We overview recent results of high-level ab initio calculations of potential energy surfaces, combined with statistical calculations of product branching ratios, aimed to unravel reaction mechanisms of the growth of nitrogen-containing polycyclic aromatic compounds under low-temperature conditions. We 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-methylene-benzenamine. 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 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 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. According to RRKM-ME calculations of reaction rate constants and product branching ratios 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, collisional stabilization of the initial CN + styrene reaction adducts may be significant, but substantial amounts of 2-vinylbenzonitrile and 2-ethynyl-N-methylene-benzenamine can still be produced and then react with C2H to form substituted azanaphthalenes. The proposed mechanism is compared with consecutive CN and C2H additions to benzene occurring through the C2H-for-H and CN-for-H exchanges and leading first to the formation of 2-ethynyl-benzonitrile, the reaction of which with C2H may produce ethynyl-azanaphthyl radicals. These reactions are also feasible at low temperatures owing to their barrierless and exothermic character, but they can form ethynyl-azanaphthyl radicals as final products only under the conditions where collisional stabilization of these radicals is significant.

Reaction mechanisms of the growth of nitrogen-containing polycyclic aromatic compounds at low temperatures: A view from theoretical calculations of potential energy surfaces

246th National Meeting (2013)

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