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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]

Among the closed-shell hydrocarbons, the carbenes c- and l-C3H2 are the lightest ones to display a permanent electric dipole moment and be detectable by rotational spectroscopy. The cyclic form, cyclopropenylidene, is ubiquitous in the InterStellar Matter (ISM) of the Milky Way and external galaxies. As such, it serves as a marker to help in characterizing the physical conditions of the ISM. The linear form, propadienylidene, is less abundant. In order to get access to their absolute and relative abundances, it is essential to understand their collisional excitation/quenching schemes. We compute here a precise ab initio potential energy surface for the interaction of c- and l-C3H2 with helium, by means of a CCSD(T)-F12a formalism and a fit onto relevant spherical harmonics functions. We conduct quantum dynamical scattering in order to get precise cross sections using a coupled-channel approach for solving the nuclear motion. We average sections to have rates for rotational quenching from 5 to 150 K. We show that these new rates are vastly different, up to more than an order of magnitude, from the older rates presented in the literature, computed with angular momentum algebra only. We expect large differences in the astrophysical analyses of C3H2, including the chemical history of those ubiquitous carbenes. [ABSTRACT FROM AUTHOR]

Methane is an ubiquitous molecule, present as a minor component in many environments, including the Earth and planet atmospheres. Its van der Waals interaction with the main gases is an important ingredient for the understanding of radiative properties for those atmospheres. We present here the first precise determination of the interaction between CH 4 and H 2 . We compute the interaction in an ab initio coupled cluster formalism, with extended atomic bases. We compare a pure CCSD(T) approach to an explicitly correlated CCSD(T)-F12a formalism. The full geometry of scattering two rigid molecules is used, resulting in a potential energy surface depending on 5 degrees of freedom. The long-range part of the ab initio computation is compared with an analytic multipolar expansion. The potential is fit onto a suitable formalism for subsequent scattering dynamics. The potential well is computed at -92.92 cm -1 , an intermediate value if compared with other methane-diatomic molecule interactions.

Among the closed-shell hydrocarbons, the carbenes c- and l-C 3 H 2 are the lightest ones to display a permanent electric dipole moment and be detectable by rotational spectroscopy. The cyclic form, cyclopropenylidene, is ubiquitous in the InterStellar Matter (ISM) of the Milky Way and external galaxies. As such, it serves as a marker to help in characterizing the physical conditions of the ISM. The linear form, propadienylidene, is less abundant. In order to get access to their absolute and relative abundances, it is essential to understand their collisional excitation/quenching schemes. We compute here a precise ab initio potential energy surface for the interaction of c- and l-C 3 H 2 with helium, by means of a CCSD(T)-F12a formalism and a fit onto relevant spherical harmonics functions. We conduct quantum dynamical scattering in order to get precise cross sections using a coupled-channel approach for solving the nuclear motion. We average sections to have rates for rotational quenching from 5 to 150 K. We show that these new rates are vastly different, up to more than an order of magnitude, from the older rates presented in the literature, computed with angular momentum algebra only. We expect large differences in the astrophysical analyses of C 3 H 2 , including the chemical history of those ubiquitous carbenes.

Isocyanic acid (HNCO) is the most stable of all its isomers; it has been observed repeatedly in many different conditions of the Interstellar Media, and its chemistry is poorly known. To quantitatively estimate the abundance of HNCO with respect to other organic molecules, we compute its rotational quenching rates colliding with H 2 , the most common gas in the gaseous Interstellar Media. We compute ab initio the van der Waals interaction HNCO-H 2 , in the rigid molecules approximation, with a CCSD(T)-F12a method. On the fitted ab initio surface, inelastic scattering cross sections and rates are calculated for a temperature range of 7-200 K, with the coupled-states quantum time-independent formalism. The critical densities are high enough to yield rotational temperatures of HNCO differing significantly from the kinetic temperature of H 2 , especially so for the shorter wavelengths observed at the ALMA interferometer. It is found that the quenching rates for collisions with ortho- or para-H 2 differ greatly, opening the possibility of far from equilibrium populations of some rotational levels of HNCO.