Structure and Magnetic Properties of Superoxide Radical Anion Complexes with Low Binding Energy at the Graphene Edges

Abstract

The complexes of superoxide radical anions formed at the zigzag edges of curved graphene sheets upon microwave excitation were studied by electron spin resonance and quantum chemical density functional theory. The binding energy of the complex decreases from 700 to 42 meV as the distance between the oxygen atom and the zigzag graphene edge increases from 1.46 to 1.64 A. The configurations and binding energies of the complexes depend both on the topology of the underlying graphene layer (single- or double-layer, flat, or curved) and on the type of atomic groups terminating the edge carbon atoms via σ-bonds. Complexes with low dissociation energy (<70 meV) can be formed only at cryogenic temperatures as a result of electronic charge transfer from the edge towards the O2 molecule. The temperature behavior of the paramagnetic spin susceptibility of the complex indicates an antiferromagnetic interaction of the spin S = 1/2 of the $${\text{O}}_{2}^{{-*}}$$ radical anions with the edge spins located outside the area “demagnetized” due to its depletion by the electronic charge. At T > 50 K, the complex irreversibly dissociates and the O2 molecule is detached from the edge. The existence of complexes with a low dissociation energy expands the views on the ionic chemical bond with a large distance between the interacting species.