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.