The Formation of Interstellar C 2 N Isomers in Circumstellar Envelopes of Carbon Stars: An Ab Initio Study

Abstract

The reaction of carbon atoms in their 3Pj electronic ground state with hydrogen cyanide, HCN (X1Σ+), is explored computationally to investigate the formation of hitherto undetected C2N isomers in the interstellar medium via a neutral-neutral reaction. Our ab initio calculations expose that the reaction has no entrance barrier and proceeds on the triplet surface via addition of the carbon atom to the π-bond, yielding a cyclic HC2N intermediate. This complex either decomposes to cyclic C2N plus atomic hydrogen or rearranges via ring opening to the HCNC or HCCN isomers. These molecules can fragment via atomic hydrogen ejection to the linear CCN (2∏) and CNC ( 2∏g) radicals. The formation of all three C 2N isomers proceeds without any exit barrier, but the reactions to form CNC, CCN, and c-C2N are found to be strongly endothermic by 52.7, 59.0, and 99.6 kJ mol-1, respectively. Based on these investigations, the neutral-neutral reaction of atomic carbon with hydrogen cyanide cannot synthesize C2N isomers in cold molecular clouds, where average translation temperatures of the reactants are only 10-15 K. However, the physical conditions in circumstellar envelopes of, for example, IRC +10216, differ strongly; close to the photosphere of the central star, temperatures can reach 4000 K, and the elevated velocity of both reactants in the long tail of the Maxwell-Boltzmann distribution can overcome the reaction endothermicity to form at least the linear CNC and CCN isomers. Therefore, these environments represent ideal targets to search for hitherto undetected CNC (2∏) and CNC (2∏g) via either infrared or microwave spectroscopy.