Essential versus accidental isochrony of diastereotopic nuclei in NMR spectroscopy
Compounds where a non-stereogenic sp 2 atom (for instance the C atom of a benzene ring) is linked to a stereogenic sp 3 atom (for instance CHFCl) have several conformations where two diastereotopic nuclei become isochronous. This also happens in some cases when the sp 3 atom is non-stereogenic, the difference being related to the weighted value of the difference of absolute shieldings. The results here described seem to be in contradiction with experimental results pointing out that all diastereotopic nuclei are anisochronous. Two examples of typical diastereotopic nuclei involving C(sp 3)–C(sp 3) bonds, a propane and an ethane, were also discussed.