Department of Chemistry, Ball State University, Muncie Indiana 47306, USA.
J Phys Chem A. 2011 Dec 29;115(51):14687-96. doi: 10.1021/jp209394d. Epub 2011 Dec 7.
The energetics and kinetics of the reaction of variously substituted benzyl radicals with a model alkene were calculated at the G3(MP2)-RAD//B3-LYP/6-31G(d) level of theory to determine whether such reactions are amenable to Hammett analysis. The reactions were studied both in the gas phase and in toluene solution in the temperature range 298-353 K; calculations include 1D-hindered rotor corrections for low frequency torsional modes, and the solvation energies were calculated using COSMO-RS at the BP/TZP level of theory. The addition reaction was found to be dominated by radical stabilization effects, but under circumstances where olefin substituent effects were decoupled from aryl substituent effects, a modest polar effect comes into play, which is enhanced by solvation. Reasonable correlations with empirical substituent parameters such as Hammett σ and σ(•) are observed for the enthalpy of activation, but additional entropic factors act to decrease the degree of correlation with respect to free energies and rate coefficients, confirming hypotheses from earlier experimental work. Substituent effects on the reverse β-fragmentation reaction, and potential cyclization of the 3-phenylpropyl radicals formed by addition are also discussed.
用 G3(MP2)-RAD//B3-LYP/6-31G(d) 理论水平计算了不同取代的苄基自由基与模型烯烃反应的能量学和动力学，以确定此类反应是否适合哈米特分析。在 298-353 K 的温度范围内，研究了气相和甲苯溶液中的反应；计算包括低频扭转模式的 1D 受阻转子校正，并用 COSMO-RS 在 BP/TZP 理论水平计算了溶剂化能。加成反应主要受自由基稳定化效应控制，但在烯烃取代基效应与芳基取代基效应解耦的情况下，会出现适度的极性效应，该效应会通过溶剂化得到增强。对于活化焓，观察到与经验取代基参数（如哈米特 σ 和 σ(•)）的合理相关性，但额外的熵因素会降低与自由能和速率系数的相关性程度，这证实了早期实验工作的假设。还讨论了取代基对β-断裂反应的逆反应以及加成形成的 3-苯基丙基自由基的潜在环化的影响。
