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A&A
Volume 655, November 2021
Article Number L1
Number of page(s) 30
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202142226
Published online 09 November 2021

© ESO 2021

1. Introduction

The QUIJOTE1 (Cernicharo et al. 2021a) and the GOTHAM2 (McGuire et al. 2018) line surveys are providing exciting results of the abundance of hydrocarbons and their ethynyl and cyano derivatives in the cold pre-stellar core Taurus Molecular Cloud 1 (TMC-1). Species such as the propargyl radical (CH2CCH), vinylacetylene (CH2CHCCH), ethynylallene (H2CCCHCCH), and cyclic hydrocarbons such as cyclopentadiene (c-C5H6), o-benzyne (C6H4), and indene (c-C9H8) (Agúndez et al. 2021a; Cernicharo et al. 2021a,b,c,d) have been detected using the QUIJOTE line survey through a line-by-line identification process. Using spectral stacking techniques, the GOTHAM line survey has provided the detections of the cyano derivatives of cyclopentadiene, benzene, and naphthalene (McGuire et al. 2018, 2021; Lee et al. 2021). These results reveal a new and unexpected chemistry that requires a profound revision of the chemical processes at work in cold dark clouds such as TMC-1. In order to provide an adequate reference for chemical models, reliable molecular abundances need to be obtained. Moreover, observing distinct ethynyl and cyano derivatives of hydrocarbons can provide important constraints on the reactivity of CCH and CN radicals with unsaturated acyclic and cyclic hydrocarbons.

In this Letter we report the discovery of two isomers of ethynyl cyclopentadiene (c-C5H5CCH) and the tentative detection of ethynyl benzene (C6H5CCH, hereinafter referred to as EBZ) towards TMC-1. From our data we also derive column densities for the corresponding cyano derivatives of benzene and cyclopentadiene, previously detected by McGuire et al. (2018), McCarthy et al. (2021), and Lee et al. (2021), providing a rigorous confirmation of the presence of these species in TMC-1 based on a line-by-line detection procedure and a coherent and homogeneous set of abundances for the CCH and CN derivatives of cyclopentadiene and benzene.

2. Observations

New receivers, built within the Nanocosmos project3 and installed at the Yebes 40 m radio telescope, were used for the observations of TMC-1 (αJ2000 = 4h41m41.9s and ). A detailed description of the system is given by Tercero et al. (2021). The receiver consists of two cold high electron mobility transistor amplifiers that cover the 31.0–50.3 GHz band with horizontal and vertical polarizations. Receiver temperatures in the runs conducted in 2020 vary from 22 K at 32 GHz to 42 K at 50 GHz. Some power adaptation in the down-conversion chains have reduced the receiver temperatures in 2021 to 16 K at 32 GHz and 30 K at 50 GHz. The backends are 2 × 8 × 2.5 GHz fast Fourier transform spectrometers with a spectral resolution of 38.15 kHz, providing the whole coverage of the Q band in both polarizations. All observations were performed in the frequency switching mode with frequency throws of 8 and 10 MHz. The main beam efficiency varies from 0.6 at 32 GHz to 0.43 at 50 GHz. Pointing corrections were derived from nearby quasars and SiO masers, and errors were always within 2–3″. The telescope beam size is 56″ and 31″ at 31 and 50 GHz, respectively. The intensity scale used in this work, antenna temperature (), was calibrated using two absorbers at different temperatures and the atmospheric transmission model ATM (Cernicharo 1985; Pardo et al. 2001). Calibration uncertainties were adopted to be 10%. All data were analysed using the GILDAS package4. Details of the QUIJOTE line survey are provided by Cernicharo et al. (2021d). The 1σ sensitivity of the survey varies between 0.17 and 0.30 mK between 31 and 50 GHz.

3. Detection of cycles in TMC-1

Line identification in this work was done using the catalogues MADEX (Cernicharo 2012), CDMS (Müller et al. 2005), and JPL (Pickett et al. 1998). By September 2021, the MADEX code contained 6377 spectral entries corresponding to the ground and vibrationally excited states, together with the corresponding isotopologues, of 1696 molecules.

The recent detection of cyclopentadiene (Cernicharo et al. 2021a) and of its cyano derivatives (McCarthy et al. 2021; Lee et al. 2021) suggests that other derivatives of cyclopentadiene could be present in this source, in particular the ethynyl ones, 1- and 2-ethynyl-1,3-cyclopentadiene (hereinafter referred to as 1-ECP and 2-ECP, respectively; see Fig. 1). These two isomers of ethynyl cyclopentadiene were observed in the laboratory by McCarthy et al. (2020). We used these data to fit the rotational and distortion constants in order to predict the frequencies of their rotational transitions within the Q band (see Sects. 3.1.1 and 3.1.2). The calculated uncertainties for these transitions are 10–25 kHz (0.1–0.3 km s−1).

thumbnail Fig. 1.

Scheme of the two lowest energy isomers of ethynyl cyclopentadiene.

3.1. The isomers of ethynyl cyclopentadiene

The substitution of a hydrogen atom by an ethynyl (CCH) group in cyclopentadiene (c-C5H6) yields three possible isomers of ethynyl cyclopentadiene. Quantum chemical calculations at the MP2/6-311++G(d,p) level of theory (Møller & Plesset 1934; Frisch et al. 1984) predict 1-ECP as the most stable isomer, followed closely by 2-ECP (6 kJ mol−1) and then 5-ethynyl-1,3-cyclopentadiene, whose energy is far higher (by 27 kJ mol−1). The structures of the two isomers 1-ECP and 2-ECP are shown in Fig. 1. The two isomers are moderately polar, with dipole moments along their a- and b-inertial axes. The predicted μa values for 1-ECP and 2-ECP are 0.81 and 1.11 D, respectively, while the μb values are 0.32 and 0.37 D, respectively, in good agreement with previous calculations (Lee & McCarthy 2019). Both isomers were observed in the laboratory by McCarthy et al. (2020) (we note that 2-ECP was named 5-ethynyl-1,3-cyclopentadiene in their work). The low value of μb for the two isomers results in b-type transitions that are around ten times weaker than the a-type ones. In the following, we start our search with the isomer with the largest dipole moment, namely 2-ECP.

3.1.1. Detection of 2-ethynyl-1,3-cyclopentadiene (2-ECP)

The laboratory data for 2-ECP, the isomer with the largest dipole moment along the a axis, cover the frequency range 6.5–25.8 GHz with Jmax = 8 (McCarthy et al. 2020). McCarthy et al. (2020) quote an uncertainty on their frequencies of 2 kHz. However, their own fit provides a standard deviation of 4.5 kHz. We thus assigned an uncertainty of 4 kHz to all their measured frequencies, except for the 71, 6–61, 5 line, for which they quote an uncertainty of 10 kHz. The resulting rotational and distortion constants with the new uncertainties do not show a significant variation with respect to their constants. However, the standard deviation of the fit improves to 3 kHz if the distortion constant δJ is also included in the fit (see Table A.3). This new set of constants was used to predict the frequencies of the rotational lines of 2-ECP in the 31–50 GHz domain.

A total of 24 a-type lines of 2-ECP were detected in TMC-1 above the 3σ level with the QUIJOTE line survey. Some of them are shown in Fig. 2. The derived line parameters are given in Appendix A (see Table A.1). A fit to the observed line profiles assuming a source diameter of 80″ (Fossé et al. 2001) provides a rotational temperature of 9.0 ± 1.0 K and a column density of (1.4 ± 0.2) × 1012 cm−2. The synthetic spectra are compared with observations in Fig. 2 (red line). With the adopted source diameter, the molecular emission fills the main beam of the telescope at all observed frequencies.

thumbnail Fig. 2.

Subset of the observed lines of 2-ECP in the 31–50 GHz frequency range towards TMC-1. Line parameters for the complete list of detected lines of 2-ECP are given in Table A.1. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of 5.83 km s−1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The red line shows the synthetic spectrum obtained from a fit to the observed line profiles, which provides Tr = 9 ± 1 K and N(2-ECP) = (1.4 ± 0.2) × 1012 cm−2. The rotational quantum numbers are indicated in each panel. Blanked channels correspond to negative features produced in the folding of the frequency switching data.

All observed lines of 2-ECP correspond to values of J between 9–14 and Ka ≤ 3; hence, the associated upper level energies cover the range 8–20 K. Rotational temperatures below 8 K underestimate the emission of the transitions arising from the higher energy levels. Nevertheless, transitions involving energy levels between 8 and 12 K are not very sensitive to the rotational temperature and can be fitted with rotational temperatures as low as 6 K with a modest variation in the column density. This effect is discussed in detail in Appendix A of Cernicharo et al. (2021e). A similar situation has been found for the cyano derivatives of cyclopentadiene and benzene (see our Appendices B and C), which have considerably larger dipole moments. The near thermalization of the rotational levels of the derivatives of cyclopentadiene and benzene is most likely due the large collisional rates we could expect for these molecules, which exhibit a much larger geometrical cross-section than linear molecules such as HC5N and HC7N, for which rotational temperatures around 8–9 K have also been found (Cernicharo et al. 2020). A similar rotational temperature of ∼9 K has been derived for indene and cyclopentadiene (Cernicharo et al. 2021a).

3.1.2. Detection of 1-ethynyl-1,3-cyclopentadiene (1-ECP)

The laboratory data for 1-ECP (McCarthy et al. 2020) cover the frequency range 6.5–24.9 GHz with Jmax = 7 and Ka ≤ 3. McCarthy et al. (2020) quote a standard deviation for their fit of 2.7 kHz. However, it is possible to reduce this value to 1.1 kHz by fitting the distortion constant δK. The new rotational and distortion constants, which are given in Table A.3, were used to predict the frequencies of the rotational lines of 1-ECP in the 31–50 GHz domain.

If the abundances of the two isomers were identical, the a-type lines of 1-ECP would be 1.9 times weaker that those of 2-ECP (the squared ratio of the a component of the dipole moment). Hence, we expect to detect only the strongest transitions of 1-ECP. A total of 14 lines of 1-ECP were detected in TMC-1 above the 3σ level with the current sensitivity of the QUIJOTE line survey. Some of them are shown in Fig. A.1. The derived line parameters are given in Appendix A (see Table A.2). A fit to the observed line profiles assuming the same source size (80″) and rotational temperature (9 K) as for 2-ECP provides a column density of (2.0 ± 0.4) × 1012 cm−2. Therefore, the isomer 2-ECP is a factor of 1.4 ± 0.5 more abundant than 1-ECP.

The detection of the two isomers of ethynyl cyclopentadiene is robust since it is based on the detection of a significant number of individual transitions. Taking into account the column density of (1.2 ± 0.3) × 1013 cm−2 derived for cyclopentadiene (Cernicharo et al. 2021a), 1-ECP and 2-ECP are less abundant than cyclopentadiene by factors of 6.0 ± 2.5 and 8.6 ± 3.3, respectively.

Finally, improved molecular constants for 1-ECP and 2-ECP resulting from a merged fit to the laboratory data and the observed frequencies in TMC-1 are provided in Appendix A.1 (see Table A.3).

3.2. Ethynyl benzene

Laboratory spectroscopy for EBZ, C6H5CCH, has been provided by different authors covering frequencies up to 340 GHz, J up to 140, and Ka up to 48 (Cox et al. 1975; Dreizler et al. 2004; Kisiel & Kraśnicki 2010). Hence, the frequency predictions in the range of our line survey are rather accurate, with calculated uncertainties below 1 kHz. The dipole moment of the molecule is low, 0.66 D (Cox et al. 1975). Hence, we could expect weak emission lines in our data. The molecule has two pairs of identical hydrogen nuclei, which introduces an ortho and para spin statistic. Ortho and para levels correspond to Ka even and odd, respectively. The ratio of statistical weights is 5/3. The lowest energy para level (11, 1) is 0.33 K above the ground ortho level (00, 0). Due to the low dipole moment and the ∼1.7 factor in the statistical weights, we do not expect to have enough sensitivity in our data to detect the para transitions. A quick examination of all Ka = 0 and 2 lines reveals only four lines at the 3σ limit of the survey. All explored lines are summarized in Appendix A.2 (see Table A.6). The four detected lines are shown in Fig. A.2. With this limited number of lines, it is not possible to claim a detection. Tentatively, we derive a column density of (2.5 ± 0.4) × 1012 cm−2 for an assumed rotational temperature of 9 K and a source size of 80″. Taking into account the density of unknown features, we consider that a stacking of our data is hazardous and that a definitive detection has to wait for the improvement of the QUIJOTE line survey. Assuming that the derived column density is a 3σ limit, the relative abundance of ethynyl cyclopentadiene (the two isomers) and EBZ is ≥1.4, which suggests that their potential cyclopentadiene and benzene precursors have abundances of the same order in TMC-1.

3.3. Cyano derivatives of cyclopentadiene and benzene

Two cyano derivatives of cyclopentadiene (hereinafter referred to as 1-CCP and 2-CCP; see Appendix B) were detected in TMC-1 using stacking techniques by McCarthy et al. (2021) and Lee et al. (2021). A few individual lines of 1-CCP were reported by Lee et al. (2021). There are some discrepancies between the column densities reported by these authors for 1-CCP. McCarthy et al. (2021) derive N(1-CCP) = (1.44 ± 0.17) × 1012 cm−2 and Trot = 6.0 ± 0.3 K, while Lee et al. (2021) find a column density of (8.3 ± 0.1) × 1011 cm−2 and a rotational temperature of 6.00 ± 0.03 K. For 2-CCP, Lee et al. (2021) derive a column density of 1.9 × 1011 cm−2. In order to provide a coherent and homogeneous set of column densities, we analyse the lines of the two isomers in Appendix B. The observed lines of 1-CCP are shown in Fig. B.1 and those of 2-CCP in Fig. B.2. Line parameters for the two species are given in Tables B.1 and B.2, respectively. We derive a rotational temperature of 9.0 ± 1.0 K for both species and column densities of (3.1 ± 0.3) × 1011 cm−2 and (1.3 ± 0.2) × 1011 cm−2 for 1-CCP and 2-CCP, respectively. The isomer 1-CCP is 2.4 times more abundant than 2-CCP, which is twice lower than the abundance ratio found by Lee et al. (2021). The discrepancies with previous works are discussed in Appendix B.

Benzonitrile, C6H5CN, was previously detected towards TMC-1 by McGuire et al. (2018) through stacking techniques and some well-detected individual lines. They obtain a rotational temperature of 7 K and a column density of 4 × 1011 cm−2. In a more recent work, Burkhardt et al. (2021) derive a column density of 1.6 × 1012 cm−2 (i.e., a factor of four higher than previously reported) and a rotational temperature of 6.1 ± 0.3 K. In Appendix C we discuss the 100 individual lines of this species detected with high sensitivity with QUIJOTE’s data. They are shown in Figs. C.1–C.4. We obtain a rotational temperature of 9.0 ± 0.5 K and a total column density of (1.2 ± 0.1) × 1012 cm−2. A rotational temperature of 6 K cannot explain the observed emission of lines with Ka ≥ 4 (see the caption of Fig. C.1 and Appendix C).

All column densities derived in this work are summarized in Table 1.

Table 1.

Abundances of ethynyl and cyano species in TMC-1.

4. Chemistry of cycles in TMC-1

It is remarkable that, given their chemical complexity, the CCH and CN derivatives of cyclopentadiene and benzene are observed with relatively large abundances in TMC-1. To understand how these species can be formed, we built a chemical model similar to that used in previous recent studies of TMC-1 (e.g., Cernicharo et al. 2021d). Briefly, we adopted typical conditions of cold dark clouds – namely, a gas 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 the so-called low-metal elemental abundances (e.g., Agúndez & Wakelam 2013) – with the exception of oxygen, for which we decreased the abundance (see below). The core of the chemical network is the RATE12 network of the UMIST database (McElroy et al. 2013), with updates from Loison et al. (2015), Marcelino et al. (2021); Agúndez et al. (2021a,b), and Cernicharo et al. (2021d). We also included a specific chemistry for the CCH and CN derivatives of c-C5H6 and C6H6. We assumed that they are destroyed through reactions with C, C+, and H+. The chemical scheme of formation of these species is based on neutral-neutral reactions and is discussed in detail in Appendix D. Briefly, reactions of CCH and CN with c-C5H6 and C6H6 lead to the CCH/CN derivatives of each cycle (Balucani et al. 1999; Jones et al. 2010). The formation of the precursor hydrocarbon cycles c-C5H6 and C6H6 relies on reactions between small hydrocarbon radicals and butadiene (CH2CHCHCH2), which acts as a key species that opens the chemistry to hydrocarbon cycles at 10 K (He et al. 2020a; Jones et al. 2011). Additional routes to the CCH/CN derivatives of c-C5H6 involve reactions of C3H/C2N with butadiene. The chemical scheme thus involves essentially neutral-neutral reactions, with the exception of benzene, which has been hypothesized to also be formed by a route involving ion-neutral reactions that result in the precursor ion C6 (see e.g., Agúndez et al. 2021a).

If we set the gas-phase elemental abundance of oxygen to O/H = 3.3 × 10−4 (C/O = 0.55), the calculated peak abundances of all CCH/CN derivatives of c-C5H6 and C6H6 remain two to three orders of magnitude below the observed values. If the gas-phase elemental abundance of oxygen is decreased so that C/O = 1 (O/H = 1.8 × 10−4), then the agreement between the chemical model and the observations is much better (see Fig. 3). Producing c-C5H6 with an abundance as high as ∼10−9 relative to H2 and CCH/CN derivatives of c-C5H6 and C6H6 with abundances in the range 10−11–10−10 seems to require a C/O elemental gas-phase ratio close to that in TMC-1. This is in agreement with a recent study on the elemental abundances in TMC-1 (Fuente et al. 2019).

thumbnail Fig. 3.

Calculated abundances of CCH and CN derivatives of c-C5H6 and C6H6 (right panel) and of their precursors (left panel). The horizontal dotted lines correspond to the abundances observed in TMC-1.

A significant failure of the chemical model concerns the abundance ratios between the CCH and the CN derivatives of both c-C5H6 and C6H6, which are below one, while the observed ones are above unity. This is ultimately caused by the overabundance of CN in the chemical model, a feature also found in previous chemical models of cold dark clouds (e.g., Agúndez & Wakelam 2013; Daranlot et al. 2013), which deserves a dedicated study. As a consequence, the calculated CCH/CN ratio is < 1 at any time (see the left panel in Fig. 3), while the observed CCH/CN ratio in TMC-1 is ∼10 (Pratap et al. 1997). This fact translates to the CCH and CN derivatives of c-C5H6 and C6H6, for which calculated CCH/CN ratios are < 1, while the observed ones are > 1 (see the right panel in Fig. 3). It is worth noting that the CCH/CN ratio observed for cyclopentadiene, (1-ECP + 2-ECP)/(1-CCP + 2-CCP) = 7.7, is close to the CCH/CN ratio itself, ∼ 10 (Pratap et al. 1997), while the CCH/CN ratio for benzene, C6H5CCH/C6H5CN ≤ 2.1, is significantly lower. This fact suggests that additional routes to the reaction CN + C6H6 could form C6H5CN in TMC-1. In the case of the CCH and CN derivatives of C3H4, the CCH/CN ratio is 3.5 (Marcelino et al. 2021; Cernicharo et al. 2021b), which is in between the values found for the derivatives of c-C5H6 and C6H6.

Concerning the abundance ratios between different isomers, for c-C5H5CCH the two isomers 1-ECP and 2-ECP are observed with similar abundances. However, the chemical model calculates 1-ECP to be much more abundant than 2-ECP (see the right panel in Fig. 3) because, in addition to the reaction C2H + c-C5H6, which produces the two isomers, the reaction between C3H and butadiene yields 1-ECP but not 2-ECP (see Appendix D). Similarly, for c-C5H5CN the chemical model calculates a 1-CCP/2-CCP ratio higher than observed because 1-CCP has more formation routes. We note, however, that the branching ratios adopted in the chemical model for the production of the different isomers are uncertain.

A success of the chemical model is that it correctly reproduces the abundance of cyclopentadiene. If the chemical scheme discussed in Appendix D is complete, the abundances of the non-polar molecules benzene and butadiene in TMC-1 should be of the order of the calculated ones. That is, benzene should be present with an abundance of some 10−9 relative to H2, similar to that of cyclopentadiene, while butadiene should be around ten times more abundant than benzene (see the left panel in Fig. 3). If butadiene is that abundant, it could be detected indirectly, for example through its protonated form or a polar derivative such as C4H5CN.

5. Conclusions

We have reported the detection in TMC-1 of two isomers of ethynyl cyclopentadiene (c-C5H5CCH), namely 1-ECP and 2-ECP, and the tentative detection of EBZ (C6H5CCH). In addition, we report an exhaustive line-by-line detection of the cyano derivatives of cyclopentadiene and benzene (c-C5H5CN and C6H5CN). This allowed us to provide a coherent set of column densities for the various CCH and CN derivatives of cyclopentadiene and benzene in TMC-1. A chemical model that includes chemical routes to these cycles based on neutral-neutral reactions is reasonably successful in explaining the order of magnitude of the observed abundances. It is predicted that benzene should have an abundance similar to that of cyclopentadiene in TMC-1.

1

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

2

GBT Observations of TMC-1: Hunting Aromatic Molecules.

Acknowledgments

We thank ERC for funding through grant ERC-2013-Syg-610256-NANOCOSMOS and Ministerio de Ciencia e Innovación of Spain (MICIU) for funding support through projects PID2019-106110GB-I00, PID2019-107115GB-C21, and PID2019-106235GB-I00. M.A. thanks MICIU for grant RyC-2014-16277.

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Appendix A: Observed lines of 1-ECP, 2-ECP, and EBZ

Line parameters were derived from a Gaussian fit to the observed lines using the GILDAS package. A velocity coverage of ± 15 km s−1 was selected for each line. Observed frequencies were derived assuming a local standard of rest velocity of 5.83 km s−1(Cernicharo et al. 2020). The predicted frequencies in MADEX (Cernicharo 2012), which arise from a fit to the laboratory data of McCarthy et al. (2020), agree within 5-30 kHz with the observed ones. These differences are always below 2×σ × Δν, where Δν is the estimated frequency uncertainty of the observed lines. The derived line parameters for 2-ECP and 1-ECP are given in Tables A.1 and A.2, respectively. Selected lines of 1-ECP and 2-ECP are shown in Figs. A.1 and 2, respectively. A merged fit to the laboratory and TMC-1 frequencies is discussed in Appendix A.1.

thumbnail Fig. A.1.

Observed lines of 1-ECP in the 31-50 GHz frequency range towards TMC-1. Line parameters for the complete list of detected lines of 1-ECP are given in Table A.2. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of 5.83 km s−1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The red line shows the synthetic spectrum obtained for an assumed Tr of 9 K and N(1-ECP) = (2.0 ± 0.4) × 1012 cm−2. The rotational quantum numbers are indicated in each panel. Blanked channels correspond to negative features produced in the folding of the frequency switching data.

Table A.1.

Observed line parameters of 2-ECP.

Table A.2.

Observed line parameters of 1-ECP.

A.1. Improved rotational constants for 1-ECP and 2-ECP

The observed lines of 2-ECP (see Table A.1) and 1-ECP (see Table A.2) were merged with the laboratory data (McCarthy et al. 2020) to provide a new set of rotational and distortion constants. They are given in Table A.3 and can be used to predict the frequencies of the rotational transitions for the two observed isomers of cyclopentadiene up to 50 GHz with an accuracy better than 15 kHz (Ka ≤ 3). The calculated and the observed-minus-calculated frequencies for 2-ECP and 1-ECP are given in Tables A.4 and A.5, respectively.

Table A.3.

Rotational and distortion constants for 1-ECP and 2-ECP.

Table A.4.

Observed and calculated line frequencies for 2-ECP.

Table A.5.

Observed and calculated line frequencies for 1-ECP.

Table A.6.

Observed line parameters of EBZ (ethynyl benzene).

A.2. Ethynyl benzene

For EBZ, the searched lines are given in Table A.6. Only four lines appear at the 3σ level and are shown in Fig. A.2. The estimated column density is discussed in Sect. 3.2.

thumbnail Fig. A.2.

Four lines of EBZ (C6H5CCH) observed in the 31-50 GHz frequency range towards TMC-1. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of 5.83 km s−1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The red line shows the synthetic spectrum obtained from a fit to the observed line profiles adopting a rotational temperature of 9 K, which provides N(EBZ) = (2.5 ± 0.4) × 1012 cm−2 (see Sect. 3.2). The rotational quantum numbers are indicated in each panel. Blanked channels correspond to negative features produced in the folding of the frequency switching data.

Appendix B: Observed lines of the two isomers of cyano cyclopentadiene (1-CCP and 2-CCP)

Two isomers of cyano cyclopentadiene, 1-CCP and 2-CCP, have been observed in the laboratory by different authors (Ford & Seitzman 1978; Sakaizumi et al. 1987; McCarthy et al. 2020). The structure of these two isomers is similar to those of 1-ECP and 2-ECP when the CCH group is changed by CN (see Fig. 1). Dipole moments for both species were measured by Ford & Seitzman (1978) and Sakaizumi et al. (1987), who provided values of μa = 4.15 ± 0.15 and 4.36 ± 0.25 D for 1-CCP and 2-CCP, respectively. Transitions of b type are expected to be much weaker due to the measured low value of the dipole moment along the b axis of 0.27 and 0.77 D, respectively (Sakaizumi et al. 1987).

The accuracy of the measurements in McCarthy et al. (2020) is much higher than those of the early microwave measurements of the two isomers, and their rotational and distortion constants, improved by additional measurements by Lee et al. (2021), were used to predict their frequencies in the domain of our QUIJOTE line survey. The two isomers were detected through stacking techniques towards TMC-1 by McCarthy et al. (2021) and Lee et al. (2021). However, as mentioned in Sect. 3.3 and below, the column densities they provide are rather uncertain, with a variation for N(1-CCP) of a factor of four between both publications. In order to provide a coherent set of column densities for the ethynyl and cyano derivatives of cyclopentadiene and benzene, we analysed their lines within our survey.

For 1-CCP, the published laboratory frequencies of McCarthy et al. (2020) cover the range 7.3-29.9 GHz, with Jmax = 9 and Ka≤3. The rotational constants provided by Lee et al. (2021) were derived with unpublished frequencies up to 36 GHz and provide frequency predictions for the 31-50 GHz domain with uncertainties ≤15 kHz. We observed 47 lines of 1-CCP with QUIJOTE, and their line parameters are given in Table B.1. Selected lines for 1-CCP are shown in Fig. B.1. The red and blue lines in Fig. B.1 show the synthetic spectrum for a column density of 3.1 × 1011 cm−2 and rotational temperatures of 9 and 6 K, respectively. It is clear, as it is robustly derived for benzonitrile in Appendix C, that the rotational temperature has to be close to 9 K in order to fit the observed parameters for lines arising from levels with Ka = 3-4. A fit to the observed line profiles, assuming a source diameter of 80″ (Fossé et al. 2001), provides Trot = 9.0 ± 1.0 K and N(1-CCP) = (3.1 ± 0.3) × 1011 cm−2. The lines shown in Fig. B.1 that involve energy levels below 10 K can be fitted with a rotational temperature of 6 K with practically the same column density, a fact that has been discussed for other species by Cernicharo et al. (2021e); it arises from the small dependence of the line opacity with Trot for these transitions as well as from the balance between the term [Trot − 2.7] and the partition function, which is proportional to Trot for linear molecules and to T for asymmetric species. Only with observations that provide a reasonable range of upper level energies for the lines is it possible to derive an accurate rotational temperature and a reliable column density.

thumbnail Fig. B.1.

Selected lines of 1-CCP in the 31-50 GHz frequency range towards TMC-1. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of 5.83 km s−1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The red line shows the synthetic spectrum obtained from a fit to the observed line profiles, which provides Trot = 9.0 ± 1.0 K and N(1-CCP) = (3.1 ± 0.3) × 1011 cm−2(see Appendix B). The blue line shows the synthetic spectrum for the same column density and a rotational temperature of 6 K. The rotational quantum numbers are indicated in each panel. Blanked channels correspond to negative features produced in the folding of the frequency switching data.

Table B.1.

Observed lines of 1-CCP towards TMC-1.

McCarthy et al. (2021) reported for 1-CCP a rotational temperature of 6.0 ± 0.3 K and a total column density of (1.44 ± 0.17) × 1012 cm−2. However, Lee et al. (2021) revised these values, also using spectral data stacking, and derive a rotational temperature of 6.00 ± 0.03 K and a total column density of (8.27 ± 0.10) × 1011 cm−2 for the same isomer (i.e. a factor of three larger than our estimation and a factor of 1.7 lower than in McCarthy et al. 2021). Lee et al. (2021) argue that they used a larger partition function for 1-CCP than McCarthy et al. (2021), resulting in a substantially lower column density for this isomer. However, the effect should be the opposite as the column density is proportional to the partition function and to the rotational temperature, which is the same in both studies. The difference in the column density between both works is probably due to the highest data sensitivity in the Lee et al. (2021) analysis. Their results correspond to a fit to four velocity components with different spatial sizes. The lack of individual lines, together with the limited spatial coverage of their data (a single position with all information on spatial sizes arising from the beam size variation with frequency), renders the fit rather optimistic, and many of the derived parameters may be strongly correlated. The lack of spatial information also applies to the QUIJOTE data. However, a rotational temperature as low as 6 K cannot explain our 47 observed lines as shown in Fig. B.1. Although the stacking procedure is a powerful method for extracting information from spectral data below the noise level, the determination of physical parameters from line stacking has to be done with extreme caution. The determination of the column density and rotational temperature of 1-CCP by McCarthy et al. (2021) and Lee et al. (2021) seems to be a bit optimistic given their small quoted uncertainties. The differences between the values determined in this work and their studies could be related to the adopted source diameter and to the beam size of both telescopes.

For 2-CCP, the laboratory frequencies measured by McCarthy et al. (2020) cover the range 7.3-18.1 GHz, with Jmax = 5 and Ka≤2; hence, the predictions in the frequency domain 31-50 GHz could be more uncertain than those of 1-CCP. The derived line parameters for all the detected lines of 2-CCP are given in Table B.2, and the lines are shown in Fig. B.2. Differences between predicted and observed frequencies reach values as high as 100 kHz. However, using the rotational and distortion constants of Lee et al. (2021), the differences between predicted and observed frequencies never exceed 20 kHz. The range of energies covered by our observations is not large enough to allow a fit to the rotational temperature, and we adopted the value derived for 1-CCP (9 K). The derived column density for 2-CCP is (1.3 ± 0.2) × 1011, a value 1.5 times lower than that of Lee et al. (2021). Hence, the abundance ratio between 1-CCP and 2-CCP, derived from a significant number of lines for each isomer, is ∼2.4, a factor of two lower than the ratio derived by Lee et al. (2021).

thumbnail Fig. B.2.

Observed lines of 2-CCP in the 31-50 GHz frequency range towards TMC-1. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of 5.83 km s−1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The red line shows the synthetic spectrum obtained from a fit to the observed line profiles, which provides N(2-CCP) = (1.3 ± 0.2) × 1011 cm−2 for a rotational temperature of 9 K. (see Appendix B). The blue line shows the synthetic spectrum for the same column density and a rotational temperature of 6 K. The rotational quantum numbers are indicated in each panel. Blanked channels correspond to negative features produced in the folding of the frequency switching data.

Table B.2.

Observed lines of 2-CCP towards TMC-1.

We used the observed line frequencies in TMC-1 and those of the laboratory (McCarthy et al. 2020) to improve the rotational and distortion constants for both isomers (see Appendix B.1).

B.1. Improved rotational constants for 1-CCP and 2-CCP

The observed lines of 1-CCP (see Table B.1) and 2-CCP (see Table B.2) were merged with the laboratory data (McCarthy et al. 2020) to provide a new set of rotational and distortion constants. They are given in Table B.3 and can be used to predict the frequencies of the rotational transitions for the two observed isomers of cyano cyclopentadiene up to 50 GHz with an accuracy better than 15 kHz (Ka ≤ 3). The calculated and the observed-minus-calculated frequencies for 1-CCP and 2-CCP are given in Tables B.4 and B.5, respectively.

Table B.3.

Molecular constants for 1-CCP and 2-CCP.

Table B.4.

Observed and calculated line frequencies for 1-CCP.

Table B.5.

Observed and calculated line frequencies for 2-CCP.

Appendix C: Benzonitrile, C6H5CN

Benzonitrile, C6H5CN, was detected in TMC-1 by McGuire et al. (2018). Assuming a rotational temperature of 7 K, these authors derived a column density of 4 × 1011 cm−2. In a more recent work by Burkhardt et al. (2021), the total derived column density is 1.6 × 1012 cm−2 (i.e. a factor of four higher than previously reported), and the derived rotational temperature is 6.1 ± 0.3 K. We explored all lines of benzonitrile in our QUIJOTE line survey. A hundred lines were detected, some of which show hyperfine splitting. They are shown in Figs. C.1, C.2, C.3, and C.4. A fit to the observed lines, assuming the same source parameters as for ECP and CCP, provides a rotational temperature of 9.0 ± 0.5 K and a column density of (1.2 ± 0.1) × 1012 cm−2. The corresponding synthetic spectrum is shown by the red line in these figures. Adopting a lower rotational temperature of 6 K, and maintaining the same column density, we could reproduce the lines reasonably well with Ka≤3 (blue line in the figures). However, the synthetic line profiles for Trot = 6 K systematically fail to reproduce the lines involving Ka≥4. Definitively, the rotational temperature has to be close to 9 K (i.e. near the kinetic temperature of the cloud). This result is consistent with the rotational temperatures derived for indene (Cernicharo et al. 2021a) and for the ethynyl and cyano derivatives of cyclopentadiene, as discussed in Sects. 3.1 and 3.3. The small discrepancy between the column densities for benzonitrile derived from GOTHAM (1.6 × 1012 cm−2) and QUIJOTE (1.2 × 1012 cm−2) data is probably related to the difference in the beam size, the assumed and/or fitted source size, and the rotational temperature used in each set of data for this species.

thumbnail Fig. C.1.

Observed lines of C6H5CN in the 31-50 GHz frequency range towards TMC-1. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of 5.83 km s−1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The red line shows the synthetic spectrum obtained from a fit to the observed line profiles, which provides Tr = 9 ± 0.5 K and N(C6H5CN) = (1.2 ± 0.1) × 1012 cm−2. The blue line shows the synthetic spectrum for a rotational temperature of 6 K and a column density of 1.5 × 1012 cm−2. The rotational quantum numbers are indicated in each panel. Blanked channels correspond to negative features produced in the folding of the frequency switching data. While the blue line produces a very good agreement with the observed intensities for Ka≤4, for higher values of Ka it underestimates the observed intensities by a factor of two.

thumbnail Fig. C.2.

Same as Fig. C.1.

thumbnail Fig. C.3.

Same as Fig. C.1.

thumbnail Fig. C.4.

Same as Fig. C.1.

Appendix D: Chemical scheme

TMC-1 represents a prototype of a cold molecular cloud, that is, an interstellar environment characterized by low average gas temperatures of about 10 K and number densities of molecular hydrogen ranging from 104 to 106 cm3 s−1. In the gas phase, these low temperatures and pressures require exoergic bimolecular reactions of the type A + B → C + D to proceed without entrance barriers, such as rapid neutral-neutral reactions (Kaiser & Hansen 2021). With respect to the newly detected 1-ECP and 2-ECP (labelled as products p1 and p2 in Fig. D.1), which resemble the product C, and atomic hydrogen as the light counter fragment D, this involves indirect reactions accessing the C7H7 potential energy surface. This C7 hydrocarbon can be formed from the A plus B reactants via bimolecular collisions of C1-C6, C2-C5, and C3-C4 hydrocarbon species at various degrees of hydrogenation, as detailed via reactions [1]-[5], [6]-[12], and [13]-[20] (see Fig. D.1):

thumbnail Fig. D.1.

Chemical scheme of formation of c-C5H6 and C6H6 and their CCH and CN derivatives. Numbers correspond to the reactions discussed in Appendix D.

Table D.1.

Reactions included in the chemical scheme of formation of c-C5H6 and C6H6 and their CCH and CN derivatives.

[1] C + C6H7

[2] CH + C6H6

[3] CH2 +C6H5

[4] CH3 +C6H4

[5] CH4 + C6H3

[6] C2 + C5H7

[7] C2H + C5H6

[8] C2H2 + C5H5

[9] C2H3 + C5H4

[10] C2H4 + C5H3

[11] C2H5 + C5H2

[12] C2H6 + C5H

[13] C3 + C4H7

[14] C3H + C4H6

[15] C3H2 + C4H5

[16] C3H3 + C4H4

[17] C3H4 + C4H3

[18] C3H5 + C4H2

[19] C3H6 + C4H

[20] C3H7 + C4.

Among these reactions, multiple bimolecular reactions potentially leading to C7H7 isomers plus atomic hydrogen are either direct or follow indirect dynamics with entrance barriers; these processes are therefore closed under the physical conditions in TMC-1. In detail, reaction [5] does not lead to any C7H7 isomer since any doublet C6H3 radical abstracts a hydrogen atom from the closed shell methane (CH4) reactant, forming the methyl radical (CH3) plus C6H4 isomers through transition states located between 5 and 30 kJ mol−1 above the separated reactants (Kaiser et al. 2011, and therein references). The direct nature and exclusive hydrogen abstraction also prohibit reaction [12] (i.e. C5H radicals forming ethyl (C2H5) radicals plus C5H2 isomer), once again through barriers from 5 and 30 kJ mol−1. The remaining reactions are indirect, via C7H7 complex formation. Among them, reactions [8]–[10] as well as [16]-[18] involve entrance barriers of addition of the doublet radical to the carbon-carbon double and triple bonds of 90 - 144 kJ mol−1 (reaction [8]; da Silva et al. 2010), 8 - 40 kJ mol−1 (reactions [9] and [10]), 44 - 130 kJ mol−1 for the propargyl radical (C3H3) reaction with vinylacetylene (C4H4) (reaction [16]; da Silva 2017, and references therein), 8 - 40 kJ mol−1 (reaction [17]), and 29 - 104 kJ mol−1 for reaction [18] of the allyl radical (C3H5) with diacetylene (C4H2) (Bodi et al. 2015, and therein references). The remaining reactions have no entrance barriers.

Here, reaction [2] was explored under single-collision conditions in crossed molecular beams as well as computationally (He et al. 2020b). The results reveal a strong energy and hence temperature dependence of the branching ratios with acetylene (C2H2) and cyclopentadienyl (C5H5) formed almost exclusively at temperatures of 10 K. Both reactions [1] and [3] access the same surface through distinct barrier-less entrance channels of carbon atom addition to the cyclic C6H7 radical (reaction [1]) followed by ring opening and/or hydrogen migration of the collision complex and carbene (CH2)–phenyl radical (C6H5) recombination that leads to the benzyl radical (C6H5CH2). Although reaction [4] has never been explored experimentally or computationally, a barrier-less methyl (CH3)–o-benzyne (C6H4) reaction leads to the o-tolyl radical (C6H4CH3), which isomerizes through hydrogen shift to the benzyl radical (C6H5CH2). These reaction intermediates are coupled to the intermediates accessed through reaction [2]. Consequently, reactions [1]–[4] are not expected to lead to 1-ECP and 2-ECP isomers (C5H5CCH).

Reactions of the carbon clusters C2, C3, and C4 (reactions [6], [13], and [20]) have not been explored computationally or experimentally. Likewise, the co-reactants C5H7, C4H7, and C3H7 have not been included in any astrochemical model. Therefore, the actual effect on the production of 1-ECP and 2-ECP isomers is unknown. Although reaction [19] is barrier-less and both the propylene (C3H6) and butadiynyl reactants (C4H) have been observed in TMC-1, the reaction is not expected to lead to 1-ECP and 2-ECP. Recent crossed molecular beam and computational studies of the ethynyl (CCH) reaction with propylene (C3H6) revealed reaction dynamics dictated by ethynyl addition – hydrogen loss mechanisms, but not to the thermodynamically most stable cyclopentadiene (C5H6) isomer (Kaiser and Mebel, in preparation). Since butadiynyl (C4H) is isolobal to ethynyl (CCH) and hence can be considered as an ethynyl-substituted ethynyl radical, the dynamics of reaction [19] are not expected to form 1-ECP and 2-ECP, but rather butadiynyl-substituted propylene isomers. Further, the reactivities of the interstellar c-C3H2 isomer cyclopropenylidene and vinylidene carbene (H2CCC) with any doublet C4H5 radical are unknown as well. However, considering the molecular structures of the cyclic and carbene-type reactants, formation of 1-ECP and 2-ECP is unlikely.

Overall, these considerations leave us with reactions [7] and [14] as the most likely pathways to forming 1-ECP and 2-ECP in TMC-1. Here, reaction [7] represents an addition–hydrogen elimination process that leads to three distinct C5H5CCH isomers, p1 to p3 (see Fig. D.1). Crossed molecular beams merged with electronic structure calculations revealed that reactions of ethynyl radicals with unsaturated hydrocarbons are barrier-less, exoergic, and lead via ethynyl addition to the carbon-carbon triple or double bond followed by hydrogen atom loss to ethynyl-substituted hydrocarbons via molecular mass growth from the bottom up (Jones et al. 2011, and references therein). In the case of vinylacetylene (C4H4) and 1,3-butadiene (C4H6), the initial addition intermediates isomerize via hydrogen migration and cyclization, leading eventually to o-benzyne (C6H4) and benzene (C6H6), respectively (Fig. D.1; Zhang et al. 2011; Jones et al. 2011). Consequently, the addition of ethynyl to the chemically non-equivalent C1 and C2 carbon atoms of cyclopentadiene followed by hydrogen elimination is expected to lead to three distinct C5H5CCH isomers, among them the astronomically observed 1-ECP and 2-ECP (p1 and p2 in Fig. D.1).

Finally, reaction [14] is worth exploring. Once again, this reaction has not been studied computationally or experimentally. The linear propynylidyne radical (C3H) is ubiquitous in TMC-1. It can be considered as an ethynyl-substituted methylidyne (CH) radical. Considering that at 10 K methylidyne (CH) reacts with 1,3-butadiene (C4H6) to cyclopentadiene (C5H6), ethynyl-substituted methylidyne radicals might react to ethynyl-substituted cyclopentadiene isomers p1 and p3 in barrier-less, overall exoergic bimolecular neutral-neutral reactions (see Fig. D.1).

Overall, reactions [7] and [14] are plausible pathways to forming the ethynyl-substituted cyclopentadiene isomers p1 to p3 under the conditions present in TMC-1 (Fig. D.1).

All Tables

Table 1.

Abundances of ethynyl and cyano species in TMC-1.

In the text
Table A.1.

Observed line parameters of 2-ECP.

In the text
Table A.2.

Observed line parameters of 1-ECP.

In the text
Table A.3.

Rotational and distortion constants for 1-ECP and 2-ECP.

In the text
Table A.4.

Observed and calculated line frequencies for 2-ECP.

In the text
Table A.5.

Observed and calculated line frequencies for 1-ECP.

In the text
Table A.6.

Observed line parameters of EBZ (ethynyl benzene).

In the text
Table B.1.

Observed lines of 1-CCP towards TMC-1.

In the text
Table B.2.

Observed lines of 2-CCP towards TMC-1.

In the text
Table B.3.

Molecular constants for 1-CCP and 2-CCP.

In the text
Table B.4.

Observed and calculated line frequencies for 1-CCP.

In the text
Table B.5.

Observed and calculated line frequencies for 2-CCP.

In the text
Table D.1.

Reactions included in the chemical scheme of formation of c-C5H6 and C6H6 and their CCH and CN derivatives.

In the text

All Figures

thumbnail Fig. 1.

Scheme of the two lowest energy isomers of ethynyl cyclopentadiene.
In the text
thumbnail Fig. 2.

Subset of the observed lines of 2-ECP in the 31–50 GHz frequency range towards TMC-1. Line parameters for the complete list of detected lines of 2-ECP are given in Table A.1. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of 5.83 km s−1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The red line shows the synthetic spectrum obtained from a fit to the observed line profiles, which provides Tr = 9 ± 1 K and N(2-ECP) = (1.4 ± 0.2) × 1012 cm−2. The rotational quantum numbers are indicated in each panel. Blanked channels correspond to negative features produced in the folding of the frequency switching data.
In the text
thumbnail Fig. 3.

Calculated abundances of CCH and CN derivatives of c-C5H6 and C6H6 (right panel) and of their precursors (left panel). The horizontal dotted lines correspond to the abundances observed in TMC-1.
In the text
thumbnail Fig. A.1.

Observed lines of 1-ECP in the 31-50 GHz frequency range towards TMC-1. Line parameters for the complete list of detected lines of 1-ECP are given in Table A.2. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of 5.83 km s−1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The red line shows the synthetic spectrum obtained for an assumed Tr of 9 K and N(1-ECP) = (2.0 ± 0.4) × 1012 cm−2. The rotational quantum numbers are indicated in each panel. Blanked channels correspond to negative features produced in the folding of the frequency switching data.
In the text
thumbnail Fig. A.2.

Four lines of EBZ (C6H5CCH) observed in the 31-50 GHz frequency range towards TMC-1. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of 5.83 km s−1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The red line shows the synthetic spectrum obtained from a fit to the observed line profiles adopting a rotational temperature of 9 K, which provides N(EBZ) = (2.5 ± 0.4) × 1012 cm−2 (see Sect. 3.2). The rotational quantum numbers are indicated in each panel. Blanked channels correspond to negative features produced in the folding of the frequency switching data.
In the text
thumbnail Fig. B.1.

Selected lines of 1-CCP in the 31-50 GHz frequency range towards TMC-1. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of 5.83 km s−1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The red line shows the synthetic spectrum obtained from a fit to the observed line profiles, which provides Trot = 9.0 ± 1.0 K and N(1-CCP) = (3.1 ± 0.3) × 1011 cm−2(see Appendix B). The blue line shows the synthetic spectrum for the same column density and a rotational temperature of 6 K. The rotational quantum numbers are indicated in each panel. Blanked channels correspond to negative features produced in the folding of the frequency switching data.
In the text
thumbnail Fig. B.2.

Observed lines of 2-CCP in the 31-50 GHz frequency range towards TMC-1. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of 5.83 km s−1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The red line shows the synthetic spectrum obtained from a fit to the observed line profiles, which provides N(2-CCP) = (1.3 ± 0.2) × 1011 cm−2 for a rotational temperature of 9 K. (see Appendix B). The blue line shows the synthetic spectrum for the same column density and a rotational temperature of 6 K. The rotational quantum numbers are indicated in each panel. Blanked channels correspond to negative features produced in the folding of the frequency switching data.
In the text
thumbnail Fig. C.1.

Observed lines of C6H5CN in the 31-50 GHz frequency range towards TMC-1. The abscissa corresponds to the rest frequency assuming a local standard of rest velocity of 5.83 km s−1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The red line shows the synthetic spectrum obtained from a fit to the observed line profiles, which provides Tr = 9 ± 0.5 K and N(C6H5CN) = (1.2 ± 0.1) × 1012 cm−2. The blue line shows the synthetic spectrum for a rotational temperature of 6 K and a column density of 1.5 × 1012 cm−2. The rotational quantum numbers are indicated in each panel. Blanked channels correspond to negative features produced in the folding of the frequency switching data. While the blue line produces a very good agreement with the observed intensities for Ka≤4, for higher values of Ka it underestimates the observed intensities by a factor of two.
In the text
thumbnail Fig. C.2.

Same as Fig. C.1.
In the text
thumbnail Fig. C.3.

Same as Fig. C.1.
In the text
thumbnail Fig. C.4.

Same as Fig. C.1.
In the text
thumbnail Fig. D.1.

Chemical scheme of formation of c-C5H6 and C6H6 and their CCH and CN derivatives. Numbers correspond to the reactions discussed in Appendix D.
In the text

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