Methyl groups as widespread Lewis bases in noncovalent interactions

Area of Science:

• Supramolecular Chemistry
• Organic Chemistry
• Computational Chemistry

Background:

• C(sp3) atoms are known Lewis acids in tetrel bonding via their sigma-holes.
• The role of methyl groups in noncovalent interactions is less understood.

Purpose of the Study:

• To investigate the potential of methyl groups to act as electron density donors in noncovalent interactions.
• To explore the Lewis basicity of methyl groups and their role in various bonding types.

Main Methods:

• Analysis of thousands of experimental crystallographic structures.
• High-level theoretical calculations to confirm interaction patterns and strength.

Main Results:

• Methyl groups act as Lewis bases, donating electron density.
• Experimental evidence shows methyl groups forming alkaline, alkaline earth, triel, tetrel, pnictogen, chalcogen, and halogen bonds.
• Theoretical calculations confirm the directionality and strength of these interactions, consistent with the electron density holes model.
• Charge transfer occurs from methyl sigma bonding orbitals into electrophilic empty orbitals.

Conclusions:

• Methyl groups exhibit significant Lewis basicity, challenging previous assumptions.
• This Lewis basicity enables methyl groups to participate in a wide range of noncovalent interactions.
• The electron density holes model provides a unified explanation for these interactions.