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1. Interstellar glycolamide: A comprehensive rotational study and an astronomical search in Sgr B2(N)

   https://doi.org/10.1051/0004-6361/202038149  

   Sanz-Novo, M. ; Belloche, A. ; Alonso, J. L. ; Kolesniková, L. ; Garrod, R. T. ; Mata, S. ; Müller, H. S. ; Menten, K. M. ; Gong, Y. ( July 2020 , Astronomy & Astrophysics)

   null (Ed.)

   Context. Glycolamide is a glycine isomer and also one of the simplest derivatives of acetamide (e.g., one hydrogen atom is replaced with a hydroxyl group), which is a known interstellar molecule. Aims. In this context, the aim of our work is to provide direct experimental frequencies of the ground vibrational state of glycolamide in the centimeter-, millimeter- and submillimeter-wavelength regions in order to enable its identification in the interstellar medium. Methods. We employed a battery of state-of-the-art rotational spectroscopic techniques in the frequency and time domain to measure the frequencies of glycolamide. We used the spectral line survey named Exploring Molecular Complexity with ALMA (EMoCA), which was performed toward the star forming region Sgr B2(N) with ALMA to search for glycolamide in space. We also searched for glycolamide toward Sgr B2(N) with the Effelsberg radio telescope. The astronomical spectra were analyzed under the local thermodynamic equilibrium approximation. We used the gas-grain chemical kinetics model MAGICKAL to interpret the results of the astronomical observations. Results. About 1500 transitions have been newly assigned up to 460 GHz to the most stable conformer, and a precise set of spectroscopic constants was determined. Spectral features of glycolamide were then searched for in the prominent hot molecular core Sgr B2(N2). We report the nondetection of glycolamide toward this source with an abundance at least six and five times lower than that of acetamide and glycolaldehyde, respectively. Our astrochemical model suggests that glycolamide may be present in this source at a level just below the upper limit, which was derived from the EMoCA survey. We could also not detect the molecule in the region’s extended molecular envelope, which was probed with the Effelsberg telescope. We find an upper limit to its column density that is similar to the column densities obtained earlier for acetamide and glycolaldehyde with the Green Bank Telescope. 

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2. Laboratory rotational spectroscopy of acrylamide and a search for acrylamide and propionamide toward Sgr B2(N) with ALMA

   https://doi.org/10.1051/0004-6361/202142448  

   Kolesniková, L. ; Belloche, A. ; Koucký, J. ; Alonso, E. R. ; Garrod, R. T. ; Luková, K. ; Menten, K. M. ; Müller, H. S. ; Kania, P. ; Urban, Š. ( March 2022 , Astronomy & Astrophysics)

   Context. Numerous complex organic molecules have been detected in the universe and among them are amides, which are considered as prime models for species containing a peptide linkage. In its backbone, acrylamide (CH 2 CHC(O)NH 2 ) bears not only the peptide bond, but also the vinyl functional group that is a common structural feature in many interstellar compounds. This makes acrylamide an interesting candidate for searches in the interstellar medium. In addition, a tentative detection of the related molecule propionamide (C 2 H 5 C(O)NH 2 ) has been recently claimed toward Sgr B2(N). Aims. The aim of this work is to extend the knowledge of the laboratory rotational spectrum of acrylamide to higher frequencies, which would make it possible to conduct a rigorous search for interstellar signatures of this amide using millimeter wave astronomy. Methods. We measured and analyzed the rotational spectrum of acrylamide between 75 and 480 GHz. We searched for emission of acrylamide in the imaging spectral line survey ReMoCA performed with the Atacama Large Millimeter/submillimeter Array toward Sgr B2(N). We also searched for propionamide in the same source. The astronomical spectra were analyzed under the assumption of local thermodynamic equilibrium. Results. We report accurate laboratory measurements and analyses of thousands of rotational transitions in the ground state and two excited vibrational states of the most stable syn form of acrylamide. In addition, we report an extensive set of rotational transitions for the less stable skew conformer. Tunneling through a low energy barrier between two symmetrically equivalent configurations has been revealed for this higher-energy species. Neither acrylamide nor propionamide were detected toward the two main hot molecular cores of Sgr B2(N). We did not detect propionamide either toward a position located to the east of the main hot core, thereby undermining the recent claim of its interstellar detection toward this position. We find that acrylamide and propionamide are at least 26 and 14 times less abundant, respectively, than acetamide toward the main hot core Sgr B2(N1S), and at least 6 and 3 times less abundant, respectively, than acetamide toward the secondary hot core Sgr B2(N2). Conclusions. A comparison with results of astrochemical kinetics model for related species suggests that acrylamide may be a few hundred times less abundant than acetamide, corresponding to a value that is at least an order of magnitude lower than the observational upper limits. Propionamide may be as little as only a factor of two less abundant than the upper limit derived toward Sgr B2(N1S). Lastly, the spectroscopic data presented in this work will aid future searches of acrylamide in space. 

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3. Millimeter wave spectrum and search for vinyl isocyanate toward Sgr B2(N) with ALMA

   https://doi.org/10.1051/0004-6361/202243627  

   Vávra, K. ; Kolesniková, L. ; Belloche, A. ; Garrod, R. T. ; Koucký, J. ; Uhlíková, T. ; Luková, K. ; Guillemin, J.-C. ; Kania, P. ; Müller, H. S. ; et al ( October 2022 , Astronomy & Astrophysics)

   Context.The interstellar detections of isocyanic acid (HNCO), methyl isocyanate (CH₃NCO), and very recently also ethyl isocyanate (C₂H₅NCO) invite the question of whether or not vinyl isocyanate (C₂H₃NCO) can be detected in the interstellar medium. There are only low-frequency spectroscopic data (<40 GHz) available for this species in the literature, which makes predictions at higher frequencies rather uncertain, which in turn hampers searches for this molecule in space using millimeter (mm) wave astronomy.

   Aims.The aim of the present study is on one hand to extend the laboratory rotational spectrum of vinyl isocyanate to the mm wave region and on the other to search, for the first time, for its presence in the high-mass star-forming region Sgr B2, where other isocyanates and a plethora of complex organic molecules are observed.

   Methods.We recorded the pure rotational spectrum of vinyl isocyanate in the frequency regions 127.5–218 and 285–330 GHz using the Prague mm wave spectrometer. The spectral analysis was supported by high-level quantum-chemical calculations. On the astronomy side, we assumed local thermodynamic equilibrium to compute synthetic spectra of vinyl isocyanate and to search for it in the ReMoCA survey performed with the Atacama Large Millimeter/submillimeter Array (ALMA) toward the high-mass star-forming protocluster Sgr B2(N). Additionally, we searched for the related molecule ethyl isocyanate in the same source.

   Results.Accurate values for the rotational and centrifugal distortion constants are reported for the ground vibrational states of trans and cis vinyl isocyanate from the analysis of more than 1000 transitions. We report nondetections of vinyl and ethyl isocyanate toward the main hot core of Sgr B2(N). We find that vinyl and ethyl isocyanate are at least 11 and 3 times less abundant than methyl isocyanate in this source, respectively.

   Conclusions.Although the precise formation mechanism of interstellar methyl isocyanate itself remains uncertain, we infer from existing astrochemical models that our observational upper limit for the CH₃NCO:C₂H₅NCO ratio in Sgr B2(N) is consistent with ethyl isocyanate being formed on dust grains via the abstraction or photodissociation of an H atom from methyl isocyanate, followed by the addition of a methyl radical. The dominance of such a process for ethyl isocyanate production, combined with the absence of an analogous mechanism for vinyl isocyanate, would indicate that the ratio C₂H₃NCO:C₂H₅NCO should be less than unity. Even though vinyl isocyanate was not detected toward Sgr B2(N), the results of this work represent a significant improvement on previous low-frequency studies and will help the astronomical community to continue searching for this species in the Universe.

    

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4. Toward the limits of complexity of interstellar chemistry: Rotational spectroscopy and astronomical search for n - and i -butanal

   https://doi.org/10.1051/0004-6361/202142848  

   Sanz-Novo, M. ; Belloche, A. ; Rivilla, V. M. ; Garrod, R. T. ; Alonso, J. L. ; Redondo, P. ; Barrientos, C. ; Kolesniková, L. ; Valle, J. C. ; Rodríguez-Almeida, L. ; et al ( October 2022 , Astronomy & Astrophysics)

   Context.In recent times, large organic molecules of exceptional complexity have been found in diverse regions of the interstellar medium.

   Aims.In this context, we aim to provide accurate frequencies of the ground vibrational state of two key aliphatic aldehydes,n-butanal and its branched-chain isomer, i-butanal, to enable their eventual detection in the interstellar medium. We also want to test the level of complexity that interstellar chemistry can reach in regions of star formation.

   Methods.We employ a frequency modulation millimeter-wave absorption spectrometer to measure the rotational features ofn- andi-butanal. We analyze the assigned rotational transitions of each rotamer separately using theA-reduced semirigid-rotor Hamiltonian. We use the spectral line survey ReMoCA performed with the Atacama Large Millimeter/submillimeter Array to search forn- andi-butanal toward the star-forming region Sgr B2(N). We also search for both aldehydes toward the molecular cloud G+0.693−0.027 with IRAM 30 m and Yebes 40 m observations. The observational results are compared with computational results from a recent gas-grain astrochemical model.

   Results.Several thousand rotational transitions belonging to the lowest-energy conformers of two distinct linear and branched isomers have been assigned in the laboratory spectra up to 325 GHz. A precise set of the relevant rotational spectroscopic constants has been determined for each structure as a first step toward identifying both molecules in the interstellar medium. We report non-detections ofn-and i-butanal toward both sources, Sgr B2(N1S) and G+0.693-0.027. We find thatn- andi-butanal are at least 2-6 and 6-18 times less abundant than acetaldehyde toward Sgr B2(N1S), respectively, and thatn-butanal is at least 63 times less abundant than acetaldehyde toward G+0.693−0.027. While propanal is not detected toward Sgr B2(N1S) either, with an abundance at least 5–11 lower than that of acetaldehyde, propanal is found to be 7 times less abundant than acetaldehyde in G+0.693−0.027. Comparison with astrochemical models indicates good agreement between observed and simulated abundances (where available). Grain-surface chemistry appears sufficient to reproduce aldehyde ratios in G+0.693−0.027; gas-phase production may play a more active role in Sgr B2(N1S). Model estimates for the larger aldehydes indicate that the observed upper limits may be close to the underlying values.

   Conclusions.Our astronomical results indicate that the family of interstellar aldehydes in the Galactic center region is characterized by a drop of one order of magnitude in abundance at each incrementation in the level of molecular complexity.

    

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5. Confirmation of gaseous methanediol from state-of-the-art theoretical rovibrational characterization

   https://doi.org/10.1039/D2CP02076A  

   Davis, Megan C. ; Garrett, Noah R. ; Fortenberry, Ryan C. ( August 2022 , Physical Chemistry Chemical Physics)

   High-level rovibrational characterization of methanediol, the simplest geminal diol, using state-of-the-art, purely ab initio techniques unequivocally confirms previously reported gas phase preparation of this simplest geminal diol in its C 2 conformation. The F12-TZ-cCR and F12-DZ-cCR quartic force fields (QFFs) utilized in this work are among the largest coupled cluster-based anharmonic frequencies computed to date, and they match the experimental band origins of the spectral features in the 980–1100 cm −1 range to within 3 cm −1 , representing a significant improvement over previous studies. The simulated spectrum also matches the experimental spectrum in the strong Q branch feature and qualitative shape of the 980–1100 cm −1 region. Additionally, the full set of rotational constants, anharmonic vibrational frequencies, and quartic and sextic distortion constants are provided for both the lowest energy C 2 conformer as well as the slightly higher C s conformer. Several vibrational modes have intensities of 60 km mol −1 or higher, facilitating potential astronomical or atmospheric detection of methanediol or further identification in laboratory work especially now that gas phase synthesis of this molecule has been established. 

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