Conformational analysis-V.: A lanthanide induced shift (L.I.S.) N.M.R. investigation of conformational isomerism in aromatic aldehydes and ketones

An L.I.S. N.M.R. investigation of conformational isomerism in meta-nitrobenzaldehyde and furan- and thiophene-3-aldehydes, and of the conformation of acetophenone has been carried out. With the diamagnetic (complexation) contributions to the observed shifts removed by means of experiments with La (fod)₃ and the application of our previously-described lanthanide ion complexing model, the pseudo-contact shifts have been used to obtain the precise conformational preferences of the aldehydes. They exist to the extent of 45%, 100% and 80% respectively in the $\text{O}$-$\text{trans}$ conformation, findings which are good agreement with the previously-reported values. The L.I.S.'s for acetophenone are in best agreement with a planar molecule, although the definition is not as good in this case. These examples suggest that the L.I.S. technique is of general applicability in the determination of conformer populations and the conformations of aromatic aldehydes and ketones.