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

Area of Science:

• Solid-state chemistry
• Supramolecular chemistry
• Computational chemistry

Background:

• Non-covalent interactions are fundamental to molecular assembly and material properties.
• Understanding weak interactions, such as C-H···X bonds, is crucial for designing functional materials.
• The directional nature and strength of interactions involving methyl groups require further investigation.

Purpose of the Study:

• To systematically evaluate the directional nature of non-covalent carbon-bonding interactions involving methyl groups (CH3).
• To quantify the interaction energies of these bonds in various chemical environments.
• To compare these interactions with known weak bonding phenomena like hydrogen bonds.

Main Methods:

• Utilized the Cambridge Structural Database (CSD) and Protein Data Bank (PDB) for structural analysis.
• Employed Density Functional Theory (DFT) calculations to determine interaction energies.
• Analyzed charge-neutral and cationic adducts with varying substituents (X).

Main Results:

• Non-covalent interactions of X-CH3 were found to be weakly directional in the solid state when X = N or O (P ≤ 1.5).
• Interaction energies were comparable to weak C-H hydrogen bonds (≤ -1.5 kcal·mol⁻¹).
• Interaction energy increased significantly with more electron-withdrawing groups (≤ -5 kcal·mol⁻¹) and in cationic species (≤ -18 kcal·mol⁻¹).

Conclusions:

• Non-covalent carbon-bonding interactions involving methyl groups exhibit subtle but measurable directionality in the solid state.
• The strength of these interactions is tunable based on the electronic nature of substituents and charge.
• These findings provide insights into weak interactions relevant to crystal engineering and supramolecular chemistry.