Experimental and theoretical study of the reaction of the ethynyl radical with nitrous oxide, C2H + N2O

Abstract

We investigated the rate constants and reaction mechanism of the gas phase reaction between the ethynyl radical and nitrous oxide (C₂H + N₂O) using both experimental methods and electronic structure calculations. A pulsed-laser photolysis/chemiluminescence technique was used to determine the absolute rate coefficient over the temperature range 570 K to 836 K. In this experimental temperature range, the measured temperature dependence of the overall rate constants can be expressed as: k(T) (C₂H + N₂O) = 2.93 × 10⁻¹¹ exp((−4000 ± 1100) K/T) cm³ s⁻¹ (95% statistical confidence). Portions of the C₂H + N₂O potential energy surface (PES), containing low-energy pathways, were constructed using the composite G3B3 method. A multi-step reaction route leading to the products HCCO + N₂ is clearly preferred. The high selectivity between product channels favouring N₂ formation occurs very early. The pathway corresponds to the addition of the terminal C atom of C₂H to the terminal N atom of N₂O. Refined calculations using the coupled-cluster theory whose electronic energies were extrapolated to the complete basis set limit CCSD(T)/CBS led to an energy barrier of 6.0 kcal mol⁻¹ for the entrance channel. The overall rate constant was also determined by application of transition-state theory and Rice–Ramsperger–Kassel–Marcus (RRKM) statistical analyses to the PES. The computed rate constants have similar temperature dependence to the experimental values, though were somewhat lower.