Tentative detection of HC₅NH⁺ in TMC-1

Abstract

Using the Yebes 40m radio telescope, we report the detection of a series of seven lines harmonically related with a rotational constant B ₀=1295.81581 ± 0.00026 MHz and a distortion constant D ₀ = 27.3 ± 0.5 Hz towards the cold dense cloud TMC-1. Ab initio calculations indicate that the best possible candidates are the cations HC₅NH⁺ and NC₄NH⁺. From a comparison between calculated and observed rotational constants and other arguments based on proton affinities and dipole moments, we conclude that the best candidate for a carrier of the observed lines is the protonated cyanodiacetylene cation, HC₅NH⁺. The HC₅N/HC₅NH⁺ ratio derived in TMC-1 is 240, which is very similar to the HC₃N/HC₃NH⁺ ratio. Results are discussed in the framework of a chemical model for protonated molecules in cold dense clouds.

1. Introduction

Although gas phase chemistry in cold interstellar clouds is dominated by ion-neutral reactions, only around 15% of the detected species are cations (see the Cologne Database for Molecular Spectroscopy; Müller et al. 2005). Observed polyatomic cations are usually protonated forms of stable molecules. The rather reduced number of them detected in space is due to the fact that a dissociative recombination with electrons is rapid and depletes cations, producing most of the neutrals observed in these objects. In addition to the widespread ions HCO⁺ and N₂H⁺, other interesting protonated species are HCS⁺ (Thaddeus et al. 1981), HCNH⁺ (Schilke et al. 1991), HC₃NH⁺ (Kawaguchi et al. 1994), HCO⁺ ₂ (Turner et al. 1999; Sakai et al. 2008), NH₃D⁺ (Cernicharo et al. 2013), NCCNH⁺ (Agúndez et al. 2015), H₂COH⁺ (Bacmann et al. 2016), and H₂NCO⁺ (Marcelino et al. 2018).

The abundance ratio between a protonated molecule and its neutral counterpart, [MH⁺]/[M], is sensitive to the degree of ionisation and to the proton affinity of the neutral. The higher the density is, the lower the ionisation fraction is and thus the lower the importance of protonated molecules. It is interesting to note that both the chemical models and the observations suggest a trend in which the abundance ratio [MH⁺]/[M] increases with an increasing proton affinity of M (Agúndez et al. 2015).

Protonated nitriles and dinitriles are observed in cold dense clouds because their neutral counterparts are abundant and have high proton affinities. The nitriles HCN and HC₃N have proton affinities of 712.9 kJ mol⁻¹ and 751.2 kJ mol⁻¹, respectively (Hunter & Lias 1998), and abundances in excess of 10⁻⁸ relative to H₂ (Agúndez & Wakelam 2013). Whereas the dinitrile NCCN has a proton affinity of 674.7 kJ mol⁻¹ (Hunter & Lias 1998) and an inferred abundance as large as that of HCN and HC₃N (Agúndez et al. 2015, 2018). The next larger members in the series of cyanopolyynes and dicyanopolyynes are also good candidates to be detected in their protonated form. Cyanodiacetylene (HC₅N) has a high proton affinity (770 ± 20 kJ mol⁻¹; Edwards et al. 2009) and it is just a few times less abundant than HC₃N in cold dark cores (Agúndez & Wakelam 2013). Dicyanoacetylene (NC₄N) also has a high proton affinity (736 kJ mol⁻¹; this work) and it is likely to be present with a high abundance based on the large inferred abundance of NCCN. Hence, we could expect the protonated forms of HC₅N and NC₄N to be present in cold dense clouds with moderately high abundances.

In this Letter, we report the detection of a new series of harmonically related lines belonging to a molecule with a ¹Σ ground electronic state towards the cold dark core TMC-1. Two of the molecular species discussed above, HC₅NH⁺ and NC₄NH⁺, could be the carriers. From ab initio calculations and the expected intensities of the lines for each of these species, we conclude that we have discovered the cation HC₅NH⁺. No laboratory data are available for it, hence, this is the first time this species has been observed. Abundance ratios between HC₃N and HC₅N and their protonated forms were derived and compared with predictions from chemical models. We searched for lines that could be attributed to NC₄NH⁺ without success. A search for NC₃NC in our data also provides upper limits to the abundance of this species.

2. Observations

New receivers, which were built within the Nanocosmos project¹ and installed at the Yebes 40m radio telescope, were used for the observations of TMC-1. The Q-band receiver consists of two HEMT cold amplifiers covering the 31.0-50.3 GHz band with horizontal and vertical polarisations. Receiver temperatures vary from 22 K at 32 GHz to 42 K at 50 GHz. The spectrometers are 2 × 8 × 2.5 GHz FFTs with a spectral resolution of 38.1 kHz, providing the whole coverage of the Q-band in both polarisations. The main beam efficiency and the half power beam width (HPBW) of the Yebes 40m telescope range from 0.6 and 55″ (at 32 GHz) to 0.43 and 37″ (at 49 GHz), respectively.

The observations leading to the line survey in the Q-band towards TMC-1 (α_J2000 = 4^h41^m41.9^s, δ_J2000 = +25°41′27.0″) were performed during several sessions between November 2019 and February 2020. The observing procedure used was frequency switching with a frequency throw of 10 MHz. The nominal spectral resolution of 38.1 kHz was used for the final spectra. A study of the velocity structure of the source (see, e.g. Lique et al. 2006; Xue et al. 2020) could require a higher spectral resolution. However, the determination of the total column density in the line of sight for a given molecule is not affected by our spectral resolution of 38.1 kHz. The sensitivity varies along the Q-band between 1 and 3 mK, which considerably improves previous line surveys in the 31-50 GHz frequency range (Kaifu et al. 2004).

The intensity scale, antenna temperature $\left({\text{T}}_A^{*}\right)$ was calibrated using two absorbers at different temperatures and the atmospheric transmission model (ATM, Cernicharo 1985; Pardo et al. 2001). Calibration uncertainties have been adopted to be 10 %. All data have been analysed using the GILDAS package².

3. Results

One of the most surprising results from the line survey in the Q-band in TMC-1 is the presence of a forest of weak lines. Most of them can be assigned to known species and their isotopologues, and only a few remain unidentified (Marcelino et al., in preparation). As previously mentioned, the level of sensitivity has been increased by a factor of 5-10 with respect to previous line surveys performed with other telescopes at these frequencies (Kaifu et al. 2004). Frequencies for the unknown lines have been derived by assuming a local standard of rest velocity of 5.83 km s⁻¹, which is a value that was derived from the observed transitions of HC₅N and its isotopologues in our line survey.

3.1. Harmonically related lines

Among the unidentified features in our survey, we have found a series of seven harmonically related lines to a precision better than 2×10⁻⁷. This strongly suggests that the carrier is a linear molecule with a ¹Σ ground electronic state. Fig. 1 shows these lines and the quantum numbers that were obtained. The derived line parameters are given in Table 1. Using the MADEX code (Cernicharo 2012), we have verified that none of these features can be assigned to lines from other species. For a linear molecule, the frequencies of its rotational transitions follow the standard expression ν(J →J - 1)=2B ₀J - 4D ₀ J ³. By fitting the frequencies of the lines given in Table 1, we derive

$$B_0=1295.81581(26)\text{MHz}$$

$$D_0=27.4(5)\times {10}^{-6}\text{MHz},$$

Fig. 1.

Observed lines of the new molecule found in the 31-50 GHz domain towards TMC-1. The abscissa corresponds to the local standard of rest velocity in km s⁻¹. Frequencies and intensities for the observed lines are given in Table 1. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The J=19-18 line is only detected at a 3.5σ level. Spectral resolution is 38.1 kHz.

Table 1. Observed line parameters for the new molecule in TMC-1.

Ju	$v_{obs}^a$ (MHZ)	$v_o-v_c^b$ (kHz)	∆vc (kms−1	${\displaystyle \int {\text{T}}_A^{*}\text{dv}}$ d (mK kms−1)	${\text{T}}_A^{*}$ * (mK)
13	33690.975	+5.2	0.80±0.05	8.2±1.0	9.7
14	36282.540	-2.6	0.73±0.04	8.0±1.0	10.4
15	38874.102	-3.3	0.59±0.05	5.9±1.0	9.4
16	41465.655	-3.1	0.46±0.05	4.5±1.0	11.9
17	44057.202	+ 1.7	0.76±0.09	5.1 + 1.0	6.3
18	46648.736	+4.5	0.68±0.08	5.3 + 1.0	7.7
19	49240.242	-8.8	0.50±0.10	2.7±1.0	5.0

Notes.

^(a)Observed frequencies for a v_LSR of 5.83 km s⁻¹. The uncertainty is 10 kHz for all lines, except for J=19-18 for which it is 20 kHz.

^(b)Observed minus calculated frequencies in kHz.

^(c)Linewidth at half intensity derived by fitting a Gaussian line profile to the observed transitions (in kms⁻¹).

^(d)Integrated line intensity in mK kms⁻¹

where values between parentheses represent the 1σ uncertainty on the parameters in units of the last digit. The standard deviation of the fit is 5.6 kHz.

The rotation constant is ~36 MHz lower than that of HC₅N (1331.332 MHz; Bizzocchi et al. 2004). It is also lower than that of dicyanoacetylene, NCCCCN (1336.7 MHz; Winther et al. 1992). The rotational constants of the ¹³C and ¹⁵N isotopologues of HC₅N are well known from laboratory measurements (Bizzocchi et al. 2004; Giesen et al. 2020). The isotopologues HC₅ ¹⁵N and H¹³CCCCCN have rotational constants of 1298.640 MHz and 1296.676 MHz, respectively, that is to say they are very close to that of the new species, suggesting that a slightly heavier species could be the carrier. The isomer HC₄NC has a larger rotational constant of 1401.182 MHz (Botschwina et al. 1998) and it can be excluded as a carrier. Moreover, this species has been detected in our line survey (Cernicharo et al. 2020) and also by Xue et al. (2020). Although the isomer HNC₅ has not been observed in the laboratory, ab initio calculations indicate that the molecule is bent with a (B + C)/2 value of ~ 1363 MHz (Gronowski & Kolos 2006; Cernicharo et al. 2020), which is ~70 MHz above the observed rotational constant.

In TMC-1, only polyatomic molecules containing H, C, N, O, and S have been found so far. We could consider a linear chain containing sulphur as a possible carrier. However, the best candidate in this case is HC₄S, which is linear but has a ²Π_i ground electronic state and a rotational constant of 1435.326 MHz that is too high (Hirahara et al. 1994). Another potential species is NCCCS, which is linear, but also with a ²Π_i ground electronic state and a rotational constant of 1439.186 MHz (McCarthy et al. 2003). The neutral species HC₅O was observed in the laboratory by Mohamed et al. (2005). It has a rotational constant of 1293.6 MHz and has been observed in TMC-1 (McGuire et al. 2017). Hence, good candidates for the carrier of the observed lines are molecular species having a structure and mass close to that of HC₅N, HC₅O, or NC₄N.

The cation HC₃NH⁺ was detected towards TMC-1 by Kawaguchi et al. (1994); additionally, protonated cyanogen, HNCCN⁺, was also detected in this source by Agúndez et al. (2015). Hence, good candidates for the carrier of the series of harmonically related lines are HC₅NH⁺ and NC₄NH⁺. An additional candidate is HC₅O⁺, which could be the product of protonation of C5O. However, C5O has not been detected so far in space and ab initio calculations (see below) indicate a rotational constant several MHz above the observed one. For protonated cyanodiacetylene, HC₅NH⁺, ab initio calculations by Botschwina et al. (1997) indicate an equilibrium rotational constant very close to ours of ~1294.1 MHz with a dipole moment of 3.811 D. It is a very good candidate indeed as the difference between the predicted and the observed rotational constant is less than 0.1%. Nevertheless, since we have a good second candidate, NC₄NH⁺, and also a third (less clear) possibility, HC₅O⁺, additional calculations are needed to help with the assignment of the lines.

3.2. Quantum chemical calculations

As mentioned before, HC₅NH⁺, NC₄NH⁺, and HC₅O⁺ are the three candidates that have a rotational constant compatible with that derived from the lines observed in TMC-1. In order to obtain precise geometries and spectroscopic molecular parameters that help with the assignment of the observed lines, we carried out high-level ab initio calculations for these three species. Other structural isomers of HC₅NH⁺ and NC₄NH⁺ species were discarded as candidates because of their rotational constant values and their energetics; all metastable isomers lie at least at 18 kcal mol⁻¹ over the HC₅NH⁺ and NC₄NH⁺ species, see Table B.3. Therefore, the calculations presented below are restricted to HC₅NH⁺, NC₄NH⁺, and HC₅O⁺ molecules. Details regarding the calculations can be found in Appendix B.

The experimental rotational parameters for HC₅N, NC₄N, and C₅O have been determined before (Bizzocchi et al. 2004; Winther et al. 1992; Ogata et al. 1995) and, therefore, they can be used to calibrate the computational results of their corresponding protonated forms. The B_e rotational constant for HC₅N, NC₄N, and C₅O, which were calculated at the CCSD(T)-F12/cc-pCVTZ-F12 level of theory, are 1329.57, 1334.69, and 1363.74 MHz, respectively. The zero-point vibrational contributions to the rotational constant at the MP2/cc-pVTZ level of theory were calculated to be 0.62, 0.72, and 1.82 MHz, respectively. Adding this contribution to the above B_e, B₀ thus takes values of 1330.19, 1335.42, and 1365.16 MHz, respectively, which are very close to the experimental values of 1331.332687(20), 1336.68433(30), and 1366.84709(6) MHz. The calculated constants show deviations from experimental values by 0.1% for all the species, which gives confidence as to the accuracy of our calculations.

The B₀ rotational constant calculated for HC₅NH⁺, NC₄NH⁺, and HC₅O⁺ were each scaled using the ratio B_exp/B_cal for HC₅N, NC₄N, and C₅O, respectively. These values are shown in Table 2 together with the estimated values for the centrifugal distortion constant (D), which were derived from the frequency calculations at CCSD/cc-pVTZ level of theory and scaled in the same manner as the rotational constants. The results of the same calculations at different levels of theory are given in Tables B.1 and B.2. Independent of the level of theory employed, our calculations provide a rotational constant value for HC₅NH⁺ around 1295.7 MHz, while that for NC₄NH⁺ is about 2.0 MHz lower, around 1293.6 MHz. In the case of HC₅O⁺, the value is larger, around 1303.0 MHz. No large differences were found for the predicted values of the centrifugal distortion constants.

Table 2. Rotational constants and electric dipole moments calculated for HC₅NH⁺ and NC₄NH⁺.

Parameter	New molecule a	HC5NH+	NC4NH+	HC5O+
B 0 (MHz)	1295.8158(3)	1295.51	1293.54	1303.12
De (MHz)	27.4(5)×10−6	26.3×10−6	27.8×10−6	31.2×10−6
μ (D)		3.26	9.47	3.10

Notes.

^(a)Values derived from the frequencies observed in TMC-1 (see Section 3.1). Values between parentheses represent the 1 σ uncertainty in units of the last digit.

4. Discussion

The calculated rotational constants of HC₅NH⁺ and NC₄NH⁺ are very close to the value of B₀ observed. A definitive assignment requires observations in the laboratory. Nevertheless, we could give some support to the assignment to HC₅NH⁺. First, the ab initio calculations at all levels of theory predict a rotational constant for HC₅NH⁺ around 1295 MHz, while that of NC₄NH⁺ is systematically 2 MHz below. Second, the rotational constant corrected for the ratio theory-to-observation of the reference species, HC₅N and NC₄N, provide an excellent match with the observed B ₀ for protonated cyanodiacetylene within 0.05%, while the difference for protonated dicyanoacetylene reaches 0.2%.

Additional support for the assignment to HC₅NH⁺ comes from the comparison of the proton affinities of HC₅N and NC₄N. As previously indicated, the abundance of a protonated species depends on the abundance and proton affinity of the neutral counterpart. A large proton affinity permits the transfer of H⁺ to the neutral species, M, through the reactions M + XH⁺ → MH⁺ + X, where XH⁺ is an abundant proton donor, such as HCO⁺ or H3. For cyanodiacetylene, HC₅N, the proton affinity was measured to be 770±20 kJ mol⁻¹ (Edwards et al. 2009). The abundance of this species is large in cold dark cores, hence, we could expect a moderately high abundance for its protonated form. Due to the lack of a permanent dipole, species such as NCCN and NC₄N have not been observed in dark clouds so far. Nevertheless, the detection of NCCNH⁺ in these objects by Agúndez et al. (2015) suggests that NCCN has an abundance as large as (1 - 10) × 10⁻⁸ relative to H₂, that is to say it is similar to that of HC_3N. Assuming an abundance for NC₄N that is similar to that of HC₅N, then the relative abundances of HC₅NH⁺ and NC₄NH ⁺ depend on the proton affinities of the neutrals and on their electronic dissociative recombination rates, which in principle could be assumed to be similar. For NC₄N, there is not an experimental value in the literature, so we calculated it at CCSD/cc-pVTZ level of theory. We used the energy balance between NC₄N + H⁺ and NC₄NH ⁺, considering NC₄N and H⁺ are independent species. We found a proton affinity value for NC₄N of 736 kJ/mol. Using the same scheme, we obtained a proton affinity value for HC₅N of 783 kJ/mol, which is very close to the experimental one (see above). Hence, the proton affinity of NC₄N is lower than that of HC₅N, which favours the protonated form of HC₅N as a carrier of our lines.

Another argument supporting this assignment concerns the very different dipole moment of the two species. While, HC₅NH⁺ has a predicted dipole moment of ~3.3 D, the corresponding value for NC₄NH⁺ is ~9.5 D (see Table 2). Hence, the abundance resulting from the observed line intensities would be significantly different if the carrier were to be one versus the other species. In Fig. 3 we show the rotational diagram obtained from the observed intensities (see Table 1) assuming that the carrier is HC₅NH⁺. We adopted a source radius of 40″ (Fossé et al. 2001). The parameter that is really interesting from this plot is the derived rotational temperature as the column density depends on the assumed dipole moment. The observed lines can be reproduced with a T_rot of 7.8 ± 0.7 K. This is a typical value of the rotational temperature for most molecules detected so far in TMC-1, but that could require a very large H₂ volume density if the dipole moment were 9.5 D, that is, if the carrier were NC₄NH⁺. The derived column density is (7.5±2.2)×10¹¹ if the carrier is HC₅NH⁺, and ~ 9 × 10¹⁰ cm⁻² if the carrier is NC₄NH⁺.

Fig. 3.

Rotational diagram of the observed lines in TMC-1 assuming a dipole moment of 3.3 D, i.e. assuming the carrier of the lines is HC₅NH⁺.

Hence, we consider that we have arguments to support the assignment of the lines to protonated cyanodiacetylene. Cernicharo et al. (2020) performed a rotational analysis to all the transitions of HC₅N observed in our line survey. They derived T_rot=8.6±0.2 K andN(HC₅N)=(1.8±0.2)×10¹⁴ cm⁻². This column density was derived from the observed weak hyperfine components in the HC₅N transitions from J=12-11 up to J=16-15 in order to take line opacity effects into account. Hence, we derived an abundance ratio of ~240 for HC₅N/HC₅NH⁺.

Another indirect indication supporting our identification could arise from the comparison of the abundances of other protonated species, in particular HC₃NH⁺ and NCCNH⁺. Both species are present in TMC-1 (Kawaguchi et al. 1994; Agúndez et al. 2015) and two rotational lines of each one are within our line survey. Fig. 2 shows the observed lines of both species. The corresponding line parameters are given in Table 3. From the observed intensities, we derived N(HC₃NH⁺)=1.0× 10¹² cm⁻² andN(HNCCN⁺)=9.0×10¹⁰ cm⁻², that is, N(HC₃NH⁺)/N(HNCCN⁺)~14. Cernicharo et al. (2020) also derived a column density for HC₃N of (2.3±0.2)×10¹⁴ cm⁻², which was corrected for line opacity effects. Therefore, the derived abundance ratio for HC₃N/HC₃NH⁺ is ≈230, which is a value that is in good agreement with the one found by Kawaguchi et al. (1994) ~160 and it is very similar to what was found above for HC₅N/HC₅NH ⁺.

Fig. 2.

Observed lines of HC₃NH⁺ (a) and of HNCCN⁺ (b). The abscissa corresponds to the local standard of rest velocity in km s⁻¹. Frequencies and intensities for the observed lines are given in Table 3. The ordinate is the antenna temperature corrected for atmospheric and telescope losses in mK. The J=4-3 line of HNCCN⁺ shows two components due to the hyperfine structure introduced by the external nitrogen atom. Spectral resolution is 38.1 kHz.

Table 3. Observed line parameters for HC₃NH⁺ and HNCCN⁺.

Ju	$v_{rest}^a$ (MHZ)	VLSR (km s−1)	∆v (km s−1)	${\displaystyle \int {\text{T}}_A^{*}\text{dv}}$ (mK km s−1)	${\text{T}}_A^{*}$ (mK)
HC3NH+
4	34631.859	5.76±0.02	0.72±0.02	16.1±1	20.9
5	43289.740	5.74±0.02	0.52±0.01	18.1±1	32.7
HNCCN+
4 b	35503.840	5.86±0.03	0.76±0.07	6.1±1	7.5
4 c	35503.981	5.82±0.01	0.82±0.03	15.7±1	18.0
5	44379.851	5.86±0.02	0.90±0.04	22.9±1	23.8

Notes.

^(a)Rest frequencies. The uncertainty is typically 2-3 kHz.

^(b)Corresponds to the F=3-2 hyperfine component.

^(c)Corresponds to the unresolved hyperfine components F=4-3 & 5-4

Between our unidentified features, we searched for lines that could be assigned to NC₄NH⁺ using our ab initio calculations without success. A search for lines of NCCCNC also provides an upper limit to its column density of ≤10¹² cm⁻².

The case for HC₅NH ⁺ is similar to that of the discovery of C3H⁺ by Pety et al. (2012), which was done from the reasonable agreement between the observed rotational constant and the best ab initio calculations available at that moment for C3H⁺.

4.1. Chemistry of protonated species

The chemistry of protonated molecules in cold dense clouds has been discussed by Agúndez et al. (2015). Here, we revisit the pseudo time-dependent gas-phase chemical model of a cold dark cloud carried out by these authors. The chemical network adopted is largely based on the UMIST RATE12 reaction network (McElroy et al. 2013). For more details on the chemical model, we refer the reader to Agúndez et al. (2015). In cold dense clouds, protonated molecules MH⁺ form mainly by the proton transfer from a proton donor XH⁺ to M :

$${\text{XH}}^{+}+\text{M}\to {\text{MH}}^{+}+\text{X},$$ (1)

while they are destroyed through the dissociative recombination with electrons

$${\text{MH}}^{+}+{\text{e}}^{-}\to \text{products}.$$ (2)

Therefore, at the steady state, the neutral-to-protonated abundance ratio is

$$\frac{[\text{M}]}{[{\text{MH}}^{+}]}=\frac{k_{DR}}{k_{PT}}\frac{[{\text{e}}^{-}]}{[{\text{XH}}^{+}]},$$ (3)

where k_DR and k_PT are the rate constants of the reactions of the dissociative recombination and proton transfer, respectively. The chemical model indicates that this scheme holds for HC₃NH ⁺ and HC₅NH ⁺, where the main proton donor XH⁺ is HCO⁺. Calculated neutral-to-protonated abundance ratios are about ten times higher than observed for HC₃NH ⁺ and HC₅NH ⁺ (see Fig. 4). That is to say, the chemical model underestimates the abundance of the protonated form with respect to its neutral counterpart. The rate constants of the dissociative recombination and proton transfer from HCO⁺ used in the chemical model are just estimates for HC₅NH ⁺; although, in the case of HC₃NH ⁺, they are well known from experiments (Anicich 2003; Geppert et al. 2004). We note that the chemical model also underestimates the abundance of HCNH⁺ (see Fig. 4) and other protonated species, such as HCS⁺ and NH₃, regardless of the cosmic-ray ionisation adopted (Agúndez et al. 2015). We suspect that the most likely reason for the underestimation of protonated molecules in the chemical model is the existence of additional formation routes that are not considered in the model.

Fig. 4.

Abundance ratios between neutral and protonated species for the series of cyanopolyynes. Calculated values correspond to a chemical model of a cold dark cloud at the steady state and observed ones to TMC-1.