Interaction of cyanogen (NCCN) with proton: A new ab initio potential energy surface

Highlights

• New ab initio potential energy surface computed for H⁺+NCCN using CCSD(T)/aug-cc-pVQZ method.

• The surface has been fitted in the form of Legendre polynomials.

• NCCNH⁺ system has a large potential well of 7 eV indicating high stability of complex.

Abstract

NCCNH⁺ is detected in interstellar medium recently. To characterize and study the collision dynamics, a new global potential energy surface for the NCCNH⁺ system has been generated using CCSD(T)/aug-cc-pVQZ method. NCCN–H⁺ has a large potential well of 7 eV indicating high stability of complex. The surface has been expanded in terms of Legendre polynomials. NCCN and NCCNH⁺ have been accurately characterized. The surface will be helpful for detailed understanding of quantum dynamics parameters such as ro-vibrational cross-sections and rate coefficients. The data obtained in this work will have perceptible impact to understand the cyanogen and protonated cyanogen chemistry in space.

Introduction

Interstellar chemistry is typically organic in nature. The majority of molecules detected in interstellar and circumstellar media consists of at least one carbon atom [1], [2], [3]. They contain many functional groups, but quarter of them consist of nitrile group or relevant organic function [4]. Particularly, some detected nitriles molecules imagined to be on the origin of life [5]. Due to their significance, they are the centre of interest of several communities working on experimental and theoretical astrochemistry for determining the collision rates with H, H⁺, H₂ and He [6], [7], [8]. Even though greater than 30 interstellar molecules consist of strong bond of cyano group, no molecules had been detected in the space which have symmetrical dicyano groups [9]. Kołos and Grabowski [10] and Petrie et al. [11] suggested that dicyanopolyynes would exist in the space. Cyanogen (NCCN) is a toxic symmetrical dicyano molecule which is thought to be highly abundant in the interstellar space [11]. Its concentration is theorised to be equal to HCN, HNC and HC₃N [9], and it possesses a low rotational constant of $0.1571$ cm⁻¹. However, due to lack of permanent dipole and low vibrational band strengths, its presence and exact abundance in space are ambiguous [12]. Nevertheless, there lies a simple solution to study such peculiar species; the study of their protonated derivative formed due to collision with protons, which are highly abundant in Interstellar medium (ISM). Protonated cyanogen (NCCNH⁺) [13], [14], which remained hidden in ISM until 2015, was detected in cold dark clouds TMC-1 and L483 [9].

Though cyanogen has remained hidden in interstellar clouds, there was no denying to its popularity when it was first detected in Titan$\prime$s atmosphere by Voyager 1 way back in 1981 through IR spectra [15]. Recently, with gas chromatography-mass spectroscopy data collected by Huygens probe launched from the Cassini spacecraft in 2005, the molecule$\prime$s concentration was measured directly and was found to be in trace amounts [16]. However, NCCN displayed a greater relative enrichment than hydrocarbons in Titan$\prime$s north pole as detected through far-infrared spectra [17]. Theoretical study regarding formation of NCCN in Titan$\prime$s atmosphere identify HNC [18] and HCCN [19] as plausible precursors. Unlike Titan, which lies within the range of our satellite and probes, there are no direct ways to probe NCCN in ISM, thus we go for the theoretical investigation to aid its study through detectable species such as NCCNH⁺. Agúndez et al. confirmed the presence of CNCN (isomer of NCCN) recently [20]. Collisional studies of NCCN with proton are therefore of interest in the interstellar medium. A lot of theoretical studies are available in the literature for NCCN molecule [14], [21], [22], [23], [24], [25], [26], [27]. Collisional dynamics mainly depend on the interaction potential between the collision partners. The potential energy surface (PES), or the interaction potential must be reliable for the understanding of collisional dynamics [28].

Protonation is a very important process [29], [30] in interstellar space and many protonated cations have already been observed in the interstellar medium including NCCNH⁺ [9]. NCCNH⁺ can be formed by proton transfer to NCCN. Therefore to understand the chemistry of NCCN and its protonated form NCCNH⁺, it is necessary to generate PES of NCCN collision with proton. In this work, PES has been generated for the ground state of NCCN collision with proton which is not available earlier. The resulting PES is then used to compute multipolar expansion coefficients. Ground state PES of NCCN collision with helium was computed in our earlier work [31], [32]. NCCN collision with proton is not studied earlier. The paper starts with a section describing the computational methodology used in the calculations. The computational methodology section consists of three subsections. In first subsection, structure details of NCCN and NCCNH⁺ by geometry optimisation has been discussed. The second subsection describes the rigid rotor PES while the third subsection provides the details of fitting of PES into a Legendre polynomial expansion. Summary and conclusions are presented in the last section.