Stabilizing Σ-Hole Dimethyl Interactions.

Methyl groups, depending on the atomthey are bound to,can behave as both Lewis acids and bases to engage in highly directionaland stabilizing dimethyl interactions. Methyl groups boundto electronegative atoms, such as N or O, arerecognized to participate in tetrel bonding as Lewis acids. On theother hand, the capability of methyl groups bound to electropositiveatoms, such as B or Al, to act as Lewis bases has been recently reported.Herein, we analyze the combination of these two behaviors to establishattractive methyl & BULL;& BULL;& BULL;methyl interactions. We have exploredthe Cambridge Structural Database to find experimental examples ofthese dimethyl-bound systems, finding a significant degree of directionalityin the relative disposition of the two methyl groups. Moreover, wehave carried out a comprehensive computational analysis at the DFTlevel of the dimethyl interactions, including the natural bond orbital,energy decomposition analysis, and topological analysis of the electrondensity (QTAIM and NCI). The dimethyl interaction is characterizedas weak yet attractive and based on electrostatics, with a non-negligiblecontribution from orbital charge transfer and polarization.