Formation of Complex Organics in the Gas Phase by Sequential Reactions of Acetylene with the Phenylium Ion

Abstract

In this paper, we report a study on the reactivity of the phenylium ion with acetylene, by measuring product yield as a function of pressure and temperature using mass-selected ion mobility mass spectrometry. The reactivity is dominated by a rapid sequential addition of acetylene to form covalently bonded C8H7(+) and C10H9(+) ions with an overall rate coefficient of 7-5 × 10(-10) cm(3) s(-1), indicating a reaction efficiency of nearly 50% at room temperature. The covalent bonding nature of the product ions is confirmed by high temperature studies where enhanced production of these ions is observed at temperatures as high as 660 K. DFT calculations at the UPBEPBE/6-31++G** level identify the C8H7(+) adduct as 2-phenyl-ethenylium ion, the most stable C8H7(+) isomer that maintains the phenylium ion structure. A small barrier of 1.6 kcal/mol is measured and attributed to the formation of the second adduct C10H9(+) containing a four-membered ring connected to the phenylium ion. Evidence for rearrangement of the C10H9(+) adduct to the protonated naphthalene structure at temperatures higher than 600 K is provided and suggests further reactions with acetylene with the elimination of an H atom and an H2 molecule to generate 1-naphthylacetylene or acenaphthylene cations. The high reactivity of the phenylium ion toward acetylene is in sharp contrast to the low reactivity of the benzene radical cation with a reaction efficiency of 10(-4)-10(-5), confirming that the first step in the cation ring growth mechanism is the loss of an aromatic H atom. The observed reactions can explain the formation of complex organics by gas phase ion-molecule reactions involving the phenylium ion and acetylene under a wide range of temperatures and pressures in astrochemical environments.

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“…In addition, peaks corresponding to ''8-C'' molecular formulae are detected at 102, 103, 104, and 105 m/z as well as two peaks corresponding to ''16-C'' structures at 203 and 205 m/z. As observed by Soliman et al (2012), the ''8-C'' molecule at 103 m/z is most likely formed from the fast, barrierless reaction between acetylene, C 2 H 2 , and the phenyl cation, C 6 H 5 + , to generate the adduct ion C 8 H 7 + :…”

Growth kinetics and formation mechanisms of complex organics by sequential reactions of acetylene with ionized aromatics

“…ions by the sequential reactions of acetylene with the phenylium ion at room temperature [25]. We also provided experimental evidence for the sequential reactions of acetylene with the pyridine and pyrimidine cations resulting in the growth of a second ring through the formation of stable NÀ ÀC bonds, thus providing a mechanism for the formation of nitrogen-containing PAHs [26].…”

“…The reactant and product ions exiting the drift cell are analyzed and detected using a second quadrupole mass spectrometer. The time-resolved studies allow the identification of primary and secondary reaction products and the measurement of rate coefficients [25,26,38]. Through our previous studies we discovered novel reactions of the benzene cation with acetylene molecules over a wide temperature range from 120 to 650 K, demonstrating that benzene ions autocatalyze the conversion of associated acetylene molecules into benzene at low temperatures (120 K), and form naphthalene-type covalent products at high temperatures (650 K) [38].…”

“…These experiments can identify the chains of reactions leading to the formation of PAHs and PANHs found in soot, combustion, organic aerosols, interstellar clouds, and meteorites [16][17][18][19][20][21][22][23][24][25][26][27]. They can also provide detailed information on the reactivity, kinetics, growth mechanisms, energy barriers, and structures of large organic ions as well as on their complexes with associated solvent molecules in different environments [28][29][30].…”

“…Ionized aromatics can also undergo a rich ion-molecule chemistry with acetylene molecules leading to the functionalized aromatics found in organic aerosols, combustion products, soot, meteorites, and interstellar medium [4,7,[13][14][15]24,42,43]. In this context, we studied the gas phase reactions of acetylene with the benzene (C 6 H 6 + ), phenylinum (C 6 H 5 + ), pyridine (C 5 H 5 N + ) and pyrimidine (C 4 H 4 N 2 + ) cations over a wide temperature range using the mass-selected ion mobility (drift cell) technique [25,26,38]. In these experiments, the reactant cations are massselected (using a quadrupole mass filter) and injected into a drift cell containing pure acetylene vapor or an acetylene-helium vapor mixture at well-defined pressures and temperatures.…”