Growth kinetics and formation mechanisms of complex organics by sequential reactions of acetylene with ionized aromatics

Abstract

In this work, we present direct experimental evidence for the formation of covalently-bonded organic ions by gas phase reactions of acetylene with the benzonitrile, phenylacetylene, and styrene radical cations, and compare these results with the previously studied acetylene reactions with the benzene, phenyl, pyridine, and pyrimidine cations. A very wide range of reaction rates has been found ranging from very slow reactions with energy barriers as in the case of benzene and styrene radical cations to reactions occurring at the collision rate as in the case of the pyrimidinium cation. In all cases, synthetic channels with increased numbers of carbon atoms are found in all the observed reactions. The resulting covalently-bonded precursor ions can undergo further growth to form polycyclic aromatic hydrocarbon ions including nitrogen-containing PAH ions. The observed chemistry under a wide range of experimental conditions, including extreme temperatures as high as 650K, strongly implies a series of ion–molecule reactions that can lead from a simple molecule such as acetylene to polycyclic complex organic ions. The product ions resulting from the sequential reactions of acetylene can, in turn be neutralized by charge transfer or recombination, to produce large neutral PAHs. Therefore, ion–molecule reactions can contribute to the formation of higher hydrocarbons and PAHs in ionizing environments. The observed reactions could explain the possible formation of complex organics under a wide range of temperatures and pressures in astrochemical environments such as the nitrogen-rich Titan's atmosphere.