Multiple-site concerted proton-electron transfer reactions of hydrogen-bonded phenols are nonadiabatic and well described by semiclassical Marcus theory.
Multiple-site concerted proton-electron transfer reactions of hydrogen-bonded phenols are nonadiabatic and well described by semiclassical Marcus theory.
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DOI：
10.1021/ja305668h
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发表时间：
2012-10-10
影响因子：
15
通讯作者：
Mayer, James M.
中科院分区：
文献类型：
--
作者：
Schrauben, Joel N.;Cattaneo, Mauricio;Day, Thomas C.;Tenderholt, Adam L.;Mayer, James M.
描述了使用激发态聚芳烃对氢键酚进行光氧化，以获得对多位点协同质子电子转移反应 (MS-CPET) 的基本理解。实验检查了在 4,6 位酚羟基和叔丁基邻位具有 -CPh2NH2、-Py 和 -CH2Py 基团的酚碱的稳定性（分别为 HOAr-NH2、HOAr-Py 和 HOAr-CH2Py；Py = 吡啶基；Ph = 苯基）。光氧化通过分子内质子从苯酚转移到悬垂碱基以及电子转移到激发的聚芳烃来进行。为了进行比较，还检查了 2,4,6-tBu3C6H2OH（一种在 2,4,6 位不带侧基和叔丁基的苯酚）。许多双分子反应速度很快，速率常数接近扩散极限。将光化学 kCPET 值与之前的热停流动力学研究的值相结合，给出了 HOAr-NH2 和 HOAr-CH2Py 氧化的数据集，其 kCPET 超过 107，驱动力 (ΔGo') 接近 0.9 eV。 log(kCPET) 与 ΔGo' 的图在每种情况下定义一条马库斯抛物线，每种抛物线都包含激发态蒽和基态铵自由基阳离子。因此，半经典 Marcus 理论很好地描述了这两个数据集，为 MS-CPET 使用该理论提供了强有力的验证。抛物线给出 λCPET ≅ 1.15–1.2 eV 和 Hab ≅ 20–30 cm−1。这些实验代表了迄今为止最直接的 MS-CPET 反应 Hab 测量。尽管速率常数仅在扩散极限以下可用，但抛物线的峰值显然远低于约的绝热极限。 6 × 1012 s−1。因此，这非常清楚地证明了反应是非绝热的。非绝热特性使反应速度减慢约 45 倍。 HOAr-Py（其中苯酚和碱共轭）的氧化结果以及 2,4,6-tBu3C6H2OH（缺乏碱）的氧化结果表明，两者均具有较低的 λ 和较大的指前项。讨论了这些结果对 MS-CPET 反应的影响。
Photo-oxidations of hydrogen-bonded phenols using excited state polyarenes are described, to derive fundamental understanding of multiple-site concerted proton-electron transfer reactions (MS-CPET). Experiments have examined phenol-bases having −CPh2NH2, −Py, and −CH2Py groups ortho to the phenol hydroxyl group and tert-butyl groups in the 4,6-positions for stability (HOAr-NH2, HOAr-Py, and HOAr-CH2Py, respectively; Py = pyridyl; Ph = phenyl). The photo-oxidations proceed by intramolecular proton transfer from the phenol to the pendent base concerted with electron transfer to the excited polyarene. For comparison, 2,4,6-tBu3C6H2OH, a phenol without a pendent base and tert-butyl groups in the 2,4,6-positions, has also been examined. Many of these bimolecular reactions are fast, with rate constants near the diffusion limit. Combining the photochemical kCPET values with those from prior thermal stopped-flow kinetic studies gives datasets for the oxidations of HOAr-NH2 and of HOAr-CH2Py that span over 107 in kCPET and nearly 0.9 eV in driving force (ΔGo′). Plots of log(kCPET) vs. ΔGo′ define a single Marcus parabola in each case, each including both excited state anthracenes and ground state aminium radical cations. These two datasets are thus well described by semi-classical Marcus theory, providing a strong validation of the use of this theory for MS-CPET. The parabolas give λCPET ≅ 1.15–1.2 eV and Hab ≅ 20–30 cm−1. These experiments represent the most direct measurements of Hab for MS-CPET reactions to date. Although rate constants are available only up to the diffusion limit, the parabolas clearly peak well below the adiabatic limit of ca. 6 × 1012 s−1. Thus, this is a very clear demonstration that the reactions are non-adiabatic. The non-adiabatic character slows the reactions by a factor of ~45. Results for the oxidation of HOAr-Py, in which the phenol and base are conjugated, and for oxidation of 2,4,6-tBu3C6H2OH, which lacks a base, show that both have substantially lower λ and larger pre-exponential terms. The implications of these results for MS-CPET reactions are discussed.
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影响因子：
62.1
作者：
Dempsey, Jillian L.;Winkler, Jay R.;Gray, Harry B.
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