Department of Chemistry, University of Rochester, Rochester, New York 14627, USA.
J Org Chem. 2012 Feb 17;77(4):1632-9. doi: 10.1021/jo202385f. Epub 2012 Jan 30.
As we reported recently, endergonic to mildly exergonic electron transfer between neutral aromatics (benzenes and biphenyls) and their radical cations in acetonitrile follows a Sandros-Boltzmann (SB) dependency on the reaction free energy (ΔG); i.e., the rate constant is proportional to 1/[1 + exp(ΔG/RT)]. We now report deviations from this dependency when one reactant is sterically crowded: 1,4-di-tert-butylbenzene (C1), 1,3,5-tri-tert-butylbenzene (C2), or hexaethylbenzene (C3). Obvious deviation from SB behavior is observed with C1. Stronger deviation is observed with the more crowded C2 and C3, where steric hindrance increases the interplanar separation at contact by 1 Å, significantly decreasing the π orbital overlap. Consequently, electron transfer (k(et)) within the contact pair becomes slower than diffusional separation (k(-d)), causing deviation from the SB dependency, especially near ΔG = 0. Fitting the data to a standard electron-transfer theory gives small matrix elements (5-7 meV) and reasonable reorganization energies. A small systematic difference between reactions of C3 with benzenes vs biphenyls is rationalized in terms of small differences in the electron-transfer parameters that are consistent with previous data. The influence of solvent viscosity on the competition between k(et) and k(-d) was investigated by comparing reactions in acetonitrile and propylene carbonate.
正如我们最近所报道的，中性芳香族化合物（苯和联苯）及其自由基阳离子之间的吸能到轻度放能电子转移在乙腈中遵循桑德罗斯-玻尔兹曼（SB）依赖性与反应自由能（ΔG）有关；即，速率常数与 1/[1 + exp(ΔG/RT)]成正比。当一个反应物受到空间位阻时，我们现在报告了这种依赖性的偏差：1,4-二叔丁基苯（C1）、1,3,5-三-叔丁基苯（C2）或六乙基苯（C3）。C1 明显偏离 SB 行为。更拥挤的 C2 和 C3 表现出更强的偏离，其中空间位阻使接触时的平面间分离增加约 1 Å，显著降低了π轨道重叠。因此，接触对中的电子转移（k(et)）比扩散分离（k(-d)）慢，导致偏离 SB 依赖性，尤其是在ΔG = 0 附近。将数据拟合到标准电子转移理论给出了较小的矩阵元（~5-7 meV）和合理的重组能。C3 与苯和联苯的反应之间的小系统差异可以用电子转移参数的微小差异来解释，这些差异与以前的数据一致。通过比较乙腈和碳酸丙烯酯中的反应，研究了溶剂粘度对 k(et)和 k(-d)竞争的影响。
