Controlling factors of oligomerization at the water surface: why is isoprene such a unique VOC?
Controlling factors of oligomerization at the water surface: why is isoprene such a unique VOC?
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水面低聚的控制因素：为什么异戊二烯是一种如此独特的挥发性有机化合物？
DOI：
10.1039/c8cp01551a
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发表时间：
2018
影响因子：
3.3
通讯作者：
and Shinichi Enami
中科院分区：
文献类型：
--
作者：
Shinnosuke Ishizuka;Tomihide Fujii;Akira Matsugi;Yosuke Sakamoto;Tetsuya Hama;and Shinichi Enami
最近的研究表明，大气中的颗粒物具有足够的酸性，可以通过引发酸催化的齐聚反应来增强对不饱和挥发性有机化合物(VOCs)的吸收。然而，水表面齐聚的控制因素仍有待阐明。在这里，异戊二烯(2-甲基-1，3-丁二烯，ISO)，1，3-丁二烯(1，3-b)，1，4-戊二烯(1，4-p)，1-戊烯(1-p)和2-戊烯(2-p)蒸汽暴露在酸性水微喷(1≤pH≤5)中，阳离子产物在∼10μS的表面产生并直接使用表面灵敏质谱仪进行检测。我们发现，在所有的情况下，碳正离子都是在空气-水界面上形成的，而齐聚的程度主要取决于结构的顺序：ISO≫1，3-b>；1，4-p≫1-p≈2-p。重要的是，异辛基的阳离子齐聚反应生成了质子化十聚体((ISO)10H+，C50种m/z为681.6)，而戊烯1-p/2-p仍然是质子化单体。我们认为，(1)(ISO)H+通过与共轭CC键对形成三级碳正离子而共振稳定，以及(2)相邻甲基诱导的π-电子富集化是唯一促进ISO齐聚的因素。在D2O和D2O- ： -H2O混合物中的实验表明，酸性水表面上的异丙基异氰酸酯齐聚反应通过两种竞争机制进行：链扩散反应和质子交换反应。此外，我们发现ISO碳正离子与相对惰性的1-p发生加成反应，生成迄今尚未表征的共低聚物。
Recent studies have shown that atmospheric particles are sufficiently acidic to enhance the uptake of unsaturated volatile organic compounds (VOCs) by triggering acid-catalyzed oligomerization. Controlling factors of oligomerization at the aqueous surfaces, however, remain to be elucidated. Herein, isoprene (2-methyl-1,3-butadiene, ISO), 1,3-butadiene (1,3-b), 1,4-pentadiene (1,4-p), 1-pentene (1-p), and 2-pentene (2-p) vapors are exposed to an acidic water microjet (1 ≤ pH ≤ 5), where cationic products are generated on its surface within ∼10 μs and directly detected using surface-sensitive mass spectrometry. We found that carbocations form at the air–water interface in all the cases, whereas the extent of oligomerization largely depends on the structure in the following order: ISO ≫ 1,3-b > 1,4-p ≫ 1-p ≈ 2-p. Importantly, the cationic oligomerization of ISO yields a protonated decamer ((ISO)10H+, a C50 species of m/z 681.6), while the pentenes 1-p/2-p remain as protonated monomers. We suggest that ISO oligomerization is uniquely facilitated by (1) the resonance stabilization of (ISO)H+ through the formation of a tertiary carbocation with a conjugated CC bond pair, and (2) π-electron enrichment induced by the neighboring methyl group. Experiments in D2O and D2O : H2O mixtures revealed that ISO oligomerization on the acidic water surface proceeds via two competitive mechanisms: chain-propagation and proton-exchange reactions. Furthermore, we found that ISO carbocations undergo addition to relatively inert 1-p, generating hitherto uncharacterized co-oligomers.