# Investigating Enols Chemistry in the Interstellar Medium: Rotational Spectroscopy and Interstellar Search of (E)-1-Propenol

Michela Nonne, Mattia Melosso, Francesca Tonolo, Luca Bizzocchi, Silvia Alessandrini, Jean-Claude Guillemin, Amavi Kpoezoun, Gnon Baba, Víctor M Rivilla, Izaskun Jiménez-Serra, et al.

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Michela Nonne, Mattia Melosso, Francesca Tonolo, Luca Bizzocchi, Silvia Alessandrini, et al. Investigating Enols Chemistry in the Interstellar Medium: Rotational Spectroscopy and Interstellar Search of (E)-1-Propenol. The Astrophysical Journal, 2026, 998 (1), pp.72. 10.3847/1538-4357/ae3729. hal-05493390

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Submitted on 4 Feb 2026

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# Investigating Enols Chemistry in the Interstellar Medium: Rotational Spectroscopy and Interstellar Search of (E)-1-Propenol

Michela Nonne<fnref n="1" />, Mattia Melosso<fnref n="2" />, Francesca Tonolo<fnref n="3" />, Luca Bizzocchi<fnref n="2" />, Silvia Alessandrini<fnref n="2" />, Jean-Claude Guillemin<fnref n="4" />, Amavi Kpoezoun<fnref n="4" />,<fnref n="5" />, Gnon Baba<fnref n="5" />, Víctor M. Rivilla<fnref n="6" />, Izaskun Jiménez-Serra<fnref n="6" />, and Cristina Puzzarini<fnref n="2" />

Received 2025 July 21; revised 2026 January 9; accepted 2026 January 9; published 2026 February 3

## Abstract

Enols represent highly reactive intermediates toward the synthesis of complex organic molecules in the interstellar medium (ISM) and serve as key tracers of nonequilibrium chemistry driven by Galactic cosmic rays. However, only few enols have been identified in the ISM, thus limiting the insights that can be derived from their observations. To expand the inventory of interstellar enols and improve our understanding of their chemistry, we provide the first spectroscopic characterization of 1-propenol, one of the simplest enols potentially observable in the ISM, which is a key step toward enabling its astronomical detection. 1-Propenol was generated through the retro Diels-Alder reaction of 3-methylnorborn-5-en-2-ol using a flash vacuum pyrolysis apparatus, and its rotational spectrum was recorded between 80 and 500 GHz using a millimeter/submillimeter-wave frequency-modulation spectrometer. High-level quantum chemical calculations supported the spectral analysis and provided accurate predictions of spectroscopic parameters. Over 2200 rotational transitions of (E)-1-propenol were successfully assigned and analyzed with the XIAM code, yielding precise rotational and centrifugal distortion constants and the methyl internal rotation barrier. Despite being essentially isoenergetic with the E form, (Z)-1-propenol could not be identified in our spectrum due to the experimental conditions. A targeted astronomical search was also carried out toward the Galactic center molecular cloud G+0.693-0.027 using ultrasensitive millimeter-wave surveys, which, however, did not reveal any hints of emission from (E)-1-propenol. Only an upper limit for its column density could be derived, which is fully consistent with ratios observed for other keto-enol pairs, indicating that more sensitive observations are required.

*Unified Astronomy Thesaurus concepts: Laboratory astrophysics (2004); Millimeter-wave spectroscopy (2252); Astrochemistry (75); Interstellar medium (847); Interstellar abundances (832)*

## 1. Introduction

Vinyl alcohol (CH₂=CHOH), (Z)-1,2-ethenediol (HOCH=CHOH), and 3-hydroxypropenal (HOCH=CHCHO) are the only three enols<fnref n="7" /> found so far in the interstellar medium (ISM). The first identification of both syn- and antivinyl alcohol was reported by B. E. Turner & A. J. Apponi (2001) toward Sagittarius B2(N). More recent observations have been obtained in the cold core of the Taurus molecular cloud (M. Agúndez et al. 2021) and in the G+0.693-0.027 molecular cloud (I. Jiménez-Serra et al. 2022), the latter being the only source where (Z)-1,2-ethenediol has been observed so far (V. M. Rivilla et al. 2022a). 3-hydroxypropenal, instead, has recently been detected toward the solar-type protostellar system IRAS 16293-2422 (A. Coutens et al. 2022; H. Müller et al. 2024).

Enols belong to the family of the so-called complex organic molecules (COMs), i.e., carbon-containing molecules comprising

six or more atoms (E. Herbst & E. F. V. Dishoeck 2009), and are regarded as crucial highly reactive intermediates in the synthesis of more complex species, including interstellar sugars. In molecular astrophysics, these species represent also key tracers of a nonequilibrium chemistry driven by Galactic cosmic rays (GCRs), which are believed to promote chemical complexity deep into the icy mantle of interstellar grains. This aspect has been investigated in detail by M. J. Abplanalp et al. (2016), who studied the impact of the energetic-electrons bombardment (mimicking the effect of secondary electrons produced by GCRs) on apolar ices composed of CH₄/CO or C₂H₆/CO. The formation of the vinyl alcohol/acetaldehyde keto-enol pair was observed in the first mixture, while the second one yielded the production of the propenol/propanal pair. In their experimental study, when considered alongside astrochemical models, M. J. Abplanalp et al. (2016) pointed out that these species are formed exclusively during the irradiation phase, with no contribution to their formation observed during the heating phase and no dependence on the kinetic temperature of the surrounding gas. The study by M. J. Abplanalp et al. (2016) also demonstrated that enols, such as vinyl alcohol and propenol, can be formed during the CO freeze-out phase and then released into the gas phase during the warm-up process.

During the last few years, a number of studies have reported on the formation of various enols on interstellar ice analogs

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<footnote n="1">¹ Scuola Superiore Meridionale, Largo San Marcellino 10, 80138 Naples, Italy</footnote>

<footnote n="2">² Dipartimento di Chimica “Giacomo Ciamician,” Università di Bologna, Via P. Gobetti 85, 40129 Bologna, Italy; mattia.melosso2@unibo.it</footnote>

<footnote n="3">³ Université de Rennes, CNRS, IPR (Institut de Physique de Rennes)—UMR 6251, 35000 Rennes, France</footnote>

<footnote n="4">⁴ Univ Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR—UMR6226, 35000 Rennes, France</footnote>

<footnote n="5">⁵ Université de Lomé, Département de Chimie, Laboratoire de Chimie Organique et des Substances Naturelles, 01 BP 1515 Lomé, Togo</footnote>

<footnote n="6">⁶ Centro de Astrobiología (CSIC-INTA), Ctra. de Ajalvir Km. 4, Torrejón de Ardoz, 28850 Madrid, Spain</footnote>

<footnote n="7">⁷ Enols are chemical species containing a hydroxyl group (OH) bonded directly to an alkene moiety (C=C).</footnote>
exposed to the effect of GCRs, including vinyl alcohol (N. F. Kleimeier & R. I. Kaiser 2021), several propenediol isomers (J. Wang et al. 2023a), acetylenediol (J. Wang et al. 2024), and ethenediol (N. F. Kleimeier et al. 2021). The case of propenol (CH3CH=CHOH), however, is of particular interest for at least two reasons: (i) the two most stable isomers of the C3H6O family to which propenol belongs, namely, acetone and propanal (A. Karton & D. Talbi 2014), have already been detected in the ISM (F. Combes et al. 1987; L. E. Snyder et al. 2002; J. M. Hollis et al. 2004; D. N. Friedel et al. 2005; M. A. Requena-Torres et al. 2008; J. Lykke et al. 2017; M. Agúndez et al. 2023); (ii) several formation routes of interstellar propenol, spanning different evolutionary stages of the star formation process, have been proposed in the literature (J. Wang et al. 2008, 2023b; D. Qasim et al. 2019; S. K. Singh et al. 2022).

In the typical conditions of dense clouds, 1-propenol (and its isomer 2-propenol, CH3C(OH)=CH2) can be formed from the grain-surface reaction between propyne (CH3CCH) and the OH radical (D. Qasim et al. 2019). In D. Qasim et al. (2019), experiments were carried out under conditions analogous to those preceding the CO freeze-out stage, that is, on an icy mantle composed mainly of water and without the aid of any energetic source. The same reaction, moreover, is capable of producing propanol, a molecule that has recently been detected in the Galactic center (A. Belloche et al. 2022; I. Jiménez-Serra et al. 2022).

In addition, 1- and 2-propenol can be formed from direct irradiation of their tautomers, i.e., propanal and acetone, respectively. S. K. Singh et al. (2022) first derived this finding by irradiating an acetaldehyde/methane mixture. The initial irradiation period resulted in the formation of acetone and propanal; then, upon extending the irradiation period, the formation of propenol and isopropenol was also observed. These results had been later confirmed by J. Wang et al. (2023b), who observed the formation of 2-propenol via direct enolization of acetone ices exposed to GCR analogs. Using a completely different approach, and with the aim of simulating the hot-core environment of a star-forming region, J. Wang et al. (2008) have shown that 1-propenol, along with a range of enols including butenol, can be generated in the discharge plasma of various alcohols, which are therefore viewed as possible enol precursors.

In this context, 1-propenol represents a good candidate for radioastronomical observations, but the required spectroscopic characterization is so far missing. To the best of our knowledge, the only high-resolution study of propenol has been reported by M. F. Shaw et al. (2017), who were able to produce both 1-propenol and 2-propenol via UV photolysis of methyl butanal and 2-pentanone, respectively, and recorded their gas-phase infrared spectra at a resolution of 0.25 cm⁻¹. While the identification of 1-propenol is undoubtedly, the spectroscopic information obtained in M. F. Shaw et al. (2017) is not sufficient to pursue an astronomical search of this species, which instead requires the accurate knowledge of its rotational spectrum.

In this work, we report (i) the first observation of the rotational spectrum of 1-propenol between 80 and 500 GHz and (ii) the astronomical search of this species toward the Galactic center molecular cloud G+0.693-0.027 (hereafter G+0.693).

## 2. Methodology

The characterization of the rotational spectrum of 1-propenol starts with the accurate quantum chemical prediction of the spectroscopic parameters of interest. These are used to simulate the rotational spectrum, which is then experimentally investigated, and to support its analysis and assignment.

### 2.1. Quantum Chemical Calculations

Since the leading terms in rotational spectroscopy are the rotational constants and they are strongly related to the molecular structure, the computational study starts from accurate structural determinations (C. Puzzarini et al. 2008). According to vibrational perturbation theory to second order (VPT2; I. M. Mills 1972), rotational constants consist of two contributions: the equilibrium one, which only depends on the equilibrium geometry and provides the largest contribution (more than 99%), and the vibrational correction (usually ~0.5%). We determined the equilibrium geometries of the most stable conformers of the two isomeric forms of 1-propenol, i.e., (E)- and (Z)-1-propenol, using the so-called CCSD(T)/CBS+CV composite scheme, where the acronym means that the coupled-cluster singles and doubles approximation augmented by a perturbative treatment of triples (CCSD(T); K. Raghavachari et al. 1989) is used in combination with the extrapolation to the complete basis set (CBS) limit and core-valence (CV) correlation effects are incorporated. In this scheme, the energy gradient to be minimized consists of three contributions: (i) the Hartree-Fock self-consistent field energy extrapolated to the CBS limit using the cc-pVTZ, cc-pVQZ, and cc-pV5Z basis sets (T. H. Dunning 1989); (ii) the CBS limit for the CCSD(T) correlation energy with the cc-pVTZ and cc-pVQZ basis sets; (iii) the CV contribution computed at the CCSD(T)/cc-pCVTZ level of theory (D. E. Woon & T. H. Dunning 1995). For more details, the reader is referred to M. Heckert et al. (2005, 2006) and C. Puzzarini et al. (2008). This level of theory provides structural parameters with an expected accuracy of 0.001 Å for bond lengths and 0.1° for angles, which implies an accuracy of about 0.1% on rotational constants (C. Puzzarini & J. F. Stanton 2023). Evaluation of vibrational corrections to rotational constants requires the application of VPT2 to a cubic force field, which has been computed using the rev-DSD-PBEP86 functional (G. Santra et al. 2019) in conjunction with the jun-cc-pVTZ basis set (T. H. Dunning 1989; E. Papajak & D. G. Truhlar 2011), and incorporating the D3 scheme (S. Grimme et al. 2010) together with the Becke-Johnson damping function (S. Grimme et al. 2011). This level of theory will be hereafter denoted as revDSD/TZ.

An accurate simulation of the rotational spectrum also requires the computation of the centrifugal distortion constants (C. Puzzarini et al. 2010). The sextic terms have been obtained at the revDSD/TZ level as a byproduct of the VPT2 calculations mentioned above. The quartic distortion constants have been derived from harmonic force field computations at the fc-CCSD(T)/cc-pVTZ level of theory (T. H. Dunning 1989; K. Raghavachari et al. 1989). From these, the equilibrium values of the dipole moment components have also been extracted, which are needed to predict line intensities. Another important point in view of correctly simulating spectral intensities is the energy difference between (E)- and (Z)-1-propenol. This has been estimated by correcting
the CCSD(T)/CBS+CV equilibrium energies for the harmo-
nic zero-point energy (ZPE) term at the fc-CCSD(T)/cc-pVTZ
level and the anharmonic revDSD/TZ ZPE contribution.

Lastly, we estimated the torsional potential V₃ due to the
internal rotation of the methyl group, defined as the first term
in the cosine expansion of the periodic hindering potential
describing the threefold barrier. This has been analyzed for
each isomer by performing a relaxed scan along the torsional
coordinate (the HCCC dihedral angle, where H is one of the
methyl hydrogens) at the revDSD/TZ level of theory. This led
to the identification of the transition state (TS) for both species.
The energetics of the torsional potential was then further
refined by means of single-point CCSD(T)/CBS+CV energy
evaluations.

All CCSD(T) calculations were performed using the CFOUR package (J. F. Stanton et al. 2016; D. A. Matthews et al. 2020), while density functional theory computations employed the Gaussian16 program suite (M. J. Frisch et al. 2019).

## 2.2. *Experimental Details*

1-Propenol has been produced as described by F. Tureček (1984), i.e., via the retro Diels–Alder reaction of 3-methylnorborn-5-en-2-ol promoted by flash vacuum pyr- olysis (FVP) at 650°C, performed with the same apparatus previously used in our laboratory to synthesize other enols (M. Melosso et al. 2019, 2022). The chemical precursor has been preheated at 40°C using a heating tape, and its vapors were conveyed in a quartz tube surrounded by a 30 cm long tubular oven. The FVP products were flown directly inside the spectrometer cell, a 3.25 m long Pyrex glass tube, closed at the ends by two high-density poly-ethylene windows. The pressure inside the cell was maintained around 1 Pa by pumping the FVP products out continuously.

The rotational spectrum of 1-propenol has been recorded in four frequency ranges, namely, 80–115, 170–230, 240–345, and 330–500 GHz, using a millimeter/submillimeter-wave frequency-modulation spectrometer (J. A. Claus et al. 2025). The millimeter-wave radiation source employed is a W-band signal generator extension module (Mini SGX, Virginia Diodes Inc., WR 10SGX-M) driven by a frequency synthesizer that emits in the interval between 9 kHz and 20 GHz and is referred to a rubidium atomic clock (which ensures a frequency accuracy of 0.001 ppb). The Mini SGX system can be coupled with active and passive frequency multipliers in cascade to obtain higher frequencies. In order to improve the signal-to-noise ratio (SNR) of the spectra, the emitted radiation is sine-wave modulated at *f* = 16 kHz, with a modulation depth chosen according to the Doppler width of the frequency region under consideration (i.e., ranging between 150 and 600 kHz). The radiation source is collimated into the cell using a parabolic mirror and focused via another parabolic mirror onto the detector. Several zero-bias Schottky-barrier diodes have been employed to detect the output radiation in the various spectral regions covered in this experiment. The detected signal is then filtered, amplified, and demodulated at twice the modulation frequency (2*f* detection scheme) by a low-noise preamplifier and a lock-in amplifier and finally digitized through a digital-to-analog converter board.

All spectra have been recorded as second derivative of the
actual absorption profiles due to the 2f detection scheme.

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Torsional potential energy barriers (in cm⁻¹) of *E*- (red curve) and Z-1-propenol (blue curve) as obtained from CCSD(T)/CBS+CV energies on top of revDSD/junTZ geometries. The green bar highlights the computed energy difference between the two isomers (18 cm⁻¹).</figcaption></figure>

Measurement uncertainties are estimated to range between 20
and 50 kHz depending on the line width and the SNR.

## 3. Analysis and Results

3.1. *Molecular Properties*

(*E*)- and (*Z*)-1-propenol are both asymmetric rotors, with the *E* isomer being much closer to the prolate limit (κ ~ −0.986) than the *Z* form (κ ~ −0.818). The internal rotation motion of the methyl group leads to a threefold potential shown in Figure 1 together with the structures of the minima and TSs along the torsional angle (HCCC) for both isomers. The energy barrier for (*E*)-1-propenol (V₃ = 743 cm⁻¹) is nearly 1 order of magnitude higher than that of the *Z* isomer (V₃ = 81 cm⁻¹). Based on the height of these barriers, one expects to observe the torsional splitting of the rotational transitions in one nondegenerate *A* component and one doubly degenerate *E* component.

As shown in Figure 1, both *E* and *Z* isomers belong to the *C*<sub>s</sub> symmetry point-group, with most of the atoms lying on the *ab*-plane of their principal axes system. Thus, they have only two nonzero dipole moment components: those along the *a*- and *b*-axes. Their magnitude is μ<sub>a</sub> = 0.74 D and μ<sub>b</sub> = 0.95 D for (*E*)-1-propenol and μ<sub>a</sub> = −1.30 D and μ<sub>b</sub> = −0.44 D for (*Z*)-1-propenol.

The computed energy difference between the *E* and *Z* isomers amounts to 18 cm⁻¹. Since this value lies below the intrinsic accuracy of the CCSD(T)/CBS+CV level of theory, the calculations do not provide a statistically significant indication of the most stable species. Consequently, from a thermodynamic point of view, the two isomers should be considered as essentially isoenergetic. However, in our experiment, the *E* form is expected to be formed more efficiently than the *Z* one because of the stereochemistry of the synthesized 3-methylnorborn-5-en-2-ol (see the Appendix for a description of its synthesis).

## 3.2. *Spectral Analysis*

The retro Diels–Alder reaction of 3-methylnorborn-5-en-2-ol leads to the formation of several products, including
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Comparison between the experimental millimeter-wave spectrum (black trace) in the 91090–91190 MHz range and the simulated one (blue trace) based on computed spectroscopic parameters. Dotted gray lines provide the correspondence between experimental and predicted transitions. The black numbers on the experimental trace denote the $K_a$ values of each line. The resolved asymmetry splitting for $K_a = 3$ transitions is shown, and the corresponding $A/E$ splitting is highlighted by the blue letters.</figcaption></figure>

cyclopentadiene, 1-propenol, its tautomer propanal, and other unidentified chemical species. This results in a high density of lines in the observed spectra. The spectroscopic parameters estimated using the computational methodology described in the previous section have been employed to simulate the rotational spectra of the target species by incorporating them in the XIAM code (H. Hartwig & H. Dreizler 1996), which exploits the internal axes method (R. Woods 1966; J. Vacherand et al. 1986). The availability of extensive literature data on cyclopentadiene (M. Bogey et al. 1988; L. Bonah et al. 2025) and propanal (J. A. Hardy et al. 1982; J. Demaison et al. 1987; O. Zingsheim et al. 2017)—readily usable thanks to the Cologne Database for Molecular Spectroscopy (H. S. Müller et al. 2005)—made it easy to recognize their presence in the recorded spectra. After having excluded the lines belonging to the known species, the high accuracy of our theoretical calculations allowed us to identify the rotational transitions relative to the most abundant isomer, that is, (E)-1-propenol.

In the initial phase, the most recognizable features based on the simulated spectrum were the R-branch ($\Delta J = +1$), a-type transitions in the frequency range around 91 GHz. The observed deviation between predicted and experimental line positions was about 35 MHz (see Figure 2 for a graphical explanation). Their identification has been facilitated by the fact that the splitting due to the methyl internal rotation is unresolved for most of the a-type transitions, with the only exception being those lines for which the asymmetry splitting is barely resolved. This is consistent with the almost coincidence between the principal a-axis and the one of the rotating methyl group, the angle between them being in the order of 20°. The assignment of these lines led to a first refinement of the estimated spectroscopic parameters, thus facilitating the assignment of some b-type transitions (belonging to both Q and R branches) involving rotational levels with $K_a = 0$ and 1. In most cases, the b-type spectrum exhibits splittings due to methyl internal rotation of a few MHz, which are thus resolved under our experimental conditions. To give an example, the splitting is ~7 MHz for the lowest J and $K_a$

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Loomis-Wood plots of the b-type, Q-branch transitions with $K_a = 5 \leftarrow 4$ of (E)-1-propenol. Each panel shows the predicted position of the A (centered, green sticks) and E (on the left, red sticks) components over a 50 MHz wide frequency window. The J values span from 32 (bottommost panel) to 43 (topmost panel). The experimental spectrum is shown in black.</figcaption></figure>

values and increases with the increasing of J and $K_a$ up to about 20 MHz (see Figure 3 for an example).

Once the set of spectroscopic parameters had been refined thanks to the analysis of 160 lines observed in the 80–115 GHz range, the measurements were extended to higher frequencies, and the assignment procedure was carried out using the Loomis-Wood Plots program by L. Bonah et al. (2022), a graphical tool particularly useful when assigning extensive sequences of lines.

(Z)-1-propenol, if present, could not be identified in our spectra. This can be rationalized based on several arguments. First, the stereochemistry of the chosen precursor inherently favors the formation of the *E* isomer, but the actual abundance ratio between the two species cannot be reliably assessed. FVP is known to be highly sensitive to a variety of experimental conditions, each of which affects the yield of the products significantly. As a result, the *Z* isomer might be generated only in trace amounts in our setup, or it may undergo rapid isomerization toward more stable species. Despite numerous attempts, no reliable assignment of its spectrum could be achieved. This failure can be attributed to several factors: (i) the huge line density of the recorded spectra, which, being acquired at room temperature, contain a large number of transitions from vibrationally excited states; (ii) the nonselective nature of the pyrolysis process, which leads to the concomitant production of many unknown chemical species, thereby exacerbating spectral congestion; and (iii) the low $V_3$ value calculated for (Z)-1-propenol together with its $\kappa$ asymmetry parameter quite different from –1 produce a sparse spectrum whose patterns are difficult to identify in the aforementioned conditions.

Overall, 2213 rotational transitions belonging to (E)-1-propenol have been assigned between 80 and 500 GHz. More specifically, 1214 distinct frequencies have been analyzed, with J and $K_a$ values up to 60 and 20, respectively, with an overall rms error of 66 kHz. This value is slightly higher than our estimate of the measurements uncertainty, but it is likely due to the limitation of the XIAM code in treating such high J and $K_a$ values. Alternative programs specifically designed for the treatment of internal rotation have been tested to assess
<table><caption>**Table 1**
Experimental Spectroscopic Constants of (E)-1-propenol Compared with Computed Values</caption><thead><tr><th>Constant</th><th>Unit</th><th>Experimental<sup>a</sup></th><th>Theoretical<sup>b</sup></th></tr></thead><tbody><tr><td>A</td><td>/ MHz</td><td>40398.919(3)</td><td>40428.365</td></tr><tr><td>B</td><td>/ MHz</td><td>3929.0129(4)</td><td>3930.301</td></tr><tr><td>C</td><td>/ MHz</td><td>3662.0952(4)</td><td>3664.101</td></tr><tr><td>D<sub>J</sub></td><td>/ kHz</td><td>0.70758(6)</td><td>0.703</td></tr><tr><td>D<sub>JK</sub></td><td>/ kHz</td><td>-3.877(2)</td><td>-4.483</td></tr><tr><td>D<sub>K</sub></td><td>/ kHz</td><td>173.19(4)</td><td>176.617</td></tr><tr><td>d<sub>1</sub></td><td>/ Hz</td><td>-64.711(6)</td><td>-65.118</td></tr><tr><td>d<sub>2</sub></td><td>/ Hz</td><td>4.605(2)</td><td>3.821</td></tr><tr><td>H<sub>JK</sub></td><td>/ mHz</td><td>-4.(1)</td><td>-3.748</td></tr><tr><td>H<sub>KJ</sub></td><td>/ mHz</td><td>-465.(3)</td><td>-420.0</td></tr><tr><td>V<sub>3</sub></td><td>/ cm<sup>-1</sup></td><td>741.3(5)</td><td>743.0</td></tr><tr><td>D<sub>pi2J</sub></td><td>/ kHz</td><td>21.(10)</td><td>...</td></tr><tr><td>D<sub>pi2K</sub></td><td>/ MHz</td><td>-3.6(5)</td><td>...</td></tr><tr><td>D<sub>pi2-</sub></td><td>/ MHz</td><td>-0.16(1)</td><td>...</td></tr><tr><td>F<sub>0</sub></td><td>/ GHz</td><td>161.19(9)</td><td>161.85</td></tr><tr><td>δ</td><td>/ rad</td><td>0.333(1)</td><td>0.359</td></tr><tr><td>ρ<sup>c</sup></td><td>...</td><td>0.237</td><td>0.243</td></tr><tr><td>No. of lines</td><td></td><td>1214</td><td></td></tr><tr><td>rms error</td><td>/ kHz</td><td>66</td><td></td></tr></tbody></table>

**Notes.**

<sup>a</sup> Numbers in parentheses represent one unit of the standard deviation in unit of the last digit.

<sup>b</sup> Equilibrium rotational constants at the CCSD(T)/CBS+CV level augmented by revDSD/TZ vibrational corrections; quartic centrifugal distortion constants at the CCSD(T)/cc-pVTZ level; sextic centrifugal distortion terms at the revDSD/TZ level; potential barrier height at the CCSD(T)/CBS+CV level on top of revDSD/TZ geometries.

<sup>c</sup> ρ represents the coupling parameter between the overall rotation of the molecule and the internal rotation of the methyl group.

possible improvements, however, without success. In any case, the accuracy achieved here with XIAM is fully adequate for the purpose of astronomical observations, as it meets the precision required to support the identification of new molecules in spectral line surveys.

Our analysis allowed the accurate determination, for the first time, of the entire set of rotational and quartic centrifugal distortion constants, the $H_{JK}$ and $H_{KJ}$ sextic terms, the $V_3$ potential energy barrier, and some other parameters associated to the internal motion, such as the angle between the internal rotation axis and the principal a-axis ($\delta$), the reciprocal of the inertia moment along the internal rotation axis ($F_0$), and the empirical internal rotation–overall rotation centrifugal distortion constants as defined by N. Hansen et al. (1999). These parameters are reported in Table 1, where the experimental values are compared with their theoretical counterpart. The computed spectroscopic constants of (Z)-1-propenol are instead given in Table 2.

The results presented in Table 1 denote a remarkable agreement between the computed and experimental set of rotational constants, the average deviation being around 0.05%. Although the quartic and sextic centrifugal distortion constants experimentally determined present higher discrepancies with respect to the theoretical values (~5% and ~7%, respectively), they are in line with what expected based on the literature on this topic, and their agreement is still quantitative. The excellent accuracy of the computed potential energy

<table><caption>**Table 2**
Computed Spectroscopic Constants of (Z)-1-propenol</caption><thead><tr><th>Constant</th><th>Unit</th><th>Computed<sup>a</sup></th></tr></thead><tbody><tr><td>A</td><td>/ MHz</td><td>17760.686</td></tr><tr><td>B</td><td>/ MHz</td><td>5576.880</td></tr><tr><td>C</td><td>/ MHz</td><td>4359.229</td></tr><tr><td>D<sub>J</sub></td><td>/ kHz</td><td>4.856</td></tr><tr><td>D<sub>JK</sub></td><td>/ kHz</td><td>24.001</td></tr><tr><td>D<sub>K</sub></td><td>/ kHz</td><td>23.327</td></tr><tr><td>d<sub>1</sub></td><td>/ kHz</td><td>-1.090</td></tr><tr><td>d<sub>2</sub></td><td>/ kHz</td><td>0.568</td></tr><tr><td>V<sub>3</sub></td><td>/ cm<sup>-1</sup></td><td>81.0</td></tr><tr><td>F<sub>0</sub></td><td>/ GHz</td><td>163.01</td></tr><tr><td>δ</td><td>/ rad</td><td>0.894</td></tr><tr><td>ρ<sup>b</sup></td><td>...</td><td>0.075</td></tr></tbody></table>

**Notes.**

<sup>a</sup> Equilibrium rotational constants at the CCSD(T)/CBS+CV level augmented by revDSD/TZ vibrational corrections; quartic centrifugal distortion constants at the CCSD(T)/cc-pVTZ level; sextic centrifugal distortion terms at the revDSD/TZ level; potential barrier height at the CCSD(T)/CBS+CV level on top of revDSD/TZ geometries.

<sup>b</sup> ρ represents the coupling parameter between the overall rotation of the molecule and the internal rotation of the methyl group.

barrier $V_3$ is noteworthy, with the relative deviation from the experimental value being as small as 0.2%.

The spectroscopic parameters derived in this work have been used to generate astronomical predictions for (E)-1-propenol. Based on a dataset extending up to $J = 60$ and $K_a = 20$, these predictions allow targeted searches over a broad frequency range up to Atacama Large Millimeter/submillimeter Array (ALMA) Band 8 (~500 GHz), with estimated uncertainties of a few kHz to few tens of kHz. Extrapolations to higher frequencies (such as ALMA Bands 9 and 10) are not recommended as some higher-order constants have uncertainties above 10%, and the large-amplitude internal motion may produce frequency shifts of several MHz outside the investigated range (M. Melosso et al. 2020).

## 4. Interstellar Search toward the G+0.693-0.027 Molecular Cloud

We have searched (E)-1-propenol toward the G+0.693 molecular cloud. This is the only interstellar source where two enols, vinyl alcohol (I. Jiménez-Serra et al. 2022) and (Z)-1,2-ethenediol (V. M. Rivilla et al. 2022a), have been identified so far. Moreover, other complex O-bearing COMs have been detected such as carbonic acid (HOCOOH; M. Sanz-Novo et al. 2023), propenol and propanal (M. A. Requena-Torres et al. 2008), and propanol ($C_3H_7OH$; I. Jiménez-Serra et al. 2022). Therefore, G+0.693 stands as a suited candidate for the detection of new O-bearing COMs in general and enols in particular.

G+0.693 is located in the Sgr B2 complex, around 55" northwest of the Sgr B2(N) hot core. There are not 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), although it has recently been proposed by L. Colzi et al. (2022, 2024) that a dense condensation in the region might be in a prestellar cluster stage on the verge of forming a new protostellar cluster. Similarly to other molecular clouds in the Galactic center
(R. Guesten et al. 1985; S. Huettemeister et al. 1993; N. J. Rodríguez-Fernández et al. 2001, 2004; A. Ginsburg et al. 2016; N. Krieger et al. 2017), G+0.693 exhibits high kinetic temperatures of 70–150 K (S. Zeng et al. 2018) and low dust temperatures of ~20 K (C. Battersby et al. 2025). The relative low H₂ densities of $10^{4-5} \text{cm}^{-3}$ (S. Zeng et al. 2020; L. Colzi et al. 2022, 2024) are responsible for the subthermal excitation of the molecules, and hence the derived excitation temperatures ($T_{\text{ex}}$) 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. 2022c). This produces a spectrum that appears less crowded than those of high-$T_{\text{ex}}$ sources such hot cores/corinos, thus alleviating line confusion and helping the identification of new interstellar molecules. Moreover, the detection and morphology of emission of SiO and Class I methanol masers indicate that the region harbors large-scale shocks, whose origin is likely a cloud–cloud collision (S. Zeng et al. 2020). These shocks can sputter the dust grains, ejecting into the gas-phase molecules formed on the ice mantles. Furthermore, the high cosmic-ray ionization rate derived in G+0.693, $10^{-14}-10^{-15} \text{ s}^{-1}$ (V. M. Rivilla et al. 2022b; M. Sanz-Novo et al. 2024a), has been proposed to trigger the formation of reactive radicals on the icy mantles, which can rapidly combine to synthesize more complex molecules via surface chemistry (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 more than 20 new interstellar species have been discovered toward this source (e.g., V. M. Rivilla et al. 2019, 2020, 2021, 2023; L. Bizzocchi et al. 2020; S. Zeng et al. 2021; I. Jiménez-Serra et al. 2022; M. Rey-Montejo et al. 2024; M. Sanz-Novo et al. 2024b, 2025) in the last few years.

We used an ultrasensitive spectral survey toward this cloud that includes Q-band observations (31.075–50.424 GHz), conducted with the Yebes 40 m radio telescope (Guadalajara, Spain), and observations across several millimeter-wavelength frequency windows (71.8–116.7 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 $\alpha = 17^h47^m22^s$, $\delta = -28^\circ21'27''$, 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 radiotelescope 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 implemented the spectroscopic entry of (E)-1-propenol generated in this work into the analysis software MADCUBA.<fnref n="8" /> Using the Spectral Line Identification and Modelling tool of the MADCUBA package (S. Martín et al. 2019), we generated the expected LTE synthetic spectra of (E)-1-propenol to search

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Transition of (E)-1-propenol 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 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>

for it in the G+0.693 spectral survey. No clear transitions of this species were identified. We have thus computed the upper limit on the column density (N) of (E)-1-propenol using the dedicated MADCUBA tool, which calculates the value of N of the LTE synthetic spectrum that corresponds to an integrated intensity of $3\sigma$, where $\sigma$ is the rms noise of the spectrum (further details can be found in Section 3.2 of S. Martín et al. 2019). We used the brightest transition according to the LTE model that appear not significantly blended with emission from other molecules and fall in a region of the spectra with low noise, so that the derived upper limit is more relevant. We used the $10_{1,9}-10_{0,10}$ transition of the A state at 44.45888 GHz, shown in Figure 4, which might be only slightly blended with a weak nearby transition of acetic acid, whose predicted intensity (<1 mK) is based on the fit of brighter lines of this species. To evaluate the upper limit on the column density of (E)-1-propenol, we assumed the same excitation temperature, velocity, and line width derived for s-propanal (V. M. Rivilla et al. 2020), which is the tautomer form of 1-propenol: $T_{\text{ex}} = 12 \text{ K}$, $v_{\text{LSR}} = 69 \text{ km s}^{-1}$, and FWHM = 21 km s$^{-1}$. We obtained $N < 2.2 \times 10^{13} \text{ cm}^{-2}$. This implies a fractional abundance with respect to molecular hydrogen of $<1.6 \times 10^{-10}$ using $N(\text{H}_2) = 1.35 \times 10^{23} \text{ cm}^{-2}$ (S. Martín et al. 2008).

## 5. Discussion and Conclusions

Motivated by the large number of studies dedicated to enols formation in the ISM, we have performed a laboratory spectroscopic characterization of 1-propenol, one of the simplest and most likely observable interstellar enols. Following the same approach previously employed for (Z)-1,2-ethenediol and its deuterated isotopologues (M. Melosso et al. 2022; M. Nonne et al. 2024), 1-propenol has been produced in the laboratory through the retro Diels–Alder reaction of a norbornene derivative promoted by FVP. This procedure has demonstrated

<footnote n="8">⁸ Madrid Data Cube Analysis on ImageJ is a software developed at the Centre of Astrobiology in Madrid: https://cab.inta-csic.es/madcuba/; see S. Martín et al. (2019).</footnote>
to be very efficient for the in situ generation of di-substituted ethenes that are sufficiently stable in the gas phase at a low pressure but not commercially available.

The rotational spectra recorded with our frequency-modulation absorption spectrometer revealed the presence of the *E* isomer of 1-propenol. Despite being essentially isoenergetic with (*Z*)-1-propenol, the *E* form was favored by the stereochemistry of the 3-methylnorborn-5-en-2-ol used as precursor. The assignment and analysis of more than 1200 distinct rotational lines allowed for the first determination of a comprehensive set of spectroscopic parameters of (*E*)-1-propenol, and the corresponding line catalog was then used for the search in the G+0.693 spectral line survey.

While (*E*)-1-propenol has not been detected and only an upper limit of its column density could be derived, a comparison with similar or chemically related species is still possible. Since other enols have been observed toward G+0.693, such as vinyl alcohol (I. Jiménez-Serra et al. 2022) and (*Z*)-1,2-ethenediol (V. M. Rivilla et al. 2022a), it is worth comparing their relative abundances with those of the associated aldehydes, namely, acetaldehyde (CH₃CHO, M. Sanz-Novo et al. 2022) and glycolaldehyde (CH₂OHCHO; V. M. Rivilla et al. 2022a), respectively. Interestingly, the derived aldehyde/enol ratios are remarkably similar, ~4 for acetaldehyde/vinyl alcohol and ~5 for glycolaldehyde/(*Z*)-1,2-ethenediol. Assuming that this ratio also applies to the propanal/1-propenol pair and using the derived column density reported for the former (*s* conformer, *s*-CH₃CH₂CHO) by V. M. Rivilla et al. (2020), the expected abundance of 1-propenol should be in the range *N* = (1.5–1.9) × 10¹³ cm⁻². If we further assume that both the *E* and *Z* isomers of 1-propenol are at the thermodynamic equilibrium, as observed for other stereoisomers (e.g., those of ethyl formate or C-cyanomethanimine; V. M. Rivilla et al. 2017, 2019), and we consider a gas kinetic temperature for G +0.693 of 100 K, the expected *Z/E* ratio would be 1.3. Therefore, the expected abundance of (*E*)-1-propenol is (6.4–8) × 10¹² cm⁻². This value is fully consistent with the upper limit derived from our data (*N* < 2.2 × 10¹³ cm⁻²), thus indicating that the current sensitivity of the observations is still not sufficient to detect this species.

While these considerations seem to suggest the *Z* isomer as a valuable alternative for detecting 1-propenol in the ISM, the experimental study by M. J. Abplanalp et al. (2016) pointed out that the relative abundance ratio between the *E* and *Z* forms (and their abundance with respect to propanal) is governed by nonequilibrium chemistry rather than thermal processes. According to the branching ratios (for propanal versus 1-propenol) derived in their work, which are 3.20 ± 0.80 for (*E*)-1-propenol and 4.23 ± 0.96 for (*Z*)-1-propenol, the expected abundance of the former is (1.9–3.1) × 10¹³ cm⁻², while the one of the latter is (1.4–2.3) × 10¹³ cm⁻². The expected abundance of (*E*)-1-propenol is again in excellent agreement with the upper limit of *N* < 2.2 × 10¹³ cm⁻² derived for this species from our observational data and points to (*E*)-1-propenol as the most abundant isomer. It is also worth noting that the branching ratio for the acetaldehyde/vinyl alcohol pair derived by M. J. Abplanalp et al. (2016), i.e., 2.68 ± 1.02, is very similar to the value of ~4 observed toward G+0.693. Altogether, these findings indicate that we are currently limited by the sensitivity of the observations and not by laboratory data.

Future observations of propenol will be crucial in order to improve our understanding of enols chemistry in the ISM and will shed some light on the capability of GCRs of driving nonequilibrium chemical processes.

## Acknowledgments

This work has been supported by MUR (PRIN grant Nos. 202082CE3T, P2022ZFNBL, and 20225228K5) and by the University of Bologna (RFO funds). M.M. thanks the European Union—Next Generation EU under the Italian National Recovery and Resilience Plan (PNRR M4C2, Investment 1.4—Call for tender n. 3138 dated 16/12/2021—CN00000013 National Centre for HPC, Big Data and Quantum Computing (HPC)—CUP J33C22001170001). We acknowledge funding from Scuola Normale Superiore (SNS) for supporting this research activity in the framework of the joint SNS-INSTM-University of Bologna “Astrochemistry—Materials under extreme conditions: clues from the gas phase” project. J.C.G. thanks the Centre National d'Etudes Spatiales (CNES) for a grant. A.K. thanks the Togolese government, the French government, the French embassy in Togo, and the Ecole Nationale Supérieure de Chimie de Rennes for research grants. V.M.R. acknowledges 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; 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 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.” The COST Action CA21101 “COSY—Confined molecular systems: from a new generation of materials to the stars” is also acknowledged.

*Facilities:* Yebes:40m, IRAM:30m.

*Software:* MADCUBA (S. Martín et al. 2019), LLWP (L. Bonah et al. 2022), XIAM (H. Hartwig & H. Dreizler 1996).

## Author Contributions

Conceptualization: M.M., V.M.R., J.C.G.; Data curation: M. N., M.M., F.T., S.A., L.B., V.M.R.; Formal analysis: M.N., M. M., F.T., S.A., L.B., V.M.R.; Funding acquisition: J.C.G., V. M.R., I.J.S., C.P.; Investigation: M.N., F.T., S.A., J.C.G., A. K., G.B., V.M.R.; Methodology: M.M., J.C.G.; Project administration: M.M., J.C.G., V.M.R., C.P.; Resources: J.C. G., V.M.R., C.P.; Software: M.N., M.M., F.T., L.B., S.A., V. M.R.; Supervision: M.M., L.B., J.C.G., V.M.R., C.P.; Validation: M.M., S.A., V.M.R., C.P.; Visualization: M.N., S.A., V. M.R., J.C.G.; Writing—original draft: M.N., M.M., F.T., S.A., V.M.R., C.P.; Writing—review & editing: all authors.

# Appendix

## Synthesis of 3-methylnorborn-5-en-2-ol

1-Propenol was synthesized from bicyclo [2.2.1] hept-5-en-2-one (E. Plettner et al. 2005; Figure 5): its methylation gave 3-methylbicyclo [2.2.1] hept-5-en-2-one (exo) (T. Rajamannar & K. K. Salasubramanian 1988), which was reduced to 3-methylbicyclo [2.2.1]hept-5-en-2-ol (2-endo,3-exo and
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Synthesis of (E)- and (Z)-1-propen-1-ol.</figcaption></figure>

2-exo,3-exo) (C. C. Lee et al. 1971). 1-Propenol is finally obtained by flash vacuum thermolysis at 650°C via a retro Diels–Alder reaction (F. Tureček 1984).

## ORCID iDs

Michela Nonne https://orcid.org/0009-0008-7595-7495

Mattia Melosso https://orcid.org/0000-0002-6492-5921

Francesca Tonolo https://orcid.org/0000-0002-9555-7834

Luca Bizzocchi https://orcid.org/0000-0002-9953-8593

Silvia Alessandrini https://orcid.org/0000-0003-3152-3261

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

Amavi Kpoezoun https://orcid.org/0009-0002-1283-2835

Gnon Baba https://orcid.org/0000-0003-0168-0362

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

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

Cristina Puzzarini https://orcid.org/0000-0002-2395-8532

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