Chirality
Direct chiral discrimination in NMR
The calculated isotropic component of the proton magnetic shielding
polarizability for hydrogen peroxide (H2O2) is shown as a function of dihedral
angle (_, as defined in the inset). The two enantiomers are R-HOOH
for dihedral angles <180° and S-HOOH for dihedral angles >180°.
σ(1)(N) is obtained from finite field
calculations at the SCF level. The solid line is to guide
the eye, and 1 ppm a.u. = 1.9446 _ 10-18 m/V.
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Nuclear magnetic resonance (NMR) spectroscopy is an important
technique for determining the structure of molecules in solution. NMR can, however, not yet
be used to determine the absolute configuration of chiral molecules in a pure liquid, as the
chemical shifts and spin-spin coupling constants are identical for the two enantiomers of a
chiral molecule. All NMR-based methods for chiral discrimination have therefore required that
the chiral solute be in the presence of a chiral reagent or solvent.
However, the electric-field perturbed chemical shift tensor, the nuclear
magnetic shielding polarizability σ(1)(N), gives rise to three chiral NMR effects in a liquid that
could make it possible to discriminate directly between the enantiomers of a chiral molecule:
The coherent precession of nuclear spins following application of a π/2 pulse to an optically
active liquid will lead to a rotating macroscopic electric polarization [1]; a
laser polarized in the plane perpendicular to the field of the magnet may in principle give rise
to chiral chemical shifts; and the application of an (oscillating) electric field
at right angles to the magnetic field of the spectrometer may give rise to a magnetization
oscillating (at a unique frequency) in the direction of the permanent magnetic field
[2]. Using finite field calculations on small chiral molecules we estimate the
magnitude of σ(1)(N) [2].
This work is in collaboration with Prof. A.D. Buckingham in the Department
of Chemistry at the University of Cambridge. Further details can be found in:
[1] A.D. Buckingham, Chem. Phys. Lett., 398 (2004) 1.
[2] A.D. Buckingham and P. Fischer, Chem. Phys., 324 (2006) 111-116.
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