Selective deuteration illuminates the importance of tunneling in the unimolecular decay of Criegee intermediates to hydroxyl radical products
Selective deuteration illuminates the importance of tunneling in the unimolecular decay of Criegee intermediates to hydroxyl radical products
复制标题
DOI：
10.1073/pnas.1715014114
复制
发表时间：
2017-11-21
影响因子：
11.1
通讯作者：
Lester, Marsha I.
中科院分区：
文献类型：
--
作者：
Green, Amy M.;Barber, Victoria P.;Lester, Marsha I.
臭氧降解烯烃是大气中羟基(OH)自由基的重要非光解来源，通过Criegee中间体的单分子衰变进行。在这里，我们报告了一个与限速氢转移步骤有关的大的动力学同位素效应，该步骤为典型的Criegee中间体CH3CHOO释放OH自由基。在红外激发下，选择性氢化的SYN-Cd3CHOO引起了氢原子的转移和自由基产物的释放。SYN-CD3CHOO的振动激活与OD产物的直接时间分辨检测相结合，测量到过渡态势垒附近的氢化时单分子衰变速度慢10倍，这一点得到了结合量子力学隧穿的微正则统计理论的证实。类似于10的相应的动力学同位素效应主要归因于由隧穿引起的D原子与H原子转移几率的降低。利用主方程模型计算了大气条件下选择性和全氘代合成甲基Criegee中间体的热单分子衰变速率。在298K(1大气压)下，隧道效应将提高合成CH3CHOO的热衰变速率，从而产生类似于50的显著动力学同位素效应。
Ozonolysis of alkenes, an important nonphotolytic source of hydroxyl (OH) radicals in the atmosphere, proceeds through unimolecular decay of Criegee intermediates. Here, we report a large kinetic isotope effect associated with the rate-limiting hydrogen-transfer step that releases OH radicals for a prototypical Criegee intermediate, CH3CHOO. IR excitation of selectively deuterated syn-CD3CHOO is shown to result in deuterium atom transfer and release OD radical products. Vibrational activation of syn-CD3CHOO is coupled with direct time-resolved detection of OD products to measure a 10-fold slower rate of unimolecular decay upon deuteration in the vicinity of the transition state barrier, which is confirmed by microcanonical statistical theory that incorporates quantum mechanical tunneling. The corresponding kinetic isotope effect of similar to 10 is attributed primarily to the decreased probability of D-atom vs. H-atom transfer arising from tunneling. Master equation modeling is utilized to compute the thermal unimolecular decay rates for selectively and fully deuterated syn methyl-substituted Criegee intermediates under atmospheric conditions. At 298 K (1 atm), tunneling is predicted to enhance the thermal decay rate of syn-CH3CHOO compared with the deuterated species, giving rise to a significant kinetic isotope effect of similar to 50.