KETO-ENOL TAUTOMERIZATION OF 2-(2'-HYDROXYPHENYL)BENZOXAZOLE AND 2-(2'-HYDROXY-4'-METHYLPHENYL)BENZOXAZOLE IN THE TRIPLET-STATE - HYDROGEN TUNNELING AND ISOTOPE EFFECTS .1. TRANSIENT ABSORPTION KINETICS
KETO-ENOL TAUTOMERIZATION OF 2-(2'-HYDROXYPHENYL)BENZOXAZOLE AND 2-(2'-HYDROXY-4'-METHYLPHENYL)BENZOXAZOLE IN THE TRIPLET-STATE - HYDROGEN TUNNELING AND ISOTOPE EFFECTS .1. TRANSIENT ABSORPTION KINETICS
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DOI：
10.1021/j100178a043
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发表时间：
1991-12-12
影响因子：
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通讯作者：
NICKEL, B
中科院分区：
文献类型：
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作者：
ALSOUFI, W;GRELLMANN, KH;NICKEL, B
在低温下粘度截然不同的三种烷烃溶剂中，研究了 2-(2'-羟基苯基)苯并恶唑 (HBO) 和 2-(2'-羟基-4'-甲基苯基)苯并恶唑 (m-MeHBO) 及其氘氧类似物 DBO 和 m-MeDBO 亚稳态三重态的氢隧道效应，温度范围为 35 至 250 K。在 HBO 的三重态中，烯醇和酮互变异构体之间的氢转移是可逆的，因为这两个三重态是等能的。在 m-MeHBO 中，氢转移实际上是单向的，因为最初填充的酮三重态具有比烯醇三重态更高的能量。尽管存在这些差异，观察到的两种化合物的氢转移速率常数具有几乎相同的值。速率常数的温度依赖性对溶剂粘度的大变化不敏感，即HBO和m-MeHBO中的氢隧道效应既不依赖于中间温度也不依赖于极低温度下的溶剂摩擦。 HBO、DBO 和 m-MeHBO、m-MeDBO 是第一个例子，其中两种同位素异构体的隧道速率常数的确定已扩展到一个温度区域，其中速率和同位素对这些速率的影响变得与温度无关。
Hydrogen tunnel effects in the metastable triplet states of 2-(2'-hydroxyphenyl)benzoxazole (HBO) and 2-(2'-hydroxy-4'-methylphenyl) benzoxazole (m-MeHBO) and their deuteriooxy analogues DBO and m-MeDBO have been investigated between 35 and 250 K in three alkane solvents that have drastically different viscosities at low temperatures. In the triplet states of HBO the hydrogen transfer between the enol and keto tautomers is reversible because the two triplet states are isoenergetic. In m-MeHBO the hydrogen transfer is virtually unidirectional because the initially populated keto triplet state has a higher energy than the enol triplet state. In spite of these differences the observed hydrogen-transfer rate constants of the two compounds have nearly the same values. The temperature dependence of the rate constants is insensitive to large changes in solvent viscosity, i.e., hydrogen tunneling in HBO and m-MeHBO depends on solvent friction neither at intermediate nor at very low temperatures. The couples HBO, DBO and m-MeHBO, m-MeDBO are tbe first examples where the determination of tunneling rate constants has been extended for both isotopomers into a temperature region where the rates and the isotope effect on these rates become temperature independent.