Interaction of CH 3 CN and CH 3 NC with He: Potential Energy Surfaces and Low-Energy Scattering.

Several nitrogen-bearing molecules, such as methyl cyanide (or acetonitrile, CH3 CN) and methyl isocyanide (CH3 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 H2 molecules at low temperatures. In this work we compute new three-dimensional potential energy surfaces for the CH3 NC-He and CH3 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 CH3 CN-He and CH3 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 CH3 CN and CH3 NC by collision with He atoms are carried out with the close-coupling method, and the collisional cross sections of ortho - and para -CH3 NC and CH3 CN are computed for kinetic energies up to 100 cm-1 . While the PESs for both complexes are qualitatively similar, that of CH3 NC-He is more anisotropic, leading to different propensity rules for rotational excitation. For CH3 NC-He, we find that |Δ j | = 1 transitions are dominant at low kinetic energy and a propensity rule that favors odd Δ j transitions is observed, whereas for CH3 CN the dominant cross sections are associated with transitions with |Δ j | = 2. We expect that the findings of this study will be beneficial for astrophysical investigations as well as laboratory experiments.