Effects of Intermolecular Interactions and Intramolecular Dynamics

Because molecules studied by NMR are not rigid isolated bodies, there will always be some effects due to the surrounding molecules and also due to small and large amplitude motions within the molecule. It may be necessary to correct for these effects. Alternatively, they can provide subtle information about the environment of a molecule as well as the dynamics of the intramolecular motional averaging that takes place in the NMR time scale. [Pg.79]

Medium effects on the molecule of interest, the homonuclear reference molecule, and on the lock substance, may need to be taken into account in an NMR experiment. These effects have been studied in the dilute gas phase, in which the shielding can be written in the form of a virial expansion in powers of density, p [Pg.79]

Contributions to the shielding of a nucleus due to the presence of a neighboring molecule are made by effects of bulk susceptibility, van der Waals interactions, electric fields, and magnetic anisotropy [Pg.79]

The bulk susceptibility contribution is the same for all nuclei in the sample and depends only on the molar magnetic susceptibility such that its contribution to ff i(A - B) Pb. for example, will depend on x of molecule B 0, is ln/7 )x or - (4tc/3)x for cylindrical gas samples perpendicular or parallel to the magnetic field. For protons and C this correction may be a large part of the medium shift. The van der Waals [Pg.79]

335 ppm for Xe in various organic liquids. The reaction field model and the equivalent solvaton modeP have been very useful in the systematic classification of some solvent effects on NMR chemical shifts, and have the advantage that only bulk properties such as refractive index are necessary in the various models. They are most successful in correlating one solute with various solvents of roughly comparable dielectric constants, less successful in interpreting larger bodies of data especially for nonpolar solvents. [Pg.80]

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