Medium and Isotope Effects

The observed shielding of a particular nucleus consists of intramolecular components (T ird (discussed in regard to protons in Section 3-2) and intermolecular components (rjnter The total shielding is given by 

Buckingham, Schaefer, and Schneider pointed out five sources of intermolecular shielding, given in the equation 

We shall consider each contribution in turn and then give several illustrations of the effects of the medium. 

The solvent has a bulk diamagnetic susceptibility that is dependent on the shape of the sample container. Thus, the solvent in a spherical container shields the solute to an extent that is slightly different from the shielding afforded by the solvent in a cylindrical container. The shielding is 

Close approach of the solute and the solvent can distort the shape of the electron cloud around a proton and deshield it, even when both components are nonpolar. Such a phenomenon (cTw) is analogous to the van der Waals effect on the chemical shift. The magnitude is rarely more than 0.1 ppm. If chemical shifts are measured from the resonance of an internal standard, the contribution from ctw should affect the solute and the standard similarly. Chemical shifts so measured should be largely independent of ctw- 

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