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=Cdme] (20b) and to ≤18 kcal·mol-1 in cationic species like [Me3O+-CH3···OH2]+ (8a).