Interaction of CH₃CN and CH₃NC with He: Potential Energy Surfaces and Low-Energy Scattering

Abstract

Several nitrogen-bearing molecules, such as methyl cyanide (or acetonitrile, CH3CN) and methyl isocyanide (CH3NC) of interest here, have been observed in various astrophysical environments. The accurate modeling of their abundance requires the calculation of rate coefficients for their collisional excitation with species such as He atoms or H2 molecules at low temperatures. In this work we compute new three-dimensional potential energy surfaces for the CH3NC–He and CH3CN–He van der Waals complexes by means of the explicitly correlated coupled cluster approach with single, double and perturbative triple excitation CCSD(T)/F12a in conjunction with the aug-cc-pVTZ basis set. We find a global minimum with D e = 55.10 and 58.61 cm–1 for CH3CN–He and CH3NC–He, respectively, while the dissociation energy D 0 of the complexes are 18.64 and 18.65 cm–1, respectively. Low-energy scattering calculations of pure rotational (de-)excitation of CH3CN and CH3NC by collision with He atoms are carried out with the close-coupling method, and the collisional cross sections of ortho- and para-CH3NC and CH3CN are computed for kinetic energies up to 100 cm–1. While the PESs for both complexes are qualitatively similar, that of CH3NC–He is more anisotropic, leading to different propensity rules for rotational excitation. For CH3NC–He, we find that |Δj| = 1 transitions are dominant at low kinetic energy and a propensity rule that favors odd Δj transitions is observed, whereas for CH3CN the dominant cross sections are associated with transitions with |Δj| = 2. We expect that the findings of this study will be beneficial for astrophysical investigations as well as laboratory experiments.

Collisional excitation of propyne (CH3CCH) by He atoms

“…We note that the PES for CH 3 CCH-He share the same qualitative behaviour that was previously observed for molecules with a threefold symmetry axis interacting with rare gas atoms (Gubbels et al 2012;Loreau et al 2014;Loreau & Van der Avoird 2015;Ben Khalifa et al 2022;Naouai et al 2021), where the global minimum occurs at ϕ = 60 • , that is, with the helium atom located between two hydrogen atoms. Because of the C 3v symmetry of the CH 3 CCH molecule, there are two more symmetry equivalent orientations for the global minimum, at ϕ = 180 • and ϕ = 300 • .…”

“…The internal rotation along the ϕ coordinate is almost free: the energy of the minimum increases between ϕ = 0 • and ϕ = 30 • (from −46.8 cm −1 at θ = 94 • to −46.6 cm −1 at θ = 98 • ) and then decreases between ϕ = 30 • and ϕ = 60 • (from −46.6 cm −1 at θ = 98 • to −51.04 cm −1 at θ = 104 • ). This behaviour was already observed for other systems with C 3v symmetry such as CH 3 CN-He and CH 3 NC-He (Ben Khalifa et al 2022) and is due to the elongated shape of the propyne molecule.…”

“…Propensity rules that favor even values of |∆ j| were already observed for the excitation of other symmetric top molecules with C 3v symmetry by He atoms, such as CH 3 CN (Ben Khalifa et al 2022) and SiH 3 CN (Naouai et al 2021). Figure 4 only illustrates cross-sections for para-and ortho-CH 3 CCH-He and for transitions with ∆ j = 0, the same behaviour is also observed for other values of ∆k.…”

“…The performance of such an approach was already tested for CH 3 CN-He, CH 3 NC-He (Ben Khalifa et al 2022) as well as SiH 3 CN-He (Naouai et al 2021). It was proven that CCSD(T)-F12 method, supplemented by the aVTZ basis set, offers an excellent accuracy for interactions involving rare gases.…”

Cold Chemistry

“…13 Specifically, they consider the fate of vibrationally excited molecules that can be prepared by novel Stark-induced adiabatic Raman passage methods, finding a wealth of resonant structures. Extending these ideas to items of astrophysical interest, Ben Khalifa et al 14 probe the question of collisional excitation of methyl cyanide or methyl isocyanide with helium. New potential energy surfaces provide useful accurate information, as well as distinct rotational selection rules for these similar isomers.…”