2002ApJ...564..787M

The reaction of carbon atoms in their ³P_j 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 (²Π) and CNC (²Π_g) radicals. The formation of all three C₂N 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 (²Π) and CNC (²Π_g) via either infrared or microwave spectroscopy.