Nuclear magnetic resonance diamagnetic shielding

Magnetic Balance and Explicit Diamagnetic Expressions for Nuclear Magnetic Resonance Shielding Tensors... 

Nuclear magnetic resonance spectroscopy gives precise information on complexation in solution. Equilibrium is rapidly established on an NMR time scale, hence only an average spectrum is observed and it is difficult to determine the spectrum of a pure complex. When complexation of a sugar or polyol with a diamagnetic ion occurs, all of the signals shift downfield. Equation (11.1) allows the variation of the shielding constant Ao- of the proton to be calculated when the nucleus is subjected to an electric field E whose projection on the C-H bond is... 

L. Visscher. Magnetic Balance and Explicit Diamagnetic Expressions for Nuclear Magnetic Resonance Shielding Tensors. Adv. Quantum Chem., 48 (2005) 369-381. 

N. F. Ramsey, Phys. Rev., 78, 699 (1950). Magnetic Shielding of Nuclei in Molecules. N. F. Ramsey, Phys. Rev., 86, 243 (1952). Chemical Effects in Nuclear Magnetic Resonance and in Diamagnetic Susceptibility. 

Selected best values of nuclear spins and moments from Table of Isotopes (Lederer and Shirley 1978) with some later data. In most cases the method identified applies only to measurements of magnetic moments (corrected for diamagnetic shielding). Values of the nuclear resonance frequency in MHz per tesla for the stable isotopes, uncorrected for diamagnetic shielding, are given in table 12. 

Values of the nuclear spin I and of the magnetic resonance frequency in MHz per tesla for the stable isotopes of the lanthanide ions. Based on the best values of the nuclear magnetic moments in table I, without the correction for diamagnetic shielding, with rounded errors, calculated from the relation y/2n = 7.62253(2)p,/f. 

We have seen that the magnetic field R, required to obtain the resonance crmdition for nucleus i at a particular irradiating rf field (Ri) is not equal to the applied static field Rq, but is instead R, = Ro(l — a) [see Eq. (20.9)], where the nuclear screening constant, a, depends on the chemical structural environment of nucleus /. The local electron density in the vicinity of the nucleus shields it from the applied field Bo by producing small local magnetic fields (diamagnetic currents). Any structural feamre that alters the electronic environment of a nucleus will affect its screening constant a and lead to an alteration in its resonance frequency or chemical shift 5,. 

Nuclear resonance is influenced in characteristic ways by the environments of the observed nuclei. However, nuclei are always surrounded by electrons and other atoms. Thus, in diamagnetic molecules the effective magnetic field at the nucleus is always smaller than the applied field 5 , i.e. the nuclei are shielded. The effect, although small, is measurable. This observation is expressed by equation 1 ... 

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