Cryogenic Infrared Action Spectroscopy of [H ₂ NCO] ⁺ and [H ₂ NCS] ⁺ , Protonated Forms of Interstellar HNCO and HNCS

HNCS, isothiocyanic acid, and HNCO, isocyanic acid, are important molecules in the interstellar medium due to their composition of the essential atoms for organic life and their possible prebiotic role. They are assumed to be formed by dissociative recombination of their protonated versions, [H₂NCO]⁺ and [H₂NCS]⁺, where protonation may occur either on the N- or the O/S-atom. Here, we report the investigation of [H₂NCO]⁺ and [H₂NCS]⁺ by broadband infrared (IR) spectroscopy in a cryogenic 22-pole ion trap instrument via tag-free leak-out spectroscopy. Infrared radiation in the range of 450-3500 cm^-1 was provided by the infrared free-electron laser FELIX-2 at HFML-FELIX. We predominantly observe the N-protonated isomers, H₂NCS⁺ and H₂NCO⁺, with potentially a small contribution of HNCOH⁺. Five fundamental transitions in the range of 450-3500 cm^-1 were observed of H₂NCO⁺ and eight fundamental and overtone/combination bands of H₂NCS⁺. The transitions are assigned with the help of quantum-chemical calculations using a combination of CCSD(T)/cc-pCVTZ for anharmonic transition energies and ωB97XD/cc-pVQZ for anharmonic transition intensities. Several modes show a rotational substructure, which is discussed in detail for the ν₅(<i>b</i>₁) bending vibrational mode of H₂NCS⁺. The data presented in this paper is the first experimental study investigating the IR spectra of these ions and also the first experimental investigation of [H₂NCS]⁺. This work provides important reference data for upcoming studies of formation mechanisms of HNCO and HNCS at cryogenic conditions.