Effect of ionic charge on the CH···π hydrogen bond

Area of Science:

• Physical Chemistry
• Supramolecular Chemistry
• Computational Chemistry

Background:

• CH···π hydrogen bonds (HBs) are crucial non-covalent interactions.
• Trimethylamine (TMA) and its cation (TMA+) are relevant in various chemical systems.
• Understanding factors influencing HB strength is key for molecular design.

Purpose of the Study:

• To investigate the strength and characteristics of CH···π hydrogen bonds involving TMA and TMA+ with various π-systems.
• To compare the HB capabilities of TMA and TMA+ with other proton donors like CF3H.
• To elucidate the roles of electrostatic potential, charge transfer, and dispersion in these interactions.

Main Methods:

• Computational chemistry methods were employed to model and calculate interaction energies.
• Analysis of electronic factors including electrostatic potentials and charge transfer.
• Assessment of the contribution of dispersion forces to the overall binding energy.

Main Results:

• CH···π HBs with TMA are weaker than those with CF3H, despite TMA having more potential donor sites.
• The positive charge in tetramethylammonium (TMA+) enhances binding energy by 4-7 times compared to TMA.
• TMA+ forms the strongest interaction with indole (15.5 kcal/mol); NH···π interactions are stronger than ionic CH···π.
• π-system conjugation and aromaticity enhance proton-accepting capacity.
• Dispersion forces are a major contributor to CH···π HBs, especially for neutral TMA.

Conclusions:

• The binding energy of CH···π HBs is significantly influenced by the charge of the proton donor and the electronic properties of the π-system.
• Electrostatic potentials, charge transfer, and dispersion collectively govern the strength of these hydrogen bonds.
• TMA+ represents a potent motif for forming strong ionic CH···π interactions, particularly with electron-rich aromatic systems like indole.