Heteronuclear double resonance spectra

EXAMPLE 12.1 Suppose we were to observe the H spectrum of CH2F2 (Section 8.6.1) at a field strength of 5.87 T, while simultaneously irradiating the fluorines, (a) Does this experiment involve homo- or heteronuclear double resonance (b) What are the values of v, v2, and the difference (Av) between them ... 

When the two sets of nuclei are of different isotopes (e.g., C and H), the experiment is described as heteronuclear double resonance. When we observe the signal from nuclei of isotope A while irradiating nuclei of isotope B, we label the resulting spectrum with the shorthand designation A (B). When the two sets of nuclei belong to the same isotope (e.g., both H), the technique is described as homonuclear double resonance. 

The use of heteronuclear double resonance is exemplified by a study of alumichrome, an analogue of ferrichrome in which iron has been replaced by aluminum (Llinas et al, 1977a). The proton spectrum, including N-H... 

By using double resonance experiments, one can greatly simplify the spectrum coupling between different types of nuclei is confirmed by both the disappearance of the peak for the saturated nuclei and the collapse of the fine structure of the coupled nuclei. Nuclei of the same type can be decoupled, as in the proton-proton example given earlier this is called homonuclear decoupling. It is of course possible to decouple unlike nuclei, such as decoupling this is called heteronuclear decoupling. 

The previous decoupling methods involved coherent spatial averaging to remove the heteronuclear dipolar coupling. The dominant line-broadening mechanism in the H magnetic resonance of solids is normally the H- H homonuclear dipolar interaction, which cannot be removed by the double-resonance experiment if the spectrum is to be observed. In order to obtain narrow H resonances of solid samples, the line-broadening effects of the homonuclear dipolar interactions must be eliminated while the chemical-shift interactions are retained. 

The experiment described above is termed selective population transfer (SPT), or more precisely in this case with proton spin inversion, selective population inversion, (SPI). It is important to note, however, that the complete inversion of spin populations is not a requirement for the SPT effect to manifest itself. Any unequal perturbation of the lines within a multiplet will suffice, so, for example, saturation of one proton line would also have altered the intensities of the carbon resonance. In heteronuclear polarisation (population) transfer experiments, it is the heterospin-coupled satellites of the parent proton resonance that must be subject to the perturbation to induce SPT. The effect is not restricted to heteronuclear systems and can appear in proton spectra when homonuclear-coupled multiplets are subject to unsymmetrical saturation. Fig. 4.20 illustrates the effect of selectively but unevenly saturating a double doublet and shows the resulting intensity distortions in the multiplet structure of its coupled partner, which are most apparent in a difference spectrum. Despite these distortions, the integrated intensity of the proton multiplet is unaffected by the presence of the SPT because of the equal positive and negative contributions (see Fig. 4.19d). Distortions of this sort have particular relevance to the NOE difference experiment described in Chapter 8. 

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