1998A&A...333.1007T

The hypothesis that the C⁺ + NH₃ -> CH₂N⁺ + H reaction contributes to an HNC/HCN abundance ratio greater than unity in dark interstellar clouds has been tested using ab initio quantum chemical techniques. The hypothesis is based on the argument that a significant fraction of the ion product is the metastable H₂NC⁺ isomer of C_2V geometry, rather than the linear HNCH⁺ structure, and that the metastable isomer subsequently recombines with electrons to form HNC preferentially. Our extensive ab initio study of the ground and excited surfaces for the C⁺ + NH₃ reaction shows, however, that this is most probably not the case. We find that the lowest energy path for reaction does lead initially to the formation of the metastable isomer in its ground singlet state, but that this product can then isomerize into the ground electronic state (¹{SIGMA}⁺) of the linear HCNH⁺ form. Dynamics calculations show that the isomerization destroys 97-98% of the product H₂NC⁺ ion. We also follow excited potential energy surfaces which lead to the excited (³B₂) electronic state of H₂NC⁺, a state which does not interconvert to the linear ion. However, the potential energy surfaces exhibit barriers on the paths to formation of H₂NC⁺(³B₂). We conclude that the H₂NC⁺ isomer is a minor product of the C⁺ + NH₃ -> CH₂N⁺ + H reaction.