Ion‐Molecule Reactions Producing HC \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $_{3}$ \end{document} NH \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $^{+}$ \end{document} in Interstellar Space: Forbiddenness of

Abstract

The reaction between cyclic C3H and the N atom is thought to be a key element in the synthesis of the interstellar molecule HC3N, which is the most abundant species in the cyanopolyyne series in dark clouds. We have examined the potential energy surfaces for the reaction between cyclic C3H and the N atom using ab initio quantum mechanical methods in order to confirm whether this reaction is feasible in interstellar space. The potential energy surface between cyclic C3H and the ground-state N atom is, however, predicted to be repulsive. The reaction between cyclic C3H and the N atom ground state (4S) is concluded not to produce HC3NH+ in interstellar space, in addition to the fact that this reaction is spin forbidden. The alternative pathway to produce HC3NH+ is the reaction between HCCH+ and HCN. This reaction is found to be very critical for the production of HC3NH+ and the H atom because this process requires several reaction steps in a complicated unimolecular rearrangement and must go through an energy barrier which is very close to the energy of the reactants. The other possible way to produce HC3NH+ is shown to be the reaction between HCCH+ and HNC. The theoretical potential energy surface suggests that the HCCNCH+ isomer of HC3NH+ cannot be formed from HCCH+ + HCN because of the endothermicity and the high energy barrier for this process. The present study supports the view that the ion-molecule reactions are not a significant source for the production of the HC3NH+, and therefore of the HC3N molecule in interstellar space. The probable pathways to form HC3NH+ are the reaction HCCH+ + HNC and the protonation of HC3N, produced mainly via neutral-neutral reactions.