Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA.
J Am Chem Soc. 2010 Mar 10;132(9):2907-13. doi: 10.1021/ja903856t.
The hydroxyl radical (HO*) is a highly reactive oxygen-centered radical whose bimolecular rate constants for reaction with organic compounds (hydrogen atom abstraction) approach the diffusion-controlled limit in aqueous solution. The results reported herein show that hydroxyl radical is considerably less reactive in dipolar, aprotic solvents such as acetonitrile. This diminished reactivity is explained on the basis of a polarized transition state for hydrogen abstraction, in which the oxygen of the hydroxyl radical becomes highly negative and can serve as a hydrogen bond acceptor. Because acetonitrile cannot participate as a hydrogen bond donor, the transition state cannot be stabilized by hydrogen bonding, and the reaction rate is lower; the opposite is true when water is the solvent. This hypothesis explains hydroxyl radical reactivity both in solution and in the gas phase and may be the basis for a "containment strategy" used by Nature when hydroxyl radical is produced endogenously.
羟基自由基（HO*）是一种具有高反应活性的含氧自由基，其与有机化合物（氢原子提取）的双分子反应速率常数在水溶液中接近扩散控制极限。本文报道的结果表明，羟基自由基在偶极非质子溶剂如乙腈中的反应性要低得多。这种反应性的降低可以根据氢提取的极化过渡态来解释，在该过渡态中，羟基自由基的氧变得高度负电性，并且可以作为氢键受体。由于乙腈不能作为氢键供体，因此过渡态不能通过氢键稳定，反应速率较低；当水为溶剂时则相反。该假设解释了羟基自由基在溶液中和气相中的反应性，并且可能是内源性产生羟基自由基时自然界使用的“遏制策略”的基础。
