Structural Aspects of Spin-Spin Coupling in Multi-NMR Spectroscopy

The observation of spin-spin coupling in NMR spectra provides a powerful method for structural elucidation. In this review the direct measurement of J is described for primarily first-order systems. The analysis of spin-spin coupling patterns is outlined for cases in which a nucleus A is coupled to a spin-1/2 nucleus B under the conditions of different natural abundance of B. The use of reduced coupling constants and the importance of relativistic effects for coupling between heavy nuclides are exemplified. The conditions under which spin-spin coupling to quadrupolar nuclei may be observed are discussed and illustrated. The effects of quadrupolar relaxation on the spectra of spin-1/2 nuclei and the analysis of the spectra in such situations are described. The less well-known phenomena, namely, isotope effects on the spin-spin coupling constant and the anisotropy of J are briefly discussed at the end of the chapter. Throughout the text numerous examples are provided and illustrated.