Fomenting proton anisochronicity in the CH₂D group

A novel C-D⋯N interaction promotes an equilibrium isotope effect that causes a large chemical shift difference (43 ppb, 22 °C) for the diastereotopic methyl protons in the (N-CH₂D)-(-)-isosparteinium cation. The observed chemical shift difference is in surprising agreement with a prediction of 44 ppb based upon an ab initio protocol. These calculations are also shown to reproduce the experimental shift difference in Anet's α-deutero-1,2-dimethylpiperidine (theory, 17 ppb; experiment, 14 ppb). The isosparteine-derived system described herein may provide an improved method for assigning the configuration and enantiomeric purity of stereogenic methyl groups (R-CHDT) by tritium NMR spectroscopy.