Bimolecular Electron Transfers That Deviate from the Sandros鈥揃oltzmann Dependence on Free Energy: Steric Effect

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• 作者：Pu Luo ; Joseph P. Dinnocenzo ; Paul B. Merkel ; Ralph H. Young ; Samir Farid
• 刊名：The Journal of Organic Chemistry
• 出版年：2012
• 出版时间：February 17, 2012
• 年：2012
• 卷：77
• 期：4
• 页码：1632-1639
• 全文大小：338K
• 年卷期：v.77,no.4(February 17, 2012)
• ISSN：1520-6904

文摘

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鈥揃oltzmann (SB) dependency on the reaction free energy (螖G); i.e., the rate constant is proportional to 1/[1 + exp(螖G/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 脜, significantly decreasing the 蟺 orbital overlap. Consequently, electron transfer (kb>etb>) within the contact pair becomes slower than diffusional separation (kb>鈥揹b>), causing deviation from the SB dependency, especially near 螖G = 0. Fitting the data to a standard electron-transfer theory gives small matrix elements (5鈥? 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 kb>etb> and kb>鈥揹b> was investigated by comparing reactions in acetonitrile and propylene carbonate.