Siddharth Joshi
CHEM 241 – Extra Credit Assignment
Professor Bradley
Self-Discrimination of Enantiomers in Hydrogen-Bonded Dimers

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.

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.
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