Satellites proton resonances

The earliest of the magnetization transfer experiments is the spin population inversion (SPI) experiment [27]. By selectively irradiating and inverting one of the 13C satellites of a proton resonance, the recorded proton spectrum is correspondingly perturbed and enhanced. Experiments of this type have been successfully utilized to solve complex structural assignments. They also form the basis for 2D-heteronuclear chemical shift correlation experiments that are discussed in more detail later in this chapter. 

Proton NMR spectra in organic molecules can be interpreted without regard to the structural carbon framework because the predominant 12C has no nuclear spin. However, 13C has a spin of V2, which not only permits its direct observation but also provides features in the H spectrum from the 13C that is present at a natural abundance of 1.1%. As we saw in Chapter 5, J(13C-H) is normally 100—200 Hz, whereas two- and three-bond coupling constants often run 5-10 Hz. Hence a resonance line from a proton attached to a 12C atom is accompanied by weak 13C satellites separated by 1J(13C-H) and placed almost symmetrically about the main line. (The departure from precisely symmetrical disposition arises from the 13C/12C isotope effect on the 1H chemical shift, as described in Section 4.8.) For example, the proton resonance of chloroform in Fig. 6.14a shows 13C satellites. 

When the molecule in question contains more than one carbon atom, the 13C satellites often become more complex. Consider, for example, the molecule CHC12CHC12, the proton resonance of which is shown in Fig. 6.14b. The ordinary spectrum is a single line because of the magnetic equivalence of the two protons. On the other hand, the approximately 2.2% of the molecules that contain one 13C and one 12C have protons that are not magnetically equivalent. The proton resonance spectrum of these molecules is an ABX spectrum in which vA v[t. A simulation of the ABX spectrum is also shown in Fig. 6.14. The value of 3Jhh (i.e., /Aij in the ABX spectrum) can readily be observed, even though it is not obtainable from the spectrum of the fully 12C molecule, which constitutes an A2 spin system. 

The main problem for the proton detection of low natural abundance heteronuclei turns out to be the suppression of the undesired proton resonances from H nuclei not coupled to the magnetically active heteronucleus of interest. Basically, the central intense resonance is of course much more difficult to suppress than the H- Sn (and H-" Sn) satellites. However, the suppression of undesired signals is far less problematic in H-" Sn than in HMQC... 

Figure 2.37. The ethene proton resonance of the platinum complex 2.4 in CDCI3 at 80 and 400 MHz showing broadening of the Pt satellites at higher field (reproduced with permission from [9]).

Figure 3.27. Dynamic range and the detection of small signals in the presence of large ones. As the digitiser resolution and hence its dynamic range are reduced, the carbon-13 satellites of the parent proton resonance become masked by noise until they are barely discernible with only 6-bit resolution (all other acquisition parameters were identical for each spectrum). The increased noise is digitisation or quantisation noise (see text below).

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. 

As noted in Section 11(c), the intensity ratio of satellite/center proton resonates in enriched systems can be used to evaluate isotopic enrichment levels. However, if FT methods are used to obtain the sn tra, a systematic error resulting in overestlmatlon of the C enrichment will be made if the system is overpulsed due to the shorter T. of the satellite resonances. 

Another limitation of the C-satellite method is the obscuring of satellite peaks by complex n.m.r. spectra in which the proton resonances either are not well resolved or exhibit a narrow range of chemical shifts. Investigation of the biosynthesis of steroids and carbohydrates would be particularly difficult because of this limitation. 

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