The conformational analysis of tetrahydronaphthoquinones using high resolution solid state 13C NMR spectroscopy

Abstract

The proton-decoupled 13C NMR spectra of five conformationally mobile cis-tetrahydronaphthoquinone derivatives were obtained in solution (CDCl 3) and in the solid state (cross polarization, magic angle spinning). The major difference between the results in the two media is that in solution the spectra consist of a series of well separated two carbon atom singlets whereas in the solid state, the singlets are replaced by doublets, with separations ranging from 0.4 to 9.3 ppm. This difference is interpreted as being due to rapid equilibrium between enantiomeric conformers in solution resulting in an average plane of symmetry. In the solid state the static spectrum of the asymmetric conformer is observed, the doublets arising because of the chemical shift differences between two conformationally distinct carbon atoms. The magnitude of the solid state doublet separations can be used to make tentative conformational assignments.