Saouma Caroline T, Mayer James M
Department of Chemistry, University of Washington, Campus Box 351700, Seattle, WA, USA.
Chem Sci. 2014 Jan 1;5(1). doi: 10.1039/C3SC52664J.
The prevalence of hydrogen atom transfer (HAT) reactions in chemical and biological systems has prompted much interest in establishing and understanding the underlying factors that enable this reactivity. Arguments have been advanced that the electronic spin state of the abstractor and/or the spin-density at the abstracting atom are critical for HAT reactivity. This is consistent with the intuition derived from introductory organic chemistry courses. Herein we present an alternative view on the role of spin state and spin-density in HAT reactions. After a brief introduction, the second section introduces a new and simple fundamental kinetic analysis, which shows that unpaired spin cannot be the dominant effect. The third section examines published computational studies of HAT reactions, which indicates that the spin state affects these reactions indirectly, primarily via changes in driving force. The essay concludes with a broader view of HAT reactivity, including indirect effects of spin and other properties on reactivity. It is suggested that some of the controversy in this area may arise from the diversity of HAT reactions and their overlap with proton-coupled electron transfer (PCET) reactions.
氢原子转移（HAT）反应在化学和生物体系中的普遍性引发了人们对确定和理解促成这种反应活性的潜在因素的浓厚兴趣。有人提出，抽象剂的电子自旋态和/或抽象原子处的自旋密度对HAT反应活性至关重要。这与从有机化学入门课程中得出的直觉相符。在此，我们对自旋态和自旋密度在HAT反应中的作用提出一种不同的观点。在简要介绍之后，第二节引入了一种新的简单基础动力学分析，该分析表明未成对自旋并非主要影响因素。第三节考察了已发表的关于HAT反应的计算研究，这些研究表明自旋态主要通过驱动力的变化间接影响这些反应。本文最后对HAT反应活性进行了更广泛的探讨，包括自旋和其他性质对反应活性的间接影响。有人认为，该领域的一些争议可能源于HAT反应的多样性及其与质子耦合电子转移（PCET）反应的重叠。
