title: Theoretical investigation of the interstellar CH3NC/CH3CN ratio.
abstract: Calculations have been performed to determine the abundance ratio of the metastable isomer CH3NC to the stable isomer CH3CN in dense interstellar clouds.  According to gas phase, ion-molecule treatments, these molecules are both synthesized via protonated ion precursors.  We have calculated the ratio of the formation rates of the protonated precursor ions-- CH3NCH+ and CH3CNH+ --synthesized via the radiative association reaction between CH3+ and HCN, which is thought to the dominant formation process of the two isomeric ions.  Our calculations, which involve both ab initio quantum chemistry and equilibrium determinations, lead to a predicted CH3NCH+/CH3CNH+ formation rate ratio between 0.1 and 0.4.  If this ratio is maintained in the neutral species formed from the precursor ions, theory predicts a sizable abundance for methyl isocyanide (CH3NC) and lends credence to its tentative observation.
authors.affiliation: Molecular Research Institute, Palo Alto, CA 94304, USA.
authors.forename: D J
authors.initials: DJ
authors.lastname: DeFrees
authors.forename: A D
authors.initials: AD
authors.lastname: McLean
authors.forename: E
authors.initials: E
authors.lastname: Herbst
mesh_terms: D000097:Acetonitriles / Q000032:analysis* / Q000138:chemical synthesis; D001256:Astronomy / Q000379:methods*; D005118:Extraterrestrial Environment; D006838:Hydrocarbons / Q000032:analysis / Q000138:chemical synthesis; D007536:Isomerism; D008956:Models, Chemical; D009570:Nitriles / Q000032:analysis* / Q000138:chemical synthesis; D011522:Protons; D011789:Quantum Theory
chemical_list: D000097:Acetonitriles; D006838:Hydrocarbons; D009570:Nitriles; D011522:Protons; C008462:methylisonitrile; C032159:acetonitrile