# The prebiotic molecular inventory of Serpens SMM1
## I. An investigation of the isomers CH₃NCO and HOCH₂CN

N. F. W. Ligterink<fnref n="1" />, A. Ahmadi<fnref n="2" />, A. Coutens<fnref n="3" />, Ł. Tychoniec<fnref n="2" />, H. Calcutt<fnref n="4" />,<fnref n="5" />, E. F. van Dishoeck<fnref n="2" />,<fnref n="6" />, H. Linnartz<fnref n="7" />, J. K. Jørgensen<fnref n="8" />, R. T. Garrod<fnref n="9" />, and J. Bouwman<fnref n="7" />

Received 8 October 2020 / Accepted 21 December 2020

## ABSTRACT

**Aims.** Methyl isocyanate (CH₃NCO) and glycolonitrile (HOCH₂CN) are isomers and prebiotic molecules that are involved in the formation of peptide structures and the nucleobase adenine, respectively. These two species are investigated to study the interstellar chemistry of cyanides (CN) and isocyanates (NCO) and to gain insight into the reservoir of interstellar prebiotic molecules.

**Methods.** ALMA observations of the intermediate-mass Class 0 protostar Serpens SMM1-a and ALMA-PILS data of the low-mass Class 0 protostar IRAS 16293B are used. Spectra are analysed with the CASSIS line analysis software package in order to identify and characterise molecules.

**Results.** CH₃NCO, HOCH₂CN, and various other molecules are detected towards SMM1-a. HOCH₂CN is identified in the PILS data towards IRAS 16293B in a spectrum extracted at a half-beam offset position from the peak continuum. CH₃NCO and HOCH₂CN are equally abundant in SMM1-a at [X]/[CH₃OH] of 5.3 × 10⁻<fnref n="4" /> and 6.2 × 10⁻<fnref n="4" />, respectively. A comparison between SMM1-a and IRAS 16293B shows that HOCH₂CN and HNCO are more abundant in the former source, but CH₃NCO abundances do not differ significantly. Data from other sources are used to show that the [CH₃NCO]/[HNCO] ratio is similar in all these sources within ~10%.

**Conclusions.** The new detections of CH₃NCO and HOCH₂CN are additional evidence for a large interstellar reservoir of prebiotic molecules that can contribute to the formation of biomolecules on planets. The equal abundances of these molecules in SMM1-a indicate that their formation is driven by kinetic processes instead of thermodynamic equilibrium, which would drive the chemistry to one product. HOCH₂CN is found to be much more abundant in SMM1-a than in IRAS 16293B. From the observational data, it is difficult to indicate a formation pathway for HOCH₂CN, but the thermal Strecker-like reaction of CN⁻ with H₂CO is a possibility. The similar [CH₃NCO]/[HNCO] ratios found in the available sample of studied interstellar sources indicate that these two species are either chemically related or their formation is affected by physical conditions in the same way. Both species likely form early during star formation, presumably via ice mantle reactions taking place in the dark cloud or when ice mantles are being heated in the hot core. The relatively high abundances of HOCH₂CN and HNCO in SMM1-a may be explained by a prolonged stage of relatively warm ice mantles, where thermal and energetic processing of HCN in the ice results in the efficient formation of both species.

**Key words.** astrochemistry – astrobiology – ISM: abundances – submillimeter: ISM – ISM: individual objects: Serpens SMM1

## 1. Introduction

Observations of molecules towards star-forming regions provide insight into the kind of species that end up in planet-forming discs. These molecules not only aid planet formation but can also seed newly formed planets with a cocktail of molecules from which larger organic molecules can be formed. Prebiotic molecules are of particular interest as they are involved in the formation of biomolecules, such as amino acids, nucleobases, proteins, and lipids (Sandford et al. 2020). In the interstellar medium (ISM) and on planets, prebiotic molecules are the building blocks from which biomolecules are made.

In the ISM, several prebiotic molecules have been detected. Examples are formamide (NH₂CHO, Rubin et al. 1971), a precursor to nucleobases and amino acids (Saladino et al. 2012), the simplest sugar-like molecule glycolaldehyde (HOCH₂CHO, Hollis et al. 2000; Jørgensen et al. 2012), methylamine (CH₃NH₂, Kaifu et al. 1974; Bøgelund et al. 2019) and aminoacetonitrile (NH₂CH₂CN, Belloche et al. 2008), building blocks of the amino acid glycine (Holtom et al. 2005; Lee et al. 2009), the peptide building blocks acetamide (CH₃C(O)NH₂) and N-methylformamide (CH₃NHCHO, Hollis et al. 2006; Halfen et al. 2011; Belloche et al. 2017, 2019; Ligterink et al. 2020), the chiral molecule propylene oxide (CH₃CHCH₂O,

<footnote n="1">¹ Physics Institute, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
e-mail: ligterink@strw.leidenuniv.nl</footnote>

<footnote n="2">² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands</footnote>

<footnote n="3">³ Laboratoire d'Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France</footnote>

<footnote n="4">⁴ Department of Space, Earth and Environment, Chalmers University of Technology, 41296 Gothenburg, Sweden</footnote>

<footnote n="5">⁵ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5,
87-100 Torun, Poland</footnote>

<footnote n="6">⁶ Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany</footnote>

<footnote n="7">⁷ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands</footnote>

<footnote n="8">⁸ Centre for Star and Planet Formation, Niels Bohr Institute & Natural History Museum of Denmark, University of Copenhagen,
Øster Voldgade 5-7, 1350 Copenhagen K., Denmark</footnote>

<footnote n="9">⁹ Departments of Chemistry and Astronomy, University of Virginia, Charlottesville, VA 22904, USA</footnote>
<figure><img src="image_1.png" /><figcaption>Fig. 1. Structures of the C₂H₃NO isomers methyl isocyanate (CH₃NCO, left) and glycolonitrile (HOCH₂CN, right).</figcaption></figure>

McGuire et al. 2016), cyanomethanimine (NHCHCN), which can oligomerise to form adenine (Zaleski et al. 2013; Rivilla et al. 2019), and the nucleobase precursors cyanamide (NH₂CN, Turner et al. 1975; Coutens et al. 2018), hydroxylamine (NH₂OH, Rivilla et al. 2020), and carbamide (also known as urea, NH₂C(O)NH₂, Belloche et al. 2019). In recent years, methyl isocyanate (CH₃NCO, Halfen et al. 2015; Cernicharo et al. 2016; Ligterink et al. 2017) has been detected in several interstellar sources, and its isomer glycolonitrile (also known as hydroxy acetonitrile, HOCH₂CN, Zeng et al. 2019) is recently identified for the first time in the ISM towards the low-mass protostar IRAS 16293–2422B (hereafter IRAS 16293B). Both of these isomers are prebiotic molecules. CH₃NCO can engage in reactions that form peptide-like structures, while HOCH₂CN is known to accelerate the oligomerisation of HCN in liquids and ice under terrestrial conditions, forming the nucleobase adenine (Schwartz & Goverde 1982; Schwartz et al. 1982).

Besides their relevance to prebiotic chemistry, CH₃NCO and HOCH₂CN are also interesting molecules for gaining insight into interstellar nitrogen chemistry. While these molecules, being isomers, have the same elemental composition (C₂H₃NO), their chemical structures differ significantly (see Fig. 1). Recent quantum chemical calculations of the stability of C₂H₃NO isomers in general reveal that CH₃NCO is the most stable of these species, followed by HOCH₂CN (Fourré et al. 2020). Therefore, observations of this isomer couple provide information on interstellar reactions involving cyanides (-CN) and isocyanates (-NCO), two important nitrogen-bearing chemical groups, as well as the physical conditions that steer or prohibit this chemistry.

Following the first detection of CH₃NCO (Halfen et al. 2015; Cernicharo et al. 2016), it was suggested that HNCO was involved in its formation due to their structural similarity and the large HNCO abundances in interstellar gas and ice (in the form of OCN⁻; Boogert et al. 2015). Various gas-phase and solid-state methylation (the addition of a CH₃ functional group to a molecule) reactions of HNCO, the OCN radical, and the OCN⁻ anion have been proposed as possible reaction pathways. Experimental investigations (Ligterink et al. 2017; Maté et al. 2018) and modelling studies (Martín-Doménech et al. 2017; Quénard et al. 2018; Majumdar et al. 2018) indicate solid-state methylation in the ice mantle as the main pathway to form CH₃NCO:

$$CH_3 + NCO \rightarrow CH_3NCO. \quad (1)$$

Variations of this pathway, such as the methylation of HNCO or OCN⁻, may be possible as well, while completely different reactions, such as the hydrogenation of HCN...CO, may also form CH₃NCO.

No gas-phase formation pathways are known for glycolonitrile, but solid-state production routes have been studied

theoretically (Woon 2001) and experimentally (Danger et al. 2012, 2014). Laboratory work indicates that the thermally activated reaction between a cyanide anion (CN⁻) and formaldehyde (H₂CO) forms HOCH₂CN:

$$[XH^+CN^-] + H_2CO \rightarrow HOCH_2CN + X, \quad (2)$$

where X is a molecule that can act as a base, such as ammonia (NH₃) or water (H₂O). This reaction is the solid-state equivalent of the Strecker synthesis, which is a sequence of chemical reactions that produce amino acids. The Strecker-like formation of HOCH₂CN is linked to the formation of aminomethanol (HOCH₂NH₂, Bossa et al. 2009) and aminoacetonitrile (NH₂CH₂CN, Danger et al. 2011). The latter of these species is detected in the ISM and known as a possible intermediate in the formation of the amino acid glycine (Belloche et al. 2008). Irradiation of HOCH₂CN results in the photoproducts formylcyanide (HC(O)CN) and ketenimine (CH₂CNH). Although not investigated, hydrogenation and oxygen additions of these two species may provide pathways to form HOCH₂CN in the ISM. The solid-state radical-radical reactions HO + CH₂CN and HOCH₂ + CN can also form glycolonitrile, but neither of these reactions has been investigated (Margulès et al. 2017). However, precursor species to these reactions can be present in ice mantles, in particular when methanol (CH₃OH) or acetonitrile (CH₃CN) are energetically processed (Allamandola et al. 1988; Hudson & Moore 2004; Bulak et al. 2020).

Methyl isocyanate and glycolonitrile can thus be used as tracers of reactions involving CN and NCO and investigating their interstellar abundances reveals information about the chemical and physical processes that drive these reactions and interstellar nitrogen chemistry as a whole. CH₃NCO and HOCH₂CN have only been detected simultaneously in the low-mass protostar IRAS 16293B (Ligterink et al. 2017; Martín-Doménech et al. 2017; Zeng et al. 2019), albeit in different observational data sets. Due to this limited sample size, it is difficult to derive correlations or variations in the C₂H₃NO isomer chemistry and therefore simultaneous identifications in other sources are required. Here, deep ALMA observations of the intermediate-mass Class 0 protostar Serpens SMM1-a (hereafter SMM1-a) are presented to derive additional constraints on CH₃NCO and HOCH₂CN chemistry.

The Serpens star-forming region contains multiple deeply embedded sources, of which SMM1 is the brightest (Casali et al. 1993). The Serpens region contains multiple outflows and jets, some of which originate from SMM1 (Dionatos et al. 2013; Hull et al. 2016). The chemistry of the SMM1 hot corino, its outflows, and the Serpens core have been characterised in various studies (e.g. White et al. 1995; Hogerheijde et al. 1999; Kristensen et al. 2010; Öberg et al. 2011; Goicoechea et al. 2012; Tychoniec et al. 2019). High-resolution continuum jet observations have shown that SMM1 consists of multiple sources, of which SMM1-a is the main one (Choi 2009; Dionatos et al. 2014; Hull et al. 2017). SMM1-a has SMM1-b as a close neighbour at ~500 au, while two other sources, SMM1-c and -d, are located further away to its north. Recent distance measurements place the Serpens core, and therefore SMM1, at a distance of 436.0 ± 10 pc (Ortiz-León et al. 2017), resulting in a luminosity estimate of the entire SMM1 source of ~100 L☉. SMM1-a is considered to be an intermediate-mass protostar (Hull et al. 2017; Tychoniec et al. 2019).

In this work, the detection and analysis of the isomers HOCH₂CN and CH₃NCO towards SMM1-a are presented and compared with literature results of IRAS 16293B and those of
<table><caption>**Table 1.** Frequency settings of ALMA SMM1 observations.</caption><thead><tr><th rowspan="2">Frequency range<br>(GHz)</th><th rowspan="2">Bandwidth<br>(GHz)</th><th colspan="2">Resolution</th></tr><tr><th>(kHz)</th><th>(km s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>217.59–217.70</td><td>0.117</td><td>488.21</td><td>0.33</td></tr><tr><td>217.97–218.09</td><td>0.117</td><td>488.21</td><td>0.33</td></tr><tr><td>218.43–218.55</td><td>0.117</td><td>488.21</td><td>0.33</td></tr><tr><td>218.92–219.03</td><td>0.117</td><td>488.21</td><td>0.33</td></tr><tr><td>219.71–219.82</td><td>0.117</td><td>488.21</td><td>0.33</td></tr><tr><td>221.30–221.42</td><td>0.117</td><td>488.21</td><td>0.33</td></tr><tr><td>221.42–221.53</td><td>0.117</td><td>488.21</td><td>0.33</td></tr><tr><td>221.53–221.65</td><td>0.117</td><td>488.21</td><td>0.33</td></tr><tr><td>231.74–231.97</td><td>0.234</td><td>488.21</td><td>0.33</td></tr><tr><td>233.41–233.65</td><td>0.234</td><td>488.21</td><td>0.33</td></tr><tr><td>234.06–235.93</td><td>1.875</td><td>1952.84</td><td>1.25</td></tr></tbody></table>

other sources. In Sect. 2 the observations towards SMM1 and the analysis method are presented. The detections of HOCH₂CN, CH₃NCO, and various other molecules are presented in Sect. 3. Section 4 discusses these detections and their likely formation pathways. The conclusions of this work are presented in Sect. 5.

## 2. Data and methods

### 2.1. *Observations and spectra of Serpens SMM1*

SMM1 was observed on 27-March-2019 during ALMA cycle 6, as part of project #20181.00836.S (PI: N.F.W. Ligterink). The region was observed using a total of 42 antennae with baselines spanning 15–332 m in configuration C43-5. The on-source integration time was 50 min, towards the phase centre $\alpha_{J2000} = 18:29:49.80$ $\delta_{J2000} = +01:15:20.6$. Spectra were recorded in select frequency windows between 217.59 and 235.93 GHz, at resolutions of 488.21 kHz (0.33 km s⁻<fnref n="2" />) and 1952.84 kHz (1.25 km s⁻<fnref n="2" />) for the continuum window (see Table 1). The data were calibrated and imaged with version 5.4.0-70 of the Common Astronomy Software Applications (CASA). Bandpass and flux calibration was conducted on J2000–1748, while phase calibration was performed on J1851+0035. The flux uncertainty was ≤20%. To reach the desired sensitivity, the measurement sets were cleaned using the Hogbom algorithm (Högbom 1974) and Briggs weighting with a robust parameter of 0.5. This resulted in an angular resolution of $1''32 \times 1''04$ and an rms noise of 2.6 mJy beam⁻<fnref n="1" /> km s⁻<fnref n="1" /> in the final spectral data cubes. The primary beam of the observations was 26''.

Due to the line richness of the source, the following procedure was followed to properly subtract the continuum from the line observations. We imaged all spectral windows without the continuum removed and used the corrected sigma-clipping method of the STATCONT package<fnref n="1" /> (Sánchez-Monge et al. 2018) to extract a continuum-subtracted line cube. STATCONT can only subtract zeroth-order polynomials, while in this data set non-zeroth-order baselines are visible. Furthermore, continuum subtraction in the uv-plane is more desirable since the deconvolution of the line emission is more robust when it is not subjected to the deconvolution errors of the brighter continuum. Therefore, the STATCONT outputs were used to identify the line-free channels in the spectra and the continuum was subtracted in the uv-plane with the uvcontsub task in CASA. Line-free channels are sparse, but for most spectral windows, at least 20%

of the bandwidth was given as input to the uvcontsub task, with the exception of two spectral windows, where only 10% of the bandwidth was line-free. From the resulting datacube, the SMM1-a hot core spectrum was extracted towards the peak continuum position $\alpha_{J2000} = 18:29:49.793$, $\delta_{J2000} = +1.15:20.200$. From the average continuum flux density (0.41 mJy beam⁻<fnref n="1" />), the background temperature was determined to be ~5.2 K.

### 2.2. PILS observations and spectra of IRAS 16293B

In this work, the column density of HOCH₂CN is determined independently from the detection presented by Zeng et al. (2019) by analysing data from the Protostellar Interferometric Line Survey (PILS). Other species relevant to this work are also searched for in the PILS data set. The observational details of the PILS survey have been presented in various other publications (e.g. Jørgensen et al. 2016) and here only the most relevant information is presented. In short, the PILS survey makes use of ALMA band 7 observations, covering a frequency range from 329 to 363 GHz at a spatial resolution of 0.′.5. To investigate the chemical inventory of IRAS 16293B, spectra are extracted at several positions. These positions are on the peak continuum, a half-beam offset from the peak continuum, and a full-beam offset from the peak continuum. Most PILS analyses of IRAS 16293B make use of the spectrum at the full-beam offset position (e.g. Jørgensen et al. 2016, 2018; Coutens et al. 2016, 2018; Ligterink et al. 2017; Persson et al. 2018; Calcutt et al. 2018). In this work, this is the main position for which molecular ratios with HOCH₂CN are determined, but the spectra of other positions are also analysed. The systemic velocity towards these positions is $V_{LSR} = 2.7$ km s⁻<fnref n="1" /> and the line width is approximately $\Delta V = 1.0$ km s⁻<fnref n="1" />. Due to the dense dust around IRAS 16293B, the background temperatures ($T_{BG}$) at these positions are higher than the cosmic microwave background radiation temperature of 2.7 K and couple with the molecular line emission (see Ligterink et al. 2018). At the full-beam offset position $T_{BG} = 21$ K, while at the half-beam offset position $T_{BG} = 52$ K.

### 2.3. *Analysis method*

The spectra were analysed with the CASSIS<fnref n="2" /> line analysis software. Spectral line lists were obtained from the JPL database for molecular spectroscopy (Pickett et al. 1998), the Cologne Database for Molecular Spectroscopy (CDMS, Müller et al. 2001, 2005), and from the literature. An overview of the spectroscopic line lists used in this work and the laboratory works they are based on is given in Appendix A. Given a spectroscopic line list as input, CASSIS produced synthetic spectra of a molecule based on parameters such as column density ($N_T$), excitation temperature ($T_{ex}$), peak gas velocity ($V_{LSR}$), line width at half maximum ($\Delta V$), and source size ($\theta_{source}$). These parameters were given as free parameters to a Monte Carlo Markov chain (MCMC) algorithm and $\chi^2$ minimisation routine. This routine found the best-fit of a synthetic spectrum to an observed spectrum over a specified parameter space, thereby determining the best-fit parameters and thus column density and excitation temperature of a molecule. For the analysis, optically thin lines ($\tau \ll 1.0$) were used. The $\tau$-value was approximated from the by-eye synthetic fit (see below) of the observed rotational lines with the CASSIS software. The molecules were assumed to be in local thermodynamic equilibrium (LTE). Errors on physical parameters take the uncertainty of the fit and the flux uncertainty as

<footnote n="2">² CASSIS has been developed by IRAP-UPS/CNRS (http://cassis.irap.omp.eu).</footnote>

<footnote n="1">¹ https://hera.ph1.uni-koeln.de/~sanchez/statcont</footnote>
<figure><img src="image_2.png" /></figure>

**Fig. 2.** Moment 0 maps of the HOCH₂CN 24<sub>8,17/16</sub>−23<sub>8,16/15</sub>, *E*<sub>up</sub> = 222 K line and the CH₃NCO *v* = 0 25<sub>0,0</sub>−24<sub>0,0</sub>, *E*<sub>up</sub> = 188 K line towards SMM1. Both lines are integrated over eight velocity bins, centred on the peak frequency of each line as determined towards SMM1-a. Positions of protostars in the SMM1 region are indicated and the beam size (1''32 × 1''04) is visualised in the bottom left corner. Dust continuum contours are given by the black dotted line at the levels of 0.02, 0.05, 0.1, 0.2, and 0.5 Jy Beam⁻¹.

input and are calculated from the spread in $\chi^2$ values around the minimum to a $3\sigma$ confidence level.

In this work, spectral lines of a molecule were identified in the observed spectra and a by-eye synthetic fit of the lines was made. For the by-eye fit, line width and peak velocity were determined from prominent spectral lines of molecules such as HNCO and CH₃OH and used as a first approximation for other molecules. An excitation temperature of 200 K was taken as an initial guess and followed by a round of adjusting column density and rotational temperature until a reasonable by-eye fit was found. The by-eye fit results were given as starting parameters for the MCMC $\chi^2$ minimisation routine. The $\chi^2$ minimisation was performed on lines that are not blended and have minimal contributions from the wings of neighbouring lines. Blending species were identified by checking the line position for other lines of known hot core/corino species with $A_{ij} > 1 \times 10^{-6}$ and $E_{up}$ of 0–1000 K. The column density was given as a free parameter over two orders of magnitude centred on the by-eye fit column density and the excitation temperature was a free parameter from 50–350 K. For SMM1-a, $\Delta V$ was a free parameter from 1.0–4.0 km s⁻¹, and the source velocity was a free parameter between 6.0–9.0 km s⁻¹. The source size was assumed to be equal to the beam size and taken to be 1''2, resulting in a beam filling factor of 0.5. A background continuum temperature ($T_{bg}$) of 5.2 K was used.

For the analysis of IRAS 16293B, $\Delta V$ and $V_{LSR}$ were fixed to 1.0 and 2.7 km s⁻¹, respectively. The source size was assumed to be equal to the beam size at 0.′5, while a background continuum temperature of $T_{bg}$ = 21 K was used.

# 3. Results

## 3.1. SMM1

In the spectra of SMM1-a, multiple unblended rotational lines of the isomers HOCH₂CN and CH₃NCO *v* = 0 are found. Moment 0 maps (the spatial mapping of the integrated line intensity of a single rotational line) of both species show that most

emission originates from SMM1-a (see Fig. 2). The identified lines towards SMM1-a are presented in Figs. 3 and 4. For HOCH₂CN, this is the second independent interstellar detection of this molecule (the first detection of HOCH₂CN is presented towards IRAS 16293B by Zeng et al. 2019), while the first detection of CH₃NCO towards SMM1-a is part of only a handful of detections of this species towards other interstellar sources.

Several other species are identified in the SMM1-a spectra as well. Rotational lines of acetonitrile (CH₃CN *v*₈ = 1), cyanamide (NH₂CN), ethanol (CH₃CH₂OH), the anti-conformer of ethylene glycol (a-(CH₂OH)₂), deuterated formaldehyde (D₂CO), isocyanic acid (HN¹²CO and HN¹³CO), methylformate (CH₃OCHO), and methanol (¹²CH₃OH and CH₃¹⁸OH) are detected. Rotational lines of the C₂H₃NO isomers methyl fulmiate (CH₃CNO) and methyl cyanate (CH₃OCN) were not identified in the spectra. The molecules aminoacetonitrile (NH₂CH₂CN), ketenimine (CH₂CNH), and formylcyanide (HC(O)CN) are searched for, but not identified. Spectra are presented in Appendix B and spectroscopic parameters of transitions are provided in Table B.1. Following the procedure detailed in Sect. 2.3, the best-fit parameters of these species are determined. For undetected species, upper limit column densities are determined by assuming $T_{ex}$ = 200 K, which is chosen as a representative excitation temperature from the molecules that are detected. The fit parameters are presented in Table 2. The main isotopologues of CH₃OH and HNCO are optically thick and their column densities are therefore determined from minor isotopologues. This is done by multiplying with the local interstellar ¹²C/¹³C ratio of 52.5 ± 15.4 (Yan et al. 2019) and ¹⁶O/¹⁸O ratio of 560 (Wilson 1999).

## 3.2. IRAS 16293B

Because the chemical inventory of IRAS 16293B is well characterised with results from the PILS survey, this data set is used to search for HOCH₂CN and related species to make an unbiased chemical comparison with SMM1-a. While HOCH₂CN is identified towards IRAS 16293B by Zeng et al. (2019), this detection cannot be confirmed in the PILS spectrum at the full-beam offset
<figure><img src="image_3.png" /><figcaption>**Fig. 3.** Identified lines of HOCH₂CN towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic spectrum overplotted in blue ($N_T = (7.4 \pm 0.9) \times 10^{14}$ cm⁻², $T_{ex} = 260 \pm 45$ K). The transition is indicated at the top of each panel and the upper state energy is given in the top left of each panel.</figcaption></figure>

position (the position commonly used for the molecular analysis of IRAS 16293B with PILS data, see Fig. 5). Figure 6 shows the HOCH₂CN a-type transitions ($J'_{0,J'}-J''_{0,J''}$) at this position covered by the PILS spectral range with a synthetic glycoloni-trile spectrum at $N_T = 1.0 \times 10^{15}$ cm⁻² and $T_{ex} = 150$ and 300 K. A synthetic spectrum of the previously identified molecules towards IRAS 16293B at the full-beam offset position is added in green. We note that this synthetic spectrum only includes molecules listed in previous publications and does not include HOCH₂CN. The molecules and parameters used in this fit are listed in Table C.1. When a rotational spectrum is experimentally measured, a-type transitions are usually the first and most accu-rately determined transitions. The assignment of these a-type

transitions, therefore, is key to claiming an unambiguous identi-fication. However, of the seven a-type transitions covered by the PILS survey, only the $39_{0,39,1}-38_{0,38,1}$ transition at 344 625 MHz is possibly detected, although this feature has a contribution of a HONO and CH₂DOH transition (Jørgensen et al. 2016, 2018; Coutens et al. 2019), which also can fully reproduce this line. Appendix C presents all the HOCH₂CN transitions at the full-beam offset position that are largely unblended and have $A_{ij} \ge 1 \times 10^{-3}$ s⁻¹. At the full-beam offset position, a large number (~40) of glycolonitrile transitions that are present in the synthetic spectrum are not seen in the observed spectrum. We note that due to the line richness of the source, the baseline subtraction is challenging, and in some cases, it can be oversubtracted. This can
<figure><img src="image_4.png" /><figcaption>**Fig. 4.** Identified lines of CH<sub>3</sub>NCO, ν=0 towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic spectrum overplotted in blue (N<sub>T</sub>=(6.4 ± 1.9) × 10<sup>14</sup> cm<sup>-2</sup>, T<sub>ex</sub> = 240 ± 60 K). The transition is indicated at the top of each panel and the upper state energy is given in the top left of each panel.</figcaption></figure>

explain why certain HOCH<sub>2</sub>CN lines are not clearly observed as the baseline at these positions dips. Furthermore, some transitions seem to be better reproduced with low excitation temperatures, whereas others require warmer temperatures. This could indicate that two gas components are traced, as also shown by Zeng et al. (2019), but at this position these components are hard to distinguish. Therefore, HOCH<sub>2</sub>CN can only tentatively be identified towards the full-beam offset position, with a column density of ~1.0 × 10<sup>15</sup> cm<sup>-2</sup> at T<sub>ex</sub> = 150 and 300 K.

At the half-beam offset position (see Fig. 5), which is closer to the continuum peak of IRAS 16293B, HOCH<sub>2</sub>CN can be identified. At this position, at least four a-type transitions are clearly detected, while the three other lines suffer from line blending or absorption features (see Fig. 7). These lines can approximately be fitted with synthetic spectra of N<sub>T</sub> = 3.0 × 10<sup>15</sup> cm<sup>-2</sup> for T<sub>ex</sub> = 150 and 300 K. In Appendix C the remaining HOCH<sub>2</sub>CN transitions at the half-beam offset position are shown, with the same selection criteria for the full-beam offset position.

In Fig. 5 the moment 0 map of the HOCH<sub>2</sub>CN 36<sub>4,32</sub>-35<sub>4,31</sub> transition towards IRAS 16293B is shown. This map shows that HOCH<sub>2</sub>CN emission towards IRAS 16293B is

compact. This explains the non-detection of glycolonitrile towards the full-beam offset position as this position misses most of the HOCH<sub>2</sub>CN emission. At the same time, this map demonstrates why Zeng et al. (2019) could detect HOCH<sub>2</sub>CN towards IRAS 16293B: These authors used a larger observational beam of 1.′′6 (with an assumed source size of 0.′′5), which covers the entire emitting area.

Because the chemical inventory at the full-beam offset position of the PILS data is characterised best, the tentative detection of HOCH<sub>2</sub>CN towards this position, with a column density of ~1.0 × 10<sup>15</sup> cm<sup>-2</sup>), is used for further analysis in this paper. However, the identification of HOCH<sub>2</sub>CN towards the half-beam offset position in combination with the moment 0 map support the detection of HOCH<sub>2</sub>CN towards IRAS 16293B by Zeng et al. (2019).

The related species CH<sub>2</sub>CNH, CH(O)CN, and NH<sub>2</sub>CH<sub>2</sub>CN are also searched for in the PILS data set, but clear and unblended lines are not identified. For these species upper limit column densities are determined at the full-beam offset position at excitation temperatures of 100 and 300 K. The upper limit column densities and abundances of all species are presented in Table 3.
<table><caption>**Table 2.** Best-fit parameters for molecules detected towards SMM1-a in a 1′′2 beam.</caption><thead><tr><th>Molecule</th><th>Lines #</th><th>N<sub>T</sub> (cm<sup>-2</sup>)</th><th>T<sub>ex</sub> (K)</th><th>V<sub>LSR</sub> km s<sup>-1</sup></th><th>ΔV km s<sup>-1</sup></th><th>[X] / [CH<sub>3</sub>OH]</th><th>[X] / [HNCO]</th></tr></thead><tbody><tr><td>HOCH<sub>2</sub>CN</td><td>18</td><td>(7.4 ± 0.9) × 10<sup>14</sup></td><td>260 ± 45</td><td>6.8 ± 0.2</td><td>2.5 ± 0.3</td><td>6.2 × 10<sup>-4</sup></td><td>7.4 × 10<sup>-2</sup></td></tr><tr><td>CH<sub>3</sub>NCO</td><td>12</td><td>(6.4 ± 1.9) × 10<sup>14</sup></td><td>240 ± 60</td><td>7.0 ± 0.4</td><td>3.0 ± 0.6</td><td>5.3 × 10<sup>-4</sup></td><td>6.4 × 10<sup>-2</sup></td></tr><tr><td>D<sub>2</sub>CO</td><td>3</td><td>(5.4 ± 2.5) × 10<sup>14</sup></td><td>[200]</td><td>7.4 ± 0.4</td><td>3.9 ± 0.2</td><td>4.5 × 10<sup>-4</sup></td><td>5.4 × 10<sup>-2</sup></td></tr><tr><td>CH<sub>3</sub><sup>18</sup>OH</td><td>4</td><td>(2.0 ± 1.1) × 10<sup>15</sup></td><td>250 ± 60</td><td>7.1 ± 0.2</td><td>2.8 ± 0.3</td><td>—</td><td>—</td></tr><tr><td><sup>12</sup>CH<sub>3</sub>OH<sup>(a)</sup></td><td>5<sup>(b)</sup></td><td>1.1 × 10<sup>18</sup></td><td>—</td><td>—</td><td>—</td><td>1.0</td><td>110</td></tr><tr><td>CH<sub>3</sub>CN<sup>(c)</sup></td><td>6</td><td>(1.3 ± 0.3) × 10<sup>15</sup></td><td>190 ± 25</td><td>7.5 ± 0.2</td><td>3.4 ± 0.3</td><td>1.1 × 10<sup>-3</sup></td><td>0.1</td></tr><tr><td>NH<sub>2</sub>CN</td><td>3</td><td>(5.1 ± 1.3) × 10<sup>13</sup></td><td>190 ± 40</td><td>7.1 ± 0.2</td><td>3.2 ± 0.4</td><td>4.3 × 10<sup>-5</sup></td><td>5.1 × 10<sup>-3</sup></td></tr><tr><td>HN<sup>13</sup>CO</td><td>3</td><td>(1.9 ± 0.3) × 10<sup>14</sup></td><td>190 ± 30</td><td>7.6 ± 0.2</td><td>3.5 ± 0.3</td><td>—</td><td>—</td></tr><tr><td>HN<sup>12</sup>CO<sup>(a)</sup></td><td>5<sup>(b)</sup></td><td>1.0 × 10<sup>16</sup></td><td>—</td><td>—</td><td>—</td><td>1.1 × 10<sup>-2</sup></td><td>1.0</td></tr><tr><td>CH<sub>3</sub>CH<sub>2</sub>OH</td><td>14</td><td>(4.1 ± 0.9) × 10<sup>15</sup></td><td>210 ± 25</td><td>7.3 ± 0.2</td><td>2.8 ± 0.6</td><td>4.1 × 10<sup>-3</sup></td><td>0.3</td></tr><tr><td>CH<sub>3</sub>OCHO</td><td>24</td><td>(7.4 ± 0.7) × 10<sup>15</sup></td><td>215 ± 20</td><td>7.3 ± 0.2</td><td>3.1 ± 0.3</td><td>7.4 × 10<sup>-3</sup></td><td>0.6</td></tr><tr><td>a-(CH<sub>2</sub>OH)<sub>2</sub></td><td>14</td><td>(1.7 ± 0.5) × 10<sup>15</sup></td><td>195 ± 70</td><td>7.2 ± 0.2</td><td>2.7 ± 0.3</td><td>1.7 × 10<sup>-3</sup></td><td>0.1</td></tr><tr><td>CH<sub>3</sub>CNO</td><td>0</td><td>≤1.0 × 10<sup>13</sup></td><td>[200]</td><td>[7.0]</td><td>[3.5]</td><td>≤9.1 × 10<sup>-6</sup></td><td>≤1.0 × 10<sup>-3</sup></td></tr><tr><td>CH<sub>3</sub>OCN</td><td>0</td><td>≤5.0 × 10<sup>13</sup></td><td>[200]</td><td>[7.0]</td><td>[3.5]</td><td>≤4.6 × 10<sup>-6</sup></td><td>≤5.0 × 10<sup>-3</sup></td></tr><tr><td>CH<sub>2</sub>CNH</td><td>0</td><td>≤1.0 × 10<sup>15</sup></td><td>[200]</td><td>[7.0]</td><td>[3.5]</td><td>≤8.3 × 10<sup>-4</sup></td><td>0.1</td></tr><tr><td>CH(O)CN</td><td>0</td><td>≤2.0 × 10<sup>14</sup></td><td>[200]</td><td>[7.0]</td><td>[3.5]</td><td>≤1.7 × 10<sup>-4</sup></td><td>≤2.0 × 10<sup>-2</sup></td></tr><tr><td>NH<sub>2</sub>CH<sub>2</sub>CN</td><td>0</td><td>≤1.0 × 10<sup>14</sup></td><td>[200]</td><td>[7.0]</td><td>[3.5]</td><td>≤8.3 × 10<sup>-5</sup></td><td>≤1.0 × 10<sup>-2</sup></td></tr></tbody></table>

**Notes.** Values in brackets are assumed. <sup>(a)</sup>Main isotopologue column densities are determined by applying the ratios <sup>16</sup>O/<sup>18</sup>O = 560 and <sup>12</sup>C/<sup>13</sup>C = 52.5 to the minor isotopologue column densities. <sup>(b)</sup>The number of lines identified of the main isotopologue in this data set. <sup>(c)</sup>The CH<sub>3</sub>CN best-fit parameters are determined from its vibrationally excited state v<sub>8</sub> = 1.

HOCH<sub>2</sub>CN - 36<sub>4,32</sub>-35<sub>4,31</sub> - E<sub>up</sub> = 324K

<figure><img src="image_5.png" /></figure>

**Fig. 5.** Moment 0 map of the HOCH<sub>2</sub>CN 36<sub>4,32</sub>-35<sub>4,31</sub>, E<sub>up</sub> = 324 K line towards IRAS 16293B. The line is integrated over eight velocity bins, centred on the peak frequency of the line. The positions of the half-beam and full-beam offset positions around IRAS 16293B are indicated and the beam size (0.′5 × 0.′5) is visualised in the bottom left corner. Dust continuum contours are given by the black dotted line at the levels of 0.02, 0.05, 0.1, 0.2, and 0.5 Jy Beam<sup>-1</sup>.

## 4. Discussion

In this work, several molecules are detected towards SMM1-a and analysed. Most notable are the detection of the C<sub>2</sub>H<sub>3</sub>NO isomers CH<sub>3</sub>NCO (methyl isocyanate) and HOCH<sub>2</sub>CN (gly-colonitrile). For HOCH<sub>2</sub>CN, this is only its second interstellar detection. CH<sub>3</sub>NCO has been detected in multiple interstellar sources, but this is the first detection towards SMM1 and

<table><caption>**Table 3.** Best-fit parameters of HOCH<sub>2</sub>CN and related species towards IRAS 16923B in the PILS data set in a 0.′5 beam.</caption><thead><tr><th>Molecule</th><th>N<sub>T</sub> (cm<sup>-2</sup>)</th><th>T<sub>ex</sub> (K)</th><th>[X] / [CH<sub>3</sub>OH]<sup>(a)</sup></th><th>[X] / [HNCO]<sup>(a)</sup></th></tr></thead><tbody><tr><td>HOCH<sub>2</sub>CN</td><td>≤1.0 × 10<sup>15</sup></td><td>[150]</td><td>≤1.0 × 10<sup>-4</sup></td><td>≤3.3 × 10<sup>-2</sup></td></tr><tr><td>HOCH<sub>2</sub>CN</td><td>≤1.0 × 10<sup>15</sup></td><td>[300]</td><td>≤1.0 × 10<sup>-4</sup></td><td>≤3.3 × 10<sup>-2</sup></td></tr><tr><td>CH<sub>2</sub>CNH</td><td>≤1.0 × 10<sup>15</sup></td><td>[100]</td><td>≤1.0 × 10<sup>-4</sup></td><td>≤3.3 × 10<sup>-2</sup></td></tr><tr><td>CH<sub>2</sub>CNH</td><td>≤2.0 × 10<sup>15</sup></td><td>[300]</td><td>≤2.0 × 10<sup>-4</sup></td><td>≤6.7 × 10<sup>-2</sup></td></tr><tr><td>CH(O)CN</td><td>≤5.0 × 10<sup>14</sup></td><td>[100]</td><td>≤5.0 × 10<sup>-5</sup></td><td>≤1.7 × 10<sup>-2</sup></td></tr><tr><td>CH(O)CN</td><td>≤5.0 × 10<sup>14</sup></td><td>[300]</td><td>≤5.0 × 10<sup>-5</sup></td><td>≤1.7 × 10<sup>-2</sup></td></tr><tr><td>NH<sub>2</sub>CH<sub>2</sub>CN</td><td>≤1.0 × 10<sup>15</sup></td><td>[100]</td><td>≤1.0 × 10<sup>-4</sup></td><td>≤3.3 × 10<sup>-2</sup></td></tr><tr><td>NH<sub>2</sub>CH<sub>2</sub>CN</td><td>≤5.0 × 10<sup>14</sup></td><td>[300]</td><td>≤5.0 × 10<sup>-5</sup></td><td>≤1.7 × 10<sup>-2</sup></td></tr></tbody></table>

**Notes.** Values in brackets are assumed. V<sub>LSR</sub> = 2.7 km s<sup>-1</sup> and ΔV = 1.0 km s<sup>-1</sup>. We note that some upper limit column densities are similar because the upper state energies of the lines cover only a narrow range of energies. <sup>(a)</sup>CH<sub>3</sub>OH and HNCO column densities were adopted from Jørgensen et al. (2018) and Coutens et al. (2016), respectively.

therefore also the first detection towards an intermediate-mass source. These new detections serve as additional evidence for a large and diverse reservoir of prebiotic molecules in star-and planet-forming regions, which can contribute to the emergence of biomolecules on planetary bodies. Of the C<sub>2</sub>H<sub>3</sub>NO isomers, CH<sub>3</sub>NCO is energetically the most favourable, followed by HOCH<sub>2</sub>CN, which has a higher relative energy of 12.1–18.6 kcal mol<sup>-1</sup> (0.5–0.8 eV molecule<sup>-1</sup> or 5800–9300 K molecule<sup>-1</sup>), depending on the level of theory used (Fourré et al. 2020). In a thermodynamic equilibrium, lower energy or more stable products are favoured, and in such a scenario, CH<sub>3</sub>NCO is expected to be more abundant than HOCH<sub>2</sub>CN by a factor of at least 1 × 10<sup>9</sup> (assuming a temperature of 300 K). The fact
<figure><img src="image_6.png" /><figcaption>HOCH₂CN - IRAS 16293-2422B - full-beam offset position</figcaption></figure>

**Fig. 6.** Overview of a-type ($J'_{0,j'} - J''_{0,j''}$) HOCH₂CN transitions covered by the PILS spectrum towards the full-beam offset position of IRAS 16293B, illustrating the non-detection of these lines in this spectrum. The observed spectrum is plotted in black and synthetic spectra for a column density of $1.0 \times 10^{15}$ cm⁻² and excitation temperatures of 150 (blue) and 300 K (red) are overplotted. The synthetic spectrum of the entire molecular inventory determined with PILS data towards this position is plotted in green. The quantum numbers of the transition are indicated at the top of each panel and the upper state energy is given in the top left of each panel.

<figure><img src="image_7.png" /><figcaption>HOCH₂CN - IRAS 16293-2422B - half-beam offset position</figcaption></figure>

**Fig. 7.** Overview of a-type ($J'_{0,j'} - J''_{0,j''}$) HOCH₂CN transitions covered by the PILS spectrum towards the half-beam offset position of IRAS 16293B, illustrating the detection of a number of these lines. The observed spectrum is plotted in black and synthetic spectra for a column density of $3.0 \times 10^{15}$ cm⁻² and excitation temperatures of 150 (blue) and 300 K (red) are overplotted. The quantum numbers of the transition are indicated at the top of each panel and the upper state energy is given in the top left of each panel.

that CH₃NCO and HOCH₂CN are found to be equally abundant is, therefore, evidence that the formation of these molecules is instead driven by kinetics.

To better understand the interstellar chemistry of HOCH₂CN and CH₃NCO, their abundances and those of several other species are compared for SMM1-a, IRAS 16293B, and several other sources for which at least some of these species are detected. For this comparison, it is important to emphasise observational differences. For example, SMM1 ($D \approx 436$ pc) is

located farther away than IRAS 16293 ($D \approx 141$ pc, Dzib et al. 2018) and the beam size used in this work (1′′2) is larger than that of the PILS survey (0′.5). Therefore, the chemical inventory of SMM1-a and the involved chemical processes are investigated on a much larger spatial scale of roughly 500 au, compared to about 70 au for IRAS 16293B. The SMM1-a observations can cover a larger range of physical environments (e.g. also the envelope or outflow), and potentially a larger temperature gradient. At the same time, the higher luminosity of SMM1 results
<figure><img src="image_8.png" /><figcaption>**Fig. 8.** Ratios of [X]/[CH₃OH] towards SMM1-a (red) and IRAS 16293B (blue) in decreasing order of SMM1-a abundance. The ‘~’ symbol indicates that these HOCH₂CN ratios have been determined with the column density of the tentative HOCH₂CN detection towards the IRAS 16293B full-beam offset position. For IRAS 16293B, column densities derived towards the full-beam offset position from the following PILS publications are used. <sup>a</sup>This work, <sup>b</sup>Ligterink et al. (2017), <sup>c</sup>Coutens et al. (2016), <sup>d</sup>Jørgensen et al. (2016, 2018), <sup>e</sup>Calcutt et al. (2018), <sup>f</sup>Persson et al. (2018), <sup>g</sup>Coutens et al. (2018).</figcaption></figure>

in a larger area where hot core conditions are present, thus compensating for the lower spatial resolution. Furthermore, the SMM1-a spectrum is extracted towards the continuum peak, but for IRAS 16293B a full-beam offset position from the continuum peak is used. Therefore, when comparing molecular ratios between the two sources, they may not only be affected by different physical conditions but also due to the way the sources were observed. Observational parameters of SMM1-a, IRAS 16293B, and other sources used for comparisons are listed in Table D.1.

HOCH₂CN is thus far only detected in two sources, SMM1-a and IRAS 16293B (Zeng et al. 2019), and therefore a chemical comparison is limited to these two objects. To use molecular ratios that are unbiased by observational parameters, only data of the chemical inventory of IRAS 16293B obtained with PILS survey data at the full-beam offset position is used for the source comparison. This means that the tentative column density of HOCH₂CN in IRAS 16293B is used. Figure 8 shows the abundances of molecules detected in this work to CH₃OH and HNCO in SMM1-a and IRAS 16293B. For IRAS 16293B, the analysis performed in this work is combined with results from Jørgensen et al. (2016, 2018); Coutens et al. (2016, 2018); Ligterink et al. (2017); Persson et al. (2018); Calcutt et al. (2018). For both the [X]/[CH₃OH] and [X]/[HNCO] ratios, all the oxygen-bearing molecules, CH₃CN, and NH₂CN are found to be more abundant in IRAS 16293B than in SMM1-a. For [X]/[CH₃OH] its ratios are generally a factor of a few lower in SMM1-a, while for the [X]/[HNCO] ratios the difference is usually more than a factor of ten.

## 4.1. SMM1-a: a HOCH₂CN-rich source

While general trends are found in the [X]/[CH₃OH] and [X]/[HNCO] ratios displayed in Fig. 8, three molecules deviate from the general trend. The abundance of CH₃NCO is found to be approximately equal in both sources, while HOCH₂CN and HNCO are more abundant in SMM1-a compared to IRAS 16293B. In particular, for HOCH₂CN a large difference is seen in its ratios to CH₃OH, which can be more than an order of magnitude different between the two sources.

<figure><img src="image_9.png" /><figcaption>**Fig. 9.** Statistical distance between molecular ratios in SMM1 and IRAS 16293B, given in $\sigma$. Larger $\sigma$ values indicate a larger difference between the two sources for a given ratio. Positive values indicate that a ratio is higher in SMM1-a, while negative values indicate that a ratio is lower in SMM1-a. In particular, all ratios of HOCH₂CN are found to be higher in SMM1-a than in IRAS 16293B.</figcaption></figure>

To gain further insight in how the chemical compositions of SMM1-a and IRAS 16293B differ, the statistical distance of molecular ratios are plotted in Fig. 9. The statistical distance indicates how significant the difference in a molecular ratio between SMM1-a and IRAS 16293B is (see Manigand et al. 2020, and Appendix E). Greater values indicate a greater difference in the molecular ratios between the two sources. A positive value indicates that the ratio of SMM1-a is greater than that of IRAS 16293B and vice versa. Figure 9 highlights that HOCH₂CN and HNCO are more abundant in SMM1-a ($\sigma$ ranging from 3–6) and CH₃NCO is moderately more abundant in SMM1-a ($\sigma$ ≥ 2).

The statistical distance results have several implications. Since the abundances of both HOCH₂CN and HNCO are enhanced in SMM1-a, this may indicate a relationship between
these two species. For CH₃NCO a much less significant enhance-
ment is seen, which can imply that both C₂H₃NO isomers form
via different chemical reactions or under different physical con-
ditions. However, it is important to stress that an abundance
correlation does not always imply a formational link between
species (Belloche et al. 2020).

The statistical distances of ratios involving CH<sub>3</sub>CN, NH<sub>2</sub>CN, a-(CH<sub>2</sub>OH), and CH<sub>3</sub>CH<sub>2</sub>OH show that there is almost no variation in these molecules between SMM1-a and IRAS 16293B. This is interesting because some of these molecules can form in reactions involving radicals from which HOCH<sub>2</sub>CN also can be formed, such as CN, CH<sub>2</sub>OH, and CH<sub>2</sub>CN. This hints that either HOCH<sub>2</sub>CN is not formed from these radicals, is not linked to the reaction networks that form the four aforementioned species, or HOCH<sub>2</sub>CN forms from the same radicals, but under different physical conditions.

Finally, it is interesting to note that ratios of D₂CO and CH₃OCHO to NH₂CN, CH₃CH₂OH, and a-(CH₂OH)₂ seem to be a bit more abundant in SMM1-a than in IRAS 16293B. This is particularly significant for the case of D₂CO since H₂CO is involved in the Strecker-like formation of HOCH₂CN (see reaction (2)). A higher abundance of H₂CO may indicate that the Strecker-like reaction can more efficiently take place. However, care needs to be taken with this interpretation since the D₂CO spectral lines in the SMM1-a spectrum are blended and thus there is a large uncertainty on its column density. At the same time, the D/H ratio of H₂CO is not known in SMM1-a, which introduces another source of uncertainty. To investigate if there is a correlation between HOCH₂CN and H₂CO, both species should be identified towards more sources and H₂CO should be observed through its minor ¹³C and ¹⁸O isotopes instead of the deuterated species. For now, however, the Strecker-like synthesis of HOCH₂CN in the ISM can neither be confirmed nor ruled out.

4.2. Interstellar formation of CH<sub>3</sub>NCO

Since its first detection in the ISM, the formation of CH<sub>3</sub>NCO
has been hypothesised to be linked to that of HNCO (see
Eq. (1)). To investigate the interstellar chemical relationship
between these two species, their gas-phase ratios towards inter-
stellar sources are plotted in Fig. 10. The result of the SMM1-a
analysis is complemented by data from the low-mass protostar
IRAS 16239B (Ligterink et al. 2017), the quiescent giant molecu-
lar cloud G+0.693 (Zeng et al. 2018), the high-mass star-forming
region Orion KL (Cernicharo et al. 2016), the high-mass hot
molecular core G10.47+0.03 (Gorai et al. 2020), and the galactic
centre source Sagittarius B2(N) (Sgr B2(N), Belloche et al. 2013,
2017; Cernicharo et al. 2016). The majority of these sources is
categorised as hot cores or corinos. In these sources, thermal
desorption of molecules from ice mantles plays an important role
to chemically enrich the gas surrounding the protostar, in partic-
ular around the desorption temperature of water ice (~100 K).
The exception is the source G+0.693, which is a molecular cloud.
Molecules observed in the gas of this cloud are assumed to
be the result of gas-phase formation reactions or non-thermal
desorption from ice mantles of dust grains.

The ratios of CH₃NCO and HNCO are generally similar at
[CH₃NCO] / [HNCO] = ~10% and vary only by a factor of a few,
in particular when the lowest and highest ratio are omitted. The
lowest ratio, found towards G+0.693, arises in a source with a
very different physical structure and can therefore not directly be
compared with the hot core and corino sources. The highest ratio
is found towards G10.47+0.03, but the analysis of HNCO in this
source is likely performed on optically thick HNCO lines and

<figure><img src="image_10.png" /><figcaption>**Fig. 10.** Ratios of [CH<sub>3</sub>NCO]/[HNCO] ratios towards SMM1-a and various other sources. Column densities from the following publications are used: Belloche et al. (2013, 2017), Cernicharo et al. (2016), Ligterink et al. (2017), Zeng et al. (2018), Gorai et al. (2020), and this work.</figcaption></figure>

the analysis seems to underestimate the HNCO column density
in this source (see Fig. 4 in Gorai et al. 2020). Removing these
results from the analysis, a correlation between CH<sub>3</sub>NCO and
HNCO is found, which may indicate a chemical link between
CH<sub>3</sub>NCO and HNCO. Furthermore, this correlation spans a
variety of sources of different masses and over four order of
magnitude in luminosities. This results in different physical con-
ditions, such as the gas and dust temperature and radiation fields
for each source. Therefore, the lack of source-to-source variation
in [CH<sub>3</sub>NCO]/[HNCO] ratio hints that the abundances of these
species are set at an early stage of star formation. Formation of
CH<sub>3</sub>NCO via reaction (1) in the ice mantles of dust grains during
the dark cloud stage or while the ice mantles are warmed up in
the hot core/corino stage are a plausible scenario.

4.3. On the formation of -CN and -NCO molecules

Figure 9 shows that HOCH₂CN and HNCO abundances are enhanced in SMM1-a compared to IRAS 16293B. At the same time, CH₃NCO is only marginally enhanced and CH₃CN and NH₂CN abundances show little difference between the two sources. Why the abundances of HOCH₂CN and HNCO are enhanced in SMM1-a and those of CH₃NCO, CH₃CN, and NH₂CN are not is not straightforward to explain. However, it is likely that the various -CN and -NCO molecules form in different chemical reactions and physical conditions.

As this work shows, CH₃NCO probably forms at an early
stage of star formation, as do CH₃CN and NH₂CN. All three
species are suggested to form in radical-radical addition reac-
tions in ice mantles. These reactions take place during the dark
cloud stage in cold (~10 K) ice mantles and significantly speed
up when the ice mantle temperature increases to ~30 K and
radicals become mobile (Garrod et al. 2008; Coutens et al.
2018). Some reactions can compete for the same radical, such as
CH₃CN and NH₂CN, both of which compete for the -CN radical.

The fact that HOCH₂CN and HNCO are enhanced in abun-
dance in SMM1-a, can indicate a link between these species.
Both molecules can be formed from HCN in ice mantles of inter-
stellar dust grains. HOCH₂CN can be formed in the Strecker-like
reaction when HCN is converted to CN⁻. HNCO and the related
anion OCN⁻ are formed when HCN:H₂O mixtures are processed with energetic UV photons or protons (Gerakines et al. 2004). These reactions are aided by high grain temperatures (>>30 K, but below the water sublimation temperature) for a prolonged time and high fluxes of photons and energetic particles. If these conditions are met in SMM1, they can explain the higher abundances of HOCH₂CN and HNCO compared to IRAS 16239B and present a formational link between some -CN and -NCO molecules. However, only circumstantial evidence can be presented for such conditions in SMM1, which is based on the fact that SMM1 hosts multiple protostellar sources and outflows, which can warm and irradiate the cloud (e.g. Choi 2009; Dionatos et al. 2014; Hull et al. 2017; Tychoniec et al. 2019).

To gain further insight into the reactions that form -CN and -NCO molecules, unbiased observations of these species towards a multitude of sources spanning different physical conditions are needed. Not only the isomers HOCH₂CN and CH₃NCO should be targeted for these observations, but also species like CH₃CN, HNCO, NH₂CN, C₂H₃CN, and C₂H₅CN. Laboratory, theoretical and modelling efforts should be focused on understanding the formation of these species. In particular, the formation of HOCH₂CN via pathways other than the thermal Strecker-like synthesis needs to be studied and a better understanding of the formation of CH₃CN is required.

## 5. Conclusions

This publication presents the simultaneous detection of the C₂H₃NO isomers methyl isocyanate (CH₃NCO) and glycolonitrile (HOCH₂CN). Both species are identified towards the intermediate-mass Class 0 protostar Serpens SMM1-a. This is only the second interstellar detection of glycolonitrile, while for methyl isocyanate it is the first detection towards an intermediate-mass protostar. Additionally, CH₃OH, HNCO, CH₃OCHO, CH₃CH₂OH, a-(CH₂OH)₂, D₂CO, and NH₂CN are detected. CH₂CNH, CH(O)CN, and NH₂CH₂CN, molecules that are related to HOCH₂CN formation and destruction, are searched for but not identified. Data from the PILS survey towards IRAS 16293B are analysed in search for HOCH₂CN and this molecule is identified in a spectrum extracted at a half-beam offset position from the continuum peak of IRAS 16239B. The molecules CH₂CNH, CH(O)CN, and NH₂CH₂CN are not identified towards this source.

The detection of CH₃NCO and HOCH₂CN towards SMM1-a is additional evidence of a large interstellar reservoir of prebiotic molecules. Delivery of these molecules to planetary surfaces may contribute to the formation of biomolecules on these objects. The column densities and abundances of CH₃NCO (N<sub>T</sub> = 6.4 × 10¹⁴ cm⁻² and [CH₃NCO]/[CH₃OH] = 5.3 × 10⁻⁴) and HOCH₂CN (N<sub>T</sub> = 7.4 × 10¹⁴ cm⁻² and [HOCH₂CN]/[CH₃OH] = 6.2 × 10⁻⁴) are found to be equal within their error bars. Since HOCH₂CN is the least energetically favourable of the two isomers, thermodynamics predicts that CH₃NCO should be more abundant. The equal ratio between both molecules is therefore evidence that the formation of these molecules is driven by kinetics.

The comparison of molecular ratios between SMM1-a and IRAS 16293B show that HOCH₂CN and HNCO are significantly more abundant in the former source. The molecular ratios of HOCH₂CN hint that the formation of this molecule does not heavily depend on solid-state radical-radical addition reactions, such as HOCH₂ + CN and HO + CH₂CN. Formation via the thermal Strecker-like reaction [X⁺CN⁻] + H₂CO in ice mantles

cannot be confirmed nor ruled out based on the current data but may be a prominent formation pathway.

To investigate the possibility that CH₃NCO formation is related to HNCO, the ratios of these molecules in SMM1-a and other sources are analysed. These ratios are found to be uniform throughout all sources at [CH₃NCO]/[HNCO] = ~10%. This indicates that there is a chemical link between both species, but also that its ratios is already set at an early stage of star formation. Presumably CH₃NCO forms via the radical-radical reaction CH₃ + NCO in ice mantles during the dark cloud stage.

It is difficult to establish a chemical link between -CN and -NCO molecules. Some may be related, such as HOCH₂CN and HNCO, which can both form from HCN at elevated (>>30 K) grain temperatures and in relatively high radiation fields. Continued observational, laboratory, and theoretical studies of -CN and -NCO molecules are required to gain further insight into their formation and links in their chemistry.

*Acknowledgements.* We thank E.G. Bøgelund, S.F. Wampfler, M.N. Drozdovskaya, B. Kulterer, and B.A. McGuire for helpful discussions on the observations, spectroscopy, and the chemistry of the C₂H₃NO isomers. The authors acknowledge assistance from Allegro, the European ALMA Regional Center node in the Netherlands. We thank the anonymous referee for their thorough review of this manuscript and helpful comments. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2018.1.00836.S and ADS/JAO.ALMA#2013.1.00278.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. NFWL is supported by the Swiss National Science Foundation (SNSF) Ambizione grant 193453. J.K.J. is supported by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme through ERC Consolidator Grant "S4F" (grant agreement No 646908). A.C. acknowledges financial support from the Agence Nationale de la Recherche (grant ANR-19-ERC7-0001-01).

## References

Allamandola, L., Sandford, S., & Valero, G. 1988, Icarus, **76**, 225

Belloche, A., Menten, K., Comito, C., et al. 2008, A&A, **482**, 179

Belloche, A., Müller, H. S., Menten, K. M., Schilke, P., & Comito, C. 2013, A&A, **559**, A47

Belloche, A., Meshcheryakov, A. A., Garrod, R. T., et al. 2017, A&A, **601**, A49

Belloche, A., Garrod, R. T., Müller, H. S. P., et al. 2019, A&A, **628**, A10

Belloche, A., Maury, A., Maret, S., et al. 2020, A&A, **635**, A198

Bocquet, R., Demaison, J., Cosléou, J., et al. 1999, J. Mol. Spectr., **195**, 345

Bøgelund, E. G., McGuire, B. A., Hogerheijde, M. R., van Dishoeck, E. F., & Ligterink, N. F. W. 2019, A&A, **624**, A82

Boogert, A. A., Gerakines, P. A., & Whittet, D. C. 2015, ARA&A, **53**, 541

Bossa, J., Theule, P., Duvernay, F., & Chiavassa, T. 2009, ApJ, **707**, 1524

Bulak, M., Paardekooper, D., Fedoseev, G., & Linnartz, H. 2020, A&A, **636**, A32

Calcutt, H., Jørgensen, J. K., Müller, H., et al. 2018, A&A, **616**, A90

Casali, M., Eiroa, C., & Duncan, W. 1993, A&A, **275**, 195

Cernicharo, J., Kisiel, Z., Tercero, B., et al. 2016, A&A, **587**, L4

Choi, M. 2009, ApJ, **705**, 1730

Christen, D., & Müller, H. S. 2003, Phys. Chem. Chem. Phys., **5**, 3600

Christen, D., Coudert, L., Suenram, R. D., & Lovas, F. J. 1995, J. Mol. Spectr., **172**, 57

Coutens, A., Jørgensen, J. K., Van der Wiel, M. H. D., et al. 2016, A&A, **590**, L6

Coutens, A., Willis, E., Garrod, R., et al. 2018, A&A, **612**, A107

Coutens, A., Ligterink, N. F. W., Loison, J.-C., et al. 2019, A&A, **623**, L13

Danger, G., Borget, F., Chomat, M., et al. 2011, A&A, **535**, A47

Danger, G., Duvernay, F., Theulé, P., Borget, F., & Chiavassa, T. 2012, ApJ, **756**, 11

Danger, G., Rimola, A., Mrad, N. A., et al. 2014, Phys. Chem. Chem. Phys., **16**, 3360

Dionatos, O., Jørgensen, J. K., Green, J. D., et al. 2013, A&A, **558**, A88

Dionatos, O., Jørgensen, J. K., Teixeira, P., Güdel, M., & Bergin, E. 2014, A&A, **563**, A28

Dzib, S., Ortiz-León, G., Hernández-Gómez, A., et al. 2018, A&A, **614**, A20

Fisher, J., Paciga, G., Xu, L.-H., et al. 2007, J. Mol. Spectr., **245**, 7
Fourré, I., Matz, O., Ellinger, Y., & Guillemin, J. 2020, A&A, 639, A16

Garrod, R. T., Weaver, S. L. W., & Herbst, E. 2008, ApJ, 682, 283

Gerakines, P., Moore, M., & Hudson, R. 2004, Icarus, 170, 202

Goicoechea, J. R., Cernicharo, J., Karska, A., et al. 2012, A&A, 548, A77

Gorai, P., Bhat, B., Sil, M., et al. 2020, ApJ, 895, 86

Halfen, D. T., Ilyushin, V., & Ziurys, L. M. 2011, ApJ, 743, 60

Halfen, D., Ilyushin, V. V., & Ziurys, L. M. 2015, ApJ, 812, L5

Hocking, W., Gerry, M., & Winnewisser, G. 1975, Can. J. Phys., 53, 1869

Högbom, J. A. 1974, A&AS, 15, 417

Hogerheijde, M. R., Van Dishoeck, E. F., Salverda, J. M., & Blake, G. A. 1999, ApJ, 513, 350

Hollis, J. M., Lovas, F. J., & Jewell, P. R. 2000, ApJ, 540, L107

Hollis, J. M., Lovas, F. J., Remijan, A. J., et al. 2006, ApJ, 643, L25

Holtom, P. D., Bennett, C. J., Osamura, Y., Mason, N. J., & Kaiser, R. I. 2005, ApJ, 626, 940

Hudson, R., & Moore, M. 2004, Icarus, 172, 466

Hull, C. L., Girart, J. M., Kristensen, L. E., et al. 2016, ApJ, 823, L27

Hull, C. L., Girart, J. M., Tychoniec, Ł., et al. 2017, ApJ, 847, 92

Ilyushin, V., Kryvda, A., & Alekseev, E. 2009, J. Mol. Spectr., 255, 32

Jacobsen, S., Jørgensen, J., Van der Wiel, M., et al. 2018, A&A, 612, A72

Jørgensen, J. K., Favre, C., Bisschop, S. E., et al. 2012, ApJ, 757, L4

Jørgensen, J. K., Van der Wiel, M. H. D., Coutens, A., et al. 2016, A&A, 595, A117

Jørgensen, J., Müller, H., Calcutt, H., et al. 2018, A&A, 620, A170

Kaifu, N., Morimoto, M., Nagane, K., et al. 1974, ApJ, 191, L135

Koivusaari, M., Horneman, V., & Anttila, R. 1992, J. Mol. Spectr., 152, 377

Kristensen, L., Van Dishoeck, E., Van Kempen, T., et al. 2010, A&A, 516, A57

Kukolich, S. G., Nelson, A., & Yamanashi, B. 1971, J. Am. Chem. Soc., 93, 6769

Lapinov, A., Golubiatnikov, G. Y., Markov, V., & Guarnieri, A. 2007, Astron. Lett., 33, 121

Lee, C.-W., Kim, J.-K., Moon, E.-S., Minh, Y. C., & Kang, H. 2009, ApJ, 697, 428

Ligterink, N., Coutens, A., Kofman, V., et al. 2017, MNRAS, 469, 2219

Ligterink, N. F. W., Calcutt, H., Coutens, A., et al. 2018, A&A, 619, A28

Ligterink, N. F., El-Abd, S. J., Brogan, C. L., et al. 2020, ApJ, 901, 37

Majumdar, L., Loison, J.-C., Ruaud, M., et al. 2018, MNRAS, 473, L59

Manigand, S., Jørgensen, J., Calcutt, H., et al. 2020, A&A, 635, A48

Margulès, L., McGuire, B., Senent, M. L., et al. 2017, A&A, 601, A50

Martín-Doménech, R., Rivilla, V., Jiménez-Serra, I., et al. 2017, MNRAS, 469, 2230

Maté, B., Molpeceres, G., Tanarro, I., et al. 2018, ApJ, 861, 61

McGuire, B. A., Carroll, P. B., Loomis, R. A., et al. 2016, Science, 352, 1449

Müller, H. S., Thorwirth, S., Roth, D., & Winnewisser, G. 2001, A&A, 370, L49

Müller, H. S., Schlöder, F., Stutzki, J., & Winnewisser, G. 2005, J. Mol. Struct., 742, 215

Müller, H. S., Brown, L. R., Drouin, B. J., et al. 2015, J. Mol. Spectr., 312, 22

Müller, H. S., Belloche, A., Xu, L.-H., et al. 2016, A&A, 587, A92

Niedenhoff, M., Yamada, K., Belov, S., & Winnewisser, G. 1995, J. Mol. Spectr., 174, 151

Öberg, K. I., Van der Marel, N., Kristensen, L. E., & Van Dishoeck, E. F. 2011, ApJ, 740, 14

Ortiz-León, G. N., Dzib, S. A., Kounkel, M. A., et al. 2017, ApJ, 834, 143

Pearson, J. C., Brauer, C. S., & Drouin, B. J. 2008, J. Mol. Spectr., 251, 394

Persson, M. V., Jørgensen, J. K., Müller, H., et al. 2018, A&A, 610, A54

Pickett, H., Poynter, R., Cohen, E., et al. 1998, J. Quant. Spectr. Rad. Transf., 60, 883

Quénard, D., Jiménez-Serra, I., Viti, S., Holdship, J., & Coutens, A. 2018, MNRAS, 474, 2796

Read, W. G., Cohen, E. A., & Pickett, H. M. 1986, J. Mol. Spectr., 115, 316

Rivilla, V., Martín-Pintado, J., Jiménez-Serra, I., et al. 2019, MNRAS, 483, L114

Rivilla, V. M., Martín-Pintado, J., Jiménez-Serra, I., et al. 2020, ApJ, 899, L28

Rubin, R. H., Swenson, G. W., J., Benson, R. C., Tigelaar, H. L., & Flygare, W. H. 1971, ApJ, 169, L39

Saladino, R., Crestini, C., Pino, S., Costanzo, G., & Di Mauro, E. 2012, Phys. Life Rev., 9, 84

Sánchez-Monge, Á., Schilke, P., Ginsburg, A., Cesaroni, R., & Schmiedeke, A. 2018, A&A, 609, A101

Sandford, S. A., Nuevo, M., Bera, P. P., & Lee, T. J. 2020, Chemical Reviews

Schwartz, A. W., & Goverde, M. 1982, J. Mol. Evol., 18, 351

Schwartz, A. W., Joosten, H., & Voet, A. 1982, Biosystems, 15, 191

Turner, B. E., Liszt, H. S., Kaifu, N., & Kisliakov, A. G. 1975, ApJ, 201, L149

Tychoniec, Ł., Hull, C. L. H., Kristensen, L. E., et al. 2019, A&A, 632, A101

White, G. J., Casali, M. M., & Eiroa, C. 1995, A&A, 298, 594

Wilson, T. 1999, Rep. Prog. Phys., 62, 143

Woon, D. E. 2001, Icarus, 149, 277

Xu, L.-H., Fisher, J., Lees, R., et al. 2008, J. Mol. Spectr., 251, 305

Yan, Y., Zhang, J., Henkel, C., et al. 2019, ApJ, 877, 154

Zakharenko, O., Motiyenko, R. A., Margulès, L., & Huet, T. R. 2015, J. Mol. Spectr., 317, 41

Zaleski, D. P., Seifert, N. A., Steber, A. L., et al. 2013, ApJ, 765, L10

Zeng, S., Jiménez-Serra, I., Rivilla, V., et al. 2018, MNRAS, 478, 2962

Zeng, S., Quénard, D., Jiménez-Serra, I., et al. 2019, MNRAS, 484, L43
## Appendix A: Spectroscopic data

In this paper, the CDMS and JPL spectroscopic databases are the primary sources of molecular line lists. In the following table,

an overview of the analysed molecules, their identifier and catalogue, and the most important publications in the literature on which these entries are based are given.

<table><caption>**Table A.1.** Spectroscopic line lists.</caption><thead><tr><th>Molecule</th><th>ID</th><th>Catalogue</th><th>Entry date</th><th>Reference</th></tr></thead><tbody><tr><td>D<sub>2</sub>CO</td><td>32 502</td><td>CDMS</td><td>Jan 2016</td><td>Bocquet et al. (1999)</td></tr><tr><td></td><td></td><td></td><td></td><td>Zakharenko et al. (2015)</td></tr><tr><td><sup>12</sup>CH<sub>3</sub>OH</td><td>32504</td><td>CDMS</td><td>May 2016</td><td>Xu et al. (2008)</td></tr><tr><td>CH<sub>3</sub><sup>18</sup>OH</td><td>34504</td><td>CDMS</td><td>Sep 2020</td><td>Fisher et al. (2007)</td></tr><tr><td>CH<sub>3</sub>CN, ν<sub>8</sub> = 1</td><td>41509</td><td>CDMS</td><td>Nov 2016</td><td>Müller et al. (2015)</td></tr><tr><td></td><td></td><td></td><td></td><td>Koivusaari et al. (1992)</td></tr><tr><td>NH<sub>2</sub>CN</td><td>42003</td><td>JPL</td><td>Jan 1991</td><td>Read et al. (1986)</td></tr><tr><td>HN<sup>12</sup>CO</td><td>43511</td><td>CDMS</td><td>May 2009</td><td>Kukolich et al. (1971)</td></tr><tr><td></td><td></td><td></td><td></td><td>Hocking et al. (1975)</td></tr><tr><td></td><td></td><td></td><td></td><td>Niedenhoff et al. (1995)</td></tr><tr><td></td><td></td><td></td><td></td><td>Lapinov et al. (2007)</td></tr><tr><td>HN<sup>13</sup>CO</td><td>44008</td><td>JPL</td><td>Jul 1987</td><td>Hocking et al. (1975)</td></tr><tr><td>CH<sub>3</sub>CH<sub>2</sub>OH</td><td>46524</td><td>CDMS</td><td>Nov 2016</td><td>Pearson et al. (2008)</td></tr><tr><td></td><td></td><td></td><td></td><td>Müller et al. (2016)</td></tr><tr><td>CH<sub>3</sub>NCO, ν = 0</td><td>57505</td><td>CDMS</td><td>Mar 2016</td><td>Cernicharo et al. (2016)</td></tr><tr><td>CH<sub>3</sub>NCO, ν = 1</td><td>57506</td><td>CDMS</td><td>Mar 2016</td><td>Cernicharo et al. (2016)</td></tr><tr><td>HOCH<sub>2</sub>CN</td><td>57512</td><td>CDMS</td><td>Mar 2017</td><td>Margulès et al. (2017)</td></tr><tr><td>CH<sub>3</sub>OCHO</td><td>60003</td><td>JPL</td><td>Apr 2009</td><td>Ilyushin et al. (2009)</td></tr><tr><td>a-(CH<sub>2</sub>OH)<sub>2</sub></td><td>62503</td><td>CDMS</td><td>Sep 2003</td><td>Christen et al. (1995)</td></tr><tr><td></td><td></td><td></td><td></td><td>Christen & Müller (2003)</td></tr></tbody></table>

## Appendix B: Supporting information for the SMM1-a analysis

<figure><img src="image_11.png" /><figcaption>**Fig. B.1.** Identified lines of CH<sub>3</sub>CN ν<sub>8</sub> = 1 towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic spectrum overplotted in blue (N<sub>T</sub> = (1.3 ± 0.3) × 10<sup>15</sup> cm<sup>-2</sup>, T<sub>ex</sub> = 190 ± 25 K). The transition is indicated at the top of each panel and the upper state energy is given in the top left of each panel.</figcaption></figure>
<figure><img src="image_12.png" /></figure>

**Fig. B.2.** Identified lines of NH₂CN towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic spectrum overplotted in blue ($N_{\text{T}} = (5.1 \pm 1.3) \times 10^{13} \text{ cm}^{-2}$, $T_{\text{ex}} = 190 \pm 40$ K). The transition is indicated at the top of each panel and the upper state energy is given in the top left of each panel.

<figure><img src="image_13.png" /></figure>

**Fig. B.3.** Identified lines of a-(CH₂OH)₂ towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic spectrum overplotted in blue ($N_{\text{T}} = (1.7 \pm 0.5) \times 10^{15} \text{ cm}^{-2}$, $T_{\text{ex}} = 195 \pm 70$ K). The transition is indicated at the top of each panel and the upper state energy is given in the top left of each panel.
<table><caption>**Table B.1.** Spectral information of molecules detected towards SMM1-a.</caption><thead><tr><th>Molecule</th><th>Database entry</th><th>Transition<br>J, K<sub>a</sub>, K<sub>c</sub>, (F)</th><th>Frequency<br>(MHz)</th><th>E<sub>up</sub><br>(K)</th><th>A<sub>ij</sub><br>(s<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="3">D<sub>2</sub>CO</td><td>32 502</td><td>4 2 3 - 3 2 2</td><td>233 650.441 (0.0500)</td><td>49.63</td><td>2.69 &times; 10<sup>-4</sup></td></tr><tr><td rowspan="2">CDMS</td><td>4 3 2 - 3 3 1</td><td>234 293.361 (0.0500)</td><td>76.62</td><td>1.58 &times; 10<sup>-4</sup></td></tr><tr><td>4 3 1 - 3 3 0</td><td>234 331.062 (0.0500)</td><td>76.63</td><td>1.58 &times; 10<sup>-4</sup></td></tr><tr><td rowspan="6"><sup>12</sup>CH<sub>3</sub>OH</td><td>32 504</td><td>15 6 9 3 - 16 5 11 3</td><td>217 642.677 (0.0220)</td><td>746</td><td>1.89 &times; 10<sup>-5</sup></td></tr><tr><td rowspan="5">CDMS</td><td>15 6 10 3 - 16 5 12 3</td><td>217 642.678 (0.0220)</td><td>746</td><td>1.89 &times; 10<sup>-5</sup></td></tr><tr><td>4 2 3 1 - 3 1 2 1</td><td>218 440.063 (0.0130)</td><td>45</td><td>5.69 &times; 10<sup>-5</sup></td></tr><tr><td>32 2 30 2 - 32 1 31 1</td><td>234 523.365 (0.1070)</td><td>1256</td><td>8.95 &times; 10<sup>-5</sup></td></tr><tr><td>4 2 3 0 - 5 1 4 0</td><td>234 683.370 (0.0120)</td><td>61</td><td>1.87 &times; 10<sup>-5</sup></td></tr><tr><td>5 4 2 2 - 6 3 3 2</td><td>234 698.519 (0.0150)</td><td>123</td><td>6.34 &times; 10<sup>-6</sup></td></tr><tr><td rowspan="6">CH<sub>3</sub><sup>18</sup>OH</td><td>34 504</td><td>5 0 5 0 - 4 0 4 0</td><td>231 758.446 (0.0300)</td><td>33</td><td>5.33 &times; 10<sup>-5</sup></td></tr><tr><td rowspan="5">CDMS</td><td>5 3 3 0 - 4 3 2 0</td><td>231 796.218 (0.0300)</td><td>83</td><td>3.41 &times; 10<sup>-5</sup></td></tr><tr><td>5 3 2 0 - 4 3 1 0</td><td>231 796.521 (0.0300)</td><td>83</td><td>3.41 &times; 10<sup>-5</sup></td></tr><tr><td>5 3 2 2 - 4 3 1 2</td><td>231 801.304 (0.0300)</td><td>81</td><td>3.42 &times; 10<sup>-5</sup></td></tr><tr><td>5 2 4 0 - 4 2 3 0</td><td>231 801.466 (0.0300)</td><td>71</td><td>4.53 &times; 10<sup>-5</sup></td></tr><tr><td>5 1 4 2 - 4 1 3 2</td><td>231 826.744 (0.0300)</td><td>54</td><td>5.33 &times; 10<sup>-5</sup></td></tr><tr><td>5 2 3 2 - 4 2 2 2</td><td>231 864.501 (0.0300)</td><td>56</td><td>4.41 &times; 10<sup>-5</sup></td></tr><tr><td rowspan="6">CH<sub>3</sub>CN, v<sub>8</sub> = 1</td><td>41 509</td><td>12 2 2 - 11 - 2 2</td><td>221 367.450 (0.0011)</td><td>649</td><td>8.98 &times; 10<sup>-4</sup></td></tr><tr><td rowspan="5">CDMS</td><td>12 - 2 2 - 11 2 2</td><td>221 367.450 (0.0011)</td><td>649</td><td>8.98 &times; 10<sup>-4</sup></td></tr><tr><td>12 4 3 - 11 - 4 3</td><td>221 380.608 (0.0011)</td><td>655</td><td>8.21 &times; 10<sup>-4</sup></td></tr><tr><td>12 - 4 3 - 11 4 3</td><td>221 380.608 (0.0011)</td><td>655</td><td>8.21 &times; 10<sup>-4</sup></td></tr><tr><td>12 1 2 - 11 1 2</td><td>221 387.271 (0.0011)</td><td>615</td><td>9.17 &times; 10<sup>-4</sup></td></tr><tr><td>12 0 2 - 11 0 2</td><td>221 394.085 (0.0011)</td><td>594</td><td>9.24 &times; 10<sup>-4</sup></td></tr><tr><td>12 3 3 - 11 3 3</td><td>221 403.521 (0.0011)</td><td>619</td><td>8.66 &times; 10<sup>-4</sup></td></tr><tr><td>12 1 3 - 11 - 1 3</td><td>221 625.840 (0.0011)</td><td>588</td><td>9.20 &times; 10<sup>-4</sup></td></tr><tr><td rowspan="3">NH<sub>2</sub>CN</td><td>42 003</td><td>11 1 11 0 - 10 1 10 0</td><td>218 461.795 (0.0200)</td><td>77</td><td>1.08 &times; 10<sup>-3</sup></td></tr><tr><td rowspan="2">JPL</td><td>11 0 11 1 - 10 0 10 1</td><td>219 719.651 (0.0200)</td><td>135</td><td>1.08 &times; 10<sup>-3</sup></td></tr><tr><td>11 1 10 0 - 10 1 9 0</td><td>221 361.160 (0.0200)</td><td>78</td><td>1.12 &times; 10<sup>-3</sup></td></tr><tr><td rowspan="6">HN<sup>12</sup>CO</td><td>43 511</td><td>10 1 10 - 9 1 9</td><td>218 985.696 (0.0192)</td><td>101</td><td>1.48 &times; 10<sup>-4</sup></td></tr><tr><td rowspan="5">CDMS</td><td>10 2 9 - 9 2 8</td><td>219 733.850 (0.0300)</td><td>228</td><td>1.35 &times; 10<sup>-4</sup></td></tr><tr><td>10 2 8 - 9 2 7</td><td>219 737.193 (0.0300)</td><td>228</td><td>1.35 &times; 10<sup>-4</sup></td></tr><tr><td>10 0 10 - 9 0 9</td><td>219 798.274 (0.0040)</td><td>58</td><td>1.47 &times; 10<sup>-4</sup></td></tr><tr><td>28 1 28 - 29 0 29</td><td>231 873.255 (0.0064)</td><td>470</td><td>6.68 &times; 10<sup>-5</sup></td></tr><tr><td>10 1 10 9 - 9 1 9 9</td><td>218 984.716 (0.1053)</td><td>101</td><td>1.63 &times; 10<sup>-6</sup></td></tr><tr><td>10 1 10 11 - 9 1 9 10</td><td>218 985.697 (0.0192)</td><td>101</td><td>1.48 &times; 10<sup>-4</sup></td></tr><tr><td>10 1 10 10 - 9 1 9 9</td><td>218 985.705 (0.0191)</td><td>101</td><td>1.46 &times; 10<sup>-4</sup></td></tr><tr><td>10 1 10 9 - 9 1 9 8</td><td>218 985.706 (0.0191)</td><td>101</td><td>1.46 &times; 10<sup>-4</sup></td></tr><tr><td>10 1 10 10 - 9 1 9 10</td><td>218 986.596 (0.0191)</td><td>101</td><td>1.48 &times; 10<sup>-6</sup></td></tr><tr><td>10 1 10 10 - 9 1 9 10</td><td>218 986.596 (0.0191)</td><td>101</td><td>1.48 &times; 10<sup>-6</sup></td></tr><tr><td>10 2 9 9 - 9 2 8 9</td><td>219 739.762 (0.0962)</td><td>231</td><td>1.60 &times; 10<sup>-6</sup></td></tr><tr><td>10 2 9 11 - 9 2 8 10</td><td>219 740.451 (0.0274)</td><td>231</td><td>1.45 &times; 10<sup>-4</sup></td></tr><tr><td>10 2 9 9 - 9 2 8 8</td><td>219 740.456 (0.0274)</td><td>231</td><td>1.43 &times; 10<sup>-4</sup></td></tr><tr><td>10 2 9 10 - 9 2 8 9</td><td>219 740.471 (0.0274)</td><td>231</td><td>1.43 &times; 10<sup>-4</sup></td></tr><tr><td>10 2 9 10 - 9 2 8 10</td><td>219 741.095 (0.0885)</td><td>231</td><td>1.45 &times; 10<sup>-6</sup></td></tr><tr><td>10 2 8 9 - 9 2 7 9</td><td>219 743.054 (0.0966)</td><td>231</td><td>1.60 &times; 10<sup>-6</sup></td></tr><tr><td>10 2 8 11 - 9 2 7 10</td><td>219 743.742 (0.0288)</td><td>231</td><td>1.45 &times; 10<sup>-4</sup></td></tr><tr><td>10 2 8 9 - 9 2 7 8</td><td>219 743.747 (0.0288)</td><td>231</td><td>1.43 &times; 10<sup>-4</sup></td></tr><tr><td>10 2 8 10 - 9 2 7 9</td><td>219 743.762 (0.0288)</td><td>231</td><td>1.43 &times; 10<sup>-4</sup></td></tr><tr><td>10 2 8 10 - 9 2 7 10</td><td>219 744.386 (0.0890)</td><td>231</td><td>1.45 &times; 10<sup>-6</sup></td></tr><tr><td>10 0 10 9 - 9 0 9 9</td><td>219 803.645 (0.1070)</td><td>58</td><td>1.67 &times; 10<sup>-6</sup></td></tr><tr><td>10 0 10 11 - 9 0 9 10</td><td>219 980.439 (0.0171)</td><td>58</td><td>1.51 &times; 10<sup>-4</sup></td></tr><tr><td>10 0 10 10 - 9 0 9 9</td><td>219 804.442 (0.0171)</td><td>58</td><td>1.50 &times; 10<sup>-4</sup></td></tr><tr><td>10 0 10 9 - 9 0 9 8</td><td>219 804.446 (0.0171)</td><td>58</td><td>1.49 &times; 10<sup>-4</sup></td></tr><tr><td>10 0 10 10 - 9 0 9 10</td><td>219 805.163 (0.0965)</td><td>58</td><td>1.51 &times; 10<sup>-6</sup></td></tr></tbody></table>
<table><caption>**Table B.1. continued.**</caption><thead><tr><th>Molecule</th><th>Database entry</th><th>Transition<br>J, K<sub>a</sub>, K<sub>c</sub>, (F)</th><th>Frequency<br>(MHz)</th><th>E<sub>up</sub><br>(K)</th><th>A<sub>ij</sub><br>(s<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="24">CH<sub>3</sub>CH<sub>2</sub>OH</td><td rowspan="12">46 524<br>CDMS</td><td>35 4 31 2 - 35 3 32 2</td><td>218 943.289 (0.0068)</td><td>560</td><td>7.38 &times; 10<sup>-5</sup></td></tr><tr><td>34 6 28 2 - 34 5 29 2</td><td>221 612.904 (0.0080)</td><td>548</td><td>8.84 &times; 10<sup>-5</sup></td></tr><tr><td>31 5 27 1 - 31 4 27 0</td><td>231 789.850 (0.0120)</td><td>506</td><td>2.24 &times; 10<sup>-5</sup></td></tr><tr><td>22 5 18 2 - 22 4 19 2</td><td>231 790.056 (0.0035)</td><td>245</td><td>8.46 &times; 10<sup>-5</sup></td></tr><tr><td>13 5 8 2 - 13 4 9 2</td><td>233 571.051 (0.0032)</td><td>108</td><td>7.54 &times; 10<sup>-5</sup></td></tr><tr><td>14 5 10 2 - 14 4 11 2</td><td>233 601.554 (0.0032)</td><td>120</td><td>7.71 &times; 10<sup>-5</sup></td></tr><tr><td>12 5 8 2 - 12 4 9 2</td><td>234 255.240 (0.0032)</td><td>97</td><td>7.39 &times; 10<sup>-5</sup></td></tr><tr><td>11 5 6 2 - 11 4 7 2</td><td>234 406.433 (0.0033)</td><td>87</td><td>7.17 &times; 10<sup>-5</sup></td></tr><tr><td>10 5 5 2 - 10 4 6 2</td><td>234 666.142 (0.0035)</td><td>78</td><td>6.89 &times; 10<sup>-5</sup></td></tr><tr><td>10 5 6 2 - 10 4 7 2</td><td>234 714.782 (0.0035)</td><td>78</td><td>6.90 &times; 10<sup>-5</sup></td></tr><tr><td>23 1 22 2 - 23 0 23 2</td><td>234 725.620 (0.0090)</td><td>233</td><td>3.17 &times; 10<sup>-5</sup></td></tr><tr><td>9 5 4 2 - 9 4 5 2</td><td>234 852.862 (0.0038)</td><td>69</td><td>6.53 &times; 10<sup>-5</sup></td></tr><tr><td>36 4 32 2 - 36 3 33 2</td><td>234 855.010 (0.0073)</td><td>591</td><td>8.70 &times; 10<sup>-5</sup></td></tr><tr><td>9 5 5 2 - 9 4 6 2</td><td>234 873.873 (0.0038)</td><td>69</td><td>6.54 &times; 10<sup>-5</sup></td></tr><tr><td>14 2 12 1 - 13 3 10 0</td><td>234 882.537 (0.0105)</td><td>155</td><td>3.64 &times; 10<sup>-5</sup></td></tr><tr><td>14 0 14 1 - 13 1 12 0</td><td>235 158.494 (0.0058)</td><td>146</td><td>1.12 &times; 10<sup>-4</sup></td></tr><tr><td rowspan="24">CH<sub>3</sub>NCO, v=0</td><td rowspan="12">57 505<br>CDMS</td><td>25 -1 0 2 - 24 -1 0 2</td><td>217 595.174 (0.0500)</td><td>194</td><td>4.84 &times; 10<sup>-4</sup></td></tr><tr><td>25 0 0 2 - 24 0 0 2</td><td>217 595.174 (0.0500)</td><td>188</td><td>4.85 &times; 10<sup>-4</sup></td></tr><tr><td>25 2 0 1 - 24 2 0 1</td><td>217 652.088 (0.0500)</td><td>171</td><td>4.82 &times; 10<sup>-4</sup></td></tr><tr><td>24 3 0 1 - 23 3 0 1</td><td>217 701.086 (0.0500)</td><td>191</td><td>4.40 &times; 10<sup>-4</sup></td></tr><tr><td>25 1 0 -3 - 24 1 0 -3</td><td>218 002.461 (0.0500)</td><td>258</td><td>4.93 &times; 10<sup>-4</sup></td></tr><tr><td>25 0 0 -3 - 24 0 0 -3</td><td>218 014.630 (0.0500)</td><td>251</td><td>4.84 &times; 10<sup>-4</sup></td></tr><tr><td>25 1 0 3 - 24 1 0 3</td><td>218 069.900 (0.0500)</td><td>257</td><td>4.93 &times; 10<sup>-4</sup></td></tr><tr><td>25 1 24 0 - 24 1 23 0</td><td>218 541.803 (0.0500)</td><td>142</td><td>4.94 &times; 10<sup>-4</sup></td></tr><tr><td>27 -1 0 1 - 26 -1 0 1</td><td>231 793.783 (0.0500)</td><td>175</td><td>6.02 &times; 10<sup>-4</sup></td></tr><tr><td>27 2 26 0 - 26 2 25 0</td><td>234 088.125 (0.0500)</td><td>181</td><td>6.05 &times; 10<sup>-4</sup></td></tr><tr><td>27 0 0 2 - 26 0 0 2</td><td>234 932.492 (0.0500)</td><td>210</td><td>6.11 &times; 10<sup>-4</sup></td></tr><tr><td>27 -3 0 2 - 26 -3 0 2</td><td>235 663.096 (0.0500)</td><td>264</td><td>6.06 &times; 10<sup>-4</sup></td></tr><tr><td>27 2 0 3 - 26 2 0 3</td><td>235 801.163 (0.0500)</td><td>296</td><td>6.12 &times; 10<sup>-4</sup></td></tr><tr><td>27 2 0 -3 - 26 2 0 -3</td><td>235 803.211 (0.0500)</td><td>297</td><td>6.12 &times; 10<sup>-4</sup></td></tr><tr><td rowspan="24">HOCH<sub>2</sub>CN</td><td rowspan="12">57 512<br>CDMS</td><td>24 2 23 1 - 23 2 22 1</td><td>218 994.156 (0.0009)</td><td>143</td><td>3.23 &times; 10<sup>-4</sup></td></tr><tr><td>24 8 17 1 - 23 8 16 1</td><td>221 334.546 (0.0009)</td><td>227</td><td>2.98 &times; 10<sup>-4</sup></td></tr><tr><td>24 8 16 1 - 23 8 15 1</td><td>221 334.546 (0.0009)</td><td>227</td><td>2.98 &times; 10<sup>-4</sup></td></tr><tr><td>24 9 15 1 - 23 9 14 1</td><td>221 344.331 (0.0010)</td><td>251</td><td>2.88 &times; 10<sup>-4</sup></td></tr><tr><td>24 9 16 1 - 23 9 15 1</td><td>221 344.331 (0.0010)</td><td>251</td><td>2.88 &times; 10<sup>-4</sup></td></tr><tr><td>24 10 14 1 - 23 10 13 1</td><td>221 372.125 (0.0010)</td><td>277</td><td>2.77 &times; 10<sup>-4</sup></td></tr><tr><td>24 10 15 1 - 23 10 14 1</td><td>221 372.125 (0.0010)</td><td>277</td><td>2.77 &times; 10<sup>-4</sup></td></tr><tr><td>24 6 19 1 - 23 6 18 1</td><td>221 406.933 (0.0009)</td><td>188</td><td>3.15 &times; 10<sup>-4</sup></td></tr><tr><td>24 6 18 1 - 23 6 17 1</td><td>221 407.170 (0.0009)</td><td>188</td><td>3.15 &times; 10<sup>-4</sup></td></tr><tr><td>24 11 13 1 - 23 11 12 1</td><td>221 413.638 (0.0010)</td><td>306</td><td>2.65 &times; 10<sup>-4</sup></td></tr><tr><td>24 11 14 1 - 23 11 13 1</td><td>221 413.638 (0.0010)</td><td>306</td><td>2.65 &times; 10<sup>-4</sup></td></tr><tr><td>24 12 12 1 - 23 12 11 1</td><td>221 466.304 (0.0011)</td><td>338</td><td>2.52 &times; 10<sup>-4</sup></td></tr><tr><td>24 12 13 1 - 23 12 12 1</td><td>221 466.304 (0.0011)</td><td>338</td><td>2.52 &times; 10<sup>-4</sup></td></tr><tr><td>24 3 22 1 - 23 3 21 1</td><td>221 466.604 (0.0009)</td><td>151</td><td>3.32 &times; 10<sup>-4</sup></td></tr><tr><td>24 8 17 0 - 23 8 16 0</td><td>221 480.199 (0.0009)</td><td>222</td><td>2.95 &times; 10<sup>-4</sup></td></tr><tr><td>24 8 16 0 - 23 8 15 0</td><td>221 480.199 (0.0009)</td><td>222</td><td>2.95 &times; 10<sup>-4</sup></td></tr><tr><td>24 9 15 0 - 23 9 14 0</td><td>221 488.785 (0.0010)</td><td>245</td><td>2.86 &times; 10<sup>-4</sup></td></tr><tr><td>24 9 16 0 - 23 9 15 0</td><td>221 488.785 (0.0010)</td><td>245</td><td>2.86 &times; 10<sup>-4</sup></td></tr><tr><td>24 7 18 0 - 23 7 17 0</td><td>221 497.997 (0.0009)</td><td>201</td><td>3.04 &times; 10<sup>-4</sup></td></tr><tr><td>24 7 17 0 - 23 7 16 0</td><td>221 498.002 (0.0009)</td><td>201</td><td>3.04 &times; 10<sup>-4</sup></td></tr><tr><td>24 10 14 0 - 23 10 13 0</td><td>221 515.800 (0.0010)</td><td>272</td><td>2.75 &times; 10<sup>-4</sup></td></tr><tr><td>24 10 15 0 - 23 10 14 0</td><td>221 515.800 (0.0010)</td><td>272</td><td>2.75 &times; 10<sup>-4</sup></td></tr><tr><td>24 13 11 1 - 23 13 10 1</td><td>221 528.501 (0.0011)</td><td>372</td><td>2.38 &times; 10<sup>-4</sup></td></tr><tr><td>24 13 12 1 - 23 13 11 1</td><td>221 528.501 (0.0011)</td><td>372</td><td>2.38 &times; 10<sup>-4</sup></td></tr><tr><td>24 5 20 1 - 23 5 19 1</td><td>221 533.058 (0.0009)</td><td>173</td><td>3.22 &times; 10<sup>-4</sup></td></tr><tr><td>24 5 19 1 - 23 5 18 1</td><td>221 542.097 (0.0009)</td><td>173</td><td>3.22 &times; 10<sup>-4</sup></td></tr><tr><td>24 11 13 0 - 23 11 12 0</td><td>221 556.802 (0.0010)</td><td>301</td><td>2.63 &times; 10<sup>-4</sup></td></tr></tbody></table>
<table><caption>**Table B.1.** continued.</caption><thead><tr><th>Molecule</th><th>Database entry</th><th>Transition<br>J, K<sub>a</sub>, K<sub>c</sub>, (F)</th><th>Frequency<br>(MHz)</th><th>E<sub>up</sub><br>(K)</th><th>A<sub>ij</sub><br>(s<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="10"></td><td rowspan="10"></td><td>24 11 14 0 - 23 11 13 0</td><td>221 556.802 (0.0010)</td><td>301</td><td>2.63 × 10<sup>-4</sup></td></tr><tr><td>24 6 19 0 - 23 6 18 0</td><td>221 557.660 (0.0009)</td><td>183</td><td>3.12 × 10<sup>-4</sup></td></tr><tr><td>24 6 18 0 - 23 6 17 0</td><td>221 557.911 (0.0009)</td><td>183</td><td>3.12 × 10<sup>-4</sup></td></tr><tr><td>24 12 12 0 - 23 12 11 0</td><td>221 609.140 (0.0010)</td><td>332</td><td>2.50 × 10<sup>-4</sup></td></tr><tr><td>24 12 13 0 - 23 12 12 0</td><td>221 609.140 (0.0010)</td><td>332</td><td>2.50 × 10<sup>-4</sup></td></tr><tr><td>25 2 23 1 - 24 2 22 1</td><td>234 584.932 (0.0300)</td><td>157</td><td>4.36 × 10<sup>-4</sup></td></tr><tr><td>25 2 23 0 - 24 2 22 0</td><td>235 112.110 (0.0300)</td><td>152</td><td>3.94 × 10<sup>-4</sup></td></tr><tr><td rowspan="20">CH<sub>3</sub>OCHO</td><td rowspan="20">60 003<br>JPL</td><td>18 10 9 3 - 17 10 8 3</td><td>219 822.126 (0.1000)</td><td>355</td><td>1.11 × 10<sup>-4</sup></td></tr><tr><td>18 10 8 3 - 17 10 7 3</td><td>219 822.126 (0.1000)</td><td>355</td><td>1.11 × 10<sup>-4</sup></td></tr><tr><td>18 11 7 0 - 17 11 6 0</td><td>221 433.019 (0.1000)</td><td>181</td><td>1.03 × 10<sup>-4</sup></td></tr><tr><td>18 11 8 0 - 17 11 7 0</td><td>221 433.019 (0.1000)</td><td>181</td><td>1.03 × 10<sup>-4</sup></td></tr><tr><td>18 10 8 2 - 17 10 7 2</td><td>221 649.411 (0.1000)</td><td>167</td><td>1.14 × 10<sup>-4</sup></td></tr><tr><td>19 4 16 4 - 18 4 15 4</td><td>231 896.060 (0.1000)</td><td>310</td><td>1.79 × 10<sup>-4</sup></td></tr><tr><td>19 8 12 4 - 18 8 11 4</td><td>233 627.478 (0.1000)</td><td>341</td><td>1.59 × 10<sup>-4</sup></td></tr><tr><td>19 10 10 0 - 18 10 9 0</td><td>234 124.883 (0.1000)</td><td>179</td><td>1.40 × 10<sup>-4</sup></td></tr><tr><td>19 10 9 0 - 18 10 8 0</td><td>234 124.883 (0.1000)</td><td>179</td><td>1.40 × 10<sup>-4</sup></td></tr><tr><td>19 10 10 1 - 18 10 9 1</td><td>234 134.600 (0.0500)</td><td>178</td><td>1.40 × 10<sup>-4</sup></td></tr><tr><td>19 6 14 3 - 18 6 13 3</td><td>234 336.107 (0.1000)</td><td>324</td><td>1.75 × 10<sup>-4</sup></td></tr><tr><td>19 5 15 3 - 18 5 14 3</td><td>234 381.269 (0.1000)</td><td>316</td><td>1.80 × 10<sup>-4</sup></td></tr><tr><td>19 7 13 4 - 18 7 12 4</td><td>234 441.264 (0.1000)</td><td>332</td><td>1.68 × 10<sup>-4</sup></td></tr><tr><td>19 9 10 2 - 18 9 9 2</td><td>234 486.395 (0.1000)</td><td>166</td><td>1.51 × 10<sup>-4</sup></td></tr><tr><td>19 9 11 0 - 18 9 10 0</td><td>234 502.241 (0.0009)</td><td>166</td><td>1.51 × 10<sup>-4</sup></td></tr><tr><td>19 9 10 0 - 18 9 9 0</td><td>234 502.432 (0.0009)</td><td>166</td><td>1.51 × 10<sup>-4</sup></td></tr><tr><td>19 9 11 1 - 18 9 10 1</td><td>234 508.614 (0.1000)</td><td>166</td><td>1.51 × 10<sup>-4</sup></td></tr><tr><td>19 8 11 2 - 18 8 10 2</td><td>235 029.952 (0.1000)</td><td>155</td><td>1.61 × 10<sup>-4</sup></td></tr><tr><td>19 8 12 0 - 18 8 11 0</td><td>235 046.493 (0.1000)</td><td>155</td><td>1.61 × 10<sup>-4</sup></td></tr><tr><td>19 8 11 0 - 18 8 10 0</td><td>235 051.378 (0.1000)</td><td>155</td><td>1.61 × 10<sup>-4</sup></td></tr><tr><td>19 8 12 1 - 18 8 11 1</td><td>235 051.378 (0.1000)</td><td>155</td><td>1.61 × 10<sup>-4</sup></td></tr><tr><td>19 6 13 5 - 18 6 12 5</td><td>235 084.738 (0.1000)</td><td>324</td><td>1.76 × 10<sup>-4</sup></td></tr><tr><td>20 3 18 4 - 19 3 17 4</td><td>235 200.422 (0.1000)</td><td>314</td><td>1.90 × 10<sup>-4</sup></td></tr><tr><td>19 5 15 4 - 18 5 14 4</td><td>235 633.058 (0.1000)</td><td>316</td><td>1.81 × 10<sup>-4</sup></td></tr><tr><td>19 7 13 0 - 18 7 12 0</td><td>235 844.544 (0.1000)</td><td>145</td><td>1.71 × 10<sup>-4</sup></td></tr><tr><td>19 7 13 1 - 18 7 12 1</td><td>235 865.969 (0.1000)</td><td>145</td><td>1.67 × 10<sup>-4</sup></td></tr><tr><td>20 2 18 3 - 19 2 17 3</td><td>235 904.655 (0.1000)</td><td>315</td><td>1.91 × 10<sup>-4</sup></td></tr><tr><td>21 2 20 3 - 20 2 19 3</td><td>235 919.352 (0.1000)</td><td>319</td><td>1.94 × 10<sup>-4</sup></td></tr><tr><td>19 7 12 0 - 18 7 11 0</td><td>235 932.379 (0.1000)</td><td>145</td><td>1.72 × 10<sup>-4</sup></td></tr><tr><td rowspan="20">a-(CH<sub>2</sub>OH)<sub>2</sub></td><td rowspan="20">62 503<br>CDMS</td><td>22 14 8 0 - 21 14 7 1</td><td>218 468.381 (0.0044)</td><td>221</td><td>1.51 × 10<sup>-4</sup></td></tr><tr><td>22 14 9 0 - 21 14 8 1</td><td>218 468.381 (0.0044)</td><td>221</td><td>1.51 × 10<sup>-4</sup></td></tr><tr><td>20 4 16 1 - 19 4 15 0</td><td>219 764.925 (0.0040)</td><td>114</td><td>2.45 × 10<sup>-4</sup></td></tr><tr><td>22 8 14 0 - 21 8 13 1</td><td>219 809.406 (0.0026)</td><td>156</td><td>2.24 × 10<sup>-4</sup></td></tr><tr><td>28 12 16 0 - 28 11 17 0</td><td>233 426.949 (0.0036)</td><td>270</td><td>3.11 × 10<sup>-5</sup></td></tr><tr><td>28 12 17 0 - 28 11 18 0</td><td>233 427.018 (0.0036)</td><td>270</td><td>3.10 × 10<sup>-5</sup></td></tr><tr><td>28 12 16 1 - 28 11 17 1</td><td>233 451.651 (0.0036)</td><td>271</td><td>3.32 × 10<sup>-5</sup></td></tr><tr><td>28 12 17 1 - 28 11 18 1</td><td>233 451.723 (0.0036)</td><td>271</td><td>3.32 × 10<sup>-5</sup></td></tr><tr><td>22 5 18 1 - 21 5 17 0</td><td>233 536.696 (0.0025)</td><td>138</td><td>2.93 × 10<sup>-4</sup></td></tr><tr><td>22 7 16 1 - 21 7 15 0</td><td>233 561.784 (0.0024)</td><td>149</td><td>2.79 × 10<sup>-4</sup></td></tr><tr><td>22 6 17 1 - 21 6 16 0</td><td>234 264.446 (0.0026)</td><td>143</td><td>2.84 × 10<sup>-4</sup></td></tr><tr><td>51 13 39 1 - 51 12 40 1</td><td>235 170.476 (0.0288)</td><td>737</td><td>4.73 × 10<sup>-5</sup></td></tr><tr><td>23 3 21 1 - 22 3 20 0</td><td>235 170.573 (0.0023)</td><td>139</td><td>2.91 × 10<sup>-4</sup></td></tr><tr><td>22 6 16 1 - 21 6 15 0</td><td>235 304.050 (0.0026)</td><td>143</td><td>2.90 × 10<sup>-4</sup></td></tr><tr><td>14 5 9 1 - 13 4 10 1</td><td>235 326.413 (0.0021)</td><td>64</td><td>2.39 × 10<sup>-5</sup></td></tr><tr><td>21 4 18 0 - 20 3 17 0</td><td>235 327.161 (0.0027)</td><td>122</td><td>6.30 × 10<sup>-5</sup></td></tr><tr><td>23 2 21 1 - 22 2 20 0</td><td>235 600.178 (0.0022)</td><td>139</td><td>3.28 × 10<sup>-4</sup></td></tr><tr><td>24 4 21 0 - 23 4 20 1</td><td>235 620.372 (0.0027)</td><td>155</td><td>2.88 × 10<sup>-4</sup></td></tr><tr><td>26 1 26 0 - 25 1 25 1</td><td>235 834.239 (0.0040)</td><td>159</td><td>3.22 × 10<sup>-4</sup></td></tr><tr><td>26 0 26 0 - 25 0 25 1</td><td>235 834.327 (0.0040)</td><td>159</td><td>3.22 × 10<sup>-4</sup></td></tr></tbody></table>
<figure><img src="image_14.png" /><figcaption>CH<sub>3</sub>CH<sub>2</sub>OH - Serpens SMM1-a</figcaption></figure>

**Fig. B.4.** Identified lines of CH<sub>3</sub>CH<sub>2</sub>OH towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic spectrum overplotted in blue ($N_{\text{T}} = (4.1 \pm 0.9) \times 10^{15} \text{ cm}^{-2}$, $T_{\text{ex}} = 210 \pm 25 \text{ K}$). The transition is indicated at the top of each panel and the upper state energy is given in the top left of each panel.
<figure><img src="image_15.png" /><figcaption>**Fig. B.5.** Identified lines of CH<sub>3</sub>OCHO towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic spectrum overplotted in blue (*N*<sub>T</sub> = (7.4 ± 0.7) × 10<sup>15</sup> cm<sup>-2</sup>, *T*<sub>ex</sub> = 215 ± 20 K). The transition is indicated at the top of each panel and the upper state energy is given in the top left of each panel.</figcaption></figure>
<figure><img src="image_16.png" /></figure>

**Fig. B.6.** Identified lines of D₂CO towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic spectrum overplotted in blue ($N_{\text{T}} = (5.4 \pm 0.5) \times 10^{14} \text{ cm}^{-2}$, $T_{\text{ex}} = [200]$ K). The transition is indicated at the top of each panel and the upper state energy is given in the top left of each panel.

<figure><img src="image_17.png" /></figure>

**Fig. B.7.** Identified lines of HN¹²CO towards SMM1. The observed spectrum is plotted in black and the line position is indicated by the red dotted line. Because these lines are optically thick, no synthetic fit is given. The transition is indicated at the top of each panel and the upper state energy is given in the top left of each panel.

<figure><img src="image_18.png" /></figure>

**Fig. B.8.** Identified lines of HN¹³CO towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic spectrum overplotted in blue ($N_{\text{T}} = (1.9 \pm 0.3) \times 10^{14} \text{ cm}^{-2}$, $T_{\text{ex}} = 190 \pm 30$ K). The transition is indicated at the top of each panel and the upper state energy is given in the top left of each panel.
<figure><img src="image_19.png" /><figcaption>**Fig. B.9.** Identified lines of ¹²CH₃OH towards SMM1. The observed spectrum is plotted in black and the line position is indicated by the red dotted line. Because these lines are optically thick, no synthetic fit is given. The transition is indicated at the top of each panel and the upper state energy is given in the top left of each panel.</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>**Fig. B.10.** Identified lines of CH₃¹⁸OH towards SMM1. The observed spectrum is plotted in black, with the best-fit synthetic spectrum overplotted in blue ($N_T = (2.0 \pm 0.7) \times 10^{15} \text{ cm}^{-2}$, $T_{ex} = 250 \pm 60$ K). The transition is indicated at the top of each panel and the upper state energy is given in the top left of each panel.</figcaption></figure>
Appendix C: Analysis of PILS data

<figure><img src="image_21.png" /><figcaption>**Fig. C.1.** Spectral lines of HOCH₂CN in the PILS spectrum towards IRAS 16293B at the full-beam offset position. The observed spectrum is plotted in black, with synthetic spectra overplotted ($N_T = [1.0 \times 10^{15}]$ cm⁻², $T_{ex} = 150$, blue, and 300 K, red). The synthetic spectrum of the entire molecular inventory determined with PILS data towards this position is plotted in green. All covered transitions with $A_{ij} \ge 1.0 \times 10^{-3}$ s⁻¹ that are not blended are shown. The transition is indicated at the top of each panel and the upper state energy is given in the top left of each panel. HOCH₂CN is not detected in the full-beam offset position spectrum towards IRAS 16293B.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>HOCH₂CN - IRAS 16293-2422B - full-beam offset position</figcaption></figure>

Fig. C.1. continued.
<figure><img src="image_23.png" /><figcaption>HOCH₂CN - IRAS 16293-2422B - full-beam offset position</figcaption></figure>

Fig. C.1. continued.
<figure><img src="image_24.png" /><figcaption>HOCH₂CN - IRAS 16293-2422B - half-beam offset position</figcaption></figure>

**Fig. C.2.** Spectral lines of HOCH₂CN in the PILS spectrum towards IRAS 16293B at the half-beam offset position. The observed spectrum is plotted in black, with synthetic spectra overplotted ($N_T = [3.0 \times 10^{15}]$ cm⁻², $T_{ex} = 150$, blue, and 300 K, red). All covered transitions with $A_{ij} \ge 1.0 \times 10^{-3}$ s⁻¹ that are not blended are shown. The transition is indicated at the top of each panel and the upper state energy is given in the top left of each panel. HOCH₂CN is detected in the half-beam offset position spectrum towards IRAS 16293B.
<figure><img src="image_25.png" /><figcaption>HOCH₂CN - IRAS 16293-2422B - half-beam offset position</figcaption></figure>

Fig. C.2. continued.
<figure><img src="image_26.png" /><figcaption>HOCH₂CN - IRAS 16293-2422B - half-beam offset position</figcaption></figure>

Fig. C.2. continued.
<table><caption>**Table C.1.** Molecules and parameters used for the IRAS 16293B synthetic spectrum.</caption><thead><tr><th>Molecule</th><th>Name</th><th>Tag</th><th>Database</th><th>N<sub>T</sub><br>(cm<sup>-2</sup>)</th><th>T<sub>ex</sub><br>(K)</th></tr></thead><tbody><tr><td>CCH</td><td>Ethynyl radical</td><td>25 501</td><td>CDMS</td><td>3.00 × 10<sup>13</sup></td><td>120</td></tr><tr><td>HCN</td><td>Hydrogen cyanide</td><td>27 501</td><td>CDMS</td><td>5.00 × 10<sup>16</sup></td><td>120</td></tr><tr><td>HNC</td><td>Hydrogen isocyanide</td><td>27 502</td><td>CDMS</td><td>5.00 × 10<sup>16</sup></td><td>120</td></tr><tr><td>H<sup>13</sup>CN</td><td>Hydrogen cyanide</td><td>28 501</td><td>CDMS</td><td>2.00 × 10<sup>14</sup></td><td>300</td></tr><tr><td>CO</td><td>Carbon monoxide</td><td>28 503</td><td>CDMS</td><td>1.00 × 10<sup>20</sup></td><td>100</td></tr><tr><td>HC<sup>15</sup>N</td><td>Hydrogen cyanide</td><td>28 506</td><td>CDMS</td><td>2.00 × 10<sup>14</sup></td><td>300</td></tr><tr><td>DNC</td><td>Hydrogen cyanide</td><td>28 508</td><td>CDMS</td><td>7.00 × 10<sup>14</sup></td><td>300</td></tr><tr><td><sup>13</sup>CO</td><td>Carbon monoxide</td><td>29 501</td><td>CDMS</td><td>3.10 × 10<sup>19</sup></td><td>100</td></tr><tr><td>C<sup>17</sup>O</td><td>Carbon monoxide</td><td>29 503</td><td>CDMS</td><td>8.00 × 10<sup>16</sup></td><td>100</td></tr><tr><td>H<sup>13</sup>C<sup>15</sup></td><td>Hydrogen cyanide</td><td>29 512</td><td>CDMS</td><td>2.00 × 10<sup>14</sup></td><td>300</td></tr><tr><td>HNCH<sub>2</sub></td><td>Methanimine</td><td>29 518</td><td>CDMS</td><td>8.00 × 10<sup>14</sup></td><td>100</td></tr><tr><td>NO</td><td>Nitrogen oxide</td><td>30 008</td><td>JPL</td><td>2.00 × 10<sup>16</sup></td><td>100</td></tr><tr><td>H<sub>2</sub>CO</td><td>Formaldehyde</td><td>30 501</td><td>CDMS</td><td>1.80 × 10<sup>18</sup></td><td>105</td></tr><tr><td>C<sup>18</sup>O</td><td>Carbon monoxide</td><td>30 502</td><td>CDMS</td><td>1.00 × 10<sup>17</sup></td><td>100</td></tr><tr><td>DCO<sup>+</sup></td><td>Formyl radical</td><td>30 510</td><td>CDMS</td><td>3.00 × 10<sup>12</sup></td><td>29</td></tr><tr><td>CH<sub>3</sub>NH<sub>2</sub></td><td>Methylamine</td><td>31 008</td><td>JPL</td><td>5.30 × 10<sup>14</sup></td><td>100</td></tr><tr><td>HDCO</td><td>Formaldehyde</td><td>31 501</td><td>CDMS</td><td>1.30 × 10<sup>17</sup></td><td>105</td></tr><tr><td>H<sup>13</sup>CO</td><td>Formaldehyde</td><td>31 503</td><td>CDMS</td><td>3.60 × 10<sup>16</sup></td><td>105</td></tr><tr><td>D<sub>2</sub>CO</td><td>Formaldehyde</td><td>32 502</td><td>CDMS</td><td>1.60 × 10<sup>16</sup></td><td>105</td></tr><tr><td>H<sub>2</sub>C<sup>18</sup>O</td><td>Formaldehyde</td><td>32 503</td><td>CDMS</td><td>2.50 × 10<sup>15</sup></td><td>105</td></tr><tr><td>CH<sub>3</sub>OH</td><td>Methanol</td><td>32 504</td><td>CDMS</td><td>2.00 × 10<sup>19</sup></td><td>300</td></tr><tr><td>CH<sub>2</sub>DOH</td><td>Methanol</td><td>33 004</td><td>JPL</td><td>7.10 × 10<sup>17</sup></td><td>300</td></tr><tr><td><sup>13</sup>CH<sub>3</sub>OH</td><td>Methanol</td><td>33 502</td><td>CDMS</td><td>4.00 × 10<sup>16</sup></td><td>300</td></tr><tr><td>NH<sub>2</sub>OH</td><td>Hydroxylamine</td><td>33 503</td><td>CDMS</td><td>3.70 × 10<sup>14</sup></td><td>100</td></tr><tr><td>D<sub>2</sub><sup>13</sup>CO</td><td>Formaldehyde</td><td>33 506</td><td>CDMS</td><td>2.20 × 10<sup>14</sup></td><td>105</td></tr><tr><td>HDC<sup>18</sup>O</td><td>Formaldehyde</td><td>33 510</td><td>CDMS</td><td>1.40 × 10<sup>14</sup></td><td>105</td></tr><tr><td>H<sub>2</sub>S</td><td>Hydrogen sulphide</td><td>34 502</td><td>CDMS</td><td>1.00 × 10<sup>18</sup></td><td>125</td></tr><tr><td>CH<sub>3</sub><sup>18</sup>OH</td><td>Methanol</td><td>34 504</td><td>CDMS</td><td>2.00 × 10<sup>16</sup></td><td>300</td></tr><tr><td>HDS</td><td>Hydrogen sulphide</td><td>35 502</td><td>CDMS</td><td>2.00 × 10<sup>16</sup></td><td>125</td></tr><tr><td>HD<sup>34</sup>S</td><td>Hydrogen sulphide</td><td>37 503</td><td>CDMS</td><td>1.00 × 10<sup>15</sup></td><td>125</td></tr><tr><td>c-C<sub>3</sub>H<sub>2</sub></td><td>Cyclopropenylidene</td><td>38 508</td><td>CDMS</td><td>2.00 × 10<sup>14</sup></td><td>100</td></tr><tr><td>CH<sub>3</sub>CCH</td><td>Propyne</td><td>40 502</td><td>CDMS</td><td>6.80 × 10<sup>15</sup></td><td>100</td></tr><tr><td>CH<sub>3</sub>CN</td><td>Acetonitrile</td><td>41 505</td><td>CDMS</td><td>4.00 × 10<sup>16</sup></td><td>120</td></tr><tr><td>CH<sub>3</sub>CN v<sub>8</sub> = 1</td><td>Acetonitrile</td><td>41 509</td><td>CDMS</td><td>4.00 × 10<sup>16</sup></td><td>120</td></tr><tr><td>CH<sub>3</sub>NC</td><td>Methyl isocyanide</td><td>41 514</td><td>CDMS</td><td>2.00 × 10<sup>14</sup></td><td>150</td></tr><tr><td>H<sub>2</sub>CCO</td><td>Ketene</td><td>42 501</td><td>CDMS</td><td>4.80 × 10<sup>16</sup></td><td>125</td></tr><tr><td>HNCNH</td><td>Carbodiimide</td><td>42 506</td><td>CDMS</td><td>2.40 × 10<sup>16</sup></td><td>300</td></tr><tr><td><sup>13</sup>CH<sub>3</sub>CN</td><td>Acetonitrile</td><td>42 508</td><td>CDMS</td><td>3.30 × 10<sup>14</sup></td><td>130</td></tr><tr><td>CH<sub>3</sub><sup>13</sup>CN</td><td>Acetonitrile</td><td>42 509</td><td>CDMS</td><td>3.00 × 10<sup>14</sup></td><td>130</td></tr><tr><td>CH<sub>3</sub>C<sup>15</sup>N</td><td>Acetonitrile</td><td>42 510</td><td>CDMS</td><td>8.70 × 10<sup>13</sup></td><td>130</td></tr><tr><td>CH<sub>2</sub>DCN</td><td>Acetonitrile</td><td>42 511</td><td>CDMS</td><td>5.60 × 10<sup>14</sup></td><td>130</td></tr><tr><td>H<sub>2</sub>C<sup>13</sup>CO</td><td>Ketene</td><td>43 505</td><td>CDMS</td><td>7.10 × 10<sup>14</sup></td><td>125</td></tr><tr><td>H<sub>2</sub><sup>13</sup>CCO</td><td>Ketene</td><td>43 506</td><td>CDMS</td><td>7.10 × 10<sup>14</sup></td><td>125</td></tr><tr><td>HDC<sub>2</sub>O</td><td>Ketene</td><td>43 507</td><td>CDMS</td><td>2.00 × 10<sup>15</sup></td><td>125</td></tr><tr><td>HNCO</td><td>Isocyanic acid</td><td>43 511</td><td>CDMS</td><td>3.70 × 10<sup>16</sup></td><td>300</td></tr><tr><td>CHD<sub>2</sub>CN</td><td>Acetonitrile</td><td>43 514</td><td>CDMS</td><td>1.20 × 10<sup>14</sup></td><td>130</td></tr><tr><td>H<sub>2</sub>N<sup>13</sup>CN</td><td>Cyanamide</td><td>43 515</td><td>CDMS</td><td>3.00 × 10<sup>13</sup></td><td>300</td></tr><tr><td>N<sub>2</sub>O</td><td>Nitrous oxide</td><td>44 004</td><td>JPL</td><td>5.00 × 10<sup>16</sup></td><td>100</td></tr><tr><td>DNCO</td><td>Isocyanic acid</td><td>44 006</td><td>JPL</td><td>3.00 × 10<sup>14</sup></td><td>300</td></tr><tr><td>HN<sup>13</sup>CO</td><td>Isocyanic acid</td><td>44 008</td><td>JPL</td><td>4.00 × 10<sup>14</sup></td><td>300</td></tr><tr><td>CS</td><td>Carbon monosulphide</td><td>44 501</td><td>CDMS</td><td>1.00 × 10<sup>16</sup></td><td>125</td></tr><tr><td>c-C<sub>2</sub>H<sub>4</sub>O</td><td>Ethylene oxide</td><td>44 504</td><td>CDMS</td><td>4.10 × 10<sup>15</sup></td><td>125</td></tr><tr><td>SiO</td><td>Silicon monoxide</td><td>44 505</td><td>CDMS</td><td>7.00 × 10<sup>13</sup></td><td>300</td></tr><tr><td>s-H<sub>2</sub>CCHOH</td><td>Vinylalcohol</td><td>44 506</td><td>CDMS</td><td>1.20 × 10<sup>17</sup></td><td>125</td></tr></tbody></table>
**Table C.1.** continued.

<table><thead><tr><th>Molecule</th><th>Name</th><th>Tag</th><th>Database</th><th>N<sub>T</sub><br>(cm<sup>-2</sup>)</th><th>T<sub>ex</sub><br>(K)</th></tr></thead><tbody><tr><td>a-H<sub>2</sub>CCHOH</td><td>Vinylalcohol</td><td>44 507</td><td>CDMS</td><td>1.20 × 10<sup>17</sup></td><td>125</td></tr><tr><td>C<sup>33</sup>S</td><td>Carbon monosulphide</td><td>45 502</td><td>CDMS</td><td>1.00 × 10<sup>14</sup></td><td>125</td></tr><tr><td>NH<sub>2</sub>CHO</td><td>Formamide</td><td>45 512</td><td>CDMS</td><td>1.00 × 10<sup>16</sup></td><td>300</td></tr><tr><td>CH<sub>3</sub>CDO</td><td>Acetaldehyde</td><td>45 524</td><td>CDMS</td><td>7.40 × 10<sup>15</sup></td><td>125</td></tr><tr><td>CH<sub>2</sub>DCHO</td><td>Acetaldehyde</td><td>45 525</td><td>CDMS</td><td>6.20 × 10<sup>15</sup></td><td>125</td></tr><tr><td>C<sup>34</sup>S</td><td>Carbon monosulphide</td><td>46 501</td><td>CDMS</td><td>3.00 × 10<sup>14</sup></td><td>125</td></tr><tr><td>t-HCOOH</td><td>Formic acid</td><td>46 506</td><td>CDMS</td><td>5.08 × 10<sup>16</sup></td><td>300</td></tr><tr><td>H<sub>2</sub>CS</td><td>Thioformaldehyde</td><td>46 509</td><td>CDMS</td><td>1.50 × 10<sup>15</sup></td><td>125</td></tr><tr><td>NH<sub>2</sub><sup>13</sup>CHO</td><td>Formamide</td><td>46 512</td><td>CDMS</td><td>1.00 × 10<sup>14</sup></td><td>300</td></tr><tr><td>CH<sub>3</sub>OCH<sub>3</sub></td><td>Dimethyl ether</td><td>46 514</td><td>CDMS</td><td>3.00 × 10<sup>17</sup></td><td>125</td></tr><tr><td>NH<sub>2</sub>CDO</td><td>Formamide</td><td>46 520</td><td>CDMS</td><td>1.40 × 10<sup>14</sup></td><td>300</td></tr><tr><td>cis-NHDCHO</td><td>Formamide</td><td>46 521</td><td>CDMS</td><td>1.40 × 10<sup>14</sup></td><td>300</td></tr><tr><td>trans-NHDCHO</td><td>Formamide</td><td>46 522</td><td>CDMS</td><td>1.20 × 10<sup>14</sup></td><td>300</td></tr><tr><td>C<sub>2</sub>H<sub>5</sub>OH</td><td>Ethanol</td><td>46 524</td><td>CDMS</td><td>2.30 × 10<sup>17</sup></td><td>300</td></tr><tr><td>HONO</td><td>Nitrous acid</td><td>47 007</td><td>JPL</td><td>9.00 × 10<sup>14</sup></td><td>100</td></tr><tr><td>t-H<sup>13</sup>COOH</td><td>Formic acid</td><td>47 503</td><td>CDMS</td><td>8.30 × 10<sup>14</sup></td><td>300</td></tr><tr><td>HDCS</td><td>Thioformaldehyde</td><td>47 504</td><td>CDMS</td><td>1.50 × 10<sup>14</sup></td><td>125</td></tr><tr><td>CH<sub>3</sub><sup>13</sup>CH<sub>2</sub>OH</td><td>Ethanol</td><td>47 511</td><td>CDMS</td><td>4.60 × 10<sup>14</sup></td><td>300</td></tr><tr><td><sup>13</sup>CH<sub>3</sub>CH<sub>2</sub>OH</td><td>Ethanol</td><td>47 512</td><td>CDMS</td><td>4.60 × 10<sup>14</sup></td><td>300</td></tr><tr><td>CH<sub>3</sub>CH<sub>2</sub>OD</td><td>Ethanol</td><td>47 515</td><td>CDMS</td><td>5.75 × 10<sup>14</sup></td><td>300</td></tr><tr><td>CH<sub>3</sub>CHDOH</td><td>Ethanol</td><td>47 516</td><td>CDMS</td><td>1.15 × 10<sup>15</sup></td><td>300</td></tr><tr><td>a-CH<sub>2</sub>DCH<sub>2</sub>OH</td><td>Ethanol</td><td>47 517</td><td>CDMS</td><td>1.34 × 10<sup>15</sup></td><td>300</td></tr><tr><td>s-CH<sub>2</sub>DCH<sub>2</sub>OH</td><td>Ethanol</td><td>47 518</td><td>CDMS</td><td>6.51 × 10<sup>14</sup></td><td>300</td></tr><tr><td>SO</td><td>sulphur monoxide</td><td>48 501</td><td>CDMS</td><td>5.00 × 10<sup>14</sup></td><td>125</td></tr><tr><td>C<sup>36</sup>S</td><td>Carbon monosulphide</td><td>48 503</td><td>CDMS</td><td>2.00 × 10<sup>13</sup></td><td>125</td></tr><tr><td>CH<sub>3</sub>SH</td><td>Methyl mercaptan</td><td>48 510</td><td>CDMS</td><td>5.50 × 10<sup>15</sup></td><td>125</td></tr><tr><td>CH<sub>3</sub><sup>35</sup>Cl</td><td>Chloromethane</td><td>50 007</td><td>JPL</td><td>3.10 × 10<sup>13</sup></td><td>125</td></tr><tr><td>HC<sub>3</sub>N</td><td>Cyanoacetylene</td><td>51 501</td><td>CDMS</td><td>1.40 × 10<sup>14</sup></td><td>100</td></tr><tr><td>CH<sub>3</sub><sup>37</sup>Cl</td><td>Chloromethane</td><td>52 009</td><td>JPL</td><td>2.20 × 10<sup>14</sup></td><td>125</td></tr><tr><td>C<sub>2</sub>H<sub>3</sub>CN</td><td>Vinyl cyanide</td><td>53 515</td><td>CDMS</td><td>4.80 × 10<sup>14</sup></td><td>110</td></tr><tr><td>C<sub>2</sub>H<sub>5</sub>CN</td><td>Ethyl cyanide</td><td>55 502</td><td>CDMS</td><td>1.50 × 10<sup>15</sup></td><td>160</td></tr><tr><td>CH<sub>3</sub>NCO</td><td>Methyl isocyanate</td><td>57 505</td><td>CDMS</td><td>3.00 × 10<sup>15</sup></td><td>300</td></tr><tr><td>CH<sub>3</sub>C(O)CH<sub>3</sub></td><td>Acetone</td><td>58 003</td><td>JPL</td><td>3.40 × 10<sup>16</sup></td><td>125</td></tr><tr><td>CH<sub>3</sub>CH<sub>2</sub>CHO</td><td>Propanal</td><td>58 505</td><td>CDMS</td><td>1.48 × 10<sup>15</sup></td><td>125</td></tr><tr><td>CH<sub>3</sub>OCHO</td><td>Methylformate</td><td>60 003</td><td>JPL</td><td>2.60 × 10<sup>17</sup></td><td>300</td></tr><tr><td>HOCH<sub>2</sub>CHO</td><td>Glycolaldehyde</td><td>60 501</td><td>CDMS</td><td>3.40 × 10<sup>16</sup></td><td>300</td></tr><tr><td>OCS</td><td>Carbonyl sulphide</td><td>60 503</td><td>CDMS</td><td>2.00 × 10<sup>16</sup></td><td>125</td></tr><tr><td>OCS v<sub>2</sub> = 1</td><td>Carbonyl sulphide</td><td>60 504</td><td>CDMS</td><td>2.00 × 10<sup>17</sup></td><td>125</td></tr><tr><td>CH<sub>3</sub>COOH</td><td>Acetic acid</td><td>60 523</td><td>CDMS</td><td>3.00 × 10<sup>15</sup></td><td>300</td></tr><tr><td>O<sup>13</sup>CS</td><td>Carbonyl sulphide</td><td>61 502</td><td>CDMS</td><td>5.00 × 10<sup>15</sup></td><td>125</td></tr><tr><td>OC<sup>33</sup>S</td><td>Carbonyl sulphide</td><td>61 503</td><td>CDMS</td><td>3.00 × 10<sup>15</sup></td><td>100</td></tr><tr><td>HOCH<sub>2</sub><sup>13</sup>CHO</td><td>Glycolaldehyde</td><td>61 513</td><td>CDMS</td><td>4.46 × 10<sup>14</sup></td><td>300</td></tr><tr><td>HO<sup>13</sup>CH<sub>2</sub>CHO</td><td>Glycolaldehyde</td><td>61 514</td><td>CDMS</td><td>4.46 × 10<sup>14</sup></td><td>300</td></tr><tr><td>CH<sub>3</sub>O<sup>13</sup>CHO</td><td>Glycolaldehyde</td><td>61 515</td><td>CDMS</td><td>6.30 × 10<sup>15</sup></td><td>300</td></tr><tr><td>DOCH<sub>2</sub>CHO</td><td>Glycolaldehyde</td><td>61 516</td><td>CDMS</td><td>4.86 × 10<sup>14</sup></td><td>300</td></tr><tr><td>HOCHDCHO</td><td>Glycolaldehyde</td><td>61 517</td><td>CDMS</td><td>1.27 × 10<sup>15</sup></td><td>300</td></tr><tr><td>HOCH<sub>2</sub>CDO</td><td>Glycolaldehyde</td><td>61 518</td><td>CDMS</td><td>6.25 × 10<sup>14</sup></td><td>300</td></tr><tr><td>a-(CH<sub>2</sub>OH)<sub>2</sub></td><td>Ethylene glycol</td><td>62 503</td><td>CDMS</td><td>1.37 × 10<sup>16</sup></td><td>300</td></tr><tr><td>s-(CH<sub>2</sub>OH)<sub>2</sub></td><td>Ethylene glycol</td><td>62 504</td><td>CDMS</td><td>3.62 × 10<sup>16</sup></td><td>300</td></tr><tr><td>OC<sup>34</sup>S</td><td>Carbonyl sulphide</td><td>62 505</td><td>CDMS</td><td>1.50 × 10<sup>16</sup></td><td>125</td></tr><tr><td><sup>18</sup>OCS</td><td>Carbonyl sulphide</td><td>62 506</td><td>CDMS</td><td>7.00 × 10<sup>14</sup></td><td>125</td></tr><tr><td>SO<sub>2</sub></td><td>sulphur dioxide</td><td>64 502</td><td>CDMS</td><td>1.50 × 10<sup>15</sup></td><td>125</td></tr><tr><td>S<sup>34</sup>O<sub>2</sub></td><td>Sulphur dioxide</td><td>66 501</td><td>CDMS</td><td>4.00 × 10<sup>14</sup></td><td>125</td></tr></tbody></table>
<table><caption>**Table C.2. HOCH₂CN lines in the PILS data towards IRAS 16293B.**</caption><thead><tr><th>Transition<br>J, K<sub>a</sub>, K<sub>c</sub>, F</th><th>Frequency<br>(MHz)</th><th>E<sub>up</sub><br>(K)</th><th>A<sub>ij</sub><br>(s<sup>-1</sup>)</th><th>Blending species</th></tr></thead><tbody><tr><td>36 11 25 1 - 35 11 24 1</td><td>331 990.003</td><td>468</td><td>1.03 × 10<sup>-3</sup></td><td>g-(CH<sub>2</sub>OH)<sub>2</sub></td></tr><tr><td>36 11 26 1 - 35 11 25 1</td><td>331 990.003</td><td>468</td><td>1.03 × 10<sup>-3</sup></td><td>g-(CH<sub>2</sub>OH)<sub>2</sub></td></tr><tr><td>36 12 24 1 - 35 12 23 1</td><td>332 038.123</td><td>500</td><td>1.01 × 10<sup>-3</sup></td><td>CH<sub>3</sub>OCHO</td></tr><tr><td>36 12 25 1 - 35 12 24 1</td><td>332 038.123</td><td>500</td><td>1.01 × 10<sup>-3</sup></td><td>CH<sub>3</sub>OCHO</td></tr><tr><td>36 8 29 1 - 35 8 28 1</td><td>332 045.021</td><td>389</td><td>1.08 × 10<sup>-3</sup></td><td>g-(CH<sub>2</sub>OH)<sub>2</sub></td></tr><tr><td>36 8 28 1 - 35 8 27 1</td><td>332 045.044</td><td>389</td><td>1.08 × 10<sup>-3</sup></td><td>g-(CH<sub>2</sub>OH)<sub>2</sub></td></tr><tr><td>36 7 30 0 - 35 7 29 0</td><td>332 409.418</td><td>363</td><td>1.09 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>36 7 29 0 - 35 7 28 0</td><td>332 410.281</td><td>363</td><td>1.09 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>36 6 31 0 - 35 6 30 0</td><td>332 678.904</td><td>345</td><td>1.10 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>36 6 30 0 - 35 6 29 0</td><td>332 699.957</td><td>345</td><td>1.10 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>36 4 33 1 - 35 4 32 1</td><td>332 797.489</td><td>323</td><td>1.14 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>36 5 31 1 - 35 5 30 1</td><td>333 147.976</td><td>335</td><td>1.13 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>37 2 36 1 - 36 2 35 1</td><td>334 177.762</td><td>318</td><td>1.15 × 10<sup>-3</sup></td><td>CH<sub>3</sub>OCHO, CH<sub>3</sub>O<sup>13</sup>CHO, C<sub>2</sub>H<sub>5</sub>CN, HO<sup>13</sup>CH<sub>2</sub>CHO</td></tr><tr><td>37 1 36 1 - 36 1 35 1</td><td>335 135.448</td><td>318</td><td>1.16 × 10<sup>-3</sup></td><td>CH<sub>3</sub>OH</td></tr><tr><td>37 1 36 0 - 36 1 35 0</td><td>335 187.943</td><td>313</td><td>1.16 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>36 4 32 1 - 35 4 31 1</td><td>335 446.009</td><td>324</td><td>1.17 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>36 2 34 1 - 35 2 33 1</td><td>335 757.704</td><td>310</td><td>1.15 × 10<sup>-3</sup></td><td>g-(CH<sub>2</sub>OH)<sub>2</sub></td></tr><tr><td>36 4 32 0 - 35 4 31 0</td><td>335 825.850</td><td>318</td><td>1.15 × 10<sup>-3</sup></td><td>g-(CH<sub>2</sub>OH)<sub>2</sub>, CH<sub>3</sub>C(O)CH<sub>3</sub>, CH<sub>3</sub>OCHO</td></tr><tr><td>38 1 38 1 - 37 1 37 1</td><td>335 871.388</td><td>323</td><td>1.17 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>38 0 38 1 - 37 0 37 1</td><td>335 909.319</td><td>323</td><td>1.17 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>38 0 38 0 - 37 0 37 0</td><td>335 985.342</td><td>318</td><td>1.17 × 10<sup>-3</sup></td><td>CH<sub>3</sub>CH<sub>2</sub>OH, g-(CH<sub>2</sub>OH)<sub>2</sub></td></tr><tr><td>36 3 33 0 - 35 3 32 0</td><td>338 263.867</td><td>311</td><td>1.07 × 10<sup>-3</sup></td><td>CH<sub>3</sub>OCHO</td></tr><tr><td>37 3 35 1 - 36 3 34 1</td><td>339 712.712</td><td>329</td><td>1.21 × 10<sup>-3</sup></td><td>NH<sub>2</sub>CHO</td></tr><tr><td>37 3 35 0 - 36 3 34 0</td><td>339 771.178</td><td>324</td><td>1.20 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>37 11 26 1 - 36 11 25 1</td><td>341 198.194</td><td>484</td><td>1.13 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>37 11 27 1 - 36 11 26 1</td><td>341 198.194</td><td>484</td><td>1.13 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>37 9 29 1 - 36 9 28 1</td><td>341 199.695</td><td>429</td><td>1.16 × 10<sup>-3</sup></td><td>g-(CH<sub>2</sub>OH)<sub>2</sub></td></tr><tr><td>37 9 28 1 - 36 9 27 1</td><td>341 199.696</td><td>429</td><td>1.16 × 10<sup>-3</sup></td><td>g-(CH<sub>2</sub>OH)<sub>2</sub></td></tr><tr><td>37 8 30 1 - 36 8 29 1</td><td>341 272.746</td><td>405</td><td>1.18 × 10<sup>-3</sup></td><td>HOCH<sub>2</sub>CHO, CH<sub>3</sub>CDO</td></tr><tr><td>37 8 29 1 - 36 8 28 1</td><td>341 272.781</td><td>405</td><td>1.18 × 10<sup>-3</sup></td><td>HOCH<sub>2</sub>CHO, CH<sub>3</sub>CDO</td></tr><tr><td>37 7 31 1 - 36 7 30 1</td><td>341 427.962</td><td>385</td><td>1.20 × 10<sup>-3</sup></td><td>CH<sub>3</sub>CHO</td></tr><tr><td>37 8 30 0 - 36 8 29 0</td><td>341 498.917</td><td>400</td><td>1.17 × 10<sup>-3</sup></td><td>CH<sub>3</sub>OCHO</td></tr><tr><td>37 8 29 0 - 36 8 28 0</td><td>341 498.954</td><td>400</td><td>1.17 × 10<sup>-3</sup></td><td>CH<sub>3</sub>OCHO</td></tr><tr><td>37 13 24 0 - 36 13 23 0</td><td>341 529.626</td><td>545</td><td>1.08 × 10<sup>-3</sup></td><td>a-(CH<sub>2</sub>OH)<sub>2</sub></td></tr><tr><td>37 13 25 0 - 36 13 24 0</td><td>341 529.626</td><td>545</td><td>1.08 × 10<sup>-3</sup></td><td>a-(CH<sub>2</sub>OH)<sub>2</sub></td></tr><tr><td>37 7 31 0 - 36 7 30 0</td><td>341 659.715</td><td>379</td><td>1.18 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>37 7 30 0 - 36 7 29 0</td><td>341 660.941</td><td>379</td><td>1.18 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>37 6 31 0 - 36 6 30 0</td><td>341 982.421</td><td>362</td><td>1.20 × 10<sup>-3</sup></td><td>CH<sub>3</sub>O<sup>13</sup>CHO</td></tr><tr><td>37 4 34 1 - 36 4 33 1</td><td>342 001.917</td><td>339</td><td>1.23 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>37 5 33 1 - 36 5 32 1</td><td>342 118.267</td><td>352</td><td>1.23 × 10<sup>-3</sup></td><td>HOCH<sub>2</sub>CHO</td></tr><tr><td>37 5 32 1 - 36 5 31 1</td><td>342 514.548</td><td>352</td><td>1.23 × 10<sup>-3</sup></td><td>CH<sub>3</sub>CDO</td></tr><tr><td>38 2 37 1 - 37 2 36 1</td><td>342 945.040</td><td>335</td><td>1.25 × 10<sup>-3</sup></td><td>H<sub>2</sub>CS, CH<sub>3</sub>O<sup>13</sup>CHO</td></tr><tr><td>38 2 37 0 - 37 2 36 0</td><td>343 045.762</td><td>330</td><td>1.24 × 10<sup>-3</sup></td><td>CH<sub>3</sub>CDO</td></tr><tr><td>39 0 39 1 - 38 0 38 1</td><td>344 625.126</td><td>339</td><td>1.27 × 10<sup>-3</sup></td><td>HONO, CH<sub>2</sub>DOH, CH<sub>3</sub>CHO</td></tr><tr><td>37 4 33 1 - 36 4 32 1</td><td>345 070.674</td><td>340</td><td>1.27 × 10<sup>-3</sup></td><td>CH<sub>3</sub>OCHO</td></tr><tr><td>37 4 33 0 - 36 4 32 0</td><td>345 471.229</td><td>335</td><td>1.25 × 10<sup>-3</sup></td><td>CH<sub>3</sub>OCHO</td></tr><tr><td>38 3 36 0 - 37 3 35 0</td><td>348 727.094</td><td>340</td><td>1.30 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>38 9 30 1 - 37 9 29 1</td><td>350 417.202</td><td>446</td><td>1.27 × 10<sup>-3</sup></td><td>a-(CH<sub>2</sub>OH)<sub>2</sub></td></tr><tr><td>38 9 29 1 - 37 9 28 1</td><td>350 417.203</td><td>446</td><td>1.27 × 10<sup>-3</sup></td><td>a-(CH<sub>2</sub>OH)<sub>2</sub></td></tr><tr><td>38 13 25 1 - 37 13 24 1</td><td>350 517.318</td><td>568</td><td>1.19 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>38 13 26 1 - 37 13 25 1</td><td>350 517.318</td><td>568</td><td>1.19 × 10<sup>-3</sup></td><td>-</td></tr><tr><td>38 10 28 0 - 37 10 27 0</td><td>350 617.089</td><td>467</td><td>1.24 × 10<sup>-3</sup></td><td>CH<sub>3</sub>O<sup>13</sup>CHO</td></tr><tr><td>38 10 29 0 - 37 10 28 0</td><td>350 617.089</td><td>467</td><td>1.24 × 10<sup>-3</sup></td><td>CH<sub>3</sub>O<sup>13</sup>CHO</td></tr></tbody></table>

Notes. List of identified and unidentified HOCH₂CN transitions with $A_{ij} \ge 1.0 \times 10^{-3}$ s⁻¹ towards IRAS 16293B in the PILS data set.
<table><caption>**Table C.2.** continued.</caption><thead><tr><th>Transition<br>J, K<sub>a</sub>, K<sub>c</sub>, F</th><th>Frequency<br>(MHz)</th><th>E<sub>up</sub><br>(K)</th><th>A<sub>ij</sub><br>(s<sup>-1</sup>)</th><th>Blending species</th></tr></thead><tbody><tr><td>38 6 33 1 - 37 6 32 1</td><td>350 983.970</td><td>384</td><td>1.32 &times; 10<sup>-3</sup></td><td>CH<sub>3</sub>OCHO, CH<sub>3</sub>CDO</td></tr><tr><td>38 6 32 1 - 37 6 31 1</td><td>351 019.318</td><td>384</td><td>1.32 &times; 10<sup>-3</sup></td><td>H<sub>2</sub>C<sup>13</sup>CO, CH<sub>3</sub>OCHO</td></tr><tr><td>38 6 32 0 - 37 6 31 0</td><td>351 269.282</td><td>378</td><td>1.30 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>38 5 34 1 - 37 5 33 1</td><td>351 402.459</td><td>369</td><td>1.33 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>39 2 38 1 - 38 2 37 1</td><td>351 704.972</td><td>352</td><td>1.35 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>39 2 38 0 - 38 2 37 0</td><td>351 809.704</td><td>346</td><td>1.34 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>38 5 33 1 - 37 5 32 1</td><td>351 897.151</td><td>369</td><td>1.34 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>38 5 33 0 - 37 5 32 0</td><td>352 181.763</td><td>363</td><td>1.32 &times; 10<sup>-3</sup></td><td>CH<sub>3</sub>O<sup>13</sup>CHO</td></tr><tr><td>39 1 38 0 - 38 1 37 0</td><td>352 488.531</td><td>346</td><td>1.35 &times; 10<sup>-3</sup></td><td>CH<sub>3</sub><sup>18</sup>OH</td></tr><tr><td>38 2 36 1 - 37 2 35 1</td><td>353 163.191</td><td>344</td><td>1.35 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>40 0 40 1 - 39 0 39 1</td><td>353 340.026</td><td>356</td><td>1.37 &times; 10<sup>-3</sup></td><td>CH<sub>3</sub>OCHO</td></tr><tr><td>40 1 40 0 - 39 1 39 0</td><td>353 395.992</td><td>351</td><td>1.37 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>40 0 40 0 - 39 0 39 0</td><td>353 420.250</td><td>351</td><td>1.37 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>38 4 34 0 - 37 4 33 0</td><td>355 133.419</td><td>352</td><td>1.36 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>39 3 37 1 - 38 3 36 1</td><td>358 294.680</td><td>363</td><td>1.33 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>39 9 31 1 - 38 9 30 1</td><td>359 634.337</td><td>463</td><td>1.37 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>39 9 30 1 - 38 9 29 1</td><td>359 634.339</td><td>463</td><td>1.37 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>39 12 27 0 - 38 12 26 0</td><td>359 881.373</td><td>545</td><td>1.30 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>39 12 28 0 - 38 12 27 0</td><td>359 881.373</td><td>545</td><td>1.30 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>39 7 33 1 - 38 7 32 1</td><td>359 919.218</td><td>419</td><td>1.41 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>39 13 26 0 - 38 13 25 0</td><td>359 947.354</td><td>579</td><td>1.28 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>39 13 27 0 - 38 13 26 0</td><td>359 947.354</td><td>579</td><td>1.28 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>39 8 32 0 - 38 8 31 0</td><td>359 967.849</td><td>434</td><td>1.38 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>39 8 31 0 - 38 8 30 0</td><td>359 967.931</td><td>434</td><td>1.38 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>39 16 23 1 - 38 16 22 1</td><td>360 033.494</td><td>705</td><td>1.21 &times; 10<sup>-3</sup></td><td>t-HCOOH</td></tr><tr><td>39 16 24 1 - 38 16 23 1</td><td>360 033.494</td><td>705</td><td>1.21 &times; 10<sup>-3</sup></td><td>t-HCOOH</td></tr><tr><td>39 14 25 0 - 38 14 24 0</td><td>360 034.284</td><td>617</td><td>1.25 &times; 10<sup>-3</sup></td><td>t-HCOOH</td></tr><tr><td>39 14 26 0 - 38 14 25 0</td><td>360 034.284</td><td>617</td><td>1.25 &times; 10<sup>-3</sup></td><td>t-HCOOH</td></tr><tr><td>39 6 33 1 - 38 6 32 1</td><td>360 303.101</td><td>401</td><td>1.43 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>39 4 36 1 - 38 4 35 1</td><td>360 369.224</td><td>373</td><td>1.45 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>40 2 39 0 - 39 2 38 0</td><td>360 565.572</td><td>364</td><td>1.44 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>39 5 35 1 - 38 5 34 1</td><td>360 684.867</td><td>386</td><td>1.44 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>40 1 39 1 - 39 1 38 1</td><td>361 063.161</td><td>369</td><td>1.46 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>39 5 34 1 - 38 5 33 1</td><td>361 297.474</td><td>386</td><td>1.45 &times; 10<sup>-3</sup></td><td>CH<sub>3</sub>CH<sub>2</sub>OH</td></tr><tr><td>39 5 34 0 - 38 5 33 0</td><td>361 594.894</td><td>381</td><td>1.43 &times; 10<sup>-3</sup></td><td>-</td></tr><tr><td>39 2 37 1 - 38 2 36 1</td><td>361 805.086</td><td>361</td><td>1.46 &times; 10<sup>-3</sup></td><td>CH<sub>3</sub>OCHO</td></tr><tr><td>41 0 41 1 - 40 0 40 1</td><td>362 053.851</td><td>374</td><td>1.47 &times; 10<sup>-3</sup></td><td>CH<sub>3</sub>C(O)CH<sub>3</sub></td></tr><tr><td>41 0 41 0 - 40 0 40 0</td><td>362 136.053</td><td>368</td><td>1.47 &times; 10<sup>-3</sup></td><td>-</td></tr></tbody></table>

**Appendix D: Source parameters**

<table><caption>**Table D.1.** Physical parameters of interstellar sources used for abundance comparison.</caption><thead><tr><th>Source</th><th>Telescope</th><th>Distance<br>(pc)</th><th>Luminosity<br>(L<sub>☉</sub>)</th><th>Beam size<br>(" × ")</th><th>Physical size<br>(au)</th><th>Reference</th></tr></thead><tbody><tr><td>IRAS 16293–2422B</td><td>ALMA</td><td>141</td><td>3<sup>(a)</sup></td><td>0.5 × 0.5</td><td>70</td><td>Jørgensen et al. (2016)</td></tr><tr><td>Serpens SMM1-a</td><td>ALMA</td><td>436</td><td>100<sup>(b)</sup></td><td>1.3 × 1.0</td><td>500</td><td>This work</td></tr><tr><td>Orion KL</td><td>ALMA</td><td>414</td><td>1 × 10<sup>5</sup></td><td>1.8 × 1.8</td><td>750</td><td>Cernicharo et al. (2016)</td></tr><tr><td>Sgr B2(N2)</td><td>ALMA</td><td>8300</td><td>4.7 × 10<sup>6</sup></td><td>1.6 × 1.2–2.9 × 1.5</td><td>1.3 × 10<sup>4</sup></td><td>Belloche et al. (2017)</td></tr><tr><td>G10.47+0.03</td><td>ALMA</td><td>8550</td><td>5 × 10<sup>5</sup></td><td>2.0 × 1.4–2.4 × 1.6</td><td>1.6 × 10<sup>4</sup></td><td>Gorai et al. (2020)</td></tr><tr><td>Sgr B2(N)</td><td>IRAM 30m</td><td>8300</td><td>4.7 × 10<sup>6</sup></td><td>3.2 × 2.8–12.2 × 4.4</td><td>3.0 × 10<sup>4</sup></td><td>Cernicharo et al. (2016)</td></tr><tr><td>G+0.693</td><td>IRAM 30m & GBT</td><td>8300<sup>(c)</sup></td><td>—</td><td>9 × 9–55 × 55</td><td>≥7.5 × 10<sup>4</sup></td><td>Zeng et al. (2019)</td></tr></tbody></table>

Notes. <sup>(a)</sup>Luminosity determined from a modelling investigation by Jacobsen et al. (2018). <sup>(b)</sup>Luminosity for the entire SMM1 source. <sup>(c)</sup>The distance to G+0.693 is assumed to be the same as Sgr B2.
## Appendix E: Statistical distance

The statistical distance of molecular ratios between SMM1-a and IRAS 16293B is calculated according to the following equation:

$$S_{X/Y} = \frac{\left(\frac{N_X}{N_Y}\right)_{\text{SMM1-a}} - \left(\frac{N_X}{N_Y}\right)_{\text{IRAS 16293B}}}{\sqrt{\sigma_{\text{SMM1-a}}^2 + \sigma_{\text{IRAS 16293B}}^2}}, \quad (E.1)$$

where $N_X$ and $N_Y$ are the column densities of two different molecules and $\sigma$ is the uncertainty on the column density

ratio $(\frac{N_X}{N_Y})$. The value of $S_{X/Y}$ is given in $\sigma$ and indicates the significance of the difference, with greater values implying that there is a more significant difference between the two sources. In this equation, positive values indicate that $(\frac{N_X}{N_Y})$ is more abundant in SMM1-a than in IRAS 16293B, while negative values indicate the opposite.