Optical rotation of noncovalent aggregates

Area of Science:

• Chiroptical spectroscopy
• Computational chemistry
• Physical organic chemistry

Background:

• Specific rotation ([alpha]D) is a fundamental property of chiral molecules.
• Self-association can influence the chiroptical properties of solutes.
• Accurate prediction of concentration-dependent [alpha]D is crucial for understanding molecular interactions.

Purpose of the Study:

• To investigate the impact of self-association on the specific rotation ([alpha]D) of (R)-(-)-pantolactone.
• To theoretically model and experimentally verify the concentration-dependent [alpha]D.
• To elucidate the role of hydrogen-bonded dimers in chiroptical property changes.

Main Methods:

• Polarimetry was used to measure the specific rotation ([alpha]D) of (R)-(-)-pantolactone solutions in CCl4 at varying concentrations.
• Theoretical calculations were performed to predict [alpha]D values for monomeric and dimeric species.
• Atomic map analysis was employed to understand the contribution of specific functional groups to [alpha]D.

Main Results:

• Experimental data showed a clear concentration-dependent specific rotation for (R)-(-)-pantolactone.
• Theoretical predictions of [alpha]D were in good agreement with experimental measurements.
• Hydrogen-bonded dimeric species were identified as the primary cause for the observed changes in [alpha]D.

Conclusions:

• Self-association significantly alters the specific rotation of chiral solutes.
• Theoretical computation of chiroptical properties can accurately predict concentration-dependent [alpha]D for self-associating systems.
• The hydrogen-bonded hydroxyl groups in dimers play a critical role in the observed chiroptical changes.