Spin-orbit coupling in the oxidative activation of H-H by FeO⁺. Selection rules and reactivity effects

Spin-orbit coupling (SOC) calculations are performed along the reaction pathway of the oxidation process, FeO⁺ + H₂ → Fe⁺ + H₂O (eq 1). Selection rules are derived for SOC between different spin situations, and are applied to understand the computed SOC patterns along the oxidation pathway, and their relationship to the electronic structure of the various species. The process involves two spin inversion (SI) junctions between sextet and quartet states: near the FeO⁺/H₂ cluster at the entrance channel, and near the Fe⁺/H₂O cluster at the exit channel. The sextet-quartet SOC is significant at the reactant extreme (for FeO⁺), but decreases at the FeO⁺/H₂ cluster and continues to decrease until it becomes vanishingly small between the ⁶D-⁴F states of Fe⁺ at the product extreme. The results show that while the quartet surface provides a low-energy path, the SI junctions reduce the probability of the oxidation process significantly. In agreement with the deductions of Armentrout et al.,(2c) the poor bond activation capability of the ⁶D ground state of Fe⁺ in the reverse reaction is accounted for by the inefficient ⁶D-⁴F state mixing due to the expected poor SOC between the respective 4s¹3d⁶ and 3d⁷ configurations. On the other hand, the ⁴F excited state of Fe⁺ can activate H₂O more efficiently since it can lead to the insertion intermediate ⁴(HFeOH⁺) in a spin-conserving manner. Other findings of Schwarz et al.(1,2a) and Armentrout et al.(2c,d) are discussed in the light of the SOC patterns. The importance of the SOC at the exit channel is highlighted by comparing the product distribution of the reaction (eq 1) with analogous reactions of MO⁺ species: when the ground state M⁺ has a 4s¹3d(n-1)(Fe⁺, Mn⁺) electronic structure as opposed to those cases where the ground state electronic structure is 3d(n) (Co⁺, Ni⁺) and where no spin inversion is required. Predictions based on the understanding of the SOC patterns are made and compared with appropriate experimental data.