Controlling factors of oligomerization at the water surface: why is isoprene such a 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 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 C[double bond, length as m-dash]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.

Reply to the ‘Comment on “The chemical reactions in electrosprays of water do not always correspond to those at the pristine air–water interface”’ by A. J. Colussi and S. Enami, Chem. Sci., 2019, 10, DOI: 10.1039/c9sc00991d

“…1. Upon intersecting isoprene gas (partial pressure, p z 0.6 atm at NTP) with electrosprays of water at bulk pH # 4, we observed mass spectral peaks of protonated isoprene (ISO$H + ) and protonated oligomers of isoprene (ISO) n H + , within $1 ms, as previously reported for isoprene 3,5 and terpenes. 4 However, we also observed the same mass spectral peaks when gaseous isoprene reacted with electrosprays of water at bulk pH > 12 and electrosprays of water comprising 10 À1 to 10 À8 M NaCl maintained at pH 5.6 ( Fig.…”

“…1 Our investigation was in response to recent ESIMS-based claims of the "superacidity" of the airwater interface of pH # 3-5 (ref. [2][3][4][5] water and recent reports of dramatic rate accelerations in chemical reactions in electrosprayed microdropletsproduced either by shearing water using jets of air 6 or applying electrical voltage to water solutions owing through metallic capillaries 7leading to the reactions and mass spectrometric characterization of the reactants/ products all within $1 ms. We wanted to understand if the role of the electrosprays leading to such fast reactions was limited only to producing large, fresh, uncontaminated water-hydrophobe interfacial areas, which is reminiscent of "on-water" chemistries in vigorously agitated oil-water emulsions, [8][9][10][11] or if the electrosprays of water entailed additional mechanisms that were responsible for the dramatic accelerations. To this end, we investigated chemical reactions between isoprene and pHadjusted water at normal temperature and pressure (NTP, 293 K, 1 atm) in water-isoprene emulsions and intersecting electrosprays of pH-adjusted water with isoprene gas.…”

Acid-Catalyzed Oligomerization at the Air–Water Interface Modified by Competitive Adsorption of Surfactants

“…Exposure of ISO(g) to the microjets produced mass signals at m/z = 69.1, 137.1, 205.2, 273.3, 341.3, 409.4, 477.5, 545.5, and 613.6, corresponding to (ISO) m H + (m = 1−9) products, respectively; this result was consistent with previous reports. 16 In the presence of the ammonium surfactants, six positive-ion peaks appeared at m/z = 74.2, 116.2, 172.2, 200.3, 228.3, and 256.3; these corresponded to Me(CH 2 ) n−1 N + Me 3 (n = 1, 4, 8, 10, 12, 14), respectively, and were accompanied by a decrease in the strength of the (ISO) m H + signals. The signals for the ammonium surfactants were also suppressed by exposure to the ISO(g) [see SI, section 2 and Figure S2], implying a competitive interfacial adsorption of ammonium surfactants versus (ISO) m H + at the air−water interface.…”

“…Theoretical calculations revealed that both the initial PT reaction and the dimerization of ISO are exothermic by ∼30 kcal mol −1 and proceed without barriers, 16 indicating that the oligomerization reactions are diffusion-controlled. Thus, we conclude that the suppression of the signals for (ISO) m H + is induced by (1) hindrance of the initial PT reaction between ISO(g) and H 3 O + (H 2 O) x<6 by the ammonium cations at the air−water interface and (2) by competitive adsorption of growing (ISO) m H + oligomers and the ammonium surfactants at the air−water interface.…”

“…14,15 Using this method, we have investigated several aspects of carbocation chemistry at the air−water interface. We found that a less hydrated hydronium species (H 3 O) + (H 2 O) x<6 that is formed in the topmost layers of a water surface of bulk pH < 4 can transfer a proton (H + ) to a gaseous unsaturated hydrocarbon of higher proton affinity (e.g., isoprene) to form a carbocation; we named this the proton-transfer (PT) reaction: 16,17…”