Laboratory astrochemical investigation of H-atom reactions and photochemistry of [H, C, N, S] isomers: astrophysical significance and implications

The astrochemically relevant reactions of [H, C, N, S] isomers were studied both theoretically and experimentally. The energetics and barriers of H-atom-addition and H-atom-abstraction reactions were computed. Experimentally, [H, C, N, S] isomers were generated via in situ photolysis in a solid para-H-2 matrix. The H-atom reaction and the photochemistry of these species were monitored by infrared spectroscopy. In the case of HNCS, the formation of HNCSH was observed. Tentatively two different conformers, syn,trans- and anti,trans-HNCSH, were identified. The subsequent H-atom-addition and H-atom-abstraction reactions did not effectively induce the isomerization of HNCS to HSCN. In the case of HCNS, the observation of the HCN product indirectly reveals H-atom addition to the S atom of HCNS, which is followed by the rupture of the N-S bond. While not explored experimentally, theoretical computations reveal negligible barrier heights for the H-atom-abstraction reactions of HSCN and HSNC. Therefore, H atoms might deplete these species in the interstellar medium (ISM). In contrast to the H-atom reactions, ultraviolet (UV) irradiation between 255 and 240 nm effectively isomerized the HNCS to HSCN and HSNC. Besides decomposition, HCNS isomerize to HNCS, HSCN, and HSNC upon exposure to UV radiation between 300 and 216 nm. In certain H-atom-free conditions, HSNC, not yet observed in the ISM, could potentially coexist in observable amounts with the already detected HNCS and HSCN in the ISM. Conversely, HCNS, another unidentified isomer in the ISM, is unlikely to accumulate there under UV radiation or in the presence of reactive radicals, including H atoms.