Rotational spectroscopic study and astronomical search for propiolamide in Sgr B2(N)
E. R. Alonso, L. Kolesniková, A. Belloche, S. Mata, R. T. Garrod, A. Jabri, I. León, J.-C. Guillemin, H. S. P. Müller, K. M. Menten, J. L. Alonso
aa r X i v : . [ a s t r o - ph . GA ] F e b Astronomy & Astrophysicsmanuscript no. 40211corr © ESO 2021February 10, 2021
Rotational spectroscopic study and astronomical search forpropiolamide in Sgr B2(N) ⋆ E. R. Alonso , , L. Kolesniková , A. Belloche , S. Mata , R. T. Garrod , A. Jabri , I. León , J.-C. Guillemin ,H. S. P. Müller , K. M. Menten , and J. L. Alonso Instituto Biofisika (UPV / EHU, CSIC), University of the Basque Country, Leioa, Spain Fundación Biofísica Bizkaia / Biofisika Bizkaia Fundazioa (FBB), Barrio Sarriena s / n, Leioa, Spain Department of Analytical Chemistry, University of Chemistry and Technology, Technická 5, 166 28 Prague 6, Czech Republice-mail: [email protected] Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany Grupo de Espectroscopia Molecular (GEM), Edificio Quifima, Área de Química-Física, Laboratorios de Espectroscopia y Bioe-spectroscopia, Parque Científico UVa, Unidad Asociada CSIC, Universidad de Valladolid, 47011 Valladolid, Spain Departments of Chemistry and Astronomy, University of Virginia, Charlottesville, VA 22904, USA Univ Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR – UMR 6226, F-35000 Rennes, France I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, GermanyReceived ; accepted
ABSTRACT
Context.
For all the amides detected in the interstellar medium (ISM), the corresponding nitriles or isonitriles have also been detectedin the ISM, some of which have relatively high abundances. Among the abundant nitriles for which the corresponding amide has notyet been detected is cyanoacetylene (HCCCN), whose amide counterpart is propiolamide (HCCC(O)NH ). Aims.
With the aim of supporting searches for this amide in the ISM, we provide a complete rotational study of propiolamide from 6GHz to 440 GHz.
Methods.
Time-domain Fourier transform microwave (FTMW) spectroscopy under supersonic expansion conditions between 6 GHzand 18 GHz was used to accurately measure and analyze ground-state rotational transitions with resolved hyperfine structure arisingfrom nuclear quadrupole coupling interactions of the N nucleus. We combined this technique with the frequency-domain room-temperature millimeter wave and submillimeter wave spectroscopies from 75 GHz to 440 GHz in order to record and assign therotational spectra in the ground state and in the low-lying excited vibrational states. We used the ReMoCA spectral line survey per-formed with the Atacama Large Millimeter / submillimeter Array toward the star-forming region Sgr B2(N) to search for propiolamide. Results.
We identified and measured more than 5500 distinct frequency lines of propiolamide in the laboratory. These lines werefitted using an e ff ective semi-rigid rotor Hamiltonian with nuclear quadrupole coupling interactions taken into consideration. Weobtained accurate sets of spectroscopic parameters for the ground state and the three low-lying excited vibrational states. We reportthe nondetection of propiolamide toward the hot cores Sgr B2(N1S) and Sgr B2(N2). We find that propiolamide is at least 50 and 13times less abundant than acetamide in Sgr B2(N1S) and Sgr B2(N2), respectively, indicating that the abundance di ff erence betweenboth amides is more pronounced by at least a factor of 8 and 2, respectively, than for their corresponding nitriles. Conclusions.
Although propiolamide has yet to be included in astrochemical modeling networks, the observed upper limit to theratio of propiolamide to acetamide seems consistent with the ratios of related species as determined from past simulations. Thecomprehensive spectroscopic data presented in this paper will aid future astronomical searches.
Key words. astrochemistry – ISM: molecules – line: identification – ISM: individual objects: Sagittarius B2 – astronomicaldatabases: miscellaneous
1. Introduction
The discovery of molecules in the interstellar medium (ISM) isgreatly facilitated by our ability to predict which compoundsare present in that environment. Many interstellar moleculesform in the icy mantles of dust grains, including complex or-ganic molecules (COMs ; Herbst & van Dishoeck (2009)). Lab-oratory simulations of the chemistry of grains in the ISM couldprovide lists of target compounds. For example, it has recently ⋆ Table 2 is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp: // cdsweb.u-strasbg.fr / cgi-bin / qcat?J / A + A / COMs are defined to contain six or more atoms; at least one of theseatoms is carbon. been shown that, under hydrogen radical bombardments at 10K, the aldehydes are not reduced into the corresponding al-cohols (Jonusas et al. 2017) except for formaldehyde whichgives methanol (Hiraoka et al. 1994; Watanabe & Kouchi 2002;Fuchs et al. 2009; Pirim & Krim 2011). Based on similar ex-periments, nitriles do not lead to amines (Krim et al. 2019) ex-cept for HCN, which provides methylamine in a very low yield(Theule et al. 2011). On the other hand, to the best of our knowl-edge, the relation between nitriles and amides by the hydrationof the former or the dehydration of the latter (see Fig. 1) hasnever been studied in such laboratory simulations. We can how-ever observe that some couples of nitrile and amide are presentin the list of compounds detected in the ISM, such as hydro-gen cyanide (HCN) and formamide (NH CHO) or acetonitrile
Article number, page 1 of 11 & Aproofs: manuscript no. 40211corr (CH CN) and acetamide (CH C(O)NH ). However, such com-parisons are di ffi cult because for most of the amides corre-sponding to the nitriles observed in the ISM, one of the mostpowerful tools for such detections, the millimeter wave spec-trum, has never been recorded. Only microwave spectra canbe found for propionamide (Marstokk et al. 1996), acrylamide(Marstokk et al. 2000), or propiolamide (Little & Gerry 1978).The particular case of glycinamide (NH CH C(O)NH ) has tobe detailed. Many attempts have tried to detect amino acids inthe ISM and particularly the simplest one, glycine, but only apotential precursor of glycine, aminoacetonitrile (NH CH CN),has been detected (Belloche et al. 2008). In the hypothesis thatthis compound could be a precursor of glycine by hydrolysis, theaddition of one molecule of water would give the glycinamideintermediate. The microwave spectrum of this compound wasrecently published (Alonso et al. 2018), but the millimeter wavespectrum is still missing. On the other hand, this is not the case ofglycolamide, a glycine isomer, whose millimeter wave spectrumwas recorded very recently (Sanz-Novo et al. 2020).
Fig. 1.
Schematic representation of the hydration of nitriles to amidesand the dehydration of amides to nitriles.
To support the hypothesis of a correlation between the pres-ence of a nitrile and its corresponding amide, we first looked atwhich nitriles are the most abundant in the ISM. Unambiguously,after hydrogen cyanide (HCN) and acetonitrile (CH CN), pro-pionitrile (C H CN) and cyanoacetylene (HC N) are the mostabundant derivatives (McGuire 2018). HC N is ubiquitous in theUniverse, having been detected in comets, the atmosphere of Ti-tan, and in many places of the ISM. Therefore, propiolamide(HCCC(O)NH ) is plausible interstellar candidate. In addition,this eight-atom molecule has fewer atoms than the detected ac-etamide and contains a carbon-carbon triple bond, a widely rep-resented functional group among the known circumstellar andinterstellar compounds such as cyanopolyynes, alkynes, alkyneradicals or anions, magnesium acetylide (MgCCH), propy-nal (HCCCHO), ethynyl isocyanide (HCCNC), oxo-penta-2,4-diynyl (HC O), and oxo-hepta-2,4,6-triynyl (HC O) (McGuire2018).Since reliable interstellar searches for propiolamide shouldbe based on rotational transitions measured directly in the lab-oratory or those predicted from a data set that covers a broadspectral range, in this work we provide a detailed rotationalstudy of this amide up to 440 GHz. Two di ff erent high-resolutionspectroscopic techniques were employed for this purpose. Nar-rowband cavity-based Fourier transform microwave (FTMW)spectroscopy was used to measure the jet-cooled spectrum ofpropiolamide up to 18 GHz and to analyze the N nuclearquadrupole hyperfine structure of the ground-state rotationaltransitions. Millimeter and submillimeter wave spectroscopictechniques were used to record the room-temperature spectrumup to 480 GHz and to identify higher- J ground-state transitionsand the pure rotational spectra in excited vibrational states. Thepresent work significantly extends the knowledge of the rota-tional spectrum of propiolamide and provides su ffi ciently pre-cise laboratory information to search for this amide in space. Wechose as a target for this interstellar search the high-mass star- forming region Sagittarius (Sgr) B2(N), which is located close tothe Galactic center. We use our latest imaging spectral line sur-vey performed toward this source with the Atacama Large Mil-limeter / submillimeter Array (ALMA) in the 3 mm atmosphericwindow, the ReMoCA survey, which recently led to the identifi-cation of urea (Belloche et al. 2019).The article is structured as follows. Details about the labo-ratory experiments are given in Sect. 2 and the analysis of therecorded spectra of propiolamide is described in Sect. 3. Thesearch for propiolamide toward Sgr B2(N) is reported in Sect. 4.We discuss our results in Sect. 5 and give our conclusions inSect. 6.
2. Experiments
Propiolamide was prepared from methyl propiolate, followingthe recipe of Miller & Lemmon (1967). The solid product wasthen evacuated overnight to remove traces of methanol and usedwithout further purifications. To record the jet-cooled rotationalspectrum in the 6 – 18 GHz frequency range, propiolamide(melting point 57 – 62 ◦ C) was heated to approximately 80 ◦ C ina pulsed nozzle, seeded in neon carrier gas (backing pressure of1 bar) and adiabatically expanded into the Fabry-Pérot resonatorof the FTMW spectrometer described elsewhere (Alonso et al.2015; León et al. 2017). A short microwave radiation pulse of0.3 ms duration was applied to polarize the molecules of propio-lamide. A free induction decay associated with molecular emis-sion was registered in the time domain and converted to the fre-quency domain by Fourier transformation. Since the molecularbeam was directed along the resonator axis, the rotational tran-sitions were observed as Doppler doublets (see Fig. 3). The res-onance frequency was subsequently obtained as the arithmeticmean of the two Doppler components.To record the room-temperature rotational spectra between75 GHz and 440 GHz, propiolamide was placed into a smallglass container that was connected directly to the free space cellof the millimeter wave spectrometer and evacuated. The sam-ple was heated with a heating gun (temperature of 80 ◦ C of theheated air) until the pressure in the cell reached the optimalvalue between 10 and 20 µ bar. The millimeter wave spectrom-eter employed in this work is based on sequential multiplica-tion of the fundamental synthesizer frequency ( ≤
20 GHz) bya set of active and passive multipliers (VDI, Inc) and has beendescribed elsewhere (Kolesniková et al. 2017a,b). For the mea-surements reported in this work, the synthesizer output was fre-quency modulated at a modulation frequency of 10.2 kHz andmodulation depth between 20 kHz and 40 kHz before it was mul-tiplied by amplifier-multiplier chains (WR10.0, WR6.5, WR9.0)in combination with additional doublers (WR4.3, WR2.2) anda tripler (WR2.8). After a double pass of the radiation throughthe cell, the signal was detected by zero-bias detectors and de-modulated by a lock-in amplifier tuned to twice the modulationfrequency. This demodulation procedure results in a shape of thelines that approximates the second derivative of a Gaussian line-profile function. All spectra were registered in 1 GHz sections inboth directions (a single acquisition cycle) and averaged. Thesesections were ultimately combined into a single spectrum andfurther processed using the AABS package (Kisiel et al. 2005,2012).
Article number, page 2 of 11lonso et al.: The rotational spectrum of propiolamide F = 27 26 ¬ J ² = 70
23 22 ¬
23 22 ¬
23 22 ¬
23 22 ¬
18 17 ¬
18 17 ¬
18 17 ¬
18 17 ¬
27 26 ¬
24 23 ¬
24 23 ¬
24 23 ¬
24 23 ¬
25 24 ¬
25 24 ¬
25 24 ¬
25 24 ¬ (b)(c) (d)(a)
69 68 67 66 65 64 F = 28 27 ¬ F = 26 25 ¬ Fig. 2.
Illustrations of the main characteristics of the room-temperature rotational spectrum of propiolamide. (a) An example of a quartet of a -typeand b -type R-branch rotational transitions between K a =
0, 1, K c = J pairs of energy levels. (b) Quadruply degenerate lines consisting of a pair of a -type and a pair of b -type transitions involving the same K a =
0, 1 pairs of levels. These strong lines appear in the spectrum approximately every6.06 GHz, which corresponds to the value of 2 C . (c) An example of a group of high- J quadruply degenerate transitions. The leading transition is J = ←
70 and the value of J decreases by 1 with each successive line running to higher frequencies. The K a quantum number is increasingaway from the leading line in such a way that the first line corresponds to four degenerate K a = , K a = ,
2, the thirdto K a = ,
3, and so on. (d) An example of N nuclear quadrupole hyperfine structure observed in the millimeter wave spectrum. The F = J ± F = J component is, in many cases, relatively well separated.
3. Rotational spectra and analysis
As revealed by Stark modulation spectroscopy, propiolamide isessentially planar with dipole moment components | µ a | = | µ b | = a -type and b -type transitions are therefore relevant in the millimeter wavespectrum. Strong lines corresponding to pairs of b -type transi-tions ( J + , J + ← J , J and ( J + , J + ← J , J could be im-mediately assigned in the spectrum. They were accompanied byweaker pairs of a -type transitions ( J + , J + ← J , J and ( J + , J + ← J , J . Figure 2a shows that the two b -type transitionsstraddle the pair of a -type transitions giving rise to characteristicquartets. These quartets, however, are not observed throughoutthe whole spectrum. As the J quantum number increases, theparticipating energy levels become near degenerate and the fourmembers of the quartets coalesce into very prominent quadru-ply degenerate lines that clearly dominate the spectrum shownin Fig. 2b. The same quartet structures and line blending are alsoobserved for higher K a transitions. With the J quantum numberfurther progressively increasing, the quadruply degenerate lineswith low values of K a quantum numbers start to form groups in which successive lines di ff er by a unit in J and K a . A typi-cal example of this feature is shown in Fig. 2c. Such a patternis characteristic for high- J a -type and b -type spectra of planarand nearly planar molecules and was observed, for example,in acrylic acid (Alonso et al. 2015), phenol (Kolesniková et al.2013), or 2-chloroacrylonitrile (Kisiel & Pszczolkowski 1994).Finally, we also localized and measured b -type Q-branch transi-tions.During the analysis procedure, we noticed that several tran-sitions exhibited small splitting. We attributed this splitting tonuclear quadrupole coupling interactions of the single N nu-cleus present in the amide group. At the current resolution ofthe spectra in Fig. 2, this typically gives rise to asymmetric dou-blets (see Fig. 2d). To correctly assign the hyperfine componentsblended together in these doublets, we measured and analyzedthe nuclear quadrupole hyperfine structure from 6 GHz to 18GHz using the cavity-based FTMW spectrometer that o ff ers sig-nificantly higher spectral resolution. One of the measured transi-tions is exemplified in Fig. 3. In total 48 hyperfine componentswere measured and analyzed using the e ff ective Hamiltonian H = H R + H Q , where H R is the standard Watson semi-rigid rotorHamiltonian in A -reduction and I r representation (Watson 1977), Article number, page 3 of 11 & Aproofs: manuscript no. 40211corr
Table 1.
Spectroscopic constants of propiolamide in the ground state and three excited vibrational states ( A -reduction, I r -representation).Ground state v in = v out = v inv = A / MHz 11417.92091 (11) a B / MHz 4135.477101 (40) 4153.949250 (78) 4146.28849 (10) 4131.104876 (83) C / MHz 3032.592907(35) 3036.029785 (61) 3041.801213 (72) 3032.360188 (56) ∆ J / kHz 0.579701 (18) 0.612639 (36) 0.594171 (59) 0.578258 (51) ∆ JK / kHz 20.86142 (18) 19.27979 (36) 21.82228 (41) 20.73217 (36) ∆ K / kHz -10.05819 (20) -16.51167 (89) -2.7538 (10) -9.93128 (41) δ J / kHz 0.1753084 (80) 0.190158 (18) 0.177915 (29) 0.174458 (25) δ K / kHz 12.16107 (24) 11.28587 (43) 13.08620 (65) 12.08963 (55) Φ J / Hz 0.0003369 (31) 0.0005094 (77) 0.000273 (17) 0.000370 (15) Φ JK / Hz 0.19762 (12) 0.16406 (25) 0.22129 (38) 0.19630 (28) Φ KJ / Hz -0.47001 (41) -0.55885 (90) -0.3560 (13) -0.46592 (78) Φ K / Hz 0.30222 (35) -0.5968 (10) 1.1713 (13) 0.30279 (69) φ J / Hz 0.0001448 (15) 0.0002278 (41) 0.0001229 (89) 0.0001680 (78) φ JK / Hz 0.095653 (64) 0.08057 (11) 0.10591 (22) 0.09532 (19) φ K / Hz 0.6385(11) 0.3538 (18) 0.9125 (29) 0.6349 (22) L JJK / mHz -0.0011188 (88) -0.001301 (27) -0.000844 (35) -0.001004 (28) L KKJ / mHz ... 0.05208 (54) -0.05083 (72) ... χ aa / MHz 1.8157 (22) 1.8157 b b b χ bb − χ cc / MHz 6.0012 (44) 6.336 (40) 6.212 (48) 5.796 (44) J min / J max /
75 3 /
71 3 /
71 3 / K a ,min / K a ,max /
32 0 /
25 0 /
25 0 / σ fit c MHz 0.023 0.032 0.032 0.026 N d Notes. ( a ) The numbers in parentheses are 1 σ uncertainties (67% confidence level) in units of the last decimal digit. The SPFIT / SPCAT programpackage (Pickett 1991) was used for the analysis ( b ) Fixed to the ground state value owing to the limited hyperfine data set in excited vibrationalstates. ( c ) Root mean square deviation of the fit. ( d ) Number of distinct frequency lines in the fit.
Frequency (MHz) F = 3 3 ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Fig. 3. N nuclear quadrupole hyperfine structure of the 3 ← rotational transition as measured by cavity-based FTMW spectrometer.Each hyperfine component, which appears as a Doppler doublet, is la-beled with the corresponding values of F quantum number. and H Q represents the nuclear quadrupole coupling Hamiltonian(Gordy & Cook 1984). The coupling scheme F = J + I N betweenthe rotational angular momentum J and the N nuclear spin an-gular momentum I N was used. A comparison of the observedasymmetric doublets with predictions based on this hyperfineanalysis made it possible to undertake a simultaneous analysisof hyperfine-split and hyperfine-free lines. Transitions measuredin the FTMW spectra and those in the millimeter wave spec-tra were adequately weighted assuming the experimental un- certainties of 5 and 30 kHz, respectively. These data were ul-timately combined with a selection of hyperfine-free transitionsfrom Little & Gerry (1978) with assigned uncertainties equal to100 kHz and were analyzed using the same e ff ective Hamilto-nian as mentioned above. The broad coverage of transition typesand quantum number values (see Table 1) guaranteed an accuratedetermination of rotational constants, centrifugal distortion con-stant along with the diagonal elements of the nuclear quadrupolecoupling tensor. All these spectroscopic parameters are summa-rized in the first column of Table 1. The list of measured transi-tions is given in Table 2 of the supplementary material. Each ground-state line in the millimeter wave spectrum wasaccompanied by satellite lines at the high- and low-frequencysides. In the neighborhood of the ground-state J = ← b -type and two a -type transitions as in the ground stateand were attributed to pure rotational transitions in excited vi-brational states. According to our theoretical calculations at theB3LYP / ++ G(d,p) level of the theory (Gaussian 16 pack-age, Frisch et al. 2016) the three low-lying vibrational modescorrespond mainly to the in-plane and out-of-plane bending mo-tions of C − C ≡ C group ν in ( A ′ ) and ν out ( A ′′ ), respectively, andto the NH inversion ν inv ( A ′′ ). Using the calculated first-ordervibration-rotation constants α i , it was possible to predict therotational constants for relevant excited states according to theequation B v = B e − P i α i ( v i + / B v and B e denote allthe three rotational constants in a given excited state and in equi-librium, respectively, and v i is the vibrational quantum numberof the i -th mode. The predictions proved to be su ffi ciently accu-rate for the unambiguous assignment of the satellite pattern, as Article number, page 4 of 11lonso et al.: The rotational spectrum of propiolamide
17 16 ¬
17 16 ¬
17 16 ¬
17 16 ¬ GS v in = 1 v out = 1 v inv = 1 Fig. 4.
Vibrational satellites accompanying the quartet of ground-state J = ←
16 transitions in the millimeter wave spectrum.
Table 2.
List of the measured transitions in the ground state and three excited vibrational states of propiolamide.Vibrational state J ′ K ′ a K ′ c F ′ J ′′ K ′′ a K ′′ c F ′′ ν obs (MHz) a u obs (MHz) b ν obs − ν calc (MHz) c Comment d G.S. 2 0 2 1 1 1 1 1 6936.603 0.005 -0.001 (1)G.S. 5 0 5 ... 4 0 4 ... 33827.730 0.100 -0.036 (2)G.S. 28 5 23 28 28 4 24 28 109907.745 0.030 -0.025 (1) v in = v in = v out = v out = v inv = v inv = Notes. ( a ) Observed frequency. ( b ) Uncertainty of the observed frequency. ( c ) Observed minus calculated frequency. ( d ) (1) This work. (2) Taken fromLittle & Gerry (1978). This table is available in its entirety in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http: // cdsweb.u-strasbg.fr / cgi-bin / qcat?J / A + A / . A portion is shown for guidance regarding its form and content. shown in Fig. 4. The two satellites displaced to high frequencyfrom the ground state were assigned to the first quanta of the twobending modes v in = v out =
1, while the low-frequencysatellite was ascribed to the first excited state of NH inversion v inv = v in = v out =
1, and v inv = a -type and b -type R-branch transitionswere identified and measured first. Then, we assigned b -type Q-branch transitions. As in the case of the ground state, severallines exhibited splitting due to N nuclear quadrupole couplinginteractions. The above Hamiltonian H = H R + H Q was thereforeused to encompass all the assigned transitions. The obtained val-ues of the rotational and centrifugal distortion constants as wellas the N nuclear quadrupole coupling constants are collected inTable 1. The list of the measured transition frequencies is givenin Table 2.Although the rotational transitions in v in = v out =
1, and v inv = v in = v out = ∆ K forthese two states ( − .
63 kHz) is very close to the ground-statevalue ( − .
058 kHz). The same behavior is also perceptible forother centrifugal distortion constants and points to the existenceof interactions between v in = v out =
1. The spectroscopicconstants reported in Table 1 should be thus taken as e ff ectiveparameters that reproduce the vibrational satellite spectrum nearthe experimental uncertainty.Since the spectroscopic line lists for interstellar detection re-quire not only reliable line frequencies but also line intensities,we provide in Table 3 partition functions of propiolamide at mul-tiple temperatures. The rotational partition function ( Q rot ) cor-responds to the ground vibrational state and was evaluated bysummation over the energy levels. We used the SPCAT program(Pickett 1991) to undertake this summation numerically, em-ploying the rotational and centrifugal distortion constants fromTable 1, dipole moment components | µ a | = | µ b | = J quantum number of 200. The vibrational-rotational parti-tion function ( Q vib-rot ) takes into consideration the ground state Article number, page 5 of 11 & Aproofs: manuscript no. 40211corr
Table 3.
Partition functions of propiolamide. T (K) Q rot Q vib-rot a Q vib Notes. ( a ) This vibrational-rotational partition function includes the con-tribution of the ground state and v in = v out =
1, and v inv = and the three observed excited vibrational states. This partitionfunction was also computed by summation over the energy lev-els with the vibrational energies of 170, 210, and 300 cm − for v in = v out =
1, and v inv =
1, respectively, and the rotationaland centrifugal distortion constants from Table 1. The values ofthe vibrational energies were roughly estimated by comparisonof spectral intensities of the same rotational transitions in thecorresponding excited vibrational state with respect to those inthe ground state. Finally, the vibrational partition function ( Q vib )was evaluated using Eq. 3.60 of Gordy & Cook (1970) by tak-ing into consideration the frequencies of 18 normal vibrationalmodes from Table A.1 of the Appendix.
4. Search for propiolamide toward Sgr B2(N)
The data set used in this work was extracted from the Re-MoCA survey performed with ALMA toward Sgr B2(N). Detailsabout the observational setup and data reduction of this imagingspectral line survey can be found in Belloche et al. (2019). Inshort, the angular resolution (HPBW) of the survey varies be-tween ∼ ′′ and ∼ ′′ , with a median value of 0.6 ′′ , whichcorresponds to ∼ − ). The equatorial coordinates of thephase center are ( α, δ ) J2000 = (17 h m s . , − ◦ ′ ′′ . ′′ or ∼ − and 1.1 mJy beam − (rms), with a medianvalue of 0.8 mJy beam − .We selected two positions for this study: the o ff set po-sition Sgr B2(N1S) located at ( α, δ ) J2000 = (17 h m s . − ◦ ′ ′′ .
48) and the secondary hot core Sgr B2(N2) at( α, δ ) J2000 = (17 h m s . − ◦ ′ ′′ . ′′ to the south of the main hot core Sgr B2(N1) and waschosen by Belloche et al. (2019) for the lower opacity of its con-tinuum emission, which allows for a deeper look into the molec-ular content of this hot core. We used a new version of our dataset for which we have improved the splitting of the continuumand line emission as reported in Melosso et al. (2020).The spectra of Sgr B2(N1S) and Sgr B2(N2) were modeledwith the software Weeds (Maret et al. 2011) under the assump-tion of local thermodynamic equilibrium (LTE). This assump-tion is appropriate because the regions where hot-core emission is detected in Sgr B2(N) have high densities ( > × cm − , seeBonfand et al. 2019). We derived a best-fit synthetic spectrum ofeach molecule separately and then added the contributions of allidentified molecules together. We modeled each species with aset of five parameters: size of the emitting region ( θ s ), columndensity ( N ), temperature ( T rot ), linewidth ( ∆ V ), and velocity o ff -set ( V o ff ) with respect to the assumed systemic velocity of thesource, V sys =
62 km s − for Sgr B2(N1S) and V sys =
74 km s − for Sgr B2(N2). We used the parameters derived for acetamide, CH C(O)NH ,by Belloche et al. (2017, 2019) to compute LTE synthetic spectraof propiolamide and search for rotational emission of the latterin the ReMoCA spectra of Sgr B2(N2) and Sgr B2(N1S), respec-tively. We only kept the column density of propiolamide as a freeparameter. We found no evidence for emission of propiolamidetoward either source. The nondetection toward Sgr B2(N1S) andSgr B2(N2) is illustrated in Figs. 5 and 6, respectively, and theupper limits to the column density of propiolamide are indicatedin Tables 4 and 5, respectively. The tables also recall the pa-rameters previously obtained for acetamide. The spectroscopiccatalog used to compute the synthetic spectra of propiolamideshown in Figs. 5 and 6 was prepared with the partition functionthat includes the vibrational ground state and the three lowestvibrationally excited states ( Q vib-rot in Table 3). The correctionfactor F vib used in Tables 4 and 5 to derive the upper limit tothe column density of propiolamide accounts for the higher vi-brational states, using the corresponding vibrational frequencieslisted in Table A.1. F vib was computed as Q vib × Q rot / Q vib-rot us-ing the values listed in Table 3.
5. Discussion
The nondetection of propiolamide toward Sgr B2(N1S) andSgr B2(N2) reported in Sect. 4.2 implies that propiolamide isat least ∼
50 and ∼
13 times less abundant than acetamide to-ward Sgr B2(N1S) and Sgr B2(N2), respectively. For compari-son, cyanoacetylene, HC N, is about six times less abundant thanmethyl cyanide, CH CN, toward Sgr B2(N2) (Belloche et al.2016) and a preliminary analysis of the ReMoCA survey yieldsa similar ratio for Sgr B2(N1S). Both pairs of molecules,the amides HCCC(O)NH / CH C(O)NH and the cyanidesHC N / CH CN, share the same structural di ff erence (an unsat-urated C H group replaced with a saturated CH group), but theabundance di ff erence between both (larger) amides is more pro-nunced than for the (smaller) cyanides, by at least a factor of 2for Sgr B2(N2) and at least a factor of 8 for Sgr B2(N1S). Existing interstellar chemical networks do not include propio-lamide; however, guided by the behavior of related species inthe chemical kinetic simulations, we may speculate on the fac-tors that could impact upon its formation and / or survival duringstar formation.In common with acetamide, it would appear plausible thatpropiolamide could be produced on grain surfaces, or withintheir ice mantles, by the reaction of the radical NH CO withanother radical; CH in the former case, C H in the latter. Un-saturated hydrocarbons such as acetylene (C H ) and associ- Article number, page 6 of 11lonso et al.: The rotational spectrum of propiolamide
Fig. 5.
Selection of transitions of HCCC(O)NH , = , = y -axis is labeled in brightnesstemperature units (K). The dotted line indicates the 3 σ noise level. Table 4.
Parameters of our best-fit LTE model of acetamide toward Sgr B2(N1S), and upper limit for propiolamide.
Molecule Status a N det b θ s c T rot d N e F vib f ∆ V g V o ff h N ref N i ( ′′ ) (K) (cm − ) (km s − ) (km s − ) CH C(O)NH (j) ⋆ d 153 2.0 160 4.1 (17) 1.16 5.0 0 . n 0 2.0 160 < . > Notes. ( a ) d: detection, n: nondetection. ( b ) Number of detected lines (conservative estimate, see Sect. 3 of Belloche et al. 2016). One line of a givenspecies may mean a group of transitions of that species that are blended together. ( c ) Source diameter (FWHM). ( d ) Rotational temperature. ( e ) Totalcolumn density of the molecule. x ( y ) means x × y . ( f ) Correction factor that was applied to the column density to account for the contributionof vibrationally excited states, in the cases where this contribution was not included in the partition function of the spectroscopic predictions. ( g ) Linewidth (FWHM). ( h ) Velocity o ff set with respect to the assumed systemic velocity of Sgr B2(N1S), V sys =
62 km s − . ( i ) Column densityratio, with N ref the column density of the previous reference species marked with a ⋆ . ( j ) The parameters were derived from the ReMoCA surveyby Belloche et al. (2019).
Table 5.
Parameters of our best-fit LTE model of acetamide toward Sgr B2(N2), and upper limit for propiolamide.
Molecule Status a N det b θ s c T rot d N e F vib f ∆ V g V o ff h N ref N i ( ′′ ) (K) (cm − ) (km s − ) (km s − ) CH C(O)NH (j) ⋆ d 23 0.9 180 1.4 (17) 1.23 5.0 1 . n 0 0.9 180 < . > Notes. ( a ) d: detection, n: nondetection. ( b ) Number of detected lines (conservative estimate, see Sect. 3 of Belloche et al. 2016). One line of a givenspecies may mean a group of transitions of that species that are blended together. ( c ) Source diameter (FWHM). ( d ) Rotational temperature. ( e ) Totalcolumn density of the molecule. x ( y ) means x × y . ( f ) Correction factor that was applied to the column density to account for the contributionof vibrationally excited states, in the cases where this contribution was not included in the partition function of the spectroscopic predictions. ( g ) Linewidth (FWHM). ( h ) Velocity o ff set with respect to the assumed systemic velocity of Sgr B2(N2), V sys =
74 km s − . ( i ) Column densityratio, with N ref the column density of the previous reference species marked with a ⋆ . ( j ) The parameters were derived from the EMoCA survey byBelloche et al. (2017). ated radicals such as C H, are typically considered “early-time”species, which become abundant in the gas phase before freecarbon has had time to become locked up in stable species suchas CO. At such a stage, the dust-grain ice mantles are still inthe process of formation. Surface radical reactions during thisperiod might provide an opportunity for propiolamide produc- tion, while gas-phase hydrocarbons remain available to adsorbonto the grain surfaces. Although the grains would be consid-ered too cold ( ∼
10 K) for surface di ff usion of large radicals todrive production, the occasional presence of reactive radicals inclose proximity to each other appears to be su ffi cient to allowa degree of complex organic molecule production even at low Article number, page 7 of 11 & Aproofs: manuscript no. 40211corr
Fig. 6.
Same as Fig. 5 but for Sgr B2(N2). temperatures (e.g., Jin & Garrod 2020). New models of hot-corechemistry (Garrod et al., in prep.) indicate that such cold, earlymechanisms may be major contributors to the production of cer-tain species that are later observed in the gas phase. This appearsto be the case with acetamide; if similar cold mechanisms werealso to contribute to propiolamide production, then the ratio ofmethane (CH ) to acetylene produced in the models could givean indication of the relative production of the larger species. Atthe end of the cold collapse stage, the Garrod et al. (in prep.)models indicate that methane is around 3 orders of magnitudemore abundant than acetylene in the ices. This ratio is consistentwith the observational ratio of acetamide to the upper limit forpropiolamide.In the models, acetamide may also be formed on the grainsthrough the addition of the radicals NH and CH CO, the lat-ter of which is a product of the destruction of acetaldehyde (CH CHO). The comparable mechanism for propiolamide pro-duction would be NH addition to the HC CO radical, whichcould be formed by the loss of a hydrogen atom from the alde-hyde group of propynal, HC CHO. As this molecule and itssurrounding chemistry are not included in our chemical models(other than in the most trivial fashion), the implications of sucha mechanism cannot be quantified.An alternative to cold surface production at early times isthe cosmic-ray-induced UV photolysis of molecules within thedust-grain ice mantles, which may occur continuously from theonset of mantle formation until their ultimate desorption into thegas phase. This mechanism provides a means by which the req-uisite radicals can be produced and thence react to form propi-olamide and acetamide. In this case, the relative abundances ofprecursors NH CHO, CH (and / or CH OH) and C H , as well Article number, page 8 of 11lonso et al.: The rotational spectrum of propiolamide as NH , CH CHO and HC CHO, would again be important indetermining ratios between acetamide and propiolamide.Assuming that propiolamide may be formed in some abun-dance on dust grains prior to the thermal desorption of dust-grain ices, it is also possible that reactions with abundant grain-surface H atoms could produce at least partial conversion topropanamide (CH CH CONH ), further reducing the abundanceof propiolamide. Hydrogenation by H atoms could similarly actto reduce the abundance of acetylene in the ice mantles, remov-ing a possible precursor for the ongoing production of propio-lamide via ice photolysis. All in all, the observed abundances andratios between propiolamide and acetamide appear to be consis-tent with the expectations from limited chemical modeling evi-dence.
6. Conclusions
Using a combination of time-domain FTMW spectroscopyand frequency-domain millimeter wave spectroscopy tech-niques, a detailed rotational study of propiolamide from 6 GHzto 440 GHz was carried out. In total, more than 5500 new rota-tional lines for the ground state and the three lowest excited vi-brational states were measured and assigned. The present worksignificantly extends the frequency coverage of the propiolamiderotational spectrum known to date and newly derived spectro-scopic parameters provide a firm base to guide a search for thismolecule using radio astronomy.Propiolamide was not detected toward the hot molecularcores Sgr B2(N1S) and Sgr B2(N2) with ALMA. The upper lim-its derived for its column density imply that it is at least 50 and13 times less abundant than acetamide toward these sources, re-spectively. This abundance di ff erence between both amides ismore pronounced than for their corresponding nitriles by at leasta factor of 8 for Sgr B2(N1S) and 2 for Sgr B2(N2).The observational results seem consistent with the low ra-tio of propiolamide to acetamide inferred from the results ofchemical kinetic models (which include the latter species butnot the former). In the proposed scenario, production of propio-lamide would occur through radical addition on dust-grain sur-faces. This mechanism may be most e ff ective at early times inthe chemical evolution when unsaturated hydrocarbons shouldbe abundant in the gas phase. Reactions with grain-surface Hatoms could also diminish the abundance of propiolamide andits precursors in the ice mantles prior to thermal desorption.More promising sources for the detection of propiolamidemay be sources where unsaturated carbon chain molecules aremore prominent than in Sgr B2(N). One possibility, for instance,is TMC1, where HC N is more than one order of magnitudemore abundant than CH CN (Gratier et al. 2016). However, theamine CH NH has not been detected toward TMC1 in theGOTHAM survey with the GBT telescope so far (B. McGuire,priv. comm. 2021), which is probably not a good sign for propi-olamide. The quiescent source G + Acknowledgements.
The funding from Ministerio de Ciencia e Innovación(CTQ2016-76393-P and PID2019-111396GB-I00), Czech Science Foundation(GACR, grant 19-25116Y), Junta de Castilla y Le´ón (Grants VA077U16 andVA244P20), and European Research Council under the European Union’sSeventh Framework Programme ERC-2013-SyG, Grant Agreement n. 610256NANOCOSMOS are gratefully acknowledged. E.R.A. thanks Fundación Biofísica Bizkaia (FBB) for a postdoctoral contract. J.C.G. thanks the programPhysique et Chimie du Milieu Interstellaire (INSU-CNRS) and the Centre Na-tional d’Etudes Spatiales (CNES). This paper makes use of the following ALMAdata: ADS / JAO.ALMA / NRAO, and NAOJ. The interferometric data are available in theALMA archive at https: // almascience.eso.org / aq / . Part of this work has been car-ried out within the Collaborative Research Centre 956, sub-project B3, fundedby the Deutsche Forschungsgemeinschaft (DFG) – project ID 184018867. RTGacknowledges support from the National Science Foundation (grant No. AST19-06489). References
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Article number, page 9 of 11 & Aproofs: manuscript no. 40211corr
Appendix A: Complementary Tables
Article number, page 10 of 11lonso et al.: The rotational spectrum of propiolamide
Table A.1.
Frequencies of normal vibrational modes of propiolamide used to calculate the vibrational partition function.Mode Frequency (cm − ) Symmetry1 3724 A ′ A ′ A ′ A ′ A ′ A ′ A ′ A ′ A ′
10 701 A ′
11 600 A ′
12 490 A ′
13 170 A ′
14 762 A ′′
15 723 A ′′
16 554 A ′′
17 300 A ′′
18 230 A ′′ Notes.
The frequencies of the three lowest modes were estimated on the basis of the experimental relative intensities measurements with uncer-tainties of 30 cm − , while the others were taken from the theoretical calculations at the B3LYP / ++++