Spin-inversion and spin-selection in the reactions FeO+ + H2 and Fe+ + N2O

Abstract

The reactions of FeO⁺ with H₂ and of Fe⁺ with N₂O were studied with respect to the production and reactivity of electronically excited ⁴Fe⁺ cations. The reaction of electronic ground state ⁶FeO⁺ with H₂ was found to predominantly produce electronically excited ⁴Fe⁺ as opposed to electronic ground state ⁶Fe⁺ corresponding to a spin-allowed reaction. ⁴Fe⁺ was observed to react with N₂O with a rate constant of 2.3 (+0.3/−0.8) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, smaller than the ground state ⁶Fe⁺ rate constant of 3.2 (±0.5) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ (at room temperature). While the overall reaction of ⁶FeO⁺ with H₂ within the Two-State-Reactivity concept is governed by efficient sextet–quartet spin-inversion in the initial reaction complex, the observation of predominant ⁴Fe⁺ production in the reaction is attributed to a much less efficient quartet–sextet back-inversion in the final reaction complex. Average spin-inversion probabilities are estimated by statistical modeling of spin-inversion processes and related to the properties of spin–orbit coupling along the reaction coordinate. The reaction of FeO⁺ with H₂ served as a source for ⁴Fe⁺, subsequently reacting with N₂O. The measured rate constant has allowed for a more detailed understanding of the ground state ⁶Fe⁺ reaction with N₂O, leading to a significantly improved statistical modeling of the previously measured temperature dependence of the reaction. In particular, evidence for the participation of electronically excited states of the reaction complex was found. Deexcitation of ⁴Fe⁺ by He was found to be slow, with a rate constant <3 × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹.