Optical activity is typically first introduced to a prospective chemist in her sophomore year organic chemistry course. Here, she is taught that optical activity is a consequence of chirality, for example, L-tartaric acid has a specific rotation of +12° at the sodium D-line. However, this leaves said chemist with a wildly skewed and rather vague understanding of the concept of optical activity. There are two major problems with the current understanding of optical activity. The first is that both theory and experiment have shown that optical activity is, in fact, not a consequence of chirality. Molecules belonging to one of four achiral point groups (Cs, C2 v, S4, and D 2d) can display optical activity in particular directions. However, measurement requires an anisotropic medium which presents major challenges. The second problem is that we lack structure-property relationships; specific rotations generally speaking are impossible to connect to molecular structure. Herein, we emphasize optical activity in achiral molecules whose high symmetry and simplified electronic structure are used to establish structure–property relationships. First, achiral optical activity is emphasized by showing that achiral polyaromatic hydrocarbons (PAH) are actually significantly more optically active than their helicene isomers. Next, small, planar, conjugated hydrocarbons are used to interpret optical activity by analysis of their π wave functions that can be intuited from structure. Finally, it is shown that aromaticity is generally deleterious for optical activity. A simple explanation is offered based on Kekule structures.
|Commitee:||Canary, James, Traaseth, Nathaniel J., Ward, Michael D., Zhang, Yingkai|
|School:||New York University|
|School Location:||United States -- New York|
|Source:||DAI-B 78/05(E), Dissertation Abstracts International|
|Keywords:||Computational chemistry, Optical activity, Organic chemistry|
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