Collisional dynamics of NC4NH+ with H2 in the interstellar medium

ABSTRACT The present work explores the quantum dynamics of protonated dicyanoacetylene (NC$_4$NH$^+$) in collision with both para and ortho-H$_2$ over the temperature range 1–100 K. Such low-temperature collisions help determine the state-to-state rate coefficients for the rotational transitions of NC$_4$NH$^+$, a recently detected species in TMC-1. To study these collisions, a four-dimensional (4D) ab initio potential energy surface (PES) for the NC$_4$NH$^+$–H$_2$ system was developed using the CCSD(T)–F12b method with an augmented correlation consistent polarized triple-zeta basis set under the rigid-rotor approximation. The PES was then augmented through a neural network (NN) fitting approach to get the dense data. It was further expanded in bispherical harmonics, for the radial coefficients to be expressed in analytical form. Based on these radial coefficients, state-to-state cross-sections and rate coefficients were calculated using the exact close-coupling method for the rotational states considering to both para and ortho-H$_2$. For rate coefficients it is observed that a strong propensity is favored for even transitions over the odd ones. The resulting rate coefficients would also help model the abundance of NC$_4$NH$^+$ in the interstellar medium under non - local thermodynamic equilibrium conditions.