，取代基对自由基反应速率的影响还不太清楚。[2]人们曾多次尝试通过动力学方法建立点（·）标度，[3]但很难评估取代基对极性明显的过渡态的自由基性质的相对较小的影响。事实上，许多自由基反应的相对速率与离子反应的取代基参数（，+，”）有相当好的相关性;它们显然对自由基稳定性的变化不敏感。4
The electron spin resonance spectra of 21 para-and meta-substituted benzyl radicals have been analyzed. A substituent constant,·, has been defined from the benzylic-hydrogen hyperfine coupling constant. The· constant reflects the component of energy that may be attributed to spin delocalization in a substituted benzyl radical in comparison to the unsubstituted radical. For the derivatives studied, para substitution is stabilizing, except when fluorine is the substituent; meta substitution destabilizes the benzyl radical. A series of radical reactions is reexamined with an extended Hammett relation. The relative importance of spin delocalization vs. polar effects is assessed by the ratio of p· to p. The importanceof choosing substituents for which· and differ widely for such an assessment is emphasized.The application of linear free energy relationships to explain and predict rates of reactions involving ions is a major achievement of modern physical organic chemistry. 1 In contrast, the effect of substituents on the rates of free radical reactions is less well understood. 2 Several attempts have been made to establish a dot (·) scale bykinetic methods, 3 but it is difficult to assess the relatively small influence of the substituent on the radical nature of a transition state that is significantly polar. In fact, relative rates of many free radical reactions correlate reasonably wellwith the substituent parameters (,+,") devised for ionic reactions; they are apparently insensitive to variation in radical stability. 4