The self-association of graphane is driven by London dispersion and enhanced orbital interactions

We investigated the nature of the cohesive energy between graphane sheets via multiple CH···HC interactions, using density functional theory (DFT) including dispersion correction (Grimmes D3 approach) computations of [n]graphane σ dimers (n = 6-73). For comparison, we also evaluated the binding between graphene sheets that display prototypical π/π interactions. The results were analyzed using the block-localized wave function (BLW) method, which is a variant of ab initio valence bond (VB) theory. BLW interprets the intermolecular interactions in terms of frozen interaction energy (ΔE_F) composed of electrostatic and Pauli repulsion interactions, polarization (ΔE_pol), charge-transfer interaction (ΔE_CT), and dispersion effects (ΔE_disp). The BLW analysis reveals that the cohesive energy between graphane sheets is dominated by two stabilizing effects, namely intermolecular London dispersion and two-way charge transfer energy due to the σ_CH → σ^∗_HC interactions. The shift of the electron density around the nonpolar covalent C-H bonds involved in the intermolecular interaction decreases the C-H bond lengths uniformly by 0.001 Å. The ΔE_CT term, which accounts for ∼15% of the total binding energy, results in the accumulation of electron density in the interface area between two layers. This accumulated electron density thus acts as an electronic "glue" for the graphane layers and constitutes an important driving force in the self-association and stability of graphane under ambient conditions. Similarly, the "double faced adhesive tape" style of charge transfer interactions was also observed among graphene sheets in which it accounts for ∼18% of the total binding energy. The binding energy between graphane sheets is additive and can be expressed as a sum of CH···HC interactions, or as a function of the number of C-H bonds.