NMR spectroscopy of stereoisomers

NMR spectroscopy techniques can determine the absolute configuration of stereoisomers such as cis or trans alkenesR or Senantiomers, and R,R or R,S diastereomers.[1][2]

In a mixture of enantiomers, these methods can help quantify the optical purity by integrating the area under the NMR peak corresponding to each stereoisomer. Accuracy of integration can be improved by inserting a chiral derivatizing agent with a nucleus other than hydrogen or carbon, then reading the heteronuclear NMR spectrum: for example fluorine-19 NMR or phosphorus-31 NMR.Mosher’s acid contains a -CF3 group, so if the adduct has no other fluorine atoms, the 19F NMR of a racemic mixture shows just two peaks, one for each stereoisomer.

As with NMR spectroscopy in general, good resolution requires a high signal-to-noise ratio, clear separation between peaks for each stereoisomer, and narrow line width for each peak. Chiral lanthanide shift reagents cause a clear separation of chemical shift, but they must be used in low concentrations to avoid line broadening.

References

  1. David Parker. “NMR determination of enantiomeric purity.” Chem. Rev. 199191, 1441–1457. [1]
  2. Frank J. Hollis. “NMR Through the Looking Glass: Uses of NMR Spectroscopy in the Analysis and Synthesis of Chiral Pharmaceuticals.” 1994. [2]

S442. NMR Project

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