An Ion-Neutral Reaction to Form Cyanobenzene and Ethynylbenzene in Titan’s Atmosphere

Abstract The ion-neutral reaction between cyanoacetylene (HC3N) and phenyl anion (C6H5–) leads to the formation of cyanobenzene (C6H5CN) and the ethynyl anion (C2H–) via a submerged, exothermic pathway. Through a different mechanism, the same two reactants also form ethynylbenzene (C6H5C2H) and CN–. These new reactions can be a foundation for the production of substituted benzenes using molecules currently present on Saturn’s largest moon, Titan. Cyclic aromatic molecules like C6H5CN and C6H5C2H are potential gateways to the formation of larger complex organic molecules and polycyclic aromatic hydrocarbons (PAHs), and these “monomers″ can lead to the molecular mass growth observed in the Titan atmosphere. A comparison of the anharmonic vibrational spectra of the five multiply substituted reactive intermediates with that of singly-substituted cyano-PAHs provides a window into the effect of the bonding environment on the vibrational frequencies of major functional groups, such as the CN stretching motion and aliphatic CH stretches. Reliable rotational parameters for each intermediate are also computed. These spectral data provide accurate guidance for future IR and microwave laboratory studies of this family of molecules. The data presented here can be useful to the astrochemical modeling and laboratory astrophysics communities, as interest in Titan continues to grow leading up to the launch of the forthcoming Dragonfly mission.