Siddharth Joshi
CHEM 241 – Extra Credit Assignment
Professor Bradley
Self-Discrimination of Enantiomers in Hydrogen-Bonded Dimers
http://pubs.acs.org/doi/abs/10.1021/ja011348c

In the above article, the stability of homochiral and heterochiral hydrogen-bonded dimers of a set of small α-amino alcohols was studied. Some model compounds from the study are show in Figure 1.

Enantiomers_Extra_Credit_Pic_1.JPG
Figure 1. Schematic example of monomers considered.
Basic questions posed, which are answered by the article:
How is the length of hydrogen-bonds related to the molecular configuration of each of the dimers?
Are heterochiral alcoholic dimers more stable than homochiral dimers?
For the nonchiral α-amino alcohol molecules considered, are the chair or boat configurations better for molecular stability?

The important points/answers to take away from this paper:
  • We can state that if the two molecules in a hydrogen-bonded dimer have the same chirality, then the molecule is homochiral. However, if the two molecules in a hydrogen-bonded dimer have opposite chirality, then it can be called heterochiral.
  • For all cases, the shortest hydrogen-bonds corresponded to the configuration with the lowest relative energy.
Enantiomers_Extra_Credit_Pic_2.JPG
Figure 2. Representation of Hydrogen-bond distance compared to interaction energy for dimers of X/Y = H/CH3 and CF3 and CH3 (circles & squares respectively)
  • In the complexes of nonchiral monomers, the chair configuration is more stable than the boat configuration by a small margin. These energetic differences increase as the size of the X group (pictured in Figure 1) increases; 0.55, 0.90, 1.02 and 2.34 kCal/mol for H, F, CH3 and CF3 respectively.
  • In chiral systems, the most favorable energetically stable dimers obtained are heterochiral in chair configuration, while the worst are homochiral in a boat configuration. The maximum difference between these two extreme configurations being 4 kCal/mol.
  • Two figures showing the lowest energy conformation for homochiral (Figure 3) and heterochiral (Figure 4) molecules are drawn below.

Figure_3.JPG Figure_4.JPG
Figure 3. Lowest Energy Homochiral Conformation Figure 4. Lowest Energy Heterochiral Conformation




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