Quantum chemical computations predict that compression of the methane dimer to an inter-nuclear separation lower than 2 A facilitates a concerted coupling and dissociation of C-H bonds of the molecules to form ethane/ethylene. In this bimolecular, concerted mechanism, ethane formation is accompanied by production of H radicals from each methane moiety that may further abstract hydrogen atoms to lead to ethylene formation. Alternatively, transformation to ethane and ethylene proceeds via stepwise molecular hydrogen elimination, with the first eliminated hydrogen molecule originating from one of the methane moieties, accompanied by an intermolecular hydrogen transfer, and the second originating from both methyl groups. (C) 2018 Elsevier B.V. All rights reserved.