Hyperconjugative effects in π-hydrogen bonding: Theory and experiment.

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• π-Hydrogen bonding plays a crucial role in molecular interactions.
• Understanding substituent effects on non-covalent interactions is vital.
• Hexamethylbenzene offers a unique platform to study steric and electronic effects.

Purpose of the Study:

• To investigate the properties of π-hydrogen bonded complexes between substituted phenols and hexamethylbenzene.
• To quantify the impact of methyl hyperconjugative effects on interaction energies and O-H stretching frequencies.
• To elucidate the mechanisms governing these methyl hyperconjugative effects.

Main Methods:

• Density functional theory (DFT) computations using the B3LYP/6-311++G(2df,2p) method.
• Infrared (IR) spectroscopy for experimental validation.
• Analysis of Hirshfeld charges, electrostatic potentials, and molecular electrostatic potential maps.

Main Results:

• Twenty π-hydrogen bonded complexes with T-shaped structures were characterized.
• Theoretical computations accurately predicted O-H stretching frequency shifts (Δν_OH).
• Δν_OH shifts in hexamethylbenzene complexes were approximately twice as large as in benzene complexes, indicating enhanced hydrogen bonding.

Conclusions:

• Methyl hyperconjugative effects significantly strengthen π-hydrogen bonding in phenol-hexamethylbenzene complexes.
• The enhanced hydrogen bonding is attributed to electronic contributions from methyl groups.
• Computational methods provide reliable predictions for these non-covalent interactions.