| dc.description | The use of NMR to characterize structure and dynamics in solids is becoming increasingly important. However, most spin-active nuclei are beset by anisotropic quadrupolar interactions which induce substantial broadening in NMR spectra of powders, leading to overlapping peaks which obscure crucial information such as quadrupole coupling constants, chemical shifts and J-couplings. Recent advances in this area have made it possible to obtain "isotropic" NMR spectra from which these data are available. The most powerful of these is multiple-quantum magic-angle spinning (MQMAS), a unique feature of which is that splittings due to J-coupling are, in some cases, amplified. This phenomenon is demonstrated by the direct observation of 1J(63/65Cu, 13C) in K3Cu(CN)4 and 1J( 11B,31P) in (PhO)3P-BH3 in NMR spectra of the quadrupolar nucleus. Theoretical considerations show that quadrupole-induced residual dipolar distortions do not influence MQMAS spectra, but may introduce differential dipolar splittings in single-quantum satellite transitions. In many situations, however, quadrupolar interactions are simply too large to be monitored directly by NMR. While their spectroscopic observation is the domain of nuclear quadrupole resonance, such nuclei can influence the appearance of NMR spectra of nearby, dipolar-coupled nuclei. For small to moderate quadrupolar interactions, first-order perturbation theory has been successful in describing these effects, but larger quadrupolar interactions demand a more rigorous treatment. The NMR lineshape of a spin-1/2 nucleus coupled to a spin-3/2 nuclide is analyzed by employing full-matrix diagonalization of the combined Zeeman-quadrupolar Hamiltonian operator for cases of axial symmetry. A highlight of this treatment is that the effective dipolar coupling constant may be obtained in favourable circumstances. In a bis(tribenzylphosphine)cuprate salt, its precise measurement facilitates an unambiguous determination of the anisotropic 63/65Cu, 31P J tensor. A careful application of this theory to NMR spectra of solid copper(I) cyanide results in direct structural insight which has not been accessible by more conventional means. | en_US |
