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Neutral HNCS was first detected in the molecular cloud Sagittarius B2 in 1979 by Frerking et al.[1] and has since been detected in several regions of the interstellar medium (ISM). [2, 3] HNCS is of particular interest because it contains elements essential for life and may act as a precursor in the formation of prebiotic molecules. Understanding its chemistry provides valuable insight into molecular processes in the ISM.
The proposed formation mechanism of HNCS involves a barrierless, exothermic reaction between NCS⁺ and H₂, forming HNCS⁺.[3, 4] However, this mechanism has not been experimentally validated. In addition, neither HNCS⁺ nor NCS⁺ has been detected in the ISM, likely due to the absence of infrared and microwave spectroscopic data. Next to this, HNCS⁺ is of spectroscopic interest due to its linear geometry and electronically degenerate 2Π ground state. This leads to coupling of the vibrational and electronic angular momenta—known as the Renner–Teller effect—which in turn leads to a breakdown of the Born-Oppenheimer approximation, posing both experimental and theoretical challenges.
To address these gaps, we measured the vibrational absorption spectrum of HNCS⁺ using the novel leak-out spectroscopy (LOS) technique[5], with the FELion cryogenic 22-pole ion trap[6] at HFML-FELIX. Additionally, experimental evidence for the proposed reaction mechanism was pursued by trapping NCS⁺ ions and introducing H₂ to promote the formation of HNCS⁺. Theoretical PES and IRC calculations using a ωB97XD/cc-pVQZ[7] supported this experimental work to help explain the observed absence of HNCS⁺ formation.
