Resumen de Effect of Anisotropy on the Chemical Shift of Vinyl Protons in trans- and cis-1,2-Dibenzoylethylenes. A Small-Group or Recitation Activity
Roosevelt Shaw, David Roane, Sean Nedd
Undergraduate students combine molecular modeling and model-building with diamagnetic anisotropy to explain why the singlet for the vinyl protons in the 1H NMR spectrum of trans-1,2-dibenzoylethylene (trans-1,4-diphenyl-2-butene-1,4-dione), (d 8.01) appears much farther downfield than that for the cis isomer (d 7.14). In the trans isomer, the vinyl protons are found in the deshielding regions of the carbonyl and phenyl groups and thus experience a more pronounced anisotropic effect. In the cis isomer, the vinyl protons appear to be deshielded only by the closer carbonyl group. 1H NMR spectra of related alkene diastereomers are also examined in an out-of-class activity.
