Chiral discrimination in NMR spectroscopy.

A. David Buckingham

doi:10.1017/s0033583515000074

Abstract

Nuclear magnetic resonance is the most important form of molecular spectroscopy in chemistry and biochemistry but it is normally blind to chirality. It was predicted in 2004 that precessing nuclear spins in chiral molecules in a liquid in a strong magnetic field induce a rotating electric polarization that is of opposite sign for enantiomers. This polarization arises from the distortion of the electronic structure by the nuclear magnetic moment in the presence of the strong magnetic field and is equivalent to the linear effect of an electric field on the nuclear shielding tensor. The polarization is strongly enhanced in dipolar molecules through the partial orientation of the permanent dipole through the antisymmetric part of the nuclear magnetic shielding tensor. Alternatively, an applied electric field will induce a chirally sensitive magnetization perpendicular to the field and to the nuclear spin. Progress towards the experimental realization of chiral discrimination by NMR is assessed.

Highlights

High-resolution nuclear magnetic resonance spectroscopy is an important technique for determining the structure of molecules in solution
The nature of the torque orienting the molecule through the antisymmetric components of σ(αNβ) can be understood as follows: the antisymmetric part of σ (N) αβ can be represented as εαβγ σ∗γ (N) where σ∗(N) is a polar vector (Buckingham, 2014; Jeffreys, 1931)
The antisymmetric shielding contribution to the nuclear shielding spin Hamiltonian σ(αNβ)m(αN)B(β0) is εαβγ σ∗γ (N) m(αN)B(β0) = σ∗(N) · (m(N) × B(0)); if σ∗z(N) is negative there is a lowering of energy when the molecular z-axis is in the direction of the vector product of the nuclear magnetic moment m(N)and the magnetic field B(0), causing the molecule, with its dipole μ(z0), to favour this direction and leading to electric polarization precessing in the plane perpendicular to B(0) at right angles to the nuclear magnetization

Summary

Introduction

High-resolution nuclear magnetic resonance spectroscopy is an important technique for determining the structure of molecules in solution. Computations of σ(1)(N) (Buckingham & Fischer, 2006; Lazzeretti et al 2008; Monaco & Zanasi 2011; Pelloni et al 2013; Zanasi et al 2007) have shown that its contribution to the electric polarization is likely to be too small for easy detection, even for heavier nuclei such as 13C and 19F.

Results

Conclusion