Collisional Dynamics of Newly Detected Protonated Dicyanoacetylene ( NC 4 NH + $$ {\mathrm{NC}}_4{\mathrm{NH}}^{+} $$ ) With He at Low Interstellar Temperatures

Cyanopolyyne and protonated-dicyanopolyyne molecules always get special attention for their detection in the interstellar medium. The rotational quantum dynamics for the collision of recently detected protonated dicyanoacetylene (${\text{NC}}_4{\text{NH}}^{+}$) with He is studied to get the inelastic rate coefficients till temperature range of 100 K. An accurate potential energy surface (PES), computed using ab initio methods, has been developed for the ${\text{NC}}_4{\text{NH}}^{+}$–He collision system. The PES was developed with the coupled cluster, that is, the CCSD(T)-F12b method in combination with the aug-cc-pVTZ basis set. The 2D PES has a global minimum with a value of −239.19 ${\text{cm}}^{-1}$. The analytical fitting of this 2D PES is done to obtain the radial coefficients, that give cross-sections for ${\text{NC}}_4{\text{NH}}^{+}$ molecule till collisional energy range of 300 ${\text{cm}}^{-1}$. The rate coefficients are achieved for the first 20 rotational transitions. An important trend is observed when comparing the de-excitation rate coefficients at different temperatures. For transitions below $\Delta j=10$, a preference for odd $\Delta j$ values is evident, which can be attributed to the anisotropy in the PES of the ${\text{NC}}_4{\text{NH}}^{+}$–He collision. This similar behavior is observed for ${\text{HC}}_3{\text{NH}}^{+}$–He collision. However, for higher transitions, a strong propensity for even $\Delta j$ transitions emerges. The results obtained in the present work will enable us to estimate the abundance of ${\text{NC}}_4{\text{NH}}^{+}$ in the ISM under non-local thermal equilibrium conditions.