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Controlling factors of oligomerization at the water surface: Why isoprene is a so unique VOC?

Abstract

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 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 C=C 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.

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Publication details

The article was received on 09 Mar 2018, accepted on 03 May 2018 and first published on 08 May 2018


Article type: Paper
DOI: 10.1039/C8CP01551A
Citation: Phys. Chem. Chem. Phys., 2018, Accepted Manuscript
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    Controlling factors of oligomerization at the water surface: Why isoprene is a so unique VOC?

    S. Ishizuka, T. Fujii, A. Matsugi, Y. Sakamoto, T. Hama and S. Enami, Phys. Chem. Chem. Phys., 2018, Accepted Manuscript , DOI: 10.1039/C8CP01551A

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