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1. The spectral features and detectability of small, cyclic silicon carbide clusters

   https://doi.org/10.3389/fspas.2022.1074879  

   Sehring, Christopher M. ; Palmer, C. Zachary ; Westbrook, Brent R. ; Fortenberry, Ryan C. ( December 2022 , Frontiers in Astronomy and Space Sciences)

   Rovibrational spectral data for several tetra-atomic silicon carbide clusters (TASCCs) are computed in this work using a CCSD(T)-F12b/cc-pCVTZ-F12 quartic force field. Accurate theoretical spectroscopic data may facilitate the observation of TASCCs in the interstellar medium which may lead to a more complete understanding of how the smallest silicon carbide (SiC) solids are formed. Such processes are essential for understanding SiC dust grain formation. Due to SiC dust prevalence in the interstellar medium, this may also shed light on subsequent planetary formation. Rhomboidal Si₂C₂is shown here to have a notably intense (247 km mol⁻¹) anharmonic vibrational frequency at 988.1 cm⁻¹(10.1 μm) forν₂, falling into one of the spectral emission features typically associated with unknown infrared bands of various astronomical regions. Notable intensities are also present for several of the computed anharmonic vibrational frequencies including the cyclic forms of C₄, SiC₃, Si₃C, and Si₄. These features in the 6–10 μm range are natural targets for infrared observation with theJames Webb Space Telescope(JWST)’s MIRI instrument. Additionally,t-Si₂C₂,d-Si₃C, andr-SiC₃each possess dipole moments of greater than 2.0 D making them interesting targets for radioastronomical searches especially sinced-SiC₃is already known in astrophysical media.

    

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2. 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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3. On the detectability of interstellar diaminomethane ((NH2)2CH2)

   https://doi.org/10.1093/mnras/stad3938  

   Watrous, Alexandria G. ; Westbrook, Brent R. ; Fortenberry, Ryan C. ( December 2023 , Monthly Notices of the Royal Astronomical Society)

   The recent synthesis of diaminomethane ((NH2)2CH2) under simulated interstellar conditions implies that this molecule is likely present in the interstellar medium (ISM), and this work provides the highly accurate quantum chemically computed rotational constants and IR frequencies for comparison to experiment that may aid in its potential future detection. This simplest geminal diamine of prebiotic note has a transition for its ν18 fundamental frequency at 702.5 cm−1 (14.23 μm) with an intensity on level with that of the antisymmetric stretch in carbon dioxide. Additionally, its 1.72 D dipole moment for the most stable C2v conformer should make it detectable with radiotelescopes as well. The C2 conformer may also be detectable in similar regions where the C2v form is present, but only warmer or higher-energy regions will allow for possible observation of the C1 conformer. In any case, rotational constants for these other two conformers are also provided in addition to the full set of fundamental vibrational frequencies for the C2v conformer. These data will assist with further laboratory classification and possible interstellar detection of this prebiotic molecule.

    

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4. Direct kinetic measurements and theoretical predictions of an isoprene-derived Criegee intermediate

   https://doi.org/10.1073/pnas.1916711117  

   Caravan, Rebecca L. ; Vansco, Michael F. ; Au, Kendrew ; Khan, M. Anwar H. ; Li, Yu-Lin ; Winiberg, Frank A. F. ; Zuraski, Kristen ; Lin, Yen-Hsiu ; Chao, Wen ; Trongsiriwat, Nisalak ; et al ( April 2020 , Proceedings of the National Academy of Sciences)

   Isoprene has the highest emission into Earth’s atmosphere of any nonmethane hydrocarbon. Atmospheric processing of alkenes, including isoprene, via ozonolysis leads to the formation of zwitterionic reactive intermediates, known as Criegee intermediates (CIs). Direct studies have revealed that reactions involving simple CIs can significantly impact the tropospheric oxidizing capacity, enhance particulate formation, and degrade local air quality. Methyl vinyl ketone oxide (MVK-oxide) is a four-carbon, asymmetric, resonance-stabilized CI, produced with 21 to 23% yield from isoprene ozonolysis, yet its reactivity has not been directly studied. We present direct kinetic measurements of MVK-oxide reactions with key atmospheric species using absorption spectroscopy. Direct UV-Vis absorption spectra from two independent flow cell experiments overlap with the molecular beam UV-Vis-depletion spectra reported recently [M. F. Vansco, B. Marchetti, M. I. Lester,J. Chem. Phys.149, 44309 (2018)] but suggest different conformer distributions under jet-cooled and thermal conditions. Comparison of the experimental lifetime herein with theory indicates only thesyn-conformers are observed;anti-conformers are calculated to be removed much more rapidly via unimolecular decay. We observe experimentally and predict theoretically fast reaction ofsyn-MVK-oxide with SO₂and formic acid, similar to smaller alkyl-substituted CIs, and by contrast, slow removal in the presence of water. We determine products through complementary multiplexed photoionization mass spectrometry, observing SO₃and identifying organic hydroperoxide formation from reaction with SO₂and formic acid, respectively. The tropospheric implications of these reactions are evaluated using a global chemistry and transport model.

    

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5. Probing Magnetic Fields in Protoplanetary Disk Atmospheres through Polarized Near-IR Light Scattered by Aligned Grains

   https://doi.org/10.3847/1538-3881/ac7ffc  

   Yang, Haifeng ; Li, Zhi-Yun ( August 2022 , The Astronomical Journal)

   Magnetic fields play essential roles in protoplanetary disks. Magnetic fields in the disk atmosphere are of particular interest, as they are connected to the wind-launching mechanism. In this work, we study the polarization of the light scattered off of magnetically aligned grains in the disk atmosphere, focusing on the deviation of the polarization orientation from the canonical azimuthal direction, which may be detectable in near-IR polarimetry with instruments such as VLT/SPHERE. We show with a simple disk model that the polarization can even be oriented along the radial (rather than azimuthal) direction, especially in highly inclined disks with toroidally dominated magnetic fields. This polarization reversal is caused by the anisotropy in the polarizability of aligned grains and is thus a telltale sign of such grains. We show that the near-IR light is scattered mostly byμm-sized grains or smaller at theτ= 1 surface and such grains can be magnetically aligned if they contain superparamagnetic inclusions. For comparison with observations, we generate synthetic maps of the ratios ofU_ϕ/IandQ_ϕ/I, which can be used to infer the existence of (magnetically) aligned grains through a negativeQ_ϕ(polarization reversal) and/or a significant level ofU_ϕ/I. We show that two features observed in the existing data, an asymmetric distribution ofU_ϕwith respect to the disk minor axis and a spatial distribution ofU_ϕthat is predominantly positive or negative, are incompatible with scattering by spherical grains in an axisymmetric disk. They provide indirect evidence for scattering by aligned nonspherical grains.

    

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