Measurements of Thermal Rate Constants and Theoretical Calculations for the N(2D, 2P) + C2H2 and C2D2 Reactions

Rate constants for the reactions N( 2 D, 2 P) + C 2 H 2 and C 2 D 2 have been measured using a technique of pulse radiolysis−resonance absorption between 220 and 293 K. Arrhenius parameters have been determined from the temperature dependence of the measured rate constants; the activation energies for the reactions of N( 2 D) were about 0.5 kcal/mol, while those for N( 2 P) were about 0.9 kcal/mol. The H/D isotope effect was found to be very small for both the N( 2 D) + C 2 H 2 and N( 2 P) + C 2 H 2 reactions. The rate constants for N( 2 D) + C 2 H 2 were found to be about 3 times as large as those for N( 2 P) + C 2 H 2 . To understand the overall reaction mechanism of the N( 2 D) + C 2 H 2 reaction, ab initio molecular orbital calculations of the lowest doublet potential energy surface have been performed. It has been found that the initial step of the reaction is the addition of the N atom to the π bond of acetylene. The rate constants have been calculated using conventional transition-state theory and compared to the experimental results. Possible reaction pathways are discussed on the basis of the ab initio results.