Enigmatic Alternatives for Epoxides in Graphene Oxide

The structural mystery of the long-known graphene oxide (GO) unfolds as one of the formidably abstract conceptual problems among nanomaterials. Generally construed as the oxidized form of graphene, it is variously proposed to host a variety of functional groups with oxygen and hydrogen. Theoretical studies are abundant on the highly-strained epoxides, while larger cyclic ethers having one or more oxygen atoms and vinylogous carbonyls are paid scant attention even though they are predicted by several structural models. The nature of the geometric and electronic structure of these alternative functional groups, the preferred distribution on the graphene lattice, comparative stability, etc., remains unexplored. Our systematic inquiry into the impact of hexagonal and periodic constraints on these mystic functional groups unveils several surprises. Among those that retain the hexagonal carbon backbone, epoxides are surprisingly more stable than larger ethers despite the excessive strain associated with their acute triangular geometry. Epoxidation conserves the planarity of the carbon backbone that allows their optimal distribution on the lattice by reducing the repulsive interactions from oxygen lone pairs and π-electrons. These findings categorically rule out the possibility of 1,3 ethers in GO and settle the long-standing debate on its existence. They face severe steric repulsion even in low dimensional systems that tear down the σ-skeleton of graphene completely apart, reducing its dimensionality. We show that selective breaking of the σ-bonds is preferred over epoxides if backed by cyclic delocalization of electrons. Particularly, 1,6-diones in trans orientation are thermodynamically favored and justify the large holes experimentally observed through microscopic imaging.