Determination of the rate constant and product channels for the radical–radical reaction NCO(X 2Π) + C2H5(X 2A″) at 293 K

Abstract

The rate constant and product branching ratios for the reaction of the cyanato radical, NCO(X ²Π), with the ethyl radical, C₂H₅(X ²A″), have been measured over the pressure range of 0.28 to 0.59 kPa and at a temperature of 293 ± 2 K. The total rate constant, k₁, increased with pressure, P(kPa), described by k₁ = (1.25 ± 0.16) × 10⁻¹⁰ + (4.22 ± 0.35) × 10⁻¹⁰P cm³ molecule⁻¹ s⁻¹. Three product channels were observed that were not pressure dependent: (1a) HNCO + C₂H₄, k_1a = (1.1 ± 0.16) × 10⁻¹⁰, (1b) HONC + C₂H₄, k_1b = (2.9 ± 1.3) × 10⁻¹¹, (1c) HCN + C₂H₄O, k_1c = (8.7 ± 1.5) × 10⁻¹³, with units cm³ molecule⁻¹ s⁻¹ and uncertainties of one-standard deviation in the scatter of the data. The pressure dependence was attributed to a forth channel, (1d), forming recombination products C₂H₅NCO and/or C₂H₅OCN, with pressure dependence: (1d) k_1d = (0.090 ± 1.3) × 10⁻¹¹ + (3.91 ± 0.27) × 10⁻¹⁰P cm³ molecule⁻¹ s⁻¹. The radicals were generated by the 248 nm photolysis of ClNCO in an excess of C₂H₆. Quantitative infrared time-resolved absorption spectrophotometry was used to follow the temporal dependence of the reactants and the appearance of the products. Five species were monitored, HCl, NCO, HCN, HNCO, and C₂H₄, providing a detailed picture of the chemistry occurring in the system. Other rate constants were also measured: ClNCO + C₂H₅, k₁₀ = (2.3 ± 1.2) × 10⁻¹³ , NCO + C₂H₆, k₂ = (1.6 ± 0.11) × 10⁻¹⁴, NCO + C₄H₁₀, k₄ = (5.3 ± 0.51) × 10⁻¹³, with units cm³ molecule⁻¹ s⁻¹ and uncertainties of one-standard deviation in the scatter of the data.