Satellites from Carbon-13 and Other Nuclides

There is a common misconception that strong coupling effects need not be considered at higher field, because accidental chemical equivalences or near equivalences are much less likely to occur than at lower magnetic fields. Of course, that is true. However, symmetric, not accidental, chemical equivalence is the basis for the effect shown in Fig. 6.13 j. 

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. 

Satellites are not restricted to 13C, but may be seen with other magnetic nuclei that are present at low abundance when the principal isotope has 1=0. Among the best known are 29Si (8.5%), nlCd and l13Cd (each about 9%), 199Hg (7.6%), and 207Pb (8.9%). Other nuclides, such as 15N, have a natural abundance so low that satellite signals are rarely observed in normal one-dimensional NMR spectra, but with polarization transfer methods described in Chapters 9 and 10, the existence of these weak satellites often permits observation of the less sensitive, low abundance nuclide by indirect detection. 

FIGURE 6.14 H NMR spectrum (300 MHz) showing l3C satellites, (a) CHC1 (/. ) CHC12CHC12, with a simulation of the AB portion of an ABX spectrum. 

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