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A&A
Volume 700, August 2025
Article Number A281
Number of page(s) 8
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/202555552
Published online 26 August 2025

© The Authors 2025

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1 Introduction

One of the most surprising discoveries made in recent years in astrochemistry has been the discovery of abundant polycyclic aromatic hydrocarbons (PAHs) in cold dark clouds. Molecules with one ring or several fused rings have been unambiguously identified in the well-known dark cloud Taurus Molecular Cloud 1 (TMC-1). These comprise derivatives of cyclopentadiene, benzene, indene, naphthalene, acenaphthylene, pyrene, and coronene (Cernicharo et al. 2021a,b, 2024a; McGuire et al. 2018, 2021; McCarthy et al. 2021; Lee et al. 2021; Burkhardt et al. 2021a; Wenzel et al. 2024, 2025). Moreover, some of these cycles have also been detected in other dark clouds similar to TMC-1 (Burkhardt et al. 2021b; Agúndez et al. 2023a).

It is nowadays a matter of debate whether these PAHs are synthesized in situ in the cold dark clouds where they are observed through a bottom-up mechanism (Cernicharo et al. 2021b; Byrne et al. 2024) or whether they result from the fragmentation of larger PAHs inherited from a previous evolutionary stage in a top-down scenario (Pety et al. 2005; Zhen et al. 2014; Burkhardt et al. 2021b; Goicoechea et al. 2025). Indeed, large PAHs with estimated sizes of 20–100 carbon atoms are inferred to be present in evolutionary stages earlier than that of cold dark clouds, such as in diffuse clouds, through the observation of so-called aromatic infrared bands (Tielens 2008; Sandstrom et al. 2023). The observation of emission bands at 4.65 and 4.35 μm in some photodissociation regions (PDRs) bright in PAH emission indicates that interstellar PAHs are strongly enriched in deuterium (Peeters et al. 2004, 2024; Onaka et al. 2014; Doney et al. 2016; Boersma et al. 2023; Pereira-Santaella et al. 2024; Yang & Li 2025; Draine et al. 2025). This fact offers a way to test the top-down scenario for the origin of PAHs in cold dark clouds. If the PAHs identified in dark environments arise from larger PAHs inherited from a previous UV-illuminated phase, we would expect them to show this enrichment in deuterium.

It is, however, difficult to put constraints on the abundance of deuterated PAHs in cold clouds because the cyano derivatives of indene, naphthalene, acenaphthylene, pyrene, and coronene are observed through relatively weak lines in TMC-1, and thus the lines from the deuterated forms are most likely below the noise level of currently available spectra. Benzonitrile (C6H5CN), however, is observed through relatively intense lines in TMC-1 (Cernicharo et al. 2021b) and, although it is not formally a PAH, it can be seen as a member of the family of aromatic rings. The search for deuterated benzonitrile in cold dark clouds is therefore feasible. We have characterized the rotational spectra of the three mono-deuterated isomers of benzonitrile, for which deuterium is substituted at the ortho, meta, and para positions, in the laboratory and searched for them in TMC-1 using the QUIJOTE (Q-band Ultrasensitive Inspection Journey to the Obscure TMC-1 Environment) line survey carried out with the Yebes 40 m telescope (Cernicharo et al. 2021c). In Sect. 2 we present the laboratory measurements of the rotational spectra of ortho, meta, and para monodeuterated benzonitrile, in Sect. 3 we describe the astronomical search for deuterated benzonitrile in TMC-1, in Sect. 4 we discuss the implications for the origin of PAHs in cold dark clouds, and in Sect. 5 we summarize our conclusions.

thumbnail Fig. 1

Insets of the recorded spectrum in the discharge of deuterated benzene and acetonitrile. The three species of deuterated benzonitrile (ortho, meta, and para) can be seen. The experimental spectra are shown in black above the x axis, while the simulated spectra for each isomer at 1 K (derived from the rotational parameters) are shown in colors below the x axis.

2 Laboratory rotational spectrum of deuterated benzonitrile

2.1 Microwave measurements

The rotational spectra of the three isomers of deuterated benzonitrile were measured in several steps across the 2–18 GHz frequency range using an electrical discharge source (Ohshima & Endo 1992; McCarthy et al. 2000), which allowed us to generate in situ the three isomers of deuterated benzonitrile. A sample of ≥97% deuterated benzene was purchased from Sigma Aldrich and used without further purification. This was used to make a 0.5% gas sample diluted in neon, and it was then passed over an external reservoir containing acetonitrile (CH3CN). The mixture was passed through a modified Parker series 9 valve, followed by the electrical discharge nozzle, and a subsequent supersonic expansion into the vacuum chamber with a pressure of 3 bar. The experimental conditions for the discharge were 1 kV and 100 mA. This resulted in the production of the ortho, meta, and para isomers of deuterated benzonitrile. For the measurements in the 2–8 GHz range, the microwave setup has been described previously (Morán et al. 2025), and the operation allows the entire range to be acquired in each acquisition. A chirped-pulse is generated by an arbitrary waveform generator, then amplified and broadcast across the chamber with a broadband horn antenna, operating in the 2–18 GHz range. The chirped-pulse interacts with the molecular signal, after which a free induction decay is produced. The free induction decay is collected with a similar horn antenna as the broadcasting antenna, amplified, and digitized on a Tektronix DPO 70804C oscilloscope. In the 8–18 GHz region, due to power limitations, targeted measurements based on the predicted transitions of the three isomers were performed. The setup for this frequency range operated in much the same way as that in the 2–8 GHz range, with some key differences. The oscilloscope was changed to a Tektronix DPO 72004, which allowed us to use a sampling rate of 50 GS/s with a maximum bandwidth of 20 GHz. The chirped-pulse amplification was carried out by a solid state amplifier (Microsemi AML218P4013) and the low-noise amplifier was substituted with a 2–18 GHz low-noise amplifier (Miteq LNAS-55-01001800-22-10P). Each measurement was 1 GHz in length, 100 000 acquisitions were acquired, and a 10 μs free induction decay was collected. In both setups, the fast frame feature of the Tektronix oscilloscopes was used to increase the overall effective repetition rate. Insets of the generated spectra can be seen in Fig. 1.

thumbnail Fig. 2

Portion of the spectra of ortho- (top), meta- (middle), and para- (bottom) deuterated benzonitrile (DBCN) recorded in the 75–110 GHz range. The experimental spectra are shown in black above the x axis, while the simulated spectra at 300 K (calculated from the rotational parameters) are shown in colors below the x axis.

2.2 Millimeter-wave measurements

Measurements of the rotational spectra of the three isomers of deuterated benzonitrile were also carried out at millimeter wavelengths (75–110 GHz), using in this case samples synthesized in the laboratory. The synthesis was achieved by palladium-catalyzed bromo-deuterium exchange (Zhang et al. 2015), starting from the commercially available ortho-, meta- and parabromobenzonitrile, respectively, and using sodium formate-D (DCOONa) as a deuterium anion source. The three deuterium-labelled benzonitriles were isolated in reasonable yields (56– 70%), as is shown by gas chromatography mass spectrometry (GC-MS). The synthesis of deuterated benzonitrile is described in detail in Appendix A.

We used a W-band instrument purchased from BrightSpec. Inc. to record the millimeter wave spectra. This instrument is equipped with a 66 cm long stainless steel cell that has a diameter of 6.5 cm and is closed on either end with teflon lenses (Zaleski et al. 2017; Arenas et al. 2017). It can be used in a static or flow cell configuration. Also, this instrument is able to operate in either fast mode, in which larger segments can be measured and stitched together (decreasing the measurement time), or in high dynamic range mode, in which smaller bandwidth segments are used to acquire the entire frequency range (reducing the spurious signal content due to the upconversion of the original chirped-pulse). Both modes are an implementation of segmented chirped-pulse spectroscopy (Neill et al. 2013). In this experiment, we used the static cell configuration and the high dynamic range mode to acquire the three spectra. Each of the synthesized samples was transferred from the original vial into a Schlenk flask. However, due to the low quantity, a few drops of commercially available benzonitrile were added to the samples to facilitate this process. The chamber was closed off from vacuum and approximately 2.5 mTorr of vapor was added to the cell. Due to outgassing from the walls of the chamber, the pressure rose during the course of the measurement to somewhere between 10-15 mTorr for each isomer. An inset of the recorded spectra at room temperature can be found in Fig. 2.

2.3 Analysis of the rotational spectra

The assignment of the rotational spectra of the three species was straightforward as the rotational transition frequencies with low Ka values were well predicted, within a few megahertz (in the millimeter wave region), by using the rotational constants reported by Bak et al. (1962) and Casado et al. (1971). As was stated before, we measured rotational transitions for the three isomers in the microwave and millimeter wave regions. Some of the transitions in the microwave region show nuclear quadrupole coupling hyperfine splittings produced by 14N, which has a nuclear spin of I = 1 (see Fig. 1). Even though deuterium also has a nuclear spin of I = 1, its nuclear quadrupole coupling effects are much smaller than the ones induced by the 14N nucleus. For the millimeter wave region, all the transitions were observed as single lines since these nuclear quadrupole coupling hyperfine splittings are smaller than the experimental broadening of the lines (see Fig. 2). A total of 657, 628, and 592 rotational transitions were observed for the ortho, meta, and para species of deuterated benzonitrile, respectively. A list with all the measured frequencies is provided (see Data Availability section).

For each isomer, all the observed transitions were analyzed in a combined fit using the SPFIT program (Pickett 1991) with the A-reduction of the Watson’s Hamiltonian in Ir representation (Watson 1977). The analysis rendered the experimental molecular constants listed in Table 1. As can be seen, all the quartic centrifugal constants could be accurately determined, except for ∆K, which was kept fixed to the value determined for benzonitrile by Zdanovskaia et al. (2018). In the same manner, we fixed the sextic centrifugal constants ϕJ, ϕJK, ϕKJ, ϕK, LJ, and LK and octic centrifugal constants LJ, LJJK, LKKJ, and LK to the values determined for benzonitrile by Zdanovskaia et al. (2018) for all three isomers. Thanks to the observation of rotational transitions in the microwave range, we could derive the values of the nuclear quadrupole coupling constants χaa and χbb, except in the case of the meta species where only χaa could be determined. In this case, χbb was fixed to the value of benzonitrile (Zdanovskaia et al. 2018).

Using the molecular constants from Table 1 we obtained accurate predictions of the rotational transition frequencies for the three isomers in the Q band, with uncertainties of less than 10 kHz for a-type R-branch transitions with Ka values <10. This represents an increase by a factor ~50 in the accuracy of the predictions in the Q band. For the intensity predictions, it was assumed that the three deuterated forms have the same dipole moment along the a axis that was measured for the parent species, 4.5152 ± 0.0068 D (Wohlfart et al. 2008). The rotational partition functions employed for each isomer are shown in Table 2. They were calculated using the SPCAT program (Pickett 1991) at a maximum value of J = 100 and not including the hyperfine constants.

Table 1

Spectroscopic parameters of the three monodeuterated isomers of benzonitrile.

3 Astronomical observations: Search for deuterated benzonitrile in TMC-1

Astronomical data from the QUIJOTE survey carried out with the Yebes 40 m telescope (Cernicharo et al. 2021c) were used to search for monodeuterated benzonitrile in TMC-1. Briefly, QUIJOTE consists of a Q-band line survey (31.0–50.3 GHz) of the cold dark cloud TMC-1 at the position where cyanopolyyne emission peaks (αJ2000 = 4h41m41.9s and ΔJ2000 = +25%41′27.0′′). The observations are carried out using the frequency-switching technique, with a frequency throw of either 8 or 10 MHz. The whole Q band is covered in one shot with a spectral resolution of 38.15 kHz in horizontal and vertical polarizations using a 7 mm dual linear polarization receiver connected to a set of 2 × 8 fast Fourier transform spectrometers (Tercero et al. 2021). The intensity scale at the Yebes 40 m telescope is the antenna temperature, T*A, which has an estimated uncertainty due to calibration of 10%. The antenna temperature can be converted to main beam brightness temperature, Tmb, by dividing T*A by Beff/Feff. The beam efficiency, Beff, is given by the Ruze formula Beff = 0.797 exp [-(𝑣/71.1)2], where 𝑣 is the frequency in GHz, and the forward efficiency, Feff, is 0.97, as measured at the Yebes 40 m telescope. The half power beam width (HPBW) can be approximated as HPBW(′′) = 1763/𝑣, where 𝑣 is the frequency in gigahertz. Observations were carried out from November 2019 to July 2024 with a total on-source telescope time of 1509.2 h, of which 736.6 h correspond to a frequency throw of 8 MHz and 772.6 h to a throw of 10 MHz. The TA root mean square (rms) noise level varies between 0.06 mK at 32 GHz and 0.18 mK at 49.5 GHz. The data were analyzed using the GILDAS software1 following the procedure described in Cernicharo et al. (2022).

The three deuterated isotopologs of benzonitrile have only a-type rotational transitions. At a rotational temperature of 9.0 K, which is the gas kinetic temperature (Agúndez et al. 2023b) and the rotational temperature of the parent species of benzonitrile in TMC-1 (Cernicharo et al. 2021b), the most favorable lines in the Q band correspond to rotational transitions with Ka = 0 and 1. The Ka = 0 and 1 lines of the three isomers of deuterated benzonitrile lying in the Q band have a modest hyperfine splitting (<20 kHz). Lines with Ka ≥ 2 have larger hyperfine splittings (30–50 kHz), similar to the values observed for the non-deuterated species in TMC-1 (Cernicharo et al. 2021b). We searched for the Ka = 0 and 1 lines of deuterated benzonitrile in our QUIJOTE data, but there is no clear evidence of them. In Fig. 3 we show the spectra at the frequencies of three selected transitions for each of the three deuterated isotopologs, ortho, meta, and para, of benzonitrile. In order to derive upper limits on the column densities of the three deuterated species, we adopted a rotational temperature of 9.0 K, derived from the analysis of 100 individual lines of the parent species of benzonitrile in TMC-1 by Cernicharo et al. (2021b), and assumed that the emission of these species would be distributed as a circle with a radius of 40 ′′, which is approximately the size measured for the parent species of benzonitrile in TMC-1 (Cernicharo et al. 2023). We derived a 3σ upper limit on the column density of ortho, meta, and para deuterated benzonitrile of 3.0 × 1010 cm−2. Adopting a column density of H2 of 1022 cm−2 (Cernicharo & Guélin 1987), the fractional abundance of each of the three deuterated isomers relative to H2 is < 3 × 10−12. The calculated line profiles adopting the 3σ upper limit derived as the column density and a full width at half maximum of 0.60 km s−1 (Agúndez et al. 2023b) are shown in Fig. 3. The column density derived for the parent species of benzonitrile in TMC-1 is 1.2 × 1012 cm−2 (Cernicharo et al. 2021b). Therefore, the abundance ratio of deuterated to non-deuterated benzonitrile is <2.5% for the ortho, meta, and para forms.

Table 2

Rotational partition function of the three monodeuterated isomers of benzonitrile at different temperatures.

thumbnail Fig. 3

Spectra of TMC-1 in the Q band at the frequencies of some of the most favorable lines of ortho, meta, and para deuterated benzonitrile. Negative artifacts produced by the frequency-switching technique have been blanked. The noise level, measured in a window of ±8 MHz around the expected position of each line with the nominal spectral resolution of 38.15 kHz, is indicated by a horizontal gray band. The red lines correspond to the line intensities calculated adopting the 3σ upper limits on the column densities derived here and a full width at half maximum of 0.60 km s−1.

4 Discussion

The D/H ratio of a given molecule is defined here as the total number of D atoms contained in the different deuterated versions of the molecule divided by the total number of H atoms present in the non-deuterated and deuterated forms of that molecule. This definition applies to small individual molecules observed at radio frequencies, such as benzonitrile, and to large unspecific PAHs observed in the infrared, in which case the D/H ratio reflects the total number of C-D bonds relative to the total number of C-H bonds. To derive the D/H ratio of benzonitrile it is necessary to take into account the statistics of the different deuterated isotopologs, where the ortho and meta forms have two equivalent positions for the deuterium atom while the para isomer has only one possible position for D. We assumed that the ortho, meta, and para isomers have statistical abundance ratios, 2:2:1, respectively. In this case, the upper limits on the column densities of the ortho, meta, and para isomers would be 3.0 × 1010 cm−2, 3.0 × 1010 cm−2, and 1.5 × 1010 cm−2, respectively. Adopting the column density of the parent species from Cernicharo et al. (2021b), we end up with a D/H ratio of <1.2% for benzonitrile in TMC-1. The upper limit is about three orders of magnitude higher than the elemental D/H ratio in the solar neighborhood, in the range from 1.5 × 10−5 to 2.3 × 10−5 (Linsky et al. 2006).

Several singly deuterated molecules have been detected in TMC-1 (see Cabezas et al. 2021a,b, 2022; Tercero et al. 2024; Cernicharo et al. 2024b). These comprise the aliphatic hydrocarbons c-C3H2, CH3CCH, C4H, H2C4, and CH3C4H and the N-bearing carbon chain molecules CH2CN, CH3 CN, HC3N, HNC3, HCCNC, CH3C3N, and HC5N. The D/H ratios derived for these molecules are in the range of 0.06–3.3% (see Table 4 of Cabezas et al. 2022, Table A.5 of Tercero et al. 2024, and Table 2 of Cernicharo et al. 2024b). The presence of these deuterated species is explained in terms of isotopic fractionation reactions that favor the incorporation of deuterium into molecules and that become efficient at the very low temperatures of cold dark clouds (Roberts et al. 2004; Roueff et al. 2005). The D/H ratio derived for benzonitrile, < 1.2%, is fully consistent with the range of D/H ratios derived for other monodeuterated molecules in TMC-1.

We were also interested in comparing the D/H ratio derived for benzonitrle in TMC-1 with the ones derived for large unspecific PAHs in UV-illuminated interstellar media. In these regions, the vibrational modes of PAHs are excited by the absorption of UV photons. This results in the emission of several strong IR bands between 3.3 and 19 μm corresponding to the different C-H and C-C bending and stretching modes (Draine & Li 2007). The D/H ratio in these PAHs was measured using the intensity ratio between the 4.65 μm band, associated with the C-D stretch in aliphatic sites, and the 3.4 μm one, corresponding to the C-H stretch counterpart, together with the intensity ratio between the 4.35 and 3.3 μm bands, which correspond to the C-D and C-H stretches, respectively, at aromatic sites. Using observations with the Infrared Space Observatory (ISO), Peeters et al. (2004) reported high D/H ratios (distributed over the aromatic and aliphatic carbon atoms) of 17% ± 3% and 36% ± 8% in the PDRs Orion Bar and M 17, respectively. However, using data from AKARI, Onaka et al. (2014) derived smaller D/H ratios of 2–3% in these two PDRs, and Doney et al. (2016) found evidence of deuterated PAHs in the spectra of only 6 of the 53 HII regions investigated, with variable D/H ratios in the range of 3–44%.

More accurate D/H ratios of PAHs have recently been obtained using James Webb Space Telescope (JWST) observations. In general, it has been found that the D/H ratio for aliphatic side groups (Dali/Hali) is significantly larger than the one for aromatic ones (Daro/Haro). The aliphatic bands are thought to be mostly produced by methyl (-CH3) and hydrogenated (-H) groups and their deuterated equivalents (e.g., Schutte et al. 1993; Bernstein et al. 1996; Buragohain et al. 2020; Pla et al. 2020; Yang & Li 2023). In the Orion Bar, Peeters et al. (2024) found Dali/Hali ratios between 4.1 and 22.5%, while in the PDR M 17, Boersma et al. (2023) measured 31 ± 12.7%. In the spiral galaxy M51, Draine et al. (2025) derived Dali/Hali of 17 ± 2%, while in the disk of the local starburst galaxy NGC 3256-S we derived a Dali/Hali ratio of ~12% following the method described in Yang & Li (2023) and using the band intensity ratio I4.65/I3.4 = 0.10 ± 0.01 observed in the JWST spectrum presented in Pereira-Santaella et al. (2024). In contrast, the deuterated aromatic band at 4.35 μm is undetected in M 17 (Daro/Haro < 0.5%; Boersma et al. 2023), in NGC 3256-S (Daro/Haro < 4% from the observed 3σ upper limit, I4.4/I3.4 < 0.03), and in the M 51 galaxy (Daro/Haro < 1.6%; Draine et al. 2025). For the Orion bar, Yang & Li (2025) report an upper limit of Daro/Haro <14%, since the 4.35 μm band might be contaminated by C-N stretches.

The high Dali/Hali ratios given above can be misleading because in astronomical PAHs, H atoms at aliphatic C sites are less abundant than at aromatic C sites. Therefore, even if the Dali/Hali ratios are relatively high, the total D/H ratio (including both aliphatic and aromatic) is substantially lower: between 3% and <17% in the Orion Bar, 2-3% in M 17 (Yang & Li 2025), 3-5% in the spiral galaxy M51, and between 1% and <5% in the starburst galaxy NGC 3256-S. The D/H ratio of <1.2% determined here for C6H5CN in TMC-1 lies just at the low edge of the range of total D/H ratios (including aliphatic and aromatic) derived in the four aforementioned UV-irradiated regions (between 1% and <17%). It probably lies below that range taking into account that it is a 3σ upper limit, which would make us conclude that the level of deuteration of PAHs in TMC-1, probed by benzonitrile, is different from that in UV-illuminated clouds. However, the D atom in deuterated benzonitrile is bonded to an aromatic C, and thus we should compare with the Daro/Haro ratios derived in the Orion Bar, M 17, M 51, and NGC 3256-S, which are all upper limits (<14, <0.5, <1.6, and <4%, respectively), since aromatic deuterium is not detected in any of these sources. In this regard, the level of deuteration of benzonitrile in TMC-1 would be consistent with the Daro/Haro ratios derived in UV-illuminated regions.

There are, however, further aspects to take into account. The PAHs detected in dark clouds have a distinctive feature compared to the ones in UV-illuminated regions. In TMC-1, the H atoms of PAHs are rarely in aliphatic sites, while in UV-irradiated regions H atoms in aliphatic sites represent a sizable fraction of the total number of H atoms, around 20% (Chiar et al. 2013; Yang et al. 2017; Hensley & Draine 2020; Draine et al. 2025). We can estimate the fraction of aliphatic H atoms in the PAHs detected to date in TMC-1. Since for many of them only the CN derivative is detected, we estimate the abundance of the pure hydrocarbon scaling up by a factor of 27, which is the abundance ratio determined for cyclopentadiene (Cernicharo et al. 2021b, 2022). The estimated column densities, in units of 1013 cm−2, are thus 1.2 for cyclopentadiene (C5H6; Cernicharo et al. 2021a), 3.2 for benzene (C6H6; Cernicharo et al. 2021b), 1.6 for indene (C9H8; Cernicharo et al. 2021a), 3.9 for naphthalene (C10H8; McGuire et al. 2021), 5.1 for acenaphthylene (C12H8; Cernicharo et al. 2024a), 9.8 for pyrene (C16H10; Wenzel et al. 2024), and 7.3 for coronene (C24H12; Wenzel et al. 2025). The column density of C-H bonds is thus 3.0 × 1015 cm−2. In the above molecules, most H atoms are at aromatic sites; only two H atoms in cyclopentadiene and two in indene are at aliphatic C sites of the hydrogenated type. The column density of H atoms in hydrogenated aliphatic C sites is therefore 5.6 × 1013 cm−2, which implies a fraction of hydrogenated aliphatic H atoms of 1.9%. Methyl groups are also thought to be an important contribution to the aliphatic fraction of H atoms in large PAHs. However, no methylated PAH has been detected in TMC-1. The simplest such molecule would be toluene (C6H5CH3), for which we estimate a 3σ upper limit on its column density of 4 × 1012 cm−2. The column density of H atoms at methyl aliphatic C sites would be <1.2 × 1013 cm−2, which implies a fraction of methyl aliphatic H atoms of <0.4%. Therefore, the fraction of aliphatic H atoms (hydrogenated plus methyl) in TMC-1 is estimated to be <2.3%, which is substantially smaller than the values inferred for large PAHs in UV-irradiated regions. Moreover, given the low fraction of aliphatic H in TMC-1, aliphatic D probably contributes little compared to aromatic D, and we can thus view the aromatic D/H ratio of <1.2% derived from benzonitrile as a proxy of the total D/H ratio.

In summary, the low deuteration level seen for benzonitrile in TMC-1 is consistent with the range of values found for other monodeuterated molecules in this same cloud, although it is on the low edge of (and very likely below) the range of D/H values derived for large PAHs in the PDRs Orion Bar and M 17 and in the galaxies M 51 and NGC 3256-S. That is, the population of PAHs in dark clouds does not seem to share the deuterium enrichment seen in PDRs. If we assume that such deuterium enhancement is also present in the large PAHs observed in diffuse clouds, the conclusion would be that PAHs in dark clouds are not inherited from the previous diffuse cloud stage. However, it is not fully clear whether this deuterium enrichment holds for PAHs in all kinds of interstellar regions. Peeters et al. (2024) argue that the D enhancement may occur in situ at the border of PDRs due to UV radiation and/or density, although it is still unknown what mechanism is behind deuterium enrichment of PAHs and whether it is a local effect that occurs in certain PDR-like regions or whether it is a characteristic of PAHs in different kinds of UV-irradiated regions, diffuse clouds included.

5 Conclusions

We have characterized in the laboratory the rotational spectra of the three isomers (ortho, meta, and para) of monodeuterated benzonitrile in the 2–18 GHz and 75–110 GHz frequency ranges. The measured frequencies have been used to predict accurate transition frequencies in the Q band and search for these three isomers in the cold dark cloud TMC-1 using data from the QUIJOTE line survey. We did not detect any of the three species, and we have derived a 3σ upper limit on the column

density of each of the three deuterated isomers of benzonitrile of 3.0 × 1010 cm−2, which implies a D/H ratio of <1.2%. This value is in line with the range of D/H ratios observed for smaller molecules in TMC-1 from radioastronomical data (0.06–3.3%) and below the range of D/H values inferred for large PAHs from infrared observations of UV-illuminated regions (between 1 and <17%). Although drawing a clear link between deuteration of PAHs in dark clouds and UV-irradiated regions is not straightforward, our results suggest that PAHs in cold dark clouds are not formed from the fragmentation of large PAHs inherited from a previous diffuse phase in a top-down scenario.

Data availability

The measured and analyzed rotational transitions and the frequency predictions for each of the three isomers of monodeuterated benzonitrile are available at zenodo.

Acknowledgements

We acknowledge funding support from Spanish Ministerio de Ciencia, Innovación, y Universidades through grants PID2021-125015NB-I00 (C.P. and A.L.), PID2022-139933NB-I00 (D.P. and J.J.), PID2023-146667NB-I00 (M.P.S. and J.R.G), and PID2023-147545NB-I00 (M.A., C.C., and J.C.). A.L.S. and M.P.S. acknowledge funding support from grants RYC2022-037922-I (A.L.S.) and RYC2021-033094-I and CNS2023-145506 (M.P.S.), funded by MCIN/AEI/10.13039/501100011033, the FSE+, and the European Union NextGenerationEU/PRTR. C.P. thanks the European Research Council for the CoG HydroChiral (Grant Agreement No 101124939). A.L.S., C.P., and A.L. also thank funding from Junta de Castilla y León, Grant INFRARED IR3032-UVA13. D.P. and J.J. thank Xunta de Galicia and the European Regional Development Fund (ERDF) for grant ED431G 2023/03 (Centro de Investigación do Sistema Universitario de Galicia accreditation 2023-2027). I.G.B. is supported by the Programa Atracción de Talento Investigador César Nombela via grant 2023-T1/TEC-29030 funded by the Community of Madrid. This work is based in part on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST; and from the European JWST archive (eJWST) operated by the ESAC Science Data Centre (ESDC) of the European Space Agency. These observations are associated with program #1328. We thank the anonymous referee for a constructive report that helped to improve this manuscript.

Appendix A Synthesis of ortho, meta, and para deuterated benzonitrile

A.1 General methods

All reactions were carried out under argon using oven-dried glassware. Anhydrous dimethyl sulfoxide (DMSO) was taken from a MBraun SPS-800 Solvent Purification System. 2-bromobenzonitrile, 3-bromobenzonitrile 4-bromobenzonitrile were purchased from BLD Pharmatech, and sodium formate from Biosynth, and were used without further purification. Thin-layer chromatography (TLC) was performed on Merck silica gel 60 F254 and chromatograms were visualized with UV light (254 and 360 nm). Column chromatography was performed on Merck silica gel 60 (ASTM 230-400 mesh). 1H and 13C nuclear magnetic resonance (NMR) spectra were recorded at 300 and 75 MHz (Varian Mercury-300 instrument). GC-MS experiments were conducted using a HP 5973 INERT series and an Agilent HP-5MS.

A.2 Experimental procedures and characterization data

The synthesis of the three isomers of deuterated benzonitrile (DBCN in Fig. A.1) was done according to an adapted procedure from the literature (Zhang et al. 2015; see Fig. A.1).

thumbnail Fig. A.1

General procedure for the synthesis of ortho, meta, and para deuterated benzonitrile.

We added Pd2(dba)3 (50 mg, 0.055 mmol), DCOONa (380 mg, 5.5 mmol) in dry DMSO (3 mL), and P(t-Bu)3 (33.3 mg, 0.165 mmol) to a stirred solution containing the corresponding bromobenzonitrile, 2-, 3-, or 4- (500 mg, 2.74 mmol). The mixture was heated at 80%C until GC-MS showed the full conversion of the starting material. After that, the reaction was quenched with saturated NH4Cl, extracted with CH2Cl2, and the organic phase was dried over Na2SO4, filtered and concentrated under high vacuum. The crude product was purified by column chromatography (SiO2, hexane) yielding the corresponding deuterated benzonitrile, ortho, meta, or para. The 1H and 13C NMR spectra are available at zenodo.

Benzonitrile-2-d (ortho deuterated benzonitrile): yield: 70%. Clear oil. 1H-RMN (300 MHz, CDCl3) Δ: 7.68 – 7.56 (m, 2H), 7.46 (ddd, J = 7.9, 5.5, 2.6 Hz, 2H) ppm. 13C-RMN-DEPT (75 MHz, CDCl3) Δ: 132.77 (CH), 132.21 (CH), 132.14 (t, J = 25.62 Hz, CD), 129.13 (CH), 129.02 (CH), 118.81 (CN), 112.40 (C) ppm.

Benzonitrile-3-d (meta deuterated benzonitrile): yield: 70%. Clear oil. 1H-RMN (300 MHz, CDCl3) Δ: 7.68 – 7.58 (m, 3H), 7.50 – 7.43 (m, 1H) ppm.13C-RMN-DEPT (75 MHz, CDCl3) Δ: 132.83 (CH), 132.27 (t, J = 23.6 Hz, CD), 132.16 (CH), 132.06 (CH), 129.18 (CH), 118.82 (CN), 112.51 (C) ppm.

Benzonitrile-4-d (para deuterated benzonitrile): yield: 56%. Clear oil. 1H-RMN (300 MHz, CDCl3)Δ: 7.68 – 7.63 (d, J=7.6 Hz, 2H), 7.47 (d, J = 7.7 Hz, 2H) ppm. 13C-RMN-DEPT (75 MHz, CDCl3) Δ: 132.48 (t, J = 22.7 Hz CD), 132.2 (2xCH), 129.0 (2xCH), 118.9 (CN), 112.4 (C) ppm.

References

  1. Agúndez, M., Marcelino, N., Tercero, B., & Cernicharo, J. 2023a, A&A, 677, L13 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  2. Agúndez, M., Marcelino, N., Tercero, B., et al. 2023b, A&A, 677, A106 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  3. Arenas, B. E., Gruet, S., Steber, A. L., et al. 2017, Phys. Chem. Chem. Phys., 19, 1751 [Google Scholar]
  4. Bak, B., Christensen, D., Dixon, W. B., et al. 1962, J. Chem. Phys., 37, 2027 [Google Scholar]
  5. Bernstein, M. P., Sandford, S. A., & Allamandola, L. J. 1996, ApJ, 472, L127 [NASA ADS] [CrossRef] [Google Scholar]
  6. Boersma, C., Allamandola, L. J., Esposito, V. J., et al. 2023, ApJ, 959, 74 [NASA ADS] [CrossRef] [Google Scholar]
  7. Buragohain, M., Pathak, A., Sakon, I., & Onaka, T. 2020, ApJ, 892, 11 [NASA ADS] [CrossRef] [Google Scholar]
  8. Burkhardt, A. M., Lee, K. L. K., Changala, P. B., et al. 2021a, ApJ, 913, L18 [Google Scholar]
  9. Burkhardt, A. M., Loomis, R. A., Shingledecker, C. N., et al. 2021b, Nat. Astron., 5, 181 [Google Scholar]
  10. Byrne, A. N., Xue, C., Van Voorhis, T., & McGuire, B. A. 2024, Phys. Chem. Chem. Phys., 26, 26734 [Google Scholar]
  11. Cabezas, C., Endo, Y., Roueff, E., et al., 2021a, A&A, 646, L1 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  12. Cabezas, C., Roueff, E., Tercero, B., et al., 2021b, A&A, 650, L15 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  13. Cabezas, C., Fuentetaja, R., Roueff, E., et al., 2022, A&A, 693, L14 [Google Scholar]
  14. Casado, J., Nygaard, L., Sørensen, O., et al., 1971, J. Mol. Struct., 8, 211 [Google Scholar]
  15. Cernicharo, J., & Guélin, M. 1987, A&A, 176, 299 [Google Scholar]
  16. Cernicharo, J., Agúndez, M., Cabezas, C., et al. 2021a, A&A, 649, L15 [EDP Sciences] [Google Scholar]
  17. Cernicharo, J., Agúndez, M., Kaiser, R. I., et al. 2021b, A&A, 655, L1 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  18. Cernicharo, J., Agúndez, M., Kaiser, R. I., et al. 2021c, A&A, 652, L9 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  19. Cernicharo, J., Fuentetaja, R., Agúndez, M., et al. 2022, A&A, 663, L9 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  20. Cernicharo, J., Tercero, B., Marcelino, N., et al. 2023, A&A, 674, L4 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  21. Cernicharo, J., Cabezas, C., Fuentetaja, R., et al. 2024a, A&A, 690, L13 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  22. Cernicharo, J., Tercero, B., Cabezas, C., et al. 2024b, A&A, 682, L13 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  23. Chiar, J. E., Tielens, A. G. G. M., Adamson, A. J., & Ricca, A. 2013, ApJ, 770, 78 [Google Scholar]
  24. Doney, K. D., Candian, A., Mori, T., et al. 2016, A&A, 586, A65 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  25. Draine, B. T., & Li, A. 2007, ApJ, 657, 810 [CrossRef] [Google Scholar]
  26. Draine, B. T., Sandstrom, K., Dale, D. A., et al. 2025, ApJ, 984, L42 [Google Scholar]
  27. Goicoechea, J. R., Pety, J., Cuadrado, S., et al. 2025, A&A, 696, A100 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  28. Hensley, B. S., & Draine, B. T. 2020, ApJ, 895, 38 [NASA ADS] [CrossRef] [Google Scholar]
  29. Lee, K. L. K., Changala, P. B., Loomis, R. A., et al. 2021, ApJ, 910, L2 [NASA ADS] [CrossRef] [Google Scholar]
  30. Linsky, J. L., Draine, B. T., Moos, H. W., et al. 2006, ApJ, 647, 1106 [Google Scholar]
  31. McCarthy, M. C., Chen, W., Travers, M. J., & Thaddeus, P. 2000, ApJS, 129, 611 [NASA ADS] [CrossRef] [Google Scholar]
  32. McCarthy, M. C., Lee, K. L. K., Loomis, R. A., et al. 2021, Nat. Astron., 5, 176 [Google Scholar]
  33. McGuire, B. A., Burkhardt, A. M., Kalenskii, S., et al. 2018, Science, 359, 6372 [Google Scholar]
  34. McGuire, B. A., Loomis, R. A., Burkhardt, A. M., et al. 2021, Science, 371, 1265 [Google Scholar]
  35. Morán, J. R., Cabezas, C., Hussain, F. S., et al. 2025, MNRAS, 538, 2084 [Google Scholar]
  36. Neill, J. L., Harris, B. J., Steber, A. L., et al. 2013, Opt. Express, 21, 19743 [Google Scholar]
  37. Ohshima, Y., & Endo, Y. 1992, J. Mol. Spectr., 153, 627 [NASA ADS] [CrossRef] [Google Scholar]
  38. Onaka, T., Mori, T. I., Sakon, I., et al. 2014, ApJ, 780, 114 [Google Scholar]
  39. Pereira-Santaella, M., González-Alfonso, E., García-Bernete, I., et al. 2024, A&A, 681, A117 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  40. Peeters, E., Allamandola, L. J., Bauschlicher, C. W., et al. 2004, ApJ, 604, 252 [NASA ADS] [CrossRef] [Google Scholar]
  41. Peeters, E., Habart, E., Berné, O., et al. 2024, A&A, 685, A74 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  42. Pety, J., Teyssier, D., Fossé, D., et al. 2005, A&A, 435, 885 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  43. Pickett H. M. 1991, J. Mol. Spectr., 148, 371 [NASA ADS] [CrossRef] [Google Scholar]
  44. Pla, P., Wang, Y., Martín, F., et al. 2020, ApJ, 899, 18 [NASA ADS] [CrossRef] [Google Scholar]
  45. Roberts, H., Herbst, E., & Millar, T. J. 2004, A&A, 424, 905 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  46. Roueff, E., Lis, D. C., van der Tak, F. F. S., et al. 2005, A&A, 438, 585 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  47. Sandstrom, K. M., Koch, E. W., Leroy, A. K., et al. 2023, ApJ, 944, L8 [NASA ADS] [CrossRef] [Google Scholar]
  48. Schutte, W. A., Tielens, A. G. G. M., & Allamandola, L. J. 1993, ApJ, 415, 397 [NASA ADS] [CrossRef] [Google Scholar]
  49. Tercero, F., López-Pérez, J. A., Gallego, J. D., et al. 2021, A&A, 645, A37 [EDP Sciences] [Google Scholar]
  50. Tercero, B., Marcelino, N., Roueff, E., et al. 2024, A&A, 682, L12 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  51. Tielens, A. G. G. M. 2008, ARA&A, 46, 289 [NASA ADS] [CrossRef] [Google Scholar]
  52. Watson, J. K. G., in Vibration Spectra and Structure, ed. J. Durig (Amsterdam: Elsevier), 6, 1 [Google Scholar]
  53. Wenzel, G., Cooke, I. R., Changala, P. B., et al. 2024, Science, 386, 810 [NASA ADS] [CrossRef] [Google Scholar]
  54. Wenzel, G., Gong, S., Xue, C., et al. 2025, ApJ, 984, L36 [Google Scholar]
  55. Wohlfart, K., Schnell, M., Grabow, J.-U., & Küpper, J. 2008, J. Mol. Spectr., 247, 119 [NASA ADS] [CrossRef] [Google Scholar]
  56. Yang, X. J., & Li, A. 2023, ApJS, 268, 12 [NASA ADS] [CrossRef] [Google Scholar]
  57. Yang, X. J., & Li, A. 2025, ApJ, 983, 136 [Google Scholar]
  58. Yang, X. J., Li, A., Glaser, R., & Zhong, J. X. 2017, ApJ, 837, 171 [Google Scholar]
  59. Zaleski, D. P., Duan, C., Carvajal, M., et al. 2017, J. Mol. Spectr., 342, 17 [Google Scholar]
  60. Zdanovskaia, M. A., Esselman, B. J., Lau, H. S., et al. 2018, J. Mol. Spectr., 351, 39 [Google Scholar]
  61. Zhang, H., Bonnesen, P. V., & Hong, K., 2015, Org. Chem. Front., 2, 1071 [Google Scholar]
  62. Zhen, J., Castellanos, P., Paardekooper, D. M., et al. 2014, ApJ, 797, L30 [CrossRef] [Google Scholar]

All Tables

Table 1

Spectroscopic parameters of the three monodeuterated isomers of benzonitrile.

In the text
Table 2

Rotational partition function of the three monodeuterated isomers of benzonitrile at different temperatures.

In the text

All Figures

thumbnail Fig. 1

Insets of the recorded spectrum in the discharge of deuterated benzene and acetonitrile. The three species of deuterated benzonitrile (ortho, meta, and para) can be seen. The experimental spectra are shown in black above the x axis, while the simulated spectra for each isomer at 1 K (derived from the rotational parameters) are shown in colors below the x axis.
In the text
thumbnail Fig. 2

Portion of the spectra of ortho- (top), meta- (middle), and para- (bottom) deuterated benzonitrile (DBCN) recorded in the 75–110 GHz range. The experimental spectra are shown in black above the x axis, while the simulated spectra at 300 K (calculated from the rotational parameters) are shown in colors below the x axis.
In the text
thumbnail Fig. 3

Spectra of TMC-1 in the Q band at the frequencies of some of the most favorable lines of ortho, meta, and para deuterated benzonitrile. Negative artifacts produced by the frequency-switching technique have been blanked. The noise level, measured in a window of ±8 MHz around the expected position of each line with the nominal spectral resolution of 38.15 kHz, is indicated by a horizontal gray band. The red lines correspond to the line intensities calculated adopting the 3σ upper limits on the column densities derived here and a full width at half maximum of 0.60 km s−1.
In the text
thumbnail Fig. A.1

General procedure for the synthesis of ortho, meta, and para deuterated benzonitrile.
In the text

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