DFT Study of Protonation of Cyanogen (NCCN) in Interstellar Medium

DFT Study of Protonation of Cyanogen (NCCN) in Interstellar Medium

The protonated cyanogen (NCCNH⁺) has been detected in the interstellar medium towards the cold dark clouds TMC-1 and L483 and in the prestellar core L1544. In the present work, the formation of NCCNH⁺ is discussed with the help of molecules that are primarily detected in the interstellar medium. We propose four plausible reaction mechanisms, studied via Density Functional Theory (DFT) using the functionals B3P86, B3PW91 and WB97XD with the 6-311G++(2d,2p) basis set. Radical–radical and ion–molecule interactions are studied to explore the possibility of formation of protonated cyanogen. The reaction of two CN radicals is proposed for the formation of NCCN, which further reacts with H⁺, H₃⁺, N₂H⁺ and H₂O⁺ to form the protonated form of cyanogen (NCCNH⁺) in the gas phase. The proton affinities and gas-phase basicities of the molecules involved in the reactions are calculated. HOMO–LUMO analysis is performed for the mechanisms.

More than 270 molecules have been detected to date in the interstellar medium. Most of the detected molecules are organic in nature; several of them served as precursors of sugars, amino acids, nucleobases [1, 2] and other prebiotic molecules. Astronomical observations report that nitriles are highly abundant in the interstellar medium [3, 4]. A very large percentage of the molecules detected so far contain a nitrile group [5], whether they are detected in cold interstellar clouds [6], in circumstellar envelopes around carbon-rich evolved stars [7], or in planetary atmospheres [8]. It has been suggested that dicyanopolyynes, which consist of a highly unsaturated linear skeleton of carbon atoms ended by a cyano group at each edge (N≡C–(C≡C)ₙ–C≡N), would exist in space [9, 10]. Due to the lack of a permanent electric dipole moment, the dicyanopolyynes are not observable in radio astronomy. Cyanogen (NCCN), the simplest symmetrical molecule with two cyano groups, is thought to be highly abundant in interstellar space [9]. The detection of CNCN (isocyanogen), a metastable isomer of cyanogen, and of the protonated form of cyanogen in the L483 cloud — and tentatively in TMC-1 [4] — supports the detection and abundance of NCCN in interstellar space.

NCCNH⁺, the protonated form of cyanogen, was first identified in the cold dark clouds TMC-1 and L483 [11], with an abundance ratio NCCNH⁺/NCCN ~ 10⁻⁴. Later, protonated cyanogen was reported to be observed in the L1544 prestellar core [12]. The detection of NCCNH⁺ is a good indication of the presence of NCCN in dark clouds. Although NCCN possesses no permanent dipole moment, its protonated form is a highly polar linear molecular cation with an electric dipole moment of 6.448 debye [13].

Protonation is a very important process in interstellar space [14, 15], and most protonated cations are suggested to be formed via proton transfer reactions, including NCCNH⁺ [11]. Protonated cyanogen can be formed by proton transfer to cyanogen. The formation of NCCNH⁺ has been studied experimentally from the reaction of NCCN with H₃⁺, N₂H⁺, H₂O⁺, C₂H₃⁺ and SO₂H⁺ using the selected-ion flow tube (SIFT) technique; the reaction rate and the proton affinity of C₂N₂ have also been determined [16].