Polycyclic aromatic nitrogen heterocycles (PAHNs) are considered as key prebiotic molecules and are found at the center of life as nucleobases in ribonucleic acid (RNA). PAHNs in the form of nucleobases have been found in meteorites and linked to the origin of life theories regarding the exogeneous delivery of key prebiotic molecules to Earth that could have engaged in prebiotic chemistry. So far the astrobiology community has identified complex prebiotic molecules to form in the solid state with matrix isolation studies. However, certain complex prebiotic molecules indicate origins predating absorption onto meteorites, and implying a potential gas phase formation route. The most favorable routes to form aromatic species in the gas phase are suggested to involve molecular weight growth processes in circumstellar envelopes (CSEs) of carbon rich Asymptotic Giant Branch (AGB) stars, where temperatures up to a few 1,000 K exist close to the photosphere of the central star. However, the mechanisms that initiate and drive PAHN formation in the gas phase have remained conjectural requiring comprehensive experimental investigations.We present the gas phase formation of a prototypical PAHN quinoline (C9H7N) through the reaction of the 2-pyridinyl radical (C5H4N) with vinylacetylene (C4H4). Here, the reaction dynamics have been investigated under single collision conditions utilizing cross beam experiments coupled with ab initiocalculations. The reactions are found to undergo a radical-hydrogen atom exchange mechanism through the formation of long-lived reaction intermediates in overall exoergic reactions. To further mimic the conditions in CSEs and AGB stars, experiments to form quinoline were investigated using a pyrolytic reactor. Here, pyrolytically generated 2-pyridinyl radicals reacted with acetylene at 1050 K in a resistively heated silicon nozzle. The products formed were VUV ionized from over a range of photon energies from 8-11 eV to generate PIE curves. The reaction was found to proceed through addition of 2-pyridinyl to acetylene, followed by a second acetylene addition, cyclization and hydrogen atom emission to form quinoline. The reaction mechanism undergoes a hydrogen-abstraction-acetylene-addition (HACA) reaction mechanism.

On the formation of quinoline and the role of gas phase reactions in the synthesis of prebiotic molecules

248th National Meeting (2014)

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