Spin−Orbit Coupling in the Oxidative Activation of H−H by FeO⁺. Selection Rules and Reactivity Effects

Abstract

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. ,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.

Density Functional Theory in Transition-Metal Chemistry: A Self-Consistent HubbardUApproach

“…This spin-allowed reaction occurs with exceedingly low efficiency (1 in every 100-1000 gas-phase collisions results in products), yet when it does proceed it is observed to be barrierless [25,26,27]. This apparent contradiction has been explained by a two-statereactivity model [5,28,29], wherein the steep reaction barriers along the spin surface of the reactants and products (sextets in both cases) preclude an efficient, exothermic reaction. Instead, the reaction must occur along a shallow but excited spin surface (here, the quartet), and the reaction bottleneck is the coupling of the two surfaces which permits the necessary spin-inversion at the entrance and exit channels.…”

“…Instead, the reaction must occur along a shallow but excited spin surface (here, the quartet), and the reaction bottleneck is the coupling of the two surfaces which permits the necessary spin-inversion at the entrance and exit channels. For several exchangecorrelation functionals, (including B3LYP) [5,29], the reaction coordinates have failed to agree qualitatively with experiments [25,26,27], higher level correlated-electron calculations [28,30], or with the established paradigm of a two-state model [29].…”

Electronic Structure Makes a Difference: Cytochrome P-450 Mediated Hydroxylations of Hydrocarbons as a Two-State Reactivity Paradigm

“…These experiments were extended [12] and led to the concept of two-state reactivity, [11,18,25] which is illustrated in Figure 2 for the reaction of an alkane with the bare FeO cation. Recent computational results [26] have verified the TSR competition paradigm for the oxidation of methane by transition metal oxide cations too.…”

“…Qualitative potential energy surface for the hydroxylation of an alkane RH by a metal oxenoid such as FeO by the low-spin (LS) and highspin (HS) routes (see refs. [11,25,26]). TS transition structure.…”

“…bond activations. [11,25] In fact, not only are the barriers lower on the low-spin pathway, but also the insertion intermediate R ± Fe ± OH is much more stable than the high-spin analogue. The concerted rebound and oxene routes are also preferred on the low-spin surface.…”