Spectroscopic evidence of the C–N covalent bond formed between two interstellar molecules (ISM): acrylonitrile and ammonia

Abstract

Acrylonitrile (AN) and ammonia (NH₃) are two important nitrogen-containing interstellar molecules in outer space, especially on Titan. Herein, we measured infrared (IR) spectra of neutral and cationic AN–NH₃ complexes by VUV single-photon ionization combined with time-of-flight mass spectrometry. On combining IR spectra with the theoretical calculations, we found that the molecules prefer to form a single-ring cyclic H-bonded structure in the neutral AN–NH₃ and (AN)₂–NH₃ clusters. However, after ionization of AN–NH₃ and (AN)₂–NH₃ clusters, a new C–N-covalent bond is confirmed to form directly between AN and NH₃, without any energy barrier in the cationic complexes. Moreover, in the ionized (AN)₂–NH₃ cluster, the covalent C–N bond prefers to form between AN and NH₃ rather than the two AN groups. These results provide spectroscopic evidence of AN forming a new molecule with NH₃, induced by VUV radiation. The formation of the new C–N bond broadens our knowledge on the evolution of the prebiotic nitrogen-containing molecules in space.