Observation of covalent and electrostatic bonds in nitrogen-containing polycyclic ions formed by gas phase reactions of the benzene radical cation with pyrimidine.

Polycyclic aromatic hydrocarbons (PAHs) and polycyclic aromatic nitrogen heterocyclics (PANHs) are present in ionizing environments, including interstellar clouds and solar nebulae, where their ions can interact with neutral PAH and PANH molecules leading to the formation of a variety of complex organics including large N-containing ions. Herein, we report on the formation of a covalently-bonded (benzene·pyrimidine) radical cation dimer by the gas phase reaction of pyrimidine with the benzene radical cation at room temperature using the mass-selected ion mobility technique. No ligand exchange reactions with benzene and pyrimidine are observed indicating that the binding energy of the (benzene·pyrimidine)˙+ adduct is significantly higher than both the benzene dimer cation and the proton-bound pyrimidine dimer. The (benzene·pyrimidine)˙+ adduct shows thermal stability up to 541 K. Thermal dissociation of the (C6D6·C4H4N2)˙+ adduct at temperatures higher than 500 K produces C4H4N2D+ (m/z 82) suggesting the transfer of a D atom from the C6D6 moiety to the C4H4N2 moiety before the dissociation of the adduct. Mass-selected ion mobility of the (benzene·pyrimidine)˙+ dimer reveals the presence of two families of isomers formed by electron impact ionization of the neutral (benzene·pyrimidine) dimer. The slower mobility peak corresponds to a non-covalent family of isomers with larger collision cross sections (76.0 ± 1.8 Å2) and the faster peak is consistent with a family of covalent isomers with more compact structures and smaller collision cross sections (67.7 ± 2.2 Å2). The mobility measurements at 509 K show only one peak corresponding to the family of stable covalently bonded isomers characterized by smaller collision cross sections (66.9 ± 1.9 Å2 at 509 K). DFT calculations at the M06-2X/6-311++G** level show that the most stable (benzene·pyrimidine)˙+ isomer forms a covalent C-N bond with a binding energy of 49.7 kcal mol-1 and a calculated collision cross section of 69.2 Å2, in excellent agreement with the value obtained from the faster mobility peak of the (benzene·pyrimidine)˙+ dimer. Formation of a C-N covalent bond displaces a hydrogen atom from a C-H bond of the benzene cation which is transferred to the second pyrimidine nitrogen atom, thus preserving the pyrimidine π system and yielding the most stable (benzene·pyrimidine)˙+ isomer. The calculations also show less stable non-covalent electrostatically bonded perpendicular isomers of the (benzene·pyrimidine)˙+ dimer with a binding energy of 19 kcal mol-1 and a calculated collision cross section of 74.0-75.0 Å2 in excellent agreement with the value obtained from the slower mobility peak of the (benzene·pyrimidine)˙+ dimer.

Stepwise association of hydrogen cyanide and acetonitrile with the benzene radical cation: structures and binding energies of (C6H6•+)(HCN)n, n = 1-6, and (C6H6•+)(CH3CN)n, n = 1-4, clusters.

Equilibrium thermochemical measurements using the ion mobility drift cell technique have been utilized to investigate the binding energies and entropy changes associated with the stepwise association of HCN and CH(3)CN molecules with the benzene radical cation in the C(6)H(6)(•+)(HCN)(n) and C(6)H(6)(•+)(CH(3)CN)(n) clusters with n = 1-6 and 1-4, respectively. The binding energy of CH(3)CN to the benzene cation (14 kcal/mol) is stronger than that of HCN (9 kcal/mol) mostly due to a stronger ion-dipole interaction because of the large dipole moment of acetonitrile (3.9 D). However, HCN can form hydrogen bonds with the hydrogen atoms of the benzene cation (CH(δ+)···NCH) and linear hydrogen bonding chains involving HCN···HCN interaction. HCN molecules tend to form externally solvated structures with the benzene cation where the ion is hydrogen bonded to the exterior of HCN chains. For the C(6)H(6)(•+)(CH(3)CN)(n) clusters, internally solvated structures are formed where the acetonitrile molecules are directly interacting with the benzene cation through ion-dipole and hydrogen bonding interactions. The lack of formation of higher clusters with n > 4, in contrast to HCN, suggests the formation of a solvent shell at n = 4, which is attributed to steric interactions among the acetonitrile molecules attached to the benzene cation and to the presence of the blocking CH(3) groups, both effects make the addition of more than four acetonitrile molecules less favorable.

Formation of nitrogen-containing polycyclic cations by gas-phase and intracluster reactions of acetylene with the pyridinium and pyrimidinium ions.

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.

Formation of complex organics in the gas phase by sequential reactions of acetylene with the phenylium ion.

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.

Formation of complex organics from acetylene catalyzed by ionized benzene.

We present direct evidence for low temperature associative charge transfer (ACT) reactions of acetylene onto the benzene cation that catalyze the conversion of acetylene molecules into polymerized cations and for high temperature addition/elimination reactions that lead to the generation of naphthalene-type ions. At low temperatures acetylene molecules bind noncovalently to the benzene cation, where partial charge transfer from the ion activates an acetylene molecule for addition polymerization with other associated acetylene molecules, thus amounting to catalytic cyclization/polymerization of the acetylene molecules. At high temperatures the barrier of the covalent addition of acetylene to the benzene cation to form a styrene-type ion is measured as 3.5 kcal/mol. The second acetylene addition followed by H elimination to form a naphthalene-type ion is calculated to be highly exothermic and without a barrier. These reactions can explain the formation of complex organics by gas phase ion-molecule reactions under a wide range of temperatures and pressures in astrochemical environments.