Singh Nakul K, Shaik Majeed S, O'Malley Patrick J, Popelier Paul L A
Manchester Interdisciplinary Biocenter, The University of Manchester, 131 Princess Street, Manchester, M1 7DN, Great Britain.
Org Biomol Chem. 2007 Jun 7;5(11):1739-43. doi: 10.1039/b703831n. Epub 2007 May 1.
Relative acidities (Delta pK(a)) of phenols and oxidation potentials (Delta E(ox)) of the phenoxide anions have been calculated for nine para-substituted phenols using density functional theory. Solvent effects were incorporated using the conductor-like polarisable continuum method. Using the calculated Delta pK(a) and Delta E(ox) values in a thermodynamic cycle, the DeltaBDE (bond dissociation enthalpy) of the phenols were also determined with all values calculated to within 1.5 kcal mol(-1) of experiment. The Delta pK(a) and Delta E(ox) values were calculated for 6-hydroxy-2,2,5,7,8-pentamethylchroman (HPMC), a model for alpha-tocopherol for which there are no known experimental values. The acidity of this compound is raised by 2.4 pK(a) units and lowered by -0.79 V relative to phenol with a calculated Delta BDE of -14.9 kcal mol(-1). There is a negative correlation (r(2) = 0.86) between the Delta pK(a) and the Delta BDE values. A stronger and positive correlation is found between the Delta E(ox) (r(2) = 0.98) and the Delta BDE values. Using these correlations it is uncovered that hydrogen abstraction of phenols, as measured by the Delta BDE, is driven by electron transfer rather than by proton transfer.
使用密度泛函理论计算了9种对取代苯酚的相对酸度（ΔpK(a)）和苯氧阴离子的氧化电位（ΔE(ox)）。采用类导体极化连续介质模型考虑溶剂效应。利用热力学循环中计算得到的ΔpK(a)和ΔE(ox)值，还确定了苯酚的ΔBDE（键解离焓），所有计算值与实验值的偏差在1.5 kcal mol(-1)以内。计算了6-羟基-2,2,5,7,8-五甲基苯并二氢吡喃（HPMC）的ΔpK(a)和ΔE(ox)值，HPMC是α-生育酚的模型化合物，尚无已知实验值。相对于苯酚，该化合物的酸度提高了2.4个pK(a)单位，氧化电位降低了-0.79 V，计算得到的ΔBDE为-14.9 kcal mol(-1)。ΔpK(a)与ΔBDE值之间存在负相关（r(2) = 0.86）。在ΔE(ox)（r(2) = 0.98）与ΔBDE值之间发现更强的正相关。利用这些相关性发现，以ΔBDE衡量的苯酚氢提取是由电子转移而非质子转移驱动的。
