title: 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.
authors.affiliation: Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284-2006, United States.
authors.forename: Abdel-Rahman
authors.initials: AR
authors.lastname: Soliman
authors.forename: Ahmed M
authors.initials: AM
authors.lastname: Hamid
authors.forename: Isaac
authors.initials: I
authors.lastname: Attah
authors.forename: Paul
authors.initials: P
authors.lastname: Momoh
authors.forename: M Samy
authors.initials: MS
authors.lastname: El-Shall
mesh_terms: D000114:Acetylene / Q000737:chemistry*; D002412:Cations / Q000138:chemical synthesis / Q000737:chemistry; D005740:Gases / Q000737:chemistry; D007477:Ions / Q000737:chemistry; D009584:Nitrogen / Q000737:chemistry*; D011083:Polycyclic Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D011726:Pyridinium Compounds / Q000737:chemistry*; D011743:Pyrimidines / Q000737:chemistry*; D011789:Quantum Theory
chemical_list: D002412:Cations; D005740:Gases; D007477:Ions; D011083:Polycyclic Compounds; D011726:Pyridinium Compounds; D011743:Pyrimidines; D009584:Nitrogen; D000114:Acetylene