Calculations Concerning Interstellar Isomeric Abundance Ratios for C 3H and C 3H 2
Abstract
The syntheses of interstellar cC_{3}H_{2}, H_{2}CCC, cC_{3}H, and HCCC, where "c" stands for the cyclic isomer, are thought to proceed via dissociative recombination of the precursor ions cC_{3}H^{+}_{3} and H_{2}CCCH^{+}, which are themselves produced mainly via the radiative association reaction between C_{3}H^{+} and H_{2}. We have utilized ab initio methods to study the potential energy surface (PES) for the association of the linear ion C_{3}H^{+} and H_{2} to form the isomers cC_{3}H^{+}_{3} and H_{2}CCCH^{+}. The overall rate coefficient for radiative association has been calculated as a function of temperature via the phase space method. Our ab initio calculations show that the H_{2}CCCH^{+} isomer is formed directly without an activation barrier from reactants, and that isomerization between the two isomers can occur readily via a lowenergy pathway consisting of two transition states (saddle points on the PES) and one intermediate (local minimum on the PES). Calculation of the equilibrium coefficient for the isomerization H_{2}CCCH^{+} ⇔ cC_{3}H_{3}^{+} as a function of energy shows that equal abundances of these two ions should be produced as relaxation proceeds, in agreement with experimental measurements at high pressure. Our results confirm the important point that a simple ionmolecule association reaction can produce a cyclic hydrocarbon. If dissociative recombination reactions involving cC_{3}H^{+}_{3} and H_{2}CCCH^{+} maintain the carbon skeletal structure of the ions and produce roughly similar C_{3}H/C_{3}>H_{2} branching ratios, then abundance ratios of unity are produced between the cyclic and noncyclic isomers of C_{3}H and C_{3}H_{2} via this mechanism. The large abundance ratio of cC_{3}H_{2} to H_{2}CCC observed in TMC1 can then be explained by differential destruction rates.
 Publication:

The Astrophysical Journal
 Pub Date:
 November 1993
 DOI:
 10.1086/173301
 Bibcode:
 1993ApJ...417..181M
 Keywords:

 ISM: MOLECULES;
 MOLECULAR PROCESSES