Formation of Nitrogen-Containing Polycyclic Cations by Gas-Phase and Intracluster Reactions of Acetylene with the Pyridinium and Pyrimidinium Ions

Abstract

Here, we present evidence from laboratory experiments for the formation of nitrogen-containing complex organic ions by sequential reactions of acetylene with the pyridinium and pyrimidinium ions in the gas phase and within ionized pyridine-acetylene binary clusters. Additions of five and two acetylene molecules onto the pyridinium and pyrimidinium ions, respectively, at room temperature are observed. Second-order rate coefficients of the overall reaction of acetylene with the pyridinium and pyrimidinium ions are measured as 9.0 × 10(-11) and 1.4 × 10(-9) cm(3) s(-1), respectively, indicating reaction efficiencies of about 6% and 100%, respectively, at room temperature. At high temperatures, only two acetylene molecules are added to the pyridinium and pyrimidinium ions, suggesting covalent bond formation. A combination of ion dissociation and ion mobility experiments with DFT calculations reveals that the addition of acetylene into the pyridinium ion occurs through the N-atom of the pyridinium ion. The relatively high reaction efficiency is consistent with the absence of a barrier in the exothermic N-C bond forming reaction leading to the formation of the C(7)H(7)N(•+) covalent adduct. An exothermic addition/H-elimination reaction of acetylene with the C(7)H(7)N(•+) adduct is observed leading to the formation of a bicyclic quinolizinium cation (C(9)H(8)N(+)). Similar chemistry is observed in the sequential reactions of acetylene with the pyrimidinium ion. The second acetylene addition onto the pyrimidinium ion involves an exclusive addition/H-elimination reaction at room temperature leading to the formation of a bicyclic pyrimidinium cation (C(8)H(7)N(2)(+)). The high reactivity of the pyridinium and pyrimidinium ions toward acetylene is in sharp contrast to the very low reactivity of the benzene cation, which has a reaction efficiency of 10(-4)-10(-5). This indicates that the presence of a nitrogen atom within the aromatic ring enhances the ring growth mechanism by the sequential addition of acetylene to form nitrogen-containing polycyclic hydrocarbon ions. The observed reactions could explain the possible formation of nitrogen-containing complex organics by gas-phase ion-molecule reactions involving the pyridinium and pyrimidinium ions with acetylene under a wide range of temperatures and pressures in astrochemical environments such as the nitrogen-rich Titan's atmosphere. The current results suggest searching for spectroscopic evidence for these organics in Titan's atmosphere.

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

“…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.…”

“…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]. In this paper we investigate the reactions of acetylene with the radical cations of benzonitrile (C 7 H 5 N), phenylacetylene (C 8 H 6 ), and styrene (C 8 H 8 ).…”

“…The product distributions at different temperatures, kinetics of the reactions at room temperature, and calculated structures of the most likely product ions are presented and discussed in Sections 3.2,3.3, and 3.4 for the acetylene reactions with benzonitrile, phenylacetylene, and styrene cations, respectively. In Section 3.5, we provide comparisons of the reactions of these three ions with acetylene with the reactions of the pyridine and pyrimidine cations that we studied previously [26]. These comparisons provide the basis for establishing trends in the reactivity of acetylene with ionized aromatics leading to the formation of polycyclic organic ions.…”