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Given its abundance in the interstellar medium recently justified by astronomical observations, the chemistry and reactivity of the metal-bearing molecule MgS require special attention. The availability of up-to-date collision data between this MgS molecule and the most abundant perturbing species H |$_2$| and He in the interstellar medium is an essential resource to derive reliable values of its molecular abundance from astronomical observations. This work aims to provide improved scattering parameters for the MgS molecular system in collision with He. A new 2D potential energy surface (2D-PES) is derived by adopting the state-of-the-art level of theory of the explicitly correlated coupled cluster with single, double, and perturbative triple excitations (RCCSD(T)-F12) method in conjunction with the aug-cc-pVTZ basis sets. This PES was used to calculate inelastic cross-sections within the framework of the close-coupling (CC) theory for total energies |$\le 500$|  cm |$^{-1}$| and J |$\le 21$|⁠. Using the Maxwell–Boltzmann distribution of kinetic energies, these cross-sections were integrated to generate the collisional (de)-excitation rate coefficients for temperatures below 100 K. We expect that the new rate coefficients will help us to analyse present and future observations of this key molecule for the interstellar chemistry. [ABSTRACT FROM AUTHOR]

The cyclic form of magnesium dicarbide molecule (c-MgC |$_2$|⁠) has been detected in the carbon-rich circumstellar envelope of IRC+10216 and is considered as a valuable tracer for characterizing the physical conditions of the surrounding gas. In order to make the most of c-MgC |$_2$| observations and accurately derive the physical conditions of the media where c-MgC |$_2$| is detected, radiative transfer modelling, including collisional and radiative (de-)excitations, have to be performed. Here, we study the excitation of c-MgC |$_{2}$| induced by collisions with He (as a proxy for H |$_2$|⁠). A new 3D potential energy surface (PES) is constructed using highly correlated ab initio methods. This PES reveals a minimum with a well depth of 20.66 cm |$^{-1}$| below the c-MgC |$_2$| -He dissociation limit. Using this PES, we compute excitation cross sections for transitions between the low-lying rotational energy levels of c-MgC |$_2$| using the time-independent quantum mechanical close-coupling formalism. These cross sections are then thermally averaged over a Boltzmann energy distribution in order to derive excitation rate coefficients at low temperatures (⁠|$T \le 30$|  K). To evaluate the impact of these new rate coefficients on the interpretation of c-MgC |$_2$| observational spectra, we perform radiative transfer calculations. We find that a very high gas density (⁠|$n \gt 10^6$| cm |$^{-3}$|⁠) is needed in order for the collisional excitation to compete with the radiative de-excitation. We also find that the excitation temperatures of the observed lines predicted by our model differ by a factor two from the value derived from the observations in IRC+10216 circumstellar envelope, indicating that the excitation of c-MgC |$_{2}$| may also be driven by a strong radiative pumping in such media. Therefore, a more sophisticated non-LTE modelling, that takes into account the collisional and radiative excitations as well as the radiative pumping, is required to accurately interpret the observational spectra of c-MgC |$_2$|⁠. [ABSTRACT FROM AUTHOR]

For a better understanding of the physico-chemistry in the interstellar medium, collisional data are needed. In this work, we provide rate coefficients for the recently detected by Cabezas and collaborators in 2023 sodium cyanoacetylide molecule NaC |$_{3}$| N(⁠|$X^{1}\Sigma ^{+}$|⁠) induced by collisions with He. A new two-dimensional potential energy surface (2D-PES) is derived by adopting the high-level theory of the explicitly correlated coupled cluster with single, double, and perturbative triple excitations (CCSD(T)-F12) in conjunction with the aug-cc-pVTZ basis sets. Our 2D-PES presents two minima: the global one with a well depth of |$-86.58$|  cm |$^{-1}$| and the second well with a |$-47.01$|  cm |$^{-1}$| depth. This was expected by the Natural Bond Orbital analysis showing two electrophilic zones located on Na and N linked Carbon with respective atomic charges : |$+0.96575$| and |$+0.28910$|⁠. This PES was used to calculate inelastic cross-sections within the framework of the close-coupling theory for total energies |$\le 170$|  cm |$^{-1}$| and |$J\le 22$|⁠. Using the Maxwell–Boltzmann distribution of kinetic energies, these cross-sections were integrated to generate the collisional (de)-excitation rate coefficients for temperatures below 30 K. In order to estimate the impact of the computed collisional rates, a non-local thermodynamical equilibrium radiative transfer calculation was performed. The new collisional data can allow accurate NaC |$_{3}$| N abundance determination in the interstellar medium. [ABSTRACT FROM AUTHOR]

Determining physical conditions in interstellar environments requires reliable estimation of collisional data for molecules detected in space. In this work, we report a rate coefficients calculation of MgC3N(X 2Σ+) induced by collision with He. This study is based on a new 2D potential energy surface (2D-PES), obtained from the explicitly correlated restricted open-shell coupled cluster approach with single, double, and perturbative triple excitation (rccsd(t)-f12) and the aug-cc-pVTZ basis sets. The MgC3N–He PES presents a global minimum with a well depth of −45.6 cm−1. Based on this interaction potential, we derived the excitation cross-sections using the close-coupling quantum time-independent formalism for total energies ≤500 cm−1 and N ≤ 40. These cross-sections were then integrated on a Maxwell–Boltzmann distribution of kinetic energies to obtain the collisional (de)-excitation rate coefficients for thermal temperature below 100 K. A non-LTE radiative transfer calculation was performed using the present collisional rates in order to estimate their impact on the abundance of MgC3N. These collisional data can help astronomers for the detection and an accurate determination of MgC3N abundance in the investigated interstellar clouds. [ABSTRACT FROM AUTHOR]

Collisional excitation of C5H+ by He was performed by mean of state-of-the-art methods. A high level of theory quantum chemical calculations were made to determine the interaction potential energy surface of C5H+ with helium. The new two-dimensional potential energy surface obtained from the RCCSD(T)-F12 ab initio approach associated with aug-cc-pVTZ basis sets, presents two minima below its dissociation limit with well depths of |$-101.8$| and |$-100.2\, \mathrm{cm}^{-1}$|. In order to derive accurate physical conditions from rotational transitions of the recently discovered C5H+ molecule in Taurus molecular cloud (TMC-1), rate coefficients calculations are performed. The C5H+(X1Σ+)–He de-excitation rates are obtained after averaging cross-sections for thermal temperature below 100 K. The integral cross-sections are computed with the close-coupling quantum time-independent formalism for |$E\le 520 \, \mathrm{cm}^{-1}$| and J ≤ 15. The new collisional data will allow accurate determination of the C5H+ abundance and will help to understand the chemistry of carbon chain ions in the interstellar gas. [ABSTRACT FROM AUTHOR]

Tricarbon and pentacarbon monoxides have been detected towards the Taurus Molecular Cloud (TMC-1) with relatively important abundances. Understanding the chemical formation of these molecules requires interpreting their observational spectra by mean of non-local thermodynamical equilibrium modelling. For this purpose, we report rate coefficients of  C3O and  C5O induced by collision with He for temperatures up to 100 K. These data are obtained by calculating inelastic cross sections for the 31 low-lying rotational levels of  C3O and  C5O using the close-coupling approach. The comparison of the new rate coefficients with those of HC3N and HC5N, previously used to interpret the observational spectra of  C3O and  C5O, reveals differences of up to an order of magnitude. The effect of the new collisional rate coefficients in radiative transfer calculations is checked by computing the excitation temperatures for some transitions and simulating the  C3O and  C5O column densities observed towards TMC-1. Our findings suggest that the use of HC n N as template for C n O may lead to local thermodynamic equilibrium conditions for gas densities as low as ∼103  cm−3. Regarding the interpretation of the observational spectra, using radiative transfer modelling based on the actual C n O collisional rate coefficients instead of rotational diagram analysis leads to underestimate the column densities reported in the literature by up to 25 per cent and accordingly the  C3O/ C5O abundance ratio by up to 50 per cent. We expect that the new rate coefficients and the radiative transfer calculations presented in this work will encourage further modellings of the C n O abundance and accordingly constrain the chemistry. [ABSTRACT FROM AUTHOR]

In order to derive accurate physical conditions of the interstellar clouds (such as TMC-1) from the rotational transitions of C2O, collisional rates are needed. Based on a new two-dimensional potential energy surface (PES), we report a new rate coefficients calculation of C2O(X 3Σ−) induced by collision with He. This PES was obtained from the explicitly correlated restricted open-shell coupled cluster with single, double, and perturbative triple excitation (rccsd(t)-f12) ab initio approach associated with aug-cc-pVTZ basis sets. The C2O–He PES presents one minimum below its dissociation limit with a well depth of 42.376 cm−1. The close-coupling quantum time-independent formalism was used to derive cross sections for E ≤ 370 cm−1 and N ≤ 10. De-excitation rate coefficients are obtained for thermal temperature below 80 K. [ABSTRACT FROM AUTHOR]

To determine the chemical composition of gases in molecular clouds, the oxygen-bearing systems C n O are needed as probe elements. The pentacarbon monoxide C5O was recently detected in TMC-1, and in order to derive accurate physical conditions from its rotational transitions, calculation of rate coefficients of C5O(1Σ+) induced by collision with He are performed for thermal temperature below 100 K. These calculations are based on a new 2D potential energy surface (2D-PES) obtained from the explicit correlated coupled cluster with single, double, and pertubative triple excitation (ccsd(t)-f12) ab initio approach associated with aug-cc-pVTZ basis sets. The C5O–He PES presents two minima below its dissociation limit with a well depths of −59.321 and −53.059 cm−1. By mean of this PES, the integral cross sections are calculated in the close-coupling quantum time independant formalism for |$E\le 500 \, {\rm cm}^{-1}$| and J ≤ 20. The de-excitation rate coefficients are obtained after averaging these cross sections at low temperatures. We expect that the new collisional data will allow accurate determination of the C5O abundance in the interstellar medium, as well as the interpretation of its emission lines. These new data are crucial to understand the chemistry of carbon chain species in the interstellar gas. [ABSTRACT FROM AUTHOR]

Despite that the tricarbon monosulfide (C3S) is among the first sulfur-containing carbon-chain molecules to be detected in the interstellar medium, no studies focused on the determination of its collisional rates. These rate coefficients are essential to estimate the abundance of C3S in the interstellar medium. Computations of the C3S( Σ + 1 ) downward rate coefficients, induced by collision with He, are performed by averaging the integral cross sections at low temperature (below 25 K ). Calculations of the cross sections in the close-coupling quantum time independent formalism for E c ≤ 110 cm − 1 and J ≤ 10 are based on a new 2-D potential energy surface. This PES is obtained from the explicit correlated coupled cluster with a single, double and perturbative triple excitation [ccsd(t)-f12] ab initio approach and the aug-cc-pVTZ basis sets. The PES have a global minimum of − 55.69 cm − 1 located at R = 6.25 bohr and θ = 94 ∘ , and a second minimum of − 36.95 cm − 1 at R = 9.35 bohr and θ = 0 ∘ . A comparison of C3S rates with those of the isoelectronic molecule C3O was made. The results indicate a great temperature dependence of the rates for transitions of Δ J > 2 . We expect that the new collisional data will allow for accurate determination of the C3S abundance in several interstellar regions. [ABSTRACT FROM AUTHOR]

The C5S molecule is the largest member of the series of sulphur-containing carbon chains C n S observed in space. Given the lack of data concerning this molecule, we computed rate coefficients of C5S(1Σ+) induced by collision with He. These rates are obtained for thermal temperature below 100 K by mean of a new two-dimensional potential energy surface (PES) calculated with the explicit correlated coupled cluster with single, double, and pertubative triple excitation (ccsd(t)-f12) ab initio approach and the aug-cc-pVTZ basis sets. The C5S–He PES presents three minimums of −59.726, −55.355, and −36.506 cm−1 below its dissociation limit. Using this PES, the integral cross-sections are performed in the close-coupling (CC) and coupled-state (CS) quantum time independent formalisms for |$E_\mathrm{ c}\le 500 \, \mathrm{ cm}^{-1}$| and J ≤ 13 (for CC) and J ≤ 50 (for CS). By averaging these cross-sections we obtained the downward rate coefficients. The new collisional data are used to simulate the excitation of C5S in the circumstellar gas. We obtain the excitation and brightness temperatures of the four lines observed towards the IRC+10216 which confirms the necessity of using radiative transfer calculations to accurately determine C5S abundance since the local thermodynamic equilibrium conditions are not fulfilled. The new collisional data should help to estimate the abundance of C5S in several interstellar regions. [ABSTRACT FROM AUTHOR]