Kinetic fall-off behavior for the Cl + Furan-2,5-dione (C4H2O3, maleic anhydride) reaction

Abstract

Rate coefficients, k, for the gas-phase Cl + Furan-2,5-dione (C₄H₂O₃, maleic anhydride) reaction were measured over the 15–500 torr (He and N₂ bath gas) pressure range at temperatures between 283 and 323 K. Kinetic measurements were performed using pulsed laser photolysis (PLP) to produce Cl atoms and atomic resonance fluorescence (RF) to monitor the Cl atom temporal profile. Complementary relative rate (RR) measurements were performed at 296 K and 620 torr pressure (syn. air) and found to be in good agreement with the absolute measurements. A Troe-type fall-off fit of the temperature and pressure dependence yielded the following rate coefficient parameters: k_o(T) = (9.4 ± 0.5) × 10⁻²⁹ (T/298)^−6.3 cm⁶ molecule⁻² s⁻¹, k_∞(T) = (3.4 ± 0.5) × 10⁻¹¹ (T/298)^−1.4 cm³ molecule⁻¹ s⁻¹. The formation of a Cl·C₄H₂O₃ adduct intermediate was deduced from the Cl atom temporal profiles and an equilibrium constant, K_P(T), for the Cl + C₄H₂O₃ ↔ Cl·C₄H₂O₃ reaction was determined. A third-law analysis yielded ΔH = −15.7 ± 0.4 kcal mol⁻¹ with ΔS = −25.1 cal K⁻¹ mol⁻¹, where ΔS was derived from theoretical calculations at the B3LYP/6-311G(2d,p,d) level. In addition, the rate coefficient for the Cl·C₄H₂O₃ + O₂ reaction at 296 K was measured to be (2.83 ± 0.16) × 10⁻¹² cm³ molecule⁻¹ s⁻¹, where the quoted uncertainty is the 2σ fit precision. Stable end-product molar yields of (83 ± 7), (188 ± 10), and (65 ± 10)% were measured for CO, CO₂, and HC(O)Cl, respectively, in an air bath gas. An atmospheric degradation mechanism for C₄H₂O₃ is proposed based on the observed product yields and theoretical calculations of ring-opening pathways and activation barrier energies at the CBS-QB3 level of theory.