Thermal Hydrogen‐Atom Transfer from Methane: The Role of Radicals and Spin States in Oxo‐Cluster Chemistry

Abstract

Hydrogen-atom transfer (HAT), as one of the fundamental reactions in chemistry, is investigated with state-of-the-art gas-phase experiments in conjunction with computational studies. The focus of this Minireview concerns the role that the intrinsic properties of gaseous oxo-clusters play to permit HAT reactivity from saturated hydrocarbons at ambient conditions. In addition, mechanistic implications are discussed which pertain to heterogeneous catalysis. From these combined experimental/computational studies, the crucial role of unpaired spin density at the abstracting atom becomes clear, in distinct contrast to recent conclusions derived from solution-phase experiments.

Methane Activation by Heterogeneous Catalysis

“…The most facile way of methane activation is the homolytic C-H bond cleavage and hydrogen atom transfer to a radical as reaction partner. Schwarz et al [9] reviewed thermal hydrogen atom transfer from methane to oxygen centered gas-phase ion clusters and all gas-phase radical-ions possessing sufficient spin density at an oxygen atom were reactive in this transformation. Results of CH 4 oxidation to CH 3 OH on Fe-ZSM-5 catalysts indicate that this mechanism could also be operational in heterogeneous catalysis [10].…”

Models in Catalysis

“…7 could be matched with the properties of a planar Au 18 cluster. Its structure is derived from a magic-size Au 19 cluster with one missing corner atom. In agreement with experiment, the highest occupied and lowest unoccupied eigenstates are of G symmetry (with four nodal planes).…”

“…In agreement with experiment, the highest occupied and lowest unoccupied eigenstates are of G symmetry (with four nodal planes). The missing atom with respect to a symmetric Au 19 gives rise to a slight asymmetry in the orbital shapes that becomes particularly evident for the lowest unoccupied eigenstate. A detailed book keeping of electrons in the quantum well states requires 22 electrons to be provided.…”

“…O Á2 gas phase cluster. Unfortunately, gas phase reactivity studies on neutral clusters are difficult, and the majority of experiments are mass spectrometric studies on charged species [19]. Since on ionization of a pure MgO cluster the same oxygen radical species is created as present in Li-doped MgO, cationic MgO clusters are valid and viable models for gas phase reactivity studies.…”

Hidden radical reactivity of the [FeO]2+ group in the H-abstraction from methane: DFT and CASPT2 supported mechanism by the example of model iron (hydro)oxide species

“…The terminal oxo center M n+ =O (where n is the oxidation state of the metal center M) is commonly considered to be the most probable candidate for this role, though there is a multiplicity of various radicaloid oxo centers in different positions. 2 The appearance of an oxo radical in the oxides implies feasibility of the electron transfer from the oxygen to the hosting metal center, e.g. as the following:…”