Magnetic resonance spectroscopies, despite being among the most useful characterization techniques due to their high sensitivity to tiny details of the geometrical and electronic structure of molecules, are blind to chirality. However, it has been recently proposed by Buckingham [1] and Buckingham and Fischer [2] that NMR could be used to achieve chiral discrimination for closed-shell chiral molecules, via the detection of a molecular electric polarization P rotating in a plane perpendicular to the NMR magnetic field [1,2], and having opposite orientation for the two enantiomers of a chiral species. In solution, the induced chiral polarization P is proportional to the pseudoscalar s(1), isotropic average of a third-rank tensor known as the shielding polarizability [1-3] However, computational estimates of s(1) in diamagnetic molecules suggest that it is generally too small to be detected [1-5].
Here we present a theory of NMR chiral discrimination which is valid for molecules in solution with a ground state of arbitrary degeneracy [6]. We describe the response of the degenerate system in terms of a generalized shielding polarizability tensor defined as an analytical third-derivative of the electronic free energy [6-9]. The proposed theory predicts previously unexplored orientational contributions to the macroscopic rotating chiral electric polarization P, which are proportional to the chiral molecule’s magnetic anisotropy, and to the square of the inverse temperature. Ab initio calculations for ten Dy3+ complexes that are also single-molecule magnets [10], show that P can be more than 1000 times larger than in diamagnetic molecules, making paramagnetic NMR chiral discrimination amenable to room temperature detection [6].