Dipolar interactions, dynamic range limitation

In general, multiple pulse techniques sufficiently average the dipolar interactions, compress the chemical shift scale, but they do not affect heteronuclear dipolar interactions and the chemical shift anisotropy. A combination of both multiple pulse techniques and magic angle spinning, so-called CRAMPS (Combined Rotational And Multiple Pulse Spectroscopy) is found to yield satisfactory results in the solid state H NMR of solids 186). The limitations of all these techniques, from the analytical point of view, arises from the relatively small chemical shift range (about 10 ppm) as compared with some other frequently studied nuclei. However, high resolution H NMR of solids is useful in studies of molecular dynamics. 

The effective matrix elements Hfj describe only the intramolecular terms associated with the chemical bonding but do not take into account long range and intermolecular interactions. For instance, the dipolar interaction between a solute and the molecules of a polar solvent are not accounted by the plain EHT matrix elements. Since semiempirical methods are much faster, the limitations imposed by the use of a continuum dielectric model for the solvent, which do not provide a good approximation for the immediate solvation shells in the vicinity of the solute or near the solid surface, can be overcome by atomistic quantum mechanical models for the solvent. Dynamic solvation effects can also be included through the semiempirical models. The hybrid QM/MM methods are also a valuable alternative to describe the dynamic effects of solvents on the quantum dynamics of the solute. The dipoles can be either intrinsic or induced. In the case of polar solvents, the electronic part of the dipole moment produced by the kth solvent molecule is f k f),... 

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