手性化合物结构鉴定

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.

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 nu clear 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. Discrimination of Chiral Compounds Using NMR Spectroscopy

Discrimination of Chiral Compounds Using NMR Spectroscopy

T. J. Wenzel

ISBN: 978-0-471-76352-9

Hardcover

576 pages

April 2007

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A comprehensive overview of the use of NMR spectroscopy for chiral discrimination Discrimination of Chiral Compounds Using NMR Spectroscopy concisely covers the broad array of reagents that make it possible to determine the optical purity and assign the absolute configuration of many classes of compounds. It describes chiral NMR derivatizing agents, solvating agents, metal-based reagents, and liquid crystals and discusses the range and types of compounds for which they can be used for analysis. After an overview of chiral reagents and methodologies, this reference:

Includes comprehensive coverage of the chiral reagents that have been reported Catalogs the range of compounds for which different reagents have been shown to be effective

Includes specialty categories such as liquid crystals, ionic liquids, and the formation of chiral aggregates from achiral building blocks that do not fit into the broader categories

Offers experimental strategies for using the reagents that are likely to improve the