Theoretical Investigation of Protonated Cyanogen Formation in the Interstellar Medium: Energetics and Spectroscopy

Protonated cyanogen (NCCNH+) has been detected in the interstellar medium toward the cold dark clouds TMC-1 and L483, as well as in the prestellar core L1544. In the present work, the formation of NCCNH+ has been discussed with the help of molecules that are primarily detected in the interstellar medium. This study explores the formation mechanisms of NCCNH+ using Density Functional Theory (DFT) at the UB3LYP-(D3-BJ)/aug-cc-pVTZ level of theory, coupled with high-accuracy CCSD(T)/aug-cc-pVQZ single-point energy calculations. The radical-radical and ion-molecule interactions were studied to explore the possibility of protonated cyanogen formation. The reaction of two CN radicals has been proposed for the formation of NCCN which further reacts with pre-detected protonated molecules to form the protonated form of cyanogen (NCCNH+) in the gas phase. All reactions were found to be barrierless and exothermic, with the calculated thermochemical parameters indicating thermodynamic favorability under the adopted computational conditions. Spectroscopic properties, particularly infrared and electronic spectra, were analyzed to characterize the protonated species and provide reference data for laboratory and astronomical studies.