Discovery of Thionylimide, HNSO, in Space: The first N-, S-, and O-bearing Interstellar Molecule

We present the first detection in space of thionylimide (HNSO) toward the Galactic center molecular cloud G + 0.693-0.027, thanks to the superb sensitivity of an ultradeep molecular line survey carried out with the Yebes 40 m and IRAM 30 m telescopes. This molecule is the first species detected in the interstellar medium containing, simultaneously, N, S, and O. We have identified numerous K a = 0, 1, and 2 transitions belonging to HNSO covering from J up = 2 to J up =10, including several completely unblended features. We derive a molecular column density of N = (8 ± 1)×1013 cm−2, yielding a fractional abundance relative to H2 of ∼6 × 10−10, which is about ∼37 and ∼4.8 times less abundant than SO and SO2, respectively. Although there are still many unknowns in the interstellar chemistry of NSO-bearing molecules, we propose that HNSO is likely formed through the reaction of the NSO radical and atomic H on the surface of icy grains, with alternative routes also deserving exploration. Finally, HNSO appears as a promising link between N, S, and O interstellar chemistry, and its discovery paves the route to the detection of a new family of molecules in space.


INTRODUCTION
In recent years, astrochemistry has witnessed an outburst of new interstellar detections, with more than 75 new species discovered since 2021. 1 These species contain the six chemical elements essential for life: carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), and sulfur (S); and many of them are considered molecular precursors of prebiotic chemistry (e.g.Belloche et al. 2008;Belloche et al. 2019;Zeng et al. 2019;Rivilla et al. 2019Rivilla et al. , 2021aRivilla et al. , 2022;;Jiménez-Serra et al. 2022).However, despite the plethora of new species, NSO compounds (i.e., molecules containing simultane-1 https://cdms.astro.uni-koeln.de/classic/moleculesously N, S and O) have attracted little attention from the astronomical community, although they appear as a promising yet unexplored link between the chemistry of N-, S-and O-bearing species in the ISM.
On Earth, the NSO chemistry plays a key biological role in transmitting signals both within and between cells and tissues (Foster et al. 2009;Miljkovic et al. 2013).In particular, several [H,N,S,O] isomers (e.g.HSNO, HONS and HNSO) are thought to connect the biochemistries of two important biological messengers, nitric oxide (NO) and hydrogen sulfide (H 2 S) (Filipovic et al. 2012;Miljkovic et al. 2013;Ivanova et al. 2014;Nava et al. 2016;Kumar & Francisco 2017).NO, as the first gasotransmitter, is involved in the regulation of vascular tone and heart function, among other physiological processes (Wu et al. 2018).Meanwhile, H 2 S plays a positive role regarding antioxidative stress and inflammation regulation (Zhao et al. 2024).NSO-bearing compounds are also very abundant in oil sands (Ji et al. 2021) and contain rich geological and geochemical data (Shi et al. 2010;Noah et al. 2015;Ziegs et al. 2018;Chang et al. 2023), excelling in recording biotic and palaeoenvironmental signatures (Yue 2023), and thus are of interest for astrobiology.
This fact triggered us to explore the chemistry of NSO compounds in the ISM.As a proof of concept, we targeted the study of thionylimide, HNSO, which can be seen as an NH-analogue of sulfur dioxide, SO 2 , where one O atom of O=S=O is replaced by a NH-group, yielding HN=S=O.This molecule has been suggested to be the parent species of interstellar NS (Barbier et al. 2006), and appears as a promising candidate since both SO and SO 2 are particularly abundant in the ISM (Snyder et al. 1975;Cernicharo et al. 2011;Vidal et al. 2017).However, despite its experimental microwave characterization being performed long ago (Kirchhoff 1969;Borgo et al. 1979;Heineking & Gerry 1993), to our knowledge, HNSO has only been searched for toward Orion KL yielding no detection (Esplugues et al. 2013).

OBSERVATIONS
We have analyzed data from an unbiased ultradeep spectral survey conducted toward the GC molecular cloud G+0.693.We covered the Q-band (31.075-50.424GHz) using the Yebes 40 m (Guadalajara, Spain) radiotelescope.Also, we covered three additional frequency windows with high sensitivity using the IRAM 30 m (Granada, Spain) radiotelescope:  GHz.For these observations, we used the position switching mode, centered at α = 17 h 47 m 22 s , δ = −28 • 21 ′ 27 ′′ , with the off position shifted by ∆α = −885 ′′ and ∆δ = 290 ′′ .The half power beam width (HPBW) of the Yebes 40 m telescope varies between ∼35 ′′ -55 ′′ (at 50 and 31 GHz, respectively; Tercero et al. 2021) and the HPBW of the IRAM 30 m radiotelescope is ∼14 ′′ −29 ′′ across the frequency range covered.Also, we assumed that the molecular emission toward G+0.693 is extended as compared to the telescope beam (Jones et al. 2012;Li et al. 2020;Zheng et al. 2024).More details of these observations (e.g., resolution and noise levels of the spectral survey) were provided in Rivilla et al. (2023) and Sanz-Novo et al. (2023).Kirchhoff (1969).The astronomical line identification of HNSO has been performed using entry 63514 of the Cologne Database for Molecular Spectroscopy (CDMS2 ; Müller et al. 2005;Endres et al. 2016), explained in detail in Appendix A, which has been implemented into the Madcuba package (Martín et al. 2019).We did not consider the hyperfine structure, since it is not resolved owing to the typical broad linewidths of the molecular line emission measured toward G+0.693 (FWHM ∼ 15−20 km s −1 ; Requena- Torres et al. 2006Torres et al. , 2008;;Zeng et al. 2018;Rivilla et al. 2022c).
We used the Spectral Line Identification and Modeling (SLIM) tool (version from 2023, November 15) within Madcuba, which, assuming a Local Thermodynamic Equilibrium (LTE) excitation condition, enables the creation of LTE synthetic spectra to be subsequently compared with the observed astronomical data.After evaluating the emission of more than 130 molecules previously identified toward G+0.693, we managed to detect numerous K a = 0, 1 and 2 transitions spanning from J up = 2 to J up = 10.The most intense unblended or slightly blended features are shown in Figure 1, and their spectroscopic information is listed in Table 1.Among them, we found six completely unblended transitions.Other transitions reproduce well the observations, once the blending with emission from other molecules previously detected toward G+0.693 is considered (see Figure 1 and Table 1).The remaining lines, blended with more prominent transitions from other species, are also in agreement with the observed spectra.
Table 1.Spectroscopic information of the unblended or slightly blended transitions of HNSO detected toward G+0.693−0.027(shown in Figure 1).

Frequency
Transition (a)  Note- (a) The rotational energy levels are labelled using the conventional notation for asymmetric tops: JK a ,Kc , where J denotes the angular momentum quantum number, and the Ka and Kc labels are projections of J along the a and c principal axes. (b) The S/N ratio is computed from the integrated signal ( T * A dv) and noise level, σ = rms × √ δv × FWHM, where δv is the velocity resolution of the spectra and the FWHM is fitted from the data.Numbers in parentheses represent the predicted uncertainty associated to the last digits.The "*" symbol designates those transitions used in the initial fit.We denote as U-line the line blending with a yet unidentified feature.
The best LTE modelling for HNSO was achieved using a two-step approach as described in San Andrés et al. (submitted, 2024).The line width (FWHM) was constrained first using exclusively the aforementioned unblended transitions (marked with a "*" symbol in Table 1), obtaining a value of FWHM = 21.5 ± 0.6 km s −1 in an initial nonlinear least-squares LTE fit using the Autofit tool within SLIM (Martín et al. 2019).Afterward, we performed a second fit that includes all the transitions shown in Figure 1 and Table 1, with the exception of the 8 0,8 -7 0,7 (slightly contaminated with an unidentified line), fixing the line width and leaving as free parameters the excitation temperature, T ex , radial velocity, v LSR , and column density, N .We derived a molecular column density of N (HNSO) = (8 ± 1) ×10 13 cm −2 , which yields a fractional abundance with respect to molecular hydrogen of (6 ± 1) × 10 −10 , using N (H 2 ) = 1.35×10 23 cm −2 as derived by Martín et al. (2008) employing C 18 O as a total H 2 column density tracer and assuming a C 18 O/H 2 abundance ratio of 1.7×10 7 , Frerking et al. (1982)).Additionally, we obtained a T ex = 11 ± 2 K and a v LSR = 68 ± 2 km s −1 , which are consistent with those found for other molecular species toward the same source (see e.g.Requena-Torres et al. 2006;Zeng et al. 2018).As shown in Table 1, the detected transitions cover a wide range of E up , which allowed us to derive an accurate estimate on the T ex .The fitted line profiles of HNSO are shown using a red line in Figure 1 overlaid with the observed spectra (in gray).In blue, we report the predicted spectrum considering all molecular species identified and analyzed toward G+0.693.
We have also performed a complementary rotational diagram analysis (Goldsmith & Langer 1999), as implemented in Madcuba, using transitions that are unblended with the emission from other molecules, together with the 8 1,7 -7 1,6 transition (at 150.0824046GHz) which is negligibly contaminated by the emission of CH 3 13 CHO, and considering the velocity-integrated intensity over the line width (Rivilla et al. 2021a).We obtained the following physical parameters for HNSO: N = (7.0 ± 0.8) ×10 13 cm −2 and T ex = 11.9 ± 0.6 K, which agree well with those derived from the AUT-  Table 1) sorted by decreasing peak intensity.The result of the best LTE fit of HNSO is plotted with a red line and the blue line depicts the emission from all the molecules identified to date in our survey, including HNSO (observed spectra shown as gray histograms).The structure of HNSO is also shown (nitrogen atom in blue; oxygen atom in red, sulfur atom in yellow and hydrogen atom in white).OFIT.The results of the analysis are depicted in Figure 2.

DISCUSSION
To understand how HNSO fits in a broader astrochemical context, we can compare its abundance with that reported for plausible precursors, such as SO (Rivilla et al. 2022b) and NS (the analysis is detailed in Appendix B), as well as other structurally related molecules also detected toward G+0.693 (see Table 2) highlightling SO 2 (analyzed in Appendix C), HNCS (Sanz-Novo et al. 2024), HNCO (Zeng et al. 2018) and HOCN (Rivilla et al. 2022c).We found a N (SO)/N (HNSO) ratio of 37 ± 4 toward G+0.693, which is significantly lower than the lower limit ratio derived toward Orion KL (N (SO)/N (HNSO) ≥ 2600; Esplugues et al. 2013).Moreover, we obtained a N (SO 2 )/N (HNSO) ratio of 4.8 ± 0.6, a N (HNCS)/N (HNSO) ratio of 0.8 ± 0.1, a N (HNCO)/N (HNSO) ratio of 40 ± 6 and a N (HOCN)/N (HNSO) ratio of 0.27 ± 0.4.Consequently, HNSO appears to be a rather abundant molecule within interstellar S-chemistry, despite having gone unnoticed until now.Our observations are in line with the results presented in Sanz-Novo et al. ( 2024) for a variety of well-known interstellar S-bearing molecules, which suggest that S is not significantly depleted toward G+0.693 compared to other astronomical sources (Martín-Doménech et al. 2016;Vidal et al. 2017;Marcelino et al. 2023;Fuente et al. 2023).
Although HNSO could be a link between N-, Sand O-interstellar chemistry, this species is missing in commonly-used chemical networks, such as the Kinetic Database for Astrochemistry (KIDA; http://kida.astrophy.u-bordeaux.fr/)or in the UMIST Database for Astrochemistry (UDfA; Millar et al. 2024).Therefore, further effort is required to account for the formation of NSO-compounds, which will contribute to achieve a better agreement between observations and chemical models of S-, N-and O-bearing species.
Concerning its production in the ISM, HNSO is most likely formed through the grain-surface reaction between the NSO radical -whose rotational signatures remain unknown and, therefore, it has not been searched for in the ISM so far-and atomic hydrogen.This hydrogenation process is feasible at the dust temperatures measured in the Galactic Center (∼20 K; Rodríguez-Fernández et al. 2000;Etxaluze et al. 2013).Once HNSO is formed on the grains, it can be subsequently transferred to the gas phase thanks to shock-induced desorption.Alternatively, if we assume a similar grain-surface formation pathway as that explored for HNCO by Quénard et al. (2018), i.e., NH + CO → HNCO (Fedoseev et al. 2016), we can propose the following alternative radical-radical diffusion reaction: NH + SO → HNSO.Note, however, that the diffusion of radicals on grain surfaces is not expected to occur efficiently at temperatures below 30 K (Garrod et al. 2008;Garrod 2013), and thus this mechanism is not expected to be dominant at the low dust temperatures measured in the Galactic Center.Nonetheless, as recently shown in the models of Garrod et al. (2022), non-diffusive chemistry could alternatively yield significant amounts of HNCO even at low dust temperatures via the reaction NH + CO → HNCO, which suggests that the same could apply to the reaction NH + SO → HNSO.
In the gas phase, the hydrogenation process has been explored theoretically using high-level coupled cluster methodologies for all possible NSO radical isomers (Kumar & Francisco 2017), suggesting that the formation of their corresponding [H,N,S,O] hydrides (i.e., HNSO, HSNO and SNOH) is exothermic and barrierless, being thus feasible under interstellar conditions.Nevertheless, these authors showed that, once formed, these hydrides will likely decompose yielding SH and NO, SN and OH or SO and NH radicals.Another possible gasphase route is the reaction between NSO and H 2 , yielding HNSO + H, which, to the best of our knowledge, remains unexplored.On the other hand, the formation of the NSO radical, a mixed oxide of nitrogen and sulfur, and also the most stable member of the [N,S,O] isomeric family (Kumar & Francisco 2017), remains uncharted as well.On grains, a possible diffusion mechanism of atomic N on the dustgrain surface cannot be a priori discarded, which, if efficient enough at the typical low temperatures of the icy grain mantles, could react with SO yielding NSO.In this context, it has recently been shown that the diffusion of atomic C is feasible at temperatures above 22 K on icy grain surfaces (Tsuge et al. 2023), which is on the order of the dust temperature of the Galactic Center.Consequently, it would be interesting to extend this investigation on plausible N-insertion reactions driven by the diffusion of N atoms on interstellar ices.Conversely, non-diffussive grain-surface reactions between N and SO may also play a role if they are formed in situ (Garrod et al. 2022) (e.g., through early photochemistry within the ices), subsequently yielding NSO.On a similar note, the reaction N(gas) + SO (grain) forming NSO on the grain surface could also be studied, since C-atom addition reactions between C coming from gas-phase and grain-surface species are proved to be efficient (Fedoseev et al. 2022).
In addition, we propose the following gas-phase reaction: NS + O 2 → NSO + O, similarly to the NCO radical, which is produced via CN + O 2 → NCO + O (reaction rate of 2.4 × 10 −11 cm −3 s −1 ; Glarborg et al. 1998;Marcelino et al. 2018), given that NS is also detected toward G+0.693 (see Appendix B).We find a molecular column density of N (NS) = (2.8 ± 0.3) × 10 14 cm −2 , which indicates that it is ∼3.5 times more abundant than HNSO, suggesting that the route starting with NS as plausible precursor may be relevant only if most of the reservoir of NS was locked up in the formation of HNSO.Nonetheless, in this case the NSO radical would need to be subsequently depleted in order to react with atomic H on the ice.All in all, NSO appears as a promising molecule to be studied by the laboratory spectroscopic community.
The feasibility of all the aforementioned routes also needs to be further explored, both theoretically and in the laboratory, to provide conclusive clues on the formation pathways of HNSO.Particularly, the spectroscopic study of other [H,N,S,O] isomers, highlighting the trans-form of HNSO, certainly merits attention, given that several steroisomers 3 have been detected toward G+0.693 (e.g., cis-and trans-formic acid, HCOOH, and the high-energy cis-conformer of carbonic acid, HO-COOH; Sanz-Novo et al. 2023).
The results presented in this letter confirm that G+0.693 is an astrochemical niche for the detection of new S-bearing species, even combined with O and/or N.This fact opens the door for the investigation of a new family of interstellar molecules.Moreover, the discovery of the first interstellar NSO-bearing species shall help in the identification of hitherto unidentified organo-sulfur molecules in the gas phase, pushing also the frontiers of known chemical complexity in the ISM and its possible contribution to prebiotic chemistry.
We are grateful to the IRAM 30 m and Yebes 40 m telescopes staff for their help during the different observing runs, highlighting project 21A014 (PI: Rivilla), project 018-19 (PI: Rivilla)   The rotational spectrum of thionylimide, HNSO, was investigated by conventional microwave spectroscopy (Kirchhoff 1969;Borgo et al. 1979) with accuracies of 100 and 50 kHz, respectively.A Fourier transform microwave (FTMW) spectroscopy was capable of resolving not only the hyperfine structure (HFS) splitting of 14 N, but also that of 1 H (Heineking & Gerry 1993).The ground state rotational parameters were improved later through ground state combination differences (GSCDs) from a high-resolution infrared study (Joo et al. 1996).
We have fitted the available rotational data by applying the rotational and centrifugal distortion parameters from Joo et al. (1996) and the HFS parameters from Heineking & Gerry (1993).Moreover, the rotational spectrum calculated from the resulting spectroscopic parameters displayed fairly large uncertainties already for transitions with low quantum numbers.The underlying GSCDs from Joo et al. (1996) were unfortunately not available anymore from the corresponding author of that study.We received GSCDs from the authors of a different high-resolution infrared study (Puskar et al. 2006).Inclusion of essentially all of these GSCDs improved the uncertainties of the rotational and centrifugal distortion parameters greatly, permitted the sextic distortion parameters determined in the earlier infrared study and even the octic L K to be fit as well.Only L KKJ was very uncertain and was kept fixed to the value from the earlier study (Joo et al. 1996).Since the molecule is rather close to the prolate symmetric limit, it is advisable to employ Watson's S reduction of the rotational Hamiltonian instead of the A reduction as used earlier.
The change in reduction had the advantage that the parameter h 3 , which is the equivalent to ϕ K in the A reduction, could be omitted in the present fit without significant deterioration of the quality of the fit.
The 1 H HFS splitting of 3.0 kHz observed in the (Heineking & Gerry 1993) could not be explained by 1 H nuclear spin-rotation coupling, but required the 1 H-14 N nuclear spin-nuclear spin coupling to be considered as well.This coupling is described by two terms of which the direct term can be calculated well from the geometry for light nuclei.Deviations are small even for moderately heavy nuclei, such as Cl, as are the contributions from the indirect term, but matter both for heavier nuclei, as shown for example in a study on BrF and IF (Müller & Gerry 1995).The HN bond is almost perfectly aligned with the b-axis, and a value of S bb = −8.02kHz was derived from the structure.The 1 H and 14 N nuclear spin-rotation parameter C ii (H) and C ii (N) were necessary in the fit, but were not determined very accurately; the uncertainties were in part as large as the values, and in the most favorable cases smaller by only by a factor of a few.Therefore, we evaluated these parameters through quantum-chemical calculations employing the commercially available program Gaussian 16 (Frisch et al. 2019).We performed B3LYP hybrid density functional (Becke 1993;Lee et al. 1988) calculations using the aug-cc-pwCVTZ basis set (i.e., a valence basis set of triple zeta quality and augmented with diffuse basis functions Dunning, Jr. 1989 and with core-correlating basis functions Peterson & Dunning, Jr. 2002).Trial fits with these parameters kept fixed or with one to all released gave fits of essentially the same quality.Also, none of the parameters released in the fit changed its value significantly outside the respective parameters.Therefore, we kept the nuclear spin rotation parameters fixed at the quantum-chemically calculated values.The resulting spectroscopic parameters are given in Table A1 together with previous values (Joo et al. 1996;Heineking & Gerry 1993).The rotational spectrum calculated from these parameters is available in the catalog section of the CDMS (entry 63514).Its accuracy is sufficient for radio astronomical observations of cold objects, where molecules show low T ex (i.e., 10-20 K), including prestellar cores and also the G+0.693 molecular cloud, and probably also for a warm source with T ex ≲ 80 K.However, this catalogue may not be accurate enough for, e.g., hot cores or hot corinos with T ex ≳ 100 K.The underlying line, parameter, and fit files along with other auxiliary files are available in the catalog archive of the CDMS (Müller et al. 2005;Endres et al. 2016).
The agreement of the rotational and centrifugal distortion parameters from this study and from Joo et al. (1996) is, unsurprisingly, good when the change in reduced Hamiltonian is taken into account.The diagonal quartic distortion parameters D J , D JK , and D K differ from the corresponding ∆ J , ∆ JK , and ∆ K by small multiples of d 2 ; δ J = −d 1 , and only the relation between d 2 and δ K are somewhat more complex.The differences in values are mostly caused by the change in reduction.Similar, albeit more complex relations apply to the sextic distortion parameter to convert from one reduction Note-a Watson's S reduction in the I r representation was used in the present fit whereas Watson's A reduction was used in the previous fit (Joo & Clouthier 1996).Hyperfine parameters were taken from Heineking & Gerry (1993).Numbers in parentheses are one standard deviation in units of the least significant figures.Numbers without uncertainties were kept fixed in the fit, see Section A. b Derived parameter.Heineking & Gerry (1993) determined χaa and χ bb − χ bb for 14 N and C bb + Ccc/2 as well as C bb − Ccc for both nuclei.
to the other.Even though our fit contains one distortion parameter less than the fit in Joo et al. (1996), most of their uncertainties are smaller by factors of a few, suggesting their unavailable data set was more extensive or more accurate than the one available to us from Puskar et al. (2006).The uncertainties of B and C are essentially the same, which is most likely a consequence of a too low weight of the two hyperfine free transition frequencies of Heineking & Gerry (1993) in the fit of Joo et al. (1996).The present nuclear quadrupole coupling parameters agree well with those from Heineking & Gerry (1993); this applies also to the nuclear spinrotation parameters if we take the relatively large uncertainties from the previous study into account.Finally, we provide in Table A2 the rotational (Q r ) partition function of the ground state of HNSO.We used SPCAT (Pickett 1991) to estimate the values of Q r by direct summation of the ground state energy levels up to J = 125 and K a = 62.These values are provided for the conventional temperatures as implemented in the JPL database (Pickett et al. 1998), and two additional temperatures of 2.725 K and 5.000 K.

B. ANALYSIS OF NS
For completeness, we also present the LTE analysis of NS ( 2 Π 1/2 state), which was performed using all the most unblended transitions (see Table B1).We obtained the spectroscopic data from the 046515 entry of the CDMS catalogue (Lee et al. 1995).We present in Figure B1 the result of the best LTE fit using the Autofit tool within SLIM.As shown, the hyperfine structure of NS is partially resolved within the astronomical data set.We derived the following physical parameters from the fit: N = (28 ± 3) ×10 13 cm −2 , T ex = (7.5 ± 0.5) K, v LSR = (70.6 ± 0.6) km s −1 and FWHM = (19 ± 1) km s −1 .The derived column density is translated into a fractional abundance with respect to H 2 of (2.1 ± 0.2) ×10 −9 .In Figure B1, we depict the fitted line profiles of NS in red, while the expected molecular emission from all the molecules detected to date toward G+0.693 is shown in blue.

C. ANALYSIS OF SO 2
We carried out the LTE analysis for SO 2 by using the most intense and unblended transitions clearly detected toward G+0.693 (see Table C1).The spectroscopic data were acquired from the 064502 entry of the CDMS catalogue (Müller & Brünken 2005).In Figure C1 we show the result of the best LTE fit derived from Autofit.We obtained the following physical parameters from the LTE fit: N = (38.4± 0.5) ×10 13 cm −2 , T ex = (18.9± 0.2) K, v LSR = (68.9± 0.1) km s −1 and FWHM = (21.1 ± 0.3) km s −1 .Thus, we derived a fractional abundance with respect to molecular hydrogen of (2.8 ± 0.3) ×10 −9 .We used the same color code for the line profiles of Figure C1 as that of Fig. 1 (i.e., fitted line profiles of SO 2 in red and the expected molecular emission from all the molecules detected to date toward G+0.693 in blue).C1).The result of the best LTE fit is shown with a red solid line, while the blue line shows the expected molecular emission from all the molecular species identified to date in our survey.The observed spectra are plotted as gray histograms.
Table B1.Spectroscopic information of the unblended transitions of NS (a) detected toward G+0.693 (shown in Figure B1).

Parity
Frequency J' -J" F ' -F " log I (300 K) gu Eup rms  Note- (a) The rotational energy levels are labelled using the quantum number J and F .The parity is also indicated.For those hyperfine components that are partially or fully coalesced, we provide the integrated intensity and S/N ratio of the mean observed line rather than the values of each component, which is given only once for each group of transitions.Numbers in parentheses represent the predicted uncertainty associated to the last digits.
Table C1.Spectroscopic information of the selected transitions of SO 2 detected toward G+0.693 (shown in Figure C1).

Frequency
Transition (a)  Note- (a) The rotational energy levels are labelled using the conventional notation for asymmetric tops: JK a ,Kc , where J denotes the angular momentum quantum number, and the Ka and Kc labels are projections of J along the a and c principal axes.Numbers in parentheses represent the predicted uncertainty associated to the last digits.C1).
The result of the best LTE fit is shown with a red solid line, while the blue line shows the expected molecular emission from all the molecular species identified to date in our survey.The observed spectra are plotted as gray histograms.

3.
DETECTION OF HNSO HNSO may occur in the cis-and in the transconfiguration, but only the former (hereafter HNSO) has been detected experimentally in the laboratory.It is an asymmetric rotor close to the prolate symmetric limit with κ = (2B − A − C)/(A − C) = −0.9190,and possesses a sizable a-dipole moment component of 0.893 D and a much weaker b-component of 0.181 D. It was firstly characterized by microwave spectroscopy more than fifty years ago by

Figure 1 .
Figure1.Transitions of HNSO identified toward the GC molecular cloud G+0.693-0.027(listed in Table1) sorted by decreasing peak intensity.The result of the best LTE fit of HNSO is plotted with a red line and the blue line depicts the emission from all the molecules identified to date in our survey, including HNSO (observed spectra shown as gray histograms).The structure of HNSO is also shown (nitrogen atom in blue; oxygen atom in red, sulfur atom in yellow and hydrogen atom in white).

Figure 2 .
Figure 2. Rotational diagram of HNSO toward G+0.693 (blue dots, including 1σ errors).The best linear fit to the data points is depicted using a gray dotted line.The derived values for the molecular column density, N , and the excitation temperature, Tex, are shown in blue.

Figure 3 .
Figure 3. Suggested chemical routes for the formation of HNSO in the ISM.We show in blue molecules that have been identified toward G+0.693 and in yellow molecules that have not been searched for toward G+0.693 because spectroscopy is not available (NSO).Surface reactions are shown in black solid lines while gas-phase reactions are depicted with dashed arrows.

Figure B1 .
Figure B1.Unblended or slightly blended transitions of NS detected toward G+0.693 molecular cloud (listed in TableC1).The result of the best LTE fit is shown with a red solid line, while the blue line shows the expected molecular emission from all the molecular species identified to date in our survey.The observed spectra are plotted as gray histograms.

Figure C1 .
Figure C1.Unblended or slightly blended transitions of SO 2 detected toward G+0.693 molecular cloud (listed in TableC1).The result of the best LTE fit is shown with a red solid line, while the blue line shows the expected molecular emission from all the molecular species identified to date in our survey.The observed spectra are plotted as gray histograms.

Table A1 .
Present and previous spectroscopic parameters a (MHz) of thionylimide.