Department of Chemical Engineering, Massachusetts Institute of Technology , 77 Massachusetts Avenue, E19-502b, Cambridge, Massachusetts 02139, United States.
J Phys Chem A. 2014 Apr 10;118(14):2667-82. doi: 10.1021/jp5009708. Epub 2014 Mar 28.
A high-level quantum chemistry investigation has been carried out for the addition and abstraction reactions by the radicals (•)OH and (•)OOH to and from the model alkenes 3-methylpyrrole and benzene. These models were chosen to reflect the functionalities contained in the side chain of the amino acid tryptophan. The W1BD procedure was used to calculate benchmark barriers and reaction energies for the smaller model system of (•)OOH addition to ethylene. It was found that the CBS-QB3 methodology compares best with the W1BD benchmark, demonstrating a mean absolute deviation (MAD) from W1BD of 3.9 kJ mol(-1). For the reactions involving the (•)OH radical and benzene or 3-methylpyrrole, addition is favored over abstraction in all cases. In particular the CBS-QB3 calculations suggest a barrierless addition reaction of the (•)OH radical to position two of 3-methylpyrrole. For the analogous addition and abstraction reactions involving the (•)OOH radical, the same order of reactivity was found, albeit with higher barriers. A number of other processes involving the addition of the (•)OOH radical were also investigated. The main findings of these studies determined that the initial (•)OOH barrier of stepwise addition to 3-methylpyrrole (+18.8 kJ mol(-1)) is significantly smaller than the concerted addition barrier (+71.5 kJ mol(-1)). This conclusion contrasts starkly with the situation for ethylene in which it is well established that the concerted process has the smaller barrier. A considerable variety of contemporary density functional theory procedures have been tested to examine their accuracy in predicting the CBS-QB3 results. It was found that the best overall performing method was UBMK with an MAD of 7.3 kJ mol(-1). A number of other functionals additionally performed well. They included UM06, RM06, UXYG3 and RXYG3, all of which have MADs of less than 8 kJ mol(-1).
已对自由基（•）OH 和（•）OOH 与模型烯烃 3-甲基吡咯和苯之间的加成和消除反应进行了高级量子化学研究。选择这些模型是为了反映氨基酸色氨酸侧链中的官能团。使用 W1BD 程序计算了较小的（•）OOH 加成到乙烯模型系统的基准势垒和反应能。结果发现，CBS-QB3 方法与 W1BD 基准最匹配，表明与 W1BD 的平均绝对偏差（MAD）为 3.9 kJ mol（-1）。对于涉及（•）OH 自由基和苯或 3-甲基吡咯的反应，在所有情况下，加成都优于消除。特别是 CBS-QB3 计算表明（•）OH 自由基对 3-甲基吡咯的第二位的加成反应是无势垒的。对于涉及（•）OOH 自由基的类似加成和消除反应，也发现了相同的反应性顺序，尽管势垒较高。还研究了涉及（•）OOH 自由基加成的许多其他过程。这些研究的主要发现确定了（•）OOH 自由基逐步加成到 3-甲基吡咯的初始（•）OOH 势垒（+18.8 kJ mol（-1））明显小于协同加成势垒（+71.5 kJ mol（-1））。这一结论与乙烯的情况形成鲜明对比，对于乙烯，已经确立了协同过程具有较小的势垒。已经测试了相当多种当代密度泛函理论程序来检查它们在预测 CBS-QB3 结果方面的准确性。结果发现，总体表现最佳的方法是 UBMK，其 MAD 为 7.3 kJ mol（-1）。还有其他一些功能也表现良好。它们包括 UM06、RM06、UXYG3 和 RXYG3，它们的 MAD 均小于 8 kJ mol（-1）。
