Chemistry and Spectroscopy of
the HNCO+ and HNCS+ Ions -
Potential Precursors for Interstellar HNCO and HNCS

Isocyanic acid (HNCO) and isothiocyanic acid (HNCS), both relevant to prebiotic chemistry, have been detected in the interstellar medium, yet the formation pathways of these species remain largely unverified experimentally. Proposed mechanisms involve their cationic precursors (HNCO⁺, HNCS⁺) and hydrogen-loss products (NCO⁺, NCS⁺), none of which have been observed astronomically due to the lack of vibrational and rotational reference data.

 

In this work, cryogenic infrared ion spectroscopy using the free-electron lasers at the HFML-FELIX facility in Nijmegen, the Netherlands, is combined with Leak Out Spectroscopy (LOS) and high-level quantum chemical calculations to obtain the first broadband vibrational spectra of HNCO⁺ and HNCS⁺. The measured spectra show good agreement with theory, enabling detailed vibrational assignments, although additional vibronic coupling effects complicate the analysis of HNCS⁺.

 

Additionally, in-situ spectroscopic reaction studies are used to experimentally test proposed formation pathways. Reactions of NCO⁺ with H₂ are shown to efficiently lead to the formation of HNCO⁺ and subsequently H₂NCO⁺, supporting its proposed interstellar formation mechanism, whereas no reaction is observed for NCS⁺ with H₂, suggesting a different pathway for the sulfur analog.

 

Overall, this work provides essential spectroscopic reference data for future astronomical searches and offers new experimental insight into the interstellar chemistry of HNCO and HNCS.