Theoretical study of the NH2+C2H2 reaction

The potential energy surface of the amino radical reaction with acetylene has been studied by the ab initio molecular orbital method, and the reaction rate has been computed by a multichannel RRKM calculation. At the highest G2M(RCC) level of theory, the barrier height for the addition step was calculated to be 8.24 kcal/mol, producing solely the HCCH-trans amino-vinyl radical. The adduct can further undergo NH2-for-H displacement decomposition and several isomerization reactions. The RRKM calculation was carried out for the lowest energy path, and the result indicates that the rate constant is expected to be pressure-dependent due to increased redissociation rate at low pressures. Kinetic modeling performed under experimental conditions reveals a substantial effect of secondary reactions on the observed NH2 decay rates. This may explain the low activation energies reported for this reaction.