The Role of Intermolecular Hydrogen Bonding and Proton Transfer in Proton-Coupled Electron Transfer
The Role of Intermolecular Hydrogen Bonding and Proton Transfer in Proton-Coupled Electron Transfer
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DOI：
10.1021/jp108339k
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发表时间：
2011-06-02
影响因子：
3.7
通讯作者：
Alvarez, Julio C.
中科院分区：
文献类型：
--
作者：
Alligrant, Timothy M.;Alvarez, Julio C.
研究了在Bronsted碱存在下，1，4-氢醌（1，4-H（2）Q）在乙腈中的电化学氧化过程中的质子耦合电子转移（PCET）.在所研究的两种碱中，带负电荷的羧酸盐、三氟乙酸盐（TFAC（-））、苯甲酸盐（BZ（-））和乙酸盐（AC（-））显示与1，4-H（2）Q的氢键，而中性胺、吡啶（PY）和N，N '-二异丙基乙胺（DIPEA）则没有。这种差异允许使用具有大致相同pK（α）（类似于12）的两种碱基（TFAC（-）和PY）在有和没有氢键影响的情况下对质子转移对PCET的影响进行独特的研究。研究表明，1，4-H（2）Q与碱TFAC（-）形成氢键，使1，4-H（2）Q的半波氧化还原电位负移（更容易氧化）相对于在相同浓度的添加的PY（其不与1，4-H（2）Q形成氢键）存在下的氧化增加0.186 V，研究的两种类型的碱，羧酸盐和胺，在1，4-H（2）Q（i）的氧化伏安中显示动力学和热力学效应的组合，然而，在HID动力学同位素实验所指示的研究条件下没有发现协同途径的证据。从拟合的数字模拟的所有基地的机制支持逐步PCET，即使在氢键的存在下，这意味着后者不占上风的过渡态，也不是速率决定。氢键通过H-1 NMR光谱进行了验证，而电化学研究通过循环伏安法进行。通过H-1 NMR测定的氢键常数和扩散系数用于拟合实验伏安图的数字模拟。
An example of proton-coupled electron transfer (PCET) comprised by the electrochemical oxidation of 1,4-hydroquinone (1,4-H(2)Q) in acetonitrile was studied in the presence of Bronsted bases in acetonitrile. Of the two types of bases studied, the negatively, charged carboxylates, trifluoroacetate (TFAC(-)), benzoate (BZ(-)), and acetate (AC(-)), showed hydrogen bonding with 1,4-H(2)Q,whereas the neutral amines, pyridine (PY) and N, N'-diisopropylethylamine (DIPEA), did not. This difference allowed a unique investigation of the effect of proton transfer on PCET with and without the influence of hydrogen bonding using two bases (TFAC(-) and PY) with approximately the same pK(a) (similar to 12). The study revealed that hydrogen bonding of 1,4-H(2)Q with the base TFAC(-) made the half wave redox potential of 1,4-H(2)Q more negative (easier to oxidize) by 0.186 V with respect to the oxidation in the presence of the same concentration of added PY, which does not hydrogen bond with 1,4-H(2)Q, Both types of bases studied, carboxylates and amines, showed a combination of kinetic and thermodynamic effects in the oxidation voltammerty of 1,4-H(2)Q(i) however, no evidence of concerted pathways was found at the conditions studied as indicated by HID kinetic isotope experiments. The mechanism from fitted digital simulations for all the bases supports a stepwise PCET, even in the presence of hydrogen bonding, implying that the latter does not prevail in the transition state nor is rate determining. Hydrogen bonding was verified by H-1 NMR spectroscopy, while the electrochemical studies were carried out by cyclic voltammetry. The hydrogen bonding constants and diffusion coefficients determined by H-1 NMR were used in digital simulations that were fitted to experimental voltammograms.