Chiral discrimination via nuclear magnetic shielding polarisabilities from NMR spectroscopy. Theoretical study of (Ra)-1,3-dimethylallene, (2R)-2-methyloxirane, and (2R)-N-methyloxaziridine

Three medium-size optically active molecules have been studied to make a guess at candidates suitablefor chiral discrimination in an isotropic medium via nuclear magnetic resonance spectroscopy. The criterion for experimentaldetection is given by the magnitude of the isotropic part of nuclear magnetic shielding polarisability tensors,related to a pseudoscalar of opposite sign for the two enantiomers. The pseudoscalar shielding polarisability atthe 17O nucleus in N-methyloxaziridine, calculated at the Hartree-Fock level, is &7.8 1017 mV1. To obtain anexperimentally observable magnetic field induced at the 17O nucleus in N-methyloxaziridine, electric fields as largeas &107–108 Vm1 should be applied to the probe. The molecular electric dipole moment induced by precession ofthe magnetic dipole of the 17O nucleus in a magnetic field of 10 T is, in absolute value, &8.8 1042 Cm. Theestimated rf-voltage at a resonance circuit is &10 nV. Smaller values have been estimated for N, C, and H nuclei in1,3-dimethylallene and 2-methyloxirane.