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.