Chiral Discrimination via Nuclear Magnetic Shielding Polarisabilities from NMR Spectroscopy:Theoretical Study of (R_a)-1,3-Dimethylallene,(2R)-2-Methyloxirane,and (2R)-N-Methyloxaziridine

摘要

Three medium-size optically active molecules have been studied to make a guess at candidates suitable for chiral discrimination in an isotropic medium via nuclear magnetic resonance spectroscopy.The criterion for experimental detection 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 at the ~17O nucleus in N-methyloxaziridine,calculated at the Hartree-Fock level,is approx = 7.8 x 10~(-17) mV~(-1).To obtain an experimentally observable magnetic field induced at the ~17O nucleus in N-methyloxaziridine,electric fields as large as approx = 10~7-10~8 Vm~(-1) should be applied to the probe.The molecular electric dipole moment induced by precession of the magnetic dipole of the ~(17)O nucleus in a magnetic field of 10 T is,in absolute value,approx 8.8 x 10~(42) Cm.The estimated rf-voltage at a resonance circuit is approx =10 nV.Smaller values have been estimated for N,C,and H nuclei in 1,3-dimethylallene and 2-mefhyloxirane.