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Issue
A&A
Volume 650, June 2021
Article Number L15
Number of page(s) 5
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202141371
Published online 21 June 2021

© ESO 2021

1. Introduction

Deuterium fractionation is a well-known process in the dense interstellar medium that can occur both in the gas phase and on the surfaces of dust particles. This process allows deuterated isotopic species of interstellar molecules to reach abundances much higher than the D/H elemental abundance ratio (1.5 × 10−5Linsky 2003). The high efficiency of deuterium fractionation allows deuterated species to achieve abundances as high as 30–40% relative to the parent species, as occurs with HDCS (Marcelino et al. 2005) and CH2DOH (Parise et al. 2006). Hence, deuterated isotopologues of abundant interstellar molecules make a significant contribution to the spectral richness of line surveys. This makes the astronomical identification of these isotopologues of utmost importance, not only to gain knowledge on their molecular formation pathways or how deuterium fractionation works, but also to assign unidentified features in line surveys.

Very sensitive broadband line surveys of astronomical sources can now be achieved thanks to new technical developments in radio astronomy. These surveys have boosted the number of new molecular identifications in recent years because weak lines arising from low-abundance species and from low-dipole moment species can be now easily detected (Agúndez et al. 2021a; Cernicharo et al. 2021a,b,c). The negative counterpart of this high sensitivity is the huge number of new lines that populate the survey, including isotopologues and vibrationally excited states, in warm environments of well-known species. Hence, discovering spectral features of new molecules requires a previous detailed analysis of the spectral contribution of known species.

Methylcyanoacetylene, CH3C3N, also known as cyanopropyne or methylpropionitrile, has been detected with high abundance in the cold dark cloud TMC-1 (Broten et al. 1984) and more recently by Marcelino et al. (2021) using a high-sensitivity line survey on TMC-1 gathered with the Yebes 40 m radio telescope (see e.g., Cernicharo et al. 2021d). Hence, the deuterated isotopologues of CH3C3N are good candidates to be observed in this source using the same line survey. We have already detected other singly deuterated isotopologues of species such as CH3CN, CH3CCH, c-C3H2, C4H, H2C4, H2CCN, HC3N, and HC5N (Cabezas et al. 2021).

In this Letter we report the identification of spectral lines of the deuterated species CH2DC3N in TMC-1. Our search for this molecule is based on the change in the rotational parameters of CH3C3N produced by the H–D exchange, which have been obtained by ab initio calculations. The derived deuterium ratios are compared to an extended chemical model including the related deuterated compounds.

2. Observations

The Q-band observations of TMC-1 (αJ2000 = 4h41m41.9s and δJ2000 = +25° 41′27.0″) described in this work were performed in several sessions between November 2019 and April 2021. They were carried out using a set of new receivers, built within the Nanocosmos project1, and installed at the Yebes 40 m radio telescope.

The Q-band receiver consists of two high-electron-mobility transitor cold amplifiers covering the 31.0–50.4 GHz band in the horizontal and vertical polarizations. The receiver temperature varies from 22 K at 32 GHz to 42 K at 50 GHz. The spectrometers formed by 2 × 8 × 2.5 GHz fast Fourier transforms (FFTs) provide a spectral resolution of 38.15 kHz and cover the whole Q band in both polarizations. The receivers and the spectrometers are described in Tercero et al. (2021).

Different frequency coverages were observed, 31.08–49.52 GHz and 31.98–50.42 GHz, which permited us to check that no spurious ghosts were produced in the down-conversion chain when the signal coming from the receiver was downconverted to 1–19.5 GHz, and was then split into eight bands with a coverage of 2.5 GHz, each of which was analysed by the FFTs.

The observing procedure used was the frequency switching mode, with a frequency throw of 10 MHz or 8 MHz (see e.g., Cernicharo et al. 2021d,e,f). The intensity scale, or antenna temperature ( T A * $ T_{\rm A}^* $), was calibrated using two absorbers at different temperatures and the atmospheric transmission model ATM (Cernicharo 1985; Pardo et al. 2001). Calibration uncertainties of 10% were adopted, based on the observed repeatability of the line intensities between different observing runs. All data were analysed using the GILDAS package2.

3. Results

The identification of most of the features from our TMC-1 Q-band line survey was done using the MADEX code (Cernicharo 2012) and the CDMS and JPL catalogues (Müller et al. 2005; Pickett et al. 1998). Nevertheless, many lines remain unidentified. Among these U lines we found a series of five lines with a harmonic relation 8:9:10:11:12 between them. This series of lines could be fitted using a Hamiltonian for a linear molecule obtaining accurate values for B and D constants: B = 1989.428172 ± 0.000610 MHz and D = 0.10950 ± 0.00269 kHz. However, a deeper inspection of the survey around the mentioned lines revealed the presence of two additional series of lines at higher and lower frequencies from the first series. The spectral pattern, taking into account all the lines, is easily recognizable as the typical a-type transition spectrum of a near-prolate molecule, with the sets of rotational transitions containing J+10, J + 1 ← J0, J, J+11, J + 1 ← J1, J, and J+11, J ← J1, J − 1 separated by B + C. All the observed lines, shown in Table 1 and Fig. 1, were analysed using an asymmetric rotor Hamiltonian in the FITWAT code (Cernicharo et al. 2018) to derive the rotational and centrifugal distortion constants shown in Table 2. With the available data we could not determine the A rotational constant, which was kept fixed to the ab initio value, as explained below.

thumbnail Fig. 1.

Observed lines of CH2DC3N in TMC-1 in the 31.0–50.4 GHz range. Frequencies and line parameters are given in Table 1. Quantum numbers for the observed transitions are indicated in each panel. The red line shows the synthetic spectrum computed for a rotational temperature of 8 K and a column density of 8 × 1010 cm−2 (see text). The additional components seen in the synthetic spectrum close to the Ka = 0 components are the Ka = 2 rotational transitions. Blanked channels correspond to negative features created in the folding of the frequency switching data. The label ‘U’ corresponds to features above 4σ.

Table 1.

Observed line parameters for CH2DC3N in TMC-1.

The identification of the spectral carrier is first based on the following points: (i) the molecule is a closed-shell species without any appreciable fine or hyperfine interaction or large amplitude motion; (ii) the determined values for B and C constants indicate that the molecule is a very slightly asymmetric rotor, because B and C values are similar; and (iii) the (B+C)/2 value (1989.43 MHz, corresponding to the rotational constant of the close symmetric species) is smaller than that of H2C5 (2295.29 MHz), but larger than that of H2C6 (1344.72 MHz), which indicates that the molecule should contain four C atoms and one atom heavier than C. With these assumptions, we excluded species with four carbon atoms and a sulfur atom because they are too heavy. Species containing four carbon atoms and oxygen, like HC4O (Kohguchi et al. 1994) and H2C4O (Brown et al. 1979), are rejected as candidates because they are too light (B = 2279.914 MHz and (B + C)/2 = 2153.75 MHz, respectively) and other species derived from them are too heavy or are open-shell species. Molecules with four carbon atoms and nitrogen could be good candidates. The HC4N molecule (Tang et al. 1999) has a rotational constant B = 2302.398 MHz, and (B+C)/2 values for the cationic species HC4NH+ and H2C4N+ in their 1Σ electronic ground states have been calculated to be 2159.3 MHz and 2194.7 MHz, respectively (CCSD/cc-pVTZ level of theory; Cížek et al. 1969; Dunning 1989). The next member of this hydrogen addition progression is CH3C3N, whose rotational constant is 2065.74 MHz, very close to our (B+C)/2 value. This prompted us to think that the spectral carrier could be the deuterated isotopologue CH2DC3N, an asymmetric rotor, because the H–D interchange breaks the C3v symmetry of CH3C3N.

We performed geometry optimization calculations for CH3C3N and CH2DC3N in order to estimate the isotopic shift on the rotational constants for the CH3C3N-CH2DC3N system. Using the ratio of experimental to theoretical values is the most common method of predicting the expected experimental rotational constants for an isotopic species of a given molecule when the rotational constants for its parent species are known. Hence, we employed the CCSD/cc-pVTZ level of theory (Cížek et al. 1969; Dunning 1989), which reproduces well the B rotational constant for CH3C3N, 2058.0 MHz versus 2065.74 MHz. The theoretical values for the rotational constants of CH2DC3N were then scaled using the experimental/theoretical ratio obtained from CH3C3N, and the results are shown in Table 2. As can be seen, the predicted values for CH2DC3N perfectly match those derived from our fit, which allows us to conclude that the spectral carrier of our lines is CH2DC3N. It should be noted that the calculations provide the equilibrium values for the rotational constants (Ae, Be and Ce), while the experimental values are the ground state rotational constants (A0, B0, and C0). Even though the equilibrium rotational constants differ slightly from the ground state constants, we can assume similar discrepancies for CH3C3N and CH2DC3N, and thus that the estimated constants for CH2DC3N are essentially unaffected.

Table 2.

Observationally derived and theoretical spectroscopic parameters (in MHz) for CH2DC3N.

Methyldiacetylene (CH3C4H) is 7.5 times more abundant than, and lines have similar intensities to, CH3C3N (Marcelino et al. 2021; Cernicharo et al. 2021c). Hence, it is straightforward to think that spectral signatures of the deuterated species of CH3C4H could also be detected in our line survey. We followed the same strategy used for CH2DC3N to predict the transition frequencies of CH2DC4H. Laboratory values are available for CH3C4D (Heath et al. 1955). We carried out geometry optimization calculations for CH3C4H. The rotational constants obtained for CH2DC4H are shown in Table 3. We found only two lines at the predicted transition frequencies corresponding to 80, 8–70, 7 and 90, 9–80, 8 with intensities of ∼1 mK. Other lines predicted in the frequency range of the line survey are below the present sensitivity. We consider that the deuterated isotopologue of CH3C4H is not detected so far (see below). The deuteration of this species is discussed in the following section.

Table 3.

Predicted spectroscopic constants (in MHz) for isotopic species of CH3C3N and CH3C4H.

Considering the intensity of the CH3C3N and CH3C4H lines in our line survey, we expected to observe the 13C isotopologues as well. The frequency transitions for these species were predicted using the rotational constants from Table 3, which were obtained using the same procedure employed for the CH2DC3N isotopic species. However, we could not find spectral signatures for any of these species around the predicted frequencies.

The column density of CH2DC3N was derived from a rotational diagram analysis of the observed intensities. We assumed a source of uniform brightness with a radius of 40″ (Fossé et al. 2001). We derive Tr = 8  ±  0.5 K and N(CH2DC3N) = (8.0 ± 0.4) × 1010 cm−2. As shown in Fig. 1, the agreement between the synthetic spectrum and the observations is excellent. The column density is not very sensitive to the adopted value of the rotational temperature between 6 and 10 K. For the normal isotopologue Marcelino et al. (2021) derived a rotational temperature for the A and E species of 6.7 ± 0.2 K and of 8.2 ± 0.6 K, respectively. They derived a total column density for CH3C3N of (1.74 ± 0.1) × 1012 cm−2. Hence, the CH3C3N/CH2DC3N abundance ratio is 22 ± 2.

The column density of CH3C4H has been derived by Cernicharo et al. (2021c) to be (1.30 ± 0.04) × 1013 cm−2. Assuming the same rotational temperature for CH2DC4H as for the main isotopologue (Cernicharo et al. 2021c), we derive a 3σ upper limit to its column density of 3.7 × 1011 cm−2. Therefore, the CH3C4H-to-CH2DC4H abundance ratio is ≥35 (3σ). For the deuterated species CH3C4D, for which laboratory spectroscopy is available (Heath et al. 1955), we derive a 3σ upper limit to its column density of 9 × 1010 cm−2. Hence, N(CH3C4H)/N(CH3C4D) ≥ 144.

4. Chemical modelling

We further investigate the chemical processes leading to deuterium insertion in methylcyanoacetylene and methyldiacetylene by extending our previous study on H2CCN and HDCCN (Cabezas et al. 2021). We only consider gas-phase mechanisms that allow quantitative predictions based on some experimental measurements and theoretical studies. The chemistry of the different C4H3N and C5H4 isomers has been discussed recently in Cernicharo et al. (2021c) and Marcelino et al. (2021), respectively, in relation with their detection in TMC-1. These two chemical families are tightly linked to the chemistry of methylacetylene, CH3CCH, and its isomer allene, CH2CCH2:

CN + CH 3 CCH → CH 3 C 3 N + H , → HCN + CH 2 CCH , → HC 3 N + CH 3 ; C 2 H + CH 3 CCH → CH 3 C 4 H + H , → H 2 CCCHCCH + H , $$ \begin{aligned} \mathrm{CN} + \mathrm{CH_3CCH}&\rightarrow \mathrm{CH_3C_3N} + \mathrm{H}, \\&\rightarrow \mathrm{HCN} + \mathrm{CH_2CCH}, \\&\rightarrow \mathrm{HC_3N} + \mathrm{CH_3}; \\ \mathrm{C_2H} + \mathrm{CH_3CCH}&\rightarrow \mathrm{CH_3C_4H} + \mathrm{H}, \\&\rightarrow \mathrm{H_2CCCHCCH} + \mathrm{H}, \end{aligned} $$

whereas

CN + CH 2 CCH 2 → CH 2 CCHCN + H , → HCCCH 2 CN + H ; C 2 H + CH 2 CCH 2 → CH 3 C 4 H + H , → H 2 CCCHCCH + H . $$ \begin{aligned} \mathrm{CN} + \mathrm{CH_2CCH_2}&\rightarrow \mathrm{CH_2CCHCN} + \mathrm{H}, \\&\rightarrow \mathrm{HCCCH_2CN} + \mathrm{H}; \\ \mathrm{C_2H} + \mathrm{CH_2CCH_2}&\rightarrow \mathrm{CH_3C_4H} + \mathrm{H}, \\&\rightarrow \mathrm{H_2CCCHCCH} + \mathrm{H}. \end{aligned} $$

Considering the deuterated analogues of these reactions introduces diverse questions, for example whether the CN reactions proceed without changing the methyl radical or lead to some scrambling of the hydrogen atoms in a quasi-stationary intermediate followed by different reaction channels. The first assumption would lead to the reactions

CN + CH 2 DCCH → CH 2 DC 3 N + H , $$ \begin{aligned} \mathrm{CN} + \mathrm{CH_2DCCH}&\rightarrow \mathrm{CH_2DC_3N} + \mathrm{H}, \end{aligned} $$(1)

CN + CH 3 CCD → CH 3 C 3 N + D , $$ \begin{aligned} \mathrm{CN} + \mathrm{CH_3CCD}&\rightarrow \mathrm{CH_3C_3N} + \mathrm{D}, \end{aligned} $$(2)

whereas the second option would introduce an additional reaction channel:

CN + CH 2 DCCH → CH 3 C 3 N + D , $$ \begin{aligned} \mathrm{CN} + \mathrm{CH_2DCCH}&\rightarrow \mathrm{CH_3C_3N} + \mathrm{D}, \end{aligned} $$(3)

CN + CH 3 CCD → CH 2 DC 3 N + H . $$ \begin{aligned} \mathrm{CN} + \mathrm{CH_3CCD}&\rightarrow \mathrm{CH_2DC_3N} + \mathrm{H}. \end{aligned} $$(4)

The case of the reactions involving C2H (C2D) is even more uncertain as an additional H(D) atom is involved, which leads to a complementary reaction channel:

C 2 D + CH 3 CCH → CH 3 C 4 H + D , $$ \begin{aligned} \mathrm{C_2D} + \mathrm{CH_3CCH}&\rightarrow \mathrm{CH_3C_4H} + \mathrm{D}, \end{aligned} $$(5)

→ CH 3 C 4 D + H , $$ &\rightarrow \mathrm{CH_3C_4D} + \mathrm{H}, \end{aligned} $$(6)

→ CH 2 DC 4 H + H ; $$ \begin{aligned} &\rightarrow \mathrm{CH_2DC_4H} + \mathrm{H}; \end{aligned} $$(7)

C 2 H + CH 2 DCCH → CH 2 DC 4 H + H , $$ \begin{aligned} \mathrm{C_2H} + \mathrm{CH_2DCCH}&\rightarrow \mathrm{CH_2DC_4H} + \mathrm{H}, \end{aligned} $$(8)

→ CH 3 C 4 D + H , $$ \begin{aligned} &\rightarrow \mathrm{CH_3C_4D} + \mathrm{H}, \end{aligned} $$(9)

→ CH 3 C 4 H + D . $$ \begin{aligned} &\rightarrow \mathrm{CH_3C_4H} + \mathrm{D}. \end{aligned} $$(10)

Similar questions arise in the deuteration mechanisms involving deuteron transfer initiated by reactions with abundant deuterated molecular ions such as H2D+ and DCO+. As an example, the products of the H2D+ + CH3C3N reaction could be CH3C3ND+ + H2, if the reaction proceeds directly, or also CH2DC3NH+ + H2, if an intermediate complex is formed3 The following step in forming deuterated methylcyanoacetylene entails dissociative recombination of the molecular ion, where an additional question arises on the branching ratios of the reaction CH2DC3NH+ + e− → CH2DC3N + H and/or →CH3C3N + D. These few examples show the multiple issues that emerge when analysing the potential chemical processes at work. We considered two different approaches. In case A we assume that the methyl radical and its deuterated form keep their structure (e.g., as in reactions 1, 2, 5, 6, 8), whereas case B involves a scrambling of the H and D atoms followed by the formation of the different products (e.g., as in reactions 3, 4, 7, 9, 10). These hypotheses are implemented in a chemical model including 320 species and more than 9000 gas-phase reactions built from previous studies (Cabezas et al. 2021; Agúndez et al. 2021b). We display in Table 4 the corresponding steady-state ratios obtained in a model adapted to TMC-1 conditions, n(H2) = 4 × 104 cm−3, T = 10 K, ζ = 1.3 × 10−17 s−1, as in Cabezas et al. (2021)4. We first note the significant sensitivity of the deuterium ratio for CH3C3N and CH3C4H to the reactivity assumptions. A low deuterium ratio, close to the observed value of CH3C3N, is favoured in the full scrambling approximation. The upper limits found for CH3C4H/CH2DC4H and CH3C4H/CH3C4D, on the other hand, are better reproduced when reactions 5, 6, and 8 are the only channels in the C2H (C2D) reactions. Whereas the other observed deuterium ratios are reasonably reproduced within a factor of 2, a significant discrepancy is still obtained for methylacetylene, CH3CCH, as already noted in Cabezas et al. (2021) and Agúndez et al. (2021b). This feature arises because the reaction of CH3CCH with H 3 + $ \rm{H}_3^+ $ (and supposedly H2D+) leads to the break up of CH3CCH into c-C3 H 3 + $ \rm{H}_3^+ $ and l-C3 H 3 + $ \rm{H}_3^+ $ (c-C3H2D+ and l-C3H2D+) rather than to C3 H 5 + $ \rm{H}_5^+ $ or C3H4D+ (CH3 CCH 2 + $ \rm{CCH}_2^+ $, CH2 DCCH 2 + $ \rm{DCCH}_2^+ $, CH3CCHD+), as found in the experimental study of Milligan et al. (2002). We did not try to include additional deuterium exchange reactions, in the absence of any theoretical or experimental information.

Table 4.

Deuteration enhancement in TMC-1 for detected molecules compared to our gas-phase chemical model.

We conclude this section by acknowledging the possible gas-phase deuteration mechanisms of cyanomethylacetylene mediated by deuteron transfer reactions with species such as H2D+ and DCO+, among other deuterated cations in low-temperature conditions, but point out the substantial uncertainties involved in the different possible reactions, so that a detailed comparison between observations and chemical modeling appears elusive. A theoretical analysis of the intermolecular interaction potentials involved in the approach of the different reactants would help to validate the various reaction mechanisms.

5. Conclusions

We have detected and unambiguously identified CH2DC3N, a new deuterated compound, in TMC-1 thanks to highly sensitive space observations of 15 different transitions and to the associated theoretical considerations and quantum mechanical calculations. Spectroscopic constants are also provided for that compound and the 13C and 15N substitutes, which should help to study those species in the laboratory as well. The observed deuterium fractions are further compared to a gas-phase model, which, despite significant uncertainties, accounts within a factor of two for the different values, except for CH3CCH. Further experimental or theoretical studies are welcome.

3

The channel CH3C3NH+ + HD is present in both cases as well.

4

The elemental values (i.e., O/H = 8  ×  10−6, C/O = 0.75, N/O = 0.5) correspond to a carbon-rich environment.

Acknowledgments

We thank ERC for funding through grant ERC-2013-Syg-610256-NANOCOSMOS. The Spanish authors thank Ministerio de Ciencia e Innovación for funding support through project AYA2016-75066-C2-1-P, PID2019-106235GB-I00 and PID2019-107115GB-C21/AEI/10.13039/501100011033. MA thanks Ministerio de Ciencia e Innovación for grant RyC-2014-16277. ER acknowledges the support of the Programme National ‘Physique et Chimie du Milieu Interstellaire’ (PCMI) of CNRS/INSU with INC/INP co-funded by CEA and CNES. Several kinetic data we used have been taken from the online databases KIDA (Wakelam et al. 2012, http://kida.obs.u-bordeaux1.fr) and UMIST2012 (McElroy et al. 2013, http://udfa.ajmarkwick.net).

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All Tables

Table 1.

Observed line parameters for CH2DC3N in TMC-1.

In the text
Table 2.

Observationally derived and theoretical spectroscopic parameters (in MHz) for CH2DC3N.

In the text
Table 3.

Predicted spectroscopic constants (in MHz) for isotopic species of CH3C3N and CH3C4H.

In the text
Table 4.

Deuteration enhancement in TMC-1 for detected molecules compared to our gas-phase chemical model.

In the text

All Figures

thumbnail Fig. 1.

Observed lines of CH2DC3N in TMC-1 in the 31.0–50.4 GHz range. Frequencies and line parameters are given in Table 1. Quantum numbers for the observed transitions are indicated in each panel. The red line shows the synthetic spectrum computed for a rotational temperature of 8 K and a column density of 8 × 1010 cm−2 (see text). The additional components seen in the synthetic spectrum close to the Ka = 0 components are the Ka = 2 rotational transitions. Blanked channels correspond to negative features created in the folding of the frequency switching data. The label ‘U’ corresponds to features above 4σ.
In the text

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