Free Access
Issue
A&A
Volume 657, January 2022
Article Number A99
Number of page(s) 17
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/202142350
Published online 19 January 2022

© ESO 2022

1 Introduction

The quest for the simplest amino acid glycine in the interstellar medium (ISM) became a never-ending story for research in the fields of astrochemistry and astrophysics. More than 40 yr have passed since the first interstellar hunt for this molecule (Brown et al. 1979), during which glycine has been extensively searched for towards various interstellar sources in both the centimetre (cm) and millimetre (mm) wave regions of the electromagnetic spectrum (Hollis et al. 1980, 2003; Snyder et al. 1983; Berulis et al. 1985; Guélin & Cernicharo 1989; Combes et al. 1996; Ceccarelli et al. 2000; Kuan et al. 2003, 2004; Belloche et al. 2008). However, its presence in the ISM has never been confirmed (Snyder et al. 2005; Jones et al. 2007; Cunningham et al. 2007), even in the era of the Atacama Large Millimeter/submillimeter Array (ALMA). This is in spite of the fact that glycine has been discovered in meteorites (Pizzarello et al. 1991; Ehrenfreund et al. 2001b; Glavin et al. 2006), dust samples from comet Wild 2 (Elsila et al. 2009), and in the coma of comet67P/Churyumov-Gerasimenko (Altwegg et al. 2016). In addition, several laboratory experiments demonstrated the synthesis of glycine, and other amino acids, when interstellar ice analogues were subjected to UV radiation (Bernstein et al. 2002; Caro et al. 2002; Lee et al. 2009; Zheng & Kaiser 2010; Kim & Kaiser 2011) or bombarded by energetic electrons (Holtom et al. 2005). Numerous studies have been undertaken to throw light on this controversy. These have focused on plausible interstellar pathways to glycine, and its detectability and survival in the hostile ISM (see, e.g. Ehrenfreund et al. 2001a; Blagojevic et al. 2003; Largo et al. 2010; Pilling et al. 2011; Lattelais et al. 2011; Rimola et al. 2012; Garrod 2013; Jimenez-Serra et al. 2014; Nhlabatsi et al. 2016; Aponte et al. 2017; Suzuki et al. 2018; Xavier et al. 2019). Particular emphasis has been placed on the formation routes of possible glycine precursors (see, e.g. Basiuk 2001; Largo et al. 2004; Koch et al. 2008; Knowles et al. 2010; Barrientos et al. 2012; Redondo et al. 2015, and references therein) which are exciting candidates for observations in the ISM as well. Some of them, such as methylamine (Kaifu et al. 1974), aminoacetonitrile (Belloche et al. 2008), and hydroxylamine (Rivilla et al. 2020)have been already detected. Rotational spectroscopic studies of potential glycine precursors hydantoin (Alonso et al. 2017; Ozeki et al. 2017) and hydantoic acid (Kolesniková et al. 2019) were reported recently. Sanz-Novo et al. (2019) further computed the spectroscopic properties of glycine isomers of which methyl carbamate (Marstokk & Møllendal 1999; Bakri et al. 2002; Ilyushin et al. 2006; Groner et al. 2007) and glycolamide (Maris 2004; Sanz-Novo et al. 2020)were studied by microwave and mm wave spectroscopy that enabled dedicated searches in the ISM (Sanz-Novo et al. 2020; Sahu et al. 2020).

In this work, we focus on glycine precursor glycinamide (NH2CH2C(O)NH2) which is predicted to be a feasible intermediate on the hydrolytic way from aminoacetonitrile to glycine (Zhu & Ho 2004; Ugliengo et al. 2011). Keeping in mind the presence of aminoacetonitrile in Sgr B2(N) (Belloche et al. 2008), glycinamide could be considered a good candidate for observations in the same source. Millimetre wave surveys of high-mass star-formingregions are known to present a forest of lines with a high level of line blending (see e.g. Tercero et al. 2010; Belloche et al. 2013). For this reason, the identification of a new molecule, such as glycinamide, in these sources has to be guaranteed by the detection of numerous features consistent with confident predictions of its spectrum over a broad frequency region. High-quality laboratory data and analysis are therefore the first and mandatory step before any interstellar search can be conducted.

It was only recently that the rotational spectrum of glycinamide was first studied. Alonso et al. (2018) investigated its conformational landscape in supersonic expansion by Fourier transform microwave spectroscopy between 6 and 16 GHz. The analysis of the spectrum revealed the existence of a single conformer whose configuration in the principal axis frame is shown in Fig. 1. In addition, an unexpected non-rigid behaviour of this conformer has been implied by abnormal values of quartic centrifugal distortion constants. Such behaviour has been attributed to a large-amplitude motion that combines C–Nt bond torsion, C–C torsion, and NtH2 inversion. As this motion is governed by tunnelling through the central barrier in a double minimum potential function, rotational transitions in the ground vibrational state are expected to be split into two components associated with two torsion/inversion sublevels, usually labelled 0+ and 0. However, due to vibrational cooling accompanying supersonic expansion, only the transitions in the lowest-lying 0+ substate were observed by Alonso et al. (2018). In order to interpret the dense mm wave surveys from interstellar sources, the rotational transitions in this 0+ substate and also in the yet experimentally unobserved 0 substate may be of utmost importance. Furthermore, severe perturbations resulting from the 0+ ↔ 0 coupling of the rotational manifolds may occur for higher J and Ka transitions in the millimetre wave region. This situation is indeed the case for cyanamide (Read et al. 1986; Kraśnicki et al. 2011; Kisiel et al. 2013; Coutens et al. 2019) in which the large-amplitude motion involves the inversion of the NH2 group. It was only possible to detect the rotational transitions of cyanamide in the 0+ and 0 substates towards solar-type protostars (Coutens et al. 2018) after a successful laboratory analysis accounting for the 0+ ↔ 0 interactions (Read et al. 1986; Kraśnicki et al. 2011; Kisiel et al. 2013). The latter studies highlighted that the common difficulties arising from extrapolations to higher frequencies are not the only problems that need to be faced in attempts to detect glycinamide in space.

In the course of the present work, the laboratory rotational spectrum of glycinamide was measured between 90 and 329 GHz. A comprehensive analysis of this mm wave spectrum made it possible to unambiguously assign the rotational transitions in the four lowest torsional/inversion substates 0+, 0, 1+ and 1. In addition, rotational transitions in 0+ and 0 substates for the v26 = 1 state were assigned and measured. Results of this work deliver novel experimental information on the double minimum potential of glycinamide and provide a firm basis for searches for this molecule in space.

Section 2 describes the laboratory experiments performed to measure the rotational spectrum of glycinamide. The analysis of the laboratory spectra is presented in Sect. 3 and the search for glycinamide towards the hot molecular core Sgr B2(N1) with ALMA is reported in Sect. 4. Section 5 discusses the spectroscopic and astronomical results, and Sect. 6 summarises our conclusions.

thumbnail Fig. 1

Geometry and electric dipole moment of the most stable conformer of glycinamide, and their orientation in the principal rotational axis frame. The dipole moment vector is confined to the ab inertial plane, and is drawn from the notional negative to the notional positive charge. The molecule has a planar amido NaH2 group, and a pyramidal amino NtH2 group. The geometry is non-planar with a small ~100 cm−1 barrier at the Cs-symmetry configuration, where all heavy atoms are in the same plane, which also bisects the CH2 and NtH2 groups. Tunnelling across the barrier between the two equivalent non-planar configurations leads to appreciable splitting into pairs ofall vibrational states, including the ground state.

thumbnail Fig. 2

Sample 1.4 GHz of the rotational spectrum of glycinamide at 50°C illustrating the absence of characteristic spectral patterns. Overbars in the experimental spectrum (top panel) mark J = 34 ← 33 transitions between rotational levels with Ka = 0 and 1,with each marked line corresponding to an overlap of a pair of stronger μa-dipole, and a pair of weaker μb-dipole transitions. The frequency span of the bar indicates the significant magnitude of the splitting between two substates for each of the three tunnelling doublets. Blue markers indicate ground-state transitions in the data set used for determining spectroscopic constants, while the bottom panel displays collected predictions made on the basis of the final fits.

2 Experiments

Pure glycinamide has the form of colourless crystals with a melting point of 63°C. A vapour pressure of 5–10 mTorr that was sufficient for measurements was generated when the sample was heated to 50°C, and signal stability was ensured using a slow flow through the three-metre free space absorption cell of the spectrometer. In a previous rotational spectroscopy study (Alonso et al. 2018), glycinamide was produced by laser ablation of a specially prepared hydrochloride glycinamide rod, whilst for this study we used pure glycinamide prepared from glycinamide hydrochloride, as purchased from Aldrich and used without further purification. Glycinamide hydrochloride (5.5 g, 50 mmol) and dry dichloromethane (100 mL) were introduced under nitrogen into a 250 ml three-necked flask. Dry ammonia was then bubbled through the stirred suspension for 30 min. The salt was filtered off and the solvent was removed under vacuum on a rotary evaporator and then on a vacuum pump (0.1 mbar, 2 h), leaving 2.5 g or 34 mmol glycinamide (68% yield). It showed long-term stability when stored in the freezer (−20°C) and there was also no discernible decomposition when heated for measurements.

Spectroscopic measurements were made in the region 90−329 GHz with the broadband backward wave oscillator (BWO) based spectrometer in Warsaw, which was described in Medvedev et al. (2004). The hardware configuration was later updated and augmented below 140 GHz with a harmonic generation source (Kisiel & Kraśnicki 2010). A frequency measurement accuracy of 50 kHz was assumed.

3 Rotational spectra and analysis

The mm-wave rotational spectrum of glycinamide posed a significant assignment challenge, which can be partly appreciated from the exampleshown in Fig. 2. Glycinamide is a rather asymmetric molecule (asymmetry parameter κ = − 0.68), expected to give rise to strong pure-rotation a-type transitions accompanied by weaker b-type transitions (calculated dipole moment components are μa = 3.8 D and μb = 1.5 D, Alonso et al. 2018). However, there is a significant lack of characteristic line patterns that normally aid assignment. The most useful starting point was provided by the fact that the strongest lines in the spectrum would be quadruply degenerate, resulting from overlaps of pairs of a-type and b-type R-branch transitions for the lowest values of Ka. Unfortunately, such lines quickly proved to be poorly treatable with a single-state Watson’s asymmetric rotor Hamiltonian (Watson 1977). On the other hand, several sequences of such strong lines were identified with the use of the AABS graphical assignment package (Kisiel et al. 2005, 2012). Initial rotational constants, reported by Kisiel et al. (2010), were confirmed by measurement of the lowest-J ground-state transitions in supersonic expansion (Alonso et al. 2018).

Complete assignment and analysis of the mm-wave spectrum became possible once it became clear that the observed sets of transitions belonged to coupled pairs of vibrational sublevels. As already mentioned, such pairs of levels typically arise through tunnelling between two symmetry equivalent structures of the molecule separated by a relatively low energy barrier. Two clear boundary conditions are possible, where tunnelling is equivalent to a torsional motion (as in phenol, Kolesniková et al. 2013), or to inversion (as in cyanamide, Kisiel et al. 2013). For glycinamide, computations indicated that tunnelling between the most stable conformation depicted in Fig. 1 and its mirror image form obtained by reflection across the ab inertial plane is subject to a barrier of only ~100 cm−1 (Alonso et al. 2018) and that such tunnelling has to involve a concerted combination of torsion and inversion. The molecule belongs to the lowest symmetry point group, C1, and the general labelling used for vibrational levels split by double minimum inversion has been adopted (see Fig. 25.2 of Papoušek & Aliev 1982). Accordingly, the vibrational ground state, normally labelled v = 0, splits into a pair of sublevels designated 0+ (lower) and 0 (upper). Similarly, the usual first excited state, v = 1, of the inversion motion, becomes a doublet labelled 1+ (lower) and 1 (upper). This splitting structure is expected to be relatively unaffected by excitation of non-inverting vibrational motions. Accordingly, the first excited state of such a motion, for example labelled va = 1, will be split by an amount comparable to that in the ground state, with labelling va = 1, 0+ (lower) and va = 1, 0 (upper). Quantum chemistry vibrational calculations at both harmonic (Li et al. 2003) and anharmonic (Alonso et al. 2018) levels indicated, in agreement, that glycinamide has two low-frequency vibrational modes ν27 and ν26 with vibrational frequencies of near 100 and 200 cm−1, respectively.Nevertheless, unambiguous attribution as to which of these modes is responsible for the tunnelling only became possible upon completion of the rotational analysis, as discussed below.

The most effective treatment of pairs of vibrational substates resulting from tunnelling is to use a (2 × 2) block diagonal Hamiltonian of the form: (1)

The two diagonal blocks, and , are set up with the standard asymmetric rotor Hamiltonian (Watson 1977) for each of the two substates. The two off-diagonal blocks, , connecting the two substates are set up with the reduced axis system (RAS) Hamiltonian (Pickett 1972). In the case where the tunnelling motion is around an axis in the ab inertial plane, but at some angle to the principal axes, the RAS blocks are: (2)

where Fbc and Fca are the main adjustable parameters describing the interaction, each of which is further expanded empirically using centrifugal distortion-type terms , , and so on. Finally, ΔE is the vibrational energy difference between the two substates, E(0) − E(0+). This approach was used, for example, for the singly deuterated species, HDNCN, of cyanamide (Kisiel et al. 2013). All fits and predictions were carried out with Pickett’s SPFIT/SPCAT package (Pickett 1991).

Assignment proceeded through analysis of R-branch transition sequences for successively higher values of Ka. Once these were understood for the ground-state tunnelling doublet, it was possible to also assign two vibrationally excited doublets. The success of the coupled state fit for each such pair of substates was critically dependent on the value of ΔE. In each case, the range of likely values had to be scanned with some care prior to using it as an adjustable parameter of fit.

The final understanding of the spectrum is illustrated in the lower part of Fig. 2. It can be seen from the pairs of lowest Ka transitions marked in the top of Fig. 2 that the frequency differences between the same rotational transitions in the substates are considerable, which reflects the relatively low barrier to the tunnelling. Extensive relative intensity measurements allowed the energies above the ground state for the two excited vibrational states (energy differences between the lower substates) to be determined as 99(13) and 201(13) cm−1, which was the initial basis for the v27 = 1 and v26 = 1 assignment marked in Fig. 2.

Spectroscopic constants resulting from the fits are summarised in Table 1 and a breakdown of the statistics for individual substates is given in Table A.1. Data for the 0+ substate also include hyperfine removed frequencies from supersonic expansion measurements reported in Alonso et al. (2018). For the two ground-state substates, some Q-branch transitions were also assigned and measured. In addition, nominal interstate transitions were observed. These result from strong mixing between perturbing rotational levels in two substates and are usually only identified in the final stages of the analysis.

For all of the identified substates, as well as for combined results for their pairs, it was possible to reproduce measured frequencies to within their assumed experimental accuracy of 50 kHz. The energy differences between the tunnelling substates were determined very precisely to sub-MHz precision. When combined with results of relative intensity measurements, these allow determination of the positions of the lowest vibrational energy levels in glycinamide, as summarised in Fig. 3. We now have further evidence concerning the proposed vibrational assignment. Tunnelling splitting is expected to increase significantly with vibrational excitation in a double minimum potential and can be successfully modelled with simple potentials, as described and carried out for cyanamide (Kisiel et al. 2013). The two splitting values in the ν27 column in Fig. 3 are consistent with a relatively low barrier, meaning that this mode is responsible for the tunnelling. On the other hand, the splitting in the ν26 tunnelling pair is very close to that in the ground state, and therefore this appears to be a standard normal vibrational mode.

The coverage of values of rotational quantum numbers by the measured transitions is relatively comprehensive and can be assessed from the data distribution plots for the three studied doublets given in Figs. A.1A.3. The values of quartic centrifugal distortion constants in the substates in each tunnelling pair are generally quite close to each other, which shows that the couplingbehaviour has been largely accounted for by the parameters in Eq. (2). Two significant exceptions are ΔK and δK for the upper substate of v26 = 1, suggesting that this substate may be interacting with some state outside the model, possibly 2+ of ν27. The dominance in magnitude of the Fca over the Fbc parameter is similar to that for HDNCN (Kisiel et al. 2013) and can accordingly be taken to be an indicator that the effective axis around which tunnelling takes place is at a relatively small angle to the b-inertial axis.

It is at this stage that the difficulties faced during the analysis and possible pitfalls in searching for a new molecule in space on the basis of incomplete fits can be appreciated. Figure 4 illustrates a typical effect of perturbations on rotational transition frequencies in the two substates. Perturbations are most efficiently followed in sequences of transitions for a given value of Ka and an unperturbed situation is characterised by smooth, near horizontal trace behaviour in a scaled frequency difference plot as in Fig. 4. The sharp spikes are due to contributions from resonances between rotational levels in the two vibrational substates that arise within a Hamiltonian matrix for a given value of the J quantum number. A specific resonance between two rotational levels in perturbing substates will have the same magnitude but the opposite sign for the two partners. Mirror image behaviour such as that seen in Fig. 4 confirms identification of the resonance partners and, if consistent with observed frequencies, is also indicative of the quality of the fit. It is notable that resonances can be of considerable magnitude (2.4 GHz for the maxima in this figure) and can also significantly affect transition frequencies for many J values around the maximum. There are many such resonances in the data for the three pairs of substates, and in Fig. 4 there is a second, smaller resonance in the sequence for the 0+ substate that has its partner in the Ka = 2 sequence for 0.

The perturbations not only affect transition frequencies, but can also significantly affect transition intensities, as visible in Fig. 5. Without perturbations, the intensity pattern for the displayed quartet of lines is expected to be as in the bottom or top traces. In the present case, both the intensity pattern and quantum number labelling of the lines in the two middle plots are significantly affected.

Finally, the number of low-lying vibrational states has a significant effect on the value of the vibration-rotation partition function Qvr that is of relevance to column density evaluations. The vibrational effect for glycinamide is documented in Table 2. It is seen that at the lowest temperatures the often-used ground-state partition function is quite satisfactory. However, at 200 K, for example, the partition function correction arising from the additional presence of the v27 = 1 and v26 = 1 doublets is 1.7. Glycinamide also has two other relatively low-frequency modes (ν25 and ν24 calculated at 303 and 384 cm−1 resp., Li et al. 2003). Combination of those modes with present information on ν27 and ν26 allows an estimate that up to 412 cm−1 it is necessary to account for populating a total of 19 sublevels, corresponding to a partition function correction of 2.4 at 200 K.

thumbnail Fig. 3

Vibrational energies of the three tunnelling doublets studied in this work. Each splitting is determined precisely from the coupled fits of measured rotational frequencies, while the relative positions of the two vibrationally excited doublets aredetermined less precisely from relative intensity measurements.

Table 1

Spectroscopic constants determined for the three assigned tunnelling doublets in glycinamide.

thumbnail Fig. 4

Perturbation shifts in selected aR-branch rotational transitions in the ground-state tunnelling doublet. The plotted quantity is the scaled difference between frequencies from the full spectroscopic model (ν) and perturbation-removed frequencies (ν0) calculated without the interstate coupling terms. The matching character of the observed resonance behaviour identifies the interacting Ka sequences in the two substates. Circles denote experimental measurements, and the perturbation contribution at the resonance peak at J″ + 1 = 30 is near 2.4 GHz. The 0+ sequence hasa smaller resonance at J″ + 1 = 19, which we identify as resulting from a resonance with a different, Ka = 2 sequence in substate 0-. All measured transitions involved in the resonances are fitted to within their experimental uncertainty.

thumbnail Fig. 5

Perturbations in the quartets of R-branch Ka = 0,1 transitions in the 0 substate for the ground state of glycinamide. At these J values, the four possible transitions are completely resolved, with the pattern of the two stronger, central a-dipole transitions, flanked by weaker b-dipoletransitions (as in the bottom and top traces). The transitions are marked by the value of Ka and transition type, and the two middle traces show significant deviations in the patterns. Triangle markers indicate transitions in the fitted data set. While this is an interesting spectroscopic curiosity, it is noted that such lowest Ka transitions are the first choice in astrophysical searches for a new molecule.

4 Search for glycinamide towards Sgr B2(N1)

4.1 Observations

We used the imaging spectral line survey ReMoCA (Re-exploring Molecular Complexity with ALMA) performed with ALMA towards Sgr B2(N). The observational setup and the method used to reduce this interferometric data set were described in Belloche et al. (2019). We summarise only the main features here. The field of view of the observations was centred between Sgr B2(N1) and Sgr B2(N2), the two main hot molecular cores of Sgr B2(N), which are separated by 4.9′′ or ~0.2 pc in projection onto the plane of the sky. The equatorial coordinates of this phase centre are (α, δ)J2000 = (17h47m19.s87, − 28°22′16.′′0). The survey has an angular resolution (HPBW) that varies between ~0.3′′ and ~0.8′′. The median angular resolution is 0.6′′ and corresponds to ~4900 au at the distance of Sgr B2 (8.2 kpc, Reid et al. 2019). Five frequency tunings of the receivers, which we call setups S1 to S5, were used to cover the frequency range from 84.1 to 114.4 GHz at a spectral resolution of 488 kHz (1.7–1.3 km s−1). The observations achieved a sensitivity per spectral channel of between 0.35 and 1.1 mJy beam−1 (rms) depending on the setup, with a median sensitivity of 0.8 mJy beam−1.

Following, Belloche et al. (2019) we selected the offset position Sgr B2(N1S) located at (α, δ)J2000= (17h47m19.s870, − 28°22′19.′′48) for this study. This position is about 1′′ to the south of the main hot core Sgr B2(N1). Its continuum emission has a lower opacity than the peak of the hot core, which allows for a deeper look into the molecular content of this source. We used an improved version of the data reduction here, as reported in Melosso et al. (2020).

We employed the software Weeds (Maret et al. 2011) to produce synthetic spectra under the assumption of local thermodynamic equilibrium (LTE). This assumption is appropriate for Sgr B2(N1S) because the regions where hot-core emission is detected in Sgr B2(N) have high densities (> 1 × 107 cm−3, see Bonfand et al. 2019). A best-fit synthetic spectrum was derived for each molecule separately, and then the contributions of all identifiedmolecules were added together. Each species was modelled with a set of five parameters: size of the emitting region (θs), column density (N), temperature(Trot), line width (ΔV), and velocity offset (Voff) with respect to the assumed systemic velocity of the source, Vsys = 62 km s−1.

4.2 Non-detection of glycinamide

We assumed a rotational temperature of 160 K, as derived for formamide, NH2CHO, by Belloche et al. (2019), an emission size of 2′′, and an FWHM line width of 5 km s−1 to compute LTE synthetic spectra of glycinamide and search for rotational emission of this molecule towards Sgr B2(N1S). We found no evidence for emission of glycinamide towards this source. This non-detection is illustrated in Fig. 6. We also searched for rotational emission from within its vibrationally excited states v27 = 1 and v26 = 1 but did not detect any clear sign of it (see Figs. 7 and 8, respectively). The upper limit that we obtain for the column density of glycinamide is indicated in Table 3 along with the parameters derived by Belloche et al. (2019) for urea, NH2C(O)NH2, and formamide, NH2CHO, and by Melosso et al. (2020) for aminoacetonitrile, NH2CH2CN.

The amides NH2CH2C(O)NH2 and NH2C(O)NH2 could be seen as the partially hydrolysed counterparts of the nitriles NH2CH2CN and NH2CN, respectively.It may therefore be instructive to compare the relative abundances of these two pairs of molecules. With this in mind, we modelled the emission spectrum of cyanamide (aminomethanenitrile, NH2CN) towards Sgr B2(N1S). The cyanamide spectroscopic information was taken from the JPL catalogue (Pickett et al. 1998). This JPL entry (TAG 42003 version 1) is based mostly on microwave data from Read et al. (1986) along with far-infrared data from Birk et al. (1993). A few transitions of cyanamide are clearly detected (see Fig. A.4), but unfortunately not enough to derive its rotational temperature from a population diagram. Therefore, we also assumed a temperature of 160 K to compute its column density, which is given in Table 3. The cyanamide line around 100.070 GHz is contaminated by absorption from HC3N J = 11–10, which is notaccurately accounted for by our current complete model of Sgr B2(N1S).

Radicals derived from methylamine, NH2CH3, may be involved in the formation of glycinamide. Therefore, we also report in Table 3 the parameters we derived by modelling the emission spectrum of this molecule. The methylamine spectroscopic information was taken from the JPL catalogue. This catalogue entry (TAG 31008 version 1) is based on the combined fit by Ilyushin et al. (2005) with data in the range of our survey mostly from that work. A dozen transitions of NH2CH3 are clearly detected towards Sgr B2(N1S), as shown in Fig. A.5. There are enough transitions to build a population diagram, which is displayed in Fig. A.6. We assumed a temperature of 230 K for the LTE modelling of the spectrum of NH2CH3 shown in Fig. A.5, slightly higher than the formal result of the fit to the population diagram (see Table 4), but within the uncertainties.

The vibrational correction factor Fvib reported in Table 3 for glycinamide corresponds to the ratio of the values given in Cols. 4 and 3 of Table 2. This is because the partition function of the spectroscopic predictions used for the astronomical search included only the vibrational contribution given in Col. 3 of that table. In proceeding like this, we neglect the contribution of vibrational levels above 412 cm−1, which is likely below a further 10% at a temperature of 160 K.

Table 2

Partition function for glycinamide(a) and its dependence on accounting for vibrational states.

5 Discussion

5.1 Laboratory spectroscopy of glycinamide

The present experimental investigation of the rotational spectrum covers the most useful ALMA bands for a molecule of this size. We satisfactorily fitted many resonances between rotational levels in the three studied tunnelling doublets allowing reliable predictions, at least when these are interpolations within the acquired data sets. At the laboratory measurement temperature of 50°C, we accounted for all of the strongest observed lines, providing a comprehensive basis for any future searches, especially at the typically significantly lower interstellar temperatures. Clearly many more outstanding lines from higher vibrational states remain to be assigned and measured in the experimental spectrum, but their analysis is expected to be even more challenging than that reported presently.

thumbnail Fig. 6

Selection of transitions of NH2CH2C(O)NH2, v = 0 covered by our ALMA survey. The synthetic spectrum of NH2CH2C(O)NH2, v = 0 used to derive the upper limit to its column density is displayed in red and overlaid on the observed spectrum of Sgr B2(N1S) shown in black. The blue synthetic spectrum contains the contributions from all molecules identified in our survey so far, but not from the species shown in red. The central frequency of each panel is indicated in MHz below its x-axis. Each panel has a width of 40 MHz, as indicated in brackets behind the central frequency. The angular resolution (HPBW) is also indicated. The y-axis is labelled in brightness temperature units (K). The dotted line indicates the 3σ noise level.

thumbnail Fig. 7

Same as Fig. 6 but for NH2CH2C(O)NH2, v27 = 1.

thumbnail Fig. 8

Same as Fig. 6 but for NH2CH2C(O)NH2, v26 = 1.

5.2 Comparison of glycinamide to related molecules in Sgr B2(N)

Our non-detection of glycinamide, NH2CH2C(O)NH2, towards Sgr B2(N1S) reported in Table 3 implies that it is at least ~1.8 times less abundant than urea, NH2C(O)NH2, which is markedly different from the pair of nitriles NH2CH2CN/NH2CN for which the longer molecule is a factor four more abundant than the shorter one. In other words, NH2C(O)NH2, the partially hydrolysed counterpart of NH2CN, has the same abundance as the latter, while NH2CH2C(O)NH2, the partially hydrolysed counterpart of NH2CH2CN, is at least seven times less abundant than the latter. The analysis reported in Sect. 4 also shows that glycinamide is at least two orders of magnitude less abundant than its potential precursors formamide and methylamine.

Table 3

Parameters of our best-fit LTE models of cyanamide, aminoacetonitrile, formamide, methylamine, and urea towards Sgr B2(N1S), along with column density upper limit for glycinamide.

Table 4

Rotational temperature of methylamine derived from its population diagram towards Sgr B2(N1S).

5.3 Glycinamide chemistry

Glycinamide is not presently included in any astrochemical models, but the results of recent chemical simulations of hot cores may nevertheless be informative. Garrod (2013) constructed a chemical network for glycine (NH2CH2COOH), which included the related species glycinal (NH2CH2CHO). These species could be formed on interstellar dust grains through the addition of radicals produced mainly by photodissocation of simpler solid-phase molecules, a process which itself is driven by the enhanced surface mobility of the radicals, as the result of grain heating induced by the star-formation process. Complex organic molecules including glycine and glycinal would later sublimate entirely from the dust grains. The chemical network included mechanisms for those molecules to be subsequently destroyed in the gas phase through ion–molecule reactions.

The more recent models by Garrod et al. (2021), which incorporate the same glycine-related chemistry, allow a broader range of grain-surface and bulk-ice kinetic processes to bring together reactive radicals and thus form complex organics. This includes the possibility of radicals reacting non-diffusively on dust-grain surfaces at very low temperatures, when the icy grain mantles are still gradually building up. In this scenario, the radicals themselves need only be formed close to each other, allowing their reactions to proceed unmediated by thermal diffusion. Furthermore, the initially translucent conditions under which the dust-grain ices build up may allow some degree of UV photoprocessing of the young ices by the ambient interstellar UV field, converting a small fraction of simple solid-phase molecules into more complex organics. The Garrod et al. models indicate that a substantial fraction of glycine production may occur at this very early stage, with glycinal acting as a precursor. Although it is not included in those models, glycinamide could plausibly be produced through a similar mechanism, such as the photodissociation of solid-phase glycinal to produce the radical NH2CH2CO, with which NH2 would react to form NH2CH2CONH2.

If such a mechanism is active in producing solid-phase glycinamide, then the observed ratio of formamide to urea might be somewhat indicative of the expected ratio of glycinal to glycinamide, which share similar molecular structures. Towards Sgr B2(N1S), the former ratio is approximately 100. Although glycinal has not yet been detected in the interstellar medium, with dedicated searches being hindered by a lack of spectroscopic predictions, the model of Garrod et al. (2021) suggests that its peak gas phase abundance should be around a factor 100 less than that of formamide. On this basis, one might therefore expect an abundance of glycinamide around 104 times lower than formamide, or 100 times lower than urea. However, it is plausible that the efficiency of the conversion of glycinal to glycinamide on the grains could be greater than the above crude comparison might suggest; in the models, glycine itself reaches a peak gas-phase abundance as great as around half that of glycinal, implying substantial conversion. This corresponds to an approximately 40 times lower abundance than that of urea produced by the model. If we were to take this abundance of glycine as the maximum possible allowed abundance for glycinamide, which assumes that glycine and glycinamide are produced in equal amounts from glycinal, then the implied 40:1 ratio of urea to glycinamide would produce a glycinamide column density of around 20 times lower than the observed upper limit. Even efficient conversion from glycinal would therefore still produce a smaller amount of glycinamide than might plausibly be detectable at this time.

However, the above production mechanism is not the only possible means by which glycinamide could form through radical addition; alternatives would include the reaction of the acetamide-related radical NH2COCH2 with NH2, or the addition of NH2CH2 to NH2CO. The latter pair of radicals can be formed through cosmic-ray-induced UV photodissociation of, or chemical H-atom abstraction from, solid-phase NH2CH3 and NH2CHO, both of which are detected towards Sgr B2(N1S) in the gas phase. A similar reaction between radicals NH2CH2 and COOH indeed produces a substantial quantity of glycine in the models; this process occurs much later, at around the time when water itself is beginning to desorb rapidly from the grains, releasing trapped radicals onto the surface of the warm ice, where they may react diffusively. The sizeable abundances of NH2CH3 and NH2CHO detected towards Sgr B2(N1S) suggest this could be a plausible pathway for glycinamide production.

The above reaction mechanisms nevertheless remain entirely conjectural, at least until they have been tested explicitly in the astrochemical models. The closure of the existing chemical network surrounding glycine in order to incorporate glycinamide seems a plausible goal for future investigation.

6 Conclusions

We performed a comprehensive laboratory rotational study of the potential glycine precursor glycinamide, up to 329 GHz. In total, over 2800 transition lines were assigned and measured for the ground state and two lowest lying excited-state tunnelling doublets. Newly derived spectroscopic constants were used to search for spectral signatures of glycinamide in Sgr B2(N) by millimetre-wave astronomy. Lists of experimental frequencies and their observed–calculated differences are provided in Tables 5–7, only available in electronic form.

Glycinamide was not detected towards the hot molecular core Sgr B2(N1S)with ALMA. The upper limit derived for its column density implies that it is at least seven times less abundant than aminoacetonitrile and 1.8 times less abundant than urea towards this source.

While glycinamide has not yet been considered in any astrochemical kinetics models, comparison with model results for related species suggests that it may be a factor of 40–100 times less abundant than urea, corresponding to a value that is a factor of 20–50 below the upper limit towards Sgr B2(N1S). This would likely be well below the spectral confusion limit of the ReMoCA survey of Sgr B2(N). Further progress in the search for glycinamide in the ISM could be made by targeting sources with a lower level of spectral confusion. The Galactic centre source G+0.693-0.027, a shocked region close to Sgr B2(N) with a rich chemical content characterised by low excitation temperatures, should be a promising target to continue the search for interstellar glycinamide.

Acknowledgements

This paper makes use of the following ALMA data: ADS/JAO.ALMA#2016.1.00074.S. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. The interferometric data are available in the ALMA archive at https://almascience.eso.org/aq/. Part of this work has been carried out within the Collaborative Research Centre 956, sub-project B3, funded by the Deutsche Forschungsgemeinschaft (DFG) – project ID 184018867. J.C.G. thanks the National Center for Space Studies (CNES), National Program “Physics and Chemistry of the Interstellar Medium” (PCMI) from CNRS / INSU, and PHC Polonium 14277ZC for financial support. L.K. and Valladolid authors acknowledge funding from the Ministerio de Ciencia e Innovación (grants PID2019-111396GB-I00, CTQ2016-76393-P), Junta de Castilla y León (grants VA244P20, VA077U16), and European Research Council under the European Union’s Seventh Framework Programme ERC-2013- SyG, Grant Agreement n. 610256 NANOCOSMOS. R.T.G. acknowledges support from the National Science Foundation (grant No. AST 19-06489). Z.K. and E.B.J. also acknowledge the use of computational resources under the grant G61-6 from the Interdisciplinary Center of Mathematical and Computer Modelling (ICM) of Warsaw University.

Appendix A Complementary Tables and figures

Table A.1 contains subset statistics for substates in each torsio- nal/inversion doublet, including numbers of fitted lines, ranges of the values of the key quantum numbers, and frequency ranges of the measurements.

The plots in Figs. A.1, A.2, and A.3 are distribution plots of obs.-calc. frequencies as a function of the valuesof J” and Ka quantum numbers, illustrating the comprehensive quantum number coverage achieved in the measurements.

Figures A.4 and A.5 show the transitions of NH2CN and NH2CH3 that are covered by the ReMoCA survey and contri- bute significantly to the signal detected towards Sgr B2(N1S). Figure A.6 shows the population diagram of NH2CH3 towards Sgr B2(N1S).

Table A.1

Subset statistics for the coupled state fits of the three tunnelling doublets for glycinamide.

thumbnail Fig. A.1

Distribution plot of quantum numbers of rotational transitions measured and fitted for the ground-state doublet of glycinamide. Symbol diameter is proportional to the value of |fobs-calc|∕σ where fobs-calc is the residual of fit for a given line and σ is its measurement uncertainty. Red colour identifies outliers with |fobs-calc|∕σ > 3. The few outliers are all for confidently assigned transitions and may be due either to blends with unassigned lines or incompletely treated perturbation contributions.

thumbnail Fig. A.2

Same as Fig. A.1 but for the v27 = 1 doublet of glycinamide.

thumbnail Fig. A.3

Same as Fig. A.1 but for the v26 = 1 doublet of glycinamide.

thumbnail Fig. A.4

Transitions of NH2CN, v = 0 covered by our ALMA survey. The synthetic spectrum of NH2CN, v = 0 is displayed in red and overlaid on the observed spectrum of Sgr B2(N1S) shown in black. The blue synthetic spectrum contains the contributions from all molecules identified in our survey so far, including the species shown in red. The central frequency of each panel is indicated in MHz below its x-axis. Each panel has a width of 40 MHz, as indicated in brackets behind the central frequency. The angular resolution (HPBW) is also indicated. The y-axis is labelled in brightness temperature units (K). The dotted line indicates the 3σ noise level.

thumbnail Fig. A.5

Same as Fig. A.4 but for NH2CH3, v = 0.

thumbnail Fig. A.6

Population diagram of NH2CH3 towards Sgr B2(N1S). The observed datapoints are shown in black while the synthetic populations are shown in red. No correction is applied in panel a. In panel b, the optical depth correction has been applied to both the observed and synthetic populations and the contamination by all other species included in the full model has been removed from the observed datapoints. The purple line is a linear fit to the observed populations (in linear-logarithmic space).

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All Tables

Table 1

Spectroscopic constants determined for the three assigned tunnelling doublets in glycinamide.

Table 2

Partition function for glycinamide(a) and its dependence on accounting for vibrational states.

Table 3

Parameters of our best-fit LTE models of cyanamide, aminoacetonitrile, formamide, methylamine, and urea towards Sgr B2(N1S), along with column density upper limit for glycinamide.

Table 4

Rotational temperature of methylamine derived from its population diagram towards Sgr B2(N1S).

Table A.1

Subset statistics for the coupled state fits of the three tunnelling doublets for glycinamide.

All Figures

thumbnail Fig. 1

Geometry and electric dipole moment of the most stable conformer of glycinamide, and their orientation in the principal rotational axis frame. The dipole moment vector is confined to the ab inertial plane, and is drawn from the notional negative to the notional positive charge. The molecule has a planar amido NaH2 group, and a pyramidal amino NtH2 group. The geometry is non-planar with a small ~100 cm−1 barrier at the Cs-symmetry configuration, where all heavy atoms are in the same plane, which also bisects the CH2 and NtH2 groups. Tunnelling across the barrier between the two equivalent non-planar configurations leads to appreciable splitting into pairs ofall vibrational states, including the ground state.

In the text
thumbnail Fig. 2

Sample 1.4 GHz of the rotational spectrum of glycinamide at 50°C illustrating the absence of characteristic spectral patterns. Overbars in the experimental spectrum (top panel) mark J = 34 ← 33 transitions between rotational levels with Ka = 0 and 1,with each marked line corresponding to an overlap of a pair of stronger μa-dipole, and a pair of weaker μb-dipole transitions. The frequency span of the bar indicates the significant magnitude of the splitting between two substates for each of the three tunnelling doublets. Blue markers indicate ground-state transitions in the data set used for determining spectroscopic constants, while the bottom panel displays collected predictions made on the basis of the final fits.

In the text
thumbnail Fig. 3

Vibrational energies of the three tunnelling doublets studied in this work. Each splitting is determined precisely from the coupled fits of measured rotational frequencies, while the relative positions of the two vibrationally excited doublets aredetermined less precisely from relative intensity measurements.

In the text
thumbnail Fig. 4

Perturbation shifts in selected aR-branch rotational transitions in the ground-state tunnelling doublet. The plotted quantity is the scaled difference between frequencies from the full spectroscopic model (ν) and perturbation-removed frequencies (ν0) calculated without the interstate coupling terms. The matching character of the observed resonance behaviour identifies the interacting Ka sequences in the two substates. Circles denote experimental measurements, and the perturbation contribution at the resonance peak at J″ + 1 = 30 is near 2.4 GHz. The 0+ sequence hasa smaller resonance at J″ + 1 = 19, which we identify as resulting from a resonance with a different, Ka = 2 sequence in substate 0-. All measured transitions involved in the resonances are fitted to within their experimental uncertainty.

In the text
thumbnail Fig. 5

Perturbations in the quartets of R-branch Ka = 0,1 transitions in the 0 substate for the ground state of glycinamide. At these J values, the four possible transitions are completely resolved, with the pattern of the two stronger, central a-dipole transitions, flanked by weaker b-dipoletransitions (as in the bottom and top traces). The transitions are marked by the value of Ka and transition type, and the two middle traces show significant deviations in the patterns. Triangle markers indicate transitions in the fitted data set. While this is an interesting spectroscopic curiosity, it is noted that such lowest Ka transitions are the first choice in astrophysical searches for a new molecule.

In the text
thumbnail Fig. 6

Selection of transitions of NH2CH2C(O)NH2, v = 0 covered by our ALMA survey. The synthetic spectrum of NH2CH2C(O)NH2, v = 0 used to derive the upper limit to its column density is displayed in red and overlaid on the observed spectrum of Sgr B2(N1S) shown in black. The blue synthetic spectrum contains the contributions from all molecules identified in our survey so far, but not from the species shown in red. The central frequency of each panel is indicated in MHz below its x-axis. Each panel has a width of 40 MHz, as indicated in brackets behind the central frequency. The angular resolution (HPBW) is also indicated. The y-axis is labelled in brightness temperature units (K). The dotted line indicates the 3σ noise level.

In the text
thumbnail Fig. 7

Same as Fig. 6 but for NH2CH2C(O)NH2, v27 = 1.

In the text
thumbnail Fig. 8

Same as Fig. 6 but for NH2CH2C(O)NH2, v26 = 1.

In the text
thumbnail Fig. A.1

Distribution plot of quantum numbers of rotational transitions measured and fitted for the ground-state doublet of glycinamide. Symbol diameter is proportional to the value of |fobs-calc|∕σ where fobs-calc is the residual of fit for a given line and σ is its measurement uncertainty. Red colour identifies outliers with |fobs-calc|∕σ > 3. The few outliers are all for confidently assigned transitions and may be due either to blends with unassigned lines or incompletely treated perturbation contributions.

In the text
thumbnail Fig. A.2

Same as Fig. A.1 but for the v27 = 1 doublet of glycinamide.

In the text
thumbnail Fig. A.3

Same as Fig. A.1 but for the v26 = 1 doublet of glycinamide.

In the text
thumbnail Fig. A.4

Transitions of NH2CN, v = 0 covered by our ALMA survey. The synthetic spectrum of NH2CN, v = 0 is displayed in red and overlaid on the observed spectrum of Sgr B2(N1S) shown in black. The blue synthetic spectrum contains the contributions from all molecules identified in our survey so far, including the species shown in red. The central frequency of each panel is indicated in MHz below its x-axis. Each panel has a width of 40 MHz, as indicated in brackets behind the central frequency. The angular resolution (HPBW) is also indicated. The y-axis is labelled in brightness temperature units (K). The dotted line indicates the 3σ noise level.

In the text
thumbnail Fig. A.5

Same as Fig. A.4 but for NH2CH3, v = 0.

In the text
thumbnail Fig. A.6

Population diagram of NH2CH3 towards Sgr B2(N1S). The observed datapoints are shown in black while the synthetic populations are shown in red. No correction is applied in panel a. In panel b, the optical depth correction has been applied to both the observed and synthetic populations and the contamination by all other species included in the full model has been removed from the observed datapoints. The purple line is a linear fit to the observed populations (in linear-logarithmic space).

In the text

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