11institutetext:  National Radio Astronomy Observatory, Charlottesville, VA
22903, USA
11email<EMAIL_ADDRESS>22institutetext: Department of Chemistry,
University of Virginia, Charlottesville, VA 22903, USA 33institutetext: Univ.
Lille, CNRS, UMR 8523 - PhLAM - Physique des Lasers Atomes et Molécules, 59000
Lille, France 44institutetext: Max-Planck-Institut für Radioastronomie, Auf
dem Hügel 69, 53121 Bonn, Germany 55institutetext: INAF, Osservatorio
Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
66institutetext: Université Grenoble Alpes, CNRS, Institut de Planétologie et
d’Astrophysique de Grenoble(IPAG), F-38000 Grenoble, France 77institutetext:
Dipartimento di Chimica, Biologia e Biotecnologie, Universitá di Perugia, Via
Elce di Sotto 8, I-06123 Perugia, Italy 88institutetext: Dipartimento di
Chimica “Giacomo Ciamician”, Universitá di Bologna,, Via Selmi 2, I-40126,
Bologna, Italy 99institutetext: Department of Astronomy, University of
Virginia, Charlottesville, VA 22903, USA 1010institutetext: Department of
Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

# Expanding the submillimeter wave spectroscopy and astronomical search for
thioacetamide (CH3CSNH2) in the ISM

A. Remijan 11 C. Xue 2211 L. Margulès 33 A. Belloche 44 R. A. Motiyenko 33 J.
Carder 22 C. Codella 55 N. Balucani 665577 C. L. Brogan 11 C. Ceccarelli 66 T.
R. Hunter 11 A. Maris 88 S. Melandri 88 M. Siebert  B. A. McGuire 9911101011

(Received XXXXX XX, 2020; accepted XXXXX XX, XXXX)

###### Abstract

Context. One of the biggest unsolved mysteries of modern astrochemistry is
understanding chemical formation pathways in the interstellar medium (ISM) and
circumstellar environments (CSEs). The detections (or even nondetections) of
molecules composed of low-abundance atomic species (such as S, P, Si, and Mg)
may help to constrain chemical pathways. Thioacetamide (CH3CSNH2) is the
sulfur analog to acetemide (CH3CONH2) and it is a viable candidate to search
for in astronomical environments - specifically toward regions where other
S-bearing molecules have been found and, if possible, that also contain a
detection of CH3CONH2. If detected, it would not only continue to expand the
view of molecular complexity in astronomical environments, but also help to
better elucidate the possible formation pathways of these types of species in
these environments.

Aims. Our aim is to expand the frequency range of the measured rotational
spectrum of CH3CSNH2 beyond 150 GHz and then to use those measurements to
extend the search for this species in the ISM. The new laboratory measurements
and expanded search cover more parameter space for determining under what
conditions CH3CSNH2 may be detected, leading to possible constraints on the
formation of large S-bearing molecules found in the ISM.

Methods. The rotational spectrum of CH3CSNH2 was investigated up to 650 GHz.
Using the newly refined spectrum of CH3CSNH2, as well as additional
spectroscopic data on the chemically related species CH3CONH2, a variety of
astronomical sources were searched including data from the following large
surveys: Prebiotic Interstellar Molecule Survey (PRIMOS) conducted with the
Green Bank Telescope (GBT); Exploring molecular complexity with ALMA (EMoCA)
conducted with the Atacama Large Millimeter/submillimeter Array (ALMA); and
Astrochemical Surveys At IRAM (ASAI) conducted with the Institut de
Radioastronomie Millimétrique (IRAM) 30m Telescope.

Results. A total of 1428 transitions from the vt=0 state with maximum values
J=47 and Ka=20 in the range up to 330 GHz, and J=95 and Ka=20 in the range
from 400–660 GHz were assigned. We also assigned 321 transitions from the vt=1
state with the maximum values J=35 and Ka=9 up to 330 GHz. We achieved a final
fit with a root-mean-square deviation of 43.4 kHz that contains 2035 measured
lines from our study and the literature for vt=0 and vt=1 states of A and E
symmetries. The final fit is based on the rho-axis-method (RAM) Hamiltonian
model that includes 40 parameters. An astronomical search for CH3CSNH2 was
conducted based on all the new spectroscopic data. No transitions of CH3CSNH2
were detected toward any of the sources contained in our survey. Using the
appropriate telescope and physical parameters for each astronomical source,
upper limits to the column densities were found for CH3CSNH2 toward each
source.

###### Key Words.:

ISM: molecules – methods: laboratory: molecular data – radio: ISM – molecular
data – submillimeter: ISM – molecular data – line: identification

##  1 Introduction

One of the biggest unsolved mysteries of modern astrochemistry is
understanding chemical formation pathways in the interstellar medium (ISM) and
circumstellar environments (CSEs). The detections (or even nondetections) of
new species are necessary to help calibrate and constrain existing chemical
models in an attempt to make them more predictive. Using low-abundance atomic
species (such as S, P, Si, and Mg) may help to constrain the chemical
pathways. This is because routes to forming molecules from these elements are
typically more limited than from species such as C, O, N, and H that 1) form
molecules easily; 2) are in large abundance; and 3) show a rich molecular
chemistry. Furthermore, while molecules that formed from these low-abundance
atomic species have primary formation pathways in the gas phase, for example,
SiO (Rivero Santamaría et al. 2017), SiS (Mota et al. 2021), and HF (Neufeld &
Wolfire 2009), grain surface chemistry is critical because grain surfaces
provide sites of absorption and desorption where atomic species can interact
and dissipate excess energy as a result of their formation. In addition,
molecular species containing sulfur have been postulated to be used as
“chemical clocks” to measure the overall evolutionary state of an interstellar
environment (for example, Matteucci 2001; Wakelam et al. 2004; Li et al.
2015). Sulfur is also extensively used in observations of solar system bodies
such as Mars, Venus, Io, and meteorites to elucidate chemical and physical
processes and the effects of solar radiation on these objects (Biver et al.
1999; Moullet et al. 2010; Belyaev et al. 2012; Franz et al. 2019). Sulfur
bearing species are also sensitive to gas temperature and density and, as
such, have been used to understand the physical environments of numerous
astronomical environments (Charnley 1997; Esplugues et al. 2014; Semenov et
al. 2018, and references therein).

Table 1: Current list of known ISM/CSE S-bearing species. Entries in bold currently do not have a detected O-substituted counterpart in the ISM/CSE. 2- | 3- | 4- | 5- | 6- | 9-atoms
---|---|---|---|---|---
CH3CH2SH
H2C2S |  C5S |
HNCS |  C4S |  H2C3S |
HCSCCH |
HCSCN |  |
SO2 |  |  |  |
S2H |  |  |  |
HCS+ |  |  |  |
|  NCS |  |  |  |

However, the main sulfur carriers on grain mantles have yet to be fully
characterized. Only OCS and SO2 have been unambiguously identified on grain
surfaces (Palumbo et al. 1995, 1997; Zasowski et al. 2009). Hydrogen sulfide
(H2S) - a main reactant in numerous chemical networks on grain surfaces - has
yet to be identified (van der Tak et al. 2003).

From the list of known interstellar S-bearing molecules (Table 1), nearly all
species have corresponding detections of their O-substituted counterparts. For
example, methanol (CH3OH) is the O-analog to methyl mercaptan (CH3SH). In
fact, the only analogous O-bearing molecules that have yet to be detected from
the list are diacetylene ether (C4O), diacetylene ketone (C5O) and
propadienone (CH2CCO). In addition, from a formation chemistry and detection
point of view, it is very surprising that (CH2CCS) (Cernicharo et al. 2021)
has been detected as well as its thioaldehyde isomer - HCCCHS (Margulès et al.
2020b). This is completely opposite from what is found in the O-analog
species. That is, the aldehyde HCCCHO is very abundant and easily detected
toward several environments (Irvine et al. 1988; Hollis et al. 2004b; Requena-
Torres et al. 2008) whereas CH2CCO has so far eluded detection, though recent
theoretical predictions offer a possible explanation (Shingledecker et al.
2019). As such, searching for S-bearing species with already identified
O-substituted counterparts in astronomical environments can provide a basis
(or proxy) for searching for more complex species. And, while it may not
always lead to a detection – or sometimes a completely different species may
be found – searching for and setting detection limits will help to calibrate
existing chemical networks that predict abundances of S-bearing species.

Acetamide (CH3CONH2) and N-methylformamide (CH3NHCHO), are two isomers which
contain a peptide bond (Hollis et al. 2006a; Belloche et al. 2017; Ligterink
et al. 2020). The detection of these molecules under interstellar conditions
is important because 1) it again increases the limit of molecular complexity
found in astronomical environments and 2) these molecules may provide a
starting point to the formation of larger, more complex, prebiotic
astronomical molecules. And, to date, the formation routes to these species
are still uncertain. Thioacetamide (CH3CSNH2) is the sulfur analog to CH3CONH2
and is a viable candidate to search for in astronomical environments -
specifically toward regions where other S-bearing molecules have been found
and, if possible, that also contain a detection of CH3CONH2. If detected, it
would not only continue to expand the limits of molecular complexity in
astronomical environments but also help to better elucidate the possible
formation pathway of these types of species in astronomical environments.

Thioacetamide is a planar, asymmetric top molecule with Cs symmetry. Its
rotational spectrum was recorded recently by free-jet absorption spectroscopy
in the 59.6–110.0 GHz frequency region (Maris et al. 2019). In addition to
detecting the most abundant isotopologue, the ^34S species was also observed
in natural abundance ($4.25\%$). Some of the rotational transition lines
showed small splittings (1-2 MHz) due to the ^14N nuclear quadrupole
interaction. Because of the internal rotation of the methyl group, all lines
are further split into A and E components. The barrier to this internal
rotation is low ($V_{3}\approx 110\,\mathrm{cm}^{-1}$), which leads to the
relatively large frequency splittings between the A and E states. That study
also conducted the first attempt at an astronomical search for this molecule
using the observational data from the unbiased Astronomical Surveys at IRAM
(ASAI111http://www.oan.es/asai/) program (Lefloch et al. 2018) toward two
sources (associated with low-mass star forming regions) that illustrated
diverse and extreme physical conditions - L1544, a cold prestellar core, and
L1157-B1, a shocked region within the ISM. These sources were also selected
because the emission regions are known to be extended with respect to the IRAM
30m beam (10$\arcsec$–30$\arcsec$), thus minimizing the affects of beam
dilution. The observations were carried out at 3 mm (72–116 GHz), 2 mm
(126–173 GHz), and 1.3 mm (200–276 GHz).

No detection of CH3CSNH2 was reported and upper limits were given on the
abundance of CH3CSNH2 toward these sources. And while several smaller sulfur
bearing species have been detected toward these regions, no larger S-bearing
molecules have been detected and there has also not been a detection of
CH3CONH2 - the O-analog of CH3CSNH2 toward these sources either.

In order to conduct a broader radio astronomical search for CH3CSNH2,
additional spectroscopic data (both from the laboratory and space) from a
wider range of sources are necessary. As such, we present here a new study
investigating the rotational spectrum of CH3CSNH2 using laboratory
measurements from 150 to 650 GHz. These measurements were done at room
temperature which enabled a global analysis of both the ground and the first
torsional excited states. This new study has permitted, for the first time, an
accurate prediction in the millimeter- and submillimeter wave ranges necessary
for a more extensive astronomical search. In addition, the existing lower
frequency data were used to search for transitions at cm wavelengths that may
potentially show maser activity. In Section 2 we describe the experimental
details of the study and the full analysis of the spectrum; in Section 3 we
describe the astronomical search for this species specifically toward regions
with previous detections of CH3CONH2 as well as other sources obtained from
archival data from various astronomical facilities; in Section 4 we discuss
the implication of the investigation in the context of the current presumed
formation pathways of CH3CONH2 in the ISM and a comparison of the abundances
found between the S- and O-substituted species already detected toward various
astronomical environments and finally, in Section 5 we provide a summary and
possible future observational work needed to better elucidate the formation of
large S-bearing species in the ISM.

##  2 Spectroscopic study

###  2.1 Experimental details

The sample of CH3CSNH2 was purchased from Aldrich and used without
purification. The absorption measurements of the rotational spectrum between
150 and 650 GHz were conducted using the fast-scanning Terahertz spectrometer
in Lille. The details of the spectrometer are described in Zakharenko et al.
(2015).

The radiation source present in the spectrometer is a commercially available
Virginia Diodes (VDI) frequency multiplication chain driven by an in-house
fabricated fast sweep frequency synthesizer. The fast sweep system is based on
the up-conversion of an AD9915 direct digital synthesizer (DDS) operating
between 320 and 420 MHz into the Ku band (12.5–18.25 GHz) by mixing the
signals from the AD9915 and an Agilent E8257 synthesizer with subsequent
sideband filtering. The DDS provides rapid frequency scanning with up to 50 ms
per point frequency switching rate. In order to obtain an optimum signal-to-
noise ratio, the spectrum was scanned with a slower rate of 1 ms per point and
with 4 scans co-averaged. The sample pressure during measurements was $\sim
15$ Pa at room temperature. Absorption signals were detected by an InSb liquid
He-cooled bolometer (QMC Instruments Ltd.). Estimated uncertainties for the
measured line frequencies are 30 kHz.

###  2.2 Analysis of the spectra

Thioacetamide is an asymmetric near-prolate rotor ($\kappa\approx$ -0.48). It
has nonzero dipole components along the $a$\- ($\mu_{a}$ = 4.1 D) and $b$-axes
($\mu_{b}$ = -1.1 D) as determined by Soundararajan (1957). The molecule is
challenging from the spectroscopic point of view as it exhibits large
amplitude methyl torsional motion. The interaction between internal rotation
of the methyl group and overall rotation of the molecule complicates the
spectral analysis. For CH3CSNH2, the barrier to internal rotation is low
($V_{3}\approx 110\leavevmode\nobreak\ \mathrm{cm}^{-1}$), and leads to
relatively large torsional $A-E$ splittings of the rotational energy levels.
The low-lying excited torsional states add more complexity through Coriolis-
type interactions with the ground state as well as with each other. As a
result, a correct interpretation of the ground state rotational spectrum of
CH3CSNH2 within a large range of $J$ and $K_{a}$ quantum numbers requires a
global analysis including excited torsional states.

In the present study, we used the theoretical model base of the rho-axis
method (RAM) (Kirtman 1962; Lees & Baker 1968; Herbst et al. 1984). The main
advantage of the RAM Hamiltonian is its general approach that simultaneously
takes into account the $A$\- and $E$-symmetry species and all torsional
levels, intrinsically taking the inter-torsional interactions into account
within the rotation-torsion manifold of energy levels. This method is
particularly suitable for a difficult case of low barrier to internal rotation
(Ilyushin et al. 2004; Neill et al. 2012) or when large coupling between
internal and overall rotation occurs (Smirnov et al. 2014), or both. Because
this method has been presented in great detail in the literature (Hougen et
al. 1994; Kleiner 2010), we do not describe it here. The RAM used for the
spectral analysis in this present study has been previously shown to be very
effective for a number of molecules that contain an internal methyl rotor
(Ilyushin et al. 2010, 2013; Smirnov et al. 2014). In particular, it has been
used for the oxygen-bearing analog of CH3CSNH2, CH3CONH2 (Ilyushin et al.
2004) which has a lower barrier to internal rotation than CH3CSNH2
($V_{3}\approx 25\leavevmode\nobreak\ \mathrm{cm}^{-1}$). For the analysis, we
employed the RAM36 (rho-axis-method for 3- and 6-fold barriers) code that uses
the RAM approach for molecules with a C3v top attached to a Cs or C2v symmetry
molecular frame and having 3- or 6-fold barriers to internal rotation,
respectively. A general expression for the RAM Hamiltonian implemented in this
code, as well as further details of the theoretical approach, is found in
Ilyushin et al. (2010) and Ilyushin & Hougen (2013). The code RAM36 used for
this analysis is publicly
available222http://www.ifpan.edu.pl/~kisiel/introt/introt.htm, and has been
successfully utilized in the spectral analysis of many molecules (Ilyushin et
al. 2010, 2013; Smirnov et al. 2014), including the recent analysis of the
sulfur-containing molecule CH3CHS (Margulès et al. 2020a).

To begin, we refit the data from the previous study (Maris et al. 2019) using
the RAM36 program. Using predictions obtained from the initial RAM36 fit, we
assigned and fit the ${}^{a}R$ transitions in the usual manner, with numerous
cycles of refinement of the parameter set while new data were gradually added.
However, it was not possible to observe the hyperfine splittings due to our
Doppler-limited resolution.

Next, we searched for the ${}^{b}R$ and ${}^{b}Q$ lines. However, these
transitions could not be assigned unambiguously due to the dense spectra and
their much smaller relative intensities. Also, several high Ka series of the
ground torsional state transitions were perturbed by the interactions with the
first excited torsional state; therefore it was necessary to get more precise
information about the positions of energy levels of this state. The band
origins of the first torsional states are $\nu^{A}=61.6589\leavevmode\nobreak\
\mathrm{cm}^{-1}$, $\nu^{E}=51.3985\leavevmode\nobreak\ \mathrm{cm}^{-1}$,
giving relative intensities to ground state at room temperature of 0.74 and
0.78, respectively. The analysis of the first torsional state also stabilized
the fit and reduced the correlation between torsional parameters of the RAM
Hamiltonian models.

Finally, we assigned 1428 transitions from the $v_{t}=0$ state with the
maximum values $J=47$ and $K_{a}=20$ in the range up to 330 GHz, and $J=95$
and $K_{a}=20$ in the range 400–660 GHz. We also assigned 321 transitions from
the $v_{t}=1$ state with the maximum values $J=35$ and $K_{a}=9$ up to 330
GHz. The final data set contains both the data from the previous study (Maris
et al. 2019), and new assignments in the millimeter- and submillimeter wave
ranges. The total of 2035 lines for $v_{t}=0$ and $v_{t}=1$ states of $A$ and
$E$ symmetries were fitted with root-mean-square deviation of 43.4 kHz
(wrms=0.90). The final fit is based on the RAM Hamiltonian model that includes
40 parameters. The values of the molecular parameters obtained from the final
fits are presented in Table 2, where they are compared with the parameters
from the initial fit of the data from the previous study (Maris et al. 2019).
The new combination of data from the $v_{t}=0$ and $v_{t}=1$ states required
23 more parameters that also improved the overall quality of the fit. In
particular, the inclusion of the $v_{t}=1$ state permitted the removal of the
strong correlation between $F$ and $\rho$ parameters of the Hamiltonian, and
to accurately determine the V6 parameter.

Table 2: Molecular parameters in $\mathrm{cm}^{-1}$ of CH3CSNH2 obtained with the RAM36 program. ntra | Parameterb | Operatorc | Valued | Value (this study)
---|---|---|---|---
220 | $F$ | $p_{\alpha}^{2}$ | 5.650542416e | 5.45339(10)f
220 | $V_{3}$ | $\frac{1}{2}(1-\cos 3\alpha)$ | 108.860(66) | 110.71340(94)
211 | $\rho$ | $J_{z}p_{\alpha}$ | 0.042368(54) | 0.0437894(17)
202 | $A_{RAM}$ | $J_{z}^{2}$ | 0.270791(38) | 0.2696379(20)
202 | $B_{RAM}$ | $J_{x}^{2}$ | 0.2207334(66) | 0.22085611(92)
202 | $C_{RAM}$ | $J_{y}^{2}$ | 0.11213403(48) | 0.112127546(92)
202 | $D_{zx}$ | $\\{J_{z},J_{x}\\}$ |  $-$0.0749884(24) |  $-$0.07505330(80)
440 | $V_{6}$ | $\frac{1}{2}(1-\cos 6\alpha)$ |  |  $-$14.66262(38)
422 | $F_{J}$ | $J^{2}p_{\alpha}^{2}$ |  | $-0.456(12)\times 10^{-6}$
422 | $F_{K}$ | $J_{z}^{2}p_{\alpha}^{2}$ |  | $-0.484(13)\times 10^{-5}$
422 | $F_{zx}$ | $\frac{1}{2}p_{\alpha}^{2}\\{J_{z},J_{x}\\}$ |  | $-0.507(93)\times 10^{-6}$
422 | $V_{3J}$ | $J^{2}(1-\cos 3\alpha)$ | $0.2781(80)\times 10^{-3}$ | $0.14296(43)\times 10^{-3}$
422 | $V_{3K}$ | $J_{z}^{2}(1-\cos 3\alpha)$ | $0.149(12)\times 10^{-2}$ | $-0.14891(40)\times 10^{-2}$
422 | $V_{3zx}$ | $\frac{1}{2}(1-\cos 3\alpha)\\{J_{z},J_{x}\\}$ | $0.1820(14)\times 10^{-2}$ | $0.20727(28)\times 10^{-2}$
422 | $V_{3xy}$ | $(J_{x}^{2}-J_{y}^{2})(1-\cos 3\alpha)$ | $0.2311(85)\times 10^{-3}$ | $0.9789(38)\times 10^{-4}$
422 | $D_{3zy}$ | $\frac{1}{2}\sin 3\alpha\\{J_{z},J_{y}\\}$ |  | $0.9823(63)\times 10^{-3}$
422 | $D_{3xy}$ | $\frac{1}{2}\sin 3\alpha\\{J_{x},J_{y}\\}$ |  | $0.450(32)\times 10^{-4}$
413 | $\rho_{J}$ | $J^{2}J_{z}p_{\alpha}$ | $-0.229(15)\times 10^{-5}$ | $-0.3828(59)\times 10^{-6}$
413 | $\rho_{K}$ | $J_{z}^{3}p_{\alpha}$ | $0.1761(49)\times 10^{-4}$ | $0.2695(19)\times 10^{-5}$
413 | $\rho_{zx}$ | $\frac{1}{2}p_{\alpha}\\{J_{z}^{2},J_{x}\\}$ |  | $-0.982(24)\times 10^{-6}$
404 | $D_{zxJ}$ | $\frac{1}{2}\\{J_{z},J_{x}\\}J^{2}$ |  | $-0.433(44)\times 10^{-7}$
404 | $D_{zxK}$ | $\frac{1}{2}\\{J_{z},J_{x}\\}J_{z}^{2}$ |  | $0.6597(45)\times 10^{-6}$
404 | $\Delta_{J}$ | $-J^{4}$ |  | $0.1137(12)\times 10^{-6}$
404 | $\Delta_{JK}$ | $-J^{2}J_{z}^{2}$ |  | $-0.3446(24)\times 10^{-6}$
404 | $\Delta_{K}$ | $-J_{z}^{4}$ |  | $0.6353(49)\times 10^{-6}$
404 | $\delta_{J}$ | $-2J^{2}(J_{x}^{2}-J_{y}^{2})$ |  | $0.1024(12)\times 10^{-6}$
404 | $\delta_{K}$ | $-\\{J_{z}^{2},(J_{x}^{2}-J_{y}^{2})\\}$ |  | $0.6668(99)\times 10^{-7}$
642 | $V_{6J}$ | $J^{2}(1-\cos 6\alpha)$ |  | $0.1254(10)\times 10^{-4}$
642 | $V_{6K}$ | $J_{z}^{2}(1-\cos 6\alpha)$ |  | $-0.1634(44)\times 10^{-3}$
642 | $V_{6zx}$ | $\frac{1}{2}\\{J_{z},J_{x}\\}(1-\cos 6\alpha)$ |  | $-0.752(33)\times 10^{-4}$
633 | $\rho_{3xy}$ | $\frac{1}{2}\\{J_{z},J_{x},J_{y},p_{\alpha},\sin 3\alpha\\}$ |  | $0.427(12)\times 10^{-5}$
624 | $V_{3JJ}$ | $J^{4}(1-\cos 3\alpha)$ |  | $-0.632(57)\times 10^{-10}$
624 | $V_{3JK}$ | $J^{2}J_{z}^{2}(1-\cos 3\alpha)$ |  | $-0.506(34)\times 10^{-8}$
624 | $F_{FJK}$ | $J^{2}J_{z}^{2}p_{\alpha}^{2}$ |  | $-0.1344(47)\times 10^{-8}$
624 | $F_{KK}$ | $J_{z}^{4}p_{\alpha}^{2}$ |  | $0.1582(46)\times 10^{-8}$
624 | $D_{3xyK}$ | $\frac{1}{2}\sin 3\alpha\\{J_{z}^{2},J_{x},J_{y}\\}$ |  | $-0.967(40)\times 10^{-7}$
615 | $\rho_{JK}$ | $J^{2}J_{z}^{3}p_{\alpha}$ |  | $0.382(32)\times 10^{-10}$
606 | $\Phi_{JK}$ | $J^{4}J_{z}^{2}$ |  | $0.1475(33)\times 10^{-11}$
606 | $\Phi_{KJ}$ | $J^{2}J_{z}^{4}$ |  | $-0.337(17)\times 10^{-11}$
606 | $\phi_{JK}$ | $J^{2}\\{J_{z}^{2},(J_{x}^{2}-J_{y}^{2})\\}$ |  | $0.820(27)\times 10^{-12}$
Number of parameters | 17 | 40
Number of lines vt=0, vt=1 | 286, 0 | 1428, 321
$F_{max}$ in GHz | 110 | 650
$J_{max},K_{a,max}$ | 15, 10 | 95, 20
rms in kHz | 107 | 43.4
wrms unitless | 0.71 | 0.90

Spectral predictions for astronomical use were calculated using the set of RAM
Hamiltonian parameters presented in last column of Table 2. The predictions
include the rotational transitions of the ground and first excited torsional
states of CH3CSNH2 calculated at 300 K for $J$ values up to 95 and in the
frequency range up to 400 GHz. In addition, we provided the tabulated values
for the partition function in Table 4, with
$Q_{\mathrm{tot}}(T)=Q_{\mathrm{vib}}(T)\times Q_{\mathrm{tr}}(T)$. The
torsion-rotational partition function $Q_{\mathrm{tr}}(T)$ was calculated from
first principles, that is, via direct summation over the rotational-torsional
states. In the presence of low-energy vibrationally excited states, the
vibrational part of the partition function should be also taken into account
to derive column densities. The vibrational partition function was calculated
with respect to the zero-point energy level using the following expression:

$Q(T)_{\mathrm{vib}}=\prod\limits_{\begin{subarray}{c}i=1\end{subarray}}^{3N-6}\frac{1}{1-e^{-E_{i}/kT}}.$
(1)

The normal modes were obtained from the harmonic force field ab initio
calculation at the B3LYP/6-311G++(3dp, 2pd) level of theory and basis set
using Gaussian 09 (Frisch et al. 2016a). The harmonic frequencies are given in
Table 3. Only the seven lowest vibrational excited state levels were
considered (mode 2 to 8 in the Table 3). The contribution of the torsional
mode is included in $Q_{\mathrm{tr}}(T)$ and thus, it is not considered for
$Q_{\mathrm{vib}}$. The remaining modes above $1000\leavevmode\nobreak\
\mathrm{cm}^{-1}$ were found to have no influence on the partition function
calculations for the temperatures up to 300 K. The results of the fit, as well
as the predictions, are available in the Supplementary Material associated
with this article.

Table 3: Harmonic vibrational frequencies up to $1000\leavevmode\nobreak\ \mathrm{cm}^{-1}$ of CH3CSNH2 determined at the B3LYP/6-311++G(3df,2pd) level of theory and basis set Mode | Frequency
---|---
| ($\mathrm{cm}^{-1}$)
1 | 36.0932
2 | 347.4387
3 | 378.6060
4 | 426.8825
5 | 518.6759
6 | 613.2849
7 | 732.7576
8 | 987.8108
9 | 1021.2864
10 | 1036.7852
11 | 1322.0596
12 | 1377.2398
13 | 1403.9333
14 | 1482.4055
15 | 1489.0705
16 | 1637.4358
17 | 3020.4774
18 | 3071.0614
19 | 3164.1245
20 | 3564.4275
21 | 3696.1525
Table 4: Rotational, vibrational, and total partition functions of CH3CSNH2 at various temperatures $T(K)$ | $Q(T)_{\mathrm{tr}}$ | $Q(T)_{\mathrm{vib}}$ | $Q(T)_{\mathrm{tot}}$
---|---|---|---
300 | 459047.9395 | 2.0239 | 929054.6665
220 | 242388.0230 | 1.3800 | 334515.0879
170 | 141743.4841 | 1.1541 | 163585.3842
150 | 109181.0704 | 1.0949 | 119537.4570
70 | 22675.8793 | 1.0014 | 22707.4107
40 | 7672.9688 | 1.0000 | 7673.0084
20 | 2332.7909 | 1.0000 | 2332.7909
10 | 784.3603 | 1.0000 | 784.3603

##  3 Expanded astronomical searches

In the initial analysis characterizing the spectroscopy of CH3CSNH2, two
astronomical sources were selected from the ASAI survey to search for
transitions of CH3CSNH2 based on the initial fit from that work (Maris et al.
2019). The ASAI survey targets sources associated with Solar-type star forming
regions over a wide range of evolutionary states, temperatures, densities and
kinematics providing a diverse sample of sources to investigate (Lefloch et
al. 2018). The protostellar core L1544 and the shocked region L1157-B1
associated with the chemically rich outflow L1157 were the two sources that
were selected. Both regions are spatially extended (with respect to the IRAM
30m beam). No transitions of CH3CSNH2 were detected above the $3\sigma$ noise
level of those observations. That study assumed a dipole moment of $\sim
4\,\mathrm{D}$ in order to calculate the upper limit to the column density of
CH3CSNH2 toward these sources. For both sources, the upper limit to the beam
averaged column density was $<10^{12}\leavevmode\nobreak\ \mathrm{cm}^{-2}$.
With the improved spectroscopic observations of this study and the inclusion
of the vibrational part of the partition function, it is now possible to
perform a more extensive search for CH3CSNH2 and to provide a more robust
constraint on column densities.

###  3.1 Search for CH3CSNH2 toward Sgr B2(N2) with ALMA

We searched for CH3CSNH2 toward the hot molecular core Sgr B2(N2). This source
belongs to the protocluster Sgr B2(N) that contains several hot cores and
(ultracompact) HII regions (for example,, Bonfand et al. 2017; Gaume et al.
1995), as well as a large number of compact continuum sources detected with
the Atacama Large Millimeter/submillimeter Array (ALMA) that are characterized
by densities above $10^{8}\leavevmode\nobreak\ \mathrm{cm}^{-3}$ and were
interpreted as being high mass young stellar objects (Sánchez-Monge et al.
2017; Ginsburg et al. 2018). Sgr B2(N) has often been targeted over the past
five decades to search for complex organic molecules thanks to the high column
densities of its hot cores that facilitate the detection of these molecules.

We used the data from the Exploring molecular complexity with ALMA (EMoCA)
imaging spectral line survey performed with ALMA in Cycles 0 and 1 to search
for CH3CSNH2 toward Sgr B2(N2). Details about the observations and data
reduction of this survey can be found in Belloche et al. (2016). In short, the
survey covers the frequency range from 84.1 to 114.4 GHz with a spectral
resolution of 488 kHz that corresponds to 1.7 to 1.3 $\mathrm{km}$
$\mathrm{s^{-1}}$ in velocity space. The median angular resolution of the
survey is $1.6\arcsec$ which corresponds to $\sim$13000 au at a distance of
8.15 kpc (Reid et al. 2019). The phase center of the interferometric
observations was located at ($\alpha,\delta$)J2000=($17^{\rm h}47^{\rm
m}19{\aas@@fstack{s}}87,-28^{\circ}22^{\prime}16{\aas@@fstack{\prime\prime}}0$).
The spectrum that is analyzed here corresponds to the peak position of Sgr
B2(N2) located at ($\alpha,\delta$)J2000=($17^{\rm h}47^{\rm
m}19{\aas@@fstack{s}}86$, $-28^{\circ}22\arcmin
13{\aas@@fstack{\prime\prime}}4$).

We used the software WEEDS (Maret et al. 2011) to model the observed spectrum
under the assumption of local thermodynamic equilibrium (LTE) and produce a
synthetic spectrum. This assumption is justified by the high densities that
characterize the region where the hot core emission is detected ($>1\times
10^{7}\ \mathrm{cm^{-3}}$, see Bonfand et al. 2019). As described in Belloche
et al. (2016), we modeled the emission of each molecule separately and then
added their contributions together. We modeled the spectrum of each species
with five parameters: size of the emitting region, column density,
temperature, line width, and velocity offset with respect to the assumed
systemic velocity of the source ($74\ \mathrm{km\ s}^{-1}$).

Figure 1: Synthetic LTE spectrum of CH3CSNH2 (in red) used to derive the upper
limit to its column density, overlaid on the ALMA spectrum of Sgr B2(N2) (in
black) and the synthetic spectrum that contains the contributions of all the
species (but not CH3CSNH2) that we have identified so far in this source (in
green). The dotted line in each panel indicates the $3\sigma$ noise level. The
transitions of CH3CSNH2 that have a modeled peak temperature lower than
3$\sigma$ and the ones that are heavily contaminated by other species are not
shown.

We did not detect emission from CH3CSNH2 toward Sgr B2(N2) with the EMoCA
survey. In order to derive an upper limit to its column density, we assumed
that its emission should trace a similar region as CH3CONH2 and we fixed the
size of the emitting region, temperature, line width, and velocity offset to
the values derived for CH3CONH2 in Sgr B2(N2) by Belloche et al. (2017). The
only free parameter was then the column density of CH3CSNH2. Figure 1 shows
the synthetic spectrum of CH3CSNH2 that we used to derive the upper limit to
its column density. This upper limit is reported in Table 5, along with the
parameters derived earlier for CH3CONH2, methanol (CH3OH), and methyl
mercaptan (CH3SH) by Belloche et al. (2017) and Müller et al. (2016) from the
EMoCA survey.

Table 5:  Parameters of our best-fit LTE model of methanol, methyl mercaptan,
and acetamide, and column density upper limit for thioacetamide, toward Sgr
B2(N2).

Molecule | Statusaa$a$d: detection, n: non-detection. |  $N_{\rm det}$bb$b$Number of detected lines (conservative estimate, see Sect. 3 of Belloche et al. 2016). One line of a given species may mean a group of transitions of that species that are blended together. | Sizecc$c$Source diameter (FWHM). |  $T_{\mathrm{rot}}$dd$d$Rotational temperature. |  $N$ee$e$Total column density of the molecule. $x$ ($y$) means $x\times 10^{y}$. |  $F_{\rm vib}$ff$f$Correction factor that was applied to the column density to account for the contribution of vibrationally excited states, in the cases where this contribution was not included in the partition function of the spectroscopic predictions. |  $\Delta V$gg$g$Linewidth (FWHM). |  $V_{\mathrm{off}}$hh$h$Velocity offset with respect to the assumed systemic velocity of Sgr B2(N2), $V_{\mathrm{sys}}=74$ $\mathrm{km/s}^{-1}$.
---|---|---|---|---|---|---|---|---
|  |  | ($\arcsec$) | (K) | (cm-2) |  | ($\mathrm{km\ s}^{-1}$) | ($\mathrm{km\ s}^{-1}$)
CH3OH, $\varv=0$ | d | 41 | 1.4 | 160 | 4.0 (19) | 1.00 | 5.4 | $-0.5$
CH3SH, $\varv=0$ | d | 12 | 1.4 | 180 | 3.4 (17) | 1.00 | 5.4 | $-0.5$
CH3C(O)NH2, $\varv=0$ | d | 10 | 0.9 | 180 | 1.4 (17) | 1.23 | 5.0 | $1.5$
CH3C(S)NH2, $\varv=0$ | n | 0 | 0.9 | 180 |  $<$ 1.6 (16) | 1.20 | 5.0 | $1.5$

333The parameters for methanol, methyl mercaptan, and acetamide were taken
from Müller et al. (2016) and Belloche et al. (2017).

Table 5 indicates that CH3CSNH2 is at least $\sim 9$ times less abundant than
CH3CONH2 in Sgr B2(N2). For comparison, CH3SH is $\sim 120$ times less
abundant than CH3OH in this source. If these two pairs of molecules have
similar abundance ratios, then the emission lines of CH3CSNH2 may be one order
of magnitude weaker than the upper limits we obtained with the EMoCA survey,
and thus most likely well below the spectral confusion limit toward this
source in this frequency range.

###  3.2 Search for CH3CSNH2 toward Sgr B2(N) with the GBT

The PRebiotic Interstellar Molecule Survey (PRIMOS) 444Access to the entire
PRIMOS data set, specifics on the observing strategy, and overall frequency
coverage information is available at http://archive.nrao.edu by searching for
GBT Program ID: AGBT07A_051 was a key science program that started in 2008
January and concluded in 2011 July taken with the Robert C. Byrd Green Bank
Telescope (GBT) currently managed by the Green Bank Observatory (GBO). The
PRIMOS project covers nearly all observable frequencies available to ground
based instrumentation from $\sim 300\ \mathrm{MHz}$ to 48 GHz at high
sensitivity ($\sim\leavevmode\nobreak\ 3-9\ \mathrm{mK}$ rms) and spectral
resolution (24.4 kHz). The pointing position for these observations were
centered on the Sgr B2(N) Large Molecule Heimat (LMH) at
($\alpha,\delta$)J2000=($17^{\rm h}47^{\rm
m}19{\aas@@fstack{s}}8,-28^{\circ}22^{\prime}17{\aas@@fstack{\prime\prime}}0$),
nearly identical to the pointing position of the EMoCA survey (see Section
3.1).

The systemic velocities found for CH3CONH2 in the original detection paper
(Hollis et al. 2006a) are +64, +73 and +82 $\mathrm{km\ s}^{-1}$. In general,
there is much less contribution from the $+73\mathrm{km\ s}^{-1}$ component
compared to $+64\ \mathrm{km\ s}^{-1}$ and $+82\ \mathrm{km\ s}^{-1}$ in the
GBT observations. However, the ALMA observations are also sensitive to the
compact emission regions of molecular gas (the resolution and associated
source sizes from the EMoCA survey are $\sim 1.6\arcsec$ whereas the GBT
observations are sensitive to a range of resolutions and source sizes,
dependent on observing frequency, from $\sim 15\arcsec$ to $\sim 80\arcsec$).
As such, from these two complementary data sets, it is possible to investigate
the hot compact regions as well as the colder more extended molecular gas in
searching for transitions of CH3CSNH2 toward the Sgr B2(N) region. Intensities
are presented on the $T_{\mathrm{A}}^{*}$ scale (Ulich & Haas 1976).

The detection of large molecules with the PRIMOS data set for the last 15
years has come as a surprise as previously, the prevailing theories for the
formation of large molecules was believed to be driven primarily by grain
surface chemistry during a warm-up phase during the process of star formation
(Garrod et al. 2008). As such, the regions surrounding hot molecular cores
would be ideal for these discoveries (See Section 3.1). Yet the PRIMOS
observations have shown that there is a large diversity of complex molecules
in the clouds surrounding the Sgr B2(N) region and not specifically centered
on the hot core regions. In addition, PRIMOS observations have also shown a
rich and complex molecular diversity among the spiral arms clouds in the
intervening gas (Corby et al. 2018) toward the Galactic Center. These
observations together indicate that gas phase chemistry may drive the
formation of molecules that cannot happen on grain surfaces (Neill et al.
2012). Even more interesting was the conclusion that most transitions of these
larger molecules (for example, CH3OCHO and CH2NH) with energy levels below 30
GHz are astronomical masers (Faure et al. 2014, 2018) leading to the detection
of new molecules which would have otherwise eluded detection (McGuire et al.
2012).

Since the initial observations from the first detection of CH3CONH2 (Hollis et
al. 2006a), additional transitions have been observed with the GBT as part of
the PRIMOS survey. Also, the initial detection did not include any of the
hyperfine structure (HFS) in the modeling of the spectral line profiles from
PRIMOS nor in the determination of the total column density. In this work,
spectral line profiles of the low frequency transitions were modeled for all
the A-state transitions below 48 GHz. In addition, transitions below 18 GHz
were modeled using the measured HFS (F. J. Lovas, private communication).
Above 18 GHz, the HFS is no longer resolved at the resolution of our
astronomical observations. We used the software MOLSIM (Lee et al. 2021) to
perform the modeling and to estimate the upper limit for the spectra analysis
of the PRIMOS and ASAI surveys. Synthetic spectra are produced with a single
excitation temperature assumption, that all transitions are thermalized, and
corrected for optical depth as discussed in Turner (1991) and Mangum & Shirley
(2015).

Acetamide (CH3CONH2) is an oblate, asymmetric top molecule ($K_{c}$ is a good
quantum number) with an internal methyl rotor (Yamaguchi et al. 2002). The
dipole moments of CH3CONH2 are $\mu_{a}=1.14D$ and $\mu_{b}=3.5D$ (Hollis et
al. 2006a). As such, b-type transitions would be inherently stronger than
a-type transitions. Several attributes of the detected transitions in PRIMOS
were immediately found. First, as expected, only b-type transitions were
detected. Second, only $\Delta J=0$, Q-branch transitions were detected with
$\Delta K_{a}=1$ and $\Delta K_{c}=-1$ for the A-symmetry states (hereafter,
the convention bQ1,-1 will be used to describe the types of transitions).
Figure 2 shows all the Q-branch, A-symmetry state, b-type transitions observed
in the PRIMOS survey. The lowest frequency transition detected near 6358 MHz
(Figure 2a) shows resolved HFS, multiple velocity components at +62, +71 and
$+81\ \mathrm{km\ s}^{-1}$ and maser activity. The model fit (red trace)
cannot reproduce the measured emission features detected from this transition.
In contrast, nearly all other transitions detected are well characterized by
the LTE model - even the weaker features. The transition near 9253 MHz (Figure
2b) has a slightly stronger $+62\mathrm{km\ s}^{-1}$ absorption component than
predicted by the model. This can be explained by possible contamination from
other spectral features near those frequencies in either absorption or
emission. For example, the H(113)$\epsilon$ recombination spectral line
profile is clearly impacted by absorption due to the acetamide transition near
21362 MHz (Figure 2g). What also differs from the initial detection paper by
Hollis et al. (2006a) is that the best fit velocity components of CH3CONH2
occur at +62, +71 and $+81\ \mathrm{km\ s}^{-1}$ and not at the notional +64,
+73 and $+82\ \mathrm{km\ s}^{-1}$ identified in other species - suggesting a
spatial differentiation of acetamide from other species such as formamide,
glycolaldehyde, propynal and cyclopropenone (Hollis et al. 2004b, a, 2006b;
Belloche et al. 2013). Table 5 presents the spectroscopic parameters for the
CH3CONH2 transitions detected in this work including the HFS for frequencies
below 18 GHz (where resolved) for the A-symmetry states and Figure 3 shows the
6358 MHz transition illustrating all the HFS and velocity components.

Figure 2: Synthetic LTE spectrum of CH3CONH2 (in green) A-symmetry state
transitions generated from the physical conditions and observing parameters
described in Section 3.2 ($\mathrm{T_{ex}}=5.8$ K, source size of $20\arcsec$,
three velocity components at +62, +71 and $+81\ \mathrm{km\ s}^{-1}$ yielding
an $\mathrm{N_{T}}\sim 7.7\times 10^{13}\ \mathrm{cm}^{-2}$ from all three
components) overlaid on the GBT spectrum of Sgr B2(LMH) (in black). Transition
quantum numbers and upper state energy levels (Table 5) are in the bottom left
corner of each spectrum. Quantum numbers with an ”$\star$” have HFS included
in the synthetic spectrum.

Figure 3: Illustrative spectrum of CH3CONH2 showing the relative intensities
of all the HFS (Table 5) and multiple velocity components at +62 (in yellow),
+71 (in blue) and $+81\ \mathrm{km\ s}^{-1}$ (in green) overlaid on the GBT
spectrum of Sgr B2(LMH) (in black). The intensity of these emission features
cannot be matched by the synthetic LTE model in this work. Table 6:
Spectroscopic properties of CH3CONH2 lines detected in PRIMOS toward Sgr
B2(LMH).

Transitions | Symm. | Frequency | $E_{up}$ | $S_{ij}\mu^{2}$ | Transition
---|---|---|---|---|---
$J^{\prime}(K_{a}^{\prime},K_{c}^{\prime})\ -\ J^{\prime\prime}(K_{a}^{\prime\prime},K_{c}^{\prime\prime})$ | $F^{\prime}\ -\ F^{\prime\prime}$ |  | (MHz) | (K) | (D2) | Type
1(1,0) - 1(0,1) | F = 1-1 | A | 6357.895(8) | 0.9988 | 1.595 |  bQ1,-1
| F = 1-2 | A | 6358.472(7) | 0.9988 | 2.658 |
| F = 2-1 | A | 6359.078(6) | 0.9989 | 2.657 |
| F = 1-0 | A | 6359.338(8) | 0.9988 | 2.126 |
| F = 2-2 | A | 6359.656(5) | 0.9989 | 7.973 |
| F = 0-1 | A | 6360.853(8) | 0.9990 | 2.125 |
2(1,1) - 1(1,0) |  | E | 9050.672(1) | 8.8659 | 4.154 |  aP0,1
2(2,0) - 2(1,1) | F = 2-1 | A | 9252.990(7) | 3.0293 | 1.247 |  bQ1,-1
| F = 2-3 | A | 9253.351(6) | 3.0293 | 1.294 |
| F = 2-2 | A | 9254.000(6) | 3.0293 | 5.771 |
| F = 3-3 | A | 9254.535(4) | 3.0293 | 10.339 |
| F = 1-1 | A | 9254.833(6) | 3.0294 | 3.738 |
| F = 3-2 | A | 9255.185(6) | 3.0293 | 1.293 |
| F = 1-2 | A | 9255.843(7) | 3.0294 | 1.246 |
1(1,1) - 1(0,1) |  | E | 13388.703(1) | 7.4374 | 7.644 |  aQ1,0
4(3,1) - 4(2,2) | F = 4-3 | A | 14209.211(10) | 9.3165 | 0.749 |  bQ1,-1
| F = 4-5 | A | 14209.393(9) | 9.3165 | 0.753 |
| F = 4-4 | A | 14210.101(7) | 9.3165 | 13.901 |
| F = 5-5 | A | 14210.440(7) | 9.3165 | 18.072 |
| F = 3-3 | A | 14210.527(7) | 9.3165 | 11.233 |
| F = 5-4 | A | 14211.147(9) | 9.3165 | 0.753 |
| F = 3-4 | A | 14211.416(10) | 9.3165 | 0.749 |
3(3,0) - 3(2,1) |  | A | 14441.706(1) | 6.1401 | 24.436 |  bQ1,-1
2(1,2) - 1(1,1) |  | E | 14651.890(1) | 8.1405 | 11.226 |  aP0,1
3(1,2) - 3(1,3) |  | E | 14960.874(1) | 10.4612 | 4.369 |  aQ0,-1
2(1,1) - 2(1,2) |  | E | 15115.748(1) | 8.8659 | 13.673 |  aQ0,-1
4(3,2) - 4(2,2) |  | E | 18507.373(2) | 15.0377 | 23.916 |  bQ1,0
6(4,2) - 6(3,3) |  | A | 19422.841(1) | 19.0778 | 65.746 |  bQ1,-1
5(3,2) - 5(2,3) |  | A | 19463.627(1) | 13.3666 | 49.517 |  bQ1,-1
3(2,1) - 3(2,2) |  | E | 20686.104(1) | 12.0580 | 23.052 |  aQ0,-1
2(2,1) - 2(1,1) |  | E | 20891.602(1) | 9.8686 | 16.739 |  bQ1,0
4(2,2) - 4(1,3) |  | A | 21362.137(1) | 8.6346 | 30.887 |  bQ1,-1
4(4,0) - 4(3,1) |  | A | 21762.725(2) | 10.3609 | 22.240 |  bQ1,-1
2(0,2) - 1(0,1) |  | E | 22095.527(1) | 7.8551 | 18.829 |  aP0,1
3(3,1) - 3(2,2) |  | A | 22769.640(1) | 6.0783 | 16.040 |  bQ1,-1
6(5,1) - 6(4,2) |  | A | 25149.383(2) | 20.2848 | 45.460 |  bQ1,-1
4(3,1) - 4(3,2) |  | E | 26196.049(1) | 16.2949 | 30.516 |  aQ0,-1
555CH3CONH2 rest frequencies from Ilyushin (2004) and F. J. Lovas, private
communication. Uncertainties in parentheses refer to the least significant
digit and are 2$\sigma$ values (type A coverage) (Taylor & Kuyatt 1994).

The situation is not as clear for the E-symmetry state transitions. Figure 4
shows the detection of ten E-state transitions. Unlike the A-state
transitions, given the difficulty in fitting the E-state, there is no
corresponding HFS data available. In addition, the astronomically detected
E-state transitions in the low frequency data are not limited to just bQ1,-1
type transitions. For example, the feature detected at 26196 MHz (Figure 4j)
is a aQ0,-1 transition; the features detected at 9050 (Figure 4a) and 14652
MHz (Figure 4c) are aP0,1 transitions; finally the feature detected at 13388
MHz (Figure 4b) is a aQ0,1 transition that appears to be a weak maser
transition. Nonetheless, the best fit parameters found for the E-state
transition are constrained to be similar to that for the A-states; that is the
model fits the data very well for an excitation temperature similar to what
was determined for the A-state but with a factor of $\sim$2 times higher
column density for those features found in absorption and not contaminated by
other spectral features. Similar to the detected A-state transitions, several
E-state transitions are contaminated by features from H-recombination lines
(H(104)$\delta$ at 22095 MHz, H(106)$\delta$ at 20891 MHz, and H(85)$\beta$ at
20686 MHz) or possibly other unidentified transitions. We note that the 15115
MHz CH3CONH2 transition (Figure 4e) only shows emission near the 71 km/s
velocity component and no other features at 62 or 81 km/s suggesting that this
feature is also a maser transition or possibly contaminated by another
species. The largest discrepancy between the data and the fit is around 14960
MHz. With no other transitions in the passband to interfere with the
detection, this feature is markedly absent in the astronomical data compared
to the other detected transitions. This transition is a aQ0,-1 similar to the
transitions shown in Figure 4e, 4g, and 4j. Looking at the energy levels of
the transitions in these figures, it appears that for transitions with upper
state energies less than 10 K, features are detected in absorption (4g, and
4j). At 15 K, the feature of this transition type is in emission (4e). And, at
$\sim$11 K, features from this transition type are absent (4d). It is possible
that features from these types of transitions pass from emission (when
$E_{u}\sim 15\mathrm{K}$) to absorption (when $E_{u}$ $\sim$ 9.87 and 9.0
$\mathrm{K}$) passing through an upper state energy around 11 K. This could
account for the transition being absent compared to the model data. The
transition is also unique in that it is a $J-K_{a}=2$ transition for both the
upper and lower states. None of the other astronomically detected transitions
are of this type. Table 5 presents the spectroscopic data for the E-state
CH3CONH2 transitions detected in the PRIMOS observations.

In total, the best fit model to the PRIMOS data was found for an excitation
temperature of 5.8 K and a source size of $20\arcsec$ for both the A- and
E-state transitions, and the total measured CH3CONH2 column density determined
from the PRIMOS data from the current analysis, is $\sim 7.7\times 10^{13}\
\mathrm{cm}^{-2}$ for the A-state transitions and $\sim 1.5\times 10^{14}\
\mathrm{cm}^{-2}$ for the E-state transitions. These values are on the same
order of magnitude with both the $\sim 8\times 10^{13}\ \mathrm{cm}^{-2}$
column density determined by Hollis et al. (2006a) at an excitation
temperature of 5 K and the $\sim 5\times 10^{13}\ \mathrm{cm}^{-2}$ determined
by Halfen et al. (2011) at an excitation temperature of 17 K for transitions
detected with upper state energy levels lower than 40 K. And, as expected, the
observed temperature and measured column densities from this current work are
much lower than what was reported by Belloche et al. (2017) toward SgrB2(N2)
from the EMoCA survey where CH3CONH2 was detected at a column density of $\sim
2\times 10^{17}\ \mathrm{cm}^{-2}$ at an excitation temperature of 180 K (see
Table 5) - clearly emission coming from a hot core region which is beam
diluted in our GBT observations. What this analysis also indicates, given the
structural similarity of CH3CONH2 and CH3CSNH2, are the types of transitions
that are most likely to be detected at lower frequencies with single dish
instruments such as the GBT.

Figure 4: Synthetic LTE spectrum of CH3CONH2 (in green) E-symmetry state
transitions generated from the physical conditions and observing parameters
described in Section 3.2 ($\mathrm{T_{ex}}$ = 5.8 K, source size of
$20\arcsec$, three velocity components at +62, +71 and $+81\ \mathrm{km\
s}^{-1}$ yielding an $\mathrm{N_{T}}\sim 1.5\times 10^{14}\ \mathrm{cm}^{-2}$
from all 3 components) overlaid on the GBT spectrum of Sgr B2(LMH) shown in
black. Transition quantum numbers and upper state energy levels (Table 5) are
in the bottom left corner of each spectrum.

Figure 5 shows the results of our attempt to detect the strongest transitions,
including possible maser transitions, from CH3CSNH2 between 8 and 24 GHz from
the PRIMOS survey. Since the fit to CH3CSNH2 was done utilizing the rho-axis
method (see Section 2.2), A- and E-symmetry state transitions from CH3CSNH2
were both initially searched for, though due to the limited Doppler resolution
used in the spectrometer, it was not possible to search for any HFS. Also,
given $\mu_{a}=4.1D$ for CH3CSNH2, the strongest transitions should be a-type
transitions ($\mu_{b}=-1.1D$). Finally, the lowest energy state transitions
could also be possible maser transitions. And, for completeness, we searched
for all A- and E-symmetry state, P- and Q-branch, a- and b-type transitions
over the entire PRIMOS observing range.

As was done when first characterizing the astronomical spectra of CH3CONH2, we
used an LTE-model666As with CH3CONH2, maser transitions are identified because
they greatly deviate from the predicted LTE-model. A full characterization of
the spectrum including this maser activity is beyond the scope of this work.
assuming a non-thermal background source of emission (Hollis et al. 2007;
McGuire et al. 2016) and simulated the spectrum of CH3CSNH2 assuming a
temperature of 5.8 K, a source size of $\sim 20\arcsec$ and a line width of
$20\ \mathrm{km\ s}^{-1}$.

However, as Figure 5 illustrates, similar to what was found in the EMoCA
survey, no CH3CSNH2 transitions (in red) were detected - although there is a
nearly frequency coincident absorption feature close to the 1(0,1) - 0(0,0) E
transition at 8189.41 MHz. The lack of exact frequency coincidence and of any
additional transitions detected however, makes this feature very unlikely to
be due to CH3CSNH2. To set an upper limit of CH3CSNH2, a rotational
temperature of 5.8 K was used and based on the lowest noise level from the
CH3CSNH2 passbands, an upper limit of $2.2\times 10^{13}\ \mathrm{cm}^{-2}$
was found - again indicating that CH3CSNH2 is at least an order of magnitude
less abundant than CH3CONH2 in the low temperature, extended regions around
Sgr B2(N).

Table 7: Spectroscopic properties of the CH3CSNH2 lines shown toward the Sgr
B2 and NGC 6334I regions.

Transition | Symm. | Frequency | $E_{up}$ | $S_{ij}\mu^{2}$ | Transition
---|---|---|---|---|---
$J^{\prime}(K_{a}^{\prime},K_{c}^{\prime})\ -\ J^{\prime\prime}(K_{a}^{\prime\prime},K_{c}^{\prime\prime})$ |  | (MHz) | (K) | (D2) | Type
1(0,1) - 0(0, 0) | E | 8189.4099(78) | 1.80 | 15.7 |  aP0,1
3(1,2) - 3(1, 3) | A | 9889.2108(51) | 2.91 | 9.3 |  aQ0,-1
| E | 10085.3720(121) | 4.30 | 9.3 |  aQ0,-1
1(1,1) - 0(0, 0) | A | 13108.4651(33) | 0.63 | 1.2 |  bP1,1
2(1,2) - 1(1, 1) | A | 15107.3571(9) | 1.35 | 23.5 |  aP0,1
| E | 15402.1503(85) | 2.73 | 22.1 |  aP0,1
2(1,1) - 1(1, 0) | E | 17958.6594(97) | 3.01 | 21.4 |  aP0,1
| A | 18421.0295(25) | 1.59 | 23.5 |  aP0,1
3(0,3) - 2(0, 2) | E | 23679.9434(25) | 3.71 | 45.9 |  aP0,1
| A | 23790.9359(13) | 2.33 | 45.3 |  aP0,1
13(*,13) - 12(*,12) | E | 90629.9722(29) | 32.74 | 428.0 |
12(3,10) - 11(3,9) | A | 96830.8697(45) | 33.60 | 171.0 |
14(*,14) - 13(*,13) v=1 | E | 96876.3625(94) | 124.33 | 463.4 |
14(*,14) - 13(*,13) | E | 97354.0297(31) | 37.41 | 461.8 |
14(*,14) - 13(*,13) | A | 97403.6841(35) | 36.05 | 461.6 |
15(*,15) - 14(*,14) | E | 104078.1290(33) | 42.40 | 495.5 |
15(*,15) - 14(*,14) | A | 104127.4707(38) | 41.05 | 495.3 |
14(2,12) - 13(2,11) | E | 110443.5401(44) | 45.08 | 203.0 |
15(2,14) - 14(2,13) | E | 110488.6550(31) | 46.76 | 224.7 |
15(1,14) - 14(1,13) | E | 110493.3785(31) | 46.76 | 224.7 |
16(*,16) - 15(*,15) | E | 110802.2128(35) | 47.72 | 529.1 |
16(*,16) - 15(*,15) | A | 110851.2160(40) | 46.37 | 529.0 |
16(*,16) - 15(*,15) v=1 | A | 111137.3603(103) | 120.00 | 528.2 |
18(*,17) - 17(*,16) | E | 130649.9350(34) | 64.60 | 577.4 |
18(*,17) - 17(*,16) | A | 130810.0540(37) | 63.35 | 576.9 |
20(*,19) - 19(*,18) | E | 144091.6978(36) | 78.11 | 644.6 |
20(*,19) - 19(*,18) | A | 144251.6541(39) | 76.88 | 644.2 |
21(*,21) - 20(*,20) | E | 144421.2557(45) | 79.15 | 697.5 |
21(*,21) - 20(*,20) | A | 144468.4149(50) | 77.81 | 697.4 |
777The approximate center frequency is listed for those transitions that are
not resolved given the spectroscopic resolution of the astronomical
observations. Each of these lines consists of two a-type and two b-type
transitions (for example, $18_{*,17}-17_{*,16}$ refers to the following 4
transitions: $18_{1,17}-17_{2,16}$, $18_{2,17}-17_{2,16}$,
$18_{1,17}-17_{1,16}$ and $18_{2,17}-17_{1,16}$ that are nearly frequency
coincident). The listed $S_{ij}\mu^{2}$ value is the sum of all four
transitions in each group. This coincidence is indicated by the asterisk
substituted for the Ka quantum number. Uncertainties in parentheses refer to
the least significant digit and are 2$\sigma$ values (type A coverage) (Taylor
& Kuyatt 1994).

Figure 5: Synthetic LTE spectrum of CH3CSNH2 (in red). The A-symmetry state
transitions are presented in the first column in panels (a)-(e) and the
E-symmetry state transitions are presented in the second column in panels
(f)-(j). The spectrum was generated from the physical conditions and observing
parameters described in Section 3.2 (that is, Tex = 5.8, source size of
$20\arcsec$, $N_{T}\sim 2.2\times 10^{13}\ \mathrm{cm}^{-2}$) overlaid on the
GBT spectrum of Sgr B2(LMH) shown in black. Transition quantum numbers and
upper state energy levels (Table 7) are at the bottom of each spectrum. No
significant emission from CH3CSNH2 transitions are detected beyond the
$3\sigma$ upper limit.

###  3.3 Search for CH3CSNH2 toward NGC 6334I with ALMA

To date, there are only 3 sources with confirmed detections of CH3CONH2. These
include the already discussed, Sgr B2(N) region, the high mass star forming
regions NGC 6334I (Ligterink et al. 2020) and G31.41+0.31 (Colzi et al. 2021).
NGC 6334I, at a distance of 1.3 kpc (Reid et al. 2014), is a massive
protocluster (Hunter et al. 2006) undergoing active star formation. Toward
this source, two distinct regions exhibit spectra typical to hot core regions,
designated MM1 and MM2 (Zernickel et al. 2012; Brogan et al. 2016). This
source was the site of the first interstellar detections of methoxymethanol
(CH3OCH2OH) (McGuire et al. 2017) and the first two vibrationally excited
torsional states of acetic acid (CH3COOH) (Xue et al. 2019) and has been the
target of several investigations of complex molecules (El-Abd et al. 2019;
Melosso et al. 2020).

ALMA observations of NGC 6334I at an angular resolution of 0.26″in ALMA Bands
4 and 7 were used to search for transitions of CH3CSNH2 toward this source.
The details of the individual data sets and their reductions are available in
Hunter et al. (2017); McGuire et al. (2017, 2018).

Figure 6 shows the results of our attempt to detect the strongest transitions
from CH3CSNH2 in ALMA Bands 4 and 7 and, in the data presented, only the most
prominent transitions are shown (Table 7). As with the data shown toward Sgr
B2(N) with the EMoCA and PRIMOS programs, there is no evidence supporting its
presence toward NGC 6334I. Similar to Figure 1, the dotted lines represent
$3\sigma$ rms values and the synthetic LTE spectrum (red trace) is overlaid on
the ALMA spectrum (black trace). Assuming Tex to be 135 K, typical of other
large molecules toward this source and the best fit temperature determined
from the detection of CH3CONH2 in Ligterink et al. (2020), the upper limit of
$N_{T}$ of CH3CSNH2 is $2\times 10^{15}\ \mathrm{cm}^{-2}$. Given the measured
CH3CONH2 column density of $4.7\times 10^{16}\ \mathrm{cm}^{-2}$, the upper
limit of the CH3CSNH2/CH3CONH2 abundance ratio is $0.04$ \- which is about a
factor of 3 smaller than what was found toward Sgr B2(N2) in a similar
frequency range from the EMoCA survey (See Section 3.1).

Figure 6: Synthetic LTE spectrum of CH3CSNH2 (in red) used to derive the upper
limit to its column density, overlaid on the ALMA spectrum of NGC 6334I (in
black). Figure labels are similar to Figure 1. The transitions of CH3CSNH2
that are weaker than 3$\sigma$ and the ones that are heavily contaminated by
other species are not shown.

###  3.4 Cursory Search for CH3CSNH2 in the ASAI sources with the IRAM 30 m

In order to complete the investigation that initially started with the search
for CH3CSNH2 toward L1544 and L1157-B from the ASAI survey, the remaining
targets from the ASAI survey were searched for the strongest transitions of
CH3CSNH2 and CH3CONH2. All the objects represent different evolutionary stages
to forming a Sun-like star - starless cores to Class 0/I sources (such as
Barnard 1, NGC1333-IRAS 4A, L1157mm, L1527, and SVS13-A) as well as jet-driven
shocked regions (such as L1448R2). The observational details of the ASAI
survey and the sources therein are fully described in Lefloch et al. (2018).
Here we present only the parameters that were used to derive the column
densities or the upper limits of both species toward these sources.

As with the searches already presented toward Sgr B2 and NGC 6334I, we find no
evidence of CH3CSNH2 in any of the ASAI sources. In addition, and quite
unfortunate, we also find no evidence for CH3CONH2 toward any of the ASAI
sources either. As such, meaningful column density ratios cannot be determined
as only upper limits to the column densities of CH3CSNH2 and CH3CONH2 can be
determined.

Similar to the analysis performed toward Sgr B2(N) and NGC 6334I, a synthetic
spectrum of each molecule was generated using the physical conditions for the
source (Table 8) and the spectroscopic line parameters measured in this work.
Then, following the methodology that was used to find the upper limits to
HCCCHS and HCCCHO (Margulès et al. 2020b), the $3\sigma$ upper limit to the
column density was derived, using the line that gave the most rigorous
constraint (that is the line that would be the highest signal-to-noise in the
event of a detection). These lines are provided in Table 9 along with the
resulting upper limits.

Table 8: Parameters for each of the ASAI sources used to search for CH3CSNH2
and CH3CONH2 .

Source | $\Delta V$ | $T_{b}$ | $T_{ex}$
---|---|---|---
| (km s-1) | (mK) | (K)
Thioacetamide
Barnard 1 | 0.8 | 6.0 | 10
IRAS 4A | 5.0 | 6.2 | 21
L1157B1 | 8.0 | 4.2 | 60
L1157mm | 3.0 | 7.0 | 60
L1448R2 | 8.0 | 7.6 | 60
L1527 | 0.5 | 5.6 | 12
L1544 | 0.5 | 6.4 | 10
SVS13A | 3.0 | 15.8 | 80
Acetamide
Barnard 1 | 0.8 | 7.4 | 10
IRAS 4A | 5.0 | 8.3 | 21
L1157B1 | 8.0 | 4.5 | 60
L1157mm | 3.0 | 7.3 | 60
L1448R2 | 8.0 | 23.3 | 60
L1527 | 0.5 | 8.0 | 12
L1544 | 0.5 | 8.2 | 10
SVS13A | 3.0 | 17.2 | 80
888In all cases, a background temperature of 2.7 K was used for each source.
$T_{b}$ is taken as the $3\sigma$ rms noise level at the location of the
target line. Furthermore, all observations were taken with the IRAM 30m
telescope and it was assumed that the source fills the beam except in the case
of SVS13A where a source size of 0.′′3 was used. For additional information on
each of the sources as well as a reference to the original citations where the
physical conditions were measured, see Margulès et al. (2020a).

Table 9: Upper limits to the abundances of CH3CSNH2 and CH3CONH2 toward the
ASAI sources and the spectral line parameters used to calculate the upper
limits in each of the sets of observations.

Source | Frequency | Transition | $E_{u}$ | $S_{ij}\mu^{2}$ |  $Q$ ($Q_{rot}$, $Q_{vib}$)a | $N_{T}$ | $N(H_{2})$ | $X_{H_{2}}$
---|---|---|---|---|---|---|---|---
| (MHz) |  ($J_{K_{a},K_{c}}^{\prime}-J_{K_{a},K_{c}}^{\prime\prime}$) | (K) |  (Debye2) |  |  (cm-2) |  (cm-2) |
CH3CSNH2
Barnard 1 | 91379.2 |  $11_{2,9}-10_{2,8}$ A | 28.9 | 156.8 | 807 (807, 1.00) | $\leq 1.06\times 10^{12}$ | $1.5\times 10^{23}$ | $\leq 7\times 10^{-12}$
IRAS 4A | 104127.5 |  $15_{*,15}-14_{*,14}$ A | 41.1 | 495.4 | 2484 (2484, 1.00) | $\leq 1.97\times 10^{12}$ | $3.7\times 10^{23}$ | $\leq 5\times 10^{-12}$
L1157B1 | 90679.9 |  $13_{*,13}-12_{*,12}$ A | 31.4 | 427.9 | 16702 (16702, 1.00) | $\leq 4.61\times 10^{12}$ | $1\times 10^{21}$ | $\leq 5\times 10^{-9}$
L1157mm | 104078.1 |  $15_{*,15}-14_{*,14}$ E | 42.4 | 495.6 | 16702 (16702, 1.00) | $\leq 2.53\times 10^{12}$ | $6\times 10^{21}$ | $\leq 4\times 10^{-10}$
L1448R2 | 97403.7 |  $14_{*,14}-13_{*,13}$ A | 36.1 | 461.6 | 16702 (16702, 1.00) | $\leq 7.48\times 10^{12}$ | $3.5\times 10^{23}$ | $\leq 2\times 10^{-11}$
L1527 | 104127.5 |  $15_{*,15}-14_{*,14}$ A | 41.1 | 495.4 | 1045 (1045, 1.00) | $\leq 4.15\times 10^{11}$ | $2.8\times 10^{22}$ | $\leq 1\times 10^{-11}$
L1544 | 91492.8 |  $10_{3,7}-9_{3,6}$ A | 26.1 | 142.5 | 807 (807, 1.00) | $\leq 5.49\times 10^{11}$ | $5\times 10^{21}$ | $\leq 1\times 10^{-10}$
SVS13A | 238335.5 |  $34_{*,33}-33_{*,32}$ A | 207.7 | 1115.6 | 29696 (29696, 1.00) | $\leq 1.82\times 10^{16}$ | $3\times 10^{24}$ | $\leq 6\times 10^{-10}$
CH3CONH2
Barnard 1 | 87632.5 |  $8_{*,8}-7_{*,7}$ E | 19.7 | 194.8 | 370 (370, 1.00) | $\leq 2.01\times 10^{11}$ | $1.5\times 10^{23}$ | $\leq 1\times 10^{-12}$
IRAS 4A | 108255.2 |  $10_{*,10}-9_{*,9}$ E | 29.7 | 254.0 | 1382 (1382, 1.00) | $\leq 1.60\times 10^{12}$ | $3.7\times 10^{23}$ | $\leq 4\times 10^{-12}$
L1157B1 | 108255.2 |  $10_{*,10}-9_{*,9}$ E | 29.7 | 254.0 | 10929 (10929, 1.00) | $\leq 4.27\times 10^{12}$ | $1\times 10^{21}$ | $\leq 4\times 10^{-9}$
L1157mm | 97943.9 |  $9_{*,9}-8_{*,8}$ E | 24.5 | 226.8 | 10929 (10929, 1.00) | $\leq 2.97\times 10^{12}$ | $6\times 10^{21}$ | $\leq 5\times 10^{-10}$
L1448R2 | 149411.2 |  $14_{*,14}-13_{*,13}$ A | 61.4 | 361.6 | 10929 (10929, 1.00) | $\leq 1.08\times 10^{13}$ | $3.5\times 10^{23}$ | $\leq 3\times 10^{-11}$
L1527 | 87632.5 |  $8_{*,8}-7_{*,7}$ E | 19.7 | 194.8 | 500 (500, 1.00) | $\leq 1.54\times 10^{11}$ | $2.8\times 10^{22}$ | $\leq 6\times 10^{-12}$
L1544 | 97943.9 |  $9_{*,9}-8_{*,8}$ E | 24.5 | 226.8 | 370 (370, 1.00) | $\leq 1.64\times 10^{11}$ | $5\times 10^{21}$ | $\leq 3\times 10^{-11}$
SVS13A | 252588.4 |  $24_{*,24}-23_{*,23}$ E | 154.4 | 632.8 | 19442 (19442, 1.00) | $\leq 9.79\times 10^{15}$ | $3\times 10^{24}$ | $\leq 3\times 10^{-9}$
999The approximate center frequency is listed for those transitions that are
not resolved given the spectroscopic resolution of the astronomical
observations. Each of these lines consists of two a-type and two b-type
transitions (for example, $15_{*,15}-14_{*,14}$ refers to the following 4
transitions: $15_{0,15}-14_{1,14}$, $15_{1,15}-14_{1,14}$,
$15_{1,15}-14_{0,14}$ and $15_{1,15}-14_{1,14}$ that are nearly frequency
coincident). The listed $S_{ij}\mu^{2}$ value is the sum of all four
transitions in each group. This coincidence is indicated by the asterisk
substituted for the Ka quantum number. The total partition functions and
column densities were calculated at the measured temperature of each source as
listed in Table 4.

Figure 7: Transitions of CH3CONH2 used to calculate the $3\sigma$ upper limits
given in Table 9. In each panel, the red trace shows the transition simulated
using the derived upper limit column density and the physical parameters
assumed for that source. The dotted line in each panel indicates the $3\sigma$
noise level. The quantum numbers and upper-level energies for each transition
is shown in the lower right of each panel. The source name is given in the
upper left of each panel. Due to the large variances between observations, the
intensity and velocity axes are not necessarily uniform between each panel.
Figure 8: Transitions of CH3CSNH2 used to calculate the upper limits given in
Table 9. Panel description is the same as in Figure 7.

##  4 Discussion

The question that remains is whether the detection (or non-detections) of
these larger molecules better calibrate existing chemical formation theories.
Earlier work suggested that the formation of NH2CHO (and possibly CH3CONH2)
can form in the gas phase via NH2 + H2CO $\longrightarrow$ NH2CHO + H (Barone
et al. 2015; Skouteris et al. 2017). An analogous reaction with CH3CHO, that
is, NH2 + CH3CHO, can lead to CH3CONH2 + H in competition with another channel
leading to NH2CHO + CH3. However, a kinetic study has shown that NH2 + CH3CHO
has an activation energy barrier (Hack et al. 1986). Following these examples,
the reactions NH2 + H2CS and NH2 + CH3CHS may lead to the formation of NH2CHS
and CH3CSNH2, respectively. Of course, dedicated work is necessary to confirm
this suggestion.

A more recent study was done to try and determine the most probable formation
route to form CH3CONH2 under interstellar conditions. Foo et al. (2018)
investigated every possible structural isomer of CH3CONH2 and as CH3CONH2 was
found to be the most stable isomer, they selected the 4 most stable
bimolecular species (and associated reaction pathways) as possible molecular
precursors to form CH3CONH2 (as well as other possible unimolecular formation
schemes whereby CH3CONH2 could form by the rearrangement of atoms from a
molecular isomer - such as CH3NHCHO). In that work, it was shown that the most
probable reaction pathways to forming CH3CONH2 would be the following gas
phase reactions:

$\displaystyle\ce{CH4}$ $\displaystyle+\ce{HNCO}$ (2) $\displaystyle\ce{H2O}$
$\displaystyle+\ce{CH3CN}$ (3) $\displaystyle\ce{CO}$
$\displaystyle+\ce{CH3NH2}$ (4) $\displaystyle\ce{NH3}$
$\displaystyle+\ce{H2CCO}.$ (5)

Of these reactions, only (3) and (5) could be feasible and the formation route
via H2O + CH3CN requires a 2-step process to form CH3CONH2 through the
isomerization of CH3NHCHO. However, both have to travel through large
transition state barriers which make these routes highly improbable except in
regions that can clearly drive nonthermal excitation - such as shocked
regions. For example, the formation (and detection) of methyl isocyanide
(CH3NC) and carbodiimide (HNCNH) is explained by the nonthermal processes from
the isomerization of methyl cyanide (CH3CN) and cyanamide (NH2CN),
respectively (Remijan et al. 2005; McGuire et al. 2012). As such, the final
formation of CH3CONH2 could be through the isomerization of CH3NHCHO.

As always, additional possibilities exist to form molecules on grain surfaces.
In recent experiments by Ligterink et al. (2018), a CH4:HNCO ice mixture at 30
K was irradiated by far UV photons to investigate the potential products
generated. The precursors used in these reactions are the same chemical
precursors as in reaction (2) only now these species are contained in an ice
matrix instead of in the gas phase. The main conclusion was that the formation
of amides can be explained by nonenergetic radical recombination reactions
with NH2. For example, the formation of formamide (NH2CHO) on a grain surface
takes place by:

The experiments successfully reproduced the abundance ratios CH3CONH2/NH2CHO
measured in the ISM. The experimental value determined is
0.4$\pm^{0.39}_{0.14}$ and while the observed values vary toward different
sources, all measured values fall within a range of 0.04-1.6 (for example,
Hollis et al. (2006a); Belloche et al. (2017, 2019); Halfen et al. (2011) and
this work). The conclusion reached based on these experiments was that
specifically toward hot cores, the abundance of CH3CONH2 can be explained by a
formation on interstellar grains with subsequent desorption from grain
surfaces. Yet, gas phase reactions under some interstellar conditions cannot
be ruled out. By observing less abundant species with similar molecular
structure, it may be possible to better determine the formation pathways - or
at least which pathway is more dominant or prevalent.

Acetamide (CH3CONH2) and other oxygen-bearing amide molecules are shown to
efficiently form on grain surfaces involving radicals. The reactant radicals
as listed in reactions (6), (7), and (8) form through the photodissociation of
methanol and ammonia on grain surfaces. Given the presumed reactions (either
in the gas phase or grain surface) for the formation of CH3CONH2, it may be
possible to obtain additional insight into the chemical kinetics by looking
for similar reactions to form CH3CSNH2. Yet, the overall excitation and
distribution of S-bearing molecules are quite different from their O-bearing
counterparts. Gas-grain models specifically do not take into account surface
reactions from many small S-bearing species as the behavior of sulfur on grain
surfaces is less well understood. Furthermore, the energetics of the
S-substituted species will be different from their O-bearing counterparts
making the chemical kinetics that is driving their formation much different
given a similar looking reaction.

Considering that a majority of large sulfur-bearing molecules have been
detected toward cold regions such as dark clouds (for example, Cernicharo et
al. 2021, and references therein), the possible gas-phase formation pathways
to forming CH3CONH2 may help to guide our understanding on the formation
mechanism of its sulfur-substitute amide, CH3CSNH2. Therefore, we propose
analogous neutral-neutral reactions to that of CH3CONH2:

$\displaystyle\ce{CH4}$ $\displaystyle+\ce{HNCS}$ (6) $\displaystyle\ce{H2S}$
$\displaystyle+\ce{CH3CN}$ (7) $\displaystyle\ce{CS}$
$\displaystyle+\ce{CH3NH2}$ (8) $\displaystyle\ce{NH3}$
$\displaystyle+\ce{H2CCS}.$ (9)

Of the precursor species in these reactions, all have been detected in the gas
phase toward a diverse sample of astronomical objects. For example, HNCS and
CH3NH2 were detected in the outer envelopes and hot cores of Sgr B2(N) (Kaifu
et al. 1974; Halfen et al. 2009; Corby et al. 2015); CH3CN is ubiquitous
toward numerous sources including hot cores (Gieser et al. 2021, and
references therein), translucent clouds (Corby et al. 2015), and dark clouds
(Willacy et al. 1993).

Finally, H2CCS has recently been detected toward cold regions (Cernicharo et
al. 2021) and CH4, H2S, CS and NH3 are all highly abundant in the gas phase in
nearly all regions of molecular gas. As such, the proposed gas-phase formation
routes to CH3CSNH2 in reactions (9-12) may be possible if energetically
favorable. To test their feasibility, quantum chemical calculations were
carried out with the Gaussian 16 suite of programs (Frisch et al. 2016b). All
geometries were optimized using Density Functional Theory with the B3LYP
method (Stephens et al. 1994) and a 6-311G++(d,p) basis set. The energetics
were corrected with free energy corrections at a temperature of 15 K at the
same level of theory. Following similar reaction paths as Foo et al. (2018),
transition states and intermediates were only found for reactions (10) and
(12) due to the potentially large activation energies of reactions (9) and
(11). The transition states were confirmed using an internal reaction
coordinate analysis, and all stationary points found were further constrained
by performing CCSD(T) single point energy calculations with the aug-cc-pVTZ
basis set. The Gibbs free energy of reactions (9), (10), (11), and (12) were
found to be -37.52, -27.62, -236.11, and -133.87 kJ/mole, respectively.
Reaction (10) was found to be a two step process with Gibbs free energy of
activations of 210.89 kJ/mole for the first step and 111.56 kJ/mole for the
second. Reaction (12) was a single step process with a Gibbs free energy of
activation of 170.62 kJ/mole. Though all proposed reactions are exothermic,
the associated energy barriers are rather large for feasible reaction paths in
cold interstellar conditions.

Also, the presumed main driver of S-chemistry on grain surfaces, H2S, has yet
to be unambigously detected on a grain surface. The grain surface reactions to
the formation of larger S-bearing molecules require that a radical containing
S (such as, HCS) or neutral species (such as, CS) be available in large
abundances on grain surfaces. Since these precursor species have yet to be
directly detected on grain surfaces, it is unlikely that analogous routes with
S-bearing species in reactions (6-8) would lead to the formation of larger
S-bearing molecules. In addition, dedicated searches for larger S-bearing
molecules have taken place over a variety of sources with several non-
detections (for example, CH3CHS(Margulès et al. 2020a), HCCCHS(Margulès et al.
2020b), and NH2CHS(Margulès et al. 2020b)) and toward locations where
comparative O-chemistry may not be present (Cernicharo et al. 2021). As such,
it is currently difficult to constrain any of the chemistry with such a
disparate set of data, yet trends comparing the relative abundances between
the O-bearing vs. S-bearing species have been done toward those sources even
if all we have are upper limits to the column densities which may eventually
lead to a better determination of the formation pathways of both families of
molecules.

##  5 Conclusions

The spectrum of CH3CSNH2 was investigated and new fits were generated using
the RAM method to assign the spectrum of CH3CSNH2 from 150 to 650 GHz. A total
of 1428 transitions from the vt=0 state with maximum values J=47 and Ka=20 in
the range to 330 GHz, and J=95 and Ka=20 in the range 400–660 GHz were
assigned. We also assigned 321 transitions from the vt=1 state with the
maximum values J=35 and Ka=9 up to 330 GHz. The final data set contains both
the data from the previous study (Maris et al. 2019), and new assignments in
the millimeter and submillimeter-wave ranges. The total of 2035 lines for vt=0
and vt=1 states of A and E symmetries were fit with root-mean-square deviation
of 43.4 kHz. The final fit is based on the RAM Hamiltonian model that includes
40 parameters. An astronomical search for CH3CSNH2 was conducted based on all
the new spectroscopic data. No transitions of CH3CSNH2 were detected toward
any source. Using the appropriate telescope and parameters for each
astronomical source, upper limits to the column densities were found for
CH3CSNH2 toward each source.

In addition, new transitions of CH3CONH2 were searched for and detected toward
Sgr B2(N-LMH) utilizing the GBT PRIMOS data set. In total, the best fit model
to the PRIMOS data was found for an excitation temperature of 5.8 K and a
source size of $20\arcsec$ for both the A- and E-state transitions, and the
total measured CH3CONH2 column density determined from the PRIMOS data from
the current analysis is $\sim 7.7\times 10^{13}\ \mathrm{cm}^{-2}$ for the
A-state transitions and $\sim 1.5\times 10^{14}\ \mathrm{cm}^{-2}$ for the
E-state transitions. As expected, the observed temperature and measured column
densities from this current work are much lower than what was reported by
Belloche et al. (2017) toward SgrB2(N2) from the EMoCA survey where CH3CONH2
was detected at a column density of $\sim 2\times 10^{17}\ \mathrm{cm}^{-2}$
and an excitation temperature of 180 K - clearly emission coming from a hot
core region which is beam diluted in our GBT observations. What this analysis
also indicates, given the structural similarity of CH3CONH2 and CH3CSNH2, are
the types of transitions that are most likely to be detected at lower
frequencies with single dish facilities such as the GBT. And while no
transitions of CH3CSNH2 were detected, the investigation of large S-bearing
species compared to their O-bearing counterparts may eventually provide
insight into possible formation routes. The gas phase formation routes to form
larger S-bearing molecules need further investigation into their energetics.
In addition, if grain surface reactions are to be considered as a viable route
to the formation of larger S-bearing species, precursors such as H2S and CS
need to be first identified within grain mantles.

###### Acknowledgements.

We would like to thank an anonymous referee for their strong support of this
work. This paper makes use of the following ALMA data:
ADS/JAO.ALMA#2011.0.00017.S, ADS/JAO.ALMA#2012.1.00012.S,
ADS/JAO.ALMA#2017.1.00370.S, and ADS/JAO.ALMA#2017.1.00661.S. ALMA is a
partnership of ESO (representing its member states), NSF (USA), and NINS
(Japan), together with NRC (Canada), NSC 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. The National Radio
Astronomy Observatory is a facility of the National Science Foundation
operated under cooperative agreement by Associated Universities, Inc. The
interferometric data are available in the ALMA archive at
https://almascience.eso.org/aq/. This paper makes use of the PRIMOS data under
GBT Archive Project Code AGBT07A-051. The Green Bank Observatory is a facility
of the National Science Foundation operated under cooperative agreement by
Associated Universities, Inc. This work is based on observations carried out
as part of the Large Program ASAI under project number 012-12 with the IRAM
30m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN
(Spain). C.X. is a Grote Reber Fellow, and acknowledges support from the
National Science Foundation through the Grote Reber Fellowship Program
administered by Associated Universities, Inc./National Radio Astronomy
Observatory and the Virginia Space Grant Consortium. Part of this work has
been carried out within the Collaborative Research Centre 956, sub-project B3,
funded by the Deutsche Forschungsgemeinschaft (DFG) – project ID 184018867.

## References

  * Barone et al. (2015) Barone, V., Latouche, C., Skouteris, D., et al. 2015, MNRAS, 453, L31
  * Belloche et al. (2019) Belloche, A., Garrod, R. T., Müller, H. S. P., et al. 2019, A&A, 628, A10
  * Belloche et al. (2017) Belloche, A., Meshcheryakov, A. A., Garrod, R. T., et al. 2017, A&A, 601, A49
  * Belloche et al. (2016) Belloche, A., Müller, H. S. P., Garrod, R. T., & Menten, K. M. 2016, A&A, 587, A91
  * Belloche et al. (2013) Belloche, A., Müller, H. S. P., Menten, K. M., Schilke, P., & Comito, C. 2013, A&A, 559, A47
  * Belyaev et al. (2012) Belyaev, D. A., Montmessin, F., Bertaux, J.-L., et al. 2012, Icarus, 217, 740
  * Biver et al. (1999) Biver, N., Bockelée-Morvan, D., Crovisier, J., et al. 1999, AJ, 118, 1850
  * Bonfand et al. (2019) Bonfand, M., Belloche, A., Garrod, R. T., et al. 2019, A&A, 628, A27
  * Bonfand et al. (2017) Bonfand, M., Belloche, A., Menten, K. M., Garrod, R. T., & Müller, H. S. P. 2017, A&A, 604, A60
  * Brogan et al. (2016) Brogan, C. L., Hunter, T. R., Cyganowski, C. J., et al. 2016, ApJ, 832, 187
  * Cernicharo et al. (2021) Cernicharo, J., Cabezas, C., Agúndez, M., et al. 2021, A&A, 648, L3
  * Charnley (1997) Charnley, S. B. 1997, ApJ, 481, 396
  * Colzi et al. (2021) Colzi, L., Rivilla, V. M., Beltrán, M. T., et al. 2021, arXiv e-prints, arXiv:2107.11258
  * Corby et al. (2015) Corby, J. F., Jones, P. A., Cunningham, M. R., et al. 2015, MNRAS, 452, 3969
  * Corby et al. (2018) Corby, J. F., McGuire, B. A., Herbst, E., & Remijan, A. J. 2018, A&A, 610, A10
  * El-Abd et al. (2019) El-Abd, S. J., Brogan, C. L., Hunter, T. R., et al. 2019, ApJ, 883, 129
  * Esplugues et al. (2014) Esplugues, G. B., Viti, S., Goicoechea, J. R., & Cernicharo, J. 2014, A&A, 567, A95
  * Faure et al. (2018) Faure, A., Lique, F., & Remijan, A. J. 2018, The Journal of Physical Chemistry Letters, 9, 3199
  * Faure et al. (2014) Faure, A., Remijan, A. J., Szalewicz, K., & Wiesenfeld, L. 2014, ApJ, 783, 72
  * Foo et al. (2018) Foo, L., Surányi, A., Guljas, A., et al. 2018, Molecular Astrophysics, 13, 1
  * Franz et al. (2019) Franz, H. B., King, P. L., & Gaillard, F. 2019, in Volatiles in the Martian Crust, ed. J. Filiberto & S. P. Schwenzer (Elsevier), 119–183
  * Frisch et al. (2016a) Frisch, M. J., Trucks, G. W., Schlegel, H. B., et al. 2016a, Gaussian˜09 Revision A.02, gaussian Inc. Wallingford CT
  * Frisch et al. (2016b) Frisch, M. J., Trucks, G. W., Schlegel, H. B., et al. 2016b, Gaussian˜16 Revision C.01, gaussian Inc. Wallingford CT
  * Garrod et al. (2008) Garrod, R. T., Widicus Weaver, S. L., & Herbst, E. 2008, ApJ, 682, 283
  * Gaume et al. (1995) Gaume, R. A., Claussen, M. J., de Pree, C. G., Goss, W. M., & Mehringer, D. M. 1995, ApJ, 449, 663
  * Gieser et al. (2021) Gieser, C., Beuther, H., Semenov, D., et al. 2021, A&A, 648, A66
  * Ginsburg et al. (2018) Ginsburg, A., Bally, J., Barnes, A., et al. 2018, ApJ, 853, 171
  * Hack et al. (1986) Hack, W., Kurzke, H., Rouveirolles, P., & Wagner, H. G. 1986, Berichte der Bunsengesellschaft für physikalische Chemie, 90, 1210
  * Halfen et al. (2011) Halfen, D. T., Ilyushin, V., & Ziurys, L. M. 2011, ApJ, 743, 60
  * Halfen et al. (2009) Halfen, D. T., Ziurys, L. M., Brünken, S., et al. 2009, ApJ, 702, L124
  * Herbst et al. (1984) Herbst, E., Messer, J. K., De Lucia, F. C., & Helminger, P. 1984, Journal of Molecular Spectroscopy, 108, 42
  * Hollis et al. (2004a) Hollis, J. M., Jewell, P. R., Lovas, F. J., & Remijan, A. 2004a, ApJ, 613, L45
  * Hollis et al. (2004b) Hollis, J. M., Jewell, P. R., Lovas, F. J., Remijan, A., & Møllendal, H. 2004b, ApJ, 610, L21
  * Hollis et al. (2007) Hollis, J. M., Jewell, P. R., Remijan, A. J., & Lovas, F. J. 2007, ApJ, 660, L125
  * Hollis et al. (2006a) Hollis, J. M., Lovas, F. J., Remijan, A. J., et al. 2006a, ApJ, 643, L25
  * Hollis et al. (2006b) Hollis, J. M., Remijan, A. J., Jewell, P. R., & Lovas, F. J. 2006b, ApJ, 642, 933
  * Hougen et al. (1994) Hougen, J. T., Kleiner, I., & Godefroid, M. 1994, Journal of Molecular Spectroscopy, 163, 559
  * Hunter et al. (2017) Hunter, T. R., Brogan, C. L., MacLeod, G., et al. 2017, ApJ, 837, L29
  * Hunter et al. (2006) Hunter, T. R., Brogan, C. L., Megeath, S. T., et al. 2006, ApJ, 649, 888
  * Ilyushin (2004) Ilyushin, V. 2004, Journal of Molecular Spectroscopy, 227, 140
  * Ilyushin et al. (2004) Ilyushin, V. V., Alekseev, E., Dyubko, S., Kleiner, I., & Hougen, J. T. 2004, Journal of Molecular Spectroscopy, 227, 115
  * Ilyushin et al. (2004) Ilyushin, V. V., Alekseev, E. A., Dyubko, S. F., Kleiner, I., & Hougen, J. T. 2004, Journal of Molecular Spectroscopy, 227, 115
  * Ilyushin et al. (2013) Ilyushin, V. V., Endres, C. P., Lewen, F., Schlemmer, S., & Drouin, B. J. 2013, Journal of Molecular Spectroscopy, 290, 31
  * Ilyushin & Hougen (2013) Ilyushin, V. V. & Hougen, J. T. 2013, Journal of Molecular Spectroscopy, 289, 41
  * Ilyushin et al. (2010) Ilyushin, V. V., Kisiel, Z., Pszczólkowski, L., Mäder, H., & Hougen, J. T. 2010, Journal of Molecular Spectroscopy, 259, 26
  * Irvine et al. (1988) Irvine, W. M., Brown, R. D., Cragg, D. M., et al. 1988, ApJ, 335, L89
  * Kaifu et al. (1974) Kaifu, N., Morimoto, M., Nagane, K., et al. 1974, ApJ, 191, L135
  * Kirtman (1962) Kirtman, B. 1962, J. Chem. Phys., 37, 2516
  * Kleiner (2010) Kleiner, I. 2010, Journal of Molecular Spectroscopy, 260, 1
  * Lee et al. (2021) Lee, K. L. K., Loomis, R. A., Xue, C., El-Abd, S., & McGuire, B. A. 2021, molsim
  * Lees & Baker (1968) Lees, R. M. & Baker, J. G. 1968, J. Chem. Phys., 48, 5299
  * Lefloch et al. (2018) Lefloch, B., Bachiller, R., Ceccarelli, C., et al. 2018, MNRAS, 477, 4792
  * Li et al. (2015) Li, J., Wang, J., Zhu, Q., Zhang, J., & Li, D. 2015, ApJ, 802, 40
  * Ligterink et al. (2020) Ligterink, N. F. W., El-Abd, S. J., Brogan, C. L., et al. 2020, ApJ, 901, 37
  * Ligterink et al. (2018) Ligterink, N. F. W., Terwisscha van Scheltinga, J., Taquet, V., et al. 2018, MNRAS, 480, 3628
  * Mangum & Shirley (2015) Mangum, J. G. & Shirley, Y. L. 2015, PASP, 127, 266
  * Maret et al. (2011) Maret, S., Hily-Blant, P., Pety, J., Bardeau, S., & Reynier, E. 2011, A&A, 526, A47
  * Margulès et al. (2020a) Margulès, L., Ilyushin, V. V., McGuire, B. A., et al. 2020a, Journal of Molecular Spectroscopy, 371, 111304
  * Margulès et al. (2020b) Margulès, L., McGuire, B. A., Evans, C. J., et al. 2020b, A&A, 642, A206
  * Maris et al. (2019) Maris, A., Calabrese, C., Favero, L. B., et al. 2019, ACS Earth and Space Chemistry, 3, 1537
  * Matteucci (2001) Matteucci, F. 2001, The chemical evolution of the Galaxy, Vol. 253 (Springer, Dordrecht)
  * McGuire et al. (2018) McGuire, B. A., Brogan, C. L., Hunter, T. R., et al. 2018, ApJ, 863, L35
  * McGuire et al. (2016) McGuire, B. A., Carroll, P. B., Loomis, R. A., et al. 2016, Science, 352, 1449
  * McGuire et al. (2012) McGuire, B. A., Loomis, R. A., Charness, C. M., et al. 2012, ApJ, 758, L33
  * McGuire et al. (2017) McGuire, B. A., Shingledecker, C. N., Willis, E. R., et al. 2017, ApJ, 851, L46
  * Melosso et al. (2020) Melosso, M., Dore, L., Tamassia, F., et al. 2020, Journal of Physical Chemistry A, 124, 240
  * Mota et al. (2021) Mota, V. C., Varandas, A. J. C., Mendoza, E., Wakelam, V., & Galvão, B. R. L. 2021, arXiv e-prints, arXiv:2109.03367
  * Moullet et al. (2010) Moullet, A., Gurwell, M. A., Lellouch, E., & Moreno, R. 2010, Icarus, 208, 353
  * Müller et al. (2016) Müller, H. S. P., Belloche, A., Xu, L.-H., et al. 2016, A&A, 587, A92
  * Neill et al. (2012) Neill, J. L., Muckle, M. T., Zaleski, D. P., et al. 2012, ApJ, 755, 153
  * Neufeld & Wolfire (2009) Neufeld, D. A. & Wolfire, M. G. 2009, ApJ, 706, 1594
  * Palumbo et al. (1997) Palumbo, M. E., Geballe, T. R., & Tielens, A. G. G. M. 1997, ApJ, 479, 839
  * Palumbo et al. (1995) Palumbo, M. E., Tielens, A. G. G. M., & Tokunaga, A. T. 1995, ApJ, 449, 674
  * Reid et al. (2019) Reid, M. J., Menten, K. M., Brunthaler, A., et al. 2019, ApJ, 885, 131
  * Reid et al. (2014) Reid, M. J., Menten, K. M., Brunthaler, A., et al. 2014, ApJ, 783, 130
  * Remijan et al. (2005) Remijan, A. J., Hollis, J. M., Lovas, F. J., Plusquellic, D. F., & Jewell, P. R. 2005, ApJ, 632, 333
  * Requena-Torres et al. (2008) Requena-Torres, M. A., Martín-Pintado, J., Martín, S., & Morris, M. R. 2008, ApJ, 672, 352
  * Rivero Santamaría et al. (2017) Rivero Santamaría, A., Dayou, F., Rubayo-Soneira, J., & Monnerville, M. 2017, Journal of Physical Chemistry A, 121, 1675
  * Sánchez-Monge et al. (2017) Sánchez-Monge, Á., Schilke, P., Schmiedeke, A., et al. 2017, A&A, 604, A6
  * Semenov et al. (2018) Semenov, D., Favre, C., Fedele, D., et al. 2018, A&A, 617, A28
  * Shingledecker et al. (2019) Shingledecker, C. N., Álvarez-Barcia, S., Korn, V. H., & Kästner, J. 2019, ApJ, 878, 80
  * Skouteris et al. (2017) Skouteris, D., Vazart, F., Ceccarelli, C., et al. 2017, MNRAS, 468, L1
  * Smirnov et al. (2014) Smirnov, I. A., Alekseev, E. A., Ilyushin, V. V., et al. 2014, Journal of Molecular Spectroscopy, 295, 44
  * Soundararajan (1957) Soundararajan, S. 1957, Trans. Faraday Soc., 53, 159
  * Stephens et al. (1994) Stephens, P. J., Chabalowski, C. F., Devlin, F. J., & Jalkanen, K. J. 1994, Chemical Physics Letters, 225, 247
  * Taylor & Kuyatt (1994) Taylor, B. N. & Kuyatt, C. E. 1994, Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, Vol. 1297 (National Institute of Standards and Technology)
  * Turner (1991) Turner, B. E. 1991, ApJS, 76, 617
  * Ulich & Haas (1976) Ulich, B. L. & Haas, R. W. 1976, ApJS, 30, 247
  * van der Tak et al. (2003) van der Tak, F. F. S., Boonman, A. M. S., Braakman, R., & van Dishoeck, E. F. 2003, A&A, 412, 133
  * Wakelam et al. (2004) Wakelam, V., Caselli, P., Ceccarelli, C., Herbst, E., & Castets, A. 2004, A&A, 422, 159
  * Willacy et al. (1993) Willacy, K., Williams, D. A., & Minh, Y. C. 1993, MNRAS, 263, L40
  * Xu et al. (2008) Xu, L.-H., Fisher, J., Lees, R., et al. 2008, Journal of Molecular Spectroscopy, 251, 305 , special issue dedicated to the pioneering work of Drs. Edward A. Cohen and Herbert M. Pickett on spectroscopy relevant to the Earth’s atmosphere and astrophysics
  * Xue et al. (2019) Xue, C., Remijan, A. J., Brogan, C. L., et al. 2019, ApJ, 882, 118
  * Yamaguchi et al. (2002) Yamaguchi, A., Hagiwara, S., Odashima, H., Takagi, K., & Tsunekawa, S. 2002, Journal of Molecular Spectroscopy, 215, 144
  * Zakharenko et al. (2015) Zakharenko, O., Motiyenko, R. A., Margulès, L., & Huet, T. R. 2015, Journal of Molecular Spectroscopy, 317, 41
  * Zasowski et al. (2009) Zasowski, G., Kemper, F., Watson, D. M., et al. 2009, ApJ, 694, 459
  * Zernickel et al. (2012) Zernickel, A., Schilke, P., Schmiedeke, A., et al. 2012, A&A, 546, A87