Surprisingly long-lived ascorbyl radicals in acetonitrile: Concerted proton-electron transfer reactions and thermochemistry
Surprisingly long-lived ascorbyl radicals in acetonitrile: Concerted proton-electron transfer reactions and thermochemistry
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DOI：
10.1021/ja802055t
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发表时间：
2008-06-18
影响因子：
15
通讯作者：
Mayer, James M.
中科院分区：
文献类型：
--
作者：
Warren, Jeffrey J.;Mayer, James M.
研究了5，6-异丙叉抗坏血酸(iAscH(-))在乙腈溶剂中的质子耦合电子转移(PCET)反应和热化学。IAscH(-)被2，4，6-TBU(3)C(6)H(2)O中心点和过量的Tempo中心点氧化生成相应的5，6-异亚丙基抗坏血酸自由基阴离子(IASC(中心点-))，该阴离子在298K的干MeCN溶液中持续数小时。IASC(中心点-)的稳定性令人惊讶，因为抗坏血酸自由基在水溶液中是瞬变的，这是由于缺乏自由基歧化所需的质子。IAscH(-)的反应符合协同质子-电子转移(CPET)机理。氧化还原电位、pK(A)和平衡测量定义了5，6-异丙叉抗坏血酸及其衍生物在MeCN中的热化学图景。这些测量给出了在MeCN中iAscH(-)的O-H键解离自由能(BDFE)为65.4+/-1.5kcal摩尔(-1)。对未烷化抗坏血酸的类似研究表明，BDFE为6708+/-1.2kcal摩尔(-1)。这些值远低于从报道数据得出的74.0+/-1.5千卡摩尔(-1)的BDFE冷杉水溶液。
Proton-coupled electron transfer (PCET) reactions and thermochemistry of 5,6-isopropylidene ascorbate (iAscH(-)) have been examined in acetonitrile solvent. iAscH(-) is oxidized by 2,4,6-tBu(3)C(6)H(2)O center dot and by excess TEMPO center dot to give the corresponding 5,6-isopropylidene ascorbyl radical anion (iAsc(center dot-)), which persists for hours at 298 K in dry MeCN solution. The stability of iAsc(center dot-) is surprising in light of the transience of the ascorbyl radical in aqueous solutions and is due to the lack of the protons needed for radical disproportionation. A concerted proton-electron transfer (CPET) mechanism is indicated for the reactions of iAscH(-). Redox potential, pK(a) and equilibrium measurements define the thermochemical landscape for 5,6-isopropylidene ascorbic acid and its derivatives in MeCN. These measurements give an O-H bond dissociation free energy (BDFE) for iAscH(-) of 65.4 +/- 1.5 kcal mol(-1) in MeCN. Similar studies on underivatized ascorbate indicate a BDFE of 6708 +/- 1.2 kcal mol(-1). These values are much lower than the aqueous BDFE fir ascorbate of 74.0 +/- 1.5 kcal mol(-1) derived from reported data.