Laboratory investigation of some ion-molecule reactions related to cyanide chemistry in dense interstellar clouds

Abstract

Laboratory rate coefficients of a number of ion–molecule reactions of charged and neutral species derived from HCN are presented. A mechanism based on these ion–molecule reactions is given for the formation of HCN and HNC via H₂CN⁺ in dense interstellar clouds. The two most important reactions for producing HCN in this mechanism are: $$\text{C}^{+}\,+\,\text{NH}_{3}\rightarrow \text{H}_{2}\text{CN}^{+}\,+\,\text{H}$$ (2a) and $$\text{CH}_{3}^{+}\,+\,\text{N}\,\rightarrow \text{H}_{2}\text{CN}^{+}\,+\,\text{H}$$ (4a) followed by dissociative recombination of H₂CN⁺ $$\text{H}_{2}\text{CN}^{+}\,+\,\text{e}\,\rightarrow \text{HCN}\,+\,\text{H}$$ $$\rightarrow \text{HNC}\,+\,\text{H}$$. (3) The structures of the intermediate species H₂CN⁺ produced in these two reactions may differ. Dissociative recombination would then give rise to both HCN and HNC from reaction (2a) and HCN from reaction (4a). If a linear HCNH⁺ structure is formed instead of the ‘formaldehyde type’ structure in both reactions, HCN and HNC will both be formed by dissociative recombination.