Kohen Amnon, Jensen Jan H
Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, USA.
J Am Chem Soc. 2002 Apr 17;124(15):3858-64. doi: 10.1021/ja016909e.
Hydrogen quantum mechanical tunneling has been suggested to play a role in a wide variety of hydrogen-transfer reactions in chemistry and enzymology. An important experimental criterion for tunneling is based on the breakdown of the semiclassical prediction for the relationship among the rates of the three isotopes of hydrogen (hydrogen -H, deuterium -D, and tritium -T). This is denoted the Swain-Schaad relationship. This study examines the breakdown of the Swain-Schaad relationship as criterion for tunneling. The semiclassical (no tunneling) limit used hereto (e.g., 3.34, for H/T to D/T kinetic isotope effects), was based on simple theoretical considerations of a diatomic cleavage of a stable covalent bond, for example, a C-H bond. Yet, most experimental evidence for a tunneling contribution has come from breakdown of those relationship for a secondary hydrogen, that is, not the hydrogen whose bond is being cleaved but its geminal neighbor. Furthermore, many of the reported experiments have been mixed-labeling experiments, in which a secondary H/T kinetic isotope effect was measured for C-H cleavage, while the D/T secondary effect accompanied C-D cleavage. In experiments of this type, the breakdown of the Swain-Schaad relationship indicates both tunneling and the degree of coupled motion between the primary and secondary hydrogens. We found a new semiclassical limit (e.g., 4.8 for H/T to D/T kinetic isotope effects), whose breakdown can serve as a more reliable experimental evidence for tunneling in this common mixed-labeling experiment. We study the tunneling contribution to C-H bond activation, for which many relevant experimental and theoretical data are available. However, these studies can be applied to any hydrogen-transfer reaction. First, an extension of the original approach was applied, and then vibrational analysis studies were carried out for a model system (the enzyme alcohol dehydrogenase). Finally, the effect of complex kinetics on the observed Swain-Schaad relationship was examined. All three methods yield a new semiclassical limit (4.8), above which tunneling must be considered. Yet, it was found that for many cases the original, localized limit (3.34), holds fairly well. For experimental results that are between the original and new limits (within statistical errors), several methods are suggested that can support or exclude tunneling. These new and clearer criteria provide a basis for future applications of the Swain-Schaad relationship to demonstrate tunneling in complex systems.
氢量子力学隧穿被认为在化学和酶学中多种氢转移反应中发挥作用。隧穿的一个重要实验标准基于对氢的三种同位素（氢-H、氘-D和氚-T）速率之间关系的半经典预测的失效。这被称为斯温-沙德关系。本研究考察了作为隧穿标准的斯温-沙德关系的失效情况。此前使用的半经典（无隧穿）极限（例如，对于H/T与D/T动力学同位素效应为3.34），是基于对稳定共价键（例如C-H键）的双原子裂解的简单理论考虑。然而，大多数隧穿贡献的实验证据来自于仲氢的这些关系的失效，也就是说，不是其键正在断裂的氢，而是其偕位氢。此外，许多报道的实验是混合标记实验，其中测量了C-H裂解的仲H/T动力学同位素效应，而D/T仲效应伴随着C-D裂解。在这类实验中，斯温-沙德关系的失效既表明了隧穿，也表明了伯氢和仲氢之间的耦合运动程度。我们发现了一个新的半经典极限（例如，对于H/T与D/T动力学同位素效应为4.8），其失效可以作为这种常见混合标记实验中隧穿的更可靠实验证据。我们研究了隧穿对C-H键活化的贡献，对此有许多相关的实验和理论数据。然而，这些研究可以应用于任何氢转移反应。首先，应用了对原始方法的扩展，然后对一个模型系统（醇脱氢酶）进行了振动分析研究。最后，考察了复杂动力学对观察到的斯温-沙德关系的影响。所有这三种方法都得出了一个新的半经典极限（4.8），高于此极限必须考虑隧穿。然而，发现对于许多情况，原来的局部极限（3.34）相当适用。对于介于原始极限和新极限之间（在统计误差范围内）的实验结果，提出了几种可以支持或排除隧穿的方法。这些新的、更清晰的标准为未来将斯温-沙德关系应用于证明复杂系统中的隧穿提供了基础。
