Five substituted cyclopropenylidene derivatives (
- Award ID(s):
- 1757220
- NSF-PAR ID:
- 10335940
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 656
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- A148
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract c -C3HX, X=CN, OH, F, NH2), all currently undetected in the interstellar medium (ISM), are found herein to have mechanistically viable, gas-phase formation pathways through neutral–neutral additions of ·X ontoc -C3H2. The detection and predicted formation mechanism ofc -C3HC2H introduces a need for the chemistry ofc -C3H2and any possible derivatives to be more fully explored. Chemically accurate CCSD(T)-F12/cc-pVTZ-F12 calculations provide exothermicities of additions of various radical species toc -C3H2, alongside energies of submerged intermediates that are crossed to result in product formation. Of the novel reaction mechanisms proposed, the addition of the cyano radical is the most exothermic at -16.10 kcal mol−1. All five products are found to or are expected to have at least one means of associating barrierlessly to form a submerged intermediate, a requirement for the cold chemistry of the ISM. The energetically allowed additions arise as a result of the strong electrophilicity of the radical species as well as the product stability gained through substituent-ring conjugation. -
ABSTRACT Nearly two decades since the detection of cyclopropenone (c-C3H2O) in the interstellar medium (ISM), the understanding of how this molecule comes to be remains incomplete. Many hypotheses place the ubiquitous hydrocarbon c-C3H2 at the centre of such discussions. However, insights into c-C3H2 chemistry are further complicated by the recent detection of ethynyl cyclopropenylidene (c-C3HC2H) and the observation of a radio line possibly belonging to methylenecyclopropene (c-C3H2CH2). In a necessary reconciliation of past and current work on the chemical capabilities of c-C3H2 in interstellar environments, the formation pathways of several functionalized cyclopropenes from c-C3H2 and a hydrogenated radical are explored. Chemically accurate CCSD(T)-F12/cc-pVTZ-F12 calculations are used to evaluate the energies of reaction and generate structures along the reaction pathway for formation products deemed chemically plausible. Potential energy scans are used to include or rule out certain paths to product formation based on conformation to the necessary requirements of cold interstellar chemistry. Four functionalized cyclopropenes in addition to c-C3H2O have net exothermic reactions when forming from c-C3H2 (c-C3H2CC, c-C3H2S, c-C3H2NH, and c-C3H2CH2). The former three are found to have reaction profiles favourable for formation in the cold ISM, while c-C3H2CH2 can only form by passage through an association barrier that must be mitigated by an energy source of some kind. c-C3H2S and c-C3H2NH are the best candidates for new spectroscopic searches. A complete detection of c-C3H2CH2 is necessary to fully understand cyclopropenylidene chemistry in the ISM.
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null (Ed.)The challenges associated with the out-of-plane bending problem in multiply-bonded hydrocarbon molecules can be mitigated in quartic force field analyses by varying the step size in the out-of-plane coordinates. Carbon is a highly prevalent element in astronomical and terrestrial environments, but this major piece of its spectra has eluded theoretical examinations for decades. Earlier explanations for this problem focused on method and basis set issues, while this work seeks to corroborate the recent diagnosis as a numerical instability problem related to the generation of the potential energy surface. Explicit anharmonic frequencies for c-(CH)C 3 H 2 + are computed using a quartic force field and the CCSD(T)-F12b method with cc-pVDZ-F12, cc-pVTZ-F12, and aug-cc-pVTZ basis sets. The first of these is shown to offer accuracy comparable to that of the latter two with a substantial reduction in computational time. Additionally, c-(CH)C 3 H 2 + is shown to have two fundamental frequencies at the onset of the interstellar unidentified infrared bands, at 5.134 and 6.088 μm or 1947.9 and 1642.6 cm −1 , respectively. This suggests that the results in the present study should assist in the attribution of parts of these aromatic bands, as well as provide data in support of the laboratory or astronomical detection of c-(CH)C 3 H 2 + .more » « less
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Context. Atomic gas in the diffuse interstellar medium (ISM) is organized in filamentary structures. These structures usually host cold and dense molecular clumps. The Galactic magnetic field is considered to play an important role in the formation of these clumps. Aims. Our goal is to explore the role of the magnetic field in the H I -H 2 transition process. Methods. We targeted a diffuse ISM filamentary cloud toward the Ursa Major cirrus where gas transitions from atomic to molecular. We probed the magnetic field properties of the cloud with optical polarization observations. We performed multiwavelength spectroscopic observations of different species in order to probe the gas phase properties of the cloud. We observed the CO ( J = 1−0) and ( J = 2−1) lines in order to probe the molecular content of the cloud. We also obtained observations of the [C ii ] 157.6 µ m emission line in order to trace the CO-dark H 2 gas and estimate the mean volume density of the cloud. Results. We identified two distinct subregions within the cloud. One of the regions is mostly atomic, while the other is dominated by molecular gas, although most of it is CO-dark. The estimated plane-of-the-sky magnetic field strength between the two regions remains constant within uncertainties and lies in the range 13–30 µG. The total magnetic field strength does not scale with density. This implies that gas is compressed along the field lines. We also found that turbulence is trans-Alfvénic, with M A ≈ 1. In the molecular region, we detected an asymmetric CO clump whose minor axis is closer, with a 24° deviation, to the mean magnetic field orientation than the angle of its major axis. The H i velocity gradients are in general perpendicular to the mean magnetic field orientation except for the region close to the CO clump, where they tend to become parallel. This phenomenon is likely related to gas undergoing gravitational infall. The magnetic field morphology of the target cloud is parallel to the H i column density structure of the cloud in the atomic region, while it tends to become perpendicular to the H i structure in the molecular region. On the other hand, the magnetic field morphology seems to form a smaller offset angle with the total column density shape (including both atomic and molecular gas) of this transition cloud. Conclusions. In the target cloud where the H i –H 2 transition takes place, turbulence is trans-Alfvénic, and hence the magnetic field plays an important role in the cloud dynamics. Atomic gas probably accumulates preferentially along the magnetic field lines and creates overdensities where molecular gas can form. The magnetic field morphology is probed better by the total column density shape of the cloud, and not its H i column density shape.more » « less