Intermolecular Non-Covalent Carbon-Bonding Interactions with Methyl Groups: A CSD, PDB and DFT Study

A systematic evaluation of the CSD and the PDB in conjunction with DFT calculations reveal that non-covalent Carbon-bonding interactions with X–CH₃ can be weakly directional in the solid state (P ≤ 1.5) when X = N or O. This is comparable to very weak CH hydrogen bonding interactions and is in line with the weak interaction energies calculated (≤ –1.5 kcal·mol⁻¹) of typical charge neutral adducts such as [Me₃N-CH₃···OH₂] (2a). The interaction energy is enhanced to ≤–5 kcal·mol−1 when X is more electron withdrawing such as in [O₂N-CH₃··O=C^dme] (20b) and to ≤18 kcal·mol⁻¹ in cationic species like [Me₃O⁺-CH₃···OH₂]+ (8a).