Laboratory observation of the valence anion of cyanoacetylene, a possible precursor for negative ions in space

Valence anions of cyanoacetylene, HCCCN^-, are synthesized by the 1,2 _-H₂⁺ abstraction reaction of O^- with acrylonitrile, H₂ C=CHCN, while the competing 1,1 _-H₂⁺ channel of the same reaction yields the cyanovinylidene anions, CCHCN-. The key to the formation of the elusive, adiabatically weakly bound HCCCN^- is the bent -̇C = ̇C-Cskeleton of the reactant. The photoelectron spectrum of HCCCN ^-, measured by means of photoelectron imaging at 532 nm, consists of a broad structureless band with a vertical detachment energy of 1.04±0.05 eV. The observed anions are stable counterparts of the low-lying anionic resonances of cyanoacetylene, which may contribute (by way of dissociative attachment) to the formation of carbon-rich and CN_-containing negative ions in extraterrestrial environments.