Hydrogenation of graphene nanoflakes and C–H bond dissociation of hydrogenated graphene nanoflakes: a density functional theory study

The Gibbs free energy change for the hydrogenation of graphene nanoflakes C _n (n = 24, 28, 30 and 32) and the C–H bond dissociation energy of hydrogenated graphene nanoflakes C _n H _m (n = 24, 28, 30 and 32; and m = 1, 2 and 3) are evaluated using density functional theory calculations. It is concluded that the graphene nanoflakes and hydrogenated graphene nanoflakes accept the orth-aryne structure with peripheral carbon atoms bonded via the most triple bonds and leaving the least unpaired dangling electrons. Five-membered rings are formed at the deep bay sites attributing to the stabilization effect from the pairing of dangling electrons. The hydrogenation reactions which eliminate one unpaired dangling electron and thus decrease the overall multiplicity of the graphene nanoflakes or hydrogenated graphene nanoflakes are spontaneous with negative or near zero Gibbs free energy change. And the resulting C–H bonds are stable with bond dissociation energy in the same range as those of aromatic compounds. The other C–H bonds are not as stable attributing to the excessive unpaired dangling electrons being filled into the C–H anti-bond orbital.