Ab initio potential energy surface of NC₄N-He: rotationally inelastic collisions and rate coefficients

The rotational dynamics at the quantum level of dicyanoacetylene (NC₄N) with helium (He) is studied in the temperature range of 1-100 K. Cross sections for the rotational transitions are obtained and used in the estimation of the rate coefficients. An ab initio-derived potential energy surface (PES) for the NC₄N-He system was generated at the CCSD(T)/aug-cc-pVTZ level of theory, considering NC₄N as a rigid rotor. The PES exhibits a global minimum with a well-depth of -43.84 cm^-1 and an angle of 67.5°. Legendre expansion coefficients were obtained through analytical fitting and subsequently employed in the calculation of rotational cross sections. The first bending vibration for the NC₄N molecule is at 107 cm^-1, and cross sections were computed for collisional energies up to 107 cm^-1 using the close-coupling method. Resonances are observed in cross sections for lower collisional energy. The rate coefficients were calculated for temperatures ranging from 1 to 100 K. In the NC₄N-He collision system, rotational transitions corresponding to Δj = 2 are the most prevalent and exhibit higher rate coefficients compared to other transitions. The new rate coefficients obtained will help to accurately interpret NC₄N rotational spectra and abundance in interstellar clouds.