# Rotational Spectroscopy and Tentative Interstellar Detection of 3-hydroxypropanal (HOCH2CH2CHO) in the G+0.693-0.027 Molecular Cloud

Zachary Fried, Roman A. Motiyenko, Miguel Sanz-Novo, Lucie Kolesniková, Jean-claude Guillemin, Laurent Margules, Tereza Uhlíková, Arnaud Belloche, Jes Jørgensen, Martin S. Holdren, et al.

► To cite this version:

Zachary Fried, Roman A. Motiyenko, Miguel Sanz-Novo, Lucie Kolesniková, Jean-claude Guillemin, et al. Rotational Spectroscopy and Tentative Interstellar Detection of 3-hydroxypropanal (HOCH2CH2CHO) in the G+0.693-0.027 Molecular Cloud. The Astrophysical Journal, 2025, 992 (2), pp.187. 10.3847/1538-4357/adf c62. hal-05346620v2

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# Rotational Spectroscopy and Tentative Interstellar Detection of 3-hydroxypropanal (HOCH₂CH₂CHO) in the G+0.693-0.027 Molecular Cloud

Zachary T. P. Fried¹, Roman A. Motiyenko², Miguel Sanz-Novo³, Lucie Kolesniková⁴, Jean-Claude Guillemin⁵, Laurent Margulès², Tereza Uhlíková⁴, Arnaud Belloche⁶, Jes K. Jørgensen⁷, Martin S. Holdren¹, Ci Xue¹, Štěpán Urban⁴, Izaskun Jiménez-Serra³, Victor M. Rivilla³, and Brett A. McGuire¹,⁸

¹Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA; zfried@mit.edu, brettmc@mit.edu

²Univ. Lille, CNRS, UMR 8523—PhLAM—Physique des Lasers Atomes et Molécules, F-59000 Lille, France

³Centro de Astrobiología (CAB), INTA-CSIC, Carretera de Ajalvir km 4, Torrejón de Ardoz, 28850 Madrid, Spain

⁴Department of Analytical Chemistry, University of Chemistry and Technology, Technická 5, 166 28 Prague 6, Czechia

⁵Univ Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR - UMR6226, F-35000 Rennes, France

⁶Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

⁷Centre for Star and Planet Formation, Niels Bohr Institute & Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen K., Denmark

⁸National Radio Astronomy Observatory, Charlottesville, VA 22903, USA

Received 2025 April 28; revised 2025 August 12; accepted 2025 August 12; published 2025 October 14

## Abstract

We synthesized the astrochemically relevant molecule 3-hydroxypropanal (HOCH₂CH₂CHO) and subsequently measured and analyzed its rotational spectrum in several frequency regions ranging from 130 to 485 GHz. We analyzed the ground vibrational state as well as the two perturbed lowest-lying vibrationally excited states. With the resulting rotational parameters, we searched for this molecule in the Sagittarius B2(N) and NGC 6334I hot cores, the IRAS 16293-2422B hot corino, and the G+0.693-0.027 and TMC-1 molecular clouds. Rotational emission of 3-hydroxypropanal was tentatively detected toward G+0.693-0.027, and a column density of (8.6 ±1.4) × 10¹² cm⁻² was determined. However, this molecule was not detected in the other sources that were investigated. The chemical implications of this tentative discovery are analyzed, and several potential chemical formation pathways of this species are discussed.

Unified Astronomy Thesaurus concepts: Astrochemistry (75); Rotational spectroscopy (2248); Interstellar molecules (849)

Materials only available in the online version of record: tar.gz files

## 1. Introduction

Recently, the C₃H₆O₂ isomers have been of particular astrochemical interest. Three of these isomers, namely, ethyl formate (A. Belloche et al. 2009; V. M. Rivilla et al. 2017; Y. Peng et al. 2019), methyl acetate (B. Tercero et al. 2013), and hydroxyacetone (Y. Zhou et al. 2020), have been detected toward various interstellar sources. That being said, the most thermodynamically stable isomer propanoic acid was not detected in searches toward Orion KL and Sagittarius (Sgr) B2 (V. V. Ilyushin et al. 2021; A. Belloche et al. 2025), thus suggesting that the gas-phase interstellar abundances do not solely follow trends of thermodynamic stability—a phenomenon observed in several other isomeric families (e.g., R. A. Loomis et al. 2015; C. N. Shingledecker et al. 2019; C. Mininni et al. 2020; M. Agúndez et al. 2023b; V. M. Rivilla et al. 2023; D. San Andrés et al. 2024). The higher-energy isomers methoxyacetaldehyde (L. Kolesniková et al. 2018) and lactaldehyde, also known as 2-hydroxypropanal (E. R. Alonso et al. 2019), were also not detected in searches toward these same star-forming regions.

An additional C₃H₆O₂ isomer of astrochemical interest is 3-hydroxypropanal (HOCH₂CH₂CHO). This species has been recommended as a strong detection target by several groups

(e.g., Z. T. P. Fried et al. 2023; J. Wang et al. 2023). Notably, J. Wang et al. (2023) recently investigated the formation of various C₃H₆O₂ isomers from methanol-acetaldehyde ices that were irradiated with energetic electrons at low temperatures, simulating molecular cloud conditions. They ultimately observed the formation of hydroxyacetone and methyl acetate (which were both detected in the interstellar medium) along with 3-hydroxypropanal. They note that 3-hydroxypropanal is formed via the recombination of the CH₂OH and CH₂CHO radicals, which may be hydrogen loss products of methanol and acetaldehyde:

$$ \mathrm{CH_2OH} + \mathrm{CH_2CHO} \rightarrow \mathrm{HOCH_2CH_2CHO}. \quad (1) $$

Prop-1-ene-1,3-diol, which is the enol tautomer of 3-hydroxypropanal, was also observed in these experiments. J. Wang et al. (2023) proposed that this species forms from keto-enol tautomerization of 3-hydroxypropanal, which has been shown to be possible in irradiated ice experiments that mimic the interaction of cosmic rays with interstellar ices under molecular cloud conditions (N. F. Kleimeier & R. I. Kaiser 2021).

Additionally, J. Wang et al. (2024) also observed that 3-hydroxypropanal was formed (along with lactaldehyde, ethyl formate, and prop-1-ene-1,3-diol) in irradiated CO-ethanol ices via Reaction (2):

$$ \mathrm{HCO} + \mathrm{CH_2CH_2OH} \rightarrow \mathrm{HOCH_2CH_2CHO}. \quad (2) $$

Moreover, 3-hydroxypropanal (HOCHCHCHO) was detected toward IRAS 16293-2422B (A. Coutens et al. 2022;

<figure><img src="image_2.png" /></figure>

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H. S. P. Müller et al. [2024]. This molecule is structurally similar to 3-hydroxypropanal, with the only difference being a single extra degree of unsaturation. The formation pathway of this molecule is also proposed to have commonalities with 3-hydroxypropanal, as they both stem from reactions with the CH₂CHO radical (A. Coutens et al. 2022). This further suggests that 3-hydroxypropanal may be generated in this source.

In order to detect this molecule in radio astronomical observations, we must first measure and analyze its rotational spectrum. For detection in the star-forming regions in which other C₃H₆O₂ isomers have been observed, an analysis of the millimeter and submillimeter wave spectrum is necessary. The warm temperatures of these regions, possibly reaching several hundred kelvin, cause the most intense rotational transitions to occur within these frequency ranges. That being said, a rotational investigation of this molecule had not been previously conducted. Therefore, in this work, we synthesized 3-hydroxypropanal and subsequently collected and fitted its millimeter and submillimeter wave rotational spectrum. This rotational data then enabled a search for the molecule in the Sgr B2(N) and NGC 6334I hot cores, the IRAS 16293-2422B hot corino, and the G+0.693-0.027 and TMC-1 molecular clouds. Section 2 outlines the quantum chemical calculations performed to support the experiments. Sections 3 and 4 then detail the experimental procedure to synthesize the molecule, along with the spectroscopic experiments and results. Finally, in Section 5, we present the observational search for 3-hydroxypropanal in the various interstellar sources and discuss some potential chemical formation pathways of this species.

## 2. Quantum Chemical Calculations

The conformational space of 3-hydroxypropanal is potentially vast due to the three single-bond rotations in its backbone. In order to determine which conformers would be present in our experimental sample, we conducted a conformational search using a mixed torsional/large-scale low-mode sampling method as implemented in MacroModel (F. Mohamadi et al. 1990). For each identified conformer, the geometries were optimized, and zero-point corrected energies were calculated using Gaussian 16 (M. J. Frisch et al. 2016) with the B3LYP/6-311++G(3df,2pd) functional and basis set. M. Kitamura et al. (2001) reported four possible forms of 3-hydroxypropanal; however, we obtained 13 conformations in the energy window of 16 kJ mol⁻¹. For the four most stable conformers, we then recalculated the relative energies with the CCSD/aug-cc-pVDZ level of theory. The relative energies and dipole moments of these four conformers are displayed in Figure 1.

Conformer 1 was predicted as the most stable form of 3-hydroxypropanal also by M. Kitamura et al. (2001). It adopts a pseudo-cyclic structure in which the hydroxyl hydrogen and carbonyl oxygen are in close proximity and thus allow for a stabilizing intramolecular hydrogen bonding interaction. This intramolecular hydrogen bonding results in Conformer 1 being 6.7 kJ mol⁻¹ (803 K) more stable than the next conformation, which has an extended structure. Therefore, it is the only conformer that is expected to have a notable population in our spectrum. Of the four displayed conformers, Conformers 1, 3, and 4 all have two equivalent configurations connected by rotations around single bonds. This effective doubling increases the statistical weight of Conformer 1, further making it the predominant species in the gas phase of our sample. To

guide its assignment, rotational constants, centrifugal distortion constants, and electric dipole moment components were calculated at three different levels of theory: B2PLYPD3/6-311++G (3df,2pd) and MP2/aug-cc-pVTZ as implemented in the Gaussian 16 software (M. J. Frisch et al. 2016), and CCSD/aug-cc-pVDZ available in the CFOUR program package (D. A. Matthews et al. 2020). The harmonic and anharmonic force field calculations were carried out in all cases. The convergence criteria for energies were set to 10⁻¹¹ atomic units. The obtained sets of spectroscopic parameters are provided in Table 1.

## 3. Experimental Description

### 3.1. Synthesis of 3-Hydroxypropanal

The synthesis of R. Roldán et al. (2018) was modified to obtain the product in the gas phase. 3,3-Diethoxy-1-propanol (1.0 g, 6.75 mmol) was slowly added to a cooled (0°C) 1 M aqueous solution of H₂SO₄ (5 ml). The reaction was stirred at 20°C for 30 minutes. This mixture was then added to a suspension of NaHCO₃ (6.3 g, 75 mmol) in dichloromethane (50 ml). At the end of bubbling, the organic phase was separated, and the residual organic compounds in the aqueous phase were extracted twice with dichloromethane (25 ml). The organic phases were mixed and dried over MgSO₄. After filtration, the solvent was removed at room temperature under vacuum until approximately 2–3 ml of solution was obtained. 3-hydroxypropanal was purified by trap-to-trap distillation under vacuum (0.1 mbar) and selective trapping in a U-tube immersed in a bath cooled at -40°C. (yield: 0.17 g, 2.3 mmol, 34%). Revaporization in the cell of the spectrometer enabled the millimeter wave spectrum to be recorded.

3-Hydroxypropanal is not very kinetically stable in pure form at room temperature in the condensed phase. The use of a crude mixture (obtained after evaporation of the solvent at 0°C under 0.1 mbar before connecting the sample to the spectrometer cell) stabilizes the product considerably, but ethanol was also observed in the spectrum. The yield of the product (based on the starting material 3,3-diethoxy-1-propanol) vaporized in the gas phase is no greater than 20%.

The following is the nuclear magnetic resonance (NMR) data for the synthesized molecule: ¹H NMR (CDCl₃, 400 MHz) δ 9.87 (t, 1H, ³JHH = 1.0 Hz, CHO), 3.96 (t, 2H, ³JHH = 5.4 Hz, CH₂O), 2.77 (2H, td, ³JHH = 5.4, 1.0 Hz, CH₂), ¹³C NMR (CDCl₃, 100 MHz) δ 202.4 (CHO), 56.5 (CH₂O), 46.1 (CH₂).

### 3.2. Spectroscopic Experiments

The rotational spectrum of 3-hydroxypropanal was measured in the frequency region 130–485 GHz using two spectrometers. The frequency-modulated absorption spectrum was first measured at room temperature over most of the frequency range spanning 150–190 GHz, 225–325 GHz, and 367–485 GHz using the Fast Lille Absorption emiSsion High resolution (FLASH) spectrometer (O. Zakharenko et al. 2015; R. A. Motiyenko et al. 2019; L. Zou et al. 2020). Its fast scan design is achieved by mixing a direct digital synthesizer signal with the signal from an Agilent synthesizer (up to 20 GHz), followed by a multiplication chain consisting of various passive and active multipliers. The FLASH spectrometer is particularly notable for incorporating the chirped-pulse technique. While this technique is well-established in the
<figure><img src="image_3.png" /><figcaption>**Figure 1.** The structures and relative zero-point-corrected energies (in kJ mol⁻¹ and kT-equivalent temperature) of the four most stable conformers of 3-hydroxypropanal. The dipole moments are listed as $\mu_a/\mu_b/\mu_c$. The point group of each conformer is also shown. See text for computational details. The red spheres denote oxygen atoms, the gray spheres are carbon atoms, and the white spheres depict hydrogen atoms.</figcaption></figure>

centimeter range, its application in the THz domain is still innovative, making the FLASH spectrometer a unique and versatile tool capable of operating up to 1.5 THz in absorption mode and 0.5 THz in chirped-pulse mode. Due to the instability of 3-hydroxypropanal, the measurements were conducted in “flow” mode. In this configuration, the spectrometer features a 220 cm long Pyrex glass tube with a 10 cm diameter and Teflon windows. The sample was continuously injected into the cell and pumped out from the opposite side.

In this specific experiment, the frequency was modulated at 40.5 kHz, and 2f frequency demodulation was employed. Once again, because of the low molecular stability, the frequency range needed to be scanned fairly quickly. Thus, a frequency step 20% larger than usual was used, ranging from 36 kHz at 150 GHz to 63 kHz at 400 GHz and above. The step size was increased at higher frequencies due to the heightened Doppler broadening of the lines. Throughout most of the frequency range, the spectra were averaged 4 times, and the scanning rate was 1 ms/point. The sample pressure and temperature during measurements were about 2–3 Pa and room temperature. Absorption signals were detected by zero-bias Schottky diodes.

The Prague semiconductor millimeter wave spectrometer (P. Kania et al. 2006) was also used to record the spectra in selected sections between 130 and 175 GHz. The required frequencies were obtained by multiplying the Agilent synthesizer-produced frequency (≤50 GHz) using a tripler or a pair of doublers. The synthesizer output was modulated at a frequency of 28 kHz. The stability of the sample signal was ensured by establishing a slow flow through the Pyrex free-space cell (8 cm in diameter, 280 cm long) at a pressure of around 4 Pa. The optical path length was doubled using a rooftop mirror. The signal was detected by zero-bias detectors

and further processed by a lock-in amplifier that was tuned to twice the modulation frequency. The final spectrum was obtained as an average of scans sweeping forward and backward in a double acquisition cycle.

The estimated frequency uncertainties over the entire measurement range were either 30 kHz, 50 kHz, or, in a small number of cases, 100 kHz. These uncertainties were primarily determined by the signal-to-noise ratio of each line. For instance, strong b-type R-branch transitions of the ground state generally had uncertainties of 30 kHz, while some very weak a-type lines were assigned uncertainties of 100 kHz.

# 4. Spectroscopic Analysis and Results

## 4.1. Ground State

The fit was initiated using a simulated spectrum that was generated with the theoretical ground state rotational constants and distortion constants using SPCAT in Pickett’s CALPGM suite of programs (H. M. Pickett 1991). Overall, the fit contains 1194 distinct transition frequencies. Including blended lines, this corresponds to 2342 transitions in the fit. The transitions range from $J' = 8-76$ and $K_a' = 0-40$. Because this molecule is a relatively prolate asymmetric top ($\kappa = -0.642$), the fit was conducted with the A reduction and $I'$ representation using ASFIT (Z. Kisiel & E. Bialkowska-Jaworska 2019). The values of each of the determined rotational and centrifugal distortion constants are listed in Table 1. Because Conformer 1 has a large electric dipole moment component along the b principal inertial axis, 1794 of the fitted transitions are b-type. As displayed in Figure 2, the strongest lines correspond to the low $K_a$ R-branch b-type transitions. Several of the high $K_a$ (e.g., $K_a' = 24-40$) Q-branch b-type transitions were also measured and fitted, albeit with much lower line intensities. The remaining 548 transitions in the fit were a-type; however, a fairly notable number of these are blended with b-type transitions (i.e., overlapping a-type and b-type R-branch transitions). The unblended a-type transitions were generally very weak due to the smaller dipole moment component along the a principal inertial axis. No c-type transitions were observed. The calculated rotational constants of Conformer 1 agree with the experimentally determined values within 3% for each of the functional/basis set combinations. The signs and magnitudes of the computed centrifugal distortion constants also agree well with the experimental values. Following the fitting using ASFIT, the determined parameters were outputted by the program in the file format required for SPFIT/SPCAT. Thus, we subsequently used these resulting files to produce a spectroscopic catalog in the standard .cat format using SPCAT to compare the fitted spectrum (including line intensities) to both our experimental and astronomical data. The spectroscopic catalog, along with both the ASFIT and SPCAT files, is provided in the data.tar.gz archive. The predicted spectra are also available in different formats, including standard .cat format (H. M. Pickett 1991) from the new database of the Lille spectroscopy group called the Lille Spectroscopic Database.<fnref n="9" /> The predictions can be generated using various options (e.g., intensity units, temperature, and frequency range), which provide additional flexibility in the data access. The partition function of 3-hydroxypropanal is listed in Table 2. The rotational partition function was calculated with SPCAT by

<footnote n="9">⁹ https://lsd.univ-lille.fr</footnote>
<table><caption>**Table 1**
Experimental Spectroscopic Parameters of Conformer 1 of 3-hydroxypropanal along with Theoretical Counterparts</caption><thead><tr><th rowspan="2">Parameter</th><th colspan="3">Experiment</th><th colspan="3">Theory<sup>a</sup></th></tr><tr><th>Ground State</th><th>v<sub>27</sub> = 1</th><th>v<sub>26</sub> = 1</th><th>I</th><th>II</th><th>III</th></tr></thead><tbody><tr><td>A (MHz)</td><td>8657.60135(34)</td><td>8693.8551(12)</td><td>8688.5405(12)</td><td>8513</td><td>8661</td><td>8606</td></tr><tr><td>B (MHz)</td><td>4116.27295(17)</td><td>4083.01(26)</td><td>4087.07(26)</td><td>3993</td><td>4054</td><td>4103</td></tr><tr><td>C (MHz)</td><td>3125.63135(11)</td><td>3112.7252(10)</td><td>3114.7592(12)</td><td>3049</td><td>3089</td><td>3118</td></tr><tr><td>Δ<sub>J</sub> (kHz)</td><td>4.010060(69)</td><td>4.4948(78)</td><td>3.6876(77)</td><td>3.663</td><td>3.675</td><td>3.834</td></tr><tr><td>Δ<sub>JK</sub> (kHz)</td><td>-9.53279(46)</td><td>-11.687(27)</td><td>-8.722(28)</td><td>-8.238</td><td>-8.280</td><td>-8.344</td></tr><tr><td>Δ<sub>K</sub> (kHz)</td><td>12.99819(71)</td><td>16.299(20)</td><td>12.487(22)</td><td>11.168</td><td>11.535</td><td>11.033</td></tr><tr><td>δ<sub>J</sub> (kHz)</td><td>1.274532(30)</td><td>1.4779(40)</td><td>1.1381(39)</td><td>1.142</td><td>1.156</td><td>1.200</td></tr><tr><td>δ<sub>K</sub> (kHz)</td><td>5.30738(43)</td><td>[5.30738]<sup>c</sup></td><td>5.7674(25)</td><td>4.851</td><td>4.966</td><td>4.937</td></tr><tr><td>Φ<sub>J</sub> (Hz)</td><td>-0.019624(13)</td><td>-0.01875(10)</td><td>-0.019507(11)</td><td>-0.013</td><td>-0.010</td><td>-0.016</td></tr><tr><td>Φ<sub>JK</sub> (Hz)</td><td>0.12908(25)</td><td>0.10550(85)</td><td>0.16475(63)</td><td>0.095</td><td>0.086</td><td>0.110</td></tr><tr><td>Φ<sub>KJ</sub> (Hz)</td><td>-0.28906(53)</td><td>-0.3020(18)</td><td>-0.3456(19)</td><td>-0.223</td><td>-0.220</td><td>-0.241</td></tr><tr><td>Φ<sub>K</sub> (Hz)</td><td>0.25706(76)</td><td>0.3476(17)</td><td>[0.25706]</td><td>0.194</td><td>0.200</td><td>0.199</td></tr><tr><td>Φ<sub>J</sub> (Hz)</td><td>-0.0091793(59)</td><td>-0.008748(50)</td><td>-0.0091793</td><td>-0.006</td><td>-0.005</td><td>-0.007</td></tr><tr><td>Φ<sub>JK</sub> (Hz)</td><td>-0.06190(11)</td><td>-0.07923(54)</td><td>-0.05572(66)</td><td>-0.037</td><td>-0.032</td><td>-0.041</td></tr><tr><td>Φ<sub>K</sub> (Hz)</td><td>-0.08147(63)</td><td>[-0.08147]</td><td>[-0.08147]</td><td>-0.037</td><td>-0.018</td><td>-0.046</td></tr><tr><td>L<sub>JK</sub> (mHz)</td><td>0.001124(38)</td><td>[0.001124]</td><td>[0.001124]</td><td>...</td><td>...</td><td>...</td></tr><tr><td>L<sub>JK</sub> (mHz)</td><td>-0.00480(11)</td><td>[-0.00480]</td><td>[-0.00480]</td><td>...</td><td>...</td><td>...</td></tr><tr><td>L<sub>KKJ</sub> (mHz)</td><td>0.00828(15)</td><td>[0.00828]</td><td>[0.00828]</td><td>...</td><td>...</td><td>...</td></tr><tr><td>L<sub>K</sub> (mHz)</td><td>-0.00664(25)</td><td>[-0.00664]</td><td>[-0.00664]</td><td>...</td><td>...</td><td>...</td></tr><tr><td>G<sub>a</sub> (MHz)</td><td>...</td><td>[1559.2]<sup>d</sup></td><td></td><td>1559.2<sup>e</sup></td><td>1544.6</td><td>1510.2</td></tr><tr><td>G<sub>b</sub> (MHz)</td><td>...</td><td>3088(54)</td><td></td><td>2433.1<sup>e</sup></td><td>2463.8</td><td>2465.3</td></tr><tr><td>G<sub>b</sub><sup>J</sup> (MHz)</td><td>...</td><td>-0.0534(23)</td><td></td><td>...</td><td>...</td><td>...</td></tr><tr><td>G<sub>b</sub><sup>K</sup> (MHz)</td><td>...</td><td>0.1065(28)</td><td></td><td>...</td><td>...</td><td>...</td></tr><tr><td>ΔE (cm<sup>-1</sup>)<sup>f</sup></td><td>...</td><td>43.8611(18)</td><td></td><td>43.5</td><td>69.3</td><td>64.1</td></tr><tr><td>|μ<sub>a</sub>| (D)</td><td>...</td><td>...</td><td>...</td><td>1.0</td><td>1.0</td><td>1.0</td></tr><tr><td>|μ<sub>b</sub>| (D)</td><td>...</td><td>...</td><td>...</td><td>2.8</td><td>2.8</td><td>2.8</td></tr><tr><td>|μ<sub>c</sub>| (D)</td><td>...</td><td>...</td><td>...</td><td>0.1</td><td>0.1</td><td>0.2</td></tr><tr><td>N<sub>lines</sub><sup>b</sup></td><td>1194</td><td>484</td><td>316</td><td>...</td><td>...</td><td>...</td></tr><tr><td>σ<sub>rms</sub> (MHz)</td><td>0.0420</td><td></td><td>0.0635</td><td>...</td><td>...</td><td>...</td></tr><tr><td>σ<sub>rms</sub>, weighted</td><td>0.84</td><td></td><td>0.97</td><td>...</td><td>...</td><td>...</td></tr></tbody></table>

**Notes.**

simulating the spectrum up to $J = 200$. The vibrational partition function was computed using the approximation formalism presented in W. Gordy & R. Cook (1970) with the anharmonic CCSD/aug-cc-pVDZ fundamental vibrational mode frequencies. The total partition function was calculated as $Q_{vib}(T) \times Q_{rot}(T)$ at each temperature.

Note that in the calculation of higher-order spectroscopic constants (e.g., centrifugal distortion and Coriolis coupling constants), the choice of quantum chemical method plays a critical role. Although B3LYP/6-311++G(3df,2pd) offers a clear advantage in terms of computational efficiency, it often falls short in accurately describing these properties due to its limited treatment of electron correlation. In a previous study on the $FSO_3$ radical (T. Uhlíková & V. Urban 2011), several

computational approaches were evaluated for the calculation of higher-order spectroscopic constants. The results demonstrated that CCSD yielded the best agreement with experimental data, further highlighting the limitations of B3LYP in this context and reinforcing the theoretical robustness of coupled-cluster methods for spectroscopic applications. Nevertheless, using the anharmonic fundamental vibrational frequencies from the other two methods in Table 1 results in vibrational partition functions that differ from CCSD by less than 2% at 300 K.

## 4.2. Vibrationally Excited States

The two lowest-energy vibrationally excited states were also clearly visible in the spectrum. These were the $v_{27} = 1$ and $v_{26} = 1$ states. The corresponding normal vibrational modes

<footnote><sup>a</sup> I: CCSD/aug-cc-pVDZ, II: B2PLYPD3/6-311++G(3df,2pd), III: MP2/aug-cc-pVTZ. Rotational and centrifugal distortion constants correspond to the ground state.</footnote>

<footnote><sup>b</sup> Number of distinct frequencies included in the fit.</footnote>

<footnote><sup>c</sup> Bracketed distortion constants were fixed to the ground state values.</footnote>

<footnote><sup>d</sup> Fixed to the value calculated at CCSD/aug-cc-pVDZ functional and basis set.</footnote>

<footnote><sup>e</sup> The $G_a$ and $G_b$ parameters from theory were determined using Equation (3) with the computed $\zeta$ and energy values and the experimental ground state rotational constants.</footnote>

<footnote><sup>f</sup> Energy difference between the $v_{27} = 1$ and $v_{26} = 1$ states.</footnote>
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Section of the room-temperature millimeter wave spectrum of 3-hydroxypropanal. Dominating lines correspond to degenerate pairs of the ground state (G. S.) a-type and b-type R-branch transitions with low $K_a$ values of Conformer 1. The leading transition is $J = 40 \leftarrow 39$, and the value of $J$ decreases by 1 with each successive line. The same pattern is observable for the $v_{27} = 1$ and $v_{26} = 1$ excited vibrational states.</figcaption></figure>

<table><caption>**Table 2**
Partition Function of Conformer 1 of 3-hydroxypropanal</caption><thead><tr><th>Temperature (K)</th><th>Q<sub>rot</sub>(T)</th><th>Q<sub>vib</sub>(T)</th><th>Q<sub>tot</sub>(T)</th></tr></thead><tbody><tr><td>300.000</td><td>83,321.061</td><td>8.99</td><td>749,274.02</td></tr><tr><td>225.000</td><td>54,070.707</td><td>4.20</td><td>227,222.08</td></tr><tr><td>150.000</td><td>29,408.229</td><td>2.06</td><td>60,609.93</td></tr><tr><td>75.000</td><td>10,391.237</td><td>1.16</td><td>12,017.88</td></tr><tr><td>37.500</td><td>3674.599</td><td>1.01</td><td>3711.28</td></tr><tr><td>18.750</td><td>1300.583</td><td>1.00</td><td>1300.67</td></tr><tr><td>9.375</td><td>460.987</td><td>1.00</td><td>460.99</td></tr></tbody></table>

are characterized by similar torsional vibrations. The $v_{27}$ mode is predominantly a torsion around the C–C bond containing the aldehyde carbon. The $v_{26}$ mode is then a torsional motion around both C–C bonds with differing proportions. The two states were calculated to be 125.0 and 168.5 cm$^{-1}$ above the ground state, respectively. This computed difference of only 43.5 cm$^{-1}$ (63K, 0.52 kJ mol$^{-1}$) resulted in some observable Coriolis coupling between the two states that impacted a fairly small number of transitions. Therefore, these states were fitted with a combined fit using the SPFIT code (H. M. Pickett 1991). The PIFORM code (Z. Kisiel 2001) was employed to format the output.

In general, the lines that displayed a notable and observable perturbation were typically the low-to-mid $K_a$ R-branch transitions. The most significant perturbations were Coriolis couplings of $\Delta K_a = 5$ beginning at $K_a' = 1$ for $v_{26} = 1$ and $K_a' = 6$ for $v_{27} = 1$. The frequency perturbations of the transition frequencies generally ranged from approximately 300 kHz to 4 MHz. An example of these perturbed transitions is displayed in the resonance plot in Figure 3. Unfortunately, however, while there were clear perturbations of $v_{26} = 1 K_a' = 5$ and 6 transitions, the perturbing counterparts $K_a' = 10, 11$, and 12 lines of $v_{27} = 1$ became too weak to observe. Thus, there were only a very small number of paired perturbing transitions that we were able to include in the fit, making it

challenging to constrain the required Coriolis coupling parameters. Additionally, since the largest observed perturbations in the transition frequencies were slightly above 4 MHz, these fairly small deviations further complicated the task of constraining the coupling parameters.

The fit was initialized with the computed energy difference between the two states, and the $G_a$ and $G_b$ parameters calculated using the following general equation:

$$G_a = \frac{\omega_{27} + \omega_{26}}{\sqrt{\omega_{27} \times \omega_{26}}} \zeta_{27,26}^a A, \quad (3)$$

where $\omega$ is the computed vibrational frequency of the two interacting modes, and $\zeta$ was computed from the anharmonic CCSD/aug-cc-pVDZ calculations. For computing $G_b$, Equation (3) is used with $\zeta_{27,26}^b$ and $B$ substituted in. The fitting procedure was carried out using a conventional bootstrap approach. Initially, the parameter $\Delta E$ was fixed to a range of values between 35 and 55 cm$^{-1}$, while the parameter $G_b$ was allowed to vary. We then searched for a global minimum of the rms deviation, following the method described by R. A. Motiyenko et al. (2018). Once the global minimum was identified, $\Delta E$ was also treated as an adjustable parameter. Given the limited number of observed perturbed transitions, the final fit was performed by fixing $G_a$ to its computed value while allowing $G_b$, $\Delta E$, $G_b^J$, and $G_b^K$ to vary. $G_b^J$ and $G_b^K$ are centrifugal distortion correction terms to the $G_b$ Coriolis coupling constant. The frequency uncertainty of the perturbed transitions was extended to 100 kHz in the fit.

Ultimately, a total of 2172 transitions were included in the excited state fit, corresponding to 800 distinct lines. The fitted rotational, distortion, and Coriolis coupling constants are displayed in Table 1. The perturbed transitions are much more accurately reproduced using the two-state fit with Coriolis coupling parameters rather than single-state fits. However, some of these lines still exhibit notable deviations from the fit, leading to a relatively large rms deviation of approximately
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Depiction of some perturbed transitions in the $v_{27} = 1$ and $v_{26} = 1$ states. The y-axis depicts the difference between the transition frequency in the excited and ground vibrational state, scaled by $J'' + 1$. All transitions are b-type R-branch lines. The scatter points depict the measured transitions. The red and blue lines connect each of the scatter points in order to more clearly display the perturbation. For the values without points, either the ground state or the excited state transition occurred at a frequency that was not measured in our experiment.</figcaption></figure>

63 kHz. The unblended a-type transitions, which were already extremely weak in the ground state, are too faint to be observed in the spectrum for the vibrationally excited states. The energy difference between the two states from the fit is exceptionally close to the difference of the computed anharmonic fundamental frequencies from the CCSD/aug-cc-pVDZ calculation. Each of the fitted centrifugal distortion constants in the excited states has the same sign and similar magnitude to the ground state values. However, as can be seen, several of the centrifugal distortion constants in the two vibrationally excited states have fairly equal and opposite differences from the ground state value. This is likely suggesting that a notable amount of the Coriolis coupling is being absorbed into the centrifugal distortion constants. If a greater signal-to-noise ratio could be achieved in future work, more perturbed transitions could be included in the fit, and the determined distortion and Coriolis coupling constants could therefore be improved. However, due to the very unstable nature of the sample, this would likely be a challenging task.

# 5. Observational Searches and Results

## 5.1. G+0.693-0.027 Molecular Cloud

We have searched for 3-hydroxypropanal toward the Galactic center molecular cloud G+0.693-0.027 (hereafter G +0.693), which is located in the Sgr B2 complex. The molecular clouds in the Galactic center exhibit high gas kinetic temperatures ranging from ~50 to ~150 K (R. Guesten et al. 1985; S. Huettemeister et al. 1993; N. J. Rodríguez-Fernández et al. 2001; A. Ginsburg et al. 2016; N. Krieger et al. 2017), low dust temperatures of ≲20–30 K (N. J. Rodríguez-Fernández et al. 2004; C. Battersby et al. 2025), and relatively low H₂

gas densities of around $10^4-10^5$ cm⁻³ (R. Guesten & C. Henkel 1983; J. Bally et al. 1987; N. J. Rodríguez-Fernández et al. 2000). The G+0.693 cloud, located around 55″ northeast of Sgr B2(N), is not an exception, and exhibits high kinetic temperatures of 70–150 K (S. Zeng et al. 2018), and densities of $10^4-5$ cm⁻³ (S. Zeng et al. 2020; L. Colzi et al. 2022, 2024).

Due to the low H₂ densities, the molecules are generally subthermally excited, and hence the excitation temperatures in most cases are significantly lower than the gas kinetic temperatures (~5–20 K; M. A. Requena-Torres et al. 2006; S. Martín et al. 2008; S. Zeng et al. 2018; I. Jiménez-Serra et al. 2020; V. M. Rivilla et al. 2022b). For this reason, the resulting spectra are less crowded than in high-Tₑx sources such as hot cores/corinos, which alleviates line confusion and helps in the identification of new interstellar molecules. Moreover, the region harbors large-scale shocks, as seen through the bright emission of SiO and HNCO, which has been attributed to a cloud-cloud collision (S. Zeng et al. 2020). These shocks have sputtered the icy mantles of dust grains, ejecting into the gas-phase molecules formed via grain surface chemistry. Furthermore, the high cosmic-ray ionization rate of the cloud, $10^{-14}-10^{-15}$ s⁻¹, inferred by the chemical modeling of molecular cations such as PO⁺ and HOCS⁺ (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 more complex molecules (V. M. Rivilla et al. 2023). From all this, G+0.693 stands as one of the most chemically rich reservoirs in the Galaxy, and nearly 25 new interstellar species have been discovered toward this source (e.g., V. M. Rivilla et al. 2020, 2021, 2023), including complex O-bearing species such as (Z)-1,2-ethenediol ((CHOH)₂; V. M. Rivilla et al. 2022c), propanol (C₃H₇OH; I. Jiménez-Serra et al. 2022), and carbonic acid (HOCOOH; M. Sanz-Novo et al. 2023). Moreover, other O-bearing molecules such as propenal and propanal are abundant toward this source (M. A. Requena-Torres et al. 2008), which makes G+0.693 a good target for the interstellar search of 3-hydroxypropanal.

We 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 several millimeter-wavelength frequency windows (83.2–115.41 GHz, 132.28–140.39 GHz, and 142.00–173.81 GHz) conducted with the 30 m IRAM radio telescope in Granada (Spain). The position switching mode was used, centered at $\alpha = 17^h47^m22^s$, $\delta = -28^\circ21'27"$, with the off position shifted by $\Delta\alpha = -885''$ and $\Delta\delta = 290''$. 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 ($T_A^*$) 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 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 note that for the frequency ranges not
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Tentative detection of 3-hydroxypropanal toward G+0.693. The selected transitions are sorted by increasing frequency and listed in Table 3. The result of the LTE model of 3-hydroxypropanal is plotted with a red line, while the blue line represents the combined emission of all molecules identified in the survey, including also 3-hydroxypropanal (observed spectra shown as gray lines and light gray-shaded histograms).</figcaption></figure>

covered by these new observations, we used data from our previous IRAM 30 m survey (see L. F. Rodríguez-Almeida et al. 2021 for additional information).

We implemented the spectroscopic entry of 3-hydroxypropanal into the analysis software MADCUBA.<fnref n="10" /> Using the Spectral Line Identification and Modeling (SLIM) tool of the MADCUBA package (S. Martín et al. 2019), we generated the expected local thermodynamic equilibrium (LTE) synthetic spectra of 3-hydroxypropanal to search for it in the G+0.693 spectral survey. We note that, given the low excitation temperatures of the molecules in G+0.693 (as described above), we did not include the vibrational contribution to the partition function ($Q_{vib}$) in this case.

In Figure 4, we show the brightest transitions of 3-hydroxypropanal, according to the LTE modeled spectra, some of which have been tentatively identified toward G +0.693. Their spectroscopic information is reported in Table 3. Besides the clear detection of several relatively bright b-type R-branch lines, only a few appear to be unblended or only slightly blended with the emission from other molecules previously detected in this cloud. We highlight the 7$_{6,2}$$-6$_{5,1}$ and 7$_{6,1}$$-6$_{5,2}$ transitions (located at

106.1483426 GHz and 106.1591837 GHz, respectively) which coalesce into a single, stronger spectral feature owing to the broad line widths of the molecular emission observed in this source (FWHM ~15–20 km s⁻¹; M. A. Requena-Torres et al. 2008; S. Zeng et al. 2018; V. M. Rivilla et al. 2022b), and the 7$_{0,7}$$-6$_{1,6}$ transition (located at 45.6874831 GHz) that is only slightly contaminated by CH$_3$CONH$_2$. Overall, the inclusion of 3-hydroxypropanal greatly enhances the agreement between the model and the observed spectra (see Figure 4). Moreover, even with a nonnegligible line blending, 3-hydroxypropanal appears to be the main carrier of most of the observed spectral features (e.g., the lines at ~81.560 GHz and ~39.066 GHz), further supporting its tentative detection. In Figures A1 and A2, we present the remaining brightest transitions of 3-hydroxypropanal covered by the survey, not shown in Figure 4, corresponding to a total of ~45 additional lines. As can be seen in both figures, the majority of these lines are severely contaminated by emission from other molecules previously identified in this survey, yet they do not show clear discrepancies between the model and the observed data.

To derive the column density, we employed the Autofit-SLIM tool (S. Martín et al. 2019) and conducted a nonlinear least-squares LTE fit of the 3-hydroxypropanal emission to the observed spectra. We used the subset of transitions reported in

<footnote n="10">¹⁰ MADCUBA on ImageJ is a software developed at the CAB in Madrid: https://cab.inta-csic.es/madcuba/; see S. Martín et al. (2019).</footnote>
<table><caption>**Table 3**
Spectroscopic Information of the Transitions of 3-hydroxypropanal Tentatively Detected toward G+0.693–0.027 (Shown in Figure 4)</caption><thead><tr><th>Frequency (GHz)</th><th>Transition <sup>a</sup></th><th>log I (nm<sup>2</sup> MHz)</th><th>g<sub>u</sub></th><th>E<sub>up</sub> (K)</th><th>Blending</th></tr></thead><tbody><tr><td>32.1896471 (13)</td><td>5<sub>0.5</sub>–4<sub>1.4</sub></td><td>–5.6326</td><td>11</td><td>5.0</td><td>Slightly blended: CH<sub>3</sub>COCH<sub>3</sub></td></tr><tr><td>32.8739195 (29)</td><td>7<sub>4.3</sub>–7<sub>3.4</sub></td><td>–5.6233</td><td>15</td><td>13.6</td><td>Slightly blended: HCCCH<sub>2</sub>CN</td></tr><tr><td>35.3495481 (13)</td><td>3<sub>2.2</sub>–2<sub>1.1</sub></td><td>–5.8750</td><td>7</td><td>3.0</td><td>U-line</td></tr><tr><td>39.0659430 (15)</td><td>6<sub>0.6</sub>–5<sub>1.5</sub></td><td>–5.3517</td><td>13</td><td>6.9</td><td>c–C<sub>6</sub>H<sub>5</sub>CN</td></tr><tr><td>40.6636428 (15)</td><td>6<sub>1.6</sub>–5<sub>0.5</sub></td><td>–5.3091</td><td>13</td><td>6.9</td><td>H<sub>2</sub>CCN</td></tr><tr><td>45.6874831 (17)</td><td>7<sub>0.7</sub>–6<sub>1.6</sub></td><td>–5.1292</td><td>15</td><td>9.1</td><td>Slightly blended: CH<sub>3</sub>CONH<sub>2</sub></td></tr><tr><td>46.5490784 (17)</td><td>7<sub>1.7</sub>–6<sub>0.6</sub></td><td>–5.1103</td><td>15</td><td>9.1</td><td>Unblended</td></tr><tr><td>78.9556730 (29)</td><td>6<sub>4.2</sub>–5<sub>3.3</sub></td><td>–4.8794</td><td>13</td><td>11.1</td><td>CH<sub>3</sub>COOH</td></tr><tr><td>81.5594297 (34)</td><td>5<sub>5.1</sub>–4<sub>4.0</sub></td><td>–4.7334</td><td>11</td><td>11.2</td><td>C<sub>2</sub>H<sub>5</sub>SH</td></tr><tr><td>81.5605329 (34)</td><td>5<sub>5.0</sub>–4<sub>4.1</sub></td><td>–4.7334</td><td>11</td><td>11.2</td><td>C<sub>2</sub>H<sub>5</sub>SH</td></tr><tr><td>88.1236563 (27)</td><td>13<sub>1.12</sub>–12<sub>2.11</sub></td><td>–4.3761</td><td>27</td><td>31.2</td><td>CH<sub>3</sub>COCH<sub>3</sub></td></tr><tr><td>96.1288496 (35)</td><td>7<sub>5.2</sub>–6<sub>4.3</sub></td><td>–4.6019</td><td>15</td><td>15.7</td><td>Unblended</td></tr><tr><td>97.0383121 (32)</td><td>9<sub>4.6</sub>–8<sub>3.5</sub></td><td>–4.7138</td><td>19</td><td>19.5</td><td>Slightly blended: C<sub>2</sub>H<sub>5</sub>NCO</td></tr><tr><td>98.8733057 (40)</td><td>6<sub>6.1</sub>–5<sub>5.0</sub></td><td>–4.4842</td><td>13</td><td>11.2</td><td>CH<sub>3</sub>OCHO</td></tr><tr><td>98.8733818 (40)</td><td>6<sub>6.0</sub>–5<sub>5.1</sub></td><td>–4.4842</td><td>13</td><td>11.2</td><td>CH<sub>3</sub>OCHO</td></tr><tr><td>102.3378081 (33)</td><td>16<sub>0.16</sub>–15<sub>1.15</sub></td><td>–4.0589</td><td>33</td><td>42.3</td><td>s–C<sub>2</sub>H<sub>5</sub>CHO</td></tr><tr><td>102.3381298 (33)</td><td>16<sub>1.16</sub>–15<sub>1.15</sub></td><td>–4.9124</td><td>33</td><td>42.3</td><td>s–C<sub>2</sub>H<sub>5</sub>CHO</td></tr><tr><td>102.3385252 (33)</td><td>16<sub>0.16</sub>–15<sub>0.15</sub></td><td>–4.9124</td><td>33</td><td>42.3</td><td>s–C<sub>2</sub>H<sub>5</sub>CHO</td></tr><tr><td>102.3388468 (33)</td><td>16<sub>1.16</sub>–15<sub>0.15</sub></td><td>–4.0589</td><td>33</td><td>42.3</td><td>s–C<sub>2</sub>H<sub>5</sub>CHO</td></tr><tr><td>106.1583426 (40)</td><td>7<sub>6.2</sub>–6<sub>5.1</sub></td><td>–4.4294</td><td>15</td><td>18.3</td><td>Unblended</td></tr><tr><td>106.1591837 (40)</td><td>7<sub>6.1</sub>–6<sub>5.2</sub></td><td>–4.4294</td><td>15</td><td>18.3</td><td>Unblended</td></tr><tr><td>113.4294989 (40)</td><td>8<sub>6.3</sub>–7<sub>5.2</sub></td><td>–4.3771</td><td>17</td><td>21.1</td><td>N<sup>34</sup>S and n–C<sub>3</sub>H<sub>7</sub>OH</td></tr><tr><td>113.4345521 (40)</td><td>8<sub>6.2</sub>–7<sub>5.3</sub></td><td>–4.3771</td><td>17</td><td>21.1</td><td>N<sup>34</sup>S and n–C<sub>3</sub>H<sub>7</sub>OH</td></tr><tr><td>133.4964353 (49)</td><td>8<sub>8.1</sub>–7<sub>7.0</sub></td><td>–4.1035</td><td>17</td><td>27.8</td><td><sup>13</sup>CH<sub>3</sub>CHO</td></tr><tr><td>133.4964356 (49)</td><td>8<sub>8.0</sub>–7<sub>7.1</sub></td><td>–4.1035</td><td>17</td><td>27.8</td><td><sup>13</sup>CH<sub>3</sub>CHO</td></tr><tr><td>138.0143053 (45)</td><td>10<sub>7.4</sub>–9<sub>6.3</sub></td><td>–4.1474</td><td>21</td><td>30.8</td><td>CH<sub>3</sub>CHO</td></tr><tr><td>138.0165330 (45)</td><td>10<sub>7.3</sub>–9<sub>6.4</sub></td><td>–4.1474</td><td>21</td><td>30.8</td><td>CH<sub>3</sub>CHO</td></tr><tr><td>140.7843737 (48)</td><td>9<sub>8.2</sub>–8<sub>7.1</sub></td><td>–4.0650</td><td>19</td><td>30.9</td><td>AgG<sup>-</sup>(CH<sub>2</sub>OH)<sub>2</sub></td></tr><tr><td>140.7843783 (48)</td><td>9<sub>8.1</sub>–8<sub>7.2</sub></td><td>–4.0650</td><td>19</td><td>30.9</td><td>AgG<sup>-</sup>(CH<sub>2</sub>OH)<sub>2</sub></td></tr><tr><td>145.2523185 (44)</td><td>11<sub>7.5</sub>–10<sub>6.4</sub></td><td>–4.1089</td><td>23</td><td>34.7</td><td>NH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>OH</td></tr><tr><td>145.2612631 (45)</td><td>11<sub>7.4</sub>–10<sub>6.5</sub></td><td>–4.1088</td><td>23</td><td>34.7</td><td>NH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>OH</td></tr></tbody></table>

**Note.**

<sup>a</sup> We used the conventional notation for asymmetric tops: *J*<sub>K<sub>a</sub>,K<sub>c</sub></sub> to label the rotational energy levels, where *J* denotes the angular momentum quantum number, and the *K*<sub>a</sub> and *K*<sub>c</sub> labels are projections of *J* along the *a* and *c* principal axes. Numbers in parentheses represent the predicted uncertainty associated with the last digits.

Table 3, taking into account the contribution to the emission from other species. We adopted the excitation temperature (*T*<sub>ex</sub>), radial velocity (*v*<sub>LSR</sub>), and line width (FWHM) values derived for *s*-propanal in M. Sanz-Novo et al. (2022): *T*<sub>ex</sub> = 12 K, *v*<sub>LSR</sub> = 69 km s<sup>-1</sup>, and FWHM = 21 km s<sup>-1</sup>, leaving the column density as the only free parameter in the fit. We obtained a *N* = (8.6 ± 1.4) × 10<sup>12</sup> cm<sup>-2</sup>, which translates into a molecular abundance with respect to H<sub>2</sub> of (6.4 ± 1.4) × 10<sup>-11</sup>, adopting a N<sub>H<sub>2</sub></sub> = 1.35 × 10<sup>23</sup> cm<sup>-2</sup> (S. Martín et al. 2008) and assuming an uncertainty of 15% of its value. The derived value not only accurately constrains the abundance of 3-hydroxypropanal in the ISM but also implies that this species is a factor of 9 ± 2 less abundant than *s*-propanal in G +0.693 (*N* = (7.4 ± 1.5 × 10<sup>13</sup> cm<sup>-2</sup>) and a factor of ~60 less abundant than acetaldehyde (5.0 ± 0.1 × 10<sup>14</sup> cm<sup>-2</sup>) (M. Sanz-Novo et al. 2022). Therefore, we observed a drop in abundance of nearly 1 order of magnitude as chemical complexity increased (i.e., with the addition of an OH group), which is in remarkable agreement with the results obtained for the related *n*-butanal toward the same molecular cloud (*s*-propanal/ct-*n*-butanal > 9; M. Sanz-Novo et al. 2022). The column densities of 3-hydroxypropanal, acetaldehyde, and propanal in each of the sources investigated in this work are displayed in Table 4.

## 5.2. Sgr B2(N) Star-forming Region

Sgr B2(N) is a high-mass star-forming protocluster located in the molecular cloud complex Sgr B2 in the central molecular zone of our Galaxy, at a distance of 8.2 kpc (M. J. Reid et al. 2019). Sgr B2(N) contains numerous hot molecular cores and H II regions (R. A. Gaume et al. 1995; M. Bonfand et al. 2017; Á. Sánchez-Monge et al. 2017). It was the target of the imaging spectral line survey Re-exploring Molecular Complexity with ALMA (ReMoCA) that was performed with the Atacama Large Millimeter/submillimeter Array (ALMA) between 84.1 and 114.4 GHz. Details about the data reduction and the method of analysis can be found in A. Belloche et al. (2019, 2025). The survey was analyzed by comparing the observed spectra to synthetic spectra computed assuming LTE with the software Weeds (S. Maret et al. 2011). For this work, we focused on the position located at (α, δ)<sub>J2000</sub> = (17h47m19s87, -28°22'19"48). This position, called Sgr B2(N1S), is about 1" to the south of the main hot core Sgr B2(N1). It was chosen by A. Belloche et al. (2019) as a compromise between reducing the dust optical depth and keeping a high enough H<sub>2</sub> column density to detect less abundant molecules.

In order to search for 3-hydroxypropanal toward Sgr B2(N1S), we assumed the same LTE parameters as
<table><caption>**Table 4**
Column Density of 3-hydroxypropanal in the Sources Investigated in This Work, along with the Source Parameters Used in This Analysis</caption><thead><tr><th>Source</th><th>3-Hydroxypropanal<sup>a</sup><br>(cm<sup>-2</sup>)</th><th>T<sub>ex</sub><sup>b</sup><br>(K)</th><th>ΔV<sup>c</sup><br>(km s<sup>-1</sup>)</th><th>v<sub>lsr</sub><br>(km s<sup>-1</sup>)</th><th>Acetaldehyde<br>(cm<sup>-2</sup>)</th><th>Ratio<sup>d</sup></th><th>Propanal<sup>e</sup><br>(cm<sup>-2</sup>)</th><th>References</th></tr></thead><tbody><tr><td>G+0.693-0.027</td><td>8.6 ± 1.4×10<sup>12</sup></td><td>12</td><td>21.0</td><td>69</td><td>5.0 × 10<sup>14</sup></td><td>58</td><td>7.4 × 10<sup>13</sup></td><td>(1)</td></tr><tr><td>Sgr B2(N)</td><td>≤7.9 × 10<sup>16</sup></td><td>250</td><td>5.0</td><td>62</td><td>6.7 × 10<sup>17</sup></td><td>≥ 8</td><td>≤1.6 × 10<sup>17</sup></td><td>(1)</td></tr><tr><td>IRAS 16293-2422B</td><td>≤1.5 × 10<sup>14</sup></td><td>125</td><td>1.0</td><td>2.6</td><td>1.2 × 10<sup>17</sup></td><td>≥ 800</td><td>2.2 × 10<sup>15</sup></td><td>(2, 3)</td></tr><tr><td>NGC 6334I</td><td>≤4.2 × 10<sup>16</sup></td><td>225</td><td>3.0</td><td>-5.2</td><td>5 × 10<sup>17</sup></td><td>≥ 12</td><td>...</td><td>This work</td></tr><tr><td>TMC-1</td><td>≤8.6 × 10<sup>11</sup></td><td>7</td><td>0.3</td><td>5.8</td><td>3.5 × 10<sup>12</sup></td><td>≥ 4</td><td>1.9 × 10<sup>11</sup></td><td>(4, 5)</td></tr></tbody></table>

**Notes.** The column densities of acetaldehyde and propanal are also noted.

<sup>a</sup> For Sgr B2(N), IRAS 16293-2422B, and NGC 6334I, the 3-hydroxypropanal column densities are corrected for vibrational and conformational contributions. These contributions are negligible under the cold conditions of G+0.693-0.027 and TMC-1.

<sup>b</sup> Excitation temperature used to determine the column density of 3-hydroxypropanal.

<sup>c</sup> Line width used in the simulation to determine the column density of 3-hydroxypropanal.

<sup>d</sup> Ratio of acetaldehyde/3-hydroxypropanal column densities.

<sup>e</sup> A conformer correction to the propanal column density was applied for the Sgr B2(N) and IRAS 16293-2422B star-forming regions. However, the value listed for the cold sources TMC-1 and G+0.693-0.027 corresponds to the most stable s-propanal conformer. **References.** For acetaldehyde and propanal column densities: (1) M. Sanz-Novo et al. (2022); (2) J. K. Jørgensen et al. (2018); (3) J. M. Lykke et al. (2017); (4) J. Cernicharo et al. (2020); (5) M. Agúndez et al. (2023a).

acetaldehyde ($T_{ex} = 250$ K, $\Delta V = 5.0$ km s$^{-1}$, see Table B1 and Table 4 for remaining parameters), keeping only the column density as a free parameter. As shown in Figure 5, 3-hydroxypropanal is not detected toward Sgr B2(N1S), and an upper limit column density of $7.9 \times 10^{16}$ cm$^{-2}$ was determined. Table B1 reports this upper limit as well as results obtained previously by M. Sanz-Novo et al. (2022) and J. Koucký et al. (2022) for acetaldehyde, lactaldehyde, 3-hydroxypropanal, propanal, and 2-hydroxyprop-2-enal. Table B1 indicates that the upper limit derived for 3-hydroxypropanal is slightly more stringent than the upper limit obtained for its structural isomer, lactaldehyde, by J. Koucký et al. (2022). We find that 3-hydroxypropanal is at least 8.5 times less abundant than acetaldehyde in Sgr B2(N1S).

### 5.3. IRAS 16293-2422B Star-forming Region

We utilized data from the ALMA Protostellar Interferometric Line Survey (PILS; J. K. Jørgensen et al. 2016) to search for 3-hydroxypropanal toward the “B” component of the Class 0 protostar IRAS 16293-2422 (IRAS 16293B). The PILS data consist of a full spectral survey of the source between 329.15 and 362.90 GHz at $0.5$ angular resolution using ALMA (for details see J. K. Jørgensen et al. 2016). We examined the spectra toward a position one beam offset from IRAS 16293B that has been the subject of several other studies as part of the PILS program: the position is characterized by bright but narrow emission lines (FWHM of approx $1$ km s$^{-1}$) that make it ideal to search for new species and provide meaningful upper limits with line blending significantly reduced. Adopting an excitation temperature of 125 K and calculating synthetic spectra for 3-hydroxypropanal, we do not see any indications of lines that can be attributed to the species down to the rms noise level of the data ($4-5$ mJy km s$^{-1}$). The $3\sigma$ upper limit to the column density for 3-hydroxypropanal derived from these fits comes out to $1.5 \times 10^{14}$ cm$^{-2}$ for the assumed temperature. All assumed source parameters, along with the determined upper limit, are displayed in Table 4. The vibrational partition function at 125 K was considered when determining this upper limit.

### 5.4. NGC 6334I Star-forming Region

NGC 6334I is a high-mass star-forming region that is host to a variety of complex organic molecules. Recently, several especially large oxygen-bearing species have been detected toward this source (B. A. McGuire et al. 2017; Z. T. P. Fried et al. 2024), thus making it a strong target to search for 3-hydroxypropanal. The observational data toward this source were collected using ALMA, with a pointing position focused on the MM1 hot core of this source (J2000 $\alpha = 17^h20^m53.387$ $\delta = -35^\circ46'57.533''$). These observations span across regions of ALMA Bands 4, 7, 9, and 10. To ensure uniformity, the data were smoothed using the standard CASA imsmooth routine to achieve a consistent angular resolution of $0.26 \times 0.26$ across all frequency bands. The observational details are described in the works of B. A. McGuire et al. (2018a), B. A. McGuire et al. (2017), and Z. T. P. Fried et al. (2024).

The molecular emission was simulated using the molsim Python package (B. A. McGuire et al. 2024). This program assumes that the molecular signal is described by a single excitation temperature. The parameters employed for the simulation toward NGC 6334I were an excitation temperature of 225 K, a $v_{lsr}$ of $-5.2$ km s$^{-1}$, and a line width of $3.0$ km s$^{-1}$ (N. F. W. Ligterink et al. 2020). The vibrational partition function and conformational correction at 225 K were taken into account when determining the upper limit column density. To establish the upper limit, we identified the 3-hydroxypropanal line in the observations with the highest signal-to-noise ratio under the given simulation parameters that was not blended with the emission of other molecular species. We then calculated the column density necessary for this peak to reach an intensity at least 3 times the rms noise level. Since the observations are line confusion limited, we visually determined the rms noise around the line used to derive the upper limit to be $13$ mJy beam$^{-1}$. Ultimately, the $3\sigma$ column density upper limit was found to be $4.2 \times 10^{16}$ cm$^{-2}$. This upper limit, along with the simulation parameters, are noted in Table 4. Figure 6 displays many of the strongest lines of 3-hydroxypropanal toward NGC 6334I simulated at the upper limit column density. It is clear that the molecular emission of 3-hydroxypropanal is not detected at this abundance.
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Selection of rotational transitions of 3-hydroxypropanal, $v=0$ covered by the ReMoCA survey. The LTE synthetic spectrum of 3-hydroxypropanal, $v=0$ 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 $3\sigma$ noise level.</figcaption></figure>

5.5. TMC-1 Molecular Cloud

TMC-1 is a cold molecular cloud that has been extensively observed via several broadband line surveys. The detected molecules in this source are most commonly unsaturated carbon-chain species (i.e., A. M. Burkhardt et al. 2018; B. A. McGuire et al. 2020; R. A. Loomis et al. 2021) or cyclic molecules (B. A. McGuire et al. 2018b, 2021; J. Cernicharo et al. 2021a; M. L. Sita et al. 2022; D. Loru et al. 2023; J. Cernicharo et al. 2024; G. Wenzel et al. 2024, 2025a, 2025b). However, more recently, several large oxygen-bearing fairly saturated species, such as propanal and ethanol, have been observed (M. Agúndez et al. 2023a).

We searched for 3-hydroxypropanal in the fifth data reduction (DR5) of the GBT Observations of TMC-1: Hunting Aromatic Molecules (GOTHAM) observations. This line survey was observed using the Robert C. Byrd Green Bank Telescope (GBT). The GOTHAM project is described by B. A. McGuire et al. (2020), while the data reduction process for DR5 is outlined by C. Xue et al. (2025). This survey has a frequency coverage spanning from 3.9 to 36.4 GHz (with some gaps). The data has an extremely high frequency resolution of 1.43 kHz and a noise level generally around 4–15 mK. The pointing position is focused on the cyanopolylyne peak of TMC-1 at (J2000) $\alpha = 04^h41^m42^s5$ $\delta = +25^\circ41'26.8''$. The molecular emission was again simulated using `molsim`, assuming an excitation temperature of 7 K, a $v_{lsr}$ of 5.8 km s$^{-1}$, and a line width of 0.3 km s$^{-1}$, parameters that are generally representative of the molecules detected in this

source (e.g., A. M. Burkhardt et al. 2018). The upper limit was determined using the same method as Section 5.4. In summary, the 3-hydroxypropanal transition with the strongest signal-to-noise ratio under the simulation parameters was determined. The column density required for this transition to have an intensity 3 times the rms noise level around this line was then derived. The upper limit column density was found to be $8.6 \times 10^{11} \text{ cm}^{-2}$. Some of the most intense lines of 3-hydroxypropanal toward TMC-1 (simulated at the upper limit column density) are displayed in Figure 7.

This, unfortunately, is not an extremely stringent column density upper limit due to the weak transitions of the molecule within the frequency range of the GOTHAM observations. In fact, propanal (which is a less complex species than 3-hydroxypropanal) was observed in the Q-band Ultrasensitive Inspection Journey to the Obscure TMC-1 Environment (QUIJOTE) survey (J. Cernicharo et al. 2021b) of TMC-1 with a column density of $(1.9 \pm 0.7) \times 10^{11} \text{ cm}^{-2}$ (M. Agúndez et al. 2023a). The frequency range of the QUIJOTE survey is 31.0–50.3 GHz. Under the assumed simulation parameters, the strongest simulated line in this range is around twice the intensity of the transition used to derive the upper limit. Moreover, the QUIJOTE survey has a noise level between 0.06 mK at 32 GHz and 0.18 mK at 49.5 GHz (M. Agúndez et al. 2025), which is more than 10 times lower than the GOTHAM survey. Thus, with additional investigation, this upper limit could likely be further refined toward TMC-1.
<figure><img src="image_8.png" /><figcaption>**Figure 6.** The strongest simulated lines of 3-hydroxypropanal in the observational data toward NGC 6334I. The simulated line profile (red) is overlaid on top of the observational data (gray). The spectrum is simulated at the $3\sigma$ upper limit column density of $4.2 \times 10^{16} \text{ cm}^{-2}$. The other simulation parameters are detailed in the text and Table 4.</figcaption></figure>

## 5.6. Discussion of Chemical Implications

As mentioned previously, one of the main formation pathways of 3-hydroxypropanal that has been proposed in the literature under astrophysically relevant conditions is the radical recombination of CH$_2$OH and CH$_2$CHO (Reaction 1). Although CH$_2$OH has not been detected in the interstellar medium, it has been shown experimentally to form from common interstellar precursors, such as hydrogenation of formaldehyde and hydrogen abstraction from methanol (A. J. Ocaña et al. 2019; J. He et al. 2022; A. Ishibashi et al. 2023). Moreover, as discussed in J. Wang et al. (2023), CH$_2$CHO could be formed via hydrogen abstraction of acetaldehyde (CH$_3$CHO). However, such production of CH$_2$CHO from acetaldehyde may be inefficient.

The C–H bond on the methyl group of acetaldehyde has a higher dissociation energy than the aldehyde hydrogen (J. Berkowitz et al. 1994). Therefore, several gas-phase experiments have shown that the CH$_3$CO radical is by far the dominant hydrogen abstraction product of acetaldehyde when reacting with OH (M. Cameron et al. 2002; N. I. Butkovskaya et al. 2004). Moreover, an investigation of interstellar ice analogs found that the predominant initial reaction of acetaldehyde and hydrogen atoms is hydrogen abstraction to form CH$_3$CO and H$_2$ (G. Molpeceres et al. 2025). Additionally, during the aforementioned experiments of J. Wang et al. (2023), the CH$_3$CO radical was detected in FTIR data of the irradiated methanol-acetaldehyde ices, whereas CH$_2$CHO was not directly observed above the detection limit.

Although it is clear that the production of CH$_3$CO is the primary product of the hydrogen abstraction of acetaldehyde, the enhanced cosmic-ray ionization rate in G+0.693 may still result in a nonnegligible formation of CH$_2$CHO. Several previous chemical modeling studies toward G+0.693 have found that a notably enhanced cosmic-ray ionization rate

<figure><img src="image_9.png" /><figcaption>**Figure 7.** Transitions of 3-hydroxypropanal with the highest signal-to-noise ratio in the GOTHAM DR5 data toward TMC-1. The simulated line profile (red) is overlaid on top of the observational data (gray). The spectrum is simulated at the $3\sigma$ upper limit column density of $8.6 \times 10^{11} \text{ cm}^{-2}$. The other simulation parameters are detailed in the text and Table 4.</figcaption></figure>

(factor of more than 100 greater than the standard Galactic disk value) is required to reproduce the observed abundances of some molecular cations in this source (V. M. Rivilla et al. 2022a; M. Sanz-Novo et al. 2024). This greater flux of cosmic rays would likely promote the production of radical species in the interstellar ices. These radicals could then recombine on the grain surface, where the surface acts to stabilize the association process and absorb the additional energy. The shocks produced by cloud–cloud collisions in G+0.693 would then cause the molecular inventory stored in interstellar ices to desorb into the gas phase, making them detectable with radio telescopes.

The column density ratio of acetaldehyde to 3-hydroxypropanal in each of the sources investigated is displayed in Figure 8. The lower limit ratio in TMC-1, Sgr B2(N), and NGC 6334I is below the tentatively derived value in G+0.693. Since these are lower limit ratios, it is challenging to draw any conclusions regarding the relative abundances in these three sources compared to G+0.693. For example, the true ratio for TMC-1, Sgr B2(N) and NGC 6334I could in theory be either above or below the tentatively determined value for G+0.693. However, this ratio is notably greater in IRAS 16293B than in G+0.693, thus indicating that there is a clear chemical differentiation in these two sources. One possible explanation is that differences in radiation environments, such as the elevated cosmic-ray ionization rate in the Galactic center, may lead to a higher rate of acetaldehyde dissociation in G+0.693, thereby enhancing the formation of CH$_2$CHO. Furthermore, the elevated cosmic-ray ionization rate could also result in a higher abundance of atomic hydrogen or carbon, potentially enhancing addition and abstraction reactions on dust grains and promoting the formation of the necessary precursor radical species to 3-hydroxypropanal (M. A. Requena-Torres et al. 2008; I. Jimenez-Serra et al. 2025). Alternatively, the destruction
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Column density ratio of acetaldehyde to 3-hydroxypropanal in each of the sources analyzed in this work. The arrows denote a column density upper limit of 3-hydroxypropanal. The values used for this figure and the related references are listed in Table 4.</figcaption></figure>

of 3-hydroxypropanal may be more efficient at the higher temperatures characteristic of the IRAS 16293B hot corino, or it may have been more extensively destroyed during ice processing in the warm-up phase of IRAS 16293B.

Another potential formation route of 3-hydroxypropanal is the insertion of singlet-excited oxygen atoms ($O(^1D)$) into the C–H bonds of the terminal $CH_3$ group of propanal ($CH_3CH_2CHO$). Singlet oxygen has been shown to form quite efficiently via the photolysis of molecules that are known to be abundant in interstellar ices. For example, experimental studies have shown that UV photolysis of $H_2O$ can produce $O(^1D)$ at around a 10% yield (T. G. Slanger & G. Black 1982; L. J. Stief et al. 1975). Moreover, $O(^1D)$ is formed at nearly 100% efficiency following the photolysis of $CO_2$ at various UV wavelengths (Y. Zhu & R. J. Gordon 1990; T. G. Slanger & G. Black 1971). Once produced, these excited oxygen atoms can efficiently insert into the bonds of hydrocarbon species (B. M. Hays & S. L. Widicus Weaver 2013; J. B. Bergner et al. 2017, 2019; A. Daniely et al. 2025). For example, it was found in astrophysical ice analogs that the insertion of $O(^1D)$ into the C–H bonds of methane and ethane can form methanol and ethanol with negligible reaction barriers (J. B. Bergner et al. 2017, 2019). Thus, it may be possible for a similar reaction to occur with propanal on interstellar ices. To the best of our knowledge, among the sources examined in this study, column densities of propanal have been previously reported in the literature for $G+0.693$ (M. Sanz-Novo et al. 2022), IRAS 16293B (J. M. Lykke et al. 2017), and TMC-1 (M. Agúndez et al. 2023a). Of note, some propanal transitions were observed in GBT observations of the Sgr B2(N-LMH) (J. M. Hollis et al. 2004). However, since the observed lines are mainly in absorption, this indicates that the observations are likely probing the colder, extended gas around the source. On the other hand, only a column density upper limit of propanal has been determined in the ReMoCA observations of Sgr B2(N) analyzed in this paper (M. Sanz-Novo et al. 2022). Additionally, as mentioned previously, the derived column density of propanal from the QUIJOTE survey of TMC-1 is below the upper limit column density of 3-hydroxypropanal that we have derived in the GOTHAM observations; thus, no useful conclusions can be drawn from this result.

The column density of propanal was found to be $2.2 \times 10^{15}$ $cm^{-2}$ toward IRAS 16293B (J. M. Lykke et al. 2017), which would suggest a propanal/3-hydroxypropanal ratio ≥14.7 in this source. On the other hand, as noted in Section 5.1, this

ratio has been tentatively determined to be around 9 in G +0.693. The high cosmic-ray flux in $G+0.693$ could result in a strong field of secondary UV photons resulting from the interaction of these cosmic rays with molecular hydrogen (S. S. Prasad & S. P. Tarafdar 1983). This UV field could therefore promote the production of singlet-excited oxygen atoms in the ices in this source, thus resulting in 3-hydroxypropanal formation.

## 6. Conclusions

In this work, we sought to detect 3-hydroxypropanal in the interstellar medium. This molecule shares chemical similarities with several previously detected interstellar species and has been predicted by multiple studies to be a promising detection target. To achieve this, we first synthesized the compound and then measured and analyzed the rotational spectrum of its most stable conformer within the 130–485 GHz range. In the process, we also fitted the rotational spectra of the two lowest lying vibrationally excited states. Due to the relatively small energy gap between these two excited states, some transition frequencies exhibited noticeable perturbations. Thus, we attempted to fit the energy difference between the states along with several Coriolis coupling parameters. Unfortunately, many of the perturbed lines were either extremely weak or below the noise level. Thus, only a small number of perturbed transitions could be incorporated into the fit, which resulted in several of the parameters only being determined with moderate precision. These parameter values could be much improved if a better signal-to-noise ratio is achieved for these transitions in future work.

After analyzing and fitting the spectrum of the ground vibrational state, we searched for this molecule in the radio astronomical observations toward the IRAS 16293-2422B hot corino, the Sgr B2(N) and NGC 6334I hot cores, as well as the $G+0.693-0.027$ and TMC-1 molecular clouds. Molecular emission of 3-hydroxypropanal was tentatively detected for the first time toward $G+0.693-0.027$, and a column density of $(8.6 \pm 1.4) \times 10^{12} \ cm^{-2}$ was derived at an excitation temperature of 12 K. This value suggests that 3-hydroxypropanal is $9 \pm 2$ times less abundant than propanal in this source.

Unfortunately, no molecular emission of 3-hydroxypropanal was observed in any of the other sources that were investigated in this work. Column density upper limits were therefore determined. We then discussed some potential formation pathways of this molecule, including radical recombination reactions and oxygen insertion processes.

Although a fairly comprehensive number of interstellar sources were analyzed in this work, the measurement of the spectrum across a broadband frequency range and into the submillimeter wave regime now makes it possible to search for 3-hydroxypropanal in various other astronomical objects. One additional potentially strong candidate would be Orion KL, where other $C_3H_6O_2$ isomers have been detected (B. Tercero et al. 2013; Y. Peng et al. 2019). Additionally, the tentative detection of 3-hydroxypropanal reported here prompts the search for other $C_3H_6O_2$ isomers toward $G+0.693$.

## Acknowledgments

We would like to sincerely thank the three anonymous referees, who each provided expert criticisms that substantially
improved the quality of this manuscript. The authors gratefully
acknowledge Dr. Isabelle Kleiner and Dr. Ha Vinh Lam
Nguyen for providing additional spectroscopic catalogs for our
analysis. This work has been funded by the Czech Science
Foundation (GACR; grant No. 24-12586S). 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. Z.T.P.F. and B.A.M.
gratefully acknowledge the support of Schmidt Family
Futures. B.A.M. gratefully acknowledges the support of the
Arnold and Mabel Beckman Foundation Beckman Young
Investigator Award. B.A.M. and C.X. gratefully acknowledge
support of the National Science Foundation grant AST-
2205126. V.M.R. acknowledges support through the grant
RYC2020-029387-I funded by MICIU/AEI/10.13039/
501100011033 and by "ESF, Investing in your future", and
from the Consejo Superior de Investigaciones Científicas
(CSIC) and the Centre of Astrobiology (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 the "European
Union NextGenerationEU/PRTR." M.S.-N. acknowledges a
Juan de la Cierva Postdoctoral Fellowship, project JDC2022-
048934-I, funded by MCIN/AEI/10.13039/501100011033
and by the European Union "NextGenerationEU/PRTR." I.J.-
S., V.M.R., and M.S.-N. acknowledge support from the grant

PID2022-136814NB-I00 by the Spanish Ministry of Science,
Innovation and Universities/State Agency of Research
MICIU/AEI/10.13039/501100011033 and by ERDF, UE. I.
J.-S. also acknowledges funding from the European Research
Council (ERC) under the European Union's Horizon Europe
research and innovation program OPENS (GA 101125858).
The National Radio Astronomy Observatory is a facility of the
National Science Foundation operated under cooperative
agreement by Associated Universities, Inc. J.-C.G. thanks the
Centre National d'Etudes Spatiales (CNES) for a grant. L.M.,
R.M., and J.-C.G. were supported by the Programme National
"Physique et Chimie du Milieu Interstellaire" (PCMI) of
CNRS/INSU with INC/INP cofunded by CEA and CNES

Statement of Efforts. All authors contributed to the writing
and editing of the manuscript.

**Appendix A**
**Complementary Figures**

In Figures A1 and A2, we report the remaining brightest
transitions of 3-hydroxypropanal that are also covered by the
survey conducted toward the molecular cloud G+0.693-0.027
but are not shown in Figure 4, spanning lines down to an
intensity comparable to the weakest transition shown in
Figure 4. The vast majority of these lines appear heavily
blended with the emission from other brighter species
previously identified in this survey or fall within a region
with lower sensitivity.
<figure><img src="image_11.png" /></figure>

**Figure A1.** Additional transitions of 3-hydroxypropanal not shown in Figure 4, which are covered by the survey toward the GC molecular cloud G+0.693-0.027. The result of the best LTE fit of 3-hydroxypropanal is plotted using a red line, while the blue line shows the emission from all the molecules identified to date in the survey, including also the target aldehyde (observed spectra shown as gray histograms and light gray-shaded area). The quantum numbers involved in each transition using the conventional notation for asymmetric tops are shown in the lower part of each panel. For the panels including multiple lines, we provide the quantum number of the lower-in-frequency transition. The transitions are sorted by decreasing intensity.
<figure><img src="image_12.png" /><figcaption>**Figure A2.** Same as in Figure A1, but for additional, weaker lines.</figcaption></figure>
## Appendix B
Complementary Tables

Table B1 summarizes key details of the search for 3-hydroxypropanal in Sgr B2(N1S), including the source parameters such as temperature, emission size, and line width, as well as the column densities determined for chemically related molecules.

<table><caption>**Table B1**
Parameters of Our Best-fit LTE Model of Acetaldehyde toward Sgr B2(N1S) and Upper Limits for Propanal, 3-hydroxypropanal, Lactaldehyde, 3-hydroxypropenal, and 2-hydroxyprop-2-enal</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>CH<sub>3</sub>CHO<sup>k*</sup></td><td>d</td><td>31</td><td>2.0</td><td>250</td><td>6.7 (17)</td><td>1.09</td><td>1.00</td><td>5.0</td><td>0.0</td><td>1</td></tr><tr><td>C<sub>2</sub>H<sub>5</sub>CHO<sup>k</sup></td><td>n</td><td>0</td><td>2.0</td><td>250</td><td>&lt;1.6 (17)</td><td>4.46</td><td>1.18</td><td>5.0</td><td>0.0</td><td>&gt; 4.3</td></tr><tr><td>HOCH<sub>2</sub>CH<sub>2</sub>CHO, v = 0</td><td>n</td><td>0</td><td>2.0</td><td>250</td><td>&lt;7.9 (16)</td><td>5.39</td><td>1.05</td><td>5.0</td><td>0.0</td><td>&gt; 8.5</td></tr><tr><td>CH<sub>3</sub>CH(OH)CHO, v = 0<sup>l</sup></td><td>n</td><td>0</td><td>2.0</td><td>250</td><td>&lt;1.7 (17)</td><td>5.68</td><td>1.01</td><td>5.0</td><td>0.0</td><td>&gt; 3.9</td></tr><tr><td>HOCHCHCHO, v = 0<sup>l</sup></td><td>n</td><td>0</td><td>2.0</td><td>250</td><td>&lt;7.0 (16)</td><td>2.00</td><td>1.00</td><td>5.0</td><td>0.0</td><td>&gt; 9.6</td></tr><tr><td>CH<sub>2</sub>C(OH)CHO, v = 0<sup>l</sup></td><td>n</td><td>0</td><td>2.0</td><td>250</td><td>&lt;7.2 (16)</td><td>2.41</td><td>1.00</td><td>5.0</td><td>0.0</td><td>&gt; 9.3</td></tr></tbody></table>

### Notes.

<sup>a</sup> d: detection, n: nondetection.

<sup>b</sup> 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.

<sup>c</sup> Emission size (FWHM).

<sup>d</sup> Rotational temperature.

<sup>e</sup> Total column density of the molecule. x (y) means $x \times 10^y$.

<sup>f</sup> 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.

<sup>g</sup> 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.

<sup>h</sup> Line width (FWHM).

<sup>i</sup> Velocity offset with respect to the assumed systemic velocity of Sgr B2(N1S), $V_{sys} = 62$ km s<sup>-1</sup>.

<sup>j</sup> Column density ratio, with $N_{ref}$ the column density of the previous reference species marked with a *.

<sup>k</sup> The parameters were derived from the ReMoCA survey by M. Sanz-Novo et al. (2022).

<sup>l</sup> The parameters were derived from the ReMoCA survey by J. Koucký et al. (2022).

## ORCID iDs

Zachary T. P. Fried https://orcid.org/0000-0001-5020-5774

Roman A. Motiyenko https://orcid.org/0000-0002-3849-1533

Miguel Sanz-Novo https://orcid.org/0000-0001-9629-0257

Lucie Kolesniková https://orcid.org/0000-0003-3567-0349

Jean-Claude Guillemin https://orcid.org/0000-0002-2929-057X

Laurent Margulès https://orcid.org/0000-0003-0629-8277

Tereza Uhlíková https://orcid.org/0000-0002-6220-2092

Arnaud Belloche https://orcid.org/0000-0003-0046-6217

Jes K. Jørgensen https://orcid.org/0000-0001-9133-8047

Martin S. Holdren https://orcid.org/0009-0008-1171-278X

Ci Xue https://orcid.org/0000-0003-2760-2119

Štěpán Urban https://orcid.org/0000-0002-4566-4930

Izaskun Jiménez-Serra https://orcid.org/0000-0003-4493-8714

Victor M. Rivilla https://orcid.org/0000-0002-2887-5859

Brett A. McGuire https://orcid.org/0000-0003-1254-4817

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