Nonspecific Solvent Effects on NMR Chemical Shifts

5 Solvent Effects on Nuclear Magnetic Resonance Spectra 6.5.1 Nonspecific Solvent Effects on NMR Chemical Shifts 

In general, two different solvent effects on NMR spectra can be distinguished (a) shifts due to a difference in the bulk volume magnetic susceptibility x of the solute and the solvent (b) shifts arising from intermolecular interactions between solute and solvent molecules. Since the bulk susceptibility effect depends on the shape of the sample and, therefore, is not of chemical interest, some form of correction for it is applied. For two 

Only shifts observed in excess of this amount may then be attributed to intermolecular interaction effects. Use of an internal standard provides an automatic compensation for the bulk susceptibility effect, but for comparison of shifts measured in this way in different solvents it must be kept in mind that the standard itself may be subject to solvent effects. These are minimized in and NMR spectroscopy by the use of tetra-methylsilane (TMS) as an internal standard. 

The intermolecular solute/solvent interactions may arise from nonspecific interaction forces such as dispersion, dipole-dipole, dipole-induced dipole, etc., as well as from specific interactions found in protic and aromatic solvents. Solvent effects on NMR spectra were first observed by Bothner-By and Glick [226] and independently by Reeves and Schneider [227] in 1957. Since then, the influence of solvent on chemical shifts (and coupling constants) has been extensively studied by scores of workers and has been thoroughly reviewed by several specialists [1-4, 288-237]. 

Some illustrative examples of solvent-dependent NMR chemical shifts for several different nuclei in a cation [238, 239], two dipolar molecules [240, 241], and an apolar molecule [242, 243] are given in Table 6-6. 

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