AN EXTENSIVE AB-INITIO STUDY OF THE C++NH3 REACTION AND ITS RELATION TO THE HNC HCN ABUNDANCE RATIO IN INTERSTELLAR CLOUDS/

The hypothesis that the C+ + NH3 --> CH2N+ + H reaction contributes to an HNC/HCN abundance ratio greater than unity in dark interstellar cl ouds 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 t han the linear HNCH+ structure, and that the metastable isomer subsequ ently recombines with electrons to form HNC preferentially. Our extens ive ab initio study of the ground and excited surfaces for the C+ + NH 3 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, bu t that this product can then isomerize into the ground electronic stat e ((1) Sigma(+)) of the linear HCNH+ form. Dynamics calculations show that the isomerization destroys 97-98% of the product H2NC+ ion. We al so follow excited potential energy surfaces which lead to the excited (B-3(2)) electronic state of H2NC+, a state which does not interconver t to the linear ion. However, the potential energy surfaces exhibit ba rriers on the paths to formation of H2NC+ (B-3(2)). We conclude that t he H2NC+ isomer is a minor product of the C+ + NH3 --> CH2N+ + H react ion.