The Formation of Interstellar C2N Isomers in Circumstellar Envelopes of Carbon Stars: An Ab Initio Study

Abstract

The reaction of carbon atoms in their electronic ground state with hydrogen cyanide, HCN 3P j (X 1&‘), is explored computationally to investigate the formation of hitherto undetected isomers in C 2 N 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 n-bond, yielding a cyclic intermediate. This complex either decomposes to cyclic plus atomic HC 2 NC 2 N 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 radicals. The formation of all three (2% g ) isomers proceeds without any exit barrier, but the reactions to form CNC, CCN, and are C 2 N c-C 2 N 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 isomers in cold molecular clouds, where average translation temperatures of the reactants are only C 2 N 10¨15 K. However, the physical conditions in circumstellar envelopes of, for example, IRC]10216, diUer 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 via either infrared or (2% g ) microwave spectroscopy.