Bond dissociation energies of O-H bonds in substituted phenols from equilibration studies
Bond dissociation energies of O-H bonds in substituted phenols from equilibration studies
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DOI：
10.1021/jo961039i
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发表时间：
1996-12-27
影响因子：
3.6
通讯作者：
Fattuoni, C
中科院分区：
文献类型：
--
作者：
Lucarini, M;Pedrielli, P;Fattuoni, C
用EPR光谱法研究了几种酚类化合物的键解离能（BDE）及其对应的苯氧基自由基的平衡。测量是在高度浓缩的溶液中进行的，在过氧化二叔丁基的存在下进行连续光解。由于在此实验条件下，苯氧基自由基的衰变比酚到苯氧基的氢转移反应慢，因此实际测量了两种自由基的平衡浓度。由于自由基种类在溶液中不断生成，因此用这种方法也可以研究产生短寿命苯氧基自由基的酚类。所有被检测的酚类化合物的O-H键都比母体化合物PhOH的键弱，它们的BDE值可以通过使用各种取代基的固定贡献的加性规则计算到一个很好的近似值。只有在空间拥挤的4-甲氧基四甲基苯酚（5b）中，对甲氧基取代基被迫留在芳香环平面之外，实验BDE才与计算值有显著差异。
Bond dissociation energies (BDE) of several phenolic compounds have been determined by studying the equilibration of couples of phenols and of the corresponding phenoxyl radicals by means of EPR spectroscopy. Measurements were carried out in highly concentrated solutions submitted to continuous photolysis in the presence of di-tert-butyl peroxide. Since under this experimental condition the decay of the phenoxyl radicals was slower than the hydrogen transfer reaction from phenols to phenoxyls, equilibrium concentrations of the two radicals were actually measured. Due to the fact that the radical species are continuously generated in solution, phenols giving short-lived phenoxyl radicals could also be investigated by this method. All of the examined phenols are characterized by O-H bonds weaker than that of the parent compound, PhOH, and their BDE values can be calculated to a good approximation by an additive rule using fixed contributions for the various substituents. Only in the case of the sterically crowded 4-methoxytetramethylphenol (5b), where the p-methoxy substituent is compelled to stay out of the plane of the aromatic ring, is the experimental BDE remarkably different from the calculated value.