Interaction of CHCN and CHNC with He : potential energy surfaces and low-energy scattering
arXiv:2309.06816 · doi:10.1021/acs.jpca.2c06925
Abstract
Several nitrogen-bearing molecules, such as methyl cyanide (or acetonitrile, CHCN) and methyl isocyanide (CHNC) 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 molecules at low temperatures. In this work we compute new three-dimensional potential energy surfaces for the CHNC-He and CHCN-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 and 58.61 cm for CHCN-He and CHNC-He, respectively, while the dissociation energy of the complexes are 18.64 and 18.65 cm, respectively. Low energy scattering calculations of pure rotational (de-)excitation of CHCN and CHNC by collision with He atoms are carried out with the close-coupling method and the collisional cross sections of and CHNC and CHCN are computed for kinetic energies up to 100 cm. While the PESs for both complexes are qualitatively similar, that of CHNC-He is more anisotropic, leading to different propensity rules for rotational excitation. For CHNC-He, we find that || = 1 transitions are dominant at low kinetic energy and a propensity rule that favors odd transitions is observed, whereas for CHCN the dominant cross sections are associated to transitions with || = 2. We expect that the findings of this study will be beneficial for astrophysical investigations as well as laboratory experiments.