CHAPTER 2-3 - THEORY OF SPIN–SPIN MULTIPLICITY

Abstract

Electron-coupled spin–spin interactions are independent of the strength of the applied field. The process of extracting the chemical shifts of the participating nuclei and the coupling constants for the interactions among them, from the experimental spectra, is usually termed the analysis of the nuclear magnetic resonance spectra. Because the process is entirely mathematical and is based ultimately on quantum–mechanical calculations, organic chemists have either ignored the important information that can be gained or have used certain simplified procedures. This chapter discusses the protons and other nuclei of spin. The spin system is simply defined as a group of magnetic nuclei that interact among each other but do not interact with any nuclei outside the spin system. The definition of strongly and weakly coupled nuclei concerns the ratio of the chemical shift differences between the interacting nuclei to the coupling constants between them, which is the critical parameter determining the appearance of the spectrum. For the purpose of analysing spin multiplets, chemical shifts are best expressed in terms of frequency, that is, in Hz. Thus, they are dependent on the operating frequency of the spectrometer.