Planar hydrocarbons more optically active than their isomeric helicenes

Area of Science:

• Physical Chemistry
• Organic Chemistry
• Spectroscopy

Background:

• Optical activity is a key property in stereochemistry, crucial for understanding molecular interactions.
• Helicenes and polyaromatic hydrocarbons (PAHs) are important classes of chiral and achiral molecules, respectively.
• Calculating optical rotation tensors provides insights into molecular structure and electronic properties.

Purpose of the Study:

• To compare the calculated optical rotation tensors of C(2v)-symmetric PAHs with their chiral helicene isomers.
• To investigate the influence of molecular symmetry and planarity on optical activity.
• To re-evaluate the factors governing the magnitude and origin of optical activity, particularly concerning solution averaging.

Main Methods:

• Computational chemistry methods were employed to calculate optical rotation tensors.
• Comparisons were made between planar, ∩-shaped PAHs and [5]-, [6]-, and [7]-helicenes.
• Analysis focused on the magnitudes of computed tensor elements and their relationship to molecular structure and symmetry.

Main Results:

• Seven ∩-shaped, planar PAHs exhibited larger computed optical rotation tensor elements than the corresponding chiral helicenes.
• The removal of solution averaging significantly altered expectations for optical activity.
• Symmetry in achiral compounds allowed for semiquantitative structure-optical rotation correlations.

Conclusions:

• Molecular symmetry and planarity play critical roles in determining optical activity.
• Computed optical rotation tensors offer valuable insights into the origins of optical activity.
• Achiral molecules can display significant optical properties, challenging traditional structure-activity relationships.