Basic Concept of Spin Decoupling

As was illustrated for the methyl and carbonyl signals in the 13C NMR spectrum of acetone (Fig. 1.10), 13C— H spin-spin coupling vanishes when proton broadband decoupling is applied. Proton broadband decoupling is the most important decoupling technique used in routine 13C NMR, simplifying 13C — H multiplet systems to spectra of up to z singlet lines for z nonequivalent 13C nuclei of a sample. 

Spin decoupling or nuclear magnetic double resonance (NMDR) is achieved by irradiating an ensemble of nuclei not only with a radio frequency fl, at resonance with the nuclei to be observed, but additionally with a second alternating field B2 at resonance with the nuclei to be decoupled e.g. H). Decoupling experiments can be carried out to convert homonuclear ( H— JH, 19F —19F) or heteronuclear multiplets (19F—1H, 

13C— lH) into singlets. NMDR spectra are often symbolized by putting the nuclei to be decoupled between brackets besides the nuclei to be observed A X. Proton decoupled 13C NMR experiments are thus referred to as 13C 1H NMR spectra. 

If B2 is applied at resonance with the nuclei X to be decoupled, ot2jy cancels B0 so that Beff. = B2, as in eq. (1.33). As a result, the magnetization vectors of the irradiated nuclei X precess perpendicularly to B0. Now, the irradiated nuclei X have their spins quantized perpendicularly to B0, while the spins of the observed nuclei A are still quantized along B0. Since the observed coupling between nuclei A and X is the scalar product of the spin quantizations IA and /x [28-30], the observed coupling is related to the angle a enclosed by the spin quantizations /A and /x. 

The observed splitting, /AX(obs, s equal to the coupling constant JAX when both spins A and X are quantized in the same direction (a = 0, cos a = 1, NMR coupled). Coupling between A and X collapses 0/Ax(ObS.) = 0) however, when the spins A and X are quantized perpendicularly to each other (a = 90 , cos a = 0, NMDR, decoupled). 

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