Free Access
Issue
A&A
Volume 654, October 2021
Article Number L9
Number of page(s) 5
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202142156
Published online 19 October 2021

© ESO 2021

1. Introduction

Cold dark clouds such as TMC-1 present a rich and complex chemistry that leads to the formation of a great variety of molecules. The list of molecular species observed in these clouds includes cations, mostly protonated forms of closed-shell abundant molecules (e.g., Agúndez et al. 2015; Marcelino et al. 2020; Cernicharo et al. 2020a, 2021a,b), and some hydrocarbon and nitrile anions (e.g., Cernicharo et al. 2020b), although most of the detected species are electrically neutral. Closed-shell species constitute only one-third of the neutral species observed in cold dark clouds, but they are the most abundant ones. The remaining two-thirds of the neutral species detected in these cold environments are open-shell radicals. Apart from OH, CH, C2H, C4H, C6H, CH2CCH, and NO, observed radicals have low abundances because, as ions, they are highly reactive species (see e.g., Agúndez & Wakelam 2013). Another fact that complicates the detection of radicals is the spectral dilution resulting from line splitting due to the interaction of the rotational angular momentum with different types of angular momenta, such as the electronic orbital, the electron spin, or the nuclear spin.

Cyanomethyl radical, CH2CN, is the simplest member of the CH2CnN (n ≥ 1) radical series. It is derived from the closed-shell species CH3CN by removing one hydrogen atom. CH2CN was detected in the cold dark cloud TMC-1 by Saito et al. (1988) and Irvine et al. (1988). Recent observations of TMC-1 using the QUIJOTE1 line survey (Cernicharo et al. 2021c) show that CH2CN is a fairly abundant radical with a CH2CN/CH3CN ratio of 3.2 ± 0.4. (Cabezas et al. 2021a). Hence, it is expected that larger members of the CH2CnN series can be observed in this source as well. CH2CCN is the next member of the series. This radical is known as α-cyanovinyl radical, and it has been characterized experimentally. Its rotational spectrum in the centimetre and millimetre regions has been observed (Tang et al. 2000; Seiki et al. 2000; Prozument et al. 2013), as has its Fourier-transform infrared emission spectrum (Letendre & Dai 2002). However, the data available in the literature do not allow the precise predictions needed to search for it in the interstellar medium (ISM) to be obtained, and its astronomical detection has not been claimed so far.

The largest member of the CH2CnN radical series that has been characterized in the laboratory is the 3-cyano propargyl radical, CH2C3N (see Fig. 1). Chen et al. (1998) detected the ortho-CH2C3N using Fourier transform microwave spectroscopy, and three years later Tang et al. (2001) improved the rotational parameters for CH2C3N by observing lines for the para-CH2C3N species using the same spectroscopy technique. Chen et al. (1998) also reported on their astronomical search for the CH2C3N radical, which they carried out using their experimental data. The obtained upper limit allowed them to estimate a column density of 2 × 1011cm−2 in TMC-1.

thumbnail Fig. 1.

Molecular structure of the CH2C3N radical.

In this Letter we report the first identification of the CH2C3N radical in space towards TMC-1, based on the laboratory data previously reported by Chen et al. (1998) and Tang et al. (2001). The derived column density for this radical is compared with analogue radicals and closed-shell species and is interpreted via chemical models to understand the chemical processes in which it is involved.

2. Observations

The data presented in this work are part of the QUIJOTE spectral line survey in the Q band towards TMC-1 (αJ2000 = 4h41m41.9s and δJ2000 = +25° 41′27.0″) that was performed at the Yebes 40m radio telescope during various observing sessions between November 2019 and April 2021. A total of 30 new molecular species have been detected using this survey (Cernicharo et al. 2020a,b,c, 2021a,b,c,d,e,f,g,h; Marcelino et al. 2020, 2021; Agúndez et al. 2021a,b; Cabezas et al. 2021a,b,c). All observations were carried out using the frequency switching technique (Cernicharo et al. 2019), with a frequency throw of 10 MHz during the two first observing runs and of 8 MHz in the later ones. This observing mode provides a S/N that is higher than the unfolded data, but on the other hand it produces negative spectral features at ±10 MHz or 8 MHz of each rotational transition. These negative features can be easily identified because of their symmetric displacement by exactly the frequency throw. As shown in Fig. 2, we blanked these channels with negative features for the sake of convenience. The selected temperature scale is . The TMB can easily be obtained by dividing the observed by the beam efficiency. Values of ηMB have been provided by Tercero et al. (2021). The was calibrated using two absorbers at different temperatures and the atmospheric transmission model ATM (Cernicharo 1985; Pardo et al. 2001).

thumbnail Fig. 2.

Observed NKa,Kc = 80, 8–70, 7, 90, 9–80, 8, and 100, 10–90, 9 lines of ortho-CH2C3N in TMC-1 in the 31.0–50.4 GHz range. The most intense hyperfine components for each rotational transition are shown. The abscissa corresponds to the rest frequency, assuming a local standard of rest velocity of 5.83 km s−1. Blanked channels correspond to negative features produced in the folding of the frequency switching data. The ordinate is antenna temperature in millikelvins. Curves shown in red are the computed synthetic spectra.

Different frequency coverages were observed, 31.08-49.52 GHz and 31.98–50.42 GHz, which permitted us to verify that no spurious ghosts were produced in the down-conversion chain. In this chain the signal coming from the receiver is down-converted to 1–19.5 GHz and then split into eight bands with a coverage of 2.5 GHz, each of which is analysed by the fast Fourier transform (FFT) spectrometers. Calibration uncertainties were adopted to be 10%, based on the observed repeatability of the line intensities between different observing runs. All data were analysed using the GILDAS package2.

3. Results

The 3-cyano propargyl radical, CH2C3N, is an asymmetric top molecule (see Fig. 1) with a doublet electronic ground state (2B1) and a fairly large dipole moment of 4.43 D (Tang et al. 2001). Due to its molecular symmetry, C2v, it is necessary to discern between ortho-CH2C3N and para-CH2C3N levels, which are described by Ka even and Ka odd, respectively. Chen et al. (1998) observed in the laboratory four rotational transitions with Ka = 0 for ortho-CH2C3N. A total of 89 hyperfine components were analysed with an effective Hamiltonian for a linear molecule in a 2Σ electronic state, and a set of linear-molecule-like constants was determined. These constants can only be used to predict the rotational transitions for ortho-CH2C3N in a hypothetical astronomical search. Tang et al. (2001) confirmed the C2v structure for CH2C3N by measuring and analysing both Ka = 1 for para-CH2C3N and those transitions previously measured by Chen et al. (1998) with Ka = 0 for ortho-CH2C3N. In this manner, Tang et al. (2001) determined a total of 15 molecular constants for the asymmetric-rotor CH2C3N radical, which properly describe the rotational spectrum of this species and allow its radio-astronomical search, since both ortho- and para-CH2C3N are observable in the ISM.

Based only on the experimental data reported by Chen et al. (1998), W. D. Langer & T. Velusamy (cited as private communication in Chen et al. 1998) searched for ortho-CH2C3N in TMC-1. An upper limit of ≤ 5 mK for the N = 5–4 transition at 21 864 MHz, averaged over the 0.20 km s−1 spectral resolution, was obtained using the position switching observing mode with an on-source integration time of 6 h. From these observations, W. D. Langer & T. Velusamy estimate that the column density of CH2C3N in TMC-1 is ≤2 × 1011 cm−1 for an assumed dipole moment of 4.42 D, a rotational temperature of 10 K, a line width of 0.5 km s−1, and under the assumption that the source fills the telescope beam, 45″. Our QUIJOTE survey has an excellent sensitivity, with a rms noise level of 0.30 mK per 38.15 kHz channel, which has allowed the detection of the 3-cyano propargyl radical.

Our search for CH2C3N is based on the frequency predictions for the ortho and para species made using the laboratory data from Chen et al. (1998) and Tang et al. (2001). These predictions are available in the CDMS catalogue (Müller et al. 2005), entry number 064509, together with other data, such as partition functions. They were implemented in the MADEX code (Cernicharo 2012) to compute column densities. We considered the ortho and para species separately as there are no radiative or collisional transitions between them. The lowest energy level of the para species (11, 1) is 13.9 K above the ortho ground level (00, 0). We adopted a dipole moment of 4.43 D, as calculated by Tang et al. (2001).

Four transitions of ortho-CH2C3N with Ka = 0 are covered by our QUIJOTE survey, at 34.9, 39.3, 43.7, and 48.1 GHz. We observed three groups of lines at the predicted frequencies of the transitions 80, 8–70, 7, 90, 9–80, 8, and 100, 10–90, 9. Figure 2 shows the lines corresponding to these rotational transitions of ortho-CH2C3N with their hyperfine structure, as observed in TMC-1. Our model predicts an intensity of 1.0 mK for the strongest hyperfine component of the 110, 11–100, 10 transition and between 0.4–0.7 mK for the others. None of these lines are detected at the 3σ (0.9 mK) detection limit of the survey at this frequency. All the hyperfine components are detected with antenna temperatures between 0.6 and 1.9 mK, and a few of them are blended with negative features produced in the folding of the frequency switching data (i.e. for the 80, 8–70, 7 transition). The hyperfine components of ortho-CH2C3N are precisely centred at the calculated frequencies with deviations in frequency smaller than 20 kHz, which is within the uncertainty given by the spectral resolution of 38.15 kHz and the error in the Gaussian fit.

A total of eight rotational transitions of para-CH2C3N with Ka = 1 are covered by our survey. However, those with an N quantum number higher than 9 are predicted at frequencies that are not detectable with the sensitivity of our survey. The spectral pattern of the Ka = 1 transitions of para-CH2C3N is very different to that of the Ka = 0 lines, where all the hyperfine components are spread over 2.0–2.5 MHz. In contrast, each Ka = 1 line is formed by two groups of hyperfine components separated by a few megahertz due to the spin-doubling interactions. This separation decreases with the N quantum number. This is illustrated in Fig. 3, where we show the four rotational transitions of para-CH2C3N with Ka = 1 observed in our survey. Six groups of lines are clearly detected with a maximum antenna temperature of 0.6 mK. The other two, 81, 8–71, 7 with J = N + 1/2 and 81, 8–71, 6 with J = N − 1/2, are affected by negative features produced in the folding of the frequency switching data and thus cannot be confirmed.

thumbnail Fig. 3.

Observed rotational transitions of para-CH2C3N in TMC-1 in the 31.0–50.4 GHz range. The most intense hyperfine components for each rotational transition are shown. The abscissa corresponds to the rest frequency, assuming a local standard of rest velocity of 5.83 km s−1. The ordinate is antenna temperature in millikelvins. Curves shown in red are the computed synthetic spectra.

An analysis of the observed intensities using a line profile fitting method (Cernicharo et al. 2021a) provides a rotational temperature of 7 ± 1 K, while the observed intensities are reproduced with column densities of (1.1 ± 0.3) × 1011 cm−2 and (4.6 ± 1.1) × 1010 cm−2 for ortho- and para-CH2C3N, respectively. The ortho/para ratio found is 2.4 ± 1.2. Chen et al. (1998) reported an estimated column density for CH2C3N in TMC-1 ≤2 × 1011cm−1, which is in accordance with our value for the total (ortho plus para) column density for CH2C3N of (1.6 ± 0.4) × 1011 cm−2.

4. Chemical modelling

To examine the chemical processes that could form the radical CH2C3N in TMC-1, we carried out chemical modelling calculations. We adopted typical conditions of cold dark clouds: a gas kinetic temperature of 10 K, a volume density of H nuclei of 2 × 104 cm−3, a cosmic-ray ionization rate of H2 of 1.3 × 10−17 s−1, a visual extinction of 30 mag, and low-metal elemental abundances (see e.g., Agúndez & Wakelam 2013). We used the chemical network RATE12 from the UMIST database (McElroy et al. 2013), with updates from Loison et al. (2014) and Marcelino et al. (2021) to include C4H3N isomers.

Since the radical CH2C3N is not included in the UMIST database, we added some reactions to account for its formation and destruction. We assumed that CH2C3N is destroyed through reactions with the neutral atoms C, N, and O, as well as through reactions with the atomic cations C+ and H+, with rate coefficients similar to those involving the radical CH2CN. As formation routes, we included the reactions

(1)

(2)

(3)

(4)

Reaction (1) has been studied using crossed molecular beam experiments and theoretical calculations (Su et al. 2005; Guo et al. 2006a). These studies indicate that the reaction is barrier-less and occurs through H atom elimination, yielding as main products the radicals 1-cyano propargyl (HCCCHCN) and 3-cyano propargyl (CH2C3N). The latter is inferred to be produced at least twice less efficiently than the former (Guo et al. 2006a), and we thus adopted a rate coefficient of 10−10 cm3 s−1 for reaction (1). Reaction (2) has not been studied to our knowledge, but some information can be extracted from the known reactivity of C2 with unsaturated hydrocarbons, in particular with CH3CCH. The reaction C2 + CH3CCH has been measured to be rapid, with a nearly constant rate coefficient of (4–5) × 10−10 cm3 s−1 in the temperature range 77–296 K (Daugey et al. 2008). Moreover, crossed molecular beam experiments and theoretical calculations indicate that the radical CH2C4H is the preferred product, with an estimated branching ratio of 0.65 (Guo et al. 2006b; Mebel et al. 2006). Based on the behaviour of the reaction C2 + CH3CCH, we adopted a rate coefficient of 3 × 10−10 cm3 s−1 for reaction (2). The radical-radical reactions (3) and (4) have not been studied to our knowledge, although based on the known reactivity of CN and C3N with unsaturated closed-shell hydrocarbons, it is plausible that they occur with no barrier, yielding CH2C3N as one of the main products. We thus adopted a rate coefficient of 10−10 cm3 s−1 for them.

We also included an additional formation route to CH2C3N through the dissociative recombination of the precursor ion CH3C3NH+ with electrons. Loison et al. (2014) estimated a branching ratio of 0.24 for the production of the radical CH2CN in the dissociative recombination of the smaller analogue ion CH3CNH+, and we thus assumed that the same behaviour holds for the formation of the radical CH2C3N in the dissociative recombination of CH3C3NH+. The reaction channel of formation of CH2C3N then reads

(5)

with a rate coefficient of 8 × 10−8 (T/300 K)−0.5 cm3 s−1.

In Fig. 4 we show, as a solid red line, the calculated fractional abundance of C2H3CN as a function of time. The peak abundance is about four times lower than the value observed in TMC-1. The main formation routes to CH2C3N are reactions (1)–(3), and (5), while reaction (4) is only a minor route.

thumbnail Fig. 4.

Calculated fractional abundances of CH3C3N and CH2C3N as a function of time. The solid red line corresponds to the abundance of CH2C3N when the reaction N + C4H3 is neglected and the dashed red line to the case in which this reaction is included. Horizontal dotted lines correspond to the abundances observed in TMC-1.

There is another potential formation route to CH2C3N. In the case of the smaller analogue radical CH2CN, the chemical model indicates that it is mainly formed through the dissociative recombination of CH3CNH+ with electrons, but also by the reaction N + C2H3, which is known to produce CH2CN as a main product (Payne et al. 1996). Similarly, it is plausible that the reaction N + C4H3 yields CH2C3N. If this reaction is implemented in the chemical model with a rate coefficient similar to that of N + C2H3, then it becomes the main route to CH2C3N and its calculated peak abundance increases by more than two orders of magnitude, lying well above the observed value (see the dashed red line in Fig. 4). However, it is currently not known whether the reaction N + C4H3 can produce CH2C3N. Moreover, the situation becomes more complicated because there are various possible isomers of the radical C4H3, although the chemical network does not distinguish between them. A dedicated study of the reaction N + C4H3 is needed to shed light on this point.

In Fig. 4 we also show the calculated abundance of the related molecule CH3C3N, which has a peak value about five times above the abundance observed in TMC-1. This behaviour is similar to that reported previously in Marcelino et al. (2021).

5. Discussion

In light of the discovery of the radical CH2C3N in TMC-1, it is worth comparing its abundance with that of chemically related molecules. In TMC-1 we have N(CH2CN) = 1.5 × 1013 cm−2 (Cabezas et al. 2021a), N(CH3CN) = 4.7 × 1012 cm−2 (Cabezas et al. 2021a), and N(CH3C3N) = 1.7 × 1012 cm−2 (Marcelino et al. 2021). Therefore, the abundance ratio CH2 C3N/CH3C3N is just 0.09, well below unity. This is in contrast with the smaller analogues, in which case the abundance ratio CH2CN/CH3CN is above one, concretely 3.2. This indicates that the chemistry of the cyanides CH2C3N and CH3C3N behaves differently to that of the smaller analogues CH2CN and CH3CN. Moreover, it is unclear whether the radical and the corresponding closed-shell molecule are connected by some common formation routes or have completely disconnected chemistries.

A common route to both CH2CN and CH3CN is the dissociative recombination of CH3CNH+. However, the yield ratio CH2CN/CH3CN is currently unconstrained (see Vigren et al. 2008). It is likely that this reaction is a major formation pathway to CH3CN but not to CH2CN. Even if some CH2CN is formed during the dissociative recombination of CH3CNH+, most CH2CN should be formed in TMC-1 through an independent and very efficient process, such as the reaction N + C2H3. It should be noted that this pathway must be much more efficient than the formation of CH3CN through CH3CNH+ + e to account for the higher abundance of CH2CN compared to CH3CN and the fact that CH2CN should be far more reactive than CH3CN. If true, this implies that the radical C2H3 should be abundant in TMC-1.

In the case of the larger cyanides CH2C3N and CH3C3N, a similar highly efficient route to the radical CH2C3N must be prevented since in this case the radical is substantially less abundant than the closed-shell molecule. This takes us to the reaction N + C4H3, which, according to our chemical model calculations, should not be a major source of CH2C3N, unlike in the case of the smaller analogue, where the reaction N + C2H3 is a major route to CH2CN. In summary, the routes to the radicals CH2CN and CH2C3N are likely chemically different.

A further point to discuss is whether the radical detected in this work could be an important intermediate to forming larger molecules. Although dedicated studies on the reactivity of CH2C3N are needed, our radical could play a role in the synthesis of large organic N-bearing molecules, such as nitrogen heterocycles. For example, benzonitrile (c–C6H5CN), known to be present in TMC-1 and other molecular clouds (McGuire et al. 2018; Burkhardt et al. 2021) and thought to be formed through the reaction of benzene with CN (Cooke et al. 2020), could be formed through the reaction of CH2C3N with allene (CH2CCH2), which is suspected to be abundant in TMC-1 (Marcelino et al. 2021; Cernicharo et al. 2021e; Agúndez et al. 2021a).

6. Conclusions

We have reported the detection of the 3-cyano propargyl radical (CH2C3N) in the cold dark cloud TMC-1. A total of seven rotational transitions with several hyperfine components were observed in our Q-band TMC-1 survey. This radical is about ten times less abundant than the corresponding closed-shell molecule CH3C3N, in contrast to the case of the smaller analogues, where the radical CH2CN is three times more abundant than CH3CN. Comparison between the chemical routes of the radicals CH2CN and CH2C3N and the corresponding closed-shell species indicates that the formation routes to the radicals CH2CN and CH2C3N are completely dissimilar.


1

Q-band Ultrasensitive Inspection Journey to the Obscure TMC-1 Environment.

Acknowledgments

This research has been funded by ERC through grant ERC-2013-Syg-610256-NANOCOSMOS. Authors also thank Ministerio de Ciencia e Innovación for funding support through projects PID2019-106235GB-I00 and PID2019-107115GB-C21/AEI/10.13039/501100011033. M. A. thanks Ministerio de Ciencia e Innovación for grant RyC-2014-16277.

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All Figures

thumbnail Fig. 1.

Molecular structure of the CH2C3N radical.

In the text
thumbnail Fig. 2.

Observed NKa,Kc = 80, 8–70, 7, 90, 9–80, 8, and 100, 10–90, 9 lines of ortho-CH2C3N in TMC-1 in the 31.0–50.4 GHz range. The most intense hyperfine components for each rotational transition are shown. The abscissa corresponds to the rest frequency, assuming a local standard of rest velocity of 5.83 km s−1. Blanked channels correspond to negative features produced in the folding of the frequency switching data. The ordinate is antenna temperature in millikelvins. Curves shown in red are the computed synthetic spectra.

In the text
thumbnail Fig. 3.

Observed rotational transitions of para-CH2C3N in TMC-1 in the 31.0–50.4 GHz range. The most intense hyperfine components for each rotational transition are shown. The abscissa corresponds to the rest frequency, assuming a local standard of rest velocity of 5.83 km s−1. The ordinate is antenna temperature in millikelvins. Curves shown in red are the computed synthetic spectra.

In the text
thumbnail Fig. 4.

Calculated fractional abundances of CH3C3N and CH2C3N as a function of time. The solid red line corresponds to the abundance of CH2C3N when the reaction N + C4H3 is neglected and the dashed red line to the case in which this reaction is included. Horizontal dotted lines correspond to the abundances observed in TMC-1.

In the text

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