Kinetics and Thermochemistry of the Reversible Combination Reaction of the Phenoxy Radical with NO

The kinetics of the association reaction of the phenoxy radical with NO were investigated using a flash photolysis technique coupled to UV absorption spectrometry. This yielded k(C6H5O + NO) = (1.65 ± 0.10) × 10-12 cm3 molecule-1 s-1, with no significant temperature effect over the temperature range 280−328 K. Experiments were performed at atmospheric pressure, and theoretical calculations using the RRKM method showed that the rate constant is at the high-pressure limit above ≈50 Torr for temperatures below 400 K. Upon increasing the temperature, the reaction was found to be reversible, and the equilibrium kinetics have been studied at seven temperatures between 310 and 423 K. The equilibrium constant can be expressed as ln(K c/cm3 molecule-1) = −(63.3 ± 1.0) + (10 140 ± 1000)K/T. Thermodynamic treatment of the data by the Third Law method of analysis yielded ΔH°298 = (−87.3 ± 8.0) kJ mol-1 (yielding ΔH°0 = (−83.8 ± 8.0) kJ mol-1 and ΔH°f,298(C6H5O(NO)) = (51.5 ± 8.0) kJ mol-1), corresponding to the calculated ΔS°298 = (−164.9 ± 8.0) J mol-1 K-1. All spectroscopic parameters necessary for RRKM calculations and for the entropy determination using statistical thermodynamics were calculated using both semiempirical (MNDO) and DFT methods. Influence of the resonance stabilization energy of radicals on R−NO bond dissociation energy is discussed.