Surfing π clouds for noncovalent interactions: arenes versus alkenes

Area of Science:

• Organic Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• Understanding noncovalent interactions is crucial in chemistry.
• Arene and alkene interactions with functional groups are key to molecular recognition.
• Intramolecular hydrogen bonding influences molecular conformation.

Purpose of the Study:

• To compare noncovalent interactions of functional groups with arenes and alkenes.
• To investigate the strength and nature of π-facial intramolecular hydrogen bonds.
• To correlate conformational free-energy differences with substituent properties and solvent effects.

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy was used for comparative analysis.
• Computational methods were employed to calculate atomic charges.
• Analysis of conformational free-energy differences was performed.

Main Results:

• Noncovalent interactions with arenes dominate over alkenes.
• A π-facial intramolecular hydrogen bond from a hydroxyl group to an arene is favored by ~1.2 kJ/mol.
• The cyano group exhibited the strongest observed interaction.
• A correlation was found between conformational free-energy differences and the calculated charge on the C(α) atom.
• Changes in free-energy differences showed a linear dependence on the solvent hydrogen bond acceptor parameter (β).

Conclusions:

• Electrostatic forces are significant in π-interactions between substituents and aromatic rings.
• The strength of these interactions is influenced by the electronic properties of the substituent and the solvent environment.
• NMR spectroscopy provides valuable insights into noncovalent interactions and molecular conformation.