Ab Initio Potential Energy Surface of NC4N-He: Rotationally Inelastic Collisions and Rate Coefficients

Abstract

Rotational dynamics at the quantum level of dicyanoacetylene (NC₄N) with Helium (He) is studied in the temperature range 1-100 K. Cross sections for rotational transitions are obtained and used in the estimation of rate coefficients. An ab initio-derived potential energy surface (PES) for the NC₄N-He system was generated at the CCSD(T)-F12b/aug-cc-pVTZ level of theory, considering NC₄N as a rigid rotor. The PES exhibits a global minimum with well-depth of -50.03 cm^-1 with 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 0 to 100 K. In the NC₄N-He collision system, rotational transitions corresponding to ∆j = 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.