Role of different isomers of the H₂CN⁺ ion in the formation of interstellar HCN and HNC

Abstract

THE H₂CN⁺ molecular ion is generally agreed^1–4 to have a critical role in the formation of interstellar HCN and HNC. The obvious problem that H₂CN⁺ has not yet been identified in interstellar space is complicated by considerable uncertainty due to the possible existence of three distinct isomers of H₂CN⁺, with nuclear arrangements Brown⁴ stated that although the third isomer had been completely neglected in previous discussion, it is likely to have a crucial role in the formation of HNC. Brown also notes that the linear isomer, which is isoelectronic with acetylene and thus expected to lie lowest of all the isomers of H₂CN⁺, may play no part at all in the formation of either HCN or HNC. This is from the hypothesis that the sequence thought most likely to lead to HCN is orfollowed by dissociative recombinationBrown argues that the formation of the more stable linear isomer (1) would require the migration of a hydrogen atom from carbon to nitrogen. As this migration is expected to be very costly energetically, the H₂CN⁺ on the right hand side of reactions (4) and (5) should be the isomer (2) above. Similarly, he argues that the sequence leading to HNC should exclusively involve isomer (3). We report here theoretical studies designed to investigate the molecular structure of (3) and its energy relative to the previously studied³ isomers (1) and (2). In addition, because reactions (4), (5) and (7) must be exothermic or very nearly so if they play a part in the interstellar medium, this question has also been assessed.