Exploring the Maze of C₂N₂H₅ Radicals and Their Fragments in the Interstellar Medium with the Help of Quantum-Chemical Computations

Abstract

Among the species discovered in the interstellar medium and planetary atmospheres, a crucial role is played by the so-called “interstellar” complex organic molecules (iCOMs) because they are the signature of the increasing molecular complexity in space. Indeed, they may represent the connection between simple molecules and biochemical species like amino acids and nucleobases. In particular, HCN and the related CN radical are the starting points of rich nitrile chemistry. In this framework, we have undertaken a computational investigation of the gas-phase reaction mechanisms involving different C2N2H5 radicals and their fragments, stemming from the addition of the cyano radical to the nitrogen atom of methylamine. Aiming at exploiting an accurate yet cost-effective protocol, a combination of CCSD­(T)-based composite schemes and density functional theory has been employed. The exploration of the plausible chemical reaction channels has led to the identification of 12 different products, as well as 28 transition states connecting reactants, intermediates, and products. Aminoacetonitrile (H2NCH2CN), proposed as an intermediate in the formation of the smallest amino acid glycine, and the CH2NH2 radical appear as products energetically accessible under astrophysical conditions.

The Role of State-of-the-Art Quantum-Chemical Calculations in Astrochemistry: Formation Route and Spectroscopy of Ethanimine as a Paradigmatic Case

“…Incorporation of the fT and pQ contributions provides negligible effects, the only exceptions being TS2, TS3 and TS4, for which deviations of about 2– 3 mol −1 are noted. As expected from our previous experience [ 24 , 33 , 61 ], the results obtained by using the ChS model are accurate. In fact, they are within mol −1 from the corresponding CBS+CV values, and within 2 mol −1 with respect to CBS+CV+fT+pQ results.…”

“…Our strategy relies on this model when larger systems are involved. In the present context, it has been employed to further demonstrate its high accuracy and reliability, which was already pointed out in References [ 24 , 33 , 61 ].…”

“…This preliminary investigation is performed at a relatively low computational level, relying on density functional theory (DFT). Usually, different pathways can be derived and only those that are feasible in the conditions typical of the astronomical environment under consideration are further investigated at a higher level [33]. Therefore, the more promising pathways are re-investigated in order to improve the structural determination of the stationary points as well as to compute the energetics at the state of the art.…”

Ab Initio Study of the Large Amplitude Motions of Various Monosubstituted Isotopologues of Methylamine (CH3-NH2)

“…Four isotopic species, 13 CH 3 NH 2 , CH 3 15 NH 2 , CH 3 NHD, and CH 2 DNH 2 , are studied and compared with the main isotopologue for predicting theoretically isotopic shifts. Recently, interstellar amines and their fragments have been studied using quantum-chemical computations ( Salta et al, 2020 ) ( Puzzarini et al, 2020 ).…”

Development and validation of a parameter-free model chemistry for the computation of reliable reaction rates

“…[24][25][26] In conjunction with geometries and harmonic frequencies issuing from double hybrid functionals, ChS has given promising results also for the activation energies of some reactions of astrochemical interest. [27][28][29][30][31] More recently, an improved variant (referred to as jun-Cheap scheme, jChS) has been introduced, which, thanks to the use of the 'june' partially augmented basis set of the 'calendar' family, 32 provides very accurate results also for non-covalent interactions. 33,34 On these grounds, in this paper we provide a comprehensive benchmark of the jChS model chemistry for several classes of reactions for which accurate reference results are available or have been purposely computed.…”