CF<sup>+</sup> excitation in the interstellar medium

Desrousseaux, Benjamin; Lique, François; Goicoechea, J. R.; Quintas‐Sánchez, Ernesto; Dawes, Richard

doi:10.1051/0004-6361/202039823

Cited by 8 publications

(4 citation statements)

References 35 publications

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“…The agreement improves, with differences less than 10 per cent for the highest values, corresponding to transitions with the smallest j 1 . Similar behaviour has also been observed in Desrousseaux et al 2021b). As previously discussed by Walker et al (2017), this close agreement between para-and ortho-H 2 rates shows that at the considered temperatures, effects of the long-range part of the interaction outweigh those of the short-range part.…”

Section: R E S U Lt Ssupporting

confidence: 87%

Inelastic rate coefficients for collisions of C4H− with H2

Balança

Quintas‐Sánchez

Dawes

et al. 2021

Monthly Notices of the Royal Astronomical Society

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Carbon-chain anions were recently detected in the interstellar medium. These very reactive species are used as tracers of the physical and chemical conditions in a variety of astrophysical environments. However, the Local Thermodynamical Equilibrium conditions are generally not fulfilled in these environments. Therefore, collisional as well as radiative rates are needed to accurately model the observed emission lines. We determine in this work the state-to-state rate coefficients of C4H− in collision with both ortho- and para-H2. A new ab initio 4D potential energy surface was computed using explicitly-correlated coupled cluster procedures. This surface was then employed to determine rotational excitation and de-excitation cross sections and rate coefficients for the first 21 rotational levels (up to rotational level j1 = 20) using the close-coupling method, while the coupled-state approximation was used to extend the calculations up to j1 = 30. State-to-state rate coefficients were obtained for the temperature range 2–100 K. The differences between the ortho- and para-H2 rate coefficients are found to be small.

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Section: R E S U Lt Ssupporting

confidence: 87%

Inelastic rate coefficients for collisions of C4H− with H2

Balança

Quintas‐Sánchez

Dawes

et al. 2021

Monthly Notices of the Royal Astronomical Society

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“…SH + critical densities (n cr = A i j / γ i j ) for inelastic collisions with H or H 2 are of the same order and equal to several 10 6 cm −3 . As for many molecular ions (e.g., Desrousseaux et al 2021), SH + -H 2 (and SH + -H) inelastic collisional rate coefficients 4 are large (γ i j 10 −10 cm 3 s −1 ). Thus, collisions with H (at low A V ) and H 2 (at higher A V ) generally dominate over collisions with electrons (γ i j of a few 10 −7 cm 3 s −1 ).…”

Section: Sh + Excitation and Column Densitymentioning

confidence: 99%

Bottlenecks to interstellar sulfur chemistry

Goicoechea

Aguado

Cuadrado

et al. 2021

A&A

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Hydride molecules lie at the base of interstellar chemistry, but the synthesis of sulfuretted hydrides is poorly understood and their abundances often crudely constrained. Motivated by new observations of the Orion Bar photodissociation region (PDR) – 1″ resolution ALMA images of SH+; IRAM 30 m detections of bright H232S, H234S, and H233S lines; H3S+ (upper limits); and SOFIA/GREAT observations of SH (upper limits) – we perform a systematic study of the chemistry of sulfur-bearing hydrides. We self-consistently determine their column densities using coupled excitation, radiative transfer as well as chemical formation and destruction models. We revise some of the key gas-phase reactions that lead to their chemical synthesis. This includes ab initio quantum calculations of the vibrational-state-dependent reactions SH+ + H2(v) ⇄ H2S+ + H and S + H2 (v) ⇄ SH + H. We find that reactions of UV-pumped H2(v ≥ 2) molecules with S+ ions explain the presence of SH+ in a high thermal-pressure gas component, Pth∕k ≈ 108 cm−3 K, close to the H2 dissociation front (at AV < 2 mag). These PDR layers are characterized by no or very little depletion of elemental sulfur from the gas. However, subsequent hydrogen abstraction reactions of SH+, H2S+, and S atoms with vibrationally excited H2, fail to form enough H2S+, H3S+, and SH to ultimately explain the observed H2S column density (~2.5 × 1014 cm−2, with an ortho-to-para ratio of 2.9 ± 0.3; consistent with the high-temperature statistical value). To overcome these bottlenecks, we build PDR models that include a simple network of grain surface reactions leading to the formation of solid H2S (s-H2S). The higher adsorption binding energies of S and SH suggested by recent studies imply that S atoms adsorb on grains (and form s-H2S) at warmer dust temperatures (Td < 50 K) and closer to the UV-illuminated edges of molecular clouds. We show that everywhere s-H2S mantles form(ed), gas-phase H2S emission lines will be detectable. Photodesorption and, to a lesser extent, chemical desorption, produce roughly the same H2S column density (a few 1014 cm−2) and abundance peak (a few 10−8) nearly independently of nH and G0. This agrees with the observed H2S column density in the Orion Bar as well as at the edges of dark clouds without invoking substantial depletion of elemental sulfur abundances.

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“…From this reaction step, an energy of ∼ 30 kcal mol −1 is required to produce ionized cyanamide. As the energy difference between that of NH 2 CN + and its 1 A' ground state is only 0.38 kcal mol −1 (calculated with CCSD(T)-F12/cc-pVTZ-F12), and as electron rate coefficients in the ISM are some orders of magnitude greater than cyanamide abundances (∼10 −6 cm 3 s −1 ; Desrousseaux et al 2020), an e − could be gained to form neutral cyanamide. On the other hand, Ra16 is composed of aminomethylidyne (•H 2 NC) and a nitrogen atom in its doublet state •H 2 NC, producing the cyanamide triplet (as in the case of Ra5) as an intermediate.…”

Section: Class B Reactionsmentioning

confidence: 96%

Gas-phase molecular formation mechanisms of cyanamide (NH₂CN) and its tautomer carbodiimide (HNCNH) under Sgr B2(N) astrophysical conditions

Ramal-Olmedo

Menor-Salván

MIYOSHI

et al. 2023

A&A

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Context. Cyanamide (NH 2 CN) and its tautomer carbodiimide (NHCHN) are believed to have been key precursors of purines and pyrimidines during abiogenesis on primitive Earth. The detection of guanine and cytosine in meteorites and comets provides evidence of their nonterrestrial formation. Although NH 2 CN has been found in several molecular clouds, NHCHN has only been detected in Sgr B2(N). Their possible molecular formation mechanisms in the gas phase and therefore their respective molecular precursors remain an open subject of investigation. Aims. The main objective of this paper is to determine which reactions can produce NH 2 CN and HNCNH in the amounts observed under the astrophysical conditions of Sgr B2(N). The determination of their most likely precursors could serve to provide new insights into possible routes to purine and pyrimidine synthesis, and by extension to nucleosides, under the astrophysical conditions of dense molecular clouds. Methods. Initially, we proposed 120 reaction mechanisms, 60 being dedicated to NH 2 CN formation and the remaining 60 to HNCNH. These mechanisms were constructed using 25 chemical species that were identified in outer space. We calculated the molecular energies of reactants and products at the CCSD(T)-F12/cc-pVTZ-F12 and MP2/aug-cc-pVDZ levels of theory, and defined the values of thermodynamic functions using the Maxwell-Boltzmann statistical quantum theory. Via an extensive literature review on the abundances of reactants and products in Sgr B2(N), in addition to a detailed kinetic study for a range of 20-300 K, we identify the most likely reaction mechanisms for both cyanamides of those proposed previously and presently.Results. From the 120 analyzed reactions, only nine for NH 2 CN and four for HNCNH could thermodynamically account for their synthesis in Sgr B2(N). The kinetic portion of our study shows that Ra60 (CH 3 NH 2 + •CN → NH 2 CN + •CH 3 ), with a modified Arrhenius expression of k T = 1.22 x 10 −9 ( T 300 ) −0.038 exp − ( −147.34 T ) cm 3 mol −1 s −1 , is the most efficient reaction at low temperatures (<60 K). Above 60 K, no reaction with known reagents in Sgr B2(N) is efficient enough. In this way, Ra37-2 (•HNCN + •NH 2 → NH 2 CN + 3 NH ) appears to be the most likely candidate, showing a modified Arrhenius constant of k T = 2.51 x 10 −11 ( T 300 ) −32.18 exp − ( −1.332 T ) cm 3 mol −1 s −1 . In the case of carbodiimide production, Rb18 (•H 2 NC + •NH 2 → HNCNH + •H) is the most efficient reaction, fitting a rate constant of k T = 4.70 x 10 −13 ( 300 T ) −3.24 exp − ( 36.28 T ) cm 3 mol −1 s −1 in Sgr B2(N). Conclusions. The detected gas-phase abundances of cyanamide (NH 2 CN) in Sgr B2(N) can be explained as: Ra60 (•CN + •CH 3 NH 2 ) from 20-60 K; Ra5: (•CN + •NH 2 ) from 60-120 K; and Ra37-2 (•HNCN + •NH 2 ) from 120-300 K. The carbodiimide (HNCNH) synthesis could proceed via Rb18 (•H 2 NC + •NH 2 ). Moreover, the presence of •HNCN and •H 2 NC in Sgr B2(N) are predicted here, making them viable candidates for future astronomical observations. The fores...

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CF⁺ excitation in the interstellar medium

Cited by 8 publications

References 35 publications

Inelastic rate coefficients for collisions of C4H− with H2

Inelastic rate coefficients for collisions of C4H− with H2

Bottlenecks to interstellar sulfur chemistry

Gas-phase molecular formation mechanisms of cyanamide (NH₂CN) and its tautomer carbodiimide (HNCNH) under Sgr B2(N) astrophysical conditions

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CF+ excitation in the interstellar medium

Cited by 8 publications

References 35 publications

Inelastic rate coefficients for collisions of C4H− with H2

Inelastic rate coefficients for collisions of C4H− with H2

Bottlenecks to interstellar sulfur chemistry

Gas-phase molecular formation mechanisms of cyanamide (NH2CN) and its tautomer carbodiimide (HNCNH) under Sgr B2(N) astrophysical conditions

Contact Info

Product

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About

CF⁺ excitation in the interstellar medium

Gas-phase molecular formation mechanisms of cyanamide (NH₂CN) and its tautomer carbodiimide (HNCNH) under Sgr B2(N) astrophysical conditions