Bond dissociation energies for Fe 2 + , Fe₂O⁺, and Fe 2 O 2 + clusters determined through threshold photodissociation in a cryogenic ion trap

Understanding and controlling the chemical behavior of iron and iron oxide clusters requires accurate thermochemical data, which, because of the complex electronic structure of transition metal clusters, can be difficult to calculate reliably. Here, dissociation energies for Fe 2 + , Fe₂O⁺, and Fe 2 O 2 + are measured using resonance enhanced photodissociation of clusters contained in a cryogenically cooled ion trap. The photodissociation action spectrum of each species exhibits an abrupt onset for the production of Fe⁺ photofragments from which bond dissociation energies are deduced for Fe 2 + (2.529 ± 0.006 eV), Fe₂O⁺ (3.503 ± 0.006 eV), and Fe 2 O 2 + (4.104 ± 0.006 eV). Using previously measured ionization potentials and electron affinities for Fe and Fe₂, bond dissociation energies are determined for Fe₂ (0.93 ± 0.01 eV) and Fe 2 − (1.68 ± 0.01 eV). Measured dissociation energies are used to derive heats of formation Δ_fH₀( Fe 2 + ) = 1344 ± 2 kJ/mol, Δ_fH₀(Fe₂) = 737 ± 2 kJ/mol, Δ_fH₀( Fe 2 − ) = 649 ± 2 kJ/mol, Δ_fH₀(Fe₂O⁺) = 1094 ± 2 kJ/mol, and Δ_fH₀( Fe 2 O 2 + ) = 853 ± 21 kJ/mol. The Fe 2 O 2 + ions studied here are determined to have a ring structure based on drift tube ion mobility measurements prior to their confinement in the cryogenic ion trap. The photodissociation measurements significantly improve the accuracy of basic thermochemical data for these small, fundamental iron and iron oxide clusters.