Gas phase kinetics and equilibrium of allyl radical reactions with NO and NO₂

Allyl radical reactions with NO and NO₂ were studied in direct, time-resolved experiments in a temperature controlled tubular flow reactor connected to a laser photolysis/photoionization mass spectrometer (LP-PIMS). In the C₃H₅ + NO reaction 1, a dependence on the bath gas density was observed in the determined rate coefficients and pressure falloff parametrizations were performed. The obtained rate coefficients vary between 0.30-14.2 × 10^-12 cm³ s^-1 (T = 188-363 K, p = 0.39-23.78 Torr He) and possess a negative temperature dependence. The rate coefficients of the C₃H₅ + NO₂ reaction 2 did not show a dependence on the bath gas density in the range used (p = 0.47-3.38 Torr, T = 201-363 K), and they can be expressed as a function of temperature with k(C₃H₅ + NO₂) = (3.97 ± 0.84) × 10^-11 × (T/300 K) ^-1.55±0.05 cm ³ s^-1. In the C₃H₅ + NO reaction, above 410 K the observed C₃H₅ radical signal did not decay to the signal background, indicating equilibrium between C₃H ₅ + NO and C₃H₅NO. This allowed the C ₃H₅ + NO ⇠C₃H₅NO equilibrium to be studied and the equilibrium constants of the reaction between 414 and 500 K to be determined. With the standard second- and third-law analysis, the enthalpy and entropy of the C₃H₅ + NO ⇠C₃H₅NO reaction were obtained. Combined with the calculated standard entropy of reaction (ΔS ₂₉₈ = 137.2 J mol^-1K^-1), the third-law analysis resulted in ΔH ₂₉₈ = 102.4 ± 3.2 kJ mol^-1 for the C ₃H₅-NO bond dissociation enthalpy.