# Millimeter-wave Spectrum Analysis and Searches for 2-aminopropenenitrile ($H_2C=C(NH_2)CN$) toward the Galactic Center Sources Sgr B2(N1) and $G+0.693-0.027$

Katerina Lukova, Lucie Kolesnikova, Jean-Claude Guillemin, Tereza Uhlíková, Jan Koucký, Frantisek Horak, Lukas Hrubcik, Karel Vavra, Patrik Kania, Štěpán Urban, et al.

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Katerina Lukova, Lucie Kolesnikova, Jean-Claude Guillemin, Tereza Uhlíková, Jan Koucký, et al. Millimeter-wave Spectrum Analysis and Searches for 2-aminopropenenitrile ($H_2C=C(NH_2)CN$) toward the Galactic Center Sources Sgr B2(N1) and $G+0.693-0.027$. The Astrophysical Journal, 2025, 994 (1), 10.3847/1538-4357/ae0d89. [hal-05423431](https://hal.science/hal-05423431v1)

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# Millimeter-wave Spectrum Analysis and Searches for 2-aminopropenenitrile ($H_2C=C(NH_2)CN$) toward the Galactic Center Sources Sgr B2(N1) and $G+0.693-0.027$

Kateřina Luková<fnref n="1" />, Lucie Kolesniková<fnref n="1" />, Jean-Claude Guillemin<fnref n="2" />, Tereza Uhlíková<fnref n="1" />, Jan Koucký<fnref n="1" />, František Horák<fnref n="1" />, Lukáš Hrubčík<fnref n="1" />, Karel Vávra<fnref n="1" />, Patrik Kania<fnref n="1" />, Štěpán Urban<fnref n="1" />, Arnaud Belloche<fnref n="3" />, Izaskun Jiménez-Serra<fnref n="4" />, and Víctor M. Rivilla<fnref n="4" />

Received 2025 April 16; revised 2025 September 14; accepted 2025 September 16; published 2025 November 17

## Abstract

We present an analysis of the millimeter-wave spectra of 2-aminopropenenitrile ($H_2C=C(NH_2)CN$), an α-aminonitrile of plausible prebiotic and astrophysical relevance. The molecule has a rotational spectrum that is in harmony with a large-amplitude motion of the amino group. Accurate rest transition frequencies are reported in selected regions between 158 and 324 GHz, along with extended and improved sets of molecular parameters for the ground state. In addition, several vibrational satellites were identified for the first time. Experimental results were used to search for 2-aminopropenenitrile toward two chemically rich interstellar sources in the Sgr B2 region of the Galactic center: the hot molecular core Sgr B2(N1S) and the molecular cloud $G+0.693-0.027$. We also searched for the reduced derivative, 2-aminopropionitrile ($CH_3CH(NH_2)CN$), in the former source. None of the molecules were detected, and the upper limits of their column densities were obtained.

*Unified Astronomy Thesaurus concepts: Millimeter-wave spectroscopy (2252); Rotational spectroscopy (2248); Catalogs (205); Interstellar medium (847); Molecular spectroscopy (2095); Experimental data (2371); Radio astronomy (1338)*

*Materials only available in the online version of record: machine-readable table*

## 1. Introduction

The α-aminonitriles are crucial building blocks of life, as they are considered prebiotic precursors of amino acids. Although Strecker synthesis is widely believed to be the origin of α-aminonitriles and subsequently of the formation of amino acids in prebiotic chemistry, the theory has its shortcomings. In particular, the traditional Strecker synthesis of cysteine, one of the sulfur-bearing amino acids, leads to an intermediate cysteine-nitrile that contains a highly unstable combination of functional groups, aminothiol and nitrile, resulting in rapid degradation of the intermediate compound and thus making the formation of free cysteine impossible. A novel plausible pathway of a prebiotic conversion from serine to cysteine residing in N-acylation of 2-aminopropenenitrile moiety ($2-APN, H_2C = C(NH_2)CN$; see Figure 1) has been recently proposed by C. S. Foden et al. (2020), who followed up on extensive studies of 2-APN reactivity conducted by G. Ksander et al. (1987) and S. Drenkard et al. (1990). The acylation of 2-APN forestalls the undesired enamine-imine tautomerization that would prevent the selective addition of hydrogen sulfide to the compound and also precludes the intermediate degradation due to the unstable combination of the aminothiol and nitrile group.

The assumption of the importance of 2-APN and its derivatives in the realm of prebiotic chemistry was further

supported by an investigation of the reactivity of N-formylated 2-APN, a coproduct of the reaction of glycolaldehyde with cyanide and formamide (N. J. Green et al. 2023). The subsequently investigated reactions of N-formylated 2-APN in water and in formamide led to numerous compounds of prebiotic relevance, including precursors of asparagine and aspartic acid and derivatives of cysteine and imidazole. Because 2-APN and its derivatives are suggested to play a pivotal role in peptide synthesis, they might represent interesting candidates for interstellar searches.

Additionally, the structural resemblance of the title molecule to formerly detected interstellar species cannot be overlooked. 2-APN is a methylidene derivative of aminoacetonitrile ($NH_2CH_2CN$) that was detected by A. Belloche et al. (2008) toward the hot core region Sgr B2(N). It can be further viewed as a –$NH_2$ derivative of vinyl cyanide ($H_2C=CHCN$), which is detected in multiple sources, and a –CN derivative of vinylamine ($CH_2=CHNH_2$). Recently, the first detection of vinylamine was confirmed toward the Galactic center cloud $G+0.693-0.027$ (hereafter $G+0.693$) by S. Zeng et al. (2021). In addition, 2-APN is a member of the $C_3H_4N_2$ isomeric family that consists of several species of astrophysical interest. Recently, interstellar searches for the five-membered ring compound imidazole (B. M. Giuliano et al. 2019) and for 3-aminopropenenitrile (D. Alberton et al. 2023) have been reported, though without successful detections. Laboratory rotational spectra of the imino tautomer of 2-APN, 2-iminopropanenitrile ($CH_3C(CN)NH$; K. Luková et al. 2022), and methyleneaminoacetonitrile ($CH_2NCH_2CN$; R. A. Motiyenko et al. 2013) have also been recorded.

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Original content from this work may be used under the terms of the [Creative Commons Attribution 4.0 licence](https://creativecommons.org/licenses/by/4.0/). Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

<footnote n="1">¹Department of Analytical Chemistry, University of Chemistry and Technology, Technická 5, 166 28 Prague 6, Czechia; katerina.lukova@vscht.cz, lucie.kolesnikova@vscht.cz</footnote>

<footnote n="2">²Univ Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR UMR6226, F-35000 Rennes, France</footnote>

<footnote n="3">³Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany</footnote>

<footnote n="4">⁴Centro de Astrobiología (CSIC-INTA), Ctra. de Ajalvir Km. 4, Torrejón de Ardoz, 28850 Madrid, Spain</footnote>
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Equilibrium geometry of 2-APN with indication of the angles used for the calculation of the potential energy scan from Figure 2. Labels *l* and *k* denote two perpendicular planes.</figcaption></figure>

The microwave spectrum of 2-APN was investigated by A. Bauder & T.-K. Ha (1983) in the frequency range of 18–58 GHz using a Stark-modulation spectrometer. Here, we extend the measurements up to 324 GHz and provide precise predictions over a wide frequency range for astrophysical use. New spectroscopic results were used to search for 2-APN in regions where the related molecules aminoacetonitrile and vinylamine were detected: the Sgr B2(N) and G+0.693 molecular clouds.

## 2. Experimental Details

### 2.1. Synthesis

The synthesis has been performed following the procedure of K. Jähnish et al. (1992) except for the last step. Potassium hydroxide in powder (840 mg, 15 mmol) was added in three parts to a –20°C cooled solution under dry nitrogen of 2-amino-3-chloropropanenitrile, hydrochloride (ClCH₂CH(CN)(NH₃)⁺, Cl⁻) (420 mg, 3.0 mmol) in anhydrous diethyl ether (10 cm³). The mixture was kept under vigorous stirring for 5 hr at room temperature. The clear solution was transferred to a flask and then the solvent was removed in vacuo (0.1 mbar or 10 Pa) at room temperature. By heating the residue up to 50°C in a vacuum line at a pressure of 0.1 mbar, 2-APN was vaporized and trapped in a U-tube immersed in a bath cooled to –30°C. The yield was 57% (116 mg, 1.71 mmol).

### 2.2. Measurements of Spectra

The room-temperature spectra were recorded using the Prague semiconductor millimeter-wave spectrometer (P. Kania et al. 2006) in three frequency ranges: 158–179, 204–238, and 292–324 GHz. These regions were acquired by multiplying the Agilent E8257D synthesizer frequency (≤50 GHz) using a chain of multipliers (multiplying factors of 4×, 8×, and 12×). The signal was detected by zero-bias detectors and

demodulated by a lock-in amplifier with a time constant of 20 ms. The lock-in amplifier was tuned to twice the modulation frequency. The roof-top mirror was used to double the optical path length to 5.6 m. The absolute pressure in the Pyrex glass cell was 15–30 µbar (1.5–3.0 Pa), and a slow continuous flow was established while the sample was kept in a bath at –40°C. The resulting spectrum was averaged from forward and backward scans. The experimental uncertainty of isolated well-developed lines was 30 kHz; broader blended lines and lines with poor signal-to-noise ratio were given uncertainties of 50 kHz. Spectra analysis was performed using the Assignment and Analysis of Broadband Spectra package (Z. Kisiel et al. 2005, 2012).

Additional measurements were performed using the TOP-TICA laser differential spectrometer. The radiation sources are two near-infrared lasers, one of which is tunable and the other fixed. The terahertz radiation at the difference frequency of both lasers is obtained in the photomixer. The sample was introduced in a 0.75 m long steel cuvette, and the total pressure was 0.3–0.5 mbar (30–50 Pa). The spectrum was recorded between 1 and 2 THz. The effective spectral resolution is estimated to be approximately 50 MHz. This corresponds to the typical performance of continuous-wave THz systems (A. J. Deninger et al. 2015).

## 3. Laboratory Results and Discussion

### 3.1. Ground State and Vibrational Satellites

For molecules of 2-APN size, the analysis of rotational spectra in the millimeter-wave region typically requires sextic centrifugal distortion constants. To estimate their values, we used coupled cluster theory with single and double substitutions (CCSD; G. D. I. Purvis & R. J. Bartlett 1982) with Dunning's correlation-consistent triple-ζ (cc-pVTZ) basis set (T. H. Dunning 1989) as implemented in the CFOUR program package (D. A. Matthews et al. 2020). The values obtained, together with those for the accompanying rotational and quartic centrifugal distortion constants and dipole moment components, are listed in Table 1. The atom coordinates are provided in the Appendix in Table 6.

Initially, a predominantly *a*-type spectrum of the ground vibrational state ($|\mu_a| = 3.76(2)$ D and $|\mu_b| = 0.71(9)$ D from A. Bauder & T.-K. Ha 1983 and $|\mu_c| = 0.98$ D from calculations) was simulated based on the experimental rotational and quartic centrifugal distortion constants determined previously (A. Bauder & T.-K. Ha 1983) and the calculated sextic ones. This prediction facilitated the assignments of <sup>a</sup>*R*-branch transitions encompassing energy levels with $K_a = 0–40$ and $J = 19–54$. The upper limit in *J* quantum number was further increased to $J = 72$ by adding several <sup>a</sup>*Q*-branches with $K_a = 0–14$ to the line list. Finally, significantly weaker *b*- and *c*-type transitions were predicted but not observed.

A. Bauder & T.-K. Ha (1983) analyzed a vibrational satellite located toward the lower frequencies with respect to the ground state, which was also observed in our records. We obtained a similarly extensive data set for this satellite as for the ground state in terms of the assigned quantum number ranges and the number of distinct frequency lines. This was enabled by its notable intensity; the intensity ratio relative to the ground state of 0.77(8) found by A. Bauder & T.-K. Ha (1983) was consistent with our observations. However, such
<table><caption>**Table 1**
Spectroscopic Constants of 2-APN (A-reduced Hamiltonian, *I*<sup>f</sup> Representation)</caption><thead><tr><th rowspan="2"></th><th colspan="3">This work<sup>a</sup></th><th colspan="2">A. Bauder & T.-K. Ha (1983)<sup>b</sup></th></tr><tr><th>0<sup>+</sup></th><th>0<sup>-</sup></th><th>Calculated<sup>c</sup></th><th>G.S.</th><th>Exc. State</th></tr></thead><tbody><tr><td>A<sub>0</sub>/MHz</td><td>9993.93728(93)</td><td>9987.6709(14)</td><td>9959.58</td><td>9994.11(10)</td><td>9988.98(43)</td></tr><tr><td>B<sub>0</sub>/MHz</td><td>4282.08442(14)</td><td>4276.44673(14)</td><td>4262.90</td><td>4282.108(6)</td><td>4276.498(19)</td></tr><tr><td>C<sub>0</sub>/MHz</td><td>3000.18939(16)</td><td>2999.12267(16)</td><td>2990.90</td><td>3000.188(5)</td><td>2999.079(15)</td></tr><tr><td>Δ<sub>J</sub>/kHz</td><td>0.715565(89)</td><td>0.711576(85)</td><td>0.68</td><td>0.749(38)</td><td>0.48(12)</td></tr><tr><td>Δ<sub>JK</sub>/kHz</td><td>16.22396(26)</td><td>16.13648(27)</td><td>15.29</td><td>17.03(32)</td><td>16.0(11)</td></tr><tr><td>Δ<sub>K</sub>/kHz</td><td>-7.7809(37)</td><td>-7.6536(58)</td><td>-6.96</td><td>-11.1(15)</td><td>-15.0(11)</td></tr><tr><td>δ<sub>J</sub>/kHz</td><td>0.241998(28)</td><td>0.239823(34)</td><td>0.23</td><td>0.273(23)</td><td>0.324(64)</td></tr><tr><td>δ<sub>K</sub>/kHz</td><td>9.98290(40)</td><td>9.95408(52)</td><td>9.35</td><td>9.45(92)</td><td>4.0(36)</td></tr><tr><td>Φ<sub>J</sub>/mHz</td><td>0.383(19)</td><td>0.387(19)</td><td>0.38</td><td>...</td><td>...</td></tr><tr><td>Φ<sub>JK</sub>/Hz</td><td>0.13550(17)</td><td>0.13635(19)</td><td>0.12</td><td>...</td><td>...</td></tr><tr><td>Φ<sub>KJ</sub>/Hz</td><td>-0.27628(46)</td><td>-0.27893(57)</td><td>-0.25</td><td>...</td><td>...</td></tr><tr><td>Φ<sub>K</sub>/Hz</td><td>[0.14]<sup>d</sup></td><td>[0.14]<sup>d</sup></td><td>0.14</td><td>...</td><td>...</td></tr><tr><td>Φ<sub>J</sub>/mHz</td><td>0.2249(53)</td><td>0.2318(66)</td><td>0.19</td><td>...</td><td>...</td></tr><tr><td>Φ<sub>JK</sub>/Hz</td><td>0.06955(11)</td><td>0.06880(14)</td><td>0.06</td><td>...</td><td>...</td></tr><tr><td>Φ<sub>K</sub>/Hz</td><td>0.33780(91)</td><td>0.3425(11)</td><td>0.31</td><td>...</td><td>...</td></tr><tr><td>Δ/uÅ<sup>2</sup></td><td>-0.141253(11)</td><td>-0.268656(12)</td><td>-0.32</td><td>-0.1397(6)</td><td>-0.2582(24)</td></tr><tr><td>|μ<sub>a</sub>|-D</td><td>...</td><td>...</td><td>3.82</td><td>3.76(2)</td><td>...</td></tr><tr><td>|μ<sub>b</sub>|-D</td><td>...</td><td>...</td><td>0.51</td><td>0.71(9)</td><td>...</td></tr><tr><td>|μ<sub>c</sub>|-D</td><td>...</td><td>...</td><td>0.98</td><td>0.0</td><td>...</td></tr><tr><td>J<sub>min</sub>/J<sub>max</sub></td><td>2/72</td><td>2/69</td><td>...</td><td>2/30</td><td>2/26</td></tr><tr><td>K<sub>a</sub><sup>min</sup>/K<sub>a</sub><sup>max</sup></td><td>0/40</td><td>0/40</td><td>...</td><td>0/9</td><td>0/8</td></tr><tr><td>N<sup>e</sup></td><td>618</td><td>572</td><td>...</td><td>43</td><td>30</td></tr><tr><td>σ<sub>fit</sub><sup>f</sup>/MHz</td><td>0.045</td><td>0.055</td><td>...</td><td>...</td><td>...</td></tr><tr><td>σ<sub>w</sub><sup>g</sup></td><td>0.90</td><td>0.84</td><td>...</td><td>...</td><td>...</td></tr></tbody></table>

**Notes.**

<sup>a</sup> The numbers in parentheses are the parameter uncertainties in units of the last decimal digits. The SPFIT/SPCAT program package (H. M. Pickett 1991) was used for the analysis.

<sup>b</sup> G.S. and the excited state correspond to our 0<sup>+</sup> and 0<sup>-</sup> states, respectively.

<sup>c</sup> CCSD/cc-pVTZ level of theory.

<sup>d</sup> Fixed at the calculated value.

<sup>e</sup> Number of distinct frequency lines in the fit.

<sup>f</sup> rms deviation of the fit.

<sup>g</sup> Unitless (weighted) deviation of the fit.

intensity is not in harmony with the lowest-lying excited vibrational state, $v_{21} = 1$, predicted at 198 cm$^{-1}$. The expected relative intensity ratio is only around 0.4.

The origin of this satellite can be understood once the geometry of 2-APN is taken into consideration and further quantum-chemical calculations are performed. Figure 1 shows that 2-APN has a pyramidal -NH₂ group with the two hydrogens unequally displaced from the plane defined by the three carbon atoms. Similar structure was also predicted by S. Saebo & L. Radom (1983) and obtained from the X-ray structure analysis (P. Seiler & J. D. Dunitz 1985). Consequently, the molecule may undergo a large-amplitude motion that involves a coupled combination of rotation and inversion movement, similar to vinylamine (K. Müller & L. D. Brown 1978). To investigate this, we performed a scan of the potential energy surface using the CCSD method with the cc-pVDZ basis set. The two angles that were varied in the scan are the pyramidal inversion angle at the nitrogen atom and the torsion around the C1-N3 bond (see Figure 1). The angles ranged from -230° to +40° and from -30° to +30° for rotation and inversion movement, respectively. All other coordinates were optimized for the lowest energy at each step. The resulting potential energy surface in the form of a 2D contour diagram is shown in Figure 2.

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Potential energy scan for the -NH₂ inversion and rotation about the C1-N3 bond. The relative energies are in cm$^{-1}$. The barrier heights hindering the inversion and rotation motions are displayed in white.</figcaption></figure>

Four energetically equivalent minima are found in Figure 2. An interconversion between these equivalent configurations requires overcoming energy barriers of different heights. Two equivalent barriers opposing the -NH₂ inversion motion are 855 cm$^{-1}$ high, and the transition states correspond to the planar structures. Two different potential barriers inhibit the
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Section of the millimeter-wave spectrum of 2-APN. Two representative lines corresponding to the overlapped *a*-type *R*-branch $K_a = 0, 1$ ($J=26 \leftarrow 25$) and 1, 2 ($J=25 \leftarrow 24$) transitions are shown for the vibrational ground state (G.S., $0^+$ and $0^-$ pair) and vibrational satellites (S1–S9). S1 and S2 correspond to the $0^+$ and $0^-$ states in the $v_{21} = 1$ excited vibrational state, respectively, while S7 and S9 correspond to the $0^+$ and $0^-$ states in the $v_{21} = 2$ state, respectively.</figcaption></figure>

–NH₂ rotation. The first one reaches its maximum in the configuration where hydrogens from the –NH₂ group are oriented toward the –CN group and has a value of 965 cm⁻¹. A configuration with these hydrogens oriented to the =CH₂ group corresponds to the transition state lying 1965 cm⁻¹ above the minima. Due to the four equivalent minima, the ground vibrational state might be split into four components. This situation was observed for molecules such as aminomalonitrile (R. A. Motiyenko et al. 2015), *gauche*-ethylamine (E. Fischer & I. Botskor 1984), and *gauche*-equatorial conformer of aminocyclobutane (L. B. Favero et al. 1998). The ground state and the intense satellite thus might be a part of a plausible multiplet. However, the relative energies of the four levels formed by splitting due to the tunneling effect depend both on the shape of the potential hypersurface and on the kinetic energy operator of the given motion. Since no information is available regarding the magnitude of the two splittings, we initially assigned the ground state to $0^+$ and the satellite to $0^-$ state (see Figure 3), similar to molecules whose spectra consist of doublets (R. Brown et al. 1990; H. Møllendal & A. Konovalov 2010).

We performed a combined fit of our data with those from A. Bauder & T.-K. Ha (1983) using the *A*-reduced effective rotational Hamiltonian (J. K. G. Watson 1977) in $I^r$ representation. The previous data were analyzed with uncertainties of 150 kHz, except for three $0^-$ state transitions (1 MHz uncertainty) and four $0^-$ state transitions that were excluded due to large deviations. The experimentally determined parameters are in Table 1, and the lists of measured transitions are given in the Appendix in Table 7. The rotational and quartic centrifugal distortion constants improved significantly over the previous study. Furthermore, the sextic ones were experimentally determined for the first time. The values obtained for the states $0^+$ and $0^-$ are in mutual consistency and in great agreement with the calculated counterparts. Differences with respect to previous results are attributable to the substantially larger data sets analyzed in this work and less precise frequencies from A. Bauder & T.-K. Ha (1983).

A further inspection of the millimeter-wave spectra revealed nine additional vibrational satellites. These are labeled S1–S9 in order of decreasing intensity in Figure 3, although the

intensities for some of them are comparable. The spectral patterns resembling those in the ground state in combination with Loomis–Wood-type plots greatly facilitated their analyses. For the most intense satellite, S1, <sup>a</sup>*R*-transitions up to $K_a = 37$ were readily assigned and eventually complemented by several <sup>a</sup>*Q*-transitions. In the spectra of S2 and S3, frequent shifts of lines were observed from their predicted frequencies in the 292–324 GHz frequency region, indicating plausible mutual rovibrational interactions. Since a proper treatment of these interactions would be difficult if not impossible with the currently available data, we decided to include only the less perturbed transitions from the 292–324 GHz frequency region. However, transition series up to $K_a = 24$ identified in the 158–179 and 204–238 GHz regions were analyzed for both states, allowing us to determine the complete set of quartic and some sextic centrifugal distortion constants. The resulting molecular parameters are in Table 2.

The analysis of the remaining satellites was even more complicated. The rotational transitions in satellites S4 and S5 are seriously perturbed. Many lines could be assigned using the Loomis–Wood-type plots but could not be treated within the scope of the semirigid rotor Hamiltonian. Consequently, we were unable to derive reliable values of the spectroscopic constants for these satellites. However, their lines are included in Figure 3 to illustrate the complete pattern of the assigned satellites. The situation was similar, though less severe, for the satellites S6–S9. In this case, we obtained the spectroscopic parameters presented in Table 2. However, their values come from the analysis of limited sets of less perturbed transitions.

The observed satellites might be associated with excited vibrational states. Our quantum-chemical calculations indicate that the lowest-frequency mode, $v_{21}$, is the in-plane C–C≡N bending motion. The next two modes, $v_{20}$ and $v_{19}$, are both out of plane. Specifically, $v_{20}$ is primarily an –NH₂ rocking vibration, while $v_{19}$ is a mixed vibration with contributions from both –NH₂ rocking and out-of-plane C–C≡N bending motions. The last mode considered here, $v_{18}$, is the in-plane C=C–N bending vibration.

The five lowest-energy excited vibrational states are $v_{21} = 1$ (198 cm⁻¹), $v_{20} = 1$ (262 cm⁻¹), $v_{19} = 1$ (300 cm⁻¹), $v_{21} = 2$ (396 cm⁻¹), and $v_{18} = 1$ (414 cm⁻¹). Their assignments to the
<table><caption>**Table 2**
Spectroscopic Constants<sup>a</sup> of Additional Vibrational Satellites Observed in the Spectra (A-reduced Hamiltonian, I' Representation)</caption><thead><tr><th></th><th>S1</th><th>S2</th><th>S3</th><th>S6</th><th>S7</th><th>S8</th><th>S9</th></tr></thead><tbody><tr><td>A/MHz</td><td>9963.9816(17)</td><td>9958.5974(50)</td><td>10020.3052(68)</td><td>10013.1967(84)</td><td>9935.4817(57)</td><td>9992.3910(78)</td><td>9929.937(14)</td></tr><tr><td>B/MHz</td><td>4299.06601(14)</td><td>4292.97252(45)</td><td>4282.49936(56)</td><td>4283.13600(62)</td><td>4315.76963(69)</td><td>4287.89787(72)</td><td>4309.2954(13)</td></tr><tr><td>C/MHz</td><td>3002.13605(11)</td><td>3001.09723(26)</td><td>3003.16068(31)</td><td>3005.46220(36)</td><td>3004.00825(32)</td><td>2999.97545(56)</td><td>3002.9991(12)</td></tr><tr><td>Δ<sub>I</sub>/kHz</td><td>0.745110(43)</td><td>0.74447(19)</td><td>0.75982(23)</td><td>0.70430(24)</td><td>0.77906(26)</td><td>0.71452(30)</td><td>0.78012(51)</td></tr><tr><td>Δ<sub>JK</sub>/kHz</td><td>15.52575(40)</td><td>15.4161(14)</td><td>16.7364(17)</td><td>16.1513(22)</td><td>14.7968(22)</td><td>16.2132(27)</td><td>14.747(10)</td></tr><tr><td>Δ<sub>K</sub>/kHz</td><td>-7.4895(58)</td><td>-7.162(38)</td><td>-7.233(51)</td><td>-8.134(58)</td><td>-6.874(35)</td><td>-7.853(59)</td><td>-6.735(99)</td></tr><tr><td>δ<sub>I</sub>/kHz</td><td>0.255468(22)</td><td>0.254876(98)</td><td>0.25292(12)</td><td>0.23719(13)</td><td>0.27120(14)</td><td>0.24137(17)</td><td>0.27168(28)</td></tr><tr><td>δ<sub>K</sub>/kHz</td><td>9.8724(10)</td><td>9.8706(31)</td><td>10.1983(39)</td><td>9.9439(44)</td><td>9.7653(40)</td><td>10.0168(47)</td><td>9.770(17)</td></tr><tr><td>Φ<sub>J</sub>/mHz</td><td>[0.375]<sup>b</sup></td><td>[0.375]<sup>b</sup></td><td>[0.375]<sup>b</sup></td><td>[0.375]<sup>b</sup></td><td>[0.375]<sup>b</sup></td><td>[0.375]<sup>b</sup></td><td>[0.375]<sup>b</sup></td></tr><tr><td>Φ<sub>JK</sub>/Hz</td><td>0.12960(13)</td><td>0.12706(84)</td><td>0.1417(10)</td><td>0.1306(13)</td><td>0.1195(13)</td><td>0.1425(17)</td><td>0.1215(55)</td></tr><tr><td>Φ<sub>KJ</sub>/Hz</td><td>-0.28274(22)</td><td>-0.2855(22)</td><td>-0.2941(28)</td><td>-0.2775(27)</td><td>-0.2926(28)</td><td>-0.2902(48)</td><td>-0.370(36)</td></tr><tr><td>Φ<sub>K</sub>/Hz</td><td>[0.14]<sup>b</sup></td><td>[0.14]<sup>b</sup></td><td>[0.14]<sup>b</sup></td><td>[0.14]<sup>b</sup></td><td>[0.14]<sup>b</sup></td><td>[0.14]<sup>b</sup></td><td>[0.14]<sup>b</sup></td></tr><tr><td>φ<sub>J</sub>/mHz</td><td>[0.2268]<sup>b</sup></td><td>[0.2268]<sup>b</sup></td><td>[0.2268]<sup>b</sup></td><td>[0.2268]<sup>b</sup></td><td>[0.2268]<sup>b</sup></td><td>[0.2268]<sup>b</sup></td><td>[0.2268]<sup>b</sup></td></tr><tr><td>φ<sub>JK</sub>/Hz</td><td>0.06642(33)</td><td>0.06525(74)</td><td>0.07189(92)</td><td>0.0656(11)</td><td>0.06407(100)</td><td>0.0654(14)</td><td>0.0985(79)</td></tr><tr><td>Φ<sub>K</sub>/Hz</td><td>[0.33789]<sup>b</sup></td><td>[0.33789]<sup>b</sup></td><td>[0.33789]<sup>b</sup></td><td>[0.33789]<sup>b</sup></td><td>[0.33789]</td><td>[0.33789]<sup>b</sup></td><td>[0.33789]</td></tr><tr><td>Δ/uÅ<sup>2</sup></td><td>0.063685(11)</td><td>-0.072342(32)</td><td>-0.163425(41)</td><td>-0.310542(50)</td><td>0.268261(39)</td><td>0.022946(54)</td><td>0.12046(11)</td></tr><tr><td>J<sub>min</sub>/J<sub>max</sub></td><td>19/54</td><td>19/54</td><td>19/54</td><td>20/52</td><td>20/54</td><td>20/51</td><td>19/38</td></tr><tr><td>K<sub>a</sub><sup>min</sup>/K<sub>a</sub><sup>max</sup></td><td>0/37</td><td>0/24</td><td>0/24</td><td>0/25</td><td>0/25</td><td>1/24</td><td>0/16</td></tr><tr><td>N<sup>c</sup></td><td>464</td><td>248</td><td>227</td><td>191</td><td>193</td><td>154</td><td>114</td></tr><tr><td>σ<sub>fit</sub><sup>d</sup>/MHz</td><td>0.029</td><td>0.028</td><td>0.031</td><td>0.035</td><td>0.035</td><td>0.039</td><td>0.048</td></tr><tr><td>σ<sub>w</sub><sup>e</sup></td><td>0.90</td><td>0.89</td><td>1.02</td><td>1.00</td><td>0.98</td><td>1.09</td><td>1.17</td></tr></tbody></table>

**Notes.**

<sup>a</sup> The numbers in parentheses are the parameter uncertainties in units of the last decimal digits. The SPFIT/SPCAT program package (H. M. Pickett 1991) was used for the analysis.

<sup>b</sup> Fixed to the 0<sup>+</sup> state value.

<sup>c</sup> Number of distinct frequency lines in the fit.

<sup>d</sup> rms deviation of the fit.

<sup>e</sup> Unitless (weighted) deviation of the fit.

<table><caption>**Table 3**
Comparison of Theoretically and Experimentally Obtained Vibrational Changes in Rotation Constants and Inertial Defects of the $v_{21} = 1$ and $v_{21} = 2$ Vibrationally Excited States</caption><thead><tr><th rowspan="2">Parameters</th><th colspan="2">Prediction<sup>a</sup></th><th colspan="4">Experiment<sup>b</sup></th></tr><tr><th>$v_{21} = 1$</th><th>$v_{21} = 2$</th><th>S1</th><th>S2</th><th>S7</th><th>S9</th></tr></thead><tbody><tr><td>$A_i - A_0$/MHz</td><td>-26.2</td><td>-52.4<sup>c</sup></td><td>-29.9557(19)</td><td>-29.0735(52)</td><td>-58.4556(58)</td><td>-57.734(14)</td></tr><tr><td>$B_i - B_0$/MHz</td><td>16.3</td><td>32.6<sup>c</sup></td><td>16.98159(20)</td><td>16.52579(47)</td><td>33.68521(70)</td><td>32.8487(13)</td></tr><tr><td>$C_i - C_0$/MHz</td><td>1.8</td><td>3.5<sup>c</sup></td><td>1.94666(19)</td><td>1.97456(31)</td><td>3.818886(36)</td><td>3.8764(12)</td></tr><tr><td>$\Delta_i - \Delta_0$/uÅ<sup>2</sup></td><td>0.22</td><td>0.43</td><td>0.204938(16)</td><td>0.196329(34)</td><td>0.409514(41)</td><td>0.38912(11)</td></tr></tbody></table>

**Notes.**

<sup>a</sup> Calculated with respect to the ground state. Rotation/inversion splitting is not accounted for.

<sup>b</sup> Values for S1 and S7 are relative to the 0<sup>+</sup> state, while those for S2 and S9 are relative to the 0<sup>-</sup> state.

<sup>c</sup> Estimated assuming $2(B_{v_{21}=1} - B_0)$.

observed satellites can be discussed by examining changes in rotational constants and inertial defects from those in the ground state. As shown in Table 3, the changes in rotational constants for the satellite S1 with respect to 0<sup>+</sup> agree with the changes predicted for $v_{21} = 1$. The changes for S2 with respect to 0<sup>-</sup> also align with these predictions (see Table 3). Therefore, it is likely that S1 and S2 correspond to the states 0<sup>+</sup> and 0<sup>-</sup> of $v_{21} = 1$, respectively. This assumption is reinforced by the inertial defect values in Table 2. More positive values compared to those for the ground state are consistent with the in-plane character of the involved mode. There is also a good consistency between the experiment and computation for inertial defect changes in Table 3. This

vibrational assignment is also consistent with the approximate relative intensities of satellite transitions. We obtained a relative intensity ratio of 0.38 for the S1/0<sup>+</sup> pair and 0.39 for the S2/0<sup>-</sup> pair, which agrees well with the prediction for the $v_{21} = 1$ state lying at 198 cm<sup>-1</sup>. The final confirmation of the vibrational assignment lies in the identification of the pair of 0<sup>+</sup> and 0<sup>-</sup> states belonging to $v_{21} = 2$. This pair belongs to the S7 and S9 satellites (see Figure 3), respectively, and as shown in Table 3, changes in rotational constants and inertial defect values with respect to those for 0<sup>+</sup> and 0<sup>-</sup> in the ground state follow a near-linear variation with the $v_{21}$ quantum number.

Although rotational transitions in $v_{20} = 1$, $v_{19} = 1$, and $v_{18} = 1$ excited vibrational states should be observable based
on intensity considerations, their assignments to satellites S3, S4, S5, S6, and S8 are not definitively established. The main complications arise from rovibrational interactions, which prevented analyses of the S4 and S5 satellites. Consequently, when comparing the experimental changes in rotational constants and inertial defects with predicted values in the Appendix in Table 8, we do not have complete information. Furthermore, there is an associated uncertainty regarding the possible splitting of the ground state into a quartet. Due to the limited experimental data, we cannot unambiguously confirm or rule out this splitting, though the number of presently observed satellites points more toward a doublet pattern. We therefore maintain the current ground-state labeling as 0⁺ and 0⁻.

### 3.2. Rotation/Inversion Splitting

In general, there are three experimental approaches to determine the energy separation $\Delta E$ between the 0⁺ and 0⁻ levels. First, it can be derived from relative intensity measurements. These measurements were performed by A. Bauder & T.-K. Ha (1983), giving the energy difference of 54(22) cm⁻¹. Second, the energy difference can be obtained when rotational levels with the same $J$ in the 0⁺ and 0⁻ states become near-degenerate and perturbations are observed. However, no signs of perturbations were found in the spectrum. Finally, $\Delta E$ can be derived from the analysis of transitions connecting the 0⁺ and 0⁻ states. In the present case, only c-type transitions are governed by the 0⁺ $\leftrightarrow$ 0⁻ selection rules because $\mu_c$ is the only dipole moment component that changes sign during the rotation/inversion motion. Unfortunately, no c-type transitions were identified in the millimeter-wave spectra despite a reasonably large value of $|\mu_c| = 0.98$ D obtained from our calculations. However, their absence is in accordance with $\Delta E = 54(22)$ cm⁻¹ from A. Bauder & T.-K. Ha (1983), due to which the most favorable transitions, cQ-branches, are shifted to the far-infrared region.

At this point, we took advantage of our laser differential spectrometer and searched for cQ-transitions in the frequency window between 1 and 2 THz. A portion of the spectrum is shown in Figure 4(a). A weak absorption feature was identified that matched the predicted rotation/inversion cQ-transitions in Figure 4(b). Strong lines of HCN, suggesting the decomposition of 2-APN, were also observed. This was further confirmed by the decreasing intensity of the plausible 2-APN signal over time, while the HCN lines increased, as shown in Figure 4(c). An estimated $\Delta E$ value of 49.685 cm⁻¹ agrees with 54(22) cm⁻¹ from A. Bauder & T.-K. Ha (1983) and is close to $\Delta E = 45.481(74)$ cm⁻¹ found for vinylamine (R. Brown et al. 1990). Despite this, because of the weakness of the signal, we consider the observation of cQ-transitions to be tentative.

Final remarks concern the planarity of 2-APN. A. Bauder & T.-K. Ha (1983) assumed a nearly planar structure, while S. Saebo & L. Radom (1983) and P. Seiler & J. D. Dunitz (1985) supported the idea of a pyramidal configuration at the amino group. Our calculated potential energy surface, along with the pairs of states observed in the millimeter-wave spectra, confirms the nonplanarity of 2-APN.

### 3.3. Partition Function

As discussed in Section 3.1, we could not definitively label all vibrational satellites and assess the contribution of a second

<figure><img src="image_7.png" /><figcaption>**Figure 4.** (a) Section of the recorded THz spectrum where a signal possibly attributable to 2-APN is labeled with a gray star. (b) Prediction of the cQ-transitions of 2-APN based on the spectroscopic constants from Table 1 and $\Delta E = 49.685$ cm⁻¹. (c) Deterioration of the plausible 2-APN signal with time.</figcaption></figure>

<table><caption>**Table 4**
Partition Functions<sup>a</sup> for 2-APN</caption><thead><tr><th>T (K)</th><th>G.S.<sup>b</sup></th><th>Four States<sup>c</sup></th></tr></thead><tbody><tr><td>300.000</td><td>138,631.48</td><td>192,222.44</td></tr><tr><td>250.000</td><td>103,260.54</td><td>136,273.62</td></tr><tr><td>200.000</td><td>71,675.38</td><td>88,910.07</td></tr><tr><td>150.000</td><td>44,387.71</td><td>51,026.63</td></tr><tr><td>75.000</td><td>13,409.56</td><td>13,709.79</td></tr><tr><td>37.500</td><td>3931.11</td><td>3933.08</td></tr><tr><td>18.750</td><td>1238.00</td><td>1238.00</td></tr><tr><td>9.375</td><td>429.51</td><td>429.51</td></tr></tbody></table>

**Notes.**

possible splitting. This limitation affects our evaluation of the partition functions. Table 4 shows two sets of values based on the experimental observation of doublet patterns. The first column presents values for the ground-state doublet (0⁺ and 0⁻). The second column includes the lowest-lying excited vibrational state $v_{21} = 1$, which is assumed to lie at 198 cm⁻¹ according to our calculations. Since $\Delta E$ is expected to be similar for the excited states of modes unrelated to rotation/inversion, we used the same $\Delta E$ value as for the ground state in our summation. It can be seen that, for example, at 200 K, the vibrational correction factor arising from the additional

<footnote><sup>a</sup> Obtained by numerical summation over the rotational levels up to $J = 200$ and $K_a = 120$.</footnote>

<footnote><sup>b</sup> Ground-state doublet with $\Delta E = 49.685$ cm⁻¹.</footnote>

<footnote><sup>c</sup> Ground-state doublet plus $v_{21} = 1$ doublet (i.e., S1 and S2 states) assuming the same $\Delta E$ value as for the ground state.</footnote>
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Selection of rotational transitions of H₂C=C(NH₂)CN, *v* = 0 covered by the ReMoCA survey. The LTE synthetic spectrum of H₂C=C(NH₂)CN, *v* = 0 used to derive the upper limit to its column density toward Sgr B2(N1S) is displayed in red and overlaid on the observed spectrum shown in black. The blue synthetic spectrum contains the contributions of all molecules identified in our survey so far. The values written below each panel correspond from left to right to the HPBW, the central frequency in MHz, the width in MHz of each panel in parentheses, and the continuum level in K of the baseline-subtracted spectra in brackets. The y-axis is labeled in brightness temperature units (K). The dotted line indicates the 3σ noise level.</figcaption></figure>

population of the states (v<sub>21</sub> = 1, 0<sup>+</sup>) and (v<sub>21</sub> = 1, 0<sup>-</sup>) is 1.24. Since no experimental information is available on the distribution of vibrational energy levels above 198 cm<sup>-1</sup>, their contributions to the partition functions are neglected. Consequently, column densities are expected to be underestimated in high-temperature interstellar regions such as hot cores/corinos.

# 4. Observational Results and Discussion

## 4.1. Sgr B2(N1S)

We used the imaging spectral line survey Reexploring Molecular Complexity with ALMA (ReMoCA) that was performed with the Atacama Large Millimeter/submillimeter Array (ALMA). Details about the observations and the method of analysis of this survey were reported in A. Belloche et al. (2019, 2022). In short, the survey targeted the high-mass star-forming region Sgr B2(N), which contains a population of hot cores and H II regions (e.g., R. A. Gaume et al. 1995; M. Bonfand et al. 2017; Á. Sánchez-Monge et al. 2017). The molecular cloud complex Sgr B2 is located in the central molecular zone of our Galaxy at a distance of 8.2 kpc

(M. J. Reid et al. 2019). The median angular resolution of ReMoCA is 0.6. The frequency range 84.1–114.4 GHz was covered with a spectral resolution of 488 kHz (1.7–1.3 km s<sup>-1</sup>). The median sensitivity of the survey is 0.8 mJy beam<sup>-1</sup> (rms), which corresponds to 0.27 K at 100 GHz. The spectra were modeled under the assumption of local thermodynamic equilibrium (LTE) with the software Weeds (S. Maret et al. 2011), which accounts for the radiative transfer and the finite angular resolution of the observations.

Until now, a reliable search for 2-APN in the ReMoCA survey was far out of reach because differences of a few MHz were identified when comparing predictions generated in this work with those from A. Bauder & T.-K. Ha (1983). Using the new spectroscopic predictions, we searched for 2-APN toward the main hot core Sgr B2(N1), which has the highest H₂ column density in Sgr B2(N). We chose the position of Sgr B2(N1S), which is located at (α, δ)<sub>12000</sub> = (17h47m19s.87, −28°22′19″.48). This position is offset from the peak of Sgr B2(N1) by about 1" to the south. It was chosen by A. Belloche et al. (2019) as a compromise between reducing the dust optical depth and keeping a high enough H₂ column density to detect less abundant molecules. To search for
<figure><img src="image_9.png" /><figcaption>Figure 6. Same as Figure 5 but for CH₃CH(NH₂)CN, v = 0.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 7.** Transition of 2-APN used to compute the column density upper limit toward G+0.693 (see text). The black line and gray histogram show the observed spectrum, the red curve is the LTE synthetic model of 2-APN using the column density upper limit derived toward G+0.693, and the blue curve indicates the emission of all the molecules previously identified toward the source.</figcaption></figure>

signatures of 2-APN emission in the ReMoCA spectrum of Sgr B2(N1S), we assumed the same LTE parameters as derived for aminoacetonitrile, NH₂CH₂CN, toward this position by M. Melosso et al. (2020), keeping only the column density of 2-APN as a free parameter. We chose aminoacetonitrile as a reference because it contains the same functional groups as 2-APN (–NH₂ and –CN) and a similar number of atoms. We did not find any evidence for 2-APN in the ReMoCA spectrum of Sgr B2(N1S) (see Figure 5). We also searched for the related molecule 2-aminopropionitrile, CH₃CH(NH₂)CN, using the spectroscopic predictions available in the Cologne Database for Molecular Spectroscopy (H. S. P. Müller et al. 2005), which are based on H. Møllendal et al. (2012). We did not detect 2-aminopropionitrile either (see Figure 6). Table 5 reports the upper limits that we derived for the column

densities of both molecules, as well as the parameters previously obtained for aminoacetonitrile. The upper limits imply that 2-APN and 2-aminopropionitrile are at least 5 and 4 times less abundant than aminoacetonitrile in this source, respectively.

With one additional carbon atom in their heavy-atom backbone, 2-APN and 2-aminopropionitrile represent a further step in molecular complexity with respect to aminoacetonitrile. Such a step is often associated with an abundance drop of at least 1 order of magnitude in Sgr B2(N)’s hot cores, as illustrated, for instance, by the case of acrylamide and propionamide with respect to acetamide (L. Kolesniková et al. 2022). With the current sensitivity of the ReMoCA survey and the level of spectral confusion in Sgr B2(N1S) at 3 mm, we are thus likely still far from a detection of 2-APN and 2-aminopropionitrile in this source. Sensitive observations of Sgr B2(N) at longer wavelengths, where the level of spectral confusion is expected to be lower, may help to make further progress.

## 4.2. G+0.693

We have also searched toward the Galactic center molecular cloud G+0.693, also located in the Sgr B2 complex. This position presents very different physical properties compared to its neighbors, Sgr B2(N) hot cores. There are no current hints of ongoing star formation activity such as compact continuum millimeter emission or molecular outflows (e.g., A. Ginsburg et al. 2018; X. Lu et al. 2019). It has recently been proposed by L. Colzi et al. (2022, 2024) that the region might be in a prestellar cluster stage on the verge of forming a new protostellar cluster. The gas kinetic temperature of G+0.693 is high (70–150 K; e.g., S. Zeng et al. 2018), in agreement with the usual gas temperatures derived in the Galactic center (e.g., N. Krieger et al. 2017; A. Ginsburg et al. 2018). However, the excitation temperatures derived from molecular line emission are usually significantly lower, with typical values ranging from 5 to 20 K (e.g., S. Zeng et al. 2018; I. Jiménez-Serra et al. 2020; V. M. Rivilla et al. 2022b). This is because the molecules are subthermally excited due to the intermediate
<table><caption>**Table 5**
Parameters of Our Best-fit LTE Model of Aminoacetonitrile and Column Density Upper Limits Obtained for 2-APN and 2-aminopropionitrile toward Sgr B2(N1S)</caption><thead><tr><th>Molecule</th><th>Status<sup>a</sup></th><th>N<sub>det</sub><sup>b</sup></th><th>Size<sup>c</sup><br>(arcsec)</th><th>T<sub>rot</sub><sup>d</sup><br>(K)</th><th>N<sup>e</sup><br>(cm<sup>-2</sup>)</th><th>F<sub>vib</sub><sup>f</sup></th><th>F<sub>conf</sub><sup>g</sup></th><th>ΔV<sup>h</sup><br>(km s<sup>-1</sup>)</th><th>V<sub>off</sub><sup>i</sup><br>(km s<sup>-1</sup>)</th><th>N<sub>ref</sub><sup>j</sup><br>N</th></tr></thead><tbody><tr><td>NH<sub>2</sub>CH<sub>2</sub>CN<sup>k,x</sup></td><td>d</td><td>23</td><td>2.0</td><td>200</td><td>1.1 (17)</td><td>1.00</td><td>1.00</td><td>5.0</td><td>0.0</td><td>1</td></tr><tr><td>H<sub>2</sub>C=C(NH<sub>2</sub>)CN</td><td>n</td><td>0</td><td>2.0</td><td>200</td><td>&lt;2.1 (16)</td><td>1.24</td><td>1.00</td><td>5.0</td><td>0.0</td><td>&gt;5.2</td></tr><tr><td>CH<sub>3</sub>CH(NH<sub>2</sub>)CN</td><td>n</td><td>0</td><td>2.0</td><td>200</td><td>&lt;2.7 (16)</td><td>2.65</td><td>1.02</td><td>5.0</td><td>0.0</td><td>&gt;4.1</td></tr></tbody></table>

**Notes.**

a d: detection; n: nondetection.

b Number of detected lines (conservative estimate; see Section 3 of A. Belloche et al. 2016). One line of a given species may mean a group of transitions of that species that are blended together.

c Source diameter (FWHM).

d Rotational temperature.

e Total column density of the molecule. x (y) means x × 10⁻⁹.

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.

g Correction factor that was applied to the column density to account for the contribution of other conformers in the cases where this contribution could be estimated but was not included in the partition function of the spectroscopic predictions.

h Line width (FWHM).

i Velocity offset with respect to the assumed systemic velocity of Sgr B2(N1S), V<sub>sys</sub> = 62.0 km s⁻¹.

j Column density ratio, with N<sub>ref</sub> the column density of the previous reference species marked with a star.

k The parameters were derived from the ReMoCA survey by M. Melosso et al. (2020).

average densities of the cloud around 10⁴ cm⁻³ (S. Zeng et al. 2020; L. Colzi et al. 2022, 2024). As a consequence, the resulting spectra are less crowded than in high-T<sub>ex</sub> sources such as hot cores, which alleviates line contamination problems and consequently helps the identification of new interstellar molecules.

Moreover, the presence of large-scale shocks has been confirmed by the bright emission of SiO of Class I CH<sub>3</sub>OH masers, which has been attributed to a cloud–cloud collision in the region (S. Zeng et al. 2020). These shocks are able to sputter the icy grain mantles and eject into the gas phase at least a fraction of the molecules formed on dust grains through surface chemistry. Furthermore, the high cosmic-ray ionization rate of the cloud, 10⁻¹⁴–10⁻¹⁵ s⁻¹, derived by the chemical modeling of ions such as HOCS⁺ and PO⁺ (V. M. Rivilla et al. 2022a; M. Sanz-Novo et al. 2024), has been proposed to trigger the formation of reactive radicals on the ices, which can rapidly combine in situ to synthesize molecules with increasing chemical complexity (V. M. Rivilla et al. 2023).

Among the more than 140 species that have been identified toward G+0.693, there are many nitriles (V. M. Rivilla et al. 2022b), and several new C- and N-bearing interstellar species have been discovered in the last few years, such as the Z-isomer of cyanomethanimine (Z-HCNHNC; V. M. Rivilla et al. 2019), HCN (V. M. Rivilla et al. 2021a), vinyl amine (C<sub>2</sub>H<sub>3</sub>NH<sub>2</sub>; S. Zeng et al. 2021), and propynimine (HCCCHNH; L. Bizzocchi et al. 2020). Moreover, several species with the –NH<sub>2</sub> group were also detected, such as hydroxylamine (NH<sub>2</sub>OH; V. M. Rivilla et al. 2020), urea (NH<sub>2</sub>CONH<sub>2</sub>; I. Jiménez-Serra et al. 2020), ethanolamine (NH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>OH; V. M. Rivilla et al. 2021b), and glycolamide (NH<sub>2</sub>COCH<sub>2</sub>OH; V. M. Rivilla et al. 2023). Therefore, this region stands as an excellent candidate to search for molecules with the –CN and –NH<sub>2</sub> functional groups, such as 2-APN.

To search for 2-APN, we have used an ultrasensitive spectral survey toward this cloud that comprised Q-band observations at centimeter wavelengths (31.075–50.424 GHz), conducted with the Yebes 40 m radio telescope (Guadalajara,

Spain), and observations across two millimeter-wavelength frequency windows (71.8–116.7 GHz, and 124.8–175.5 GHz), conducted with the 30 m IRAM radio telescope in Granada (Spain). The position switching mode was used, centered at α = 17h47m22s, δ = −28°21′27″, with the off position shifted by Δα = −885″ and Δδ = 290″. The half-power beamwidth (HPBW) of the Yebes 40 m telescope varies between ~35″ and 55″ (at 50 and 31 GHz, respectively; F. Tercero et al. 2021), and the HPBW of the IRAM 30 m radio telescope is ~14″–29″ across the frequency range covered. The line intensity of the spectra is presented in units of antenna temperature (T<sub>A</sub><sup>*</sup>) since the molecular emission toward G +0.693 is extended beyond the telescope primary beams (S. Brünken et al. 2010; P. A. Jones et al. 2012; J. Li et al. 2020; M. G. Santa-Maria et al. 2021; L. Colzi et al. 2024). Further details on these observations, including the resolution and final noise levels of the molecular line survey, are given in V. M. Rivilla et al. (2023) and M. Sanz-Novo et al. (2023).

We implemented the spectroscopic entry of 2-APN into the analysis software MADCUBA.<fnref n="5" /> Using the Spectral Line Identification and Modeling tool of the MADCUBA package (S. Martín et al. 2019), we generated the expected LTE synthetic spectra of 2-APN to search for it in the G+0.693 spectral survey. No clear transition of the species was identified. We have thus computed integrated intensity 3σ column density upper limits. We used the brighter transitions according to the LTE model that appear not blended with emission from other molecules and assumed the same T<sub>ex</sub>, line width, and v<sub>LSR</sub> that were derived for vinylamine, of which 2-APN is a CN derivative (10.8 K, 18 km s⁻¹, and 67 km s⁻¹, respectively; S. Zeng et al. 2021). Figure 7 shows the spectrum around the 5<sub>1,5</sub>–4<sub>1,4</sub> transition at 32.532027 GHz, which was used to compute the upper limit, and the LTE synthetic spectrum considering the value of the N upper limit. We derived an upper limit of N < 6.9 × 10¹¹ cm⁻², which translates into a molecular abundance of <5.1 × 10⁻¹², using

<footnote n="5">⁵ Madrid Data Cube Analysis on ImageJ is software developed at the Centre of Astrobiology in Madrid (https://cab.inta-csic.es/madcuba/; see S. Martín et al. 2019).</footnote>
the N(H₂) of 1.35 × 10²³ cm⁻² from S. Martín et al. (2008). This upper limit is a factor of ~130 lower than the abundance derived for vinyl cyanide (H₂C=CHCN, usually written as C₂H₃CN; S. Zeng et al. 2018). We note that according to the upper limit reported here, the line intensities of the 2-APN transitions are expected to be extremely weak, <0.5 mK, which indicates that its detection will surely require significantly higher-sensitivity observations.

## 5. Conclusions

The millimeter-wave spectrum of 2-APN has been recorded in the 158–179, 204–238, and 292–324 GHz frequency regions. Over 2700 lines belonging to pure rotational transitions have been measured and assigned for the ground-state 0⁺ and 0⁻ doublet and seven vibrational satellites. Using the new frequency predictions, rotational transitions of 2-APN were searched for toward the hot core Sgr B2(N1) and the molecular cloud G+0.693. Additionally, emission lines from the reduced derivative 2-aminopropionitrile were searched for in the former source. We did not observe any spectral signatures of these molecules in any of these sources, and the upper limits for the column densities were derived. Despite this, the spectroscopic data presented in this work serve as a tool for future searches of 2-APN in interstellar space.

## Acknowledgments

This work has been funded by the Czech Science Foundation (GACR, grant No. 24-12586S). The Prague authors gratefully acknowledge this financial support. Computational resources were provided by the e-INFRA CZ project (ID:90254), supported by the Ministry of Education, Youth and Sports of the Czech Republic. J.-C.G. thanks the Centre National d'Etudes Spatiales (CNES) for a grant. I.J.-S. and

V.M.R. acknowledge support from the grant PID2022-136814NB-I00 funded by the Spanish Ministry of Science, Innovation and Universities/State Agency of Research MICIU/AEI/10.13039/501100011033 and by “ERDF/UE.” V.M.R. also acknowledges funding from the grant RYC2020-029387-I funded by MICIU/AEI/10.13039/501100011033 and by “ESF, Investing in your future”; from the Consejo Superior de Investigaciones Científicas (CSIC) and the Centro de Astrobiología (CAB) through the project 20225AT015 (Proyectos intramurales especiales del CSIC); and from the grant CNS2023-144464 funded by MICIU/AEI/10.13039/501100011033 and by “European Union NextGenerationEU/PRTR.” I.J.-S acknowledges funding from the ERC grant OPENS (project number 101125858) funded by the European Union. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2016.1.00074.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 interferometric data are available in the ALMA archive at https://almascience.eso.org/aq/.

## Appendix
Complementary Tables

Table 6 lists the optimized atomic coordinates of 2-APN and Table 7 lists measured transitions of 2-APN. Table 8 contains experimentally determined values of vibrational changes in rotational constants and inertial defects of the vibrational satellites S3, S6, and S8 and calculated values for the $v_{20} = 1$, $v_{19} = 1$, and $v_{18} = 1$ states.

<table><caption>**Table 6**
The Atomic Coordinates of 2-APN Optimized at the CCSD/cc-pVTZ Level of Theory</caption><thead><tr><th rowspan="2">Atom</th><th colspan="3">Coordinates (in bohr)</th></tr><tr><th>X</th><th>Y</th><th>Z</th></tr></thead><tbody><tr><td>C1</td><td>0.95157960</td><td>-0.14559343</td><td>-0.01450125</td></tr><tr><td>C2</td><td>2.13610913</td><td>-2.37874305</td><td>-0.01793554</td></tr><tr><td>N3</td><td>2.10018613</td><td>2.23381885</td><td>0.11131089</td></tr><tr><td>H4</td><td>3.94117283</td><td>2.19000419</td><td>-0.37026824</td></tr><tr><td>H5</td><td>1.17356656</td><td>3.57849845</td><td>-0.86639490</td></tr><tr><td>C6</td><td>-1.78270862</td><td>-0.06603102</td><td>0.01201407</td></tr><tr><td>N7</td><td>-3.96397559</td><td>0.04546006</td><td>-0.00459043</td></tr><tr><td>H8</td><td>1.06829849</td><td>-4.11054575</td><td>0.04408766</td></tr><tr><td>H9</td><td>4.17493108</td><td>-2.48399427</td><td>-0.04706602</td></tr></tbody></table>

<figure><img src="image_11.png" /></figure>
<table><caption>**Table 7**
List of Measured Transitions of 2-APN</caption><thead><tr><th>Species</th><th>J'</th><th>K'<sub>a</sub></th><th>K'<sub>c</sub></th><th>J''</th><th>K''<sub>a</sub></th><th>K''<sub>c</sub></th><th>ν<sub>obs</sub><sup>a</sup></th><th>u<sub>obs</sub><sup>b</sup><br>(MHz)</th><th>ν<sub>obs</sub> - ν<sub>calc</sub><sup>c</sup><br>(MHz)</th><th>(ν<sub>obs</sub> - ν<sub>calc</sub>)<sub>b</sub><sup>d</sup><br>(MHz)</th><th>Weight<sup>e</sup></th></tr></thead><tbody><tr><td>0<sup>+</sup></td><td>49</td><td>0</td><td>49</td><td>48</td><td>0</td><td>48</td><td>296,901.4626</td><td>0.03</td><td>-0.0354</td><td>-0.0354</td><td>0.46</td></tr><tr><td>0<sup>+</sup></td><td>49</td><td>0</td><td>49</td><td>48</td><td>1</td><td>48</td><td>296,901.4626</td><td>0.03</td><td>-0.0354</td><td>-0.0354</td><td>0.04</td></tr><tr><td>0<sup>+</sup></td><td>49</td><td>1</td><td>49</td><td>48</td><td>0</td><td>48</td><td>296,901.4626</td><td>0.03</td><td>-0.0354</td><td>-0.0354</td><td>0.04</td></tr><tr><td>0<sup>+</sup></td><td>49</td><td>1</td><td>49</td><td>48</td><td>1</td><td>48</td><td>296,901.4626</td><td>0.03</td><td>-0.0354</td><td>-0.0354</td><td>0.46</td></tr><tr><td>0<sup>-</sup></td><td>46</td><td>6</td><td>40</td><td>45</td><td>6</td><td>39</td><td>314,888.2676</td><td>0.03</td><td>0.0251</td><td>0.0463</td><td>0.50</td></tr><tr><td>0<sup>-</sup></td><td>46</td><td>7</td><td>40</td><td>45</td><td>7</td><td>39</td><td>314,888.2676</td><td>0.03</td><td>0.0675</td><td>0.0463</td><td>0.50</td></tr><tr><td>S1</td><td>33</td><td>5</td><td>28</td><td>32</td><td>5</td><td>27</td><td>231,575.6862</td><td>0.03</td><td>0.0398</td><td>...</td><td>1.00</td></tr><tr><td>S1</td><td>34</td><td>5</td><td>29</td><td>33</td><td>5</td><td>28</td><td>237,529.0901</td><td>0.03</td><td>-0.0107</td><td>...</td><td>1.00</td></tr><tr><td>S2</td><td>30</td><td>9</td><td>22</td><td>29</td><td>9</td><td>21</td><td>228,592.5799</td><td>0.03</td><td>0.0046</td><td>...</td><td>1.00</td></tr><tr><td>S2</td><td>31</td><td>9</td><td>23</td><td>30</td><td>9</td><td>22</td><td>235,803.8973</td><td>0.03</td><td>-0.0104</td><td>...</td><td>1.00</td></tr><tr><td>S3</td><td>28</td><td>14</td><td>14</td><td>27</td><td>14</td><td>13</td><td>208,348.0046</td><td>0.05</td><td>-0.0231</td><td>-0.0088</td><td>0.50</td></tr><tr><td>S3</td><td>28</td><td>14</td><td>15</td><td>27</td><td>14</td><td>14</td><td>208,348.0046</td><td>0.05</td><td>0.0054</td><td>-0.0088</td><td>0.50</td></tr><tr><td>S6</td><td>31</td><td>6</td><td>25</td><td>30</td><td>6</td><td>24</td><td>226,750.8208</td><td>0.03</td><td>0.0277</td><td>...</td><td>1.00</td></tr><tr><td>S6</td><td>32</td><td>6</td><td>26</td><td>31</td><td>6</td><td>25</td><td>232,460.7665</td><td>0.03</td><td>-0.0232</td><td>...</td><td>1.00</td></tr><tr><td>S7</td><td>22</td><td>15</td><td>8</td><td>21</td><td>15</td><td>7</td><td>163,242.4566</td><td>0.05</td><td>-0.0338</td><td>-0.0338</td><td>0.50</td></tr><tr><td>S7</td><td>22</td><td>15</td><td>7</td><td>21</td><td>15</td><td>6</td><td>163,242.4566</td><td>0.05</td><td>-0.0338</td><td>-0.0338</td><td>0.50</td></tr><tr><td>S8</td><td>38</td><td>10</td><td>28</td><td>37</td><td>10</td><td>27</td><td>301,396.4649</td><td>0.03</td><td>0.0107</td><td>...</td><td>1.00</td></tr><tr><td>S8</td><td>30</td><td>13</td><td>18</td><td>29</td><td>13</td><td>17</td><td>224,867.2130</td><td>0.03</td><td>0.0649</td><td>...</td><td>1.00</td></tr><tr><td>S9</td><td>23</td><td>12</td><td>12</td><td>22</td><td>12</td><td>11</td><td>171,781.1055</td><td>0.05</td><td>0.0397</td><td>-0.0183</td><td>0.50</td></tr><tr><td>S9</td><td>23</td><td>12</td><td>11</td><td>22</td><td>12</td><td>10</td><td>171,781.1055</td><td>0.05</td><td>-0.0763</td><td>-0.0183</td><td>0.50</td></tr></tbody></table>

**Notes.**

a Observed frequency.

b Uncertainty of the observed frequency.

c Observed minus calculated frequency.

d Observed minus calculated frequency for blends.

e Intensity weighting factor for transitions derived using calculated dipole moment components from Table 1.

(This table is available in its entirety in machine-readable form in the online article.)

<caption>**Table 8**</caption>

Experimentally Obtained Vibrational Changes in Rotational Constants and Inertial Defects of the Vibrational Satellites S3, S6, and S8 in Comparison with Calculated Values for the $v_{20} = 1$, $v_{19} = 1$, and $v_{18} = 1$ States

<table><thead><tr><th rowspan="2">Parameters</th><th colspan="3">Prediction<sup>a</sup></th></tr><tr><th>$v_{20} = 1$</th><th>$v_{19} = 1$</th><th>$v_{18} = 1$</th></tr></thead><tbody><tr><td>$A_i - A_0$/MHz</td><td>18.7</td><td>-2.2</td><td>1.7</td></tr><tr><td>$B_i - B_0$/MHz</td><td>-0.8</td><td>2.3</td><td>7.6</td></tr><tr><td>$C_i - C_0$/MHz</td><td>1.6</td><td>3.7</td><td>0.1</td></tr><tr><td>$\Delta_i - \Delta_0$/uÅ<sup>2</sup></td><td>-0.02</td><td>-0.16</td><td>0.21</td></tr></tbody><thead><tr><th rowspan="2">Parameters</th><th colspan="3">Experimental Values Relative to the 0<sup>+</sup> State</th></tr><tr><th>S3</th><th>S6</th><th>S8</th></tr></thead><tbody><tr><td>$A_i - A_0$/MHz</td><td>26.3679(69)</td><td>19.2594(85)</td><td>-1.5463(79)</td></tr><tr><td>$B_i - B_0$/MHz</td><td>0.41494(58)</td><td>1.05158(64)</td><td>5.81345(73)</td></tr><tr><td>$C_i - C_0$/MHz</td><td>2.97129(35)</td><td>5.27281(39)</td><td>-0.21394(58)</td></tr><tr><td>$\Delta_i - \Delta_0$/uÅ<sup>2</sup></td><td>-0.022158(42)</td><td>-0.169289(51)</td><td>0.164199(55)</td></tr></tbody></table>

<table><thead><tr><th rowspan="2">Parameters</th><th colspan="3">Experimental Values Relative to the 0<sup>-</sup> State</th></tr><tr><th>S3</th><th>S6</th><th>S8</th></tr></thead><tbody><tr><td>$A_i - A_0$/MHz</td><td>32.6343(69)</td><td>25.5258(85)</td><td>4.7201(79)</td></tr><tr><td>$B_i - B_0$/MHz</td><td>6.05263(58)</td><td>6.68927(64)</td><td>11.45114(73)</td></tr><tr><td>$C_i - C_0$/MHz</td><td>4.03801(35)</td><td>6.33953(39)</td><td>0.85278(58)</td></tr><tr><td>$\Delta_i - \Delta_0$/uÅ<sup>2</sup></td><td>0.105245(43)</td><td>-0.041886(51)</td><td>0.291602(55)</td></tr></tbody></table>

**Note.**

a Calculated with respect to the ground state. Rotation/inversion splitting is not accounted for.

**ORCID iDs**

Kateřina Luková https://orcid.org/0000-0002-0087-0626
Lucie Kolesniková https://orcid.org/0000-0003-3567-0349
Jean-Claude Guillemin https://orcid.org/0000-0002-2929-057X

Tereza Uhlíková https://orcid.org/0000-0002-6220-2092
Jan Koucký https://orcid.org/0000-0001-9498-9400
František Horák https://orcid.org/0000-0001-9782-8928
Lukáš Hrubčík https://orcid.org/0009-0003-6258-2245
Karel Vávra https://orcid.org/0000-0002-6282-3710
Patrik Kania https://orcid.org/0000-0001-8338-3569
Štěpán Urban https://orcid.org/0000-0002-4566-4930
Arnaud Belloche https://orcid.org/0000-0003-0046-6217
Izaskun Jiménez-Serra https://orcid.org/0000-0003-4493-8714
Víctor M. Rivilla https://orcid.org/0000-0002-2887-5859

**References**

Alberton, D., Lattanzi, V., Endres, C., et al. 2023, *APJ*, 951, 108

Bauder, A., & Ha, T.-K. 1983, *CPL*, 97, 135

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

Belloche, A., Garrod, R. T., Zingsheim, O., Müller, H. S. P., & Menten, K. M. 2022, *A&A*, 662, A110

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

Belloche, A., Müller, H. S. P., Garrod, R. T., & Menten, K. M. 2016, *A&A*, 587, A91

Bizzocchi, L., Prudenzano, D., Rivilla, V. M., et al. 2020, *A&A*, 640, A98

Bonfand, M., Belloche, A., Menten, K. M., Garrod, R. T., & Müller, H. S. P. 2017, *A&A*, 604, A60
Brown, R., Godfrey, P., Kleibomer, B., Pierlot, A., & McNaughton, D. 1990, *JMoSp*, 142, 195

Brünken, S., Belloche, A., Martín, S., Verheyen, L., & Menten, K. M. 2010, *A&A*, 516, A109

Colzi, L., Martín-Pintado, J., Rivilla, V. M., et al. 2022, *ApJL*, 926, L22

Colzi, L., Martín-Pintado, J., Zeng, S., et al. 2024, *A&A*, 690, A121

Deninger, A. J., Roggenbuck, A., Schindler, S., & Preu, S. 2015, *JIMTW*, 36, 269

Drenkard, S., Ferris, J., & Eschenmoser, A. 1990, *HChAc*, 73, 1373

Dunning, T. H. 1989, *JChPh*, 90, 1007

Favero, L. B., Corbelli, G., Velino, B., Caminati, W., & Favero, P. G. 1998, *CP*, 228, 219

Fischer, E., & Botskor, I. 1984, *JMoSp*, 104, 226

Foden, C. S., Islam, S., Fernández-García, C., et al. 2020, *Sci*, 370, 865

Gaume, R. A., Claussen, M. J., de Pree, C. G., Goss, W. M., & Mehringer, D. M. 1995, *ApJ*, 449, 663

Ginsburg, A., Bally, J., Barnes, A., et al. 2018, *ApJ*, 853, 171

Giuliano, B. M., Bizzocchi, L., Charmet, A. P., et al. 2019, *A&A*, 628, A53

Green, N. J., Russell, D. A., Tanner, S. H., & Sutherland, J. D. 2023, *JAChS*, 145, 10533

Jähnish, K., Weigt, E., & Bosies, E. 1992, *Synthesis*, 12, 1211

Jiménez-Serra, I., Martín-Pintado, J., Rivilla, V. M., et al. 2020, *AsBio*, 20, 1048

Jones, P. A., Burton, M. G., Cunningham, M. R., et al. 2012, *MNRAS*, 419, 2961

Kania, P., Střířeská, L., Šimečková, M., & Urban, Š. 2006, *JMoSt*, 795, 209

Kisiel, Z., Pszczółkowski, L., Drouin, B. J., et al. 2012, *JMoSp*, 280, 134

Kisiel, Z., Pszczółkowski, L., Medvedev, I. R., et al. 2005, *JMoSp*, 233, 231

Kolesniková, L., Belloche, A., Koucký, J., et al. 2022, *A&A*, 659, A111

Krieger, N., Ott, J., Beuther, H., et al. 2017, *ApJ*, 850, 77

Ksander, G., Bold, G., Lattmann, R., et al. 1987, *HChAc*, 70, 1115

Li, J., Wang, J., Qiao, H., et al. 2020, *MNRAS*, 492, 556

Lu, X., Mills, E. A. C., Ginsburg, A., et al. 2019, *ApJS*, 244, 35

Luková, K., Kolesniková, L., Koucký, J., et al. 2022, *A&A*, 665, A9

Maret, S., Hily-Blant, P., Pety, J., Bardeau, S., & Reynier, E. 2011, *A&A*, 526, A47

Martín, S., Martín-Pintado, J., Blanco-Sánchez, C., et al. 2019, *A&A*, 631, A159

Martín, S., Requena-Torres, M. A., Martín-Pintado, J., & Mauersberger, R. 2008, *ApJ*, 678, 245

Matthews, D. A., Cheng, L., Harding, M. E., et al. 2020, *JChPh*, 152, 214108

Melosso, M., Belloche, A., Martin-Drumel, M. A., et al. 2020, *A&A*, 641, A160

Møllendal, H., & Konovalov, A. 2010, *JPCA*, 114, 2151

Møllendal, H., Margulès, L., Belloche, A., et al. 2012, *A&A*, 538, A51

Motiyenko, R. A., Margulès, L., Alekseev, E. A., & Guillemin, J.-C. 2015, *JPCA*, 119, 1048

Motiyenko, R. A., Margules, L., & Guillemin, J.-C. 2013, *A&A*, 559, A44

Müller, H. S. P., Schlöder, F., Stutzki, J., & Winnewisser, G. 2005, *JMoSt*, 742, 215

Müller, K., & Brown, L. D. 1978, *HChAc*, 61, 1407

Pickett, H. M. 1991, *JMoSp*, 148, 371

Purvis, G. D. I., & Bartlett, R. J. 1982, *JChPh*, 76, 1910

Reid, M. J., Menten, K. M., Brunthaler, A., et al. 2019, *ApJ*, 885, 131

Rivilla, V. M., García de La Concepción, J., Jiménez-Serra, I., et al. 2022a, *FrASS*, 9, 829288

Rivilla, V. M., Jiménez-Serra, I., García de la Concepción, J., et al. 2021a, *MNRAS*, 506, L79

Rivilla, V. M., Jiménez-Serra, I., Martín-Pintado, J., et al. 2021b, *PNAS*, 118, e2101314118

Rivilla, V. M., Jiménez-Serra, I., Martín-Pintado, J., et al. 2022b, *FrASS*, 9, 876870

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

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

Rivilla, V. M., Sanz-Novo, M., Jiménez-Serra, I., et al. 2023, *ApJL*, 953, L20

Saebo, S., & Radom, L. 1983, *J. Mol. Struct.: THEOCHEM*, 105, 119

Sánchez-Monge, Á., Schilke, P., Schmiedeke, A., et al. 2017, *A&A*, 604, A6

Santa-Maria, M. G., Goicoechea, J. R., Etxaluze, M., Cernicharo, J., & Cuadrado, S. 2021, *A&A*, 649, A32

Sanz-Novo, M., Rivilla, V. M., Jiménez-Serra, I., et al. 2023, *ApJ*, 954, 3

Sanz-Novo, M., Rivilla, V. M., Jiménez-Serra, I., et al. 2024, *ApJ*, 965, 149

Seiler, P., & Dunitz, J. D. 1985, *HChAc*, 68, 2093

Tercero, F., López-Pérez, J. A., Gallego, J. D., et al. 2021, *A&A*, 645, A37

Watson, J. K. G. 1977, in Vibrational Spectra and Structure, ed. J. R. Durig, Vol. 6 (Amsterdam: Elsevier), 1

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

Zeng, S., Jiménez-Serra, I., Rivilla, V. M., et al. 2021, *ApJL*, 920, L27

Zeng, S., Zhang, Q., Jiménez-Serra, I., et al. 2020, *MNRAS*, 497, 4896