School of Life Science, Shandong University, Binhai Road 72, Qingdao 266237, China.
Phys Chem Chem Phys. 2019 Sep 18;21(36):20296-20307. doi: 10.1039/c9cp03612a.
The BrO + HO2 reaction, which participates in the cycle of ozone removal via BrOH formation, was explored both in the absence and in the presence of water using ab initio calculations. Two main sets of products, (i) HBr + O3 and (ii) BrOH + O2, are formed regardless of the presence of water, following a hydrogen abstraction mechanism. The HBr + O3 products are formed from the intermediate BrOOOH adduct, whereas BrOH + O2 are formed either from the intermediate OBrOOH adduct or via a barrierless hydrogen transfer from HO2 to BrO. Owing to the formation of molecular oxygen that can bear different spin configurations, the formation of BrOH + O2 products was examined both on the singlet and the triplet surfaces. Under relevant atmospheric temperatures and pressure, the formation of products (i) is energetically and kinetically less favorable than that of products (ii). The rate coefficient at 298 K for the HBr + O3 formation was determined to be 2.00 × 10-20 cm3 molecule-1 s-1, and found to decrease by 1-2 orders of magnitude when one or both reactants are clustered with water. For the formation of BrOH + O2, a rate coefficient of 2.21 × 10-11 cm3 molecule-1 s-1 is determined on both singlet and triplet surfaces in the absence of water. Though this rate coefficient slightly decreases for the hydrated reactions, the fractions of the reactants that are effectively complexed with water are not high enough to shift the overall BrOH + O2 formation rate. The current study further indicates that humidity plays a negligible role in ozone removal via the BrO + HO2 reaction.
溴过氧自由基（BrO）+ 氢过氧自由基（HO2）的反应，通过 BrOH 的形成参与臭氧去除循环，本研究使用从头算计算分别在无水和有水条件下对其进行了探讨。无论是否存在水，都通过氢提取机制形成两种主要产物（i）HBr + O3 和（ii）BrOH + O2。HBr + O3 产物是由中间产物 BrOOOH 加合物形成的，而 BrOH + O2 产物是由中间产物 OBrOOH 加合物形成的，或者是通过 HO2 向 BrO 的无势垒氢转移形成的。由于形成的分子氧可以承载不同的自旋构型，因此在单重态和三重态表面上都检查了 BrOH + O2 产物的形成。在相关的大气温度和压力下，产物（i）的形成在能量上和动力学上都不如产物（ii）有利。在 298 K 时，HBr + O3 形成的速率系数确定为 2.00 × 10-20 cm3 分子-1 s-1，当一个或两个反应物与水形成簇时，该速率系数会降低 1-2 个数量级。对于 BrOH + O2 的形成，在无水条件下，在单重态和三重态表面上确定的速率系数分别为 2.21 × 10-11 cm3 分子-1 s-1。尽管该速率系数对于水合反应略有降低，但与水有效络合的反应物分数不足以改变整体 BrOH + O2 形成速率。目前的研究进一步表明，湿度在通过 BrO + HO2 反应去除臭氧方面的作用可以忽略不计。
