Iyer Siddharth, He Xucheng, Hyttinen Noora, Kurtén Theo, Rissanen Matti P
Department of Chemistry, and ‡Department of Physics, University of Helsinki , Helsinki, 00100, Finland.
J Phys Chem A. 2017 Sep 14;121(36):6778-6789. doi: 10.1021/acs.jpca.7b01588. Epub 2017 Aug 29.
The HO radical is an important atmospheric molecule that can potentially influence the termination of autoxidation processes of volatile organic compounds (VOCs) that lead to the formation of highly oxygenated multifunctional compounds (HOMs). In this work, we demonstrate the direct detection of the HO radical using an iodide-based chemical ionization mass spectrometer (iodide-CIMS). Expanding on the previously established correlation between molecule-iodide binding enthalpy and iodide-CIMS instrument sensitivity, the experimental detection of the HO radical was preceded by the quantum chemical calculation of the HOI cluster (PBE/aug-cc-pVTZ-PP level), which showed a reasonably strong binding enthalpy of 21.60 kcal/mol. Cyclohexene ozonolysis intermediates and closed-shell products were next detected by the iodide-CIMS. The ozone-initiated cyclohexene oxidation mechanism was perturbed by the introduction of the HO radical, leading to the formation of closed-shell hydroperoxides. The experimental investigation once again followed the initial computational molecule-iodide binding enthalpy calculations. The quantum chemical calculations were performed at the PBE/aug-cc-pVTZ-PP level for radicals and DLPNO-CCSD(T)/def2-QZVPP//PBE/aug-cc-pVTZ-PP level for the closed-shell products. A comparison between the iodide-CIMS and nitrate-CIMS spectra with identical measurement steps revealed that the iodide-CIMS was able to detect the low-oxidized (O/C ratio 0.5 and 0.66) cyclohexene ozonolysis monomer products more efficiently than nitrate-CIMS. Higher-oxidized monomers (O/C ratio 1 to 1.5) were detected equally well by both methods. An investigation of dimers showed that both iodide- and nitrate-CIMS were able to detect the dimer compositions possibly formed from reactions between the peroxy radical monomers considered in this study. Additionally, iodide-CIMS detected organic ions that were formed by a previously suggested mechanism of dehydroxylation of peroxy acids (and deoxygenation of acyl peroxy radicals) by HOI clusters. These mechanisms were computationally verified.
羟基自由基（HO 自由基）是一种重要的大气分子，它可能会影响挥发性有机化合物（VOCs）自氧化过程的终止，而这些过程会导致形成高度氧化的多功能化合物（HOMs）。在这项工作中，我们展示了使用基于碘化物的化学电离质谱仪（碘化物 - CIMS）直接检测 HO 自由基。基于先前建立的分子 - 碘化物结合焓与碘化物 - CIMS 仪器灵敏度之间的相关性，在对 HO 自由基进行实验检测之前，先对 HOI 簇进行了量子化学计算（PBE/aug-cc-pVTZ-PP 水平），结果显示其结合焓为 21.60 kcal/mol，相当强。接下来，碘化物 - CIMS 检测了环己烯臭氧化中间体和闭壳产物。HO 自由基的引入扰乱了臭氧引发的环己烯氧化机制，导致形成闭壳型氢过氧化物。实验研究再次遵循了最初的计算分子 - 碘化物结合焓计算。对于自由基，量子化学计算在 PBE/aug-cc-pVTZ-PP 水平进行；对于闭壳产物，在 DLPNO-CCSD(T)/def2-QZVPP//PBE/aug-cc-pVTZ-PP 水平进行。在相同测量步骤下，对碘化物 - CIMS 和硝酸盐 - CIMS 光谱进行比较，结果表明碘化物 - CIMS 比硝酸盐 - CIMS 更能有效地检测低氧化态（O/C 比为 0.5 和 0.66）的环己烯臭氧化单体产物。两种方法对高氧化态单体（O/C 比为 1 至 1.5）的检测效果相当。对二聚体的研究表明，碘化物 - CIMS 和硝酸盐 - CIMS 都能够检测到可能由本研究中考虑的过氧自由基单体之间的反应形成的二聚体组成。此外，碘化物 - CIMS 检测到了通过 HOI 簇对过氧酸进行脱羟基化（以及酰基过氧自由基脱氧）的先前提出的机制形成的有机离子。这些机制通过计算得到了验证。
