Interaction of CH$_3$CN and CH$_3$NC with He : potential energy surfaces and low-energy scattering

Abstract

Several nitrogen-bearing molecules, such as methyl cyanide (or acetonitrile, CH$_3$CN) and methyl isocyanide (CH$_3$NC) 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 H$_2$ molecules at low temperatures. In this work we compute new three-dimensional potential energy surfaces for the CH$_3$NC-He and CH$_3$CN-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 CH$_3$CN-He and CH$_3$NC-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 CH$_3$CN and CH$_3$NC by collision with He atoms are carried out with the close-coupling method and the collisional cross sections of $ortho-$ and $para-$CH$_3$NC and CH$_3$CN are computed for kinetic energies up to 100 cm$^{-1}$. While the PESs for both complexes are qualitatively similar, that of CH$_3$NC-He is more anisotropic, leading to different propensity rules for rotational excitation. For CH$_3$NC-He, we find that |$\Delta j$| = 1 transitions are dominant at low kinetic energy and a propensity rule that favors odd $\Delta j$ transitions is observed, whereas for CH$_3$CN the dominant cross sections are associated to transitions with |$\Delta j$| = 2. We expect that the findings of this study will be beneficial for astrophysical investigations as well as laboratory experiments.