Rotational (de-)excitation of CH3CN in collisions with H2 on an accurate potential energy surface

Abstract

Observations of molecules with C_3v symmetry, such as CH₃CN, are particularly valuable in molecular clouds as the rotational transition selection rules of these molecules allow them to serve as gas thermometers. Interpreting their spectra in non-local thermodynamic equilibrium (non-LTE) conditions requires accurate collisional rate coefficients, especially for interactions with common interstellar species like H₂. In this work, we present a five-dimensional potential energy surface for CH₃CN in van der Waals interaction with H₂ (¹Σ⁺), computed using the CCSD(T)/F12 method and the aug-cc-pVTZ basis set. This potential energy surface is fitted with analytical functions suited for scattering calculations. Cross sections for rotational transitions in collisions between ortho- and para-CH₃CN and para-H₂ (j₂ = 0) are computed using the close-coupling quantum scattering method, across energies from threshold up to 150 cm⁻¹. These data are essential for interpreting interstellar CH₃CN emission lines and advancing our understanding of diverse astronomical environments.