Attraction versus Repulsion between Methyl and Related Groups: (CH₃NHCH₃)₂ and (CH₃SeBr₂CH₃)₂

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Supramolecular Chemistry

Background:

• Investigated crystal structures with methyl group interactions in homodimers.
• Utilized Quantum Theory of Atoms in Molecules (QTAIM), Non-Covalent Interactions (NCI), and Natural Bond Orbital (NBO) analyses.
• Observed discrepancies between suggested attractive interactions and calculated negative interaction energies.

Purpose of the Study:

• To examine the ability of methyl groups to interact with each other using Density Functional Theory (DFT) calculations.
• To explore the nature and strength of C⋅⋅⋅C tetrel bonds in homodimers.
• To identify factors influencing the strengthening of intermethyl bonds.

Main Methods:

• Performed DFT calculations on a series of (CH3PnHCH3)2 homodimers, varying the Pn atom (N, P, As, Sb).
• Analyzed interaction energies of C⋅⋅⋅C tetrel-bonded species.
• Investigated the effect of electron-withdrawing and electron-donating substituents on bond strength.

Main Results:

• Calculated interaction energies for C⋅⋅⋅C tetrel bonds were initially low (<1 kcal/mol).
• Interaction energies increased significantly (up to ~3 kcal/mol) when the carbon atom was replaced by a heavier tetrel element.
• Asymmetric substitution (donor-acceptor) strengthened the C⋅⋅⋅C intermethyl bonds.
• Observed attractive forces despite coulombic repulsion between σ-holes.

Conclusions:

• Methyl groups can engage in tetrel bonding, though typically weak.
• Heavier tetrel atoms and asymmetric substitution enhance C⋅⋅⋅C bond strength.
• QTAIM, NCI, and NBO analyses should be interpreted cautiously, as they may indicate bonding where only repulsive potentials exist.