A study of the reactions of Fe and FeO with NO2, and the structure and bond energy of FeO2

Abstract

The reaction Fe+NO₂→FeO+NO was studied by the pulsed two-photon dissociation at 248 nm of ferrocene vapour to produce Fe atoms in an excess of NO₂ and He bath gas. Atomic Fe was then monitored by time-resolved laser-induced fluorescence spectroscopy at 248.3 nm [Fe(x⁵F⁰₅–a⁵D₄)], yielding k(192<T/K<471)=(4.84_-1.10^+1.24)×10^-10 exp[-(2.60±0.56) kJ mol^-1/RT] cm³ molecule^-1 s^-1, where the quoted uncertainties are a combination of the 2σ standard errors in the kinetic data and the systematic experimental errors. For the reaction FeO+NO₂→FeO₂+NO, FeO was formed from the pulsed photolysis of a ferrocene–NO₂ mixture in N₂ bath gas, and then monitored by time-resolved LIF in the Orange system at 582.0 nm [FeO(D⁵Δ₄–X⁵Δ₄)]. This yielded k(196<T/K<527)=(5.04_-1.08^+1.20)10^-10 exp[-(3.64±0.58) kJ mol^-1/RT] cm³ molecule^-1 s^-1. The preexponential factors of both reactions are shown to be in good accord with the collision frequencies calculated by applying the orbiting criterion on their respective long-range potentials. The small activation energy of FeO+NO₂ implies that the bond energy D₀(Fe–O₂)>198 kJ mol^-1. Abinitio calculations on FeO₂, performed with hybrid density functional/Hartree–Fock theory, show that the isomeric forms of the molecule that are stable enough to be produced in this reaction are the triplet and quintet states of dioxo (inserted) FeO₂, with D₀(Fe–O₂)=284±38 kJ mol^-1.