Low-Temperature Mechanisms for the Formation of Substituted Azanaphthalenes through Consecutive CN and C₂H Additions to Styrene and N-Methylenebenzenamine: A Theoretical Study

Abstract

Ab initio G3(MP2,CC)/B3LYP/6-311G** calculations of potential energy surfaces (PESs) for the reactions of cyano and ethynyl radicals with styrene and N-methylenebenzenamine have been performed to investigate a possible formation mechanism of the prototype nitrogen-containing polycyclic aromatic compounds: (substituted) 1- and 2-azanaphthalenes. The computed PESs and molecular parameters have been used for RRKM and RRKM-Master Equation calculations of reaction rate constants and product branching ratios under single-collision conditions and at pressures from 3 to 10(-6) mbar and temperatures of 90-200 K relevant to the organic aerosol formation regions in the stratosphere of a Saturn's moon Titan. The results show that ethynyl-substituted 1- and 2-azanaphthalenes can be produced by consecutive CN and C2H additions to styrene or by two C2H additions to N-methylenebenzenamine. All CN and C2H radical addition complexes are formed in the entrance channels without barriers, and the reactions are computed to be exothermic, with all intermediates and transition states along the favorable pathways residing lower in energy than the respective initial reactants. The reactions are completed by dissociation of chemically activated radical intermediates via H losses, so that collisional stabilization of the intermediates is not required to form the final products. These features make the proposed mechanism viable even at very low temperatures and under single-collision conditions and especially significant for astrochemical environments. In Titan's stratosphere, collisional stabilization of the initial CN + styrene reaction adducts may be significant, but substantial amounts of 2-vinylbenzonitrile and 2-ethynyl-N-methylenebenzenamine can still be produced and then react with C2H to form substituted azanaphthalenes.

Formation Mechanisms of Naphthalene and Indene: From the Interstellar Medium to Combustion Flames

“…Recent studies exploiting a pyrolytic reactor revealed indeed that the reaction of pyridyl radicals with acetylene does lead to the formation of (iso)­quinoline, the nitrogen-substituted counterparts of naphthalene involving HACA-type reaction pathways . Theoretically, we demonstrated that NPAH can be also formed at low temperatures through consecutive CN and C 2 H additions to styrene and N -methylenebenzenamine . Therefore, these findings provide new insights into the formation mechanisms of aromatic molecules incorporating nitrogen atoms through gas phase radical mediated reactions in high-temperature circumstellar envelopes, and hopefully in future studies through vinylacetylene- or C 2 H/CN-mediated reactions in low-temperature interstellar environments such as in cold molecular clouds.…”

“…109 Theoretically, we demonstrated that NPAH can be also formed at low temperatures through consecutive CN and C 2 H additions to styrene and N-methylenebenzenamine. 110 Therefore, these findings provide new insights into the formation mechanisms of aromatic molecules incorporating nitrogen atoms through gas phase radical mediated reactions in high-temperature circumstellar envelopes, and hopefully in future studies through vinylacetylene-or C 2 H/CN-mediated reactions in low-temperature interstellar environments such as in cold molecular clouds. The experimental and theoretical results discussed in the present work demonstrate very significant pressure dependence for the rate constants, especially the product-channel specific rate constants.…”

Gas Phase Synthesis of (Iso)quinoline and Its Role in the Formation of Nucleobases in the Interstellar Medium

“…This can be achieved by formally replacing the phenyl radical reactant (C 6 H 5 ) by a pyridyl radical (C 5 H 4 N) formed via hydrogen abstraction from pyridine (C 5 H 4 N). The latter is predicted to be synthesized at high temperatures via the reaction of HCN with two acetylene molecules, which are abundant in circumstellar envelopes of carbon stars (Cherchneff 2011;Landera & Mebel 2013). However, as of now, the formation mechanisms of even the prototypical representatives of NPAHs-quinoline and its isomer isoquinoline as detected in the Murchison meteorite (Plows et al 2003)-remain elusive.…”

Free radical routes for prebiotic formation of DNA nucleobases from formamide

“…Recently, free radical mechanisms using CN radicals toward the formation of azanaphthalenes have been proposed as a viable route on Titan. 10,11 Many studies indicated formamide to be active in photolytic reactions. For example, formamide adds to olefins under photolytic conditions in the presence of acetone.…”