Interstellar Molecule HNSO Builds up on Dust, Offering Clues to Life’s Origins

Scientists are elucidating the formation pathways of thionylimide (HNSO), a molecule recently detected in interstellar space. Juan Carlos del Valle, Miguel Sanz-Novo, and Johannes Kästner, from the University of Stuttgart and the Centro de Astrobiología (CAB), CSIC-INTA, alongside Kenji Furuya et al., demonstrate that HNSO efficiently forms on dust grain surfaces via reactions between atomic oxygen and nitrogen with NS and SO radicals. This research is significant because it identifies a key intermediate, NSO, and highlights the importance of surface diffusion of oxygen and nitrogen atoms in driving HNSO production. Their astrochemical models predict solid HNSO abundances comparable to those of icy OCS, establishing it as a major sulfur-bearing species in interstellar ices and prompting further investigation into the prevalence of HNOS-containing molecules in astronomical environments.

Dust grain surface chemistry drives thionylimide formation and abundance in interstellar ices, impacting their spectral features

Scientists have uncovered a novel pathway for the formation of thionylimide (HNSO), a recently detected molecule in interstellar space. This research details how HNSO efficiently arises on the surfaces of dust grains through reactions involving atomic oxygen and nitrogen, utilising the radicals NS and SO to create NSO as a crucial intermediate.
Subsequent hydrogenation of NSO yields HNSO, with a pronounced preference for the lowest energy cis configuration, while the trans form appears to be less stable under typical interstellar conditions. Astrochemical models predict that solid HNSO can achieve abundances comparable to those of icy OCS, establishing it as a major sulfur-bearing species within interstellar ices.

In contrast, gas-phase abundances of HNSO remain lower than those observed for OCS. The implementation of a multibinding scheme within these models clarifies the significant role of diffusive chemistry in the early production of HNSO, leading to improved alignment with observational data. These findings demonstrate that reactions involving diffusing oxygen and nitrogen atoms on icy grains contribute substantially to sulfur chemistry and potentially to more complex molecular formation in dense clouds.
This work suggests that the interplay of atomic addition and surface diffusion is critical for building molecular complexity in challenging astronomical environments. The discovery motivates further investigation into molecules containing hydrogen, nitrogen, oxygen, and sulfur in other astronomical settings, expanding our understanding of interstellar chemical inventories. This research provides a new perspective on the origins of heteroatomic molecules in space and their potential links to prebiotic chemistry.

Computational methods for modelling HNSO formation on amorphous solid water are increasingly sophisticated and detailed

Density functional theory calculations underpin the investigation into the formation of the HNSO molecule on interstellar ice grains. Geometry optimizations and vibrational frequency calculations were performed at the ωB97M-D4-gCP/def2-SVP level, followed by single-point energy refinements using ωB97M-D4/def2-TZVPPD to enhance accuracy.

A broken-symmetry DFT approach was employed to describe low-spin reactive channels, balancing computational cost with the need to account for multiconfigurational character in the reactions. This methodology successfully duplicated previously characterised potential energy surfaces for radical NSO and SNO, confirming the absence of appreciable barriers along the reaction coordinate.

The reactivity was modelled using a representative cluster of 33 water molecules representing amorphous solid water, a structure previously used in related studies. SO and NS radicals were initially placed on three distinct binding sites within the ASW cluster, Pocket, dH, and Pentamer, each representing a characteristic hydrogen-bonding environment.

Binding energies were then derived using the equation BE = Hadsorbate + Hcluster − Hcluster+adsorbate, where H represents the enthalpy at zero Kelvin, calculated from electronic and zero-point energies. This protocol involved relaxing the structure after placing the radicals on the pre-optimised water cluster, with Hadsorbate obtained from gas-phase calculations.

Twelve distinct reactions were then investigated, considering attacks on both atomic centres of SO and NS at each of the three adsorption sites. Potential energy surface scans were performed for each reaction coordinate, starting with an initial distance of approximately 4.0Å between the incoming atom and the adsorbate.

Transition states were located via saddle-point optimisation for reactions exhibiting barriers, and reaction and activation energies were computed using established thermodynamic equations to quantify the energetic favourability of each pathway. Subsequent reactions were then examined, beginning with the products of the initial set, to further elucidate the formation mechanism of HNSO.

HNSO Formation Pathways on Icy Grain Surfaces and Astrochemical Implications are explored through laboratory experiments and modeling

Researchers detail the formation of the recently detected molecule HNSO through combined chemical calculations on ices and astrochemical models. The study reveals that HNSO is efficiently produced on grain surfaces via reactions involving atomic oxygen and nitrogen with the radicals NS and SO, establishing NSO as a crucial intermediate.

Subsequent hydrogenation of NSO preferentially yields the lowest energy cis conformer of HNSO, while the trans form is considered metastable and likely short-lived within interstellar environments. Models predict that solid HNSO can achieve abundances comparable to icy OCS, positioning it among the dominant sulfur-bearing species in interstellar ices.

In contrast, gas-phase abundances of HNSO remain lower than those observed for OCS. Implementation of a multibinding scheme within the models clarifies the role of diffusion in HNSO production at early times, improving the agreement between model predictions and observational data. Quantum chemical calculations employed the ωB97M-D4-gCP/def2-SVP level of theory for geometry optimizations and vibrational frequency calculations, with single-point energy refinements performed using ωB97M-D4/def2-TZVPPD.

These calculations utilized a broken-symmetry DFT approach to describe low-spin reactive channels, balancing accuracy with computational cost. The research team explored HNSO formation on amorphous solid water using a representative cluster of 33 water molecules, examining three distinct binding sites: Pocket, dH, and Pentamer, each representing a characteristic hydrogen-bonding environment.

Binding energies were derived from enthalpy calculations at zero Kelvin, considering both the adsorbate and water cluster energies. Twelve distinct reactions were investigated, involving attacks on both atomic centers of SO and NS radicals at each of the three adsorption sites. Reaction and activation energies were computed using established thermodynamic equations, allowing for detailed analysis of reaction pathways and barriers. Exploratory simulations were also conducted to assess the feasibility of cis, trans isomerization of HNSO, focusing on tunneling-mediated interconversion between the two conformers.

HNSO Formation via Surface Reactions and Astrochemical Implications are explored herein

Scientists have demonstrated that the recently detected molecule, HNSO, forms efficiently on icy grain surfaces through reactions involving atomic oxygen and nitrogen with the radicals NS and SO. These calculations and astrochemical models reveal that NSO is a key intermediate in this process, subsequently hydrogenated to form HNSO, with a preference for the lowest energy cis conformer.

The models predict that solid HNSO can achieve abundances comparable to icy OCS, establishing it as a major sulfur-bearing species within interstellar ices, although gas-phase abundances remain lower. The implementation of a multibinding scheme in the models clarifies the role of diffusion in HNSO production, particularly at early times, and improves the agreement between model predictions and observational data.

These findings indicate that reactions involving diffusing oxygen and nitrogen atoms on icy grains significantly contribute to the formation of sulfur-containing molecules, and potentially others containing hydrogen, nitrogen, oxygen, and sulfur, in dense clouds. The authors acknowledge limitations in their description of certain potential energy surfaces, specifically the quartet state involved in some reactions, deferring further investigation to future work. Future research should focus on exploring these more complex energy surfaces and searching for molecules containing combinations of hydrogen, nitrogen, oxygen, and sulfur in diverse astronomical environments to further validate these findings.