Spectroscopic characterization and photochemistry of HC3N- and CH3C3N-: implications for ion chemistry in Titan's atmosphere

Molecular ions are key intermediates in the build-up of chemical complexity in interstellar clouds. Among the more than 300 interstellar molecules, only eight negative ions, i.e. C2n+1N- (n = 0-3) and HC2n- (n = 2-5), have been astronomically observed. Understanding the formation mechanism of these ions under the interstellar conditions is essential for astrochemical modelling and establishing the astrochemical networks. Cyanopolyynes including the parent molecule HC3N are carbon-chain molecules that have been observed in a variety of astronomical objects such as the Titan's atmosphere. Herein, two cyanoacetylene anions HC3N- and CH3C3N- were generated in solid Ne matrix at 3 K and characterized with matrix-isolation infrared spectroscopy, as aided by isotopic substitutions and the ab initio calculations at the UCCSD(T)-F12a/cc-pVTZ-F12 level using second-order vibrational perturbation theory. Upon red-light irradiation at 625 nm, both ions undergo electron detachment by reformation of the neutral species. Importantly, the concomitant dehydrogenation of HC3N- has also been observed in the matrix, providing new insight into the intriguing mechanism for the formation of C3N- in the upper atmosphere of Titan.